Displays

The formation of a solid-state material from solution is a ubiquitous process of fundamental importance not only for synthesis in solid-state chemistry but for a wide range of disciplines such as geosciences and biology. However, established classical crystallization theories fall short in explaining the outcome of crystallization and mineralization processes in complex environments, such as in biomineralizing organisms or geochemical and industrial settings.  The misfit between classical textbook knowledge and the plurality of conflicting experimental evidence facilitated the advent of an array of new crystallization concepts. These so-called nonclassical crystallization processes are fuelled by the attachment of multiatomic assemblies rather than by attachment of single ions drive crystal formation. Some of these models, such as oriented attachment, were unequivocally backed by experimental evidence and thus accepted by the science community. Other models have encountered distinct resistance from peers. At the centre of this intense dispute, we find the calcium carbonate system, which is of crucial importance for a range of disciplines. For this system, in particular, the existence of prenucleation clusters in the form of dynamically ordered liquid-like polyoxoanions (DOLLOP) has been suggested, and it has been claimed that nonclassical nucleation processes take place. However several groups have challenged this claim, claiming an entirely classical crystallization behaviour

Based on our results, we will draw a different picture of calcium carbonate formation. We show that the issues with this very systems root in its solute chemistry and the fact that this renders a calcium carbonate solution into a multicomponent system. We show liquid-liquid phase separation of near-neutral calcium carbonate solutions along with the first ultrastructural model of amorphous calcium carbonate (ACC). This findings give insight into the formation mechanisms of calcium carbonate under kinetically controlled conditions. Our findings further demonstrate that the formation of a liquid-condensed mineral precursor phase is not solely a “quirk of the peculiar calcium carbonate system” but a general phenomenon: it is an early stage precursor in the formation pathway of calcium carbonate under geo- and biochemical relevant conditions. Moreover, we show that this unexpected demixing behaviour is widespread, many inorganic components go through spinodal decomposition, when the reaction conditions are kinetically controlled and the solution chemistry disadvantage burst nucleation. Our data suggest that it is not the misconception and oversimplification of classical theories but our oversimplification of the solution chemistry which causes the current dispute on classical vs nonclassical nucleation of inorganic compounds. Currently, we see no need for invoking “non-classical” notions of nucleation since our exceptional observations can entirely be explained by established physicochemical concepts apart from CNT. Our results raise the awareness that a supramolecular solution and coordination chemistry provides the key to a thorough understanding of the genesis of inorganic solids under kinetically controlled conditions.

How to cite: Wolf, S.: From solutes to solids: towards a supramolecular view on mineralization processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21861, https://doi.org/10.5194/egusphere-egu2020-21861, 2020

Sediment deposition along continental margins and especially close to the outlets of major river systems is highly dynamic and influenced by changing environmental conditions, such as sea-level variations and the shifting of ocean currents.
The upper slope of the Mozambique margin (SE Africa) receives its sediments from the Zambezi River and is the largest river-fed deposition center along the Eastern African Margin. Global sea level rise during the last glacial-Holocene transition led to a re-routing of the Zambezi River sediment plume. This caused order-of-magnitude changes in sedimentation rates along the shelf break of the Mozambique margin. The variable sediment input as well as changing organic matter load and quality resulted in non-steady state early diagenesis leading to changes in formation and upward flow of methane. This is reflected in temporally and spatially variable formation conditions of authigenic minerals (such as pyrite), especially at the sulfate-methane transition zone (SMTZ) where upward-diffusing methane is anaerobically oxidized by sulfate. Pyrite accumulations in sediment cores can be used to define the past positioning of SMTZs. The isotopic composition of sulfur in pyrite can provide information about the geochemical and environmental factors (e.g., availability of methane, sulfate, reactive iron) controlling the formation of these authigenic minerals during different times of sediment deposition.

We present geochemical data from sediment cores acquired in 2015 during the PAMELA-MOZ4 campaign onboard R/V Pourquoi Pas? offshore Mozambique. A reactive transport model is used to simulate the evolution of early diagenetic conditions over the time of sediment deposition (i.e., the last 27,000 years). By reproducing the currently observed mineral accumulations, the temporal development of methane generation and upward flux, and the past positioning of the SMTZ, can be reconstructed. With this, we are able to put a time constraint on past events of authigenic mineral accumulation and reveal their response to sedimentation rate changes caused by sea-level rise. We further discuss isotope signatures of small-scale diagenetic processes at the Mozambique margin.

This research was co-funded by TOTAL and IFREMER as part of the PAMELA scientific project.

How to cite: Zindorf, M., Rooze, J., Meile, C., Jouet, G., März, C., Newton, R., Rouxel, O., Pelleter, E., Brandily, C., Gayet, N., and Pastor, L.: Authigenic pyrite formation in iron-dominated marine sediments of the Mozambique Margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8680, https://doi.org/10.5194/egusphere-egu2020-8680, 2020

Directly west of the San Rafael Swell on the Colorado Plateau in the western U.S., the Jurassic Entrada Sandstone is intruded by a ~2 km long mafic dike. The dike is Miocene; however, the area is also crosscut by Laramide (~50Ma) clusters of deformation bands that are up 500 m long and up to ~3 m wide. The mafic intrusions infused the area with fluids that bleached the red sandstone directly surrounding the dike. On one side of the dike, the bleached area terminates at an adjacent deformation band set ~475 m south of the dike. Field observations suggest that the dike acted as a baffle preventing fluids from migrating further into the sandstone. Spheroidal calcite and iron (oxyhydr)oxide concretions are present in the bleached host rock, although calcite concretions (1-3 cm diameter) are present throughout the area on both sides of the deformation bands and in both red and white host rock. Iron (oxyhydr)oxide concretions (1-5 cm diameter) are limited to the uppermost bleached section between the dike and the deformation band set. Some iron concretions have solid interiors, and some have well-cemented rinds with interiors depleted of cement. Additionally, some iron concretions are nucleated on individual deformation bands that are ~2 mm wide and iron (oxyhydr)oxide cemented joint faces are also present. Thermochemical modeling shows the infiltrating Miocene fluids were CO₂-bearing, but near neutral pH. The restricted location of the iron (oxyhydr)oxide concretions and relation to the calcite concretions suggest that stagnation of fluid is needed for spheroidal iron oxyhydroxide concretion formation. Calcite concretion nucleation and growth may be quicker resulting in more widespread occurrences, and/or may have preceded the Miocene fluids that infiltrated the unit. The evidence presented here shows that recently proposed models calling for calcite concretion precursors and acidic fluids for iron (oxyhydr)oxide concretion formation may not be correct.

How to cite: Potter-McIntyre, S., Filiberto, J., Schwenzer, S., Crandall, J., Perl, S., and Baharier, B.: New insight into spheroidal iron (oxyhydr)oxide concretion formation models: Diagenetic concretion nucleation associated with neutral fluids from a mafic intrusion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2104, https://doi.org/10.5194/egusphere-egu2020-2104, 2020

In the semi-arid region south of Sulaimani city, paleosols can be found. The genesis of these paleosols needs to be elucidated. Here, we investigated a section of an Oligocene paleosol from that region. The section is about 7m thick with lower, middle and upper horizons, which consisted of partially weathered dolomitic limestone; conglomerate or lithified pebbly paleosol; and sandy-silty claystone or lithified clayey paleosol, respectively. These horizons were studied mineralogically and stratigraphically using X-ray diffraction, scanning electron microscopy, Simultaneous Thermal Analysis (STA) and granulometric analysis. Palygorskite content was higher in the conglomerate (lithified gravely paleosol) and in the partially weathered dolomitic limestone than in clayey paleosol.

Palygorskite is a trace mineral that allows to estimate climatic conditions during soil genesis. In hand specimen, palygorskite occurs as green patches of crystalline coating that covers pores and cavities on limestone and dolostone. Under scanning electron microscope, it appears as linear and fibrous loose bundles that occupy the interstice between the dolomite crystals and is mainly associated with dolostone and limestone facies.

We thus conclude that the climate was arid, and that the terrestrial land cover in this part of the Oligocene Basin in Northern Iraq was limestone and dolostone.

On this land, soil genesis and intermittent stream and sheet erosion was occurring during the entire Oligocene and it is well known, stratigraphically, as Oligocene Unconformity.

The possible origin of the palygorskite was the development during Oligocene by upward accumulation under hydrothermal condition in partially high weathered dolomitic limestone of Pila Spi formation during burial. Palygorskite occurs in marine, lacustrine and soil environments. Limited occurrences are associated with hydrothermal activity, in both marine and continental environments. Palygorskite-containing soils are limited almost exclusively to arid and semiarid areas of the world and are rather unstable in humid conditions.

The present paleosol was developed on Oligocene terrestrial land that bordered the sea covering Middle and Southern Iraq. Due to non-deposition weathering and mass wasting, calcareous gravely soil (limestone conglomerate) was generated. Sandy and clay soil were developed on the terrestrial land which stratigraphically formed an unconformity. This land was covered by water of a closed lagoon. Limestones are deposited as Lower Fars Formation.

These occurrences are associated with aquatic conditions characterized by alkaline solutions with high activities of Si and Mg. The most common setting for lacustrine palygorskite genesis are playa deposits, ancient lacustrine terraces, or closed-basin deposits of other types. While traces of palygorskite can be identified in a wide variety of soils, significant amounts of the pedogenically formed mineral are commonly associated with one specific situation of soil genesis like soils have been affected by fluctuating ground water, soil morphology that includes distinct and sharp textual transitions. This groups includes many paleosols. Most of these Paleosols are non-saline or only slightly saline.

How to cite: Mentler, A., Khanaqa, P., Karim, K., Ottner, F., Schomakers, J., Keiblinger, K., and Kral, R.: Palygorskite in a Paleosol from Zagros mountain belt, NE Iraq, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19949, https://doi.org/10.5194/egusphere-egu2020-19949, 2020

Spectroscopy based methods have proven great potential in efficient, non-invasive material characterization. Recording the material-specific optical properties delivers instant information on the composition of an investigated sample surface without chemical sample preparation and may be operated in spatially continuous mode. In minerals, laser-induced fluorescence (LiF) provides a promising method to address the challenges of robust and efficient rare-earth element (REE) detection. The method is based on the characteristic electronic transitions within the incompletely filled f-shell of REE. The corresponding emission shows distinct features (spectral fingerprints) in the visible and near-infrared (VNIR) range of the electromagnetic spectrum that allow to distinguish between individual REE and mineral matrix luminescence. Only REE with completely filled or empty f-orbitals miss characteristic luminescence (i.e. Y³⁺, Sc³⁺, La³⁺, Lu³⁺), while the emissions of Gd³⁺ lie at lower wavelengths than the observed VNIR range.

We test the suitability of LiF in applications of REE identification by (1) building a spectral LiF library from a sample set of luminescent REE phosphates and (2) evaluating observed emissions in samples of non-luminescent REE, and (3) comparing indicated REE cross-contamination to results of neutron activation analysis (NAA). As samples, we use the Smithsonian REE phosphate standards for electron microprobe analysis. The synthetic material delivers a simple, well-defined host composition, is well investigated and NAA results are available on additional trace REE concentration resulting from the material production procedure. The trace REE concentrations are at the order of 10^-4 given in mass fraction. We employ laser-induced fluorescence at three commonly used laser wavelengths (325 nm, 442 nm, 532 nm) to acquire our REE sample spectra and record LiF signals in the visible to near-infrared spectral range (350 – 1080 nm).

The comparison of spectra from non-luminescent REE phosphates shows clear similarities in emission patterns that can be assigned to specific luminescent REE using the spectral LiF library. Our results demonstrate the suitability of LIF for REE detection along with the benefits of selective element excitation and highlight the high sensitivity of the LiF method. The detected emissions in the non-luminescent samples indicate a detection limit below mass fractions of 10^-4, when compared to NAA results, but also show that not all REE are equally responsive. Here, the co-existence of REE with complex interactions such as charge transfer contributes to the observed emission pattern. Adding to the spectral LiF library data and expanding investigations to further mineral hosts will facilitate new applications of LIF for REE analysis in natural samples and its implementation in raw material exploration.

How to cite: Fuchs, M. C., Beyer, J., Lorenz, S., Sharma, S. K., Renno, A. D., Heitmann, J., and Gloaguen, R.: Laser-induced fluorescence (LiF) spectroscopy for detecting REE cross-contaminations in the Smithsonian rare-earth element phosphate standards , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11006, https://doi.org/10.5194/egusphere-egu2020-11006, 2020

The diameter of framboidal pyrites was widely used as a measure of redox condition in modern and ancient sedimentary environments, the proposed critical values of average size and standard deviation of framboids are about 8μm and 3μm respectively. However, a few reports proposed that the exceptionally large size and standard deviation of framboidal pyrites in cored sediments from northeastern South China Sea is closely related to the anaerobic oxidation of methane (AOM) processes mainly dominated in sulfate-methane transition zones (SMTZ). Here we investigate the occurrence of framboidal pyrites in two cored sediments of sites SC-W02B-2017 and SC-W03B-2017 at Shenhu area during the first offshore gas hydrate production test in northern South China Sea. Combined with the statistics of size and standard deviation of framboidal pyrites, the relative concentrations and sulfur isotopic compositions of bulk pyrites, we verified that the AOM could enhance the framboidal pyrite formation. Our data show that both the size and the standard deviation of framboidal pyrite present an unusual positive excursion in cored sediment column. By interpreting the coupling occurrence of positive excursions both pyrite concentrations and sulfur isotopes, four main paleo-sulfate-methane transition zones (Paleo-SMTZ) are roughly recognized in depths around 50 meter below seafloor (mbsf), 90-100 mbsf, 135-225 mbsf and 180 mbsf, where unusual strong AOM and unusual methane releases might happened. The morphology shows most of the pyrite framboids occur in framboidal cluster with a rod-like, irregular block shape and secondary overgrowth. The size of pyrite framboids in site W02B ranges from 8.1μm to 40.1μm with maximal about 40.1μm and in site W03B from 8.6μm to 25.3μm with maximal about 101.2μm (n=2686 from 13 samples). Our data show the average size and the standard deviation of pyrite framboids are more than 20μm and 3.0μm respectively, and the higher δ³⁴S value and larger size of framboid mainly occur near the intervals of paleo-SMTZs in marine sediment columns. Therefore, we propose again that the enhancing AOM in SMTZs could flourish the growth of pyrite framboids and enlarge the standard deviation of framboidal size, which might be implication for more precise interpretation of redox condition of sedimentary environments using framboidal pyrite diameter.

How to cite: Wang, J. and Wei, Q.: Framboidal pyrites flourished in sulfate-methane transition zones of cored sediments in the northern South China Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3979, https://doi.org/10.5194/egusphere-egu2020-3979, 2020

Amorphous calcium carbonate (ACC) has been observed, or inferred to exist, in the majority of the major phyla of marine calcifying organisms. The CaCO₃ produced by these organisms represents one of the largest long-term carbon sinks on Earth’s surface, such that identifying how calcification will respond to anthropogenic climate change is an urgent priority. A substantial portion of our knowledge of the biomineralisation process of these organisms is derived from inferences based on skeletal geochemical data, yet such models typically do not include an ACC component because little is known about trace element and isotope fractionation into ACC. In order to address this, we present, to our knowledge, the first structural and geochemical data of ACC precipitated from seawater under varying carbonate system conditions, seawater Mg/Ca ratios, and in the presence of three of the most common intracrystalline amino acids (aspartic acid, glutamic acid, and glycine). Based on these data we identify the carbonate system conditions necessary to produce ACC from seawater [Evans et al., 2019], and identify the dominant controls on ACC geochemistry. As an example, we utilise these data to build a simple biomineralisation model for the low-Mg (e.g. planktonic) foraminifera, based on precipitation of low-Mg calcite through an ACC precursor phase in a semi-enclosed pool. This exercise demonstrates that the observed shell geochemistry of this group of organisms can be fully reconciled with a model that includes an ACC component, and moreover that constraints can be placed on the degree of ACC utilisation and the ACC-calcite transformation process. More broadly, the exercise demonstrates that knowledge of the characteristics and geochemistry of ACC is important in the development of a process-based understanding of marine calcification.

Evans, D., Webb, P., Penkman, K. Kröger, R., & Allison, N. [2019] The Characteristics and Biological Relevance of Inorganic Amorphous Calcium Carbonate (ACC) Precipitated from Seawater. Crystal Growth & Design 19: 4300.

How to cite: Evans, D., Gray, W., Rae, J., Greenop, R., Webb, P., Penkman, K., Kröger, R., and Allison, N.: Building amorphous calcium carbonate into geochemical biomineralisation models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15902, https://doi.org/10.5194/egusphere-egu2020-15902, 2020

Over the last decennia, alkaline leachates from the weathering of legacy steel slag disposal sites have affected the surrounding soils and drainage streams. The hyperalkaline and hypersaline conditions around these sites are comparable to extreme paleo environments such as alkaline lakes in rift volcanic settings. Investigating the carbonate deposits forming in these man-made systems provides a unique opportunity to link the ongoing physical and microbial processes to their resultant carbonate morphologies.

Here we present data from 3 sites across Dene Burn, a slag drainage stream in Consett, County Durham, UK. After 100 years, iron and steel production ceased in 1980, leaving over 20 million tons of slag in the form of several large mounds. Analysis showed Dene Burn to be typical of slag drainage waters with an elevated pH (>9) and saturated with different secondary phase minerals- particularly calcite. However, the physical distribution of carbonates is more comparable with estimated local kinetic precipitation rate than it is to thermodynamic saturation, indicating that the fundamental control on carbonate formation arises from crystal surface processes. A microbial community comprising predominantly Proteobacteria (Alpha-, Gamma-, Beta- and Deltaproteobacteria), Cyanobacteria, Bacillariophyta (diatoms) and Bacteroidetes (Flavobacterium) was identified at the 3 sites. The microbial communities and an abundance of extracellular polymeric substances (EPS) were shown in close association with the mineral phases detected at the sites. The presence and composition of these biofilms appears to control local carbonate mineralisation rates and carbonate morphologies.

Drainage streams from steel slag provide a unique opportunity to study carbonate mineral formation under extreme environmental conditions. Furthermore, maximising carbonate formation at such sites could be utilised as a remediation and carbonate sequestration technique.

How to cite: van der Land, C., Cowan, K., Sherry, A., Bastianini, L., Gray, N., Rogerson, M., Mercedes-Martin, R., Prior, T., Cesar, E., and Mayes, W.: Microbial control on Anthropocene carbonates in slag drainage waters, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19690, https://doi.org/10.5194/egusphere-egu2020-19690, 2020

Cyanobacteria are an abundant and diverse group of photosynthetic bacteria that have shaped Earth’s environment for billions of years and play a vital role in the cycling of numerous elements such as carbon, calcium, and phosphorus. In particular, their impact on the global carbon cycle is of significant interest in the context of carbon capture and climate change, as they sequester atmospheric CO₂ into organic carbon and biogenic calcium carbonates (CaCO₃) through a process called calcification.  The process of calcification has long been considered as extracellular and non-biologically controlled. However, recently, several cyanobacterial species have been reported to form intracellular amorphous calcium carbonate (ACC) inclusions. These cyanobacteria were found in diverse environments and accumulate high concentrations of AEE (Ca, Ba and Sr) from solutions undersaturated with respect to AEE-carbonate phases. Moreover, one of these cyanobacteria species, G. lithophora was shown to selectively accumulate stable and radioactive alkaline earth elements (AEE) within the intracellular amorphous carbonates and/or polyp inclusions (Mehta et al., 2019). Recently, it was confirmed that cyanobacteria forming intracellular ACC contained a much higher content of alkaline earth elements (AEE) than all other cyanobacteria (DeWever et al., 2019). The high concentration of Ba and Sr within these intracellular inclusions was surprising because Ba and Sr have usually been considered as having no physiological role at all. The high concentration of Ca within these intracellular inclusions was directly in contrast with the traditional paradigm of cells maintaining a state of homeostasis with respect to Ca. Furthermore, Sr/Ca and Ba/Ca ratios in these ACC inclusions were very different from those expected from abiotic precipitation in the solution surrounding the cells (Cam et al. 2015). To understand the biological driver behind these observations, first, I will present a review of the above mentioned “vital effects” in the context of intracellular calcification in cyanobacteria. Second, using batch incubation experiments, I will show that high Ca concentrations are vital not only for the growth of G. lithophora, but also for the uptake of Ba by G. lithophora. Lastly, I will examine Ca homeostasis in ACC forming cyanobacterial strains by using an antagonist/inhibitor of a known channel/transporter involved in Ca transport.  Overall, these insights will shed some light on the role of cyanobacteria forming intracellular ACC on carbonate (bio)mineralization, in both modern and ancient Earth’s environment. 

Reference:

N Mehta, K Benzerara, B Kocar, V Chapon, Sequestration of radionuclidesRadium-226 and Strontium-90 by cyanobacteria forming intracellular calcium carbonates, ES&T 2019

De Wever, A.; Benzerara, K. et al. Evidence of High Ca Uptake by Cyanobacteria Forming Intracellular CaCO 3 and Impact on Their Growth. Geobiology 2019

Cam, N., Georgelin, T., Jaber, M., Lambert, J.-F., and Benzerara, K, In vitro synthesis of amorphous Mg-, Ca-, Sr- and Ba-carbonates: what do we learn about intracellular calcification by cyanobacteria? Geochim. Cosmochim. Acta 2015

How to cite: Mehta, N., Panet, F.-S., and Benzerara, K.: Biogeohemical significance of Intracellular calcification by Cyanobacteria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20253, https://doi.org/10.5194/egusphere-egu2020-20253, 2020

The abundance and isotopic content of boron in carbonate biominerals provide our best records of ocean carbon chemistry and pH, which have proved instrumental in studying past episodes of CO₂-induced climate change. The boron proxies are based on the theory that carbonates solely incorporate B(OH)₄^- in proportion to seawater B(OH)₄^-/HCO₃^- or B(OH)₄^-/CO₃^2-, capturing both the state of the ocean C system and the pH-dependent isotopic composition of B(OH)4-. However, models of biomineralisation invoke significant modification of internal carbon chemistry to facilitate calcification, and substantial proton export has been observed during carbonate formation. The pH, carbon and boron chemistry at the site of calcification cannot be the same as that of external seawater. How, then, do biominerals appear to record seawater B(OH)₄^-? While unanswered, this question raises serious problems for our interpretation and use of the B proxies.

We explore this question using a quantitative model of B transport and incorporation in biomineralisation. Three key fluxes dominate biomineral formation: CaCO₃ precipitation, the exchange of seawater with the external environment, and ion transport across membranes by diffusion or active pumping. By reducing the problem to the balance between these three key fluxes, it is possible to explore a wide range of biomineralisation scenarios with minimally restrictive assumptions. Within this framework, we consider both the transport of B(OH)₄^-, and the transport and passive diffusion of membrane-permeable B(OH)₃, allowing us to explore a comprehensive range of candidate biomineralisation scenarios and B transport processes.

By explicitly including the independent transport of both B species, our model offers two key insights into the mechanisms behind the boron proxies and biomineralisation:

1. We identify biomineralisation mechanisms that allow B geochemistry to record external seawater conditions, despite the modified chemistry at the calcification site.

2. We constrain the dynamics of the calcification environment (e.g. ‘closed’ vs. ‘open’ or Rayleigh- vs. transport-dominated system) by inverting the model to consider paired B/Ca and δ¹¹B data, offering key new constraints on ion transport processes in biomineralisation.

How to cite: Branson, O. and Gagnon, A.: Boron proxies and biomineralisation: the possible, the impossible and the likely., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21988, https://doi.org/10.5194/egusphere-egu2020-21988, 2020

Understanding the atmosphere-continent-ocean carbon cycle and its associated oceanic carbon system is one of the keystones to face the Anthropocene’s climate change. Since the 1990s the isotopic ratio of boron (δ¹¹B) in calcitic shells of planktonic foraminifera has proven to be a powerful geochemical proxy to determine the oceanic paleo-pH and its link to atmospheric CO₂ level over geological times¹, whereas the ratio B/Ca as proxy of the seawater carbonate chemistry is still questionable^2,3.

However, the use of planktonic foraminifera in paleoclimatic reconstructions requires calibrations of the pH – δ¹¹B relationships to correct what is known as « vital effect »⁴: each species controls differently its calcification process and consequently slightly modifies the seawater chemistry during biomineralization^5,6. Moreover, shell size effect on δ¹¹B has been reported for some symbiont-bearing species due to photosynthetic increase of pH^7,8.

Calibrations for the symbiont-barren Globigerina bulloides have been already determined^9,10 but sparse data have been reported so far for the test size effect on δ¹¹B ¹¹.

Here we measured the δ¹¹B of three different fractions (250-315, 315-400 and >400 μm) of G. bulloides sampled along the coretop PS97-122 from the Chilean margin (54.10°S, 74.91°W), by using a new protocol developed at IPGP and dedicated to small samples which couple a microsublimation technique and a micro-direct injection device (μ-dDIHEN¹²). Our preliminary results show significantly higher δ¹¹B values for the large fractions compared to the small ones, as found for symbiont-bearing planktonic species such as Globigerinoides sacculifer⁷ and Globigerinoides ruber⁸.

• (1) Pearson & Palmer, 2000, Nature 406, 695-699
• (2) Yu et al., 2007, Paleoceanography 22, PA2202
• (3) Allen et al., 2012, EPSL 351-352, 270-280
• (4) Urey et al., 1951, Soc. Am. Bull. 62, 399-416
• (5) Erez, 2003, Rev. in Min. and Geochem. 54 (1), 115-149
• (6) de Nooijer et al., 2014, Earth-Science Reviews 135, 48-58
• (7) Hönisch & Hemming, 2004, Paleoceanography 19, PA4010
• (8) Henehan et al., 2013, EPSL 364, 111-122
• (9) Martínez-Botíet al., 2015, Nature 518, 219-222
• (10) Raitzsch et al., 2018, EPSL 487, 138-150
• (11) Henehan et al., 2016, EPSL 454, 282-292
• (12) Louvat et al., 2019, JAAS 8, 1553-1563

How to cite: Buisson, M., Louvat, P., Karancz, S., Tian, R., Raitzsch, M., Bijma, J., and Rollion-Bard, C.: δ11B and B/Ca ontogenetic variability within Globigerina bulloides , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19994, https://doi.org/10.5194/egusphere-egu2020-19994, 2020

Despite being some of the largest bio-constructions on the planet, coral reefs are made by many millions of cm- to mm-sized polyps of Scleractinian corals. Calcification occurs in a micron sized space sandwiched between the coral animal and the existing skeleton, known as the extra cellular medium (ECM). The coral animal has a tight control on the carbonate system in this space through deploying enzymatic pumps (e.g. Ca-ATPase) and secreting acidic-rich proteins. Tracking the state of the carbonate system in the ECM is therefore key to forming a mechanistic understanding of how environmental change, such as ocean acidification, influences skeletal formation and ultimately the growth and resilience of these important ecosystems.

Traditional means to examine ECM composition is through the use of micro-electrodes. While these approaches have revealed many key insights they are, by their nature, invasive.  They also only provide snap shots of information for corals grown in the laboratory. The boron isotopic composition of the coral skeleton and its boron content (expressed as B/Ca ratio) have recently emerged as a viable alternative approach to fully characterise the carbonate system in the ECM.  However, most studies employ bulk sampling techniques which require averaging across both structural elements of the coral skeleton and many months to years of growth. Laser ablation MC-ICP-MS approaches are now available as an alternative sampling protocol (e.g. Standish et al. 2019), and along with B/Ca (and other trace element) measurements this not only allows a reconstruction of the full carbonate system of the ECM from an analysis of the skeleton of any coral (cultured or wild) at unprecedented spatial and temporal resolution, but it also allows an examination of the influence of the carbonate system in the ECM on trace element incorporation. 

Here we present boron isotope and trace element analyses of several tropical, reef-building, corals to examine the nature and magnitude of fine scale variation in ECM composition.  By studying corals from locations where external seawater is well known we also gain insights into trace element incorporation and whether external seawater pH can be accurately reconstructed from the boron-based proxies at weekly (or better) resolution. 

Standish, C.D., Chalk, T.B., Babila, T.L., Milton, J.A., Palmer, M.R., Foster, G.L. (2019) The effect of matrix interferences in situ boron isotope analysis by laser ablation MC-ICP-MS, Rapid Communications in Mass Spectrometry 33: 959–968 https://doi.org/10.1002/rcm.8432

How to cite: Foster, G. L., Chalk, T. B., and Standish, C. D.: Boron isotope analysis of coral skeletons by laser ablation MC-ICP-MS: new insights into calcification and environmental reconstruction at high temporal resolution , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8202, https://doi.org/10.5194/egusphere-egu2020-8202, 2020

A widespread phenomenon in biogenic and inorganic carbonates that are formed out of isotopic equilibrium is a nearly ubiquitous co-variation (slope) of carbon vs. oxygen isotopes, in e.g., speleothem and cryogenic carbonates, shells and skeletons of foraminifera, corals etc. For proxy calibrations, it is critical to understand such isotope variations (often labeled kinetic or vital effects) in proxies widely used for paleo-reconstructions. Given that this phenomenon is observed in inorganic carbonates and biogenic carbonates across different phyla suggest a common underlying mechanism, possibly independent of biological controls, that is, likely of inorganic origin. Here we present results from laboratory experiments on synthetic carbonate precipitation to constrain the kinetic isotope fractionation factor (KFF) of carbon and oxygen during CO2 hydration. We used an experimental setup similar to that of an earlier study but with important modifications and tight temperature and pH control. The average d13C and d18O values of our carbonate samples (BaCO3) produced at 25 deg C and pH = 8.0 (NBS) are -29.7 +- 0.71 per mil (VPDB) and 18.8 +- 0.56 per mil (VSMOW), respectively. From the isotope data, we calculate our experimental 13KFF and 18KFF, which refer to the 13C/12C and 18O/16O fractionation between CO2(g) and BaCO3, where the d13C and d18O values of CO2(g) were calculated using known equilibrium fractionation factors. Our results show that our KFFs are the largest values compared to previously reported experimental KFFs (except for one study), suggesting that our values are closest to the full isotopic disequilibrium during CO2 hydration. Based on our KFFs, we will present the expected slope of carbon vs. oxygen isotopic disequilibrium from kinetic effects during CO2 hydration. We will also discuss the expected slope from equilibrium effects of solution pH on oxygen isotopes. Comparison with field and culture data will reveal the origin of the slope of carbon vs. oxygen isotopic disequilibrium in biogenic and inorganic carbonates.

How to cite: Zeebe, R., Yumol, L., and Uchikawa, J.: Solution to an enigma: Explaining the slope of carbon vs. oxygen isotopic disequilibrium in biogenic and inorganic carbonates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10573, https://doi.org/10.5194/egusphere-egu2020-10573, 2020

Reaction kinetics involved in the precipitation of carbonates can introduce large scatter and inaccuracies in the temperatures derived from their δ¹⁸O and ∆₄₇ values. Advances in mass spectrometry instrumentation recently enabled high-precision analysis of the ¹⁸O–¹⁸O clumping in carbonate minerals (∆₄₈), despite the relatively low natural abundance of ¹²C¹⁸O¹⁸O, the main isotopologue contributing to the ∆₄₈ signal (1). Measurements of ∆₄₈, when combined with ∆_47, can yield additional insights into kinetic effects and the carbonate formation environment (2).

Here we report high-precision ∆₄₇ and ∆₄₈ values of speleothem carbonates, modern coral skeletons, a brachiopod, and a belemnite. We constrained equilibrium in ∆₄₇ vs ∆₄₈ space by anchoring empirically derived ∆₄₇ vs temperature and ∆₄₈ vs temperature relationships to a Devils Hole mammillary calcite, known to be precipitated at extremely slow rates at a constant 33.7(±0.8) °C and water oxygen isotope composition. Our results, compared to theoretical predictions, provide the most substantial evidence to date that the isotopic disequilibrium commonly observed in speleothems and scleractinian coral skeletons is inherited from the dissolved inorganic carbon pool of their parent solutions. Data from an ancient belemnite imply it precipitated near isotopic equilibrium and confirm the warmer-than-present temperatures at Early Cretaceous southern high latitudes. The presence of similar kinetic departure in a brachiopod but not in a belemnite suggests that the current discrepancy between belemnite and brachiopod-based temperature estimates in the geologic record is most likely related to a greater kinetic bias in the isotopic composition of brachiopod shells.

We demonstrate that the combined clumped isotope method makes it possible to identify carbonates that did not precipitate in thermodynamic equilibrium from their parent water. Our results highlight the potential that the combined clumped isotope analyses hold for accurate paleoclimate reconstructions and the identification of the kinetic fractionation processes dominant in carbonate (bio)mineralisation.

(1) Fiebig et al. (2019), https://doi.org/10.1016/j.chemgeo.2019.05.019

(2) Guo, W. (2020), https://doi.org/10.1016/j.gca.2019.07.055

How to cite: Bajnai, D., Guo, W., Löffler, N., Methner, K., Krsnik, E., Coplen, T. B., Gischler, E., Hansen, M., Henkel, D., Price, G. D., Raddatz, J., Scholz, D., and Fiebig, J.: Combined clumped isotope measurements resolve kinetic biases in carbonate formation temperatures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3051, https://doi.org/10.5194/egusphere-egu2020-3051, 2020

The Mg/Ca paleotemperature proxy in planktic foraminifera is one of the most widely-used proxies for sea surface temperature. However, this ratio is not constant throughout the test, varying systematically by several fold independent of temperature between faster and slower growing diurnal bands. This phenomenon has yet to be explained mechanistically, however, changing calcification rates may be a contributing factor. Observing the relationship between calcification rate and trace metal incorporation for multiple proxies at the scale of this banding will allow us to better understand the contribution of kinetic effects to heterogeneity. In this study, we examine Me/Ca ratios on a diurnal cycle in Orbulina universa, utilizing a novel approach based on multiple isotopic spikes that allows us to measure Sr/Ca, Li/Ca and Mg/Ca with the precision of isotope dilution while still maintaining the time resolution of microanalytical techniques. Using independently measured growth rates derived from NanoSIMS measurements of diurnal Mg/Ca heterogeneity, we examine the effect of crystal growth rate on foraminiferal Sr/Ca and Li/Ca.  We observe that Sr/Ca ratios in foraminifera are ~3% higher during the night than during the day, which initially appears opposite to the expected signal based on growth rate. However, we also observe a positive correlation between Sr and Mg in foraminiferal calcite, which falls on the same mineralogical line as the Sr/Ca and Mg/Ca of other biogenic and inorganic calcites. We attribute offsets in calcite composition from this mineralogical relationship to kinetics. Interpreted within that framework, day Sr/Ca ratios appear more affected by kinetics than night Sr/Ca ratios, which is consistent with observed calcification rates. The difference between any given data point and the mineralogical line can be explained by kinetic processes, and correlates with oceanographic properties in cultured foraminifera, which could help separate temperature from growth rate effects in the paleorecord.

How to cite: Bonnin, E., Spero, H., and Gagnon, A.: Testing the effect of crystal growth rate on foraminiferal calcite microchemistry using Sr/Ca of individual day/night bands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13960, https://doi.org/10.5194/egusphere-egu2020-13960, 2020

Bivalve shells serve as excellent high-resolution archives of marine paleoclimate. Recently, ultrastructural features of the shells were investigated as potential temperature proxies that can overcome the limitations of the stable oxygen isotope method (i.e., missing data on past seawater oxygen isotope signature and diagenetic overprint). According to previous studies, the size of individual biomineral units of prismatic, nacreous and crossed-lamellar ultrastructures in cross-sections along the axis of maximum growth was solely related to water temperature. Despite being present in 90% of all mollusks, the crossed-lamellar ultrastructure was only studied for environmental relationships in one species (Glycymeris bimaculata) until now. To determine whether this new proxy can be applied to other bivalves with crossed-lamellar ultrastructure, further studies are needed.

We analyzed the shells of other Glycymerididae collected at near-shore and shelf environments (G. nummaria and G. pilosa: Adriatic Sea, Croatia; G. glycymeris: Iroise Sea, France; Glycymeris sp: Southern Pacific, New Zealand) by means of SEM, using a previously developed automatic image analysis procedure. Morphological changes of the biomineral units of the shells were assessed for relationships with temperature, salinity and food availability. Additionally, the crossed-lamellar architectures of phylogenetically more distantly related taxa (Venus verrucosa and Callista chione: Adriatic Sea, Croatia) were assessed.

Our results show that all studied Glycymerididae species, irrespective of environmental setting and locality, formed larger biomineral units in warmer waters. However, biomineral properties of ontogenetically old shell portions are more difficult to interpret, because declining growth rates condense the shell record and aggravate ultrastructural analyses. The crossed-lamellar shell layers of V. verrucosa and C. chione exhibited hierarchical organizations very similar to those of the Glycymerididae. The ultrastructural temperature proxy can therefore be applied to crossed-lamellar shells of bivalves from a wide range of coastal settings, preferably in ontogenetically young shell portions.

How to cite: Höche, N., Peharda, M., Thébault, J., and Schöne, B. R.: Exploring the biomineral morphology of crossed-lamellar bivalve shells as a water temperature proxy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7573, https://doi.org/10.5194/egusphere-egu2020-7573, 2020

Phytoplankton is one of the most important producers of oxygen, and plays an important role in the export of large amounts of carbon to the deeper ocean. Since phytoplankton is also the basis of most food webs in the ocean, understanding the dynamic system of phytoplankton is a crucial part to understand past carbon- and nutrient cycles and paleoclimatic changes. The export of nutrients is also an important factor impacting cold-water coral (CWC) reefs and may play a role in controlling their distribution. Here we present laser ablation inductively coupled mass spectrometer (LA-ICP-MS) Element/Ca measurements from Acesta excavata, a file clam, often associated with cold-water coral reefs along the European continental margin. Environmental parameters were recorded with lander systems directly deployed in the CWC reefs, which allows us to compare our geochemical data to in-situ ocean data.

Our results reveal, that Ba/Ca ratios show stable baseline values with intermittent sharp peaks. The location of these peaks in between major growth lines and temperature reconstructions with Mg/Sr ratios (Schleinkofer et al., submitted) show that these peaks occur during Winter and are repeatable between samples from the same location. This indicates a strong external forcing mechanism and allows cross-dating of different bivalve shells. While the occurrence of Ba/Ca peaks correlates with phytoplankton maxima, the absolute Ba/Ca ratio does not correlate with the phytoplankton abundance.

Mn/Ca ratios show similar trends as Ba/Ca ratios but the peaks are phase shifted and occur slightly delayed. These peaks could be triggered by decreasing oxygen concentrations in the water caused by the decomposition of organic material.

As A. excavata does not show easily distinguishable growth lines under the light microscope despite of Mutvei staining or fluorescence microscopy, we hypothesize that P/Ca ratios might be usable to locate highly phosphorylated shell areas that usually correlate with major growth lines. P/Ca ratios show no perceivable features in the vicinity of major growth lines. Instead we recognize that Ba/Ca peaks follow a minimum in P/Ca which is possibly caused by the uptake of phosphor by plankton.

These results suggest that A. excavata have potential as a promising tool for high resolution paleoenvironmental reconstructions of both intermediate and overlying surface water masses.

References

Schleinkofer N, Raddatz J, Evans D, Gerdes A, Flögel S, Voigt S, et al. Elemental to calcium ratios in the marine bivalve Acesta excavata: an archive for high-resolution paleoceanographic reconstructions of intermediate water masses. PLoS One. Submitted

How to cite: Schleinkofer, N., Raddatz, J., Evans, D., Gerdes, A., Voigt, S., and Wisshak, M.: Ba/Ca, P/Ca, Li/Ca and Mn/Ca ratios in the deep-sea bivalve Acesta excavata: Valuable tools to reconstruct plankton dynamics in cold-water coral ecosystems?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4724, https://doi.org/10.5194/egusphere-egu2020-4724, 2020

The Cenozoic period encompasses the last transition from the “greenhouse” climate of the late Early Eocene (~50 Ma) to our modern “icehouse” climate with its much lower CO₂ levels, significant polar glaciation and major sea level drop. The Eocene-Oligocene transition (EOT), that marks the first major ice-sheet build-up on Antarctica, has been extensively studied as it represents the entrance into an icehouse mode. Identification of this major step of Antarctic ice-sheet build-up strongly relies on δ¹⁸O and Mg/Ca benthic foraminifera records from ODP / DSDP sites. By contrast, few records currently exist from coastal environments despite the presence of abundant fossil archives, like bivalve shells. Yet palaeoenvironmental records from these peculiar coastal sites could bring information on how they react to global climate changes and help to further understand the behavior of our climate system. In this study, we applied a multi-proxy strategy coupling δ¹⁸O, δ¹³C, clumped isotopes (Δ₄₇), strontium isotopes (⁸⁷Sr/⁸⁶Sr) analyses on aragonitic and calcitic bivalves and sediments recovered from the Isle of Wight (London-Paris Basin, Northeastern Atlantic Ocean) to provide additional constrain on environmental changes in this region across the Eocene-Oligocene Transition (~37.8–33 Ma).

Our new coupled δ¹⁸O and Δ₄₇ dataset highlights a marked decrease in local seawater temperatures (~ 8°C) coupled to a drop in local seawater δ¹⁸O, likely linked to the sea level drop associated with ice-cap formation and an evolution toward more proximal, brackish environment in this region (that is apparent from sediment facies evolution). We estimate the salinity decrease recorded at the local scale from the Eocene to the Oligocene as reaching about 6 PSU, from 31 to 25 PSU. Strontium isotope analyses of the bivalves support this interpretation, showing values close to that of seawater up to the EOT but a marked deviation from contemporaneous global seawater ⁸⁷Sr/⁸⁶Sr values toward more radiogenic values afterward. This positive deviation is in agreement with an evolution toward more proximal environments, subjected to larger freshwater inputs.

How to cite: Briard, J., de Rafélis, M., Vennin, E., Daëron, M., Chavagnac, V., Emmanuel, L., Merle, D., and Pucéat, E.: Paleotemperature and paleosalinity evolution across Eocene-Oligocene Transition in North Atlantic Ocean: Insights from geochemical analysis of bivalve shells., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18358, https://doi.org/10.5194/egusphere-egu2020-18358, 2020

Zinc (Zn) is an important bioessential trace element. Its distribution in the modern oceans reflects a combination of biological uptake, remineralization and the physical ocean circulation. Furthermore, the partitioning behaviour of Zn (D_Zn) and its isotopes (δ⁶⁶Zn) in carbonates has been linked to ambient seawater carbonate chemistry [1-3].

Development of Zn isotopes in carbonates as a palaeoceanographic tool has been hampered by the high concentrations of Zn in contaminating material, such as lithogenic or authigenic (e.g. Fe-Mn oxide) phases. However, deep-sea corals are large enough to be subjected to aggressive physical and chemical cleaning, enabling effective removal of contaminating phases. They also have several other advantages over traditional palaeoclimate archives, including the ability to assign precise absolute ages to individual specimens based on uranium-series dating [4].

Here we present Zn/Ca and δ⁶⁶Zn data for a suite of modern and recent (<1000 yr) deep sea corals from six ocean regions spanning the far North Atlantic to the Tasman Sea. We observe what appears to be species-specific Zn partitioning behaviour, but no clear links between D_Zn or coral δ⁶⁶Zn and ambient seawater carbonate chemistry. Overall, there is good agreement between measured or best-guess modern seawater δ⁶⁶Zn and coral aragonite δ⁶⁶Zn values, suggesting that corals of species Desmophyllum dianthus and genus Caryophyllia do not significantly fractionate Zn isotopes during calcification. Deep sea corals may thus provide a useful archive of the past ocean Zn isotope composition and its spatial variability.

[1] Marchitto T. M., Curry W. B. and Oppo D. W. (2000). Paleoceanography 15, 299–306. 
[2] van Dijk, I., de Nooijer, L. J., Wolthers, M., & Reichart, G. J. (2017). Geochimica et Cosmochimica Acta 197, 263-277. 
[3] Mavromatis, V., González, A. G., Dietzel, M., & Schott, J. (2019). Geochimica et Cosmochimica Acta 244, 99-112. 
[4] Robinson, L. F., Adkins, J. F., Frank, N..., & van de Flierdt, T. (2014), DSR Part II 99, 184-198.

How to cite: Little, S., van De Flierdt, T., Wilson, D., Rehkämper, M., Adkins, J., and Robinson, L.: Zn isotopes in deep sea corals: a useful palaeoceanographic archive? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7732, https://doi.org/10.5194/egusphere-egu2020-7732, 2020

The chemical composition of foraminiferal calcite is widely used for studying past environmental conditions and biogeochemistry. However, high rate of microbial-derived organic matter degradation and abundant dissolved metal sources in sediment and pore waters may impede the application of paleoenvironmental proxies due to formation of secondary carbonates on the outside and/or inside of foraminiferal tests. Secondary carbonate precipitation severely alters the foraminiferal geochemistry and can be difficult to eliminate through standard cleaning procedures for foraminiferal trace element analyses. Here we present results of the mineral composition and formation sequence of diagenetic coatings on the tests of foraminifera formed under extreme anoxic conditions in the Baltic Sea deepest basin (the Landsort Deep, IODP Exp. 347, Site M0063), as well as changing trace element concentrations of authigenic carbonates on the test on a millennial time-scale. The focus is on the diagenetic carbonates present on the tests of the low-oxygen tolerant benthic foraminiferal species Elphidium selseyensis and Elphidium clavatum. We applied geochemical and imaging methods by using scanning electron microscope imaging (SEM) and energy dispersive spectroscopy (EDS), synchrotron-based x-ray fluorescence microscopy (nano-XRF), RAMAN spectroscopy and laser ablation (LA)-ICP-MS, in order to ascertain the sedimentary diagenetic processes, and the foraminiferal authigenic mineral formation sequence. The authigenic carbonates were enriched in Mg, Mn, Fe and Ba, depending on the redox environmental conditions when the authigenic carbonates were precipitated. In particular, concentrations of redox-sensitive elements such as Mn and Fe were increased in bottom waters and sedimentary pore waters under oxygen-depleted conditions in the Landsort Deep, which resulted in Mn- and Fe-enriched carbonate formation. The diagenetic alteration on foraminiferal tests provides potential opportunity to investigate past sedimentary redox environment and primary productivity in the Baltic Sea.

How to cite: Ni, S., Quintana Krupinski, N., Groeneveld, J., Knudsen, K. L., Persson, P., Somogyi, A., Brinkmann, I., Seidenkrantz, M.-S., and Filipsson, H. L.: Early diagenesis in benthic foraminifera under anoxic conditions from the Landsort Deep, Baltic Sea (IODP Site M0063), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8653, https://doi.org/10.5194/egusphere-egu2020-8653, 2020

Diagenetic carbonates often show large variations in their carbon isotope compositions. Variations are mainly the result of isotope fractionation effects during microbial metabolic processes, and these processes themselves may induce carbonate formation. Inorganic carbon from dissimilatory microbial activity shows negative carbon isotope values (d¹³C), in particular if methane is used as a carbon source. In turn, inorganic carbon produced during methanogenesis shows positive d¹³C values. The range of isotope values preserved in the carbonate phase ultimately depends on the reservoir sizes, diffusive mixing of different carbon sources, and episodic formation of carbonate (Meister et al., 2019; Meister and Reyes, 2019). The carbon-isotope signature of diagenetic carbonates therefore represents an archive of past biogeochemical activity in the subsurface.

References:

Meister, P. and Reyes, C. (2019) The carbon-isotope record of the sub-seafloor biosphere. In: "Tracking the Deep Biosphere through Time" (Eds. H. Drake, M. Ivarsson, C. Heim), Geosciences 9, 507, 1-25. https://doi:10.3390/geosciences9120507

Meister, P., Liu, B., Khalili, A., Böttcher, M.E., and Jørgensen, B.B. (2019) Factors controlling the carbon isotope composition of dissolved inorganic carbon and methane in marine porewater: An evaluation by reactive-transport modelling. J. Marine Systems 200, 103227, 1-18. https://doi.org/10.1016/j.jmarsys.2019.103227

How to cite: Meister, P. and Reyes, C.: The carbon-isotope signature of diagenetic carbonates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2094, https://doi.org/10.5194/egusphere-egu2020-2094, 2020

A numerical reaction transport model was developed to simulate the effects of microbial activity and mineral reactions on the composition of the porewater in a 150-m-thick sedimentary interval drilled in the Peruvian deep-sea trench (Ocean Drilling Program, Site 1230). This site shows a zone of intense methanogenesis below 10 m sediment depth. The simulation shows that microbial activity accounts for most alkalinity production of up to 150 mmol/l, while the excess of CO₂ produced during methanogenesis causes a strong acidification of the porewater. Ammonium production from organic matter degradation significantly contributes to alkalinity production, whereby ion exchange was simulated to compensate for hidden ammonium production not otherwise accounted for. Although clay minerals are reacting far too slowly to equilibrate with the porewater over millions of years, additional alkalinity is provided by alteration of chlorite, illite, and feldspar to kaolinite. Overall, alkalinity production in methanogenic zones is sufficient to prevent dissolution of carbonates and to induce carbonate formation either continuously as disseminated cryptic dolomite or episodically as hard lithified beds along a supersaturation front. The simulation presented here provides fundamental insight into the diagenetic effects of the deep biosphere and may also be applicable for the long-term prediction of the stability and safety of deep CO₂ storage reservoirs.

How to cite: Herda, G., Petrishcheva, E., Gier, S., Liu, B., and Meister, P.: Microbial alkalinity production and clay mineral alteration in marine methanogenic sediments: implications for diagenetic carbonate formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9810, https://doi.org/10.5194/egusphere-egu2020-9810, 2020

Continental carbonates are a repository of exceptional climate and environmental changes at scales from sub-annual to decadal to millennial. Their fabrics and chemistry encapsulate information about temperature and rainfall variability, volcanic eruptions, earthquakes, vegetation changes, as well as microbial interaction. Yet, fabric and chemical properties are influenced by the crystallization pathways and, crucially, growth mechanisms and diagenesis of the carbonate crystals. Here we present examples from diverse continental settings and discuss why fabrics are extremely important to determine the accuracy of preservation of a “primary” signal.

Most continental carbonate formation is driven by degassing. This is the case of cave carbonate deposits (speleothems), which allowed tremendous breakthrough in palaeoclimate science. Speleothems form in the dark, from drip waters poor in nutrients and organic compounds. Their most common fabric consist of columnar crystals. Nanoscale investigation shows that speleothem crystals have diverse growth pathways, including particle attachment (Frisia et al., 2018). The distribution of climate-sensitive trace elements, thus, rather than following crystallographic sector zoning, follows “parallel growth layers” reflecting environmental changes. The critical parameter in growth process is the drip rate. By contrast, subglacial and cryogenic carbonates, which also grow in the dark and consist of columnar crystals, form in micro-phreatic environment where supersaturation is not attained by degassing, but by concentration of elements by slow freezing.  In this situation trace elements are incorporated following crystallographic faces and provide exceptional information of subglacial processes including volcanic eruptions (Frisia et al., 2017).

Lacustrine, spring and fluvial carbonates grow at Earth’s surface, being exposed to Sun’s light. These carbonates’ precipitation, similarly to speleothems, is promoted by degassing, but also by the presence of photosynthetic organisms and high substances organic interaction. Their fabrics are commonly characterized by micrite, which is rare in caves and in subglacial samples.

Evaporitic lake (Great Salt Lake, GSL) and spring deposits described in Della Porta (2015) were observed by TEM. One typical microfabric is peloidal micrite. The GSL peloidal micrite consists of calcite nanocrystals, and the peloids are associated with aragonite and filaments. Spring deposits peloidal micrite also consists of nanocrystal aggregates surrounded by filaments. 

Most speleothems and spring/lake carbonates document a phase of growth that involves nanocrystal aggregation, which we did not observe in the phreatic subglacial samples.

Implications for palaeoenvironmental research: In speleothems, Ostwald ripening likely transforms nanoparticle aggregates into larger crystals. Critically, in speleothems, Ostwald ripening processes result in removal of some tracers, such as Si, associated to first growth phases, and preservation of those that we use to reconstruct palaeo hydrology. In lake and spring deposits it would seem that micrite preserves the original environmental data, because micrite means that the crystals were protected from ripening by the organic part of the deposit. In subglacial carbonates, growth appears to follow a classical ion attachment at growth sites, thus, their fabrics preserve pristine primary signals.   

References:

Della Porta, G. (2015) Geological Society, London, Special Publications 418, 17-68.

Frisia, S. et al (2018) Earth-Science reviews 178, 68-91.

Frisia, S.,  et al. (2017) Nature Communications 8.

How to cite: Frisia, S., Borsato, A., and Della Porta, G.: Continental carbonates growth pathways, fabrics and diagenesis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3170, https://doi.org/10.5194/egusphere-egu2020-3170, 2020

Carbonate-based reconstructions of environmental conditions in the Precambrian rely heavily on shallow-water and typically microbially-mediated carbonates. This is because Precambrian carbonate rocks formed either microbially or abiotically. Consequently, organo-sedimentary carbonate structures (microbialites) are extensively used as archives of physico-chemical conditions of early Earth environments using traditional isotopes such as stable carbon and oxygen isotopes. When post-depositional alteration is carefully evaluated, valuable information on local seawater chemistry may be gained. More recently, non-traditional isotope systems are applied to microbialites for reconstructing, for example, the redox evolution of our planet. However, interpretations of non-traditional isotope data are challenging, and information on diagenetic alteration is crucial. We present geochemical analyses of modern and ancient microbialites, which are part of an ongoing study on the chromium isotope systematics in modern and fossil microbialites (1). Here, we focus on stable C- and O-isotope data and diagenetic alteration of the analysed microbialites. This approach aims to build a framework for interpreting paleo-environmental reconstructions using non-traditional isotope systems. First results of powdered sub-samples of the modern microbialites show that stable C- and O-isotope data reliably reflect the environmental conditions of their depositional setting: high δ¹³C values (+2 to +8 ‰) indicate extensive microbial activity and high δ¹⁸O values point to evaporative settings. One set of Precambrian microbialite samples also has high δ¹³C values (~+4 ‰), similar to the modern microbialites, but in comparison to modern samples, relatively low δ¹⁸O values (~-3 ‰). Yet, another set of Precambrian microbialite samples display both low δ¹³C (~-0.5 ‰) and δ¹⁸O values (-3 to -6 ‰). The results are interpreted to indicate a different depositional environment and/or more likely, a stronger degree of post-depositional diagenetic alteration that might also explain the comparatively low δ⁵³Cr values of these samples.

How to cite: Rodler, A. S., Bruggmann, S., Goderis, S., and Claeys, P.: Stable isotope data of modern and ancient microbialites , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3040, https://doi.org/10.5194/egusphere-egu2020-3040, 2020

Stromatolites are laminated, presumably microbial structures, consisting largely of an authigenic precipitate, thus, providing potential geochemical archives of early Earth aqueous environments and their habitability. In this study, we report trace element and Sm/Nd isotope data from Palaeoarchean stromatolites and adjacent cherts of the Strelley Pool Formation (NW Australia), obtained by ICP-MS and TIMS, to test their reliability as archives for palaeo-environmental reconstruction and to understand authigenic mineral formation.

Stromatolitic carbonates plot together with the stratigraphically underlying Marble Bar cherts on a linear Sm-Nd regression line yielding an age of 3253 ±320 Ma.. In contrast, associated crystal-fan carbonates yield 2718 ±220 Ma, suggesting that their Sm-Nd isotope system was altered after deposition. Geochronological information via Sm-Nd dating of black and white cherts is limited, probably due to a reset of the isotope system during an unknown Paleoproterozoic or younger alteration event.

Carbonates, as well as white cherts, show shale-normalized rare earth element and yttrium patterns (REY_SN; except for redox-sensitive Ce and Eu) parallel to those of modern seawater, indicating a seawater-derived origin. Positive Eu_SN anomalies (2.1 - 2.4), combined with heterogeneous ɛNd_3.35Ga values (-3.2 to +5.8) within alternating stromatolite laminae, support that seawater chemistry was variably affected by both continental weathering and high-temperature hydrothermal fluids contributing elements of both young mafic or older felsic rocks. In contrast, black cherts show non-seawater like REY_SN patterns and significant amounts of elements leached from the surrounding rocks, masking the pristine geochemical composition of ancient seawater. In conclusion, Archaean stromatolites indeed preserve pristine authigenic phases at the mm-scale that contain signatures representative of the water chemistry prevailing in the depositional environment.

How to cite: Viehmann, S., Hohl, S. V., Tepe, N., Van Kranendonk, M., Reitner, J., Hofmann, T., Koeberl, C., and Meister, P.: Carbonate and chert genesis in the 3.35 Ga old Strelley Pool Formation (Australia): Insights from trace metals and Sm/Nd dating , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6909, https://doi.org/10.5194/egusphere-egu2020-6909, 2020

During the Lutetian (middle Eocene, 48-41 Ma), Earth’s climate was in transition from greenhouse to icehouse conditions, sometimes referred to as a “doubthouse climate”. These circumstances allowed the Paris Basin (France) and Hampshire Basin (UK) to be hotspots for marine biodiversity, hosting a diverse assemblage of molluscs, including members of the Conidae family. Most species within the family are known to live for multiple years, possibly up to a decade, in fully marine conditions and mostly in shallow waters. Under these fully marine conditions, Conidae shells would be excellent recorders of sea water temperatures, allowing paleotemperature reconstruction for the two basins. However, climatic parameters such as temperature extremes or seasonality have not been well documented in the two basins during the Lutetian, with only a handful of studies available [Andreasson & Schmitz 2000, Huyghe et al. 2015]. Here, we made longitudinal and latitudinal comparisons between the two basins, using carbon and oxygen stable isotope data measured on different Conidae species, in order to provide seasonality reconstructions in north-western Europe. The focus of this research is mainly on assessing isotopic variation of seasonality within a basin and comparison between basins, including previously published data. SEM and cold cathodoluminescence shows that for both basins the preservation of the mollusc carbonate is sufficient to allow for approximations of the original environmental conditions. Three specimens from each basin were sampled by means of manual drilling along the growth axis of the shells. Obtained stable carbon and oxygen isotope data were used to reconstruct variation in paleotemperature and productivity. Following the methodology of Kobashi & Grossman 2003, patterns in the isotopic signature throughout the life of each specimen give an indication of the environmental reconstruction and any internal variability. By comparing existing and newly collected data from the same localities and family, we examine whether differences in seasonality are species-specific, due to climatic variation, or reflect environmental differences. 
Andreasson, F.P., Schmitz, B. (2000) Temperature seasonality in the early middle Eocene North Atlantic region: Evidence from stable isotope profiles of marine gastropod shells, GSA Bulletin, 112, 628-640. 
Huyghe, D., Lartaud., F., Emmanuel, L., Merle, D., Renard, M. (2015) Palaeogene climate evolution in the Paris Basin from oxygen stable isotope (δ18O) compositions of marine molluscs. Journal of the Geological Society, 172, 576-587. 
Kobashi, T., Grossman, E.L. (2003) The oxygen isotopic record of seasonality in Conus shells and its application to understanding late middle Eocene (38 Ma) climate, Paleontological Research, 7, 343-355. 

How to cite: Clark, A., Vellekoop, J., Keleman, Z., and Speijer, R.: Lutetian conid snails from the Paris and Hampshire Basins as seasonality archives of the middle Eocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5285, https://doi.org/10.5194/egusphere-egu2020-5285, 2020

The carbonate skeletons of marine organisms are unique archives for high-resolution climate reconstructions. Well-preserved specimens potentially allow for seasonal to even daily scale variability reconstructions of climate and environment  in deep time (pre-Quaternary), providing otherwise unavailable snapshots of climate variability during greenhouse periods (e.g. Steuber et al., 2005; Ivany et al., 2008; de Winter et al., 2017). However, uncertainties on past seawater compositions hamper use of the popular stable oxygen isotope ratio (δ¹⁸O) as proxy for paleotemperature reconstructions. The use of the independent carbonate clumped isotope (Δ₄₇) paleothermometer, which is insensitive to changes in seawater composition, on these promising fossil archives is complicated because of sample size limitations (Fernandez et al., 2017; Bernasconi et al., 2018).

In an attempt to circumvent these issues and use the δ¹⁸O and Δ₄₇ measurements jointly for accurate seasonal reconstructions of temperature and seawater isotope composition, we present a novel data reduction approach that combines Δ₄₇ measurements of small (~100 µg) serially sampled aliquots to estimate summer and winter temperatures in mollusk shell records. When applied on Δ₄₇ and δ¹⁸O measurements in the same specimens, combined with accurate shell chronologies, this approach reconstructs seasonal differences in temperature and seawater composition in a coastal site from the Campanian (Late Cretaceous) high-latitudes.

To test the robustness of these reconstructions, we apply different approaches of combining δ¹⁸O and Δ₄₇ data on a wide range of simulated data representing various scenarios of variability in growth rate, temperature and sea water composition typical for the natural shallow marine environments of carbonate-producers. This approach tests how choices such as sampling resolution and the method of data collection and reduction influence the accuracy and reproducibility of (paleo)seasonality reconstructions in these scenarios.

Finally, we present preliminary data of δ¹⁸O and Δ₄₇ analyses on bivalve specimens grown under controlled temperature conditions that allow us to calibrate the techniques above for temperature reconstructions. Together, these investigations pave the way for accurate, high-resolution climate reconstructions in deep time. These reconstructions provide valuable information on the dynamics of greenhouse climates, against which climate models can be compared to improve predictions of future climate.

References

Bernasconi, S. M., et al. Geochemistry, Geophysics, Geosystems, 19(9), 2895–2914, 2018.

Fernandez, A. et al. Geochemistry, Geophysics, Geosystems, 18(12), 4375–4386, 2017.

Ivany et al. Geological Society of America Bulletin, 120(5–6), 659–678, 2008.

Steuber, T. et al. Nature, 437(7063), 1341–1344, 2005.

de Winter, N. J. et al. Palaeogeography, Palaeoclimatology, Palaeoecology, 485, 740–760, 2017.

How to cite: de Winter, N., Witbaard, R., Ullmann, C., Soerensen, A., Thibault, N., Müller, I., Kocken, I., Claeys, P., and Ziegler, M.: Absolute temperature seasonality from skeletal carbonates—Techniques and limitations of oxygen- and clumped isotope analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2842, https://doi.org/10.5194/egusphere-egu2020-2842, 2020

The shells of oysters (Family Ostreidae) are predominantly composed of two different calcite microstructures: A dense foliated structure consisting of sheet-like folia (“foliated” microstructure) and a more porous microstructure consisting of less well organized leaf-shaped crystals (“chalky” microstructure). These unique characteristics of oyster shells have been subject to a number of studies, with some authors hypothesizing that the chalky structures are mineralized by bacteria living in the shell (Vermeij, 2014). The formation of these microstructures is of great interest, because the phenomenon is unique in the mollusk phylum and because the shells of oysters are popular archives for paleoclimate and paleoenvironment reconstructions (e.g. Bougeois et al., 2018; de Winter et al., 2018). Previous authors have challenged the bacterially mediated mineralization hypothesis through microstructural observations of different parts of the oyster shell (Checa et al., 2018).

Here, we expand on this structural evidence by adding detailed observations of differences in chemical composition between the foliated and chalky microstructures. We combine information on trace element concentrations with stable carbon, oxygen, nitrogen and sulfur isotope ratios as well as carbonate clumped isotope analyses of samples from foliated and chalky structures in multiple modern specimens of Magallana gigas, the Pacific oyster. These analyses shed light on the chemical variability within the oyster shell and how it relates to the occurrence of various calcite microstructures. Given the unique isotopic signature of bacterially mediated calcite, our isotopic analysis results allow us to definitively conclude whether the chalky shell structure in modern oysters was precipitated via symbiotic microbes. Furthermore, the degree of intra-shell chemical variability has implications for paleoclimate and paleoenvirionmental reconstructions from fossil oyster shells, for which the applied trace element and isotope systems function as important proxies. The results of this study therefore yield important recommendations for sampling fossil oyster shells for reconstructions, and provide a baseline for the investigation of chemical variability between shell microstructures throughout the Ostreidae family and the mollusk phylum.

References

Bougeois, L., Dupont-Nivet, G., De Rafélis, M., Tindall, J. C., Proust, J.-N., Reichart, G.-J., de Nooijer, L. J., Guo, Z. and Ormukov, C.: Asian monsoons and aridification response to Paleogene sea retreat and Neogene westerly shielding indicated by seasonality in Paratethys oysters, Earth and Planetary Science Letters, 485, 99–110, 2018.

Checa, A. G., Harper, E. M. and González-Segura, A.: Structure and crystallography of foliated and chalk shell microstructures of the oyster Magallana: the same materials grown under different conditions, Scientific reports, 8(1), 7507, 2018.

Vermeij, G. J.: The oyster enigma variations: a hypothesis of microbial calcification, Paleobiology, 40(1), 1–13, 2014.

de Winter, N., Vellekoop, J., Vorsselmans, R., Golreihan, A., Soete, J., Petersen, S., Meyer, K., Casadio, S., Speijer, R. and Claeys, P.: An assessment of latest Cretaceous Pycnodonte vesicularis (Lamarck, 1806) shells as records for palaeoseasonality: a multi-proxy investigation, Climate of the Past, 14(6), 725–749, 2018.

How to cite: Dämmer, L. K., de Winter, N. J., Falkenroth, M., Reichart, G.-J., Moretti, S., Martinez-García, A., Höche, N., Rodiouchkina, K., Goderis, S., Vanhaecke, F., and Ziegler, M.: A chemical investigation of microstructural changes in oyster (Magallana gigas) shells, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4506, https://doi.org/10.5194/egusphere-egu2020-4506, 2020

Bivalves offer outstanding potential as environmental archives. However, vital effects exert a strong control on the incorporation of many trace and minor elements into the shell so that their use as environmental proxies is currently limited. Furthermore, Sr and Mg show a strong relationship to the micrometer-sized shell architecture (shell microstructure), i.e., near growth lines, which are typically dominated by irregular simple/spherulitic prismatic microstructures, the concentrations of these elements are significantly higher than in portions between growth lines (= growth increments, which are microstructurally more complex). In contrast, Ba is uncoupled from the prevailing shell microstructure. To shed more light on these issues, we conducted a combined element chemical (in-situ analysis by means of LA-ICP-MS) and microstructural analyses (using SEM) of shells of Arctica islandica collected alive in NE Iceland.

According to our findings, (1) contemporaneous shell portions in the hinge and ventral margin (both belonging to the outer shell layer) within individual specimens showed nearly identical Sr/Ca and Mg/Ca values, but Ba/Ca was 1.5 – 2.5 times higher in the ventral margin than in the hinge. (2) In agreement with previous studies, Sr and Mg were strongly elevated near annual growth lines. (3) Along an isochronous transect from the inner portion of the outer shell layer near the myostracum toward the outer shell surface (in the ventral margin), Si/Ca values increased, on average, by 75% ± 11%, whereas Na/Ca values decreased by 7% ± 1%. Along this transect, the shell microstructure gradually changed from crossed-acicular to homogeneous suggesting that Si and Na are linked to the prevailing nanometer-sized shell architecture or underlying physicochemical processes controlling their formation. (4) In the hinge, Ba/Ca, Sr/Ca, Mn/Ca and Mg/Ca attained highest values along the axis of maximum growth, but gradually decreased in slower growing (contemporaneous) shell portions away from that axis. (5) In contemporaneous shell portions (in either the hinge or the ventral margin), the concentration of some elements varied significantly among specimens, whereas others showed little variability. For example, in similar and contemporaneous shell portions of different specimens, Na/Ca values exhibited only little variation (17.4 – 23.7 mmol/mol), whereas Sr/Ca and B/Ca differed more severely (0.3 – 1.6 mmol/mol and 0.04 – 0.07 mmol/mol, respectively; both within growth increments). Despite these inter-specimen chemical differences, the shell microstructure remained largely invariant.

Our findings firstly suggest that the extrapallial fluid, if it exists at all, is chemically inhomogeneous. This could result from differences in the efficiency of transmembrane ion transport or to differences in shell formation rate along the growing margin (e.g., faster growth in the outer portion of the outer shell layer than in portions closer to the myostracum). Secondly, chemical differences among specimens may be attributed to physiological differences. Thirdly, some elements such as Ba are uncoupled to microstructural properties, but co-vary strongly among specimens suggesting an environmental control on the uptake and incorporation of this element into the shell.

How to cite: Schnabel, E., Shirai, K., Murakami-Sugihara, N., Jochum, K. P., Höche, N., Nishida, K., and Schöne, B. R.: Links between shell chemistry and microstructure – A case study using Arctica islandica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10873, https://doi.org/10.5194/egusphere-egu2020-10873, 2020

There are numerous similarities between the otolith (an acellular calcium carbonate aggregate in the inner ears of fishes) and the shells of freshwater bivalves. Since both grow during a lifetime of the individuals, and show small increments or growth zones (in daily or subdaily periods), they are excellent time-keepers. By this capability they provide information about both the life history of the individuals and the geochemical evolution of their environments (Schulz-Mirbach et al., 2018; Cerrato, 2000). Changes of major and trace elements between the different growth zones have been studied thoroughly, but structural features, particularly those of otoliths, are not well known. We used scanning and scanning transmission electron microscopy (SEM and STEM) to study oriented ion-milled sections of an otolith and samples of Dreissena shells from Lake Balaton, a large, shallow lake in Hungary. SEM observations confirm that the growth zones within the otolith are built up of small increments, and they have a radially asymmetric appearance, whereas STEM images show that the small increments are terminated by tiny holes. Selected-area electron diffraction (SAED) patterns and HRTEM images show that the aragonite material of both samples is highly defective, with dense arrays of planar defects occurring in distinct areas, and grains joining along low-angle boundaries (around 1°). In addition, areas with multiple (double and triple) periodicities along the [110]* directions occur in both samples. Based on these preliminary observations, nanostructural features could provide important details about the growth of biogenic aragonite and the structural properties of distinct growth zones.

Cerrato R. M. (2000): What fish biologist should know about bivalve shells, Fisheries Research, 46, 39-49.

Schulz-Mirbach T., Ladich F., Plath M., Heß M. (2018): Enimgatic ear stones: what we know about the functional role and evolution of the fish otoliths, Biological Reviews, 94 (2), 457-482.

Acknowledgments: The research was supported by the ÚNKP-19-3 new national excellence program of the ministry for innovation and technology. 

How to cite: Molnár, Z., Pekker, P., Jakab, M., Dódony, I., Vitál, Z., and Pósfai, M.: Nanostructure of biogenic aragonite: a study of otoliths and bivalve shells from a freshwater environment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21996, https://doi.org/10.5194/egusphere-egu2020-21996, 2020

Brachiopods present a key taxon for Phanerozoic palaeo-climatic and palaeo-environmental reconstructions, owing to their good preservation and abundance in the geological record. Yet to date, only little is known on the mechanisms that control the incorporation of some key elements into their calcitic shells, as well as the mechanisms behind the biomineral formation itself, especially in thecideid brachiopods. To evaluate the distribution and controls on Mg, Ca, and Sr we examined the composition of natural Pajaudina atlantica Logan, 1988 (Thecideidae, Brachiopoda) originating from Canary Islands, Spain as well as specimens cultured experimentally under various pH-pCO₂ and temperature conditions [1]. At a high-spatial resolution, electron microprobe analyses (EMP) revealed substantial intrashell and intraspecific Mg and Ca heterogeneities that seemed to be principally linked to growth features and different microstructures rather than changes in temperature. Strontium, on the other hand, appeared uniform across the shell and related to the culture medium or seawater Sr content. After almost two years of culturing, however, the new shell production was only minimal and cryptic, and difficult to evaluate by visual inspections. By combining culture-specific geochemical fingerprints with radiocarbon dating of natural samples, we estimated the growth rates to be on the order of several tens to few hundreds of µm per year, which may potentially suggest a large life span and slow growth of this species, and if true, would certainly make them a highly interesting archive for inferring past ocean variabilities.

[1] Jurikova H., et al. (2019) Geochim. Cosmochim. Acta 248, 370–386.

How to cite: Liebetrau, V., Jurikova, H., Gutjahr, M., Henkel, D., Hiebenthal, C., Krause, S., and Eisenhauer, A.: Magnesium, calcium, strontium and radiocarbon in the shell of the brachiopod Pajaudina atlantica: implications for growth, biomineralisation and palaeo-proxy application, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19006, https://doi.org/10.5194/egusphere-egu2020-19006, 2020

Bamboo corals are calcitic octocorals dwelling in a broad range of water depths and in all ocean basins. Their skeletons could give insight into the temporal variability of environmental parameters at their growth locations, in areas where long-time observations are often lacking. A thorough understanding of calcification mechanisms is essential to interpret the chemical composition of their high-magnesium calcite skeleton regarding environmental fluctuations of the deeper ocean. To address this issue, we employed electron microprobe analysis, confocal Raman spectroscopy, laser ablation-ICPMS and solution based multi collector-ICPMS that together provide insights into the fine-scale spatial heterogeneity of the coral chemical composition. We investigate the spatial distribution of Na, S, and Ca, as well as organic matter in skeletal sections of specimens of Keratoisis grayi (family Isididae) from the Atlantic Ocean. Two bamboo coral samples from the Atlantic and Pacific Ocean were further used to create laser ablation-based maps of δ¹¹B and boron to carbon ratios (B/C) over the sample radii. These maps are compared with results obtained via solution based δ¹¹B analyses on drilled samples.

An inverse correlation between Na and S is observed while S seems to be positively correlated with organic matter. We will discuss the ability of a qualitative physicochemical model to explain the observed Na and S distribution and the potential role of organic matter and amorphous calcium carbonate. Our results indicate that skeletal Na/Ca in bamboo corals is largely driven by physiological processes rather than environmental salinity variability. The spatial distribution of δ¹¹B shows a positive correlation with B/C. The observed range of bulk δ¹¹B - partly falling below the theoretical borate fractionation curve in seawater - is larger than the conventional measured δ¹¹B of the calcite fraction alone. The latter cannot be explained with a spatial smoothing of the distribution during sample drilling but is rather associated with a loss of an isotopically highly variable B fraction during sample bleaching. Potential reasons for the observed differences in B isotopic range and their implications will be presented. We conclude that skeletal δ¹¹B as a proxy for pH_SW is dependent on the applied technique and investigated material fraction.

How to cite: Flöter, S., Fietzke, J., Gutjahr, M., Farmer, J., Hönisch, B., Nehrke, G., and Eisenhauer, A.: Isotopic and elemental mapping of bamboo corals – reference to calcification mechanism and proxy applications , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10590, https://doi.org/10.5194/egusphere-egu2020-10590, 2020

Fossil coccospheres provide a wealth of information on cellular traits that can be compared directly to the living coccosphere such as cell size. Cell size is critical to ecosystem dynamics and particle sinking which has implications on the carbon cycle. However, cell size reconstruction is hindered by the poor preservation of coccospheres as coccospheres often disintegrate into individual coccoliths. Although palaeoecological information can be attained from individual coccoliths, assumptions must be made when correlating cell size to coccolith size. We demonstrate a novel technique using imaging flow cytometry to rapidly and reliably sort coccospheres from marine sediment by exploiting their unique optical and morphological properties. Imaging flow cytometry combines the functional insight of morphological information provided by microscopy with high sample numbers that are associated with flow cytometry. High throughput imaging overcomes the constraints of laborious manual microscopy enabling the analysis of sediments containing low concentrations of coccospheres that would simply not be feasible to manually hunt for coccospheres. By applying this technique to the fine fraction of sediments, hundreds of coccospheres can be isolated without the need for additional sample processing. Morphological information of individual coccospheres is obtained and graphical and statistical information can be extracted. This approach lends itself perfectly to rapid processing of down-core sediment samples or high spatial coverage from core-top samples and may prove valuable in investigating the interplay between a changing climate and coccolithophore response.

How to cite: Langley, B., Halloran, P., Power, A., Rickaby, R., and Love, J.: Palaeo cell size: A Novel Technique to Investigate the Coccosphere Fossil Record using Imaging Flow Cytometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17869, https://doi.org/10.5194/egusphere-egu2020-17869, 2020

Despite their omnipresence in pelagic carbonate sediments, the coccoliths, the calcite biominerals produced by the coccolithophores, have historically been under-exploited in palaeoenvironmental studies. This is due, in part, to their small size (2-20 microns), which makes them difficult to isolate from other particles, and to the large differences in isotopic composition existing between coccolith calcite and equilibrium conditions. This so-called “vital effect” complicates the use of coccolith geochemistry to derive paleoclimatic signals with confidence. Recent studies from cultured and fossil coccoliths have shown that the oxygen and carbon isotopic compositions of the coccoliths are particularly sensitive to the availability of CO₂ in the environment, upon which the coccolithophores rely for their photosynthetic activity. Therefore, our approach here is to test whether the coccolith geochemistry can be used as a novel proxy for surface ocean and atmospheric CO₂ concentrations.

In this study, different size fractions of coccoliths were extracted from carbonate sediments of site MD95-2037 in the Northern Atlantic Ocean and run for isotopic analysis. Using calibrations between coccolith vital effects and seawater [CO₂] from culture studies, we present a seawater [CO₂] curve for site MD95-2037 across Termination II (130 kyrs). The curve was in turn translated into atmospheric pCO₂ estimates taking into account changes in ancillary parameters (such as temperature). Coccolith-derived CO₂ concentrations yield comparable values, both for the absolute numbers and trends, to the record from Vostok ice cores. This coherency is confirmed by a 80 ppm-shift in pCO₂ concentrations in the North Atlantic between glacial and interglacial times reconstructed from the coccolith record.

Altogether, these datasets confirms that coccolith geochemistry can indeed be used to reconstruct past changes in [CO₂]_sw. Perspectives for this study include providing the scientific community with a new record of pCO₂ for periods extending beyond the Vostok record, in particular the Mid-Pleistocene Transition, where a decrease in global pCO₂ has been put forward to explain the shift from 41 kyr- to 100 kyr-cycles in glacial-interglacial cycles.

How to cite: Godbillot, C., Hermoso, M., and Minoletti, F.: Probing the use of coccolith geochemistry as a proxy for past carbon dioxide concentrations - Insights from Termination II in the Northern Atlantic Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7471, https://doi.org/10.5194/egusphere-egu2020-7471, 2020

Coccolithophores play an important dual role in ocean biogeochemistry: they use dissolved inorganic carbon (DIC) in the surface for both photosynthesis and coccolith calcification. Stable isotopes in coccoliths are the result of various effects, including different vital effects, allowing hypotheses about the varying active carbon acquisition strategies in response to changing environmental conditions. Understanding the physiological mechanisms that cause these changes remains challenging.

The MIS 12 to MIS 9 interval is a crucial climatic period encompassing changing glacial-interglacial cyclicity and pronounced variations in atmospheric CO₂ concentration. Different paleorecords indicate that coccoliths were an important component of the carbonate fraction during this interval, with the outstanding worldwide dominance of the highly calcified coccolithophore species Gephyrocapsa caribbeanica.

The carbon isotopic fractionation during photosynthesis (εp) in alkenones, biomarkers produced by coccolithophores, is a proxy to reconstruct past aqueous CO₂ concentration. Here we present a new εp reconstruction spanning this glacial/interglacial interval (460 to 330 kyr) at ODP Site 925 in the western tropical Atlantic. We aim to evaluate the interplay of CO₂ and productivity effects on coccolith calcification and stable isotopes (δ¹⁸O and δ¹³C) in coccolith calcite integrating these data with the size and thickness of coccolith platelets and the geochemical Sr/Ca record.

The comparison of mean coccolith size with coeval samples from the deeper ODP Site 929 allows the evaluation of the degree of nannofossil dissolution across the interval.

How to cite: González-Lanchas, A., Stoll, H. M., Flores, J.-A., Sierro, F. J., Hernandez-Almeida, I., and Guitian, J.: The interplay of glacial/interglacial climate-CO2 and productivity effects on coccolith calcification and vital effects across the MIS 12 to MIS 9 in the western tropical Atlantic , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19299, https://doi.org/10.5194/egusphere-egu2020-19299, 2020

We cultured two species of Amphistegina under four pCO₂ concentrations yielding variable pH (8.1 -7.6) and DIC (2340-2570 μM) at constant temperature (25 ºC) and alkalinity (~ 2500 μM). To mark the newly grown shells under the experimental conditions we spiked the culture seawater with ¹³⁵Ba. The variability of trace elements within the foraminiferal shells was measured on three individuals of each species for each treatment using LA-ICPMS in the knob area. Sharp transition zones were observed between the natural and the ~tenfold increased ¹³⁵Ba in the shells. The shape of the transition zone is best described by a logistic equation for population growth. We propose that this reflects the dynamics of seawater vacuoles population that serve the biomineralization process and provide Ca and DIC for calcification of Amphistegina as described in previous publications (e.g. Bentov et al., 2009). In individuals that showed significant growth (identified by ¹³⁵Ba-enriched shell), B, Na and Sr showed a significant increase with DIC, while K and Mg were slightly lower or unchanged. LA-ICPMS profiles in the central knob (~70 µm depth) also revealed previously described cyclical changes in concentration of Mg, each apparently representing a growth of a new chamber. Additional elements such as K, Na and U showed similar cycles with the same frequency and phase as the Mg cycles. Sr showed variability with similar frequency but not in-phase with those of the Mg. These multi-element cycles were found both in the newly grown calcite (elevated-¹³⁵Ba and pCO₂) and in the natural skeleton regardless of the pCO₂ treatments. These high Mg and multi-element cycles seem to be an essential part of the calcification process. They may originate from the interaction with the organic matrix resulting in elevated Mg and other elements in the primary calcite while secondary calcite of the lamination process shows lower concentrations. It is also possible that primary calcite is enriched in trace elements if an Amorphous CaCO₃ (ACC) or vaterite precursors are involved. In addition, Rayleigh fractionation from a semi-closed reservoir, the presence of high Mg in the lattice or any combination of the previous causes may explain the trace elements enrichment. While changes in the pCO₂ did change the average concentrations of B, Na, and Sr, they did not affect the banding of trace elements in these foraminifera, suggesting that these cycles are inherent to the biomineralization process.

Bentov, S., Brownlee, C., and Erez, J. (2009). The role of seawater endocytosis in the biomineralization process in calcareous foraminifera. Proc. Natl. Acad. Sci. U.S.A. 106, 21500–21504. doi: 10.1073/pnas.0906636106

How to cite: Levi, A., Müller, W., and Erez, J.: The effect of high pCO2 on trace elements and intrashell variability: A culture experiment with live benthic foraminifera., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13080, https://doi.org/10.5194/egusphere-egu2020-13080, 2020

Na is incorporated into marine carbonate minerals and was recently proposed as a proxy for paleo-salinity. However we demonstrated that Na/Ca ratio in foraminiferal shells (Na/Ca_shell) is actually a novel proxy for past ocean Ca concentrations (Ca_sw) in benthic foraminifera (Hauzer et al., 2018). In the present study, we determined the extent to which foraminiferal Na/Ca (and other elements) change with salinity for the high-Mg large benthic foraminifer Operculina ammonoides. This laboratory culture experiment was conducted under four different salinities 32.9, 36.1, 40.65 and 43.0 PSU, at a constant temperature of 25 °C and pH of ~8.17. O. ammonoides specimens were labeled with the fluorescent dye Calcein (40 µM) for five days prior to the experiment. Experimental seawater was prepared from filtered Gulf of Eilat seawater (40.65 PSU) and the salinity was modified by the addition of deionized water or by the evaporation to the desired value at room temperature. All experimental seawater were spiked with ¹³⁵Ba in order to unambiguously determine newly grown CaCO₃ during spatially-resolved analysis of the shell. Six specimens of each treatment were selected according to the presence of non-fluorescent chambers past the Calcein mark. The CaCO₃ shells were analyzed using the LA-ICPMS. Water chemistry was analyzed using ICP-OES and ICP-MS. Experimental foraminifera added 90-160% of their original weight, based on alkalinity-depletion measurements during the experiment. The elemental ratios of Na, Mg and Li to Ca in O. ammonoides shells increased linearly with increasing seawater salinity. In contrast, Sr/Ca_shell showed no resolvable change with salinity. Since Na/Ca_shell does correlate with salinity, it appears that it could be used as a paleosalinity proxy. However, when variations of Na/Ca_shell due to salinity are compared to variations due to Ca_sw, it is clear that salinity has a minor effect compared to the Ca concentrations. Thus, when reconstructing paleosalinity, Na/Ca_shell will produce accurate results only for samples that are within the residence time of Ca_sw (~1My). Furthermore, regional and global changes in ocean salinity over geological time can only slightly affect the use of Na/Ca_shell as a proxy for past changes in Ca_sw.

Hauzer, H., Evans, D., Müller, W., Rosenthal, Y., & Erez, J. (2018). Calibration of Na partitioning in the calcitic foraminifer Operculina ammonoides under variable Ca concentration: Toward reconstructing past seawater composition. Earth and Planetary Science Letters, 497, 80-91.

How to cite: Hauzer, H., Evans, D., Müller, W., Rosenthal, Y., and Erez, J.: The effect of salinity on Na/Ca in cultured shells of the foraminifer Operculina ammonoides, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17249, https://doi.org/10.5194/egusphere-egu2020-17249, 2020

The calcite shells of planktonic foraminfera are a key archive for palaeoceanic reconstruction and represent one of the largest sinks of carbon from the surface ocean. Therefore, understanding the biomineralisation process of these organisms, and how responsive it is to ocean acidification, is an important part of accurately predicting the carbon cycle response to past and future climate change events. To date, the majority of the direct observational evidence on which foraminifera biomineralisation models are based comes from shallow-dwelling benthic species. Whilst this has provided a large amount of important information, it is not known how applicable these models are to the low-Mg planktonic foraminifera. In particular, key questions regarding the relative importance of seawater vacuolisation (SWV) versus calcium transmembrane transport (TMT) remain unresolved. We present the results of fluorescent labelling experiments on intact, decalcified planktonic foraminifera (Globigerinoides ruber and Globigerinella siphonifera) using the cell-impermeable dyes calcein, FITC-dextran, and SNARF-dextran, enabling direct observation of seawater vacuoles within the cell via confocal microscopy. Our results indicate that seawater endocytosis plays a dominant role in the calcification process. Seawater vacuoles can make up a large proportion of the intracellular volume, with a residence time on the order of hours. Moreover, we show that the skeleton is labelled with fluorescent dyes such that seawater derived from these vacuoles must be present at the calcification site. Along with inferences based on geochemical data [Evans et al., 2018], our results strongly argue that biomineralisation models centred on seawater endocytosis are applicable to the planktonic foraminifera.

Evans, D., Erez, J., Müller, W. [2018] Assessing foraminifera biomineralisation models through trace element data of cultures under variable seawater chemistry. GCA 236:198.

How to cite: Evans, D., Erez, J., and Müller, W.: Understanding the role of seawater vacuolisation in the biomineralisation of planktonic foraminifera using confocal microscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18090, https://doi.org/10.5194/egusphere-egu2020-18090, 2020

Our understanding of how atmospheric pCO₂ varied over the Cenozoic has been steadily improving, thanks in part to ever more numerous and more refined estimates from boron isotopes in foraminiferal calcite. However, the challenge of understanding how foraminiferal physiology and ecology might have influenced measured boron isotope-pH values becomes larger as we move towards older, extinct species that may be ever more different relative to well-studied modern descendants. For instance, shell morphology in itself may have effected differences in early Cenozoic foraminiferal carbon isotopes [1], while elsewhere some data suggest Eocene vital effects in boron isotopes may have been weaker than today [2]. To successfully extend boron isotope-derived pCO₂ estimates further back into the Cretaceous, where most clades have no Cenozoic descendants, necessitates a thorough approach to understanding symbiont and depth ecology in these foraminifera. Some such information can be gleaned from trends in oxygen and carbon isotopes with size [e.g. 3], but this alone cannot fully elucidate differences in physiology and biomineralisation pathways.

Here we present new insights into the physiology and palaeoecology of several key late Cretaceous planktic foraminiferal species from element/Ca ratios, measured as depth-profiles by laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS). While single-chamber Mg/Ca ratios support some depth migration patterns indicated from oxygen isotopes [3], our observed trends in boron incorporation with ontogeny often run counter to predictions based on carbon isotopes. Moreover, B/Ca ratios in Cretaceous foraminifera are strongly species-dependent, with studied trochospiral taxa recording far higher B/Ca ratios than co-habiting Heterohelicids, perhaps indicating fundamental differences in trace element incorporation mechanisms (and perhaps biomineralisation pathways) across different clades. We discuss the implications of these findings for proxy reconstructions in the Cretaceous, with a particular focus on expanding the horizons of palaeo-CO₂ and palaeotemperature reconstruction. 

[1] Gaskell, D. E. and Hull, P. M. (2019) Symbiont arrangement and metabolism can explain high δ¹³C in Eocene planktonic foraminifera. Geology 47 (12): 1156–1160.

[2] Houston, R. M., Huber, B. T., and Spero, H. J. (1999) Size-related isotopic trends in some Maastrichtian planktic foraminifera: methodological comparisons, intraspecific variability, and evidence for photosymbiosis. Marine Micropaleontology 36: 169–188.

[3] Anagnostou, E., John, E., Edgar, K. M., Foster, G. L., Ridgwell, A. J., Inglis, G. N., Pancost, R. D., Lunt, D. J. & Pearson, P. N. (2016) Changing atmospheric CO₂ concentration was the primary driver of early Cenozoic climate. Nature 533: 380-384.

How to cite: Henehan, M., Evans, D., Müller, W., and Hull, P.: Latest Cretaceous foraminiferal ecology and palaeoceanographic inferences from chamber-specific LA-ICPMS analysis., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19424, https://doi.org/10.5194/egusphere-egu2020-19424, 2020

Amorphous calcium carbonate (ACC) is one of the six well-known CaCO₃^.nH₂O (0 ≤ n ≤ 6) solids and is of vast interest in the development of advanced materials. ACC offers enhanced performance compared to its crystalline equivalents due to its high solubility, specific surface and porosity. A large body of studies has been devoted to the applicability of ACC in pharmaceutical and industrial domains, pointing out material porosity to be a key property for its application. However, less is known about the material porosity evolution during ACC transformation into crystalline calcium carbonate (e.g. calcite or vaterite).

In this study we investigate the transformation of ACC in air and the effect of three additives (magnesium chloride, activated carbon and xanthan) at distinct humidities on the properties of the final crystalline product. ACC standard material was synthesized in either pure form or together with one of the above additives, stamped into a pellet, and exposed to 40 or 75 % RH. Mineralogical characterization of the crystalline products exhibits individual quantitative polymorph distribution induced by different additives and humidities. The most prominent result of the present study is the highly dissimilar pore size distribution when the ACC pellets were exposed to different humidities. Scanning electron microscopy combined with an image analysis software revealed 75 % RH to cause an increase of pore size of the final product by a factor of 10. These findings have significant implications to tailor and improve ACC nanomaterial designs and syntheses for pharmaceutical and industrial applications.

How to cite: Goetschl, K., Spirk, T., Purgstaller, B., and Dietzel, M.: Transformation of Amorphous Calcium Carbonate in Air - The Role of Additives and Humidity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8966, https://doi.org/10.5194/egusphere-egu2020-8966, 2020

The crystallization pathways of amorphous into crystalline orthocalciumphosphate phases is a widely discussed topic, with processes not yet entirely understood. Current research focuses on medical applications as well as natural sedimentary systems, for example bone-tissue-engineering, bio-mineralization and phosphogenesis, with inorganic precipitation experiments under controlled ambient conditions being the first step to improve our understanding of the fundamental formation processes. By mixing of stock solutions with CaCl₂/MgCl₂ and NaHPO₄ we created a supersaturated solution in respect to CaPO₄-phases and varied the pH by adding different amounts of NaOH. Continuous sampling was performed over the period of 24 hours, with sampling intervals after 1 min, 10 min, 60 min and 24 h. In order to record temporal changes in mineralogical and chemical composition, samples (solids and fluids) were investigated by XRD, FTIR, SEM and ICP-OES, respectively. Our experiments yield considerable differences concerning the time of amorphous calcium phosphate (ACP) transformation into hydroxyapatite (HAP), heavily depending on the pH, Ca/P ratio and Mg content of the stock solution. Main results show that a higher pH stabilizes the ACP over a period of the first 60 min, whereas at lower pH the transformation of ACP into the crystalline phase already starts at 10 min after mixing. Increasing the Ca/P ratio of the stock solution results in ACP being less stable and the transformation into HAP occurs earlier. In contrast, the presence of Mg seems to delay the formation of HAP via ACP. After 24 hours the experiments showed nano-crystalline HAP and most likely some other phases as octacalcium phosphate.

How to cite: Hippler, D., Schnedlitz, T., and Purgstaller, B.: Investigating the reaction pathway of crystalline orthocalciumphosphate formation via amorphous precursors in respect to different pH, Ca/P ratios and Mg presence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20510, https://doi.org/10.5194/egusphere-egu2020-20510, 2020