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Advances in geochemical proxy development and application: from biomineral archives to past global changes

Reliable information on past environmental and climatic conditions is crucial for understanding the evolution of life and the Earth System as a whole. Skeletal components of marine or aquatic organisms are among the most important and widely-used natural archives capturing information about the environment and fluid chemistry during precipitation in the form of geochemical signatures and/or specific mineralogies or micromorphologies. Over the past decades, a refined understanding of (bio)mineralisation, together with the development of new isotopic and elemental proxies (e.g. clumped isotopes Δ47, boron isotopes δ11B, or elemental ratios such as Li/Mg), has led to numerous breakthroughs in palaeoclimate research (e.g. on the evolution of seawater chemistry, causes and consequences of mass extinctions, or greenhouse vs. icehouse climate sensitivities). Simultaneously, geochemical, petrographic and crystallographic approaches have brought novel insights into (bio)mineral formation processes and alteration pathways of a variety of organisms. Critically, however, our knowledge of the incorporation of elements into the crystal lattice, and the quality and reliability of extracted climatic and environmental records, depends on careful proxy calibrations, and evaluation of secondary controls such as kinetic or vital effects and diagenetic influences.

This session seeks contributions on geochemical proxy development, including but not limited to new proxies, calibrations, modelling frameworks, and analytical or methodological advances. We invite experimental and observational studies dealing with biogenic but also inorganic mineral precipitation, transformation and alteration, including interface geochemistry, geomicrobiology or new perspectives on biomineralisation from culturing of calcifying organisms. We also welcome examples on how mechanistic understanding of marine or terrestrial carbonates and/or application of novel approaches results in an improved understanding of the global carbon(ate) cycle and Earth history. The aim of this session is to synthesize recent advances in geochemistry and (bio)mineralisation to further palaeo-proxy development and application that will result in a comprehensive understanding of past global changes.

Co-organized by CL5.1/OS3/SSP1
Convener: Hana JurikovaECSECS | Co-convener: Michael Henehan
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Wed, 28 Apr, 14:15–15:00

Chairpersons: Hana Jurikova, Michael Henehan

5-minute convener introduction

Patrick Martin et al.

The flux of terrigenous dissolved organic carbon (tDOC) from land to the coastal ocean is an important component of the global carbon cycle, and can impact coastal marine ecosystems. A potentially large fraction of this tDOC flux can be oxidised to CO2, resulting in coastal ocean acidification and ultimately degassing to the atmosphere. tDOC is also rich in chromophoric dissolved organic matter (CDOM) which is the fraction of dissolved organic matter that absorbs light. CDOM plays an important role in aquatic systems by absorbing sunlight and reducing its transmission through the water column. This is a partially beneficial effect since photodegradation reactions release nutrients and protect biota from harmful UV radiation. However, CDOM light absorption also reduces the light available for primary producers. Long running tDOC and CDOM measurements from North America and Europe show that tDOC fluxes have been increasing in the 20th century in response to climate and land-use change. However, despite the biogeochemical and ecological significance of tDOC, there are few long-term records of tDOC, and none at all from tropical shelf sea environments. This severely limits our understanding of its drivers and processes. 

Here, we show that luminescence green-to-blue (G/B) ratios in coral skeleton cores are an accurate proxy for tDOC concentration in seawater. Coral luminescence is generated by humic-like substances, which are highly fluorescing compounds that are incorporated by corals into their skeletons during growth, forming sub-annual growth layers that luminesce under UV light. These humic-like substances are an integral component of tDOC and are an important constituent of the CDOM pool. We used solution fluorescence excitation emission matrix (EEM) measurements of dissolved powders of coral skeletons collected from multiple locations on the Sunda Sea Shelf along with abiogenic aragonite growth experiments to show that coral luminescence G/B is quantitatively related to the fluorescence intensity of terrestrial humic substances. We then combined a satellite-retrieved time series of CDOM with an analysis of a coral core section from an area of Borneo affected by run-off from tropical peatlands. We show that coral G/B ratio is a quantitative proxy for CDOM concentration at monthly resolution over a period of 12 years at this site (R2 = 0.57). Furthermore, we examine data from a multi-year biogeochemical time series in Singapore combined with recently collected coral cores from the monitoring site. These results show that coral luminescence G/B ratios are highly correlated with terrestrial CDOM absorption across the ultraviolet and visible wavelength spectrum, as well as with the tDOC concentration as estimated from stable isotopes of dissolved organic carbon. Taken together, our results show that coral G/B ratios can be a powerful proxy to reconstruct bio-optical and biogeochemical variability resulting from tDOC input. Corals can therefore allow us to potentially reconstruct tDOC flux variability across tropical seas over past centuries, and therefore to investigate seasonal to inter-annual drivers of tDOC dynamics.

How to cite: Martin, P., Kaushal, N., Tanzil, J., Sanwlani, N., Yang, L., Zhou, Y., Cherukuru, N., Sahar, S., Müller, M., Mujahid, A., Lee, J. N., and Goodkin, N.: Coral skeletal luminescence records changes in terrigenous dissolved organic matter (tDOM) parameters in tropical coastal waters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6864, https://doi.org/10.5194/egusphere-egu21-6864, 2021.

Jassin Petersen et al.

Before proxy records can be reliably employed in palaeoclimate research, calibration studies have to be conducted to assess the confidence intervals of the respective proxies. Here, we use shells of the fast growing Pacific oyster Magallana gigas from the Central Wadden Sea, North Sea, a temperate barrier island-backbarrier tidal flat-salt marsh system with large seasonal changes of water mass-properties, for the calibration of geochemical proxies. M. gigas represents a non-native invasive species that rapidly develops oyster reefs. Calcite shells of two specimens from the intertidal and subtidal zones were sampled in high resolution yielding sub-monthly data sets. The time period represented in the shell, based on δ18O age modelling, was estimated at 8-10 years and the growth of the shells was restricted from (late) spring to (early) autumn of each year. Mg/Ca, Mn/Ca and Sr/Ca ratios of the intertidal and subtidal specimens show similar seasonal patterns. Mg/Ca and Sr/Ca ratios are investigated as high-resolution sea surface temperature (SST) proxies. Important ontogenetic effects (i.e., increasing time-averaging with increasing age) as well as intra-species variability are discussed as limiting factors for the proxy development. Intertidal Mg/Ca ratios show only a significant correlation to the high-resolution SST record of the Central Wadden Sea when the early ontogenetic stage is considered. Sr/Ca ratios were comparable in terms of absolute values and amplitudes to those of M. gigas in the Northern Wadden Sea, but amplitudes were decreasing with increasing ontogeny. These findings seriously hamper the application of Mg/Ca and Sr/Ca for reliable palaeotemperature reconstructions regardless of ontogenetic stage. The Mn/Ca ratios were investigated as proxy for Mn cycling in tidal basins, where it is interrelated with seasonal changes in primary production. In addition to the generally observed seasonal variability of the Mn/Ca records, the subtidal Mn/Ca is significantly elevated compared to intertidal Mn/Ca. The subtidal Mn/Ca offset likely reflects differences in Mn cycling in tidal settings and could, therefore, serve in the palaeorecord as indicator to differentiate inter- and subtidal habitats in the same embedding sedimentary facies. This habitat effect has to be considered as an important factor besides environmental change when interpreting the high-resolution proxy record of fossil oysters.

How to cite: Petersen, J., Titschack, J., Groeneveld, J., Wehrmann, A., Hebbeln, D., and Freiwald, A.: Reef-building oysters record seasonal variations in water mass-properties of tidal basins from the Central Wadden Sea (North Sea), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6523, https://doi.org/10.5194/egusphere-egu21-6523, 2021.

Melita Peharda et al.

Pinna nobilis is a large bivalve endemic to the Mediterranean Sea that lives in shallow coastal areas. Due to its size and relatively fast shell growth rates, it is an interesting taxon for high resolution geochemical and sclerochronological research. Subsequently to previous analyses of δ18O and δ13C in P. nobilis shells, here, we investigate nitrogen isotopes in the carbonate-bound organic matrix (δ15NCBOM) of this species. Our objectives were to test if P. nobilis shells (i) can be used as an indicator of the isotopic baseline of the system, and (ii) is a good candidate for obtaining high-resolution temporal data on environmental δ15N variability. Due to the multiple mass mortality events of P. nobilis spreading throughout the Mediterranean, including the Adriatic Sea, we also tested if (iii) P. nobilis geochemistry changes as a response to diseases.

Shells were opportunistically collected by skin diving from 4 shallow coastal localities in the eastern Adriatic, as a part of a project on mortality monitoring. Specimens from Lim channel (October 2019), Kaštela Bay (January 2020) and Mali Ston Bay (November 2019) were collected alive, while in Pag Bay, shells of three recently dead specimens were collected in September 2020. Tissue and epibionts were removed and shells carefully cleaned and air-dried. Shell powder was collected by milling sample swaths by hand using a DREMEL Fortiflex drill equipped with a 300 μm tungsten carbide drill bit. For δ15NCBOM analysis, three shells from each locality were processed and three replicas were collected from each of these shells by milling shallow lines parallel to the growth axis from the internal shell surface. In addition, high-resolution δ15NCBOM data were obtained for one shell from Kaštela by milling lines (N=40) perpendicular to the major growth axis from the external shell surface. From this shell we also collected shell powder for δ18Oshell and δ13Cshell analysis to enable placing δ15NCBOM into temporal context. Isotope samples were analyzed Union College on an elemental analyzer - isotope ratio mass spectrometer.

Results indicate significant differences in δ15NCBOM between sampling localities, with lowest values recorded for shells from Pag Bay (3.73±0.36‰), and highest for shells sampled in Lim channel (7.04±0.63‰). High-resolution δ15NCBOM data obtained from the shell collected from Kaštela Bay corresponded to a time interval from spring 2018 to spring 2019. These data showed relatively small variations (5.02±0.33‰). However, δ15NCBOM values increased to 8.65±1.61‰ closest to the shell margin, and were coupled with a decrease in δ13Cshell values, indicating that this animal was experiencing stressful conditions several months prior to its death. According to our findings, δ15NCBOM values serve as an indicator of the isotopic baseline of the ecosystem as well as a potential powerful tool to study bivalve physiology.

Research was the supported by the Croatian Science Foundation, research project BivACME.

How to cite: Peharda, M., Gillikin, D., Schöne, B., Verheyden-Gillikin, A., Uvanović, H., Markulin, K., Šarić, T., and Župan, I.: Nitrogen isotope sclerochronology - insights into coastal environmental conditions and Pinna nobilis ecology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-454, https://doi.org/10.5194/egusphere-egu21-454, 2021.

Hana Uvanović et al.

The Smooth clam Callista chione is a commercially important venerid bivalve. It is widely distributed in the eastern Atlantic Ocean and the Mediterranean Sea and inhabits sandy sediments in coastal waters at depths down to 180 m. With specimens that can reach 10 cm of shell length and with a lifespan of more than four decades, C. chione represents an interesting archive for sclerochronological research. The aim of this study was to analyse possible variations in δ18Oshell and δ13Cshell values between C. chione specimens collected in different parts of the Mediterranean Sea.

Callista chione shells were collected alive from three localities: (1) Caleta de Vélez in the north region of the Alborán Sea, Spain; (2) Gulf of Venice, Italy, North Adriatic, and (3) west coast of the Istrian peninsula, Croatia, also in the North Adriatic. At the first two localities, specimens were obtained from catch of the commercial fishing vessels, while in Istria they were collected by SCUBA diving. Prior to analysis, the external shell surface was physically cleaned by grinding. Shell powder for δ18Oshell and δ13Cshell analysis was then collected by milling narrow sample swaths in the outer shell surface and processed at Mainz University using a GasBench II - IRMS.

Modelled daily temperature and salinity values were obtained for each locality and used for calculating the predicted δ18Oshell values. For Caleta de Vélez, daily temperature and salinity values were obtained by MEDSEA model; for the Venetian region by the AdriSC climate model, and for Istria by the 3D numerical model ROMS. Temporal alignment of measured δ18Oshell values was conducted manually in Excel by best-fitting measured isotope data to predicted δ18Oshell curves.

Seasonal δ18Oshell cycles were observed in all studied specimens. Temporal alignment of measured and modelled δ18Oshell values clearly showed that C. chione grew fast during the warm part of the year, while slower growth occurred during the winter months. Samples collected in Caleta de Vélez had the narrowest range of δ18Oshell values (-0.43 to +1.73 ‰), while δ18Oshell values in C. chione from Istria showed the largest amplitudes (-1.61 to +2.67 ‰). Growth patterns varied between sampling localities.

The δ13Cshell values varied strongly between localities and specimens. Highest δ13Cshell values were obtained for C. chione shells from Caleta de Vélez (0.51 ± 0.03 ‰, range -0.19 to +1.06 ‰). The broadest range of δ13Cshell values (-3.37 to -0.08 ‰) were measured in shells from the Venetian region. These samples also had the lowest mean (-1.42 ± 0.14 ‰). Shells from Istria had δ13Cshell values ranging from -1.57 to +0.38 ‰ (mean: -0.42 ± 0.28 ‰). Observed differences between localities are statistically significant (Kruskal Wallis H = 150.4, p < 0.001).  Isotope data obtained for this study were compared with data from a previous study on the same species in the Eastern Adriatic.

Research was supported by the Croatian Science Foundation, research project BivACME.

How to cite: Uvanović, H., Schöne, B. R., Janeković, I., Denamiel, C., Mazzoldi, C., Baro, J., Markulin, K., and Peharda, M.: Callista chione – geochemical archive of δ18O and δ13C data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7559, https://doi.org/10.5194/egusphere-egu21-7559, 2021.

Konstantin Pustovoytov and Simone Riehl

The tribe Lithospermeae (fam. Boraginaceae) represents one of very few taxa vascular plants that accumulate appreciable amounts of calcium carbonate in their tissues. The CaCO3 is localized in the pericarp sclerenchyma, which makes their small fruits (nutlets) mechanically durable and provides their good preservation in sediments and cultural layers. Fossil Lithospermeae fruits appear as whitish, slightly elongated entities, 3-5 mm in length.  At archaeological sites, the nutlets can be of diverse origin: in most contexts they represent carpological evidence for weed flora of the past, however, some findings suggest that they were used for decorative purposes (beads etc.).  

Here we overview the potential use of fruit carbonate of Lithospermeae in paleoecological research.   

14C-dating: Fruit carbonate of the taxon can be successfully dated with radiocarbon.  

The 14C concentration in the CaCO3 fraction of modern nutlets is well-correlated to the recent atmospheric 14C levels. Radiocarbon ages of old nutlets are in good correspondence with the age ranges of archaeological contexts. Obviously, fruit carbonate can represent a geochemically closed system for millennia in sediment environments.               

δ18O values: Our data based on an array of herbarium exemplars of Lithospermeae, suggest that the δ18O of fruit carbonate is distinctively sensitive to the amount of atmospheric precipitation during the warm season. The degree of correlation between δ18O and local air temperatures is lower.

We further performed an experiment on gromwell (Buglossoides arvensis (L.) I.M.Johnst), irrigated by water with different oxygen isotope signatures. The δ18O values of fruit CaCO3 showed correlation to the δ18O of irrigation water. The oxygen isotope fractionation in fruit carbonate turned out to be surprisingly low with 1000lnα = 4.72±3.49, which is relatively close to foraminiferal CaCO3.

 δ13C values: In contrast to the oxygen isotope signature, we did not find a strong correlation of the δ13C values of fruit carbonate to precipitation and temperature.  However, the photosynthetic origin of carbon in fruit CaCO3 admits a possibility of some links of δ13C to ambient factors.  


How to cite: Pustovoytov, K. and Riehl, S.: Fruit carbonate in Lithospermeae: 14C, stable isotope composition and potential as a paleoenvrionmental proxy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11778, https://doi.org/10.5194/egusphere-egu21-11778, 2021.

Siyuan Sean Chen et al.

Benthic nepheloid layers (BNLs) are particle-rich layers that can extend over a thousand meter or more above the seafloor and are thought to be produced by the resuspension of fine sediments from strong bottom currents. They can often be subdivided into two sublayers: (i) a lower sublayer in contact with the seabed, where particle concentrations are the largest and which roughly coincides with the bottom mixed layer (BML); and (ii) an upper sublayer in which particle concentration decreases up to a clear water minimum (CWM). Although BNLs have long been recognised in vertical traces of optical instruments lowered to abyssal depths, their influence on ocean biogeochemical cycles – on the cycling of particle-reactive metals in particular – remains poorly understood.

In this study, we characterize the BNLs observed between the New England continental shelf and Bermuda and explore their influence on the cycling of 230Th and 231Pa – two naturally-occurring particle-reactive radionuclides that have found different applications in chemical oceanography and paleoceanography. To this end, we use concomitant measurements of temperature, salinity, particle concentration derived from light beam transmissometry, and 230Th and 231Pa activities in the dissolved and particulate fractions, which have been collected along the western segment of the U.S. GEOTRACES GA03 transect. We estimate that the thickness of strong BNLs (particle concentration > 20 µg l-1) varied from about 72 to 1358 m between different deep stations. At all stations, particle concentrations below the CWM were the highest in the BML, whose thickness ranged from 95 to 320 m, and decreased generally with height above the seafloor. A simplified model of particle-radionuclide cycling in the deep water column, which includes a particle source representing sediment resuspension at topographic reliefs and their subsequent lateral transport, is fitted to observed profiles of particle concentration and radionuclide activities at two selected stations. The model can reproduce simultaneously the increase of particle concentration with depth, the low dissolved activities in the BNLs, and the extremely large particulate activities near the bottom. Analysis of 230Th and 231Pa budgets reveals that the behaviour of both radionuclides in the BNL is fundamentally different from that envisioned in reversible exchange theory. Sensitivity tests with the model suggest that lateral particle sources near continental slopes and similar reliefs can produce significant biases in the paleoceanographic applications of both radionuclides, including the 230Th-normalization method and the interpretation of sediment 231Pa/230Th records.

How to cite: Chen, S. S., Marchal, O., Lerner, P., McCorkle, D., and Rutgers van der Loeff, M.: Benthic Nepheloid Layers along the U.S. GEOTRACES GA03 Transect in the Western North Atlantic: Characterization and Influence on 230Th and 231Pa Cycling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6510, https://doi.org/10.5194/egusphere-egu21-6510, 2021.

David Bajnai and Daniel Herwartz

In any reaction involving water, the educt oxygen is either derived from H2O or OH- (i.e., the hydroxide ion). For example, during carbonate precipitation the relative proportion of (de)hydration (CO2 + H2O ⇔ H+ + HCO3-) and (de)hydroxylation (CO2 + OH- ⇔ HCO3-) reactions is pH-dependent. When modelling this system, the oxygen isotopic composition of water can be measured directly, but the oxygen isotopic composition of hydroxide must be calculated from the respective fractionation factor (1000lnαH2O–OH-). Experimental studies from the 1960s determined 1000lnαH2O–OH- to be 39.22(±2.88)‰ at 25 °C and estimated its temperature dependence at ‑0.5‰ °C-1 (1-3). These empirical observations were recently questioned by a theoretical study that implied a much lower fractionation factor of 23.18–18.91‰ at 25 °C as well as a lower temperature dependence of ‑0.05‰ °C‑1 (4).

To provide new experimental data to solve this controversy, we performed quantitative witherite (BaCO3) precipitation experiments. Tank CO2 gas of known oxygen and carbon isotopic composition was injected into saturated Ba(OH)2 solution of known oxygen isotopic composition. Following the hydroxylation of the CO2, BaCO3 instantly precipitated from the high pH (>12) solution. Since the precipitate directly inherited 1/3 of its oxygen from the hydroxide ion and 2/3 from the tank CO2, the δ18O value of the OH- can be calculated via mass balance. Subsequently, the 1000lnαH2O–OH- value can be derived. Altogether 18 experiments were performed at a range of temperatures (1–80 °C) and using solutions of different oxygen isotopic composition (range of ca. 20‰). Minor variations between the δ13C values (< 1‰) of the BaCO3 and the tank CO2 attests the quantitative precipitation of the reference gas.

Our 1000lnαH2O–OH- values show a similar temperature dependence as the recent theoretical study of Zeebe (4), but our fractionation factor at 25 °C is much closer to the values reported in the 1960s. Reasons for the discrepancies between our study and previous publications in terms of 1000lnαH2O–OH- and its temperature dependency will be discussed. One of our hypotheses is that the H2O ⇒ OH- + H+ reaction introduces a large kinetic effect as isotopically light H2O is pyrolysed more frequently. In contrast, the back reaction proceeds rapidly without an isotopic preference. Hence, the self-ionisation of water cannot be described as a classic equilibrium. Alternative explanations such as unidentified kinetic isotope effects in our precipitation experiments (i.e., on the crystal surface) cannot be ruled out.

(1) H. R. Hunt, H. Taube, The relative acidity of H2O18 and H2O16 coördinated to a tripositive ion. J. Phys. Chem. 63, 124-125 (1959).
(2) E. R. Thornton, Solvent isotope effects in H2O16 and H2O18. J. Am. Chem. Soc. 84, 2474-2475 (1962).
(3) M. Green, H. Taube, Isotopic fractionation in the OH-–H2O exchange reaction. J. Phys. Chem. 67, 1565-1566 (1963).
(4) R. E. Zeebe, Oxygen isotope fractionation between water and the aqueous hydroxide ion. Geochim. Cosmochim. Acta 289, 182-195 (2020).

How to cite: Bajnai, D. and Herwartz, D.: Oxygen isotope (18O/16O) fractionation between water and hydroxide ion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4909, https://doi.org/10.5194/egusphere-egu21-4909, 2021.

Jeroen van der Lubbe et al.

Biogenic and inorganic carbonates are widely used to reconstruct past temperatures and fluid compositions. For decades, calcification temperatures have been inferred from oxygen isotope composition (δ18O) of calcium carbonates (CaCO­3) assuming the δ18O of the parental fluid and isotopic equilibrium precipitation conditions. The development of the clumped isotope (Δ47) thermometer allows for reconstructing equilibrium calcification temperatures without requiring a priori knowledge of the water δ18O values.

Carbonate minerals can also contain several weight percentages of water, which are typically trapped within microscopic pores. These fluid-inclusions may preserve remnants of the parental fluid, which can be analyzed for the δ18O as well as hydrogen isotopic (δ2H) composition. Subsequently, the δ18O of fluid-inclusion and host carbonate may allow for the determination of paleotemperatures by providing constraint on the δ18O water value.

Reasonable equilibrium temperatures can be obtained for speleothem calcites from cave systems. On the contrary, anomalously high temperatures are derived from δ18O fluid-inclusion and calcite pairs in soil carbonates possibly suggesting diffusion of trapped water from host CaCO3. Deeply-buried and subsequently exhumed (inorganic) calcite veins have yielded discrepant paleotemperature estimates between fluid-inclusion and Δ47 thermometers. The distinctly lower fluid-inclusion derived temperatures might be attributed to kinetic fraction during initial vein cementation and/or isotopic re-equilibration between fluid-inclusions and CaCO3 at lower temperatures during uplift.

Heating experiments demonstrate that the oxygen isotope exchange between fluid inclusions and host carbonate is limited for inorganic calcite and aragonite at high temperatures (175oC) for short timescales (90 minutes). In contrast, considerable positive shifts in the δ18O of fluid inclusions have been recorded in biogenic aragonites during experimental heating, which coincide with lower carbonate δ18O values (albeit to a lesser extent due to the overwhelming amount of oxygen in the CaCO3), indicative of re-equilibration between host carbonate and pore fluids. This effect leads to apparently high equilibrium temperatures. In conjunction, the Δ47 derived temperatures do not change significantly after heating of inorganic aragonite, whereas a considerable higher Δ47 temperature is derived from aragonitic bivalve samples after heating. The positive shift in both thermometers has interpreted to reflect re-crystallization of CaCO3 and isotopic re-equilibration between the host carbonate and fluid-inclusions. This exchange might be facilitated by extremely small fluid-inclusions present in biogenic carbonates and/or water associated with organic substances.

Importantly, these isotopic exchange processes in biogenic aragonites took place in the absence of an external fluid and below the temperature thresholds for solid-state-reordering and the aragonite-to-calcite transition. The novel application of combined fluid-inclusion and clumped isotope thermometry has a proven utility in determining equilibrium precipitation temperatures, monitoring preservation of the primary fluid-inclusions and re-crystallization processes during diagenesis. However, additional experiments and analytical improvements are needed to further constrain the diagenetic behavior of this proxy.


How to cite: van der Lubbe, J., Nooitgedacht, C., Staudigel, P., and Ziegler, M.: Application of combined fluid-inclusion and clumped isotope thermometry to biogenic and inorganic carbonates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16113, https://doi.org/10.5194/egusphere-egu21-16113, 2021.

Michael Henehan et al.

Boron isotope ratios, as measured in marine calcium carbonate, are a proven tracer of past seawater and calcifying fluid pH and thus a powerful tool for the reconstruction of past atmospheric CO2 and monitoring of coral physiology. For such applications, understanding the inorganic baseline upon which foraminiferal vital effects or coral pH upregulation are superimposed should be an important prerequisite. Yet, investigations into boron isotope fractionation in synthetic CaCOpolymorphs have often reported variable and even conflicting results, implying that we may not fully understand pathways of boron incorporation into calcium carbonate.  Here we address this topic with experimental data from calcite and aragonite precipitated across a range of pH in the presence of both Mg and Ca. We confirm the results of previous studies that the boron isotope composition of inorganic aragonite precipitates closely reflects that of aqueous borate ion, but that calcites display a higher degree of scatter, and diverge from the boron isotope composition of borate ion at low pH. We discuss these findings with reference to the simultaneous incorporation of other trace and minor elements, and highlight a number of mechanisms by which crystal growth mechanisms may influence the concentration and isotope composition of boron in CaCO3. In particular, we highlight the potential importance of surface electrostatics in driving variability in published synthetic carbonate datasets. Importantly for palaeo-reconstruction, however, these electrostatic effects are likely to play a much more minor role during natural precipitation of biogenic carbonates.

How to cite: Henehan, M., Klein-Gebbinck, C., Foster, G., Wyman, J., Hain, M., and Kim, S.-T.: No ion is an island: Multiple ions involved during boron incorporation into CaCO3, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10652, https://doi.org/10.5194/egusphere-egu21-10652, 2021.

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