Displays

Phytoliths are a major source of plant-available Si in weathered soils, particularly for crops with high Si demand, such as rice. Yet, not much is known about the evolution of Si release from phytoliths under real soil conditions. The extraction of phytoliths from soil is difficult and usually leads to changes in phytolith surface chemistry. Paddy rice cultivation induces oscillations in redox potential by alternating submergence and drainage. These oscillations may have a major impact on the evolution of phytolith Si release. For instance, reduced Fe²⁺, abundantly in solution under low redox potential may sorb onto negatively charged phytolith surfaces and form iron oxide coatings when redox potential rises after drainage. We thus hypothesise that phytolith Si release decreases with time in soil as phytolith surfaces are increasingly coated with oxides and organic matter. To test the effect of oscillating redox potential on phytolith surface chemistry and implicit changes in Si release we conduct experiments with phytoliths extracted from rice straw by dry ashing. Extracted phytoliths are sequentially exposed to soil solutions with contrasting redox potentials (anoxic vs. oxic), using either alternating anoxic-oxic solutions or exclusively oxic solutions. Anoxic exposure is conducted in Ar atmosphere (< 1% O₂ partial pressure). After each exposure events the filtrate is analysed for pH and redox potential, Fe²⁺ with the Ferrozine method, and total Fe, Al and Si with inductive-coupled plasma-optical emission spectrometry. Filter residues are sampled and analysed after 1, 2, 4, and 8 exposure steps (each lasting 2 hours), respectively. Surface chemical composition is analysed with X-ray photoelectron spectroscopy. Specific surface area is determined with N₂ gas adsorption at 77 K and surface charge is measured by determining electrophoretic mobility using dynamic light scattering. Batch dissolution experiments in mini-reactors are carried out for assessing the Si release of untreated and treated phytoliths. The experimental results will provide important information on the changes of phytolith surface chemistry and Si release from phytoliths in systems with alternating redox potentials such as rice paddies.

How to cite: Koebernick, N., Kaiser, K., Klotzbücher, A., Mikutta, R., Vetterlein, D., and Klotzbücher, T.: Do oscillating redox conditions affect long-term Si release from phytoliths?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3050, https://doi.org/10.5194/egusphere-egu2020-3050, 2020.

Soil contamination with inorganic contaminants such as lead (Pb), copper (Cu) and cadmium (Cd) is a major environmental issue, especially concerning food and groundwater security. Various studies demonstrated positive effects of Si regarding resilience of some crops towards these inorganic contaminants. One reason could be a complexation reaction of Si and the metal cations. However, this process has not been systematically investigated yet. Thus, our research contributes to reducing the mobility of Cd, Cu and Pb in contaminated soils and to decreasing their transfer into aquifers or plants.

The main goal of this study is to elucidate the extent and the mechanisms of the interactions between Pb²⁺, Cd²⁺ and Cu²⁺ and silicic acid, including the long-term kinetics, and to investigate whether the metals are bound by silicic acid. We carried out a series of precipitation experiments in aqueous solution at room temperature to understand these processes.

We used Tetraethoxysilane (TEOS) as Si source and Pb(NO₃)₂, Cd(NO₃)₂ and Cu(NO₃)₂ with an initial concentration of 10 mmol l^-1 for synthesis. Selectivity of Si towards the metals was tested in an equimolar solution of all three salts and TEOS. Time-dependency of particle growth was examined at sixteen different dates using dynamic light scattering (DLS) and transmission electron microscopy (TEM). We measured the Si and metal concentrations in the dialyzed aliquots using microwave plasma-atomic emission spectrometry (MP-AES). Spectroscopic analysis of the dialyzed and freeze dried solid phase, was performed using FTIR and ²⁹Si-NMR spectroscopy.

DLS and TEM analyses showed that the metals had an accelerating effect on the polymerization reaction of silicic acid [Cu²⁺ > (Cu²⁺, Pb²⁺, Cd²⁺) > Cd²⁺ > Pb²⁺]. Particle growth followed initial formation of nanoparticles through homogenous nucleation. Particle growth in the control synthesis (TEOS in aqueous solution) stopped after 124 days at a size of 34 nm (Z-Average). Particles in the syntheses with the metals kept growing until the experiment was completed after 211 days. The final particle sizes depended on the metal present, reaching a final size of 260 nm (Cu), 96 nm (Pb) and 196 nm (Cd). Final concentrations of up to 15, 10 and 13 µmol l^-1 of Cu, Pb and Cd, respectively, remained in the dialyzed aliquots. The Si concentrations in these aliquots increased continuously until an equilibrium was reached after 112 days at different concentrations (Cu, 7.3 mmol l^-1; Pb, 6.9 mmol l^-1; Cd, 4.8 mmol l^-1). The FTIR spectra showed a shift of the Si-O stretching vibration by 10 to 32 cm^-1 towards lower wavenumbers, which could indicate an incorporation of the metals in the polymeric network of the silicic acid. ²⁹Si-NMR relaxation experiments showed a shortening effect of Cu²⁺-ions on the relaxation time of the Si nuclei. It appears that the proportion of the rapidly relaxing components decreases for the Si-atoms deep inside the silicate matrix. This indicates that the Cu centres are located predominantly at the huge surfaces (up to 667 m² g^-1) of the Si matrix. Future extraction experiments will show how strong the metals are bound to the Si polymeric network.

How to cite: Stein, M., Georgiadis, A., Gudat, D., and Rennert, T.: Formation and properties of inorganic Si-contaminant compounds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9979, https://doi.org/10.5194/egusphere-egu2020-9979, 2020.

As oil-palm plantations are expanding rapidly in SE Asia, it is essential to ensure that soil functions are sustained after land-use transformation. This includes the maintenance of well-balanced soil nutrient levels to prevent soil degradation as well as understanding soil silicon (Si) dynamics to optimize oil-palm management. However, studies on the influence of oil-palm cultivation on soil Si pools have not yet been undertaken, although it is known that oil palms accumulate Si in their biomass and should thus affect Si pools and cycling. We hypothesized that under oil-palm monocultures, Si losses may exceed Si input into soils, due to (1) erosion of phytolith-enriched topsoils, (2) increased Si uptake by oil palms, (3) harvest and palm-frond management. The aim of this study is to compare Si pools in Acrisols of Sumatra (Indonesia) under rainforest and oil-palm plantations to assess whether these soil Si pools are significantly depleted under oil-palm plantations. We included both well-drained and riparian sites, hypothesizing that riparian sites are less prone to net Si depletion, as they receive additional Si through regular flooding and slope water from higher areas. Soil samples (1 g) from soil profiles (≤ 1 m, n = 4 for each land-use type and topographic position) were subjected to sequential Si extraction to determine mobile Si, adsorbed Si, Si in soil organic matter, Si occluded in pedogenic oxides and hydroxides, and biogenic Si.

Si in soil organic matter (SOM) and biogenic Si represent the largest Si pools in the Acrisols. Our preliminary results suggest that these pools are controlled by land use rather than by topographic position (riparian versus well-drained). Ah horizons under oil-palm plantations have lower contents of Si in SOM (0.052-1.04 mg g^-1) than those under rainforest (0.59-1.5 mg g^-1). There is no significant difference between well-drained and riparian sites, as Si input by slope water and flooding does not affect Si in SOM. Besides, the concentrations of biogenic Si are lower in soils under oil-palm plantations than under rainforest. The contents of both mobile and adsorbed Si in soils are similar to marginally higher in riparian soils (5-30 µg g^-1), compared to well-drained soils (5-20 µg g^-1), with no clear difference between land-use types. These Si fractions unlike Si in SOM are most directly influenced by Si input through slope water and flooding.

How to cite: Greenshields, B., von der Lühe, B., Hughes, H. J., Tjoa, A. B., and Sauer, D.: Impact of rainforest transformation into oil-palm plantations on Si pools in soils, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13788, https://doi.org/10.5194/egusphere-egu2020-13788, 2020.

Silicon (Si), non-essential but beneficial to plants, plays a crucial role in maintaining plant functions by alleviating a number of biotic and abiotic stresses. Applying manure, lime and chemical fertilizers to soils may impact the pool of plant available Si, but their impact over decades to century is unknown.

Here, we determined the evolution of the content of plant available Si in a silty soil derived from Quaternary loess (Haplic Luvisol), submitted to a long-term bare fallow experiment initiated in 1928 in Versailles (INRA, France). On this bare fallow soil, different treatments were applied annually since 1929, among which, manure, lime (CaCO₃), NaNO₃ and (NH₄)₂SO₄) and compared to control soil. Archived soil samples were already characterized for their basic properties (pH, CEC, OC, N, oxalate-extractable Al, Fe and Si, DCB extractable Fe, particle size distribution, elemental analysis). Here, we computed the total reserve in bases (TRB), and we determined the content of plant available Si (CaCl₂-Si) through a kinetical extraction using 0.01 M CaCl₂.

TRB was 110 cmol (+) kg^-1 in 1929. During the 90 years period, TRB (cmol (+) kg^-1) remained constant in manured plots, decreased to 96 in control/NaNO₃ plots and to 84 in the (NH₄)₂SO₄ plot whereas it increased to 160 in the CaCO₃ plot. The initial CaCl₂-Si content did not differ between the treatments, as it ranged between 25 and 30 mg kg^-1 in 1929. Annual manure supply resulted in the progressive increase of CaCl₂-Si up to 60 mg kg^-1. In this treatment, CaCl₂-Si (30 to 60 mg kg^-1) and OC (18 to 40 g kg^-1) contents were strongly and positively correlated, suggesting the continuous silicon through manure supply (probably phytoliths), and their dissolution at pH 6.6-7.6. In the four other treatments, OC content regularly decreased from 18 to 5 g kg^-1 from 1929 to 2019, but CaCl₂-Si largely differed between them. Our data suggest a strong impact of pH on CaCl₂-Si as well as the occurrence of two sources of bioavailable Si: phytoliths in limed plots (pH 6.6 to 8.8) and clay minerals in acidified plots submitted to annual (NH₄)₂SO₄ application (pH from 6 to 3.5).

Our preliminary results show that, in a given soil type, the pool of bioavailable silicon is strongly affected by soil properties, especially soil pH, OC content and weathering stage.

How to cite: Li, Z., Meunier, J.-D., Van-Oort, F., Keller, C., and Delvaux, B.: Plant available silicon in bare fallow soils after 90 years of annual supplies of manure, lime and fertilizers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20657, https://doi.org/10.5194/egusphere-egu2020-20657, 2020.

Intense irrigation along with extensive use of fertilizers significantly effects the hydrological and biogeochemical cycles in shallow aquifers. Land use changes associated with human activities are known to be a major controlling factor of the terrestrial silicon cycle, altering silicon fluxes to surface and groundwater. In the present study we determined dissolved silicon concentration (DSi) and δ³⁰Si of shallow groundwater samples collected from bore wells and piezometers of two watersheds in Southern India under contrasting land use: one intensely cultivated (Berambadi) and one forested (Mule Hole).

Intense groundwater irrigation in the Berambadi region leads to water table depletion, progressive salinization and occurrence of nitrate hotspots in groundwater. We collected groundwater samples during two periods, during the summer (dry) season in March and during the South-West monsoon season in August from both watersheds. DSi values ranged from  410 µM to 1487 µM, with a lower value during August sampling indicating dilution effects caused by monsoon precipitation. Mule Hole and Berambadi aquifer recharge mostly occurs through surface water percolation or from lateral flow. Groundwater composition thus exhibits seasonal variation depending on precipitation which can be traced using water isotopes (δ¹⁸O and δ²H). The depleted values in Berambadi groundwater (average δ¹⁸O of -2.99 ‰ and δ²H of -15.86 ‰) compared to forested watershed in Mule Hole indicate higher contribution from meteoric water likely due to quicker turnover resulting from continuous irrigation.

Silicon isotope fractionation in natural waters is majorly controlled by soil-water interaction consisting in dissolution of primary minerals and formation of secondary minerals and also from biogenic sources and uptake.  Preliminary results show no significant differences in δ³⁰Si signatures in groundwater from the two watersheds (1.1 ± 0.3 ‰) in dry season despite higher and more variable DSi concentration in cultivated watershed (1100 ± 260 µM vs. 790 ± 120 µM for the forest). Assuming similar discharge, higher DSi concentration in Berambadi during both seasons indicates increased export/mobilization of Si into aquifer when compared to forested landscape.

We will further refine our understanding of Si biogeochemistry in groundwater and the changes associated with land use by comparing the water and silicon isotopes with the germanium/silicon ratio and major element compositions in comparison with surface water data.

How to cite: Pullyottum Kavil, S., Cardinal, D., Riotte, J., Dapoigny, A., Ruiz, L., Baud, B., Vedula VSS, S., Kumar, B. S. K., Vaury, V., and Chakrabarti, R.: Assessing seasonal controls in silicon cycle and isotopic signatures of groundwater under anthropogenic stress in tropical watershed, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21873, https://doi.org/10.5194/egusphere-egu2020-21873, 2020.

Little data currently exist on the chemistry of soils on the island of San Cristóbal, Galápagos, despite the importance of this data in understanding how the island has weathered through time. We sought to resolve this lack of data by surveying soils from different elevations and in different climate zones across the island. We collected soil samples from transects and sites across a precipitation-gradient in order to describe the mineralogy and chemistry of the soils, and to understand how soils have weathered in different precipitation regimes across the island. We used a mass balance approach, coupled with chemical weathering indices, to understand profile-scale to site-scale differences in weathering.

Climate-dependent shifts in soil characteristics are apparent: at the wettest sites, the soils have the lowest pH, the highest percentage of amorphous material, and the highest loss on ignition values. We compared the saprolite, the basal material from the soil pits in which the basalt bedrock’s texture was still apparent but the material was extremely friable, and previously reported unweathered bedrock data, showing that the saprolite was highly weathered relative to the unweathered bedrock. Using the mass balance approach, we show that while base cations have been lost from soils relative to the parent material underlying the profiles, aluminum and iron concentrations have remained the same or have increased.

 We used chemical indices of weathering as evidence for the relationship between weathering intensity and precipitation, with greater weathering intensity observed in the very humid highlands compared to the less intense weathering that has occurred in the arid lowlands. The windward side of the island shows higher intensities of weathering than the leeward side. Our findings conform with other soil chemistry studies on the islands of Santa Cruz and Isabela, also in the Galápagos archipelago, showing that more intense weathering, accompanied by a greater loss of mobile elements, is observed at wetter sites.

How to cite: Percy, M. and Benninger, L.: Mineralogical and chemical variability of soils across a tropical ocean island climate gradient, San Cristóbal, Galápagos, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4266, https://doi.org/10.5194/egusphere-egu2020-4266, 2020.

Soil mineralogy plays an important role in stabilizing soil organic carbon (SOC) against decomposition by forming organo-mineral complexes with reactive mineral surfaces. However, few studies take the influence of parent material geochemistry on the development of C stabilization mechanisms into account. In addition, studies evaluating C stabilization in soil are often limited to temperate climate zones with young to intermediate aged soils. This is not representative for older, deeply weathered and leached tropical systems and limits our understanding of the relationship between geology, soil formation and their effect on C stabilization.

Here, we study the relationship between soil carbon stabilization and the geochemical properties of soils developed on different parent material along geomorphic transects in pristine tropical forest systems under comparable climate. Our study is located in the eastern part of the Congo basin along the East African Rift Mountain System where we sampled 36 one meter soil cores along nine geomorphic transects on geologies ranging from mafic to felsic geochemistry.

Carbon stocks ranged between 2.67 tC ha^-1 to 85.75 tC ha^-1 and were on average composed of 4.5% (±5.3% SD) coarse particulate organic matter, 46.0% (±10.3% SD) (micro)aggregates associated C and 49.6% (±11.2% SD) free silt and clay associated C. Our analysis shows that the topographic position of the investigated soils had no effect on SOC stocks and the distribution of soil C fractions. Regression models and partial correlation analysis reveal that strong correlations of SOC stocks exists to geochemical properties of the solid phase of soil but not to the distribution of soil C fractions. SOC decreased strongly with soil depth on soils developed on felsic parent material, but less so on mafic or intermediate parent material. In addition, mafic geochemistry shows significantly higher SOC stocks compared to their felsic counterparts.

We conclude that despite long-lasting weathering, the contrasting geochemistry of the underlying parent material leaves a footprint in soil geochemistry that affects C stocks but less so on stabilization mechanisms. We hypothesis that carbon dynamics in these undisturbed tropical forest systems are more driven by C input and nutrient recycling than by variation in C stabilization potential.  

How to cite: Reichenbach, M., Fiener, P., Wilken, F., Six, J., Kidinda, L., Mujinya, B., and Dötterl, S.: Soil organic carbon stocks in tropical soil systems under rainforests controlled by geochemistry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12993, https://doi.org/10.5194/egusphere-egu2020-12993, 2020.

Good time for soil scientists, bad time for soils? Join me at my Soil System Sciences - OECS award lecture where I will highlight how Global Change affects soils across ecosystems and what this means for future plant-soil inter­actions and biogeochemical cycles in a warming, crowded world out of balance.

Global Change from the Arctic to the Tropics has accelerated drastically in recent decades, subsequently effecting ecosystems everywhere. Soils and biogeochemical cycling within are no exception. For example, how carbon and nutrients are stabilized in and released from soil is highly affected by changing land use and climate. Despite these changes, soil in earth system models is not represented mechanistically, but rather given a mostly budgetary “black box” function. No methodological framework is available that accounts for the combined effects of climate, geochemistry and disturbance on soil dynamics at larger scales. In addition, most of our process understanding of biogeochemical cycling in soils is derived from data-rich temperate regions. This data has limited applicability in low latitudinal (tropics) or high latitudinal (boreal/subpolar) climate zones, where soils have different properties and drastically different developmental histories.

In my talk I will illustrate with a few examples how the gaps in our understanding of soil processes across climate zones and dismissing lateral soil fluxes leads to large uncertainties in predicting future trajectories of the global carbon cycle. I will highlight how the interactions of weathering and disturbance can influence and dominate biogeochemical cycles and microbial processes in soils. I will also discuss some directions where geochemical proxies that are available at the global scale can be useful to model the spatial and temporal patterns of soil carbon storage and turnover.

How to cite: Doetterl, S.: Soil resource dynamics in a changing world, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3445, https://doi.org/10.5194/egusphere-egu2020-3445, 2020.

The weathering of continental silicate rocks is a main sink of CO₂ at the geological timescale. As it is dependent on the climatic conditions (more weathering in a warmer world), the silicate weathering acts as a negative feedback on the carbon cycle, limiting the amplitude of past climatic changes.

Many contributions have shown that silicate weathering efficiency (the « weatherability ») is strongly correlated to the physical erosion. Because of this tight link, many works have focused on the role of mountain ranges in the climatic evolution, because those areas are characterized by intense physical denudation, thus potentially boosting chemical weathering. Simply speaking, periods of active mountain building are suspected to generate cold conditions.

Conversely, little attention has been paid to the role of large and flat continental areas. Due to the lack of physical erosion in those flat areas, the weathering processes will generate thick regoliths, progressively shielding the bedrock and ultimately decreasing the weatherability. Periods of limited mountain building activity might generate very high CO₂ level and warm climatic episodes.

However, this simple scheme, defining two extreme poles for the surficial Earth system (one mountainous and cold, the other flat and warm) raises several questions:

• the two modes (mountainous and flat) generally co-exist. Their relative role in the control of the climate is probably dependent on the continental configuration, and on the location of tectonically active and non-active areas in latitude and longitude.
• the dynamics of the thick regolith is not well constrained. How long does it take to generate thick regoliths? What is the response time of thick regoliths to a perturbation?
• what about the horizontal transfer of sediments? Recent works have shown that sediments are exported from mountain ranges and weathered in plains at the feet of the mountains. How can we incorporate this into numerical models? 

We will explore the role of the regolith thickness with the spatially-resolved GEOCLIM model. We will focus on the consequences of the colonization of the continents by vascular land plants over the course of the Devonian. This event is suspected to have impacted the weatherability of all the continental surfaces in the same direction (increase in weatherability). We will show that the way atmospheric CO₂ is responding is depending on the initial state of the weathering system, prior to the colonization event. We will also explore the response time of the regolith cover to the global environmental change. We show that short glacial events can be generated in the direct vicinity of the colonization event, if the response time of the regolith layer is long and the colonization is fast. This cold overshoot disappears when the colonization time is assumed to be long (10 Myr), and the continental configuration becomes a critical factor impacting the CO₂ evolution.

How to cite: Godderis, Y., Maffre, P., and Donnadieu, Y.: Thick regoliths and the geological history of climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19235, https://doi.org/10.5194/egusphere-egu2020-19235, 2020.

Continental weathering is a major sink of atmospheric CO₂ over long time scale (> 1Ma) through CO₂ consumption during chemical weathering of silicate minerals. Yet the importance of this process in climate evolution remains debated. The Late Cretaceous period records a pronounced decrease in temperatures at a global scale between 90 and 65 million years, that marks the first step of the progressive climatic decline leading to our modern climate. This cooling is concomitant to a major tectonic uplift of the African and South American continents (Friedrich et al., 2012; Gallagher et al., 1999).

The main objective of this work is to bring new constraints on the links between tectonic, weathering and climate processes, in order to explore the potentially determinant impact of this tectonic uplift on the late Cretaceous long-term cooling. We use here a new proxy of silicate weathering, based on the coupled Lu-Hf and Sm-Nd isotope systems in clays. This proxy has been recently calibrated in modern environments (Bayon et al., 2016) but has only been scarcely applied to deep-time environments. This approach was coupled to clay mineralogy, assessing the evolution of the intensity of physical erosion linked to the uplift. In this study, these coupled weathering and erosion proxies have been applied on clays recovered from DSDP site 356 (Brazil margin).

A change in detrital clay material is recorded at the Santonian-Campanian transition (83.6 Ma), characterized by a decrease in primary clay minerals (illite, chlorite) proportions and an increase of smectite. We interpret this change as reflecting an increase in chemical weathering forming pedogenic smectites, which would have followed an episode of intense mechanical erosion from the Turonian to the Santonian. Enhanced chemical weathering, lasting until the Maastrichtian, was likely associated to locally increased hydrolysing conditions, that would be consistent with the observed decrease in palygorskite proportions, a clay mineral commonly formed in arid conditions.

We interpret the ɛ_Nd decrease  observed (at 87 Ma) as reflecting a change of sources with a possibly decreasing contribution of basalts from the Parana-Etendeka traps associated to an increasing contribution of old crustal material. Δɛ_Hf values, which represents the deviation of the sample’s ɛ_Hf compared to the clay array (Bayon et al., 2016), highlight a marked increase in the intensity of chemical weathering at the transition between the Santonian and the Campanian, that is coherent with the concomitant evolution of clay mineral assemblages.

Our new results point to the existence of a relatively arid local climate in the Turonian to Santonian interval, which would have favoured the physical disaggregation of rocks during the uplift of the Brazilian margin. The new relief would thereafter have favoured, from the Campanian onward, locally enhanced precipitations and more hydrolysing conditions, and thus intensified chemical weathering. In the context of the Brazilian margin, the observed chemical weathering increase would then represent the consequence of the active uplift.

Bayon et al. (2016) EPSL 438, p. 25-36.

Friedrich et al. (2012) GSA 40(2), p. 107-110.

Gallagher and Brown (1999) Geol. Soc. Londond Sepc. Pub 153(1), p.41-53

How to cite: Corentin, P., Pucéat, E., Pellenard, P., Freslon, N., Guiraud, M., Blondet, J., and Bayon, G.: Evolution of the continental weathering of the South American margin during the Late Cretaceous: a new look upon the tectonic-climate links., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7022, https://doi.org/10.5194/egusphere-egu2020-7022, 2020.

Laterite formations are deep regoliths, up to one hundred of meters thick, that represent about 80% of the global soil volume. Formed under tropical conditions, laterites result from successive chemical weathering reactions over long periods up to tens of millions of years. Laterites can thus be seen as both an actor of the long-term carbon cycle, through CO₂ consumption by silicate weathering and witness of the long-term climate evolution. Indeed, secondary minerals found nowadays in lateritic profiles may have recorded past environmental conditions that prevailed at the time of their formation. Despite the large distribution of lateritic formations around the world, their timing and processes of formations as well as their preservation over long period of time remain unclear.

Here, we investigate an entire weathering profile developed on the Guiana Shield, in Brownsberg mountains, Suriname. The sampling region has remained in equatorial position for the last 100 Myr and has seen lateritic development since early Tertiary [1]. Such latitudinal stability offers the possibility to look at links between long-term climate evolution or climatic events and long-term chemical weathering processes.

The lateritic profile shows a strong loss in both alkali and alkaline-earth elements as well as a desilication, and an enrichment in Fe, particularly in the duricrust. The study of trace elements and rare earth elements highlights various geochemical processes behind the development of a lateritic – bauxitic profile.

 (U-Th-Sm)/He ages of iron oxides from the duricrust show the presence of multiple generations of Fe oxides, demonstrating that the Brownsberg profile underwent multiple dissolution and recrystallization phases since its formation, at least 19.9 ± 1.8 Ma ago. These successive weathering processes may have led to the particular enrichment in the profile such as the one observed for Fe and V in the duricrust. Measurement of d18O – dD on secondary minerals, i.e. kaolinite and Fe-oxides s.l., will help to connect mineralogical and geochemical variations with the environmental conditions that prevailed at the time of their formation [2].

[1] Theveniaut and Freyssinet, 2002. Pal. Pal. Pal., 178, 91-117

[2] Girard et al., 2000. GCA, 64 n°3, 409 – 426

How to cite: Ansart, C., Calmels, D., Gautheron, C., Monvoisin, G., Agrinier, P., Couëffe, R., Roig, J.-Y., and Quantin, C.: Combining (U-Th-Sm)/He dating and geochemical budget to understand laterite formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9521, https://doi.org/10.5194/egusphere-egu2020-9521, 2020.

Lateritic soils are deep weathering profiles, developed in tectonically quiescent areas under tropical conditions and over long timescales. Laterites are key components in the regulation of element cycle in the Earth’s history but, the timing between climatic changes and lateritic weathering episodes remains unconstrained. The combination of chronometric and weathering proxies is one way to build a comprehensive story of laterite formation.

In this study, two lateritic vertical profiles were targeted on the outer part of the Guyana Shield in the Amazon Basin. This region is tectonically stable and subjected to a rainy tropical climate since the Cretaceous. The first soil profile, located in the Brownsberg Mountains, Suriname, is developed on Proterozoic Greenstone [1]. The second lateritic cover, already studied and dated using EPR technique [2], is developed over the Cretaceous sedimentary Alter do Chao formation, Brazil. Both lateritic profiles are characterized by 1/ a total depletion of soluble elements and weathering of primary minerals at the base of the profile and 2/ a desilication followed by the formation of Fe and Al duricrusts on top. Here, traditional geochemical budgets are seconded by measurements of Si isotopes in both soils (bulk and/or clay fractions) and laterite draining streams. Silicon isotopes (δ³⁰Si) are known to be an excellent weathering proxy, fractionated during clay mineral formation [3].

In Suriname bulk soils, heavier δ³⁰Si is associated with lateritization due to the “buffering” quartz exerts on bulk δ³⁰Si. However, if clay fractions are isolated, the observed strong enrichment in light Si (Δδ³⁰Si_{clay fraction-bedrock} up to -0.9‰) is in line with the weathering of primary minerals and the formation of kaolinite. The dating of this intense weathering episode is c.a. 2-9 Ma based on preliminary EPR dating of kaolinites.

Regarding the Brazilian laterite, the material forming the Alter do Chao formation already suffered weathering episodes before deposition. The combination of EPR dating [2] and δ³⁰Si measurements on the clay fraction reveals two distinct formation phases. First, chemical weathering is limited to the 37-22 Ma period. Second, the progressive depletion of δ³⁰Si from the bottom to the top of the lateritic profile highlights a replacement of a first kaolinite generation by a second population through dissolution-reprecipitation around 6 Ma, as previously inferred by EPR dating [2].

These results, in combination with elemental mass budgets, give us better constraints to estimate the intensity and the timing of element mass transfers during laterite formation. 

[1] Monsels & van Bergen (2017) Journal of Geochemical Exploration 180, 71-90. [2] Balan et al. (2005) GCA 69 (9), 2193-2204. [3] Opfergelt & Delmelle (2012) Comptes Rendus Geoscience 334 (11), 723-738.

How to cite: Guinoiseau, D., Bouchez, J., Fekiacova, Z., Allard, T., Ansart, C., and Quantin, C.: Silicon isotopes as tracers of laterite formation processes through time and space, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13820, https://doi.org/10.5194/egusphere-egu2020-13820, 2020.

Laterites are developing under intense chemical weathering and low physical erosion rates. Despite their large extension at the Earth’s surface, there is still a lack of time constraints for their formation, evolution and relation with climatic change. Nevertheless, several chronological studies show that they represent a geological record at least all along the Cenozoic Era. Indeed, laterite samples often contain several coexisting generations of iron oxides and oxyhydroxides that indicate successive weathering processes due to the dissolution of previously formed phases followed by reprecipitation. This study focuses on the condition and chronology of weathering in Northeastern French Guiana which generated pedogenic iron crusts on Paleoproterozoic mafic and intermediate rocks. It offers the opportunity to document the evolution of this part of the Guyana Shield, known as a tectonically stable area since the Cretaceous. The two sampling sites, Kaw and Baduel, are paleosurfaces at 300m and 100m elevations, respectively, that have been dated previously by paleomagnetism, providing Eocene ages for both sites, albeit with some substantial uncertainties and dispersion [1].

Since the duricrust (top layer) of the lateritic profile is enriched in hematite and goethite, we aim to date those mineral phases using the (U-Th-Sm)/He method. Older ages are from Oligocene and Miocene epochs for the Kaw and Baduel sites, respectively, with a large dispersion in the age values, as expected from the presence of several generations of Fe-minerals. Identification of petrological relationship between these different generations is hindered by their intimate mixing. In order to overcome this difficulty and to identify the episodes of weathering and mineral precipitation, we coupled a number of mineralogical and geochemical analyses, namely through powder and single grain X-ray diffraction, energy dispersive X-ray spectrometry (SEM-EDS) and solution- and LA-ICP-MS. Data on formation ages of secondary iron phases will be discussed by reference to literature, in terms of geodynamic and paleoclimatic forcing.

[1] Théveniaut, H., and Freyssinet, P. (2002): Timing of lateritization on the Guiana Shield: synthesis of paleomagnetic results from French Guiana and Suriname. Palaeogeography, Palaeoclimatology, Palaeoecology (178) 91-117

How to cite: Heller, B., Bressan Riffel, S., Gautheron, C., Allard, T., Morin, G., Roig, J.-Y., and Coueffe, R.: (U-Th-Sm)/He dating of supergene Fe duricrusts in NE French Guiana: implications of a multiproxy approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11900, https://doi.org/10.5194/egusphere-egu2020-11900, 2020.

It is well-understood that iron redox dynamics can lead to both organic matter persistence—through the stabilization of organic matter in iron mineral associations or in Fe-cemented aggregate structures—as well as organic matter decomposition—through microbial respiration on ferric iron and through the production of hydroxyl radicals during the oxidation of ferrous iron (i.e., Fenton chemistry). However, we do not understand how the relative impact of each of these processes manifests during redox fluctuations. For instance, we do not understand how the net decomposition of organic matter via Fenton chemistry during the oxidation of ferrous iron compares with the net protection of organic matter via newly formed short-range-ordered (SRO) ferric minerals; nor do we understand how much of that recently-protected organic matter will be lost during a transient anoxic event. Certainly, some of the key parameters determining the balance of iron-mediated OM protection vs. decomposition include the timescales of the redox fluctuations (the duration of the oxic or anoxic periods), the rates of iron oxidation, and critically, the dynamics of the resident microbial community. Here, we explore these parameters using upland soils from the Calhoun and Luquillo Critical Zone Observatories in laboratory experiments. (1) We quantified Fe-stimulated OM protection vs. decomposition by amending ¹³C-labeled dissolved OM (DOM) and ⁵⁷Fe-labeled Fe^II_aq to soil slurries incubated under either static oxic or fluctuating redox conditions. (2) We tracked the rates of Fe reduction, CO2 production, and CH4 production from soils during multiple redox fluctuations with three different lengths of O2 exposure and equal lengths of anoxia. From these experiments we find that (1) the addition of iron only conferred net protection to newly added organic matter and only under strict oxic conditions, whereas in treatments without added DOC or that were exposed to transient anoxia, the addition of iron stimulated net organic matter decomposition. (2) That the length of O₂ exposure altered the balance of Fe reduction and methanogenesis during the anoxic periods with longer O2 exposure suppressing Fe reduction and enhancing methanogenesis. These findings suggest iron redox dynamics will likely tend to enhance organic matter decomposition in soils. But, importantly, these studies have specifically focused on localized iron dynamics and biogeochemical coupling with organic matter by using well-mixed systems. Spatial heterogeneity and soil structural features have yet to be evaluated in this context.

How to cite: Thompson, A.: Dynamics of organic matter decomposition during iron redox fluctuations in soils, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12540, https://doi.org/10.5194/egusphere-egu2020-12540, 2020.

In soils and sediments, poorly-crystalline, short-range order (SRO) iron minerals constitute one of the most abundant and reactive components. With high surface areas, SRO minerals like ferrihydrite (Fe₁₀O₁₄(OH)₂+mH₂O) influence the biogeochemical cycling of trace elements and nutrients, particularly in redox dynamic environments. While under oxic conditions SRO iron mineral adsorption capacity is high, in the absence of O₂, Fe^III acts as an electron acceptor during microbial respiration. Electron transfer induces transformations in pure iron minerals, impacting the release and re-distribution of SRO-associated trace elements and nutrients.

In nature, however, pure SRO iron minerals rarely form. Rather, the ubiquitous presence of natural organic matter (OM) in soils and sediments promotes the formation mineral-organic associations. Coprecipitation of ferrihydrite with OM decreases particle size and alters the mineral susceptibility towards microbial reduction. Thus, under reducing conditions, an increased rate and extent of mineral transformation could be expected for OM-associated ferrihydrite. However, in the presence of abiotic reductants, mineral transformation rates and extents in OM-associated ferrihydrite are markedly inhibited when compared to that of a pure ferrihydrite. Using polygalacturonic acid (PGA) as a proxy for acid carbohydrate fraction found in exopolymeric substances, we reacted ferrihydrite-PGA coprecipitates of varying C:Fe molar ratios (0-2.5) with ferrous Fe (Fe(II), 0.5-5.0 mM) at neutral pH for up to 5 weeks. Through a combination of XRD and ⁵⁷Fe Mössbauer spectroscopy, we showed that at all Fe(II) concentrations, the kinetics and extent of mineral transformation decreased with increasing C content of the coprecipitates. Similarly, ferrihydrite-OM coprecipitates comprising PGA, citric acid (CA), or galacturonic acid (GA) of similar C:Fe molar ratios (~0.6) also showed inhibited mineral transformations compared to a pure ferrihydrite, whereby the extent of inhibition of mineral transformations followed the order GA>>CA>PGA. In addition, electron microscopy imaging showed that the crystal morphology of the secondary mineral phases varied with the varying chemical structure of the coprecipitating organic ligands. Despite this, applications of stable Fe isotope tracers revealed that all OM-associated ferrihydrite actively partook in iron atom exchange, suggesting that the presence of OM inhibited crystal growth of more crystalline phases, therefore again leading to SRO phases during iron atom exchange. Collectively, the stabilization of high surface-area ferrihydrite under reducing conditions via recrystallization has implications for the release and re-distribution of ferrihydrite-associated trace elements and nutrients in redox-dynamic environments.

How to cite: ThomasArrigo, L. K. and Kretzschmar, R.: Ferrihydrite mineral transformations in the presence of Fe(II) and organic ligands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21278, https://doi.org/10.5194/egusphere-egu2020-21278, 2020.

Electron acceptors (NO₃^–, SO₄^2–, Fe³⁺, Mn⁴⁺) play a crucial function in the oxidation of soil recalcitrant organic compounds. Soils that present large amount of total Fe (8-57 g kg^-1soil) and organic (C) (10-110 g kg^-1soil), iron-reducing bacteria (IRB)  may play a importan role. In the present study we hypothesized that IRB which reduce Fe(III)(oxyhydr)oxide of low solubility to soluble Fe(II), can contribute substantially to the degradation of lignin from soil organic matter (SOM). The aim of this study was to isolate IRB and evaluate their importance in lignin degradation. IRB were obtained from topsoils of different climates (humid temperate, cold temperate, subpolar), vegetation type (steppe, rainforest) and parent materials (granitic, volcanic, fluvio-glacial, basaltic-Antartic and metamorphic). The potential of IRB to reduce Fe(III) was assessed with lactate substrate as source of carbon (C) and anthraquinone-2,6-disulfonate (AQDS) as electron acceptor. The contribution of IRB to lignin degradation was assessed in an anaerobic microcosms experiment for 36 h. The CO₂ efflux from sterilized and reinoculated soil with IRB was compared with sterilized (abiotic), non-sterilized (biotic) and induced Fenton reaction. Lignin degradation by IRB was examined by: 1) bacterial growth containing alkali lignin and alkali lignin disappearance during incubation, 2) Lignin peroxidase and manganese peroxidase activities originated from IRB, 3) cells abundance estimated from ATP synthase from bacteria growing in alkali lignin and 4) lignin degradation monitored by fluorescence disappearance intensity. The major microbial group for Fe(III) reduction, as essayed by PLFA and nested-PCR and sequencing different species were Geobactericeae-strains (G. metallireducens and G. lovleyi) in all studied. The CO₂ respiration in reinoculated soils was 140% higher than the CO₂ release by abiotic and Fenton reaction and, 40% lower than biotic treated soil. The Fe(II) extractable in HCl in soil derived from basaltic-Antarctic parent material showed 362 % more Fe(II) solubilisation than that of biotic treatment. Fluorescence intensity decreased during lignin degradation and it was closely correlated with CO₂ release in the same sample. We conclude that IRB community such as Geobacter spp. Uses intensively Fe(III) as an electron acceptor to oxidize lignin compounds, and this process is especially active in Fe rich soils.

How to cite: Merino, C., Matus, F., Kuzyakov, Y., Jofré, I., and Najera, F.: Iron-reducing bacteria play a key role in lignin degradation by electron transferring from soil organic matter, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11949, https://doi.org/10.5194/egusphere-egu2020-11949, 2020.

Iron-Organic Matter (Fe-OM) aggregates produced by redox alternation in wetlands are a key factor in the control of metallic pollutants mobility. Their ability to adsorb metal(loid)s depends on the size, morphology and structural arrangement between Fe and OM phases, which are mainly controlled by the OM occurrence. The physical, chemical and morphological organization of such aggregates is influenced by the physico-chemical conditions prevailing in the environment. Calcium (Ca) is a common major ion in natural waters which exhibits high affinity for OM. It can thus modify the size and the structural organization of Fe-OM aggregates and, subsequently, their  ability to bind metal(loid)s. Among metal(loids), arsenic (As) is of major importance because of its high toxicity and its high affinity towards Fe(III)-oxyhydroxides. Moreover, Fe-OM aggregates are an important factor controlling the mobility of arsenic (As) in the environment.

Mimetic natural Fe-OM aggregates were synthesized at various Fe/OM and Ca/Fe ratios. After a fine characterization of the size and structural organization, Fe-OM-Ca associations were used to perform As binding sorption experiments at 2 As/Fe ratios. The suspensions were stirred during 24h and subsequently filtrated ant ultra-filtrated.

Our study demonstrates that Ca strongly influences the Fe-OM aggregates physical organisation. For low Ca/Fe ratio, Fe phases exhibit a fractal organization in which Fe phases are composed of oligomers, and primary nano-aggregates (around 6 nm) which aggregate in larger Fe secondary aggregates (>200 nm). Both are embedded in the OM matrix composed of isolated molecules and OM aggregates. For high Ca/Fe ratios, OM, Fe oligomers and primary nano-aggregates form a large continuous network where Fe phases are connected by OM large molecules. With the increasing Ca/Fe ratio, the amount of Fe oligomers decreases to the benefit of larger primary nano-aggregates (increase of their geometrical radius). Ultrafiltration experiments demonstrated that DOC, Fe, Ca and As follow the same size distribution. Surprisingly, As sorption increases with the increasing size and amount of primary nano-aggregates and the formation of the large network. SAXS analyses revealed that in such network, the distance between primary nano-aggregates increases as compared to their distance in secondary aggregates. All this results suggest that, with the increasing Ca/Fe ratio, although the primary nano-aggregates size increase, their structural distance allows to rise the availability of their binding site for As.

This study demonstrates that Ca not only controls the Fe-OM structural organization but also its subsequent capacity to bind toxic elements such as As. These results are of major importance since such parameter was never so clearly evidence. They show that the actual representation of the physical organisation of Fe-OM aggregates and its reactivity have to be renewed as well as the geochemical models.

How to cite: Beauvois, A., Vantelon, D., Jestin, J., Bouhnik-Le Coz, M., Catrouillet, C., Rivard, C., Dupont, A., Briois, V., Bizien, T., and Davranche, M.: How does Ca modify the surface reactivity of Fe-OM aggregates against Arsenic binding?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14593, https://doi.org/10.5194/egusphere-egu2020-14593, 2020.

Ferrihydrite (Fh) is a short-range ordered Fe(III) oxyhydroxide which is often associated with significant amounts of trace metals in soils and sediments. Fh is frequently observed to be unstable under reducing conditions and can be transformed into secondary Fe minerals, during which associated trace metals are either redistributed in the minerals or released into solution. Natural organic matter (NOM), often coexisting with Fe minerals, is known to alter the transformation pathways of Fh, however, its effect on associated trace metals is not well known. Here we investigated how cadmium (Cd) is redistributed when Fh undergoes microbial Fe(III) reduction in the presence of NOM. Incubation with the Fe(III)-reducing bacteria Geobacter sulfurreducens showed that the rate and extent of reduction of Cd-loaded Fh were enhanced by increasing concentrations of NOM (i.e. increasing C/Fe ratio). Under low C/Fe ratios, only 3-5% of Fe(III) was reduced, but around 70% of pre-adsorbed Cd was released into the aqueous phase due to Fh transformation to lepidocrocite. At high C/Fe ratio (1.6), the Fe(III) reduction rate in the first 6 hours became nearly 3 times faster than in the absence of NOM, and more than 35% of Fe(III) was reduced over 5 days, possibly because the adsorbed NOM decreased the size of aggregates and the residual NOM in solution worked as electron shuttle. No Fh transformation was observed (using Mössbauer spectroscopy or X-ray diffraction) suggesting NOM could impede Fh crystal growth, and there was only negligible Cd release into solution. Lower concentrations of aqueous Cd lowered the metal's toxicity toward Geobacter sulfurreducens thus enabling more prolonged microbial reduction. The negligible Cd released during microbial Fh reduction might be due to recapture of Cd (initially bound to Fh) by NOM adsorbed on Fh. In summary, our study suggests the presence of NOM can be beneficial for the stability of Cd adsorbed to Fh under reducing conditions.

How to cite: Zhou, Z., Muehe, E. M., Tomaszewski, E. J., Kappler, A., and Byrne, J. M.: Influence of Natural Organic Matter on the Fate of Cadmium During Microbial Ferrihydrite Reduction , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2938, https://doi.org/10.5194/egusphere-egu2020-2938, 2020.

Redox-induced release dynamics of arsenic (As) in an abandoned geogenic arsenic-contaminated gold mine spoil in Ghana has never been studied. Therefore, our aim was to investigate the effects of varied soil redox conditions on mobilisation and speciation of As from an abandoned highly contaminated gold mine spoil (with 4,283 mg As/kg soil) using an automated biogeochemical microcosm set-up. We also studied the impact of redox potential (E_H)-dependent changes of pH, Fe, Mn, Al, S, Cl^-, SO₄^2-, DOC, DIC, DC, DN and SUVA on the release dynamics of As. As mineralogical composition and speciation were further determined using a synchrotron-based X-ray absorption spectroscopy (XANES). Linear combination fits of XANES results indicated that scorodite (FeAsSO₄) and arsenopyrite (FeAsS) are the two major As-containing minerals in the studied mine spoil. Geochemical fractionation using sequential extraction procedure indicated greater proportions of the extracted As in the amorphous iron oxide fraction III (1390.13 mg kg^-1, 32.5% of the total As) and residual fraction V (2591.67 mg kg^-1, 60.5% of the total As). Concentrations of dissolved Fe and SUVA were higher during reducing conditions and decreased under oxidising conditions and both showed negative significant relationships with E_H (E_H and SUVA: r = -0.76, P < 0.01; E_H and Fe: r = -0.75). Mobilisation of As was greater under reducing conditions (dissolved As = 136.68 mg/L) than in oxidising environments (dissolved As = 8.06 mg/L). The release of As under low E_H can be explained by the associated reductive dissolution of Fe oxides, as demonstrated by the high positive significant relationship between Fe and As (r = +0.97, P < 0.01). Dissolved As release dynamics can also be linked to desorption of aromatic carbon compounds on the surfaces of dissolved organic carbon, as demonstrated by the high positive significant correlation between SUVA and As (r = +0.573, P < 0.01). Further, the release dynamics of dissolved As was also affected by changes in pH (r = -0.4, P < 0.05), but were not affected by redox-induced dynamics of Mn, Al, S, Cl^-, SO₄^2-, DOC, DIC, DC, DN. We conclude that conditions such as flooding and high rainfall in this contaminated mine spoil could create reducing conditions, leading to reductive dissolution of the arsenopyrite As-bearing primary mineral and may lead to higher As release into the groundwater, translocation into the food chain with potential impacts on human health.

Keywords: Arsenopyrite, redox chemistry, arsenic mobilisation, gold mine spoil, reductive and oxidative dissolution.

How to cite: Mensah, A. K., Marschner, B., Wang, J., Shaheen, S. M., and Rinklebe, J.: Mobilization, release and speciation of arsenic in an As-contaminated gold mine spoil under varied soil redox conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1360, https://doi.org/10.5194/egusphere-egu2020-1360, 2020.

How to cite: Van Thuyne, J.: Are Fungus-Growing Termites Major Contributors To Sediment Grain-Size Distributions In Subtropical Environments? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1280, https://doi.org/10.5194/egusphere-egu2020-1280, 2020.

The role of Arbuscular mycorrhizal fungi (AMF) in conditioning soils is achieved by its metabolite glomalin. However, glomalin has not been biochemically defined, it has often been quantified in terms of Glomalin-related soil protein (GRSP). Therefore, as a proxy for AMF, GRSP has been widely used to explore the role of AMFs in various ecosystems around the world. However, information on AMF-carbon-weathering interactions is limited. To evaluate the relationship among the AMF, carbon content, nutrients and chemical index of alteration (CIA), GRSP in 133 surface sediment samples and the major components, nutrient content and the grain size of 304 surface sediment samples were analyzed in the wetlands of the Liaohe Delta (LHD), including the upper delta plain wetlands (UDPW) and its adjacent shallow sea wetlands (SSW). The GRSP concentrations averaged over 133 samples were 2.30 ± 0.17 mg g^-1, in a range between 0.11 and 11.31mg g^-1, and significantly affected by the land use pattern. The ratios of organic carbon in GRSP (GRSP-C) to soil organic carbon (SOC) ranged between 0.71 and 25.34%, with an average of 10.34 ± 0.52%, confirmed that GRSP was an important part of the sediment carbon pool in the LHD. In addition, it is worth noting that the carbon dynamics in these wetlands were closely related to human activities. The CIA values varied from 18.97 to 68.75, and were significantly higher in the UDPW than in the SSW (p<0.05). In order to explore the effect of AMF on weathering process, triangle maps were constructed to analyze the weathering characteristics of sediment samples with different GRSP concentrations. The results indicated that biologically AMF-mediated weathering in this area leads to the formation of clay minerals. Moreover, The CIA was significantly correlated with GRSP concentrations (r=0.43, p<0.01), and both the CIA and GRSP were significantly correlated with nutrient concentrations (SOC, TN, P, and Fe) (p<0.01). These results indicate that AMF excursions in wetland ecosystems enhance carbon sequestration and mineral weathering, and in turn they alter retention of at least some nutrients.

How to cite: Pei, L., Ye, S., Yuan, H., Pei, S., Xie, S., and Wang, J.: The distributions of Glomalin-related soil protein in the coastal wetlands of the Liaohe Delta, Northeast China: Implications for mineral weathering and carbon sequestration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2496, https://doi.org/10.5194/egusphere-egu2020-2496, 2020.

Chestnut soils are an obligatory component of solonetz complexes in the northern part of the Caspian lowland, differ from solonetzes by the morphological properties of the horizons, although these soils are usually located at a distance of several meters.

For the chestnut soil we can see the following vegetation: forbs-fescue-feather association with spirea, sometimes with a thin moss cover. Above the solonetz dominates Kochia prostrata and Artemisia pauciflora with Myosúrus sp. and rare Poa sp. curtains.

The aim of the study is to identify the mineralogical composition of clay fraction (<1 μm) of chestnut soil and to compare it with the mineralogical composition of the solonetz in the area with unexpressed microrelief.

To separate soil fractions <1 μm samples were rubbed into a thick paste and sedimented. Oriented preparations of fractions were examined by XRD method.

The balance of the mineral phases of clay in soils and parent rocks is the same - mixed-layered minerals prevail over illite. An exception is only the upper horizons of the compared soils, in which the content of illite prevails over mixed-layer minerals. In this case, the thickness of the surface horizons differs more significantly (5 times) than the difference in the content of illite in the clay fraction of the solonetz in SEL (0-5 cm) horizon. This soils are also have certain features of similarity in the crystallochemical shape: the imperfection of the kaolinite structure and the superdispersed shape of the mixed-layer phase at the surface horizons, as well as the appearance of individual smectite and chlorite packets in the mixed-layer phase in the lower horizons (BC and C).

The work was supported by RFBR 18-016-00129-а.

How to cite: Churilin, N., Lebedeva, M., and Varlamov, E.: Mineralogical composition of solonetzic complex with unexpressed micro-relief in the northern part of the caspian lowland., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3332, https://doi.org/10.5194/egusphere-egu2020-3332, 2020.

‘Machair’ describes a landscape form that is present along the Atlantic seaboard of Scotland and Ireland, and that is characterized by a gently sloping coastal plain developed from aeolian carbonate and quartzose sand. We characterized three grassland soil profiles along a coastal transect on Harris (Outer Hebrides, Scotland) by standard methods (colour, texture, pH, wet-chemical extractions), infrared spectroscopy, X-ray diffractometry, X-ray fluorescence spectrometry and differential scanning calorimetry. Our aim was to understand the impacts of humankind, matter input, weathering and accumulation of soil organic matter (SOM) on chemical processes and soil properties. One of the profiles differed distinctly from the other two, in particular regarding depth, texture, carbonate and SOM contents, and properties of SOM (relative content of rather labile permanganate-oxidizable SOM, transformation state), presumably caused by earlier land use as arable land. We classified the soil with the least depth as Hypereutric Leptosol, and the others as Cambic and Calcaric Phaeozem. Thermally stable SOM was present in all samples, likely pointing to pyrogenic SOM, i.e. black carbon. The mineralogical composition differed among the profiles and reflected the intermediate character of the local rocks. In all topsoil horizons, we identified Mg-hydroxy-interlayered minerals (HIMs), which are rather rare, given the commonly low abundance of Mg ions in the soil solution relative to Ca, or Al in acidic soil. The share of Mg-HIMs of the total minerals in the clay fraction ranged from 25% in a subsoil to 71% in a topsoil horizon. We suggest that sea spray is the source of subsequently intercalated Mg. This composition of the clay fraction, which is possibly typical of soil on certain machair sites, and is the result of a pedogenic process, surely affects soil properties and processes such as cation exchange capacity and SOM storage and thus element cycles.

How to cite: Rennert, T. and Herrmann, L.: Sea spray and land use affect clay mineral and soil organic matter properties in soil on machair (Harris, Scotland), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4425, https://doi.org/10.5194/egusphere-egu2020-4425, 2020.

Flooding caused by snowmelt runoff in the spring and early summer and heavy rainfall in the summer could enhance P release into nearby surface water bodies causing eutrophication. Six soil amendments were tested for their effectiveness  in reducing P release from flooded-soils. Soils were collected from the flood-prone fields in the Red River Valley region in Manitoba, Canada. The tested amendments were gypsum, magnesium sulphate, alum, ferric chloride, zeolite and manganese oxides. Intact soil columns were subjected to flooding for 8 weeks at 4^oC simulating the snowmelt in the spring and the early summer and at 22^oC simulating flooding occurrences in the summer.  Release of soil P into soil solution and floodwater was higher at 22^oC than that at 4^oC. Gypsum, magnesium sulphate, alum and ferric chloride were effective in reducing the concentrations of P in the pore- and flood-water at various capacities. Ongoing research on zeolite and manganese oxide suggests that manganese oxide was more effective in reducing soluble P concentrations in soils at early days of flooding.

How to cite: Attanayake, C., Kumaragamage, D., Weerasekara, C., Vitharana, U., Dharmakeerthi, S., Van, E., Goltz, D., and Indraratne, S.: Soil amendments reduce P release from flooded soils: Incubation studies simulating snowmelt and summer flooding, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11873, https://doi.org/10.5194/egusphere-egu2020-11873, 2020.

Due to intensified land use (agriculture, forestry) humans directly influence silicon (Si) cycling on a global scale. In this context, especially Si exports by harvested crops (most of them are Si accumulators) and increased erosion rates generally lead to a Si loss in agricultural soils (anthropogenic desilication). Harvesting of field crops can cause Si losses of up to 100 kg Si ha^-1 per year. On a global scale about 35% of total phytogenic Si is synthesized by field crops due to their relatively high Si contents as well as biomasses and this proportion is going to increase with increased agricultural production within the next decades. In order to avoid (natural) limitations of plant available Si and enhance plant growth and resistance against abiotic and biotic stresses, Si fertilization is widely used, especially in (sub)tropical agricultural systems. In this context, specific Si fertilization, for example, in the form of recycled organic siliceous materials (e.g., straw, biochar), might be a promising strategy for both increasing crop yields and decreasing desilication of agricultural soils. However, most studies focus on rice and sugarcane production and there is still only little knowledge about Si cycling in agricultural systems of the temperate zone. We analyzed soil and plant samples from an ongoing long-term field experiment (established 1963, randomized block design: plots with low, medium, and high mineral NPK fertilization rates, plots with straw fertilization in addition to NPK fertilization, control plots) in NE Germany to answer the following questions: (i) Can we observe a significant desilication (indicated by a decrease in plant available Si in soils) of agricultural systems in the temperate zone in the long term?, (ii) Is this potential desilication affected by NPK fertilization rates?, (iii) Is this potential decrease of plant available Si in soils reflected in Si concentrations of the grown plants (e.g., wheat)?, and (iv) Can we prevent potential anthropogenic desilication by straw fertilization? Here we present our first results to answer these questions. The answers to these questions will help us to obtain a deeper understanding of Si cycling in agricultural biogeosystems in the temperate zone in general and to derive practice-oriented recommendations for a more environmentally friendly and sustainable crop production in particular.

How to cite: Puppe, D., Kaczorek, D., and Sommer, M.: Anthropogenic desilication of agricultural soils – Results from a long-term field experiment in NE Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8241, https://doi.org/10.5194/egusphere-egu2020-8241, 2020.

Silicon (Si) is not considered an essential element for plant growth but improves blade stability and enhances the plant’s ability to resist metal toxicities. In soil solution, dissolved Si can be present as monomeric and polymeric silicic acid. Pre-experiments showed that monomeric Si is much less competitive than dissolved organic carbon (DOC) in sorption to Fe oxides, suggesting that monomeric Si can easily be leached from upper soil layers. However, drying of soil can increase Si pore water concentrations, thus facilitating the formation of Si polymers. We tested the sorption of monomeric versus polymeric silicic acid to goethite (a-FeOOH) at pH 4.5, and presumed stronger binding and less desorption by DOC for polymeric Si because of its multidentate mineral surface attachment. Equilibrium solutions were analysed for dissolved Si by optical emission spectrometry and Si species present at the mineral surfaces were revealed by X-ray photoelectron spectroscopy. Adsorption experiments indicated that the initial binding of polymeric Si was followed by surface polymerization at higher Si loads. Due to surface polymerization, the sorption of polymeric Si greatly exceeded that of monomeric Si. Besides adsorption experiments, desorption experiments using a DOC solution produced from an organic soil surface layer will provide information on the remobilization potential of the sorbed Si species. We hypothesize that (1) the displacement of monomeric as well as of polymeric Si by organic compounds depends on its surface loading and (2) polymeric Si is less desorbable than monomeric Si. Knowledge on the resistance of monomeric and polymeric Si against desorption by DOC will improve our understanding of processes controlling Si leaching and phytoavailability in soil.

How to cite: Dobritzsch, J., Klotzbücher, A., Klotzbücher, T., Kaiser, K., Mikutta, C., and Mikutta, R.: Competition of monomeric and polymeric silicic acid with natural organic matter for binding sites at goethite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10140, https://doi.org/10.5194/egusphere-egu2020-10140, 2020.

All plants contain some silicon (Si), but some species take it up passively through the transpiration stream while others additionally actively accumulate Si by producing transporters. Here, we review the literature, both qualitatively and quantitatively, to investigate the importance of transpiration for Si uptake across diverse plant groups with different accumulation capacities.  We will use variation among species in terms of phylogeny, habitat (e.g. aquatic vs. terrestrial), and environmental conditions (e.g. water or nutrient stress) to tease apart the roles of transporters and transpiration in controlling rates of Si accumulation, and make use of published manipulative experiments to explore how Si availability impacts the importance of these two uptake mechanisms.

How to cite: Cooke, J. and Carey, J.: Transpiration and transporters: teasing apart passive and active transport of plant silicon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15907, https://doi.org/10.5194/egusphere-egu2020-15907, 2020.

Silicon (Si) is known to have beneficial effects on plants, in particular on rice, which is a strong Si accumulator. Si helps mitigate environmental stresses and nutrient deficits of plants. In some regions, the limited plant-available Si in soils might have detrimental effects on rice cultivation. Crop-residue recycling can help to maintain the amount of plant-available Si in soils. However, the effect of crop-residue management practices on the soil-plant Si cycle and on Si availability to plants remains largely understudied. Here, we contribute to fill this knowledge gap by reporting a study on the effects of three different rice-residue management practices on Si-depleted paddy rice systems from northern Vietnam. The rice-residue management practices were (1) direct incorporation of rice residues into the soils, (2) burning in the field, and (3) use as fodder for animals, followed by composting of the obtained manure, and subsequent application of the composted manure to the field. We analyzed different Si reservoirs in soils and plant-Si contents under these different practices. Our results show a strong correlation between the different soil Si reservoirs and plant Si contents. We found no significant difference with respect to plant-available Si in soils and plant-Si contents between the different management practices. Moreover, our data suggest that Si-depleted rice-cultivation systems proportionally lose Si through grain harvest faster than less Si-depleted systems, because of enhanced relative Si accumulation in the grains. This loss cannot be mitigated by straw recycling. It may be one of the reasons why straw recycling has only a limited effect in the extremely Si-depleted rice-cultivation systems that were analysed in this study. Such information is critical in finding ways to maintain an appropriate level of plant-available Si in cultivated soils.

How to cite: Hughes, H., Trong Hung, D., and Sauer, D.: Silicon recycling through rice-residue management does not prevent silicon depletion in paddy rice cultivation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16603, https://doi.org/10.5194/egusphere-egu2020-16603, 2020.

The amount of water available to leach solutes from soil is one of the major features determining mineral weathering, secondary mineral synthesis and soil properties. The occurrence of gibbsite in soils denotes strong desilication.

Here, we quantify the reservoirs of bioavailable Si and phytolithic Si in wet tropical Andosols rich in gibbsite along a topoclimosequence where mean annual rainfall (MAR) increases from 2650 to 4400 mm with increasing altitude (65-375m above sea level) in Basse-Terre, Guadeloupe. We assessed bioavailable Si through CaCl₂ extraction in soil and the pool of soil phytoliths through Na₂CO₃ extraction and heavy liquid (hl) separation (followed by XRD quantification). The Na₂CO₃ extraction was performed on both the bulk soil and oxalate–treated soil (ox-Na₂CO₃) cleared of its amorphous aluminosilicates.

The Andosols have reached an advanced weathering stage. Their secondary products included (Al, Fe)-humus complexes, ferrihydrite, gibbsite and aluminous allophanic substances. The contents of organic C, metal-humus, ferrihydrite and gibbsite increased in wettest conditions (>3000mm) whereas allophane content concomitantly decreased. Ox-Na₂CO₃ Si (2-7 g kg^-1) contents were below hl Si contents (2-22 g kg^-1), and were negatively correlated to each other (r = -0.88) suggesting the occurrence of two pools of phytoliths: (i) free and fresh phytoliths, (ii) aged phytoliths entrapped in soil aggregates. Yet, bioavailable Si content in soil decreased from 63 to 12 mg kg^-1 with increasing MAR (r = -0.92), and was strongly correlated (r = +0.95) to that of phytolithic Si as assessed after ox-Na₂CO₃ extraction. The Si/Al ratio of the ox-Na₂CO₃ extract regularly decreased from 1.06 to 0.37 with increasing MAR, hence corroborating strongest desilication in wettest conditions. In these highly leached, gibbsitic Andosols, rainfall is thus the major driver of plant Si availability.

How to cite: Vander Linden, C., Li, Z., Iserentant, A., and Delvaux, B.: Rainfall as the major driver of plant Si availability in gibbsitic Andosols, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19728, https://doi.org/10.5194/egusphere-egu2020-19728, 2020.

Clogging of subsurface pipe drainage systems by rust precipitates is a problem in many cultivated areas and especially on the coast of Ostrobothnia, northwestern Finland. The subsurface drainage pipes need to be flushed every few years to remove the rust, which causes additional maintenance costs. These problems are particularly common in acid sulphate (AS) soils that have peat horizons on top of sulfidic materials. These soils are often wet, and the drainage water contains high dissolved iron concentration, commonly above 20 mg l^-1. Reducing conditions prevail in certain horizons and oxidation of sulfidic minerals and low pH are typical of the horizons above, all resulting in mobilization of several elements. Upon entering the aerobic drainage pipe dissolved iron is oxidized and readily precipitates as rust. In dry summers, the precipitate is typically hardened and the whole pipe drainage system can be blocked. Minerals containing sulphur (S) may also be precipitated in the pipes. The fresh precipitates can adsorb heavy metals that occur in substantial concentrations in AS drainage waters. In this study, 10 rust samples were collected from ditches and wells. All sites, except one, had a 20-70 cm peaty topsoil. A comprehensive chemical analysis was carried out and the precipitates were investigated with a scanning electron microscope (SEM). Colours of the samples were strong brown or reddish yellow (Munsell notation 7.5YR 5/6-6/8). Silicon content was only 0.3-0.9%, indicating the absence of actual soil material in the precipitates. The material contained 27-49% organic matter (1.9 x C), co-precipitated from the humic substances of drainage water. Iron was by far the most abundant element. If all Fe is contained in ferrihydrite (66% Fe), this mineral constituted 35-63% (mean 46%) of the precipitate while aluminium hydroxide (34% Al) constituted 0.7-9% (mean 5%). Even though most drainage waters were rich in S (commonly above 40 mg l^-1, the maximum S concentration of the precipitates was only 1.9% and the mean at 0.7%. Sulphur-containing minerals jarosite and schwertmannite were not detected in the SEM images, either, suggesting that these minerals are not precipitated from AS drainage waters. Dissolved heavy metals are leached from AS soils but they were not markedly co-precipitated in our samples. The mean concentration of Cd was only 1 mg kg^-1 and Ni 12 mg kg^-1, Cr 33 mg kg^-1, Cu and Zn 32 mg kg^-1 while Mn was more abundant, 355 mg kg^-1. In our peaty AS soils there is thus substantial mobilization of Fe and a flux out of the soil and a new solid phase is formed in the drainage pipes and ditches constituting mostly of iron hydroxide and humic substances. If dredged, application of this material onto the fields seems not to pose major environmental hazards.

How to cite: Yli-Halla, M., Kekkonen, J., Lötjönen, T., and Marttila, H.: Rust precipitates in drainage systems of peaty acid sulphate soils in Finland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2534, https://doi.org/10.5194/egusphere-egu2020-2534, 2020.

When acid sulfate soils dry, they generate large amounts of sulfuric acid due to oxidation of iron (Fe) sulfides (e.g., pyrite), causing formation of Fe sulfates such as jarosite and strong acidification (pH < 4). After re-saturation of these sulfuric soils and re-establishment of reduced conditions, activity of Fe- and sulfate-reducing bacteria promote re-formation of Fe sulfides and pH increase. However, many reducing bacteria are heterotrophic and require sufficient available organic carbon (OC). Despite the general knowledge about positive impacts of OC addition to ameliorate sulfuric soils, little is known about the reduction of Fe sulfates (here: jarosite) to Fe sulfides and the formation of mineral-organic associations after establishing anoxic conditions.

We investigated the remediation of a sandy, jarosite-containing sulfuric soil (initial pH = 3.0, initial redox values approx. 400 mV) in a 20-week anoxic laboratory incubation experiment under re-submerged conditions. We used a control without OC addition plus treatments with wheat straw addition as substrate for reducing bacteria. Besides the natural sulfuric soil, an artificial acid sulfate soil composed of synthesized jarosite mixed with quartz sand was used to simulate a simple, mineralogically well-characterized model of the natural soil. To ensure similar conditions, the artificial soil was submerged with soil solution from the natural sulfuric soil. We monitored pH and redox values in the soil suspension weekly. After 20 weeks, concentrations of OC, Fe, and S were analysed in bulk soils and soil solutions. The mineral composition was characterised by X-ray diffraction (XRD).

Addition of wheat straw to the natural acid sulfate soil led to quick changes in redox and pH values, reaching pH ≥ 6.0 and redox values ≤ -100 mV within three weeks. XRD analyses revealed complete loss of jarosite during incubation. Addition of wheat straw to the artificial acid sulfate soil led to slightly lower pH and higher redox values than for the natural soil, resulting in approx. pH 5.7 and redox values ≤ 0 mV after three weeks. Some of the jarosite was reduced, but it is still detectable after incubation. Without wheat straw addition, for both soils pH values remained low (pH ≤ 4.0) and redox values remained high (≥ 300 mV). Jarosite concentration did not change during the incubation without straw. The results showed that microbial reduction of acid sulfate soils requires supply of sufficient organic matter, which effectively triggers the reduction of jarosite to sulfides.

How to cite: Koelbl, A., Kaiser, K., Mosley, L., Fitzpatrick, R., Marschner, P., and Mikutta, R.: Loss of jarosite during remediation of a sandy acid sulfate soil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6736, https://doi.org/10.5194/egusphere-egu2020-6736, 2020.

Natural colloids composed of iron (Fe) and organic matter (OM) are a key factor controlling metallic pollutants mobility according to their high adsorption capacities, consequence of their high binding sites density. The physico-chemical condition in which the Fe-OM nanoaggregates are formed influences their structural organization, and more specifically the Fe speciation. In this study, we probe the influence of three major cations, present in high quantity in natural systems: Calcium (Ca), Aluminum (Al) and Silicon (Si). Ca is known to have a huge affinity toward OM, as well as Al which also easily get into Fe hydroxides structure. For its part, Si is known to restrain Fe oxides growth and crystallinity, despite mechanisms remain unknown. Ca, Al and Si are thus expected to modify Fe-OM nanoaggregates organization and impact Fe speciation.

Mimetic environmental Fe-OM-cation nanoaggregates were synthesized with different Fe/OM and cation/Fe ratios. They were observed by TEM. The Fe speciation was characterized by XAS as well as the cations interactions with the components of the Fe-OM colloids. The size and arrangement of Fe-nanoparticles were determined by SAXS. Results show that Fe speciation is complex and variable according to Fe and cation contents relative to OM. Fe phases appear to be composed of oligomers and ferrihydrite nanoparticles, both embedded in the OM matrix. The Fe-nanoparticles are forming a fractal network which organization is controlled by the OM. When the Fe/OM ratio increases, oligomers content decreases to the benefit of Fe-nanoparticles which size increases. Adding cations, this phenomenon is strongly modified, either increased, with the addition of Ca and Al, or decreased, with the addition of Si. These modifications result from the different interactions we could evidence between the cations and the different Fe-OM network components.

These results clearly highlight the dramatic effect of Al, Si and Ca cations on the Fe-OM colloidal network, impacting both Fe speciation and OM organization. These structural modifications directly impact the capabilities of Fe-OM nanoaggregates to trap and transport pollutants.

How to cite: Vantelon, D., Beauvois, A., Jestin, J., and Davranche, M.: The dramatic variability of cations impacts on Fe speciation in Fe-OM nanoaggregates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8131, https://doi.org/10.5194/egusphere-egu2020-8131, 2020.

Peat swamps contain substantial accumulations of organic matter due to waterlogging and slower decomposition rates. Peat swamps can be underlain by sulfidic sediments where there is  abundant iron and sulfate for reduction to form a range of sulfidic minerals, primarily pyrite (FeS₂). Sulfidic sediments can acidify to produce sulfuric acid, similar to acid mine drainage (AMD) and acid sulfate soil (ASS) environments when oxidised, which can occur when water levels drop due to drainage or periods of drought. Discharging surface and shallow groundwater can therefore acidify adjacent lakes and waterways. These swamps can also present significant fire hazards when drying occurs. 

This study identified the chemical and mineralogical changes in sulfidic peat swamp sediments along a temperature gradient to simulate the effects of fire. We found that fire induced changes in the Fe-minerals to form a range iron (oxy)hydroxides and iron oxides such as magnetite, mghemite and haematite in increasing crystallinity  with increasing temperatures. pH initially decreased on drying a minimum of pH 3.15, before increasing with increasing temperature to 650^oC to pH 4.86, which can mobilise environmentally important pH-sensitive metals. 

Peat swamps are highly susceptible to the effects of fire when surface- and shallow groundwater levels decrease as a result of extended drought or drainage. Fire can irreversibly alter underlying soil properties to induce changes in soil minerals and potentially impact the surrounding environment. 

How to cite: Wong, V., Williamson, T., Etschmann, B., and Wilson, S.: The effects of fire on sulfidic peat swamp sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11956, https://doi.org/10.5194/egusphere-egu2020-11956, 2020.

Redox-driven changes in Fe crystallinity and speciation may affect soil organic matter (SOM) stabilization and carbon (C) turnover, with consequent influence on global terrestrial soil organic carbon (SOC) cycling. Under reducing conditions, increasing concentrations of Fe(II) released in solution from the reductive dissolution of Fe (hydr)oxides may accelerate ferrihydrite transformation, although our understanding of the influence of SOM on these transformations is still lacking. 

Here, we evaluated abiotic Fe(II)-catalyzed mineralogical changes in Fe (hydr)oxides in bulk soils and size-fractionated SOM pools (for comparison, fine silt plus clay, FSi+Cl, and fine sand, FSa) of an agricultural soil, unamended or amended with biochar, municipal solid waste compost, and a combination of both. 

FSa fractions showed the most significant Fe(II)-catalyzed ferrihydrite transformations with the consequent production of well-ordered Fe oxides irrespective of soil amendment, with the only exception being the compost-amended soils. In contrast, poorly crystalline ferrihydrite still constituted ca. 45% of the FSi+Cl fractions of amended soils, confirming the that the higher SOM content in this fraction inhibits atom exchange between aqueous Fe(II) and the solid phase. Building on our knowledge of Fe(II)-catalyzed mineralogical changes in simple systems, our results evidenced that the mechanisms of abiotic Fe mineral transformations in bulk soils depend on Fe mineralogy, organic C content and quality, and organo-mineral associations that exist across particle-size SOM pools. Our results underline that in the fine fractions the increase in SOM due to organic amendments can contribute to limiting abiotic Fe(II)-catalyzed ferrihydrite transformation, while coarser particle-size fractions represent an understudied pool of SOM subjected to Fe mineral transformations. 

How to cite: Giannetta, B., Balint, R., Said-Pullicino, D., Plaza, C., Martin, M., and Zaccone, C.: Fe(II)-catalyzed transformation of Fe (hydr)oxides in particle-size soil organic matter fractions from amended agricultural soils , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14873, https://doi.org/10.5194/egusphere-egu2020-14873, 2020.

The biogeochemical cycle of iron is fundamentally important in natural systems and facilitates processes ranging from the carbon cycle to the immobilisation of potentially toxic elements. In the absence of oxygen, iron is the most abundant terminal electron acceptor for microbial respiration, which produces both Fe(II) and oxidised organic matter. Thus, in low-sulfur reducing environments it is likely that conditions favouring the precipitation of the ferrous carbonate mineral siderite are abundant. Previous research has suggested that the oxidation of siderite in the presence of arsenic produced a combination of goethite and siderite that had a sorption capacity for arsenic that was an order of magnitude higher than either siderite or synthetic goethite alone¹. Furthermore, the oxidation of siderite may produce reactive oxygen species, such as hydroxyl radicals, that are capable of oxidising recalcitrant contaminants and may influence CO₂ release in soils^2,3. However, despite the clear environmental importance, little is currently known about the oxidative transformation of siderite under environmentally relevant conditions.

Here, we used a series of batch experiments (2 g L^-1 mineral suspension, pH 7.5) to characterise siderite oxidation kinetics under oxic conditions, in the presence and absence of the organic ligands citrate, EDTA, tiron, and salicylate (10 mM). We selected these ligands to be representative of small organic acids that are likely ubiquitous in environments where siderite forms and to contain a range of interesting functional groups, namely carboxylates, catechols and thiols. Alongside batch experiments, we used a combination of Raman microspectroscopy and X-ray diffraction for mineral characterisation.

Our results show that synthetic siderite oxidises extremely quickly and undergoes a complete transformation to poorly crystalline goethite in less than 6 hours. However, in the presence of an organic ligand, up to 50 % of structural Fe(II) remains after 300 hours, which we propose is due to surface passivation of the mineral by the organic ligand. We found that the rate and products of oxidation are dependent on the ligand structure. For the carboxylate ligands citrate and EDTA we found that siderite remained the dominant phase whereas ferrihydrite and lepidocrocite/magnetite predominated in the presence of tiron and salicylate respectively.

Our findings are important for understanding iron dynamics in periodically reducing environments, as siderite may be more stable in the presence of oxygen than previously thought and therefore iron redox cycling may occur at a slower rate than would be otherwise accounted for in biogeochemical models.

(1)           Guo, H.; Ren, Y.; Liu, Q.; Zhao, K.; Li, Y. Enhancement of Arsenic Adsorption during Mineral Transformation from Siderite to Goethite: Mechanism and Application. Environ. Sci. Technol. 2013, 47 (2), 1009–1016.

(2)           Tong, M.; Yuan, S.; Ma, S.; Jin, M.; Liu, D.; Cheng, D.; Liu, X.; Gan, Y.; Wang, Y. Production of Abundant Hydroxyl Radicals from Oxygenation of Subsurface Sediments. Environ. Sci. Technol. 2015, 50 (1), 214–221.

(3)           Trusiak, A.; Treibergs, L. A.; Kling, G. W.; Cory, R. M. The Role of Iron and Reactive Oxygen Species in the Production of CO2 in Arctic Soil Waters. Geochim. Cosmochim. Acta 2018, 224, 80–95.

How to cite: Rothwell, K. and Kretzschmar, R.: Siderite Oxidation in the Presence of Organic Ligands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16894, https://doi.org/10.5194/egusphere-egu2020-16894, 2020.

Floodplain soils experience highly dynamic wet and dry cycles that trigger changes in redox conditions and as such play a crucial role for environmental nutrient cycling and pollutant fate.

To elucidate the effects of varying water saturation on the predominant biogeochemical processes and their dynamics we simulated a heavy rain fall event with subsequent steady rain over ten consecutive days at a plot of arable soil in a floodplain near Tübingen, southwest Germany. We monitored how soil redox conditions, redox sensitive soil constituents and microbial communities responded to changing water saturation.

The experiment design was fully randomized comprising irrigated plots mimicking rain events and dry controls.

Multi-level redox probes recorded in situ redox potentials at 10 cm intervals down to 90 cm depth on irrigated and dry plots. The initially dry soil showed redox potentials of +600 mV. The simulated heavy rain fall provoked a drop in redox potentials within hours in depths down to 40 cm and within a delay of 1 to 2 days in depths down to 60 cm. Subsequent steady rain lead to a decrease of the redox potentials to a minimum of -200 mV to -300 mV in depths of 20 to 30 cm and -100 mV in depths of 40 to 50 cm.

Soil cores were retrieved throughout the experiment to identify microbial communities and to determine depth profiles of nitrate, ammonium, adsorbed and poorly crystalline iron as well as total iron, and sulfide and sulfate in the pore water and the solid phase.

The high resolution temporal data on changes in redox potential, soil chemistry and soil microbial communities will be presented and discussed in terms of the predominant biogeochemical processes in the soil profile.

How to cite: Schlögl, J., Cramaro, L., Griebler, C., and Haderlein, S. B.: Dynamics of redox potential and nutrient turnover in dry floodplain soils during a simulated rain fall event, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19106, https://doi.org/10.5194/egusphere-egu2020-19106, 2020.

Dissolution of iron(oxyhydr)oxides is a key biogeochemical process that affects the cycling and bioavailability of iron (Fe). Recently, we demonstrated that submicromolar concentrations of Fe(II) accelerate dissolution of Fe(III)(hydr)oxides with the synthetic ligands ethylenediaminetetraacetate (EDTA) and hydroxybenzyl ethylenediaminediacetic acid (HBED) and also with the biogenic ligand desferrioxamine-B (DFOB) in anoxic conditions at circumneutral pH. The catalytic effect of Fe(II) was explained by electron transfer (ET) to surface Fe(III) and accelerated detachment of surface Fe(III)-ligand complexes. However, the extent of ET on the mineral surface before and during accelerated dissolution remained unclear. Here we describe the extent of ET by investigating dissolution and isotope exchange with lepidocrocite (Lp) and goethite (Gt) and varying concentrations of Fe(II), ⁵⁷Fe(II), and DFOB. Most experiments were conducted under anoxic conditions at pH 7.0 in bicarbonate-CO₂-buffered suspensions.

Our results show that in anoxic carbonate-buffered suspensions, 1-5 µM Fe(II) increased the rates of Lp dissolution at pH 7.0 by up to 60-fold. The addition of 20 or 50 µM DFOB after ⁵⁷Fe(II) led to accelerated detachment of ⁵⁶Fe(III) from Lp and release of already adsorbed/exchanged ⁵⁷Fe into the solution. A kinetic model considering exchange of charge on the surface between ⁵⁷Fe(II) and ⁵⁶Fe(III), before and during dissolution, was developed to explain the observed results. The rates for ET and isotope exchange before and during accelerated dissolution are very different for Lp and Gt, presumably reflecting the differences in structure and mineralogy.

This study contributes to the quantification of ET from added Fe(II) to the surface of Fe(III)(hydr)oxides and of the acceleration of overall non-reductive dissolution by traces of Fe(II) in anoxic environments. In this presentation, the key findings of the isotope exchange and dissolution studies with Lp and Gt will be presented in order to highlight the importance of interfacial Fe(II)/Fe(III) ET  processes occurring at (sub)oxic-anoxic interfaces of soils and sediments.

How to cite: Biswakarma, J., Kang, K., Schenkeveld, W. D. C., Kraemer, S. M., Hering, J. G., and Hug, S. J.: Linking Isotope Exchange and Fe(II)-Catalyzed Ligand-Controlled Dissolution of Iron(hydr)oxides , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20789, https://doi.org/10.5194/egusphere-egu2020-20789, 2020.

Ferric iron (Fe(III)) minerals, such as ferrihydrite and lepidocrocite, can be reduced to ferrous iron (Fe(II)) through microbial reductive dissolution under reducing soil conditions, to form dissolved Fe(II) or mixed Fe(II)-Fe(III) mineral phases. The dissolved Fe(II) catalyses iron mineral transformation to more crystalline iron phases. Silica (Si), in the form of silicic acid, is an ubiquitous component of natural soil solutions and is known to hinder the iron mineral transformation process. However, the mechanisms and the mineral phases that are formed during ferrihydrite and lepidocrocite transformation in the presence of Si remain unclear. We reacted ferrihydrite, Si-ferrihydrite co-precipitates, lepidocrocite and Si-adsorbed lepidocrocite with 0.3 mM and 3 mM isotopically labelled ⁵⁷Fe(II) for four weeks. At six time points, we sampled the solid and the aqueous phase, to follow iron mineral transformation by X-ray diffraction and dissolved Fe(II) dynamics. In addition, we tracked the iron atom exchange between the aqueous and the solid phase by measuring the ^57/56Fe isotope ratio in filtrates and dissolved solid phases. Our data demonstrates the hindering effect of Si on Fe(II) catalysed ferrihydrite and lepidocrocite transformation. The presence of Si decreased the initial Fe(II) adsorption and strongly slowed down the iron atom exchange, especially in the lepidocrocite treatment. Collectively, the results of this study demonstrate, how Si can impact iron mineral transformation in soils with different Fe(II) release potentials under reducing conditions.

How to cite: Schulz, K., ThomasArrigo, L. K., Rothwell, K. A., and Kretzschmar, R.: How does silica affect Fe(II)-catalysed transformation of ferrihydrite and lepidocrocite?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22500, https://doi.org/10.5194/egusphere-egu2020-22500, 2020.

The landscape in lowland Amazonia is shaped by large rivers, whose depositional-erosive dynamics built fluvial terraces covered by upland forests. Thus, fluvial deposits distributed across lowland Amazonia are of crucial relevance since they represent the best accessible archives to study the history of environment and climate change. The timing of the assembly of the modern transcontinental Amazon River is considered a key event in the landscape evolution of Amazonia, however, proposed ages range from Miocene, early Pliocene to Pliocene/Pleistocene. Therefore, regional stratigraphic correlations need to improve to ensure a better understanding of reconstructions of past conditions in Amazonia during the Cenozoic. Yet, these are difficult due to the lack of absolute ages to constrain phases of sediment deposition or erosion and weathering. In lowland central Amazonia, past environmental conditions are recorded in the Alter do Chão and Novo Remanso Formations. Both units are dominated by sandy and highly oxidized sediments with scarce paleontological remains complicating the application of biostratigraphy dating methods. The Alter do Chão and Novo Remanso Formations are well exposed in the left margin of the Solimões-Amazon River main stem and show remarkable zones rich in supergene iron weathering products, which has been used to define the stratigraphic boundaries among the Alter do Chão Formation, Novo Remanso Formation and overlying sediments. In this study, we use the (U-Th-Sm)/He dating method on goethite and hematite grains to determine the age of iron-enrichment layers and duricrusts that mark boundary surfaces used to define the stratigraphic framework of the Alter do Chão and Novo Remanso formations. The (U-Th-Sm)/He ages allow to improve chronological constraints for both formations and to discuss the timing of fluvial terraces building and weathering conditions in central Amazonia through time.

How to cite: Gautheron, C., Cabriolu, C., Pupim, F., Parra, M., Schwartz, S., Pinna-Jamme, R., Horbe, A., Kern, A. K., and Oliveria Sawakuchi, A.: Paleogene to Quaternary geodynamical evolution of the lowland Central Amazonia inferred by weathering phases dating, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8407, https://doi.org/10.5194/egusphere-egu2020-8407, 2020.

Approximately 70% of the emerged relief on the Earth is characterized by erosional low-gradient topography also known as planation surfaces (PS). Many geomorphologists defend the idea that some of these surfaces could be relics of old reliefs uplifted and preserved from erosion for tens of millions years. Some of the highest PS of Southeast Africa (> 2000 m) were considered by King (1962) as remnants of an ante-Cretaceous paleorelief called “Gondwana Surface”. Specifically, the Nyika Plateau (Northern Malawi, 2200 m) is one of the largest potential relics of the “Gondwana Surface” in Southeastern Africa. This PS overlooks the stripped etchplain of the Malawian Plateau, a potential Late Cretaceous PS about 1200 m of elevation.

However, the preservation of such ancient reliefs is controversial, particularly under a tropical wet-dry climate. Doubts about the ages of these PS exist mainly due to the lack of a precise chronology of these objects on a continental scale. In detail, African PS are often covered by preserved or partly eroded tropical weathering covers such as unconsolidated laterites and/or duricrusts. Under these climatic conditions, lateritic duricrusts can be preserved for millions of years and thus contain several generations of iron oxides witnesses of past local paleoenvironment and geodynamic evolution. In order to understand the formation and preservation of the Southeast African highest PS and date them, we decided to apply (U-Th)/He dating of iron oxides on selected duricrust samples. The exploration of the Nyika Plateau allowed the discovery of an outcropping duricrust and a depositional area of eroded duricrust blocks from different origins. We study duricrust samples from these two areas in order to find some clues about the plateau antiquity and to improve our knowledge about the local paleoclimatic and geodynamic history.

Samples from the in situ duricrust levels, outcropping on the plateau, are polygenic and are formed by three main types of zones: preserved and degraded hematite-rich zones, that are considered to correspond to the initial generation of iron oxides, and a goethitic matrix. The preserved hematites have a Mesozoic (U-Th)/He ages, whereas the goethite-rich matrix of this duricrust formed during the Quaternary. The degraded hematite-rich parts, also rich in quartz, have more dispersed ages ranging from the Mesozoic to the Tertiary. In the detrital accumulation zone, blocks from a similar duricrust were found as well as blocks of another type of duricrusts: a pisolithic one rich in goethite. This last type of duricrust was eroded from a more recent duricrust level, as their iron oxides have Late Tertiary/Quaternary ages. These dating proved the Nyika Plateau relative stability since the Mesozoic period, confirming that duricrusting of reliefs in tropical area can also protect old emerged landscapes from total erosion.

King L.C. (1962) Morphology of Earth, Oliver and Boyld, Edinburg.

How to cite: Mathian, M., Baby, G., Ferry, J.-N., Guillocheau, F., Allard, T., Rafiki N Chindandali, P., Ruffet, G., Quantin, C., Pinna-Jamme, R., and Gautheron, C.: First proofs of preservation of a Mesozoic paleorelief in Southeast Africa: Insights from the (U-Th)/He dating of iron oxides from Malawian duricrusts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8846, https://doi.org/10.5194/egusphere-egu2020-8846, 2020.

Lateritic peofiles results from weathering processes involving coeval warm temperature and high precipitation seasons rates in intertropical context. Through geological times, the laterite will develop and an indurated duricrust iron oxide enriched horizon may develop on the profile top. The duricrusts can be used as excellent lateritic surface markers, that can be preserved from erosion. Nevertheless, the relationship between the timing of their development, the geomorphology and the processes involved are still debated.

To this end, we have investigated duricrusts of the large Roraima plateau, in the Guyana shield, Northern Brazil, attributed to the “Gondwana” surface of purported “Jurassic-Cretaceous” age. Four duricrust samples have been collected from ~840 to 950 m.a.s.l in the same area, with both 1.9 Ga sandstone and 1.75 Ga gabbro as parent rock respectively. The duricrust samples exhibit different textures, including pisoliths and massive banded type textures.

Samples mineralogy have been characterized using classical methods such as optical properties, texture and DRX. Several generations have been finally identified. The SEM analysis revealed different porosities and microstructures. Each identified generation collected has been dated by the (U-Th)/He method using micrometric aliquots.

All the samples present quite homogenous dated generations, presenting mostly Paleocene/Eocene as well as early and late Miocene ages. In addition, oldest identified generations in the pisolithic sample suggest remaining of older Cretaceous weathering event. Our geochronological results are not directly correlated to the sample altitudes but more likely to the duricrust structure. Finally, the (U-Th)/He age distribution reveal that the Roraima landscape underwent several weathering episodes in the Cenozoic times and are younger than initially supposed.

How to cite: Sanchez, C., Gautheron, C., Pinna-Jamme, R., Haurine, F., Roig, J.-Y., and Coueffë, R.: First Cenozoic ages from the Roraima’s region landscape (Northern Brazil): insights from hematite and goethite (U-Th)/He dating, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12777, https://doi.org/10.5194/egusphere-egu2020-12777, 2020.

Ferruginous duricrusts record a part of the Earth’s geodynamical and climatic history in tropical area, because they can be formed over a wide geologic period. However, the events and processes related to their formation, transformation and distribution are still obscure. This is mainly due to the complexity arising from their finely divided and polycrystalline nature together with the coexistence of various generations of supergene minerals, such as iron and aluminum oxides, oxyhydroxides or hydroxides (e.g. goethite, hematite and gibbsite) and kaolinite, even at microscopic scale. Classical mineralogical investigations are often realized using powders samples, which hinders subsequent analyses on the same sample, such as SEM or (U-Th)/He dating. Thus, the aim of this study was to propose a new way to investigate the mineralogy of supergene ferruginous samples on micrometric grains that will be analyzed by (U-Th)/He dating method. Prior to this analysis, we first compare the X-ray diffraction data of grains and small amounts of powders looking to reveal the mineralogical composition of populations of secondary minerals of a ferruginous duricrust by taking into account the heterogeneity of the material. Samples were collected from a ferruginous duricrust with pisolitic structure developed over epiclastic conglomerates and sandstones deposited by alluvial fan and fluvial streams from the Upper Cretaceous at the western Minas Gerais state (Brazil). The geomorphology of the study area is delineated by remnants of paleosurface (up to 1,000 m a.s.l.), which comprises an important record of long-term Brazilian continental history.Macroscopic facies recognized on duricrusts sections were described, which allowed the identification of various populations of secondary minerals. After this detailed description, grains (size < 0.5 mm) were collected and powder samples of each population were prepared by crushing. Overall, the results point out that the grain and powder samples could be used to identify mineralogical composition at fine resolution of secondary minerals from ferruginous duricrusts. In addition, XRD results are similar for both types of sample preparation, however the < 0.5 mm grain samples are more advantageous because they are not destructive and thus allow to get a finer description of the mineralogy of different populations and can subsequently be used for e.g. (U-Th)/He dating, providing critical information for interpreting and discussing the ages of iron oxides.

Grant: 19/10708-7; 17/22292-4; 17/20788-2, São Paulo Research Foundation (FAPESP)

References

Allard, T., Gautheron, C., Riffel, S.B., Balan, E., Soares, B.F., Pinna-Jamme, R., Derycke, A., Morin G., Bueno, G.T., Nascimento, N., 2018. Combined dating of goethites and kaolinites from ferruginous duricrusts. Deciphering the Late Neogene erosion history of Central Amazonia. Chemical Geology 479, 136-150.

Monteiro, H.S., Vasconcelos, P.M.P., Farley, K.A., Spier, C.A., Mello, C.L., 2014. (U-Th)/He geochronology of goethite and the origin and evolution of cangas. Geochim. Cosmochim. Acta  131, 267–289.

Vasconcelos, P.M., Heim, J.A., Farley, K.A., Monteiro, H.S., Waltenberg, K., 2013. ⁴⁰Ar/³⁹Ar and (U–Th)/He - ⁴He/³He geochronology of landscape evolution and channel irondeposit genesis at LynnPeak, Western, Australia. Geochim. Cosmochim. Acta 117, 283-312.

How to cite: Marques, K., Allard, T., Morin, G., Baptiste, B., Gautheron, C., and Vidal-Torrado, P.: Discussing the dating of ferruginous duricrusts: promises from mineralogy of supergene minerals with non-destructive microsampling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17822, https://doi.org/10.5194/egusphere-egu2020-17822, 2020.

At the global scale and on geological time scales, mechanical erosion and chemical weathering budgets are linked. Together, these processes contribute to the formation and the degradation of the Earth’s critical zone and to the biogeochemical cycles of elements. While the weathering of hot and humid shields areas exhibit low weathering rates because of the depth of the mature depleted soil mantle there, shields areas dominate the continents areas over intertropical regions and, therefore, represent a significant proportion of the global delivery of dissolved matter to the oceans. In addition, these environments are under supply-limited conditions (the weathering rate is limited by the low rates of the erosion) and thus particularly sensitive to long-term variability erosion rates. Despite this importance, weathering-erosion budgets and rates estimation in these environments is sparse, and generally performed at a local scale (soil profiles) or, when performed at a larger catchment scale, the intra cratonic characteristics variabilities (e. g. the diversity of mechanical erosional regimes) are usually not singled out.

In the present study, we explored the variability of the weathering intensity of the Ogooué sub-basins (Western central Africa, Gabon) as a function of their geomorphologic, tectonic and lithological setting variability. We analyzed major and trace elements concentration and the strontium and neodymium isotopes of water, suspended matter sediments and bedload sampled in 24 Ogooué tributaries (September 2017 campaign). Our results show that shield areas exhibit a high variability of chemical weathering intensity, which follows the erosional regime characteristics of the studied sub-basins, likely related to their tectonic activity. Three regions can be distinguished: The Bateke plateau (East sub-basins - PB), is composed of pure sandstones (quartz) and is inert in term of tectonic activity and therefore in term of erosion and weathering budget; the northern sub-basins (NB) are subjected to low tectonic activity and exhibit slightly higher erosion and weathering intensity than PB region and, by comparison, southern sub-basins (SB) exhibits uplift activity which is traduced by more intensive erosion and weathering processes.

The annual dissolved solid budget of the Ogooué basin is ~2.52 t.yr^-1 for a rate of 11.7 t.km^-2.yr^-1. According to the source discrimination method performed based on the geochemical analysis, the atmospheric inputs contributes to around 20% to the TDS, the silicate weathering contribution dominates the dissolved exports throughout 70% of its production while the carbonates weathering lowly contributes to the TDS production.

By comparison to the other large shields rivers, this basin exhibit a lower range of chemical silicate weathering rate than most of the world’s large rivers, with values similar to those of the Congo River. This new dataset provides a key information to complete the World River chemistry database, which is limited for inter-tropical regions, especially in tectonically quiescent environments. Moreover, this study provides new data for tropical shields contexts allowing for the exploration of the interactions between erosion rates and climate in the control of continental weathering rates, and their relationships with long-term carbon cycle and short-term biogeochemical cycles.

How to cite: Moquet, J.-S., Bouchez, J., Braun, J.-J., Bogning, S., Mbonda, A., Bricquet, J.-P., Carretier, S., Regard, V., Paiz, M. C., and Gaillardet, J.: Supply-limited weathering regime in a tropical shields basin (Ogooué River basin, Gabon), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18181, https://doi.org/10.5194/egusphere-egu2020-18181, 2020.

The aftermath of the end-Permian mass extinction is marked by large and recurrent perturbations of the environment and of the biosphere, which are thought to have delayed the recovery of marine ecosystems. A potential widespread loss of vegetation cover linked to destabilization of terrestrial ecosystems along with the climate warming that persisted for several million years after the Permian-Triassic boundary likely contributed to the markedly enhanced soil erosion and intensified continental chemical weathering recorded in the Early Triassic (Algeo and Twitchett, 2010). As continental weathering delivers nutrients to the oceans, this process could have played a major role in the repeated development of anoxic conditions by sustaining primary productivity and export of organic matter to the seafloor (Algeo and Twitchett, 2010; Sun et al., 2018). Yet our knowledge of the importance of this process in triggering anoxic conditions is currently hampered by the lack of proxies providing chemical weathering records at a local scale. In this study, we tested a novel proxy of chemical weathering intensity at the local scale, based on the coupled isotopic composition of hafnium and neodymium in clay minerals, to explore the links between chemical weathering, climate fluctuations, and anoxia in the western USA basin during the Early Triassic. This proxy has been recently calibrated in modern environments (Bayon et al., 2016) but has only been scarcely applied to deep-time environments.

We analyzed clay sediments for their Hf and Nd isotope composition from 5 sections within the western USA basin (that encompasses the Smithian-Spathian boundary (SSB). The well -stablished bio-chemo-stratigraphical frame of this basin allows the exploration of the respective timing of anoxia establishment and variations in chemical weathering of the continental masses adjacent to the basin at a high temporal resolution. Our new dataset highlights the existence of a decrease in chemical weathering of the continents surrounding the Basin at the Smithian-Spathian boundary, during the development of anoxic conditions marked by enhanced organic matter burial in the sediments. Our new data therefore bring new light on the links between nutrient inputs linked to modifications in continental weathering and the establishment of anoxic conditions in the western USA basin. The decrease in continental chemical weathering depicted in our data set occurs during the global cooling event identified by conodont  d¹⁸O records in other regions of the word (Goudemand et al., 2019). This cooling may have promoted a decrease in the intensity of the hydrological cycle and the establishment of more arid conditions in the western USA basin, impeding chemical weathering in the studied area.  

Algeo, T. J. & Twitchett, R. J. (2010). Geology, 38(11), 1023-1026.

Bayon, G. et al. (2016). Earth and Planetary Science Letters, 438, 25-36.

Goudemand, N. et al. (2019). Earth-Science Reviews.

Sun, H. et al. (2018). Proceedings Nation. Academy of Sciences, 115(15), 3782-3787.

How to cite: Freslon, N., Pucéat, E., Brayard, A., and Bayon, G.: Climate control of silicate weathering intensity through the Smithian/Spathian boundary in the western USA basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18466, https://doi.org/10.5194/egusphere-egu2020-18466, 2020.

Revealing the age of secondary minerals derived from weathering participates to an increasingly detailed understanding of evolution of continental surfaces. Among weathered materials, laterites represent 1/3 of emerged continents area and 80 % of global soil volume [1]. At a global scale, the Amazon Basin is a major basin where the chronology of weathering covers, witnesses of its past geodynamic and paleoclimatic evolution, is still poorly documented. It was demonstrated that kaolinites from laterites can be dated using their concentration in radiation-induced paramagnetic defects and that their ages can be discussed in terms of geodynamic or paleoclimatic episodes [2] [3] [4] [5] [6]. This complements Fe (oxyhydr)oxides and Mn oxides dating in weathering covers.

In a first part, recently upgraded steps of the methodology for kaolinite dating will be presented. They include an improved fitting strategy of dosimetry curves and a better assessment of radiation dose rate in the investigated samples, accounting for an heterogeneous spatial distribution of radionuclides and for some degree of geochemical opening due to Rn mobility.

In a second part, contrasting examples of kaolinite dating in laterites (from loose horizons or iron duricrusts) occurring in the Amazon Basin will be presented. These data highlight that weathering episodes revealed by kaolinite dating mostly range over late Miocene to Quaternary periods. Discussion will first point to profile genesis, showing that rejuvenation of kaolinites or erosion may have prevailed and obliterated more ancient generations, and that variation of ages along profiles allow simple models of weathering fronts propagation to be proposed. In addition, kaolinite ages will be discussed by comparison to the geochronology of main geodynamic or paleoclimatic events in the region.

[1] Nahon, D.B., (2003). Comptes Rendus Acad. Sci. Geoscience, 335, 1109-1119.

[2] Balan E.; Allard T., Fristch E., Selo M., Falguères C., Chabaux F., Pierret M.C., Calas G. (2005) Geochim. Cosmochim. Acta 69(9), 2193-2204.

[3] Allard T., Gautheron C., Bressan Riffel S., Balan E., Fernandes Soares B., Pinna-Jamme R., Derycke A., Morin G., Taitson Bueno G., do Nascimento N. (2018) Chem. Geol., 479, 136-150.

[4] Allard T., Pereira L., Mathian M., Balan E., Taitson Bueno G., Falguères C., do Nascimento N.R. (2020) Palaeogeogr. Palaeoclimatol. Palaeoecol. (submitted)

[5] Mathian M., Aufort J., Braun J.J., Riotte J., Selo M., Balan E., Fritsch E., Bhattacharya S., Allard T. (2019) Gondwana Res., 69, 89-105.

[6] Mathian M., Taitson Bueno T. G., Balan E., Fritsch E., do Nascimento N.R., Selo M., Allard T. (2020) Geoderma (in revision).

How to cite: Allard, T., Mathian, M., Taitson Bueno, G., Pereira, L., Fernances Soares, B., Regina Montes, C., Balan, E., and Regina do Nascimento, N.: Recent developments and applications of kaolinite dating: examples of weathering covers from the Amazon Basin (Brazil), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21770, https://doi.org/10.5194/egusphere-egu2020-21770, 2020.