Displays

A number of studies have shown that iron reduction in marine sediments is not confined to sulfate- or sulfide-containing depths but may also affect deep methanic intervals. In particular dynamic depositional settings often show the release of dissolved iron below the sulphate-methane transition (SMT). The specific process behind this deep iron release is not well understood. It has been suggested that anaerobic oxidation of methane (AOM) mediated by Fe oxide reduction plays an important role. So there might be a close, so far unaccounted link between the Fe and C cycles in deep marine sediments.

Here we present a compilation of inorganic geochemical data including δ⁵⁶Fe values of pore water and reactive Fe fractions for sediments of the Helgoland mud area (North Sea) for which a coupling between deep iron reduction and AOM has been proposed [1]. The sediments show a shallow SMT and increasing dissolved Fe concentrations of up to 400 µM further below. High sedimentation rates led to a fast burial and preservation of reactive Fe (oxyhydr)oxides, enabling deep iron reduction as we observe it today.

Isotopic fractionation of Fe has been demonstrated for DIR in culture experiments and in shallow marine sediments. Such studies build upon the principle that microbes preferentially utilize light Fe isotopes (⁵⁴Fe) causing a fractionation between solid ferric and dissolved ferrous iron. For alternative biotic Fe reduction pathways in methanic environments, there are practically no data. We hypothesized that any microbially mediated iron reduction process would result in a similar preferential release of ⁵⁴Fe and, thus, shift pore water δ⁵⁶Fe towards negative values. Furthermore we hypothesized that the microbial utilization of a specific Fe (oxyhydr)oxide pool would result in a relative enrichment of ⁵⁶Fe in the residual ferric substrate.

Close to the sediment-water interface pore water δ⁵⁶Fe in the mud area is generally negative and shows a downward trend towards positive values as it can be expected for in-situ dissimilatory iron reduction (DIR) [2]. The Fe isotope signal close to the sulfidic interval is ~1‰ heavier than above and below as Fe sulfide precipitation preferentially removes ⁵⁴Fe from pore water. A pronounced downward shift of pore-water δ⁵⁶Fe to more negative values within the methanic zone is a clear indication for microbial Fe reduction coupled to organic matter degradation. However, this shift does not coincide with the main interval of Fe release for which potential for Fe-AOM had been demonstrated [1]. In this deeper interval, the released Fe has an isotopic composition that matches that of the ferric substrates. We conclude that either 1) Fe-AOM plays a subordinate role for Fe release at depth or 2) does not go along with significant Fe isotope fractionation, which might be explained by different ways of electron transfer between microbe and the iron oxide compared to DIR.

[1] Aromokeye, D. et al., 2019. Frontiers in Microbiology, doi: 10.3389/fmicb.2019.03041.

[2] Henkel, S. et al., 2016. Chemical Geology 421: 93-102.

How to cite: Henkel, S., Liu, B., Staubwasser, M., Kasemann, S., Meixner, A., and Kasten, S.: Stable iron isotope signals as indicators for iron reduction pathways in deep methanic sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9273, https://doi.org/10.5194/egusphere-egu2020-9273, 2020.

Several important isotopic biomarkers derive at least part of their signal from the stable isotope composition of leaf water (e.g., leaf wax δ²H, cellulose δ²H and δ¹⁸O, lignin δ¹⁸O). In order to interpret these isotopic proxies, it is therefore helpful to know which environmental variable most strongly controls a given leaf water stable isotope signal. We collated observations of the stable isotope compositions of leaf water, xylem water, and atmospheric vapour, along with air temperature and relative humidity, to test whether the dominant driver of leaf water ²H concentration could differ from that of ¹⁸O concentration. Our dataset comprises 690 observations from 35 sites with broad geographical coverage. We limited our analysis to daytime observations, when the photosynthetic processes that incorporate the leaf water isotopic signal primarily take place. The Craig-Gordon equation was generally a good predictor for daytime bulk leaf water stable isotope composition for both δ²H (R²=0.86, p<0.001) and δ¹⁸O (R²=0.63, p<0.001). It showed about 10% admixture of source water was caused by unenriched water pools such as leaf veins or the Péclet effect. Solving the Craig-Gordon equation requires knowledge of relative humidity, air temperature, and the stable isotope compositions of source water and atmospheric vapour. However, it is not possible to invert the Craig-Gordon equation to solve for one of these parameters unless the others are known. Here we show that the two isotopic signals of δ²H and δ¹⁸O are predominantly driven by different environmental variables: leaf water δ²H correlated most strongly with the δ²H of source water (R²=0.68, p<0.001) and atmospheric vapour (R²=0.63, p<0.001), whereas leaf water δ¹⁸O correlated most strongly with air relative humidity (R²=0.46, p<0.001). We conclude that these two isotopic signals of leaf water are not simply mirror images of the same environmental information, but carry distinct signals of different climate factors, with crucial implications for the interpretation of downstream isotopic biomarkers.

How to cite: Cuntz, M. and Cernusak, L. A. and the Isotopists: The dominant environmental driver of leaf water stable isotope enrichment differs for H-2 compared to O-18, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3343, https://doi.org/10.5194/egusphere-egu2020-3343, 2020.

While the global budget of nitrous oxide (N₂O) is rather well constrained from a “top-down” perspective considering the change in the atmospheric burden and stratospheric N₂O destruction, estimates of the various sources such as natural/agricultural soils, coastal areas or fossil fuel burning and industry remain uncertain. The isotopic composition of N₂O, i.e., the relative abundances of the four most abundant isotopic species (¹⁴N¹⁴N¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, and ¹⁴N¹⁴N¹⁸O) have been identified as instrumental tools for attributing emissions to the corresponding production-consumption processes and to estimate the global budget. During the past two decades, N₂O isotopic composition of individual sources has been investigated, and temporal trends in the isotopic composition of atmospheric N₂O have been studied using and firn air and archived air samples collected in Antarctica. With regard to ¹⁵N and ¹⁸O in atmospheric N₂O, a decreasing trend was consistently observed across studies, but contradictory results have been obtained for site preference (SP), i.e., the difference in the abundances of ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁵N¹⁶O relative to ¹⁴N¹⁴N¹⁶O. In addition, N₂O isotopic composition for natural or agricultural soils rely on a limited amount of studies and usually cover only parts of the annual cycle.

Since instruments used for optical isotope ratio spectroscopy (OIRS) can be deployed in the field, OIRS offers the opportunity to better characterize individual sources through long-term data in high temporal resolution. However, application of OIRS is challenging and, thus, remains scarce with regard to spatial resolution. For this reason, model-based regional estimates are pertinent to overcome the lack of regional estimates of N₂O isotopic composition, to analyze trends, and to provide data for a refinement of the global budget.

To obtain regional-scale (Switzerland) model-based estimates of N₂O isotopic composition, we used data sets of measured N₂O isotopic composition of two sites that are based on OIRS, and applied the Stable Isotope MOdel for Nutrient cyclEs, SIMONE in conjunction with the biogeochemical model LandscapeDNDC. Our results show that SIMONE/LandscapeDNDC was capable of reflecting especially SP, but also ¹⁵N-N₂O at sites with different soil properties. For agricultural soils, our simulations revealed an annual cycle in SP, with higher values during the growing season, but not for ¹⁵N-N₂O. We will also discuss effects of agricultural management on N₂O emissions as well as temporal trends.

How to cite: Wolf, B., Haas, E., Kraus, D., Kiese, R., and Butterbach-Bahl, K.: Regional characterization of N2O isotopic composition emitted from soils in view of land cover, agricultural management and annual cycles based on measurements and modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15250, https://doi.org/10.5194/egusphere-egu2020-15250, 2020.

The global Shuram anomaly records the longest and most negative carbonate carbon isotopic excursion in Earth history. It took place during the late Ediacaran (c. 570 – c. 551 Ma) with δ¹³C_carbvalues down to −12‰. In South China, Doushantuo Formation Member IV (c. 555-551 Ma) consists mainly of organic-rich black shales and records the recovery of this anomaly, with values going from –6‰ to +0.5‰. The origin of this anomaly is thought to be related to the existence of a vast pool of dissolve organic carbon (DOC) in the ocean that was episodically oxidized thereby providing a source of ¹³C-depleted inorganic carbon. However, the main processes that ultimately drove to its recovery remain elusive. Here, we present new δ¹³C_organd δ¹⁵N dataset along a shelf-to-basin transect of the Nanhua basin (South China) as robust organic proxies to reconstruct the spatial and temporal evolution of paleoproductivity at basin scale. In addition, Raman spectroscopy is used to assess the thermal maturity of the samples. These new results define areas of high primary productivity and suggest the existence of an oxygen minimum zone (OMZ) together with other reduced oxic areas. From base to top of Member IV, the observed increasing and covariant trends in δ¹³C_carb and δ¹³C_orgdata together with a decreasing drift in δ¹⁵N values in platform and mid-lower slope environments are interpreted as areas where primary productivity became the main source of organic matter. Conversely, decreasing trends in δ¹³C_carb and δ¹³C_org data together with invariant δ¹⁵N values in the upper slope and deep basin environments are interpreted as areas where reduced DOC dominated as the principal source or organic carbon. Based on that, we propose that a new balance was established between primary and secondary paleoproductivity, whereby the former succeeded to the latter as one of the principal contributors that led to the carbon isotope recovery in carbonates. This new model represents a plausible solution to the enigmatic negative δ¹³C_carbisotopic excursion of the late Ediacaran.

How to cite: Cañadas Blasco, F., Papineau, D., Shields, G., Zhu, M., Li, C., and Leng, M. J.: The recovery of the Shuram anomaly and paleoproductivity balance, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15179, https://doi.org/10.5194/egusphere-egu2020-15179, 2020.

Madagascar shows very high erosion rates and, in some regions, the landscape is dotted by major gullies called “lavaka”. Lavaka are also very frequent in the surroundings of Lake Alaotra, a large, shallow lake in the Malagasy highlands. A central question with respect to landscape evolution in Madagascar is to what extent human impact has triggered environmental change in terms of vegetation, erosion rates (lavaka formation) and sediment dynamics. Sedimentary archives in lakes such as lake Alaotra can be of great help to resolve this question provided that we understand how sediment and carbon are mobilised and transported through the landscape.   

In this study, we traced pathways of sediment and carbon fluxes through this eroding landscape, from the eroded hillslopes over various sediment deposition zones (floodplains, reservoirs, marshes..) to Lake Alaotra. Detailed profiles taken along convex hillslope transects (grasslands and primary forest), in the marsh peat, floodplains and lake were analyzed for carbon and nitrogen content, texture, and stable carbon isotope ratios (δ¹³C). Along the grassland hillslopes, soil OC content is extremely low, from 0.4 to 1.8% in the top layer and rapidly decreasing to <0.2 % below 100  cm depth. The current vegetation predominantly consists of C4 grasses (δ¹³C ~-13 ‰), yet soil δ¹³C ranges between -24 and -18‰, and most profiles show a decrease in δ¹³C with the depth – in contrast to observations in most C3-dominated systems. Contrary to our expectations, Lake Alaotra was found not to be a major sink of hillslope-derived sediments and/or carbon. Sediment cores from different parts of the lake have high OC contents (5 to 18%) and contain only minor amounts of sand, the dominant grain size class on the hillslopes. The high OC content of the lake sediments, in combination with data on C/N ratios and δ¹³C indicate that the OC in the lake sediments is mainly derived from the surrounding marshes and in situ primary production rather than from terrestrial C eroded from the catchment. Floodplains are likely a key sink for soil-derived sediments: similar to hillslope soils, sediment profiles in the floodplains show a low %OC and relatively high δ¹³C values ranging between -21 and -14‰.

We conclude that most of the detritic sediments and carbon mobilised on the hillslopes through erosion do not reach lake Alaotra, even though erosion rates in the landscape are extremely high. Studying sedimentary profiles in the lake may provide information on environmental change (e.g. through changes in carbon contents and/or characteristics) but is insufficient to understand the entire sediment and carbon cascades in the Malagasy landscape.

How to cite: Razanamahandry, V. F., Brosens, L., Dewaele, M., Jacobs, L., Benjamin, C., Broothaerts, N., Verstraeten, G., Razafimbelo, T., Rafolisy, T., Govers, G., and Bouillon, S.: Sediment mobilisation in Lake Alaotra catchment, Madagascar , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8122, https://doi.org/10.5194/egusphere-egu2020-8122, 2020.

Stable sulfur isotope analysis is potentially applicable in various fields in forensics and environmental analytics to investigate the sources and degradation of organic compounds, many of them being priority pollutants in groundwater and the atmosphere. A broader use of sulfur isotopes of organic compounds in environmental studies is still hampered by the availability of precise and easy-to-use techniques. Here we present a method for the determination of stable sulfur isotope ratios using gas chromatography coupled with multiple-collector inductively coupled plasma mass spectrometry (GC-MC-ICPMS) which can be used for both δ³⁴S and δ³³S analysis. The method was evaluated using the reference materials IAEA-S-1, IAEA-S-2 and IAEA-S-3 which were converted offline to SF₆ prior to analysis. Standardization was carried out by using a two-point calibration approach. The δ³⁴S values obtained by our method are in good agreement (within analytical uncertainty) with the results obtained by the conventional dual inlet method. Additionally, the impact of the used mass resolution (low and medium), the influence of auto-protonation of sulfur isotopes and the effect of isobaric interferences of O₂⁺ on the obtained isotopic ratios was investigated. The analytical precision (1σ) for δ³⁴S and δ³³S values was usually better than ±0.1 ‰ for analytes containing >0.1 nmol S. Thus, the presented compound-specific online method should be sufficiently precise to address a wide variety of research questions involving mass independent isotope effects of sulfur-containing organic compounds to discriminate sources or biological and chemical reactions in the environment.

How to cite: Steffen, K., Faina, G., Axel, H., Harald, S., and Matthias, G.: Compound Specific Stable Sulfur Isotope Analysis (δ34S and δ33S) of Organic Compounds Using Gas Chromatography Hyphenated with Multiple Collector Inductively Coupled Plasma Mass Spectrometry (GC-MC-ICPMS), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13835, https://doi.org/10.5194/egusphere-egu2020-13835, 2020.

Sulfate isotopes in a heavily polluted river network

Tobias Goldhammer¹, Juliane Lenz²

¹Leibniz-Institute for Freshwater Ecology and Inland Fisheries, Department of Chemical Analytics and Biogeochemistry, Berlin, Germany

²University of Bremen, Department of Geosciences, Germany

The Spree is the major river system in NE Germany, with about 380km in length and a catchment of more than 10,000 km². While intensive open pit lignite mining in the upper catchment has significantly altered the hydrology and hydrochemistry over the last century, River Spree is at the same time a critical supplier of drinking water to the city of Berlin. Acid mine drainage is the major contributor to the river water sulfate load, which frequently exceeds the drinking water limit of 250mg L^-1. Increasing summer drought and low-flow regimes are projected to intensify this situation in the future. The sulfate pollution in River Spree has induced a significant shift in biogeochemical regimes, in particular in those compartments of the river network where low flow velocity is supportive to sediment accumulation and bacterial sulfate reduction. Secondary effects include the mobility of iron and phosphorus, and entail critical consequences for the aquatic ecosystem.

In this contribution, we discuss the results of an integrated study of hydrochemistry and sulfate and water isotopes in the Spree river network. We put particular emphasis on

(1) Differentiating major geographic and functional sulfate sources and sinks in the Spree river network based on sulfur and oxygen isotopes in river sulfate

(2) Quantifying these sources and sinks by simple endmember models, and identifying limitations of this approach

(3) The role of biogeochemical sulfur cycling (reduction/reoxidation cycles and intermediates) in retention spaces of the river network and the consequence for prevailing isotope signatures.

How to cite: Goldhammer, T. and Lenz, J.: Isotope insights into sulfate loads and sulfur cycling in a heavily polluted river network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14997, https://doi.org/10.5194/egusphere-egu2020-14997, 2020.

Stable isotope analysis of biogenic carbonates has remained one of the most important tools in paleoceanography since Emiliani (1955) first described the fluctuation of oxygen isotopes in planktic foraminifers over the Pleistocene. Many laboratories now possess equipment with the capability to analyse foraminifer specimens singularly, at least for larger planktic forms.

Being able to run single specimens of planktic foraminifers is significant, because it yields entirely different information than when one would analyse multiple specimens from the same species. Planktic foraminifers have an average life span of about one month, so analysing single specimens, makes paleoceanographic data at seasonal resolution available (e.g. Ganssen et al., 2011; Metcalfe et al 2019, and references therein).

Most modern equipment for stable isotope analysis of CaCO₃ samples yields good precision down to 10 microgram sample size. The smallest samples are generally measured with a dual inlet technique, because that quantitatively collects the CO₂ gas sample in a cold trap before analysis, leading to a more efficient use of the sample gas. Modern dual inlet equipment has a sample size limit somewhere between 10 and 6 microgram CaCO₃ sample weight, and in that range usually operates at increased analytical uncertainty when compared to larger samples (e.g. Ganssen et al., 2011). Smaller samples are problematic, because at small amounts of sample gas, the dual inlet system is not able to maintain viscous flow conditions required for precise isotope analysis. To circumvent that barrier, one can use continuous-flow (CF) mass spectrometry because in CF systems the carrier gas ensures proper flow conditions even if there is (virtually) no sample gas produced. Doing so has previously allowed for the  isotope analysis of CaCO₃ samples in the 10 – 6 microgram range at an external precision (1SD) of ~0.12‰  for both δ¹⁸O and δ¹³C (e.g. Metcalfe et al 2019).

To further improve the performance of CF mass spectrometry for small CaCO₃ samples, we ran experiments on a Thermo GASBENCH system, equipped with a cold trap (cf. Fiebig et al 2005) and interfaced with a Delta-V mass spectrometer. The experiments consisted of replicate analysis of CaCO₃ standards between 10 and 3 micrograms in weight, which is the weight range of many of the smaller specimens of planktic foraminifers.

Several hardware modifications were implemented to improve system stability and remove observed effects of contribution of blank CO₂ building up in the sample vials. With these modifications, external reproducibility of the set-up for carbonate standard aliquots between 10 and 4 microgram reached a precision of ~0.10 ‰ for both δ¹⁸O and δ¹³C (1SD). This is similar to precisions typically attained for routine analysis of much larger samples in standard operation on the same equipment, and demonstrates that precise stable isotope analysis of smaller single-specimen planktic foraminifers than we could achieve so far is within reach of CF mass spectrometry.

References:

Emiliani, C. 1955, DOI: 10.1086/626295

Fiebig, J., et al. 2005, DOI: 10.1002/rcm.2060

Ganssen, G.M., et al., 2011, DOI:10.5194/cp-7-1337-2011

Metcalfe, B., et al., 2019, DOI: 10.1029/2018PA003475

How to cite: Vonhof, H., de Graaf, S., Spero, H., Schiebel, R., Verdegaal-Warmerdam, S., Metcalfe, B., and Haug, G.: High-precision stable isotope analysis of less than 5 microgram carbonate samples by continuous-flow mass spectrometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19889, https://doi.org/10.5194/egusphere-egu2020-19889, 2020.

Food products of certain geographic origin are more valued by consumers than the same commodities from other regions. Therefore, there is the risk and fear that incorrect labelling and declaration of geographic origin can occur to increase profit. Thus, a control of declared geographic origin is required to ensure correct labelling and to identify fraud.

For this purpose, apricot samples of the recent vintage (2019) are investigated to differentiate samples from different apricot-producing regions in Austria, Slovakia and other countries. The isotope composition of the elements hydrogen (H), carbon (C), nitrogen (N) and oxygen (O) of fruit pulp (H, C, N, O), fruit stone (H, C, O) and fruit juice (O) is analysed to find appropriate parameters for the differentiation of geographic origin. The investigation of different sample tissues (pulp, stone, juice) supports a better differentiation of geographic origin due to different seasonal intervals influencing the respective commodities.

Within the frame of the project 3 vintages will be investigated and analysed for stable isotopes as well as other analytical techniques (molecular markers). The combination of all sample data (including previously accumulated data, e.g. Horacek 2017, Horacek 2019) will lead to an improved differentiation and identification of geographic origin.

This work is a contribution to the Interreg project IDARPO partially funded by the EU-Interreg program.

References:

Horacek, M., 2017, Isotope investigation of apricots from the Wachau-area/Lower Austria („Wachauer Marille“) to control the declared geographic origin: A pilot study – first results. Mitteilungen Klosterneuburg. 67, p. 219-228.

Horacek, M., 2019, Stable isotope analysis for control of declared geographic origin of Austrian apricots. EGU 2019, Vienna.

How to cite: Horacek, M.: Stable isotope analysis for control of declared geographic origin of Austrian and Slovak apricots: The IDARPO-Interreg-Project (AT-SK) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20095, https://doi.org/10.5194/egusphere-egu2020-20095, 2020.

A detailed understanding of the carbon and oxygen isotope ratios of modern pollen is crucial for the interpretation of fossil δ¹³C_pollen and δ¹⁸O_pollen values. To broaden our knowledge of pollen-isotope ratios we investigated the isotope ranges of nine abundant tree species from central and northern Europe (vegetation periods 2015 and 2016).

In general, the isotope values of modern pollen are highly species-specific and yield site-specific patterns. Trees of different locations revealed distinct δ¹³C_pollen and δ¹⁸O_pollen patterns for maritime and continental growing conditions and for high and low altitudes. Furthermore, pollen-isotope ratios reflect the time of blossoming. δ¹³C_pollen values of broad-leaved species flowering before leaf proliferation (Janurary to March; Alnus glutinosa and Corylus avellana) are on average 2.6‰ lower in comparison to broad-leaved and coniferous trees flowering during late spring and early summer (April to June; Acer pseudoplatanus, Betula pendula, Carpinus betulus, Fagus sylvatica, Picea abies, Pinus sylvestris and Quercus robur). δ¹⁸O_pollen values yielded similar results, which are on average 3.1‰ lower for species flowering early in the year. An intra-annual analysis of Betula pendula and Pinus sylvestris pollen revealed increased δ¹⁸O_pollen values during the last stages of pollen-maturation, whereas δ¹³C_pollen values of both species remain consistent during late pollen development. Additionally, pollen-isotope values vary markedly within individual trees. Circumferential and height-dependent variations within single trees can be as high as 3.5‰ for δ¹³C_pollen and 2.1‰ for δ¹⁸O_pollen.

Our results suggest that local environmental conditions are generally reflected in the carbon and oxygen pollen-isotopes, but some species seem to reflect the conditions more closely than others. The data indicate that it may even be feasible to reconstruct intra-annual climate conditions by analysing isotopes of species whose pollen develop during different seasons throughout the year.

How to cite: Müller, C., Hethke, M., Riedel, F., and Helle, G.: Stable carbon and oxygen isotope variability of modern pollen from nine abundant European tree species., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8056, https://doi.org/10.5194/egusphere-egu2020-8056, 2020.

Forest soils are important for retaining nitrogen (N), especially in areas where anthropogenic activities have led to historically high inputs of N. As forests age and their N demands for biomass accumulation decline, the capacity for N retention of soils may change as well, although little work has been done to further our understanding of this process. We conducted a mineral soil reciprocal transplant study in three northern hardwood forests of different ages (young, recently mature, and old growth) in New Hampshire, USA to determine how the retention of isotopically labeled nitrogen from leaf litter would differ depending on characteristics of the incubated soil’s origin and destination. After 18 months of incubating the soil bags below the ¹⁵N-labeled litter, we did not find retention of litter-derived N to be related to the age of the incubation site forest, but rather that it differed based on the origin of the incubated soil.  We found that the soil C content was the strongest predictor of how much of the tracer was recovered in the transplanted soil bags. Furthermore, the C content of soils changed during incubation and tended to change in the direction of equilibrating with the soil C concentration of the incubation site. This finding suggests that site characteristics are important in determining soil C concentrations and consequently N retention capacities.

How to cite: Fuss, C., Lovett, G., Goodale, C., Ollinger, S., Lang, A., and Ouimette, A.: Tracing leaf litter-derived 15N to mineral soil organic matter in forests of different ages, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13976, https://doi.org/10.5194/egusphere-egu2020-13976, 2020.

In this study, we investigated if variations in the stable Tl isotope ratios in soil samples from different profiles can be linked to data on the extractability and speciation of soil Tl and whether the isotopic data allow drawing conclusions on the geochemical processes linked with soil formation/rock weathering. We observed a significant accumulation of the heavy ²⁰⁵Tl isotope in the B horizons, with ε²⁰⁵Tl values that were up to 7 higher than in the underlying bedrock. This ²⁰⁵Tl enrichment, however, was neither reflected in the speciation of Tl nor its chemical fractionation. Furthermore, exchangeable soil Tl in the B horizons was found to be much isotopically lighter than the bulk soil Tl. Our findings suggest that the observed isotopic shift may be linked to cyclic Tl mobilization and immobilization processes over the period of rock weathering and soil formation. Oxidative Tl uptake by Mn-oxides associated with a ²⁰⁵Tl enrichment, continuous weathering of the Tl(III)-containing phases, followed by a Tl(I) remobilization (leading to enrichment in ²⁰⁵Tl) are suggested to be responsible for the binding of the heavy Tl isotope fraction into other phases, mainly illite (a dominant Tl host), which is not normally expected. We therefore conclude that the use of the Tl isotopic data for phase or sorption mechanism identification in a dynamic multi-phase (soil) system can be very complicated, but, in contrast, suggesting their efficient use as a proxy for redox-controlled processes.

How to cite: Vanek, A., Voegelin, A., Mihaljevic, M., Ettler, V., Trubac, J., Drahota, P., Vankova, M., Oborna, V., Penizek, V., Pavlu, L., Drabek, O., Vokurkova, P., Zadorova, T., and Holubik, O.: Stable thallium isotope fractionation in soils as affected by pedogenesis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2399, https://doi.org/10.5194/egusphere-egu2020-2399, 2020.

Due to its historical Pb, Zn, Ag, Cd and Cu mines and associated smelter, Kabwe in Zambia is known to be one of the most polluted cities in the world.

Contamination by Pb, Zn and Cu was assessed in four soil profiles around the smelter and remote locations, using Q-ICPMS for trace metal elemental and Pb isotopic measurements. A sequential extraction procedure (SEP) approach was used to obtain a detailed understanding of the vertical behaviour of the contaminants and its availability for plant uptake. Slags Pb isotopic ratios were also determined. Furthermore, tree rings of local pine trees (Pinus Montezumae) were collected and analysed for the same contaminants and Pb isotopes coupled with C isotopes. Results were compared to the smelter production historical records to assess the viability of these trees as environmental archive.

Results show that contamination is exclusive to the top layers of soil and is greater in soils closer to the smelter, which are highly contaminated (max: 16000 mg/kg Pb; 140000 mg/kg Zn; 600 mg/kg Cu). Remote soils have much lower topsoil concentrations (min: 61 mg/kg Pb; 351 mg/kg Zn; 21 mg/kg Cu). Interestingly, the greatest contaminant concentrations were found in the tree furthest from the source of pollution (max.: Pb, 6.48 mg/kg; Zn, 10.6 mg/kg; Cu, 10.2 mg/kg). Particle size of wind-blown dump dust decreases with distance. A hypothesis is considering that these would be more easily adsorbed and absorbed by tree bark and leaves. This suggests that above-ground tree uptake is more important than soil uptake for the selected elements.

Slag Pb isotopic ratios average at ²⁰⁶Pb/²⁰⁷Pb = 1.15; ²⁰⁸Pb/²⁰⁶Pb = 2.15; for tree rings; both sites: ²⁰⁶Pb/²⁰⁷Pb = 1.15; ²⁰⁸Pb/²⁰⁶Pb = 2.13; and in top soils, close to smelter: ²⁰⁶Pb/²⁰⁷Pb = 1.15; ²⁰⁸Pb/²⁰⁶Pb = 2.12; and in remote location: ²⁰⁶Pb/²⁰⁷Pb =1.14; ²⁰⁸Pb/²⁰⁶Pb = 2.15. Isotopic ratios confirm the mine and smelter to be the main source of contamination.

Smelter production records show three major shifts in production amount; increase from the late 1950s to early 1970s and a subsequent decrease till the closure of the smelter in 1994 with a peak in production in the early 1980s. There seems to be a correlation between Pb production and Pb uptake and Pb and C isotopic ratio variations within a 5 to 10 years delay.

This study was supported by the Czech Science Foundation project (GAČR 19-18513S) and received institutional funding from the Center for Geosphere Dynamics (UNCE/SCI/006). Part of the equipment used for this study was purchased from the Operational Program Prague - Competitiveness (Project CZ.2.16/3.1.00/21516).

How to cite: Baieta, R., Mihaljevič, M., Ettler, V., and Vaněk, A.: Environmental impact assessment of a Pb, Zn smelter using soil, slag and tree ring elemental and isotopic geochemistry in Kabwe, Zambia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8211, https://doi.org/10.5194/egusphere-egu2020-8211, 2020.

This study aims to develop a simplified denitrification method for the δ¹⁵N and δ¹⁸O analysis of nitrate (NO₃^–) in natural groundwater samples following Zhu et al. (Sci Total Environ. 2018; 633: 1370–1378) and Sigman et al. (Anal Chem. 2001; 73: 4145–4153). With the help of Pseudomonas aureofaciens bacteria, the simplified method induced denitrification of the sample and completely converted the NO₃^– into measurable N₂O while avoiding sample fractionation. In contrast to the classic denitrification method (Sigman et al., 2001), which is based on anaerobic cultivation, the bacteria are cultivated aerobically in the simplified method (Zhu et al., 2014). In this study, aerobic cultivation was performed in a nitrate-free medium. Unlike the other two methods, aerobic cultivation was performed without the addition or removal of nitrate in the liquid medium. This eliminates the need for another preparation step, saving time. There was no contamination with external NO₃^–. After further optimising the influencing factors, the method yielded high accuracy and precision (standard deviations were generally ≤ 0.7‰ for δ¹⁸O and ≤ 0.3‰ for δ¹⁵N), confirming the suitability of this procedure. Finally, the potential applicability of the method was demonstrated by measuring the isotopic composition of NO₃^– in natural groundwater samples.

How to cite: Stock, P., Roder, S., and Burghardt, D.: Applying the denitrification method to 15N and 18O analysis of nitrate in natural groundwater samples, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4864, https://doi.org/10.5194/egusphere-egu2020-4864, 2020.

Mareza Spring is used for the water supply of Podgorica (capital of Montenegro) since over 70 years. It is located in the central part of Montenegro in the north-western part of Podgorica Valley. The recharge area and origin of groundwater of this karstic source are not known well. This is primarily due to the fact that drainage divides in karst terrains are the unknown and insufficiently examined segments. There are a few hypotheses about the origin of water: 1. from the Zeta River which flows few kilometres north-east from Mareza Spring, 2. from the Morača River which partly sinks at the exit of the canyon (around 10 km east from Mareza Spring), and 3. from the Prekornica Mountain recharge area that is located 10-20 km north-east from the spring (that is a karst plateau with average altitude around 1,000 m asl). Therefore, the isotopic techniques (altitude effect, comparison) could be useful for testing these assumptions. In the present study monitoring of stable isotopes (²H, ¹⁸O) in precipitation, surface water and groundwater of this area is carried out to determine the origin of water and adequate protection of Mareza Spring.

How to cite: Radulovic, M., Horacek, M., Sekulic, G., Ćipranić, I., Živaljević, S., Stumpp, C., and Wyhlidal, S.: An application of stable isotope techniques for the investigation of geographic origin of water–investigation of the Mareza Spring near Podgorica (Montenegro), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18220, https://doi.org/10.5194/egusphere-egu2020-18220, 2020.

Sediments in the coastal ocean can play an important role in nutrient regeneration and in recharging the water column with dissolved inorganic nutrients. This function, however, depends on various variables, such as physical characteristics, but also on biological traits like fauna composition and activity. To unravel and quantify these effects, we investigated nutrient fluxes and nitrate stable isotope composition in water samples along a North Sea – Skagerrak – Baltic Sea gradient during the Maria S. Merian cruise MSM 50 in January 2016.

Especially in the North Sea and the Skagerrak region, d¹⁵N values of nitrate were unexpectedly high, suggesting that underlying sediments with relatively enriched isotope signatures were a source of nitrate. This nitrification signal, however, resembled an autumn situation rather than the expected winter values. Parallel sediment incubations confirm that the benthic rates of oxygen consumption and nutrient turnover were indeed very similar to respective rates in autumn and that the sediment was a source of recycled nitrate. From the North Sea towards the Baltic Sea, we found, in accordance with previous studies, a depletion in nitrate stable isotope values. This is indicative of different nitrate sources in the respective basins: in the North Sea region, N of anthropogenic origin leads to high N values in surface sediments and in newly generated nitrate. Due to a higher share of nitrogen fixation, the nitrogen stable isotope signal of surface sediments in the Baltic Sea was depleted, which in turn was mirrored in lower nitrate isotope values in the water column above the sediment.

Overall, the data highlight the importance of nitrate regeneration. Parallel flux measurements reveal that faunal activity shifts the nutrient balance from sequestration to regeneration. Seasonal differences enable us to unravel seasonal effects of fauna and microbiota on nutrient budgets.

How to cite: Dähnke, K., Neumann, A., and Sanders, T.: Nutrient regeneration and benthic fluxes in the Coastal Baltic and North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15863, https://doi.org/10.5194/egusphere-egu2020-15863, 2020.

The sedimentary sequence from Lake Emanda (65°17′N; 135°45′E; 675 m a.s.l), one large freshwater body (33.1 km²) in the continuous permafrost of the Verkhoyansk Mountains, has been investigated within the German-Russian ‘Paleolimnological Transect’ (PLOT) project. It provided important insight into the environmental and climate dynamics in northeastern Siberia.

Well preserved diatoms occur only in the upper 125-cm interval of a 6.1-m sediment core (Co1412) covering the last ca. 13.4 cal. ka BP, and are mostly dominated by Cyclotella iris (up to 84%). The diatom succession is enriched by fragilarioid assemblages in the interval from ca. 11.0 to 13.0 cal. ka BP, while Aulacoseira ambigua is more frequent between 8.5 and 6.5 cal. ka BP. Diatoms were purified to > 98% SiO₂ and < 0.8% Al₂O₃ suitable for oxygen isotope (δ¹⁸O_diatom) analysis. The δ¹⁸O_diatom values were corrected for contamination and range between +22.5‰ and +27.3‰. Maximum δ¹⁸O_diatom values (+26.7 to +27.3‰) are registered between 9.0 and 9.9 cal. ka BP and probably reflect a thermal maximum and/or very dry conditions in Early Holocene. The absolute minimum (+22.5‰) in the δ¹⁸O_diatom record is marked at 0.4 cal. ka BP and likely corresponds to the Little Ice Age. In general, a gradual depletion of 4.8‰ in δ¹⁸O_diatom is observed within the last 10 cal. ka, in line with an overall Holocene temperature decrease.

Our conclusions are based on a comprehensive investigation of both the modern hydrological system and diatom species analyses. The most recent δ¹⁸O_diatom = +24.2‰ combined with the present day lake water isotope composition (mean δ¹⁸O_lake = −16.5‰), indicates a reasonable water−silica isotope fractionation (α = 1.0414) yielding a water temperature of 12 °C. The data demonstrate that the δ¹⁸O_diatom variability is associated with changes in the lake water isotopic composition rather than with lake temperature. The present water isotopic composition of Lake Emanda displays substantial evaporation effects, likely further influenced by air temperature and atmospheric circulation.

How to cite: Kostrova, S., Meyer, H., Pestryakova, L., Biskaborn, B., Fernandoy, F., and Baumer, M.: Environmental and climate dynamics in northeastern Siberia according to diatom oxygen isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8574, https://doi.org/10.5194/egusphere-egu2020-8574, 2020.

The interface of land and sea is of particular interest regarding the exchange of elements, like nutrients, carbon and sulfur. Submarine groundwater discharge (SGD) is an important pathway for element exchange from the terrestrial to the marine environment and vice versa. The discharging water can not only consist of fresh ground water but also of a considerable proportion of recirculated often brackish seawater.

Here, we followed the water and element exchange and associated biogeochemical transformation processes in front of a rewetted peatland at the southern Baltic Sea. Vertical pore water profiles were retrieved via up to 5 m long multi-port pore water samplers on a seasonal base. An extrodinary storm event in early 2019 not only led to the partial flooding of an associated coastal peatland with brackish water but also pushed Baltic Sea water into the coastal aquifers allowing to investigate the time-dependent return to previous subterrestrial ‚normal‘ conditions via SGD-induced freshening. Weekly sampling was carried out to follow the changes after the storm event in the sediments in front of a coastal peatland. Here we present new results of the pre- and after storm event pore water profiles. A focus was set on the investigation of tracers for concentration gradients of major and redox-sensitive trace elements, nutrients and the stable isotope composition (H, C, S, O) of water, dissolved inorganic carbon (DIC) and sulfate to understand the mixing processes and superimposing biogeochemical transformation reactions.

We found evidence for a strong control of the bottom-pore water exchange by lithology and a high activity of dissimilatory sulfate-reducing microorganisms in the coastal sediments leading to the accumulation of substantial DIC superimposed by corrosion of sedimentary carbonates.

Acknowledgement: This study is supported by the DFG research training group Baltic TRANSCOAST and the Leibniz IOW.

How to cite: Jenner, A.-K., Westphal, J., Liu, B., Schmiedinger, I., and Böttcher, M.: Dynamics in the isotope biogeochemistry of a SGD-impacted coastal aquifer after a storm event, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7563, https://doi.org/10.5194/egusphere-egu2020-7563, 2020.

Excessive consumption of petroleum and crude oil for energy purposes has resulted in the contamination of many natural systems and waterways. However, determining the presence and level of contamination has been difficult due to the presence of naturally occurring hydrocarbons and to the complexity of the molecular fingerprint of petroleum and crude oils. Naturally occurring straight-chain n-alkanes and polycyclic aromatic hydrocarbons (PAHs) in sediments are both commonly used to determine organic matter sources through diagnostic and isotope ratios, and these ratios are affected by the presence of petroleum and crude oil. As such, they offer a potential avenue for determining whether crude oil contaminants are present in natural systems. The purpose of this project was to determine whether diagnostic ratios of n-alkanes and of PAHs as well as compound-specific isotope ratios of n-alkanes (∂²H_alk and ∂¹³C_alk) could be used to detect crude oil or petroleum contamination, and at what level of contamination the difference becomes significant. This was accomplished by separating the aliphatic and aromatic fractions of the natural and crude oil hydrocarbons by column chromatography, spiking natural sediment hydrocarbons with crude oil hydrocarbons at different levels, and analyzing the samples by GC-MS (Gas Chromatography-Mass Spectrometry) and by GC-IRMS (Isotope-Ratio Mass Spectrometry). The isotopic ratios and the hydrocarbon concentrations were determined by external standard calibration, and the diagnostic ratios were then calculated from the concentrations. Both ratios were then evaluated for their efficiency in detecting the presence of crude oil contamination.

How to cite: Dupont, L., Imfeld, A., Ouellet, A., and Gélinas, Y.: Detection of Crude Oil Contamination in St-Lawrence Estuary Sediments Using n-Alkanes and PAHs Diagnostic and Isotopic Ratios, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5931, https://doi.org/10.5194/egusphere-egu2020-5931, 2020.

Interstitial waters extracted from long sediment cores retrieved during expedition 369 (Sites U1512-U1516) of the International Ocean Drilling Program (IODP) were analysed for the stable water isotopic (O and H isotopes) composition to constrain hydrographic changes in this region prior to modern time and possible changes due to water-rock interaction and fluid mixing. Dissolved sulfate (S and O isotopes), and sulfide (S isotopes) were analyzed to characterize, in concert with concentration measurements, diagenetic microbial and water-rock interaction processes in the sulfur cycles. The measurements demonstrate substantial downcore variations in the water oxygen isotope composition. Net microbial sulfate reduction with depth was observed at all sites, but sulfate was only found to be consumed completely, within the investigated core lengths at Site U1512, that is located off southern Australia. Whereas associated sulfur isotope fractionation is characteristic for medium range fractionation factors, the oxygen isotope composition provides evidence for a much more complex story of sulfur diagenesis at the investigated sites: At Site U1516, for instance, the oxygen isotope composition of dissolved sulfate is equilibrated with pore water, although sulfate concentrations remain above 20 mM. This indicates an intense re-oxidative sulfur cycle. At Site U1513, on the other hand, the oxygen isotope composition remains out of isotope exchange equilibrium although sulfate concentrations fall below 20 mM, indicating that the net decrease in dissolved sulfate is dominantly caused by authigenic gypsum precipitation at depth, which is further confirmed by the dissolved Ca concentration.

How to cite: Böttcher, M. E., Brumsack, H.-J., Schmiedinger, I., and Quan, T.: Stable isotopes evidencing microbial activity, mineral authigenesis and fluid mixing in deep interstitial fluids off South-Western Australia (IODP Leg 369), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13867, https://doi.org/10.5194/egusphere-egu2020-13867, 2020.

The deep subseafloor biosphere represents one of the Earth’s largest, but also least understood ecosystems with diverse species and mostly uncharacterized microbial communities. International Ocean Discovery Program (IODP) Expedition 370 (Temperature Limit of the Deep Biosphere off Muroto) established Site C0023 down to 1180 mbsf in the Nankai Trough off Japan to explore the upper temperature limit of microbial life in the deep sedimentary biosphere [1]. Site C0023 is characterized by a complex lithostratigraphic and depositional history with strongly changing sedimentation rates. Volcanic ash layers are ubiquitous in all lithological units. However, the highest abundance of ash layers could be observed between 400 and 700 mbsf. Previous studies have shown that volcanic ashes represent hotspots for microbial life [2] and are commonly characterized by high Fe(III) and Mn(IV) contents [3]. Onboard measurements show a release of dissolved Fe in the depth interval associated with the highest abundance of ash layers [1]. Therefore, we hypothesized that the release is related to microbial Fe reduction fueled by the mineralogy of the volcanic ash. In order to identify the source and reaction pathway of the liberated Fe, we applied sequential extractions of differently reactive Fe oxide pools on mud rock and ash layer samples as well as stable iron isotope (δ⁵⁶Fe) analyses on pore-water and solid-phase samples. Microbial Fe reduction leads to Fe isotope fractionation with an enrichment of light isotopes in the released Fe and a respective enrichment of heavy isotopes in the residual ferric substrate. Therefore, the δ⁵⁶Fe signals of different reactive Fe pools and the pore water are used to identify the pools actually involved in microbial respiration processes. Our results show that the total Fe content in mud rock of Site C0023 is relatively constant at ~4.2 wt%. Reactive Fe oxides represent 25% of the total Fe. The bulk Fe content in the ash layers varies between 1.4 and 6.8 wt%. Surprisingly, most ash samples contain less total Fe (3.35 wt% on average) compared to the surrounding mud rock. Similarly, the contents of the reactive Fe oxides are significantly lower. This indicates that either (1) ash layers do not represent the energy substrate for microbial Fe reduction, or (2) reactive Fe in ash samples has already been used up by microbes. The bulk Fe content in recent volcanic material from an active volcano on the Japanese island arc is ~4.4 wt% [4]. The higher Fe content in fresh volcanic material compared to ash samples at Site C0023 might suggest that reactive Fe in ash layers is already reduced. Alternatively, the dissolved Fe release might be related to microbial reduction of structural Fe(III) in smectite promoting the smectite-to-illite transition, which has previously been proposed for Site C0023 [5].

References:
[1] Heuer, V.B. et al., 2017. In Proc. IODP Volume 370.
[2] Inagaki, F. et al., 2003. AEM 69: 7224-7235.
[3] Torres, M.E. et al., 2015. Geobiology 13: 562-580.
[4] Vogel, A. et al., 2017. J. Geophys. Res. Atmos. 122: 9485-9514.
[5] Kim, J. et al., 2019. Geology 47: 535-539.

How to cite: Köster, M., Manners, H. R., Heuer, V. B., Morono, Y., Inagaki, F., Kasten, S., and Henkel, S.: Tracing Fe reaction pathways in tephra-rich deep subseafloor sediments from the Nankai Trough offshore Japan by using sequential extractions and stable Fe isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9160, https://doi.org/10.5194/egusphere-egu2020-9160, 2020.