Intra-shell boron isotope ratios in the symbiont-bearing benthic foraminiferan Amphistegina lobifera: Implications for δB vital effects and paleo-pH reconstructions

The boron isotope composition of marine carbonates is considered to be a seawater pH proxy. Nevertheless, the use of δ¹¹B has some limitations such as the knowledge of the fractionation factor (α_4-3) between boric acid and the borate ion and the amplitude of “vital effects” on this proxy that are not well constrained. Using secondary ion mass spectrometry (SIMS) we have examined the internal variability of the boron isotope ratio in the shallow water, symbionts bearing foraminiferan Amphistegina lobifera. Specimens were cultured at constant temperature (24 ± 0.1 °C) in seawater with pH ranging between 7.90 and 8.45. Intra-shell boron isotopes showed large variability with an upper limit value of ≈30‰. Our results suggest that the fractionation factor α_4-3 of 0.97352 (Klochko et al., 2006) is in better agreement with our experiments and with direct pH measurements in seawater vacuoles associated with the biomineralization process in these foraminifera. Despite the large variability of the skeletal pH values in each cultured specimen, it is possible to link the lowest calculated pH values to the experimental culture pH values while the upper pH limit is slightly below 9. This variability can be interpreted as follows: foraminifera variably increase the pH at the biomineralization site to about 9. This increase above ambient seawater pH leads to a range in δ¹¹B (Δ¹¹B) for each seawater pH. This Δ¹¹B is linearly correlated with the culture seawater pH with a slope of −13.1 per pH unit, and is independent of the fractionation factor α_4-3, or the δ¹¹B_sw through time. It may also be independent of the pK_B (the dissociation constant of boric acid) value. Therefore, Δ¹¹B in foraminifera can potentially reconstruct paleo-pH of seawater.     

Boron isotope geochemistry of Paleozoic brachiopod calcite: Implications for a secular change in the boron isotope geochemistry of seawater over the Phanerozoic

Boron isotope composition of marine carbonates has been proposed as a paleo-pH proxy and potential tool to reconstruct atmospheric pCO₂. The precise knowledge of the boron isotopic composition of ancient seawater represents the fundamental prerequisite for any paleo-pH reconstruction. This contribution presents boron isotope values for Silurian to Permian brachiopod calcite that might be used to reconstruct pH or boron isotope composition of past oceans. All brachiopod shells were screened for diagenetic recrystallization by means of cathodoluminescence microscopy, trace element geochemistry (B, Fe, Mn, Sr) as well as SEM. Only nonluminescent shells revealing well-preserved microstructures, high strontium and boron concentrations as well as low iron and manganese contents were accepted for boron isotope analysis. The boron isotope ratios of Silurian, Devonian, Pennsylvanian and Permian brachiopod calcite range from 6.8 to 11.0‰, 7.3 to 14.9‰, 12.4 to 15.8‰ and 10.1 to 11.7‰, respectively. These δ¹¹B values are significantly lower in comparison to δ¹¹B values of modern biogenic carbonates and indicate that the Paleozoic oceans were depleted in ¹¹B by up to 10‰. Box modeling of the boron geochemical cycle suggests that the significant depletion of ¹¹B in the oceanic reservoir may have been initiated by an enhanced continental boron discharge. Our data support the earlier made conclusion that boron isotopes may not be used in the geological past as reliable paleo-pH proxy unless the boron isotopic composition of ancient oceans can be constrained by further studies.     

A critical evaluation of the boron isotope-pH proxy: The accuracy of ancient ocean pH estimates

The boron isotope-pH technique is founded on a theoretical model of carbonate δ¹¹B variation with pH that assumes that the boron isotopic composition of carbonates mirrors the boron isotopic composition of borate in solution (δ¹¹B_carb = δ¹¹B_borate). Knowledge of the fractionation factor for isotope exchange between boric acid and borate in solution (α_4-3), the equilibrium constant for the dissociation of boric acid (pK_B*), as well as the isotopic composition of boron in seawater (δ¹¹B_sw) are required parameters of the model.The available data suggests that both the value of α_4-3 and the history of δ¹¹B_sw are poorly constrained. However, if one assumes that δ¹¹B_carb = δ¹¹B_borate, an empirical value for α_4-3 can be estimated from the results of inorganic carbonate precipitation experiments. This exercise yields an α_4-3 value of ∼0.974 in accordance with recent theoretical estimates, but substantially deviates from the theoretical value of 0.981 often used to estimate paleo-ocean pH. Re-evaluation of ocean pH using an α_4-3 value of 0.974 and published foraminiferal δ¹¹B values for the Cenozoic yield pH estimates that are relatively invariant, but unrealistically high (∼8.4-8.6). Uncertainty increases as foraminiferal ‘vital effects’ are considered and different models for secular changes in seawater δ¹¹B are applied.The inability to capture realistic ocean pH possibly reflects on our understanding of the isotopic relationship between carbonate and borate, as well as the mechanism of boron incorporation in carbonates. Given the current understanding of boron systematics, pH values estimated using this technique have considerable uncertainty, particularly when reconstructions exceed the residence time of boron in the ocean.     

古海水pH值代用指标——海洋碳酸盐硼同位素研究进展

仪器测量的海水pH记录太短，无法评估海水pH自然变化的频率和幅度,并预测未来大气CO₂急剧增加后海水酸度的响应。海相碳酸盐的硼同位素是目前恢复古海洋pH的有效途径，倍受古气候—环境学家的重视。评述了近年来海洋碳酸盐的硼同位素的最新研究成果和研究现状，重点探讨了海相碳酸盐的硼同位素的测定方法、硼同位素—pH模型和古海水pH恢复等前沿内容，旨在提供一个系统的海洋碳酸盐硼同位素—pH系统的基本概念及研究思路，以利于气候学、地质学界了解这一交叉领域的发展动态。     

Assessing scleractinian corals as recorders for paleo-pH: Empirical calibration and vital effects

Laboratory experiments on the branching, symbiont-bearing coral genus Porites and Acropora have been carried out to determine the dependence of the skeletal boron isotopic composition (δ¹¹B) on the pH of seawater. The results show a clear relationship similar to previously established empirical calibrations for planktonic foraminifera and inorganic calcite. A −0.6‰ offset exists between P. cylindrica and A. nobilis which is systematic over the pH range of 7.7-8.2. To test whether the δ¹¹B of coral skeletons changes with physiological processes such as photosynthesis and respiration, corals were grown along a depth transect in their natural environment and under controlled conditions in the laboratory at varying light intensities and food supply. Although we also observe an isotopic offset between P. compressa and Montipora verrucosa, neither experimental treatment systematically changed the δ¹¹B of the two species. These findings are encouraging for using the boron isotope paleo-pH proxy in corals, because it appears that seawater pH is the dominant control on the boron isotopic composition in corals.     

Controlling factors of the δ 11B-pH proxy and its research direction

Significant boron isotope fractionation occurs in nature (?70 ‰ to +75 ‰) due to the high geochemical reactivity of boron and the large relative mass difference between ¹⁰B and ¹¹B. Since the 1990s, reconstruction of ancient seawater pH using the isotopic composition of boron in bio-carbonates (δ ¹¹B_carb), and then calculation of the past pCO₂ have become important issues for the international isotope geochemistry community, and are called the δ ¹¹B-pH proxy. Although many achievements have been made by this proxy, various aspects of boron systematics require rigorous evaluation. Based on the previous researches, mechanism of boron isotope fractionation, variation of boron isotope (δ ¹¹B) in nature (especially in bio-carbonates) and controlling factors of the δ ¹¹B-pH proxy, such as the dissociation constant of B(OH)₃ in seawater (pK_a), the δ ¹¹B of seawater (δ ¹¹B_SW), the boron isotopic fractionation factor between B(OH) ₄ ^? and B(OH)₃ (α _4–3), and the incorporated species of boron into bio-carbonates, are reviewed in detail and the research directions of this proxy are proposed. Generally, the controversy about pK_a, δ ¹¹B_sw, and α _4–3 is relatively less, but whether boron incorporated into bio-carbonates only in the form of B(OH) ₄ ^? remains doubtful. In the future, it is required that the physicochemical processes that control boron incorporation into carbonates be rigorously characterized and that the related chemical and isotopic fractionation be quantified. It is also necessary and important to establish a “best-fit empirically equation” between δ ¹¹B_carb and pH of seawater based on the precipitation experiments of inorganic or culture experiments of corals or foraminifera. In addition, extended application of the δ ¹¹B-pH proxy to the earlier part of the Phanerozoic relying on the Brachiopods is worthy of studying. Like other geochemical indicators, there are limiting factors of δ ¹¹B; however, it remains a very powerful tool in the reconstruction of past seawater pH at present.     

NIST RM 8301 Boron Isotopes in Marine Carbonate (Simulated Coral and Foraminifera Solutions): Inter‐laboratory δ11B and Trace Element Ratio Value Assignment

The boron isotopic ratio of ¹¹B/¹⁰B (δ¹¹B_SRM951) and trace element composition of marine carbonates are key proxies for understanding carbon cycling (pH) and palaeoceanographic change. However, method validation and comparability of results between laboratories requires carbonate reference materials. Here, we report results of an inter‐laboratory comparison study to both assign δ¹¹B_SRM951 and trace element compositions to new synthetic marine carbonate reference materials (RMs), NIST RM 8301 (Coral) and NIST RM 8301 (Foram) and to assess the variance of data among laboratories. Non‐certified reference values and expanded 95% uncertainties for δ¹¹B_SRM951 in NIST RM 8301 (Coral) (+24.17‰ ± 0.18‰) and NIST RM 8301 (Foram) (+14.51‰ ± 0.17‰) solutions were assigned by consensus approach using inter‐laboratory data. Differences reported among laboratories were considerably smaller than some previous inter‐laboratory comparisons, yet discrepancies could still lead to large differences in calculated seawater pH. Similarly, variability in reported trace element information among laboratories (e.g., Mg/Ca ± 5% RSD) was often greater than within a single laboratory (e.g., Mg/Ca < 2%). Such differences potentially alter proxy‐reconstructed seawater temperature by more than 2 °C. These now well‐characterised solutions are useful reference materials to help the palaeoceanographic community build a comprehensive view of past ocean changes.     

Modelling the geochemical cycle of boron: Implications for the long-term δ11B evolution of seawater and oceanic crust

The boron geochemical cycle has been simulated using a time-dependent geochemical box model that was coupled to a one-dimension model of seawater–oceanic crust interactions. Boron elemental and isotopic compositions of oceanic rocks as a function of depth were calculated by mass balance, using the temperature and porosity profiles of the crust as well as the available experimental and empirical distribution coefficients and fractionation factors between mineral and water. Ranges of boron elemental and isotopic variations of seawater were calculated for crust–seawater interactions that take place from the ridge-axis to the off-axis closure of the hydrothermal system. The present-day δ¹¹B of seawater (40‰) could represent a steady-state value. However, depending on crustal permeability, lifetime of water–rock interactions, and expansion rate of the oceanic ridge, the δ¹¹B of seawater may vary from 30‰ to 50‰ at the 10 million year scale.Some boron isotope compositions of Cretaceous biogenic carbonates and ophiolitic serpentinites from Oman are comparable to modern rock samples, suggesting that the δ¹¹B of Cretaceous seawater was close to the present-day value. Low δ¹¹B values of some biogenic carbonates cannot be attributed to low pH values of past seawater, but more probably to δ¹¹B variations of seawater or diagenetic alteration by crustal aqueous fluids. Boron isotope composition of hydrothermally altered serpentines could be considered as a promising proxy of the seawater composition.     

An approach to estimate Lower Jurassic seawater oxygen‐isotope composition using δ18O and Mg/Ca ratios of belemnite calcites (Early Pliensbachian,northern Spain)

Palaeotemperature estimates from the oxygen‐isotope compositions of belemnites have been hampered by not knowing ancient seawater isotope compositions well enough. We have tackled this problem using Mg/Ca as a proxy for temperature and here, we present a ~2 Ma record of paired Mg/Ca and δ¹⁸O measurements of Jurassic (Early Pliensbachian) belemnites from the Asturian basin as a palaeo‐proxy of seawater oxygen‐isotope composition. From the combined use of the two approaches, we suggest a δ¹⁸O_w composition of about ?0.1‰ for the Jamesoni–Ibex zones. This value may have been increased by about 0.6‰ during the Davoei Zone due to the effect of waters with a different δ¹⁸O_w composition. These findings illustrate the inaccuracy of using a globally homogeneous ice‐free value of δ¹⁸O_w = ?1‰ for δ¹⁸O_carb‐based palaeotemperature reconstructions. Our data suggest that previous palaeotemperatures calculated in the region from δ¹⁸O values of belemnites may have been underestimated as the seawater oxygen isotopic composition could have been higher.     

Antiphase hydrogen and oxygen isotope periodicity in chert nodules

Oxygen and hydrogen isotope analyses were made of Jurassic-age chert nodules from the Holy Cross Mountains, SE Poland, along radial transects at high spatial resolution. There is a radial “sigmoidal” periodicity for both isotope ratios, but the two are out of phase, with high δD values corresponding to low δ¹⁸O values. Periodicity for a 100- to 120-mm diameter nodule is approximately 16 mm, increasing slightly toward the rim, with amplitudes approaching 20 and 3.0‰ for hydrogen and oxygen, respectively. The combined hydrogen-oxygen isotope data for one nodule fall on a published curve for chert forming in equilibrium with seawater (Knauth and Epstein, 1976); the range of delta values corresponds to temperature variations of ∼10°C. Data for a second chert fall on a subparallel δD-δ¹⁸O line with δD values that are almost 50‰ lower. The δD-δ¹⁸O patterns for the nodules cannot be explained by periodic mixing of meteoric and ocean water because the hydrogen and oxygen isotope data are out of phase. Two possible explanations for the antiphase periodicity are (a) cyclical temperature variations, perhaps related to an unstable convection system (e.g., Bolton et al., 1999), and (b) self-organizing catalytic precipitation (e.g., Wang and Merino, 1990). The systematic isotopic variations are difficult to explain by diagenesis and strongly suggest that primary isotopic compositions are preserved. The isotopic data provide important information on the thermal history of the sedimentary basin, if temperature variations are the cause of the isotopic periodicity.     

Re-evaluating boron speciation in biogenic calcite and aragonite using B MAS NMR

Understanding the partitioning of aqueous boron species into marine carbonates is critical for constraining the boron isotope system for use as a marine pH proxy. Previous studies have assumed that boron was incorporated into carbonate through the preferential uptake of tetrahedral borate B(OH)₄⁻. In this study we revisit this assumption through a detailed solid state ¹¹B magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic study of boron speciation in biogenic and hydrothermal carbonates. Our new results contrast with those of the only previous NMR study of carbonates insofar as we observe both trigonal and tetrahedral coordinated boron in almost equal abundances in our biogenic calcite and aragonite samples. In addition, we observe no strict dependency of boron coordination on carbonate crystal structure. These NMR observations coupled with our earlier re-evaluation of the magnitude of boron isotope fractionation between aqueous species suggest that controls on boron isotope composition in marine carbonates, and hence the pH proxy, are more complex that previously suggested.     

A stable isotope-based approach to tropical dendroclimatology

We describe a strategy for development of chronological control in tropical trees lacking demonstrably annual ring formation, using high resolution δ¹⁸O measurements in tropical wood. The approach applies existing models of the oxygen isotopic composition of alpha-cellulose (Roden et al., 2000), a rapid method for cellulose extraction from raw wood (Brendel et al., 2000), and continuous flow isotope ratio mass spectrometry (Brenna et al., 1998) to develop proxy chronological, rainfall and growth rate estimates from tropical trees lacking visible annual ring structure. Consistent with model predictions, pilot datasets from the temperate US and Costa Rica having independent chronological control suggest that observed cyclic isotopic signatures of several permil (SMOW) represent the annual cycle of local rainfall and relative humidity. Additional data from a plantation tree of known age from ENSO-sensitive northwestern coastal Peru suggests that the 1997-8 ENSO warm phase event was recorded as an 8‰ anomaly in the δ¹⁸O of α-cellulose. The results demonstrate reproducibility of the stable isotopic chronometer over decades, two different climatic zones, and three tropical tree genera, and point to future applications in paleoclimatology.     

Production and Certification of a Unique Set of Isotope and Delta Reference Materials for Boron Isotope Determination in Geochemical,Environmental and Industrial Materials

Isotopic reference materials are essential to enable reliable and comparable isotope data. In the case of boron only a very limited number of such materials is available, thus preventing adequate quality control of measurement results and validation of analytical procedures. To address this situation a unique set of two boron isotope reference materials (ERM‐AE102a and ‐AE104a) and three offset δ¹¹B reference materials (ERM‐AE120, ‐AE121 and ‐AE122) were produced and certified. The present article describes the production and certification procedure in detail. The isotopic composition of all the materials was adjusted by mixing boron parent solutions enriched in ¹⁰B or ¹¹B with a boron parent solution having a natural isotopic composition under full gravimetric control. All parent solutions were analysed for their boron concentration as well as their boron isotopic composition by thermal ionisation mass spectrometry (TIMS) using isotope dilution as the calibration technique. For all five reference materials the isotopic composition obtained on the basis of the gravimetric data agreed very well with the isotopic composition obtained from different TIMS techniques. Stability and homogeneity studies that were performed showed no significant influence on the isotopic composition or on the related uncertainties. The three reference materials ERM‐AE120, ERM‐AE121 and ERM‐AE122 are the first reference materials with natural δ¹¹B values not equal to 0‰. The certified δ¹¹B values are ?20.2‰ for ERM‐AE120, 19.9‰ for ERM‐AE121 and 39.7‰ for ERM‐AE122, each with an expanded uncertainty (k = 2) of 0.6‰. These materials were produced to cover about three‐quarters of the known natural boron isotope variation. The ¹⁰B enriched isotope reference materials ERM‐AE102a and ERM‐AE104a were produced for industrial applications utilising ¹⁰B for neutron shielding purposes. The certified ¹⁰B isotope abundances are 0.29995 for ERM‐AE102a and 0.31488 for ERM‐AE104a with expanded uncertainties (k = 2) of 0.00027 and 0.00028, respectively. Together with the formerly certified ERM‐AE101 and ERM‐AE103 a unique set of four isotope reference materials and three offset δ¹¹B reference materials for boron isotope determination are now available from European Reference Materials.     

Boron isotopic composition of atmospheric precipitations and liquid-vapour fractionations

Boron isotope compositions (δ¹¹B) and B concentrations of rains and snows were studied in order to characterize the sources and fractionation processes during the boron atmospheric cycle. The ¹¹B/¹⁰B ratios of instantaneous and cumulative rains and snows from coastal and continental sites show a large range of variations, from −1.5 ± 0.4 to +26.0 ± 0.5‰ and from −10.2 ± 0.5 to +34.4 ± 0.2‰, respectively. Boron concentrations in rains and snows vary between 0.1 and 3.0 ppb. All these precipitation samples are enriched in ¹⁰B compared to the ocean value (δ¹¹B = +39.5‰). An empirical rain-vapour isotopic fractionation of +31‰ is estimated from three largely independent methods. The deduced seawater-vapour fractionation is +25.5‰, with the difference between the rain and seawater fractionations principally reflecting changes in the speciation of boron in the liquid with ∼100% B(OH)₃ present in precipitations. A boron meteoric water line, δD = 2.6δ¹¹B − 133, is proposed which describes the relationship between δD and δ¹¹B in many, but not all, precipitations. Boron isotopic compositions of precipitations can be related to that of the seawater reservoir by the seawater-vapour fractionation and one or more of (1) the rain-vapour isotopic fractionation, (2) evolution of the δ¹¹B value of the atmospheric vapour reservoir via condensation-precipitation processes (Rayleigh distillation process), (3) any contribution of vapour from the evaporation of seawater aerosols, and (4) any contribution from particulate matter, principally sea salt, continental dust and, perhaps more regionally, anthropogenic sources (burning of biomass and fossil fuels). From the δ¹¹B values of continental precipitations, a sea salt contribution cannot be more than a percent or so of the total B in precipitation over these areas.     

Physiological and isotopic responses of scleractinian corals to ocean acidification

Uptake of anthropogenic CO₂ by the oceans is altering seawater chemistry with potentially serious consequences for coral reef ecosystems due to the reduction of seawater pH and aragonite saturation state (Ω_arag). The objectives of this long-term study were to investigate the viability of two ecologically important reef-building coral species, massive Porites sp. and Stylophora pistillata, exposed to high pCO₂ (or low pH) conditions and to observe possible changes in physiologically related parameters as well as skeletal isotopic composition. Fragments of Porites sp. and S. pistillata were kept for 6-14 months under controlled aquarium conditions characterized by normal and elevated pCO₂ conditions, corresponding to pH_T values of 8.09, 7.49, and 7.19, respectively. In contrast with shorter, and therefore more transient experiments, the long experimental timescale achieved in this study ensures complete equilibration and steady state with the experimental environment and guarantees that the data provide insights into viable and stably growing corals. During the experiments, all coral fragments survived and added new skeleton, even at seawater Ω_arag < 1, implying that the coral skeleton is formed by mechanisms under strong biological control. Measurements of boron (B), carbon (C), and oxygen (O) isotopic composition of skeleton, C isotopic composition of coral tissue and symbiont zooxanthellae, along with physiological data (such as skeletal growth, tissue biomass, zooxanthellae cell density, and chlorophyll concentration) allow for a direct comparison with corals living under normal conditions and sampled simultaneously. Skeletal growth and zooxanthellae density were found to decrease, whereas coral tissue biomass (measured as protein concentration) and zooxanthellae chlorophyll concentrations increased under high pCO₂ (low pH) conditions. Both species showed similar trends of δ¹¹B depletion and δ¹⁸O enrichment under reduced pH, whereas the δ¹³C results imply species-specific metabolic response to high pCO₂ conditions. The skeletal δ¹¹B values plot above seawater δ¹¹B vs. pH borate fractionation curves calculated using either the theoretically derived α_B value of 1.0194 (Kakihana et al. (1977) Bull. Chem. Soc. Jpn.50, 158) or the empirical α_B value of 1.0272 (Klochko et al. (2006) EPSL248, 261). However, the effective α_B must be greater than 1.0200 in order to yield calculated coral skeletal δ¹¹B values for pH conditions where Ω_arag ? 1. The δ¹¹B vs. pH offset from the seawater δ¹¹B vs. pH fractionation curves suggests a change in the ratio of skeletal material laid down during dark and light calcification and/or an internal pH regulation, presumably controlled by ion-transport enzymes. Finally, seawater pH significantly influences skeletal δ¹³C and δ¹⁸O. This must be taken into consideration when reconstructing paleo-environmental conditions from coral skeletons.     

Evidence for ocean acidification in the Great Barrier Reef of Australia

Geochemical records preserved in the long-lived carbonate skeleton of corals provide one of the few means to reconstruct changes in seawater pH since the commencement of the industrial era. This information is important in not only determining the response of the surface oceans to ocean acidification from enhanced uptake of CO₂, but also to better understand the effects of ocean acidification on carbonate secreting organisms such as corals, whose ability to calcify is highly pH dependent. Here we report an ∼200 year δ¹¹B isotopic record, extracted from a long-lived Porites coral from the central Great Barrier Reef of Australia. This record covering the period from 1800 to 2004 was sampled at yearly increments from 1940 to the present and 5-year increments prior to 1940. The δ¹¹B isotopic compositions reflect variations in seawater pH, and the δ¹³C changes in the carbon composition of surface water due to fossil fuel burning over this period. In addition complementary Ba/Ca, δ¹⁸O and Mg/Ca data was obtained providing proxies for terrestrial runoff, salinity and temperature changes over the past 200 years in this region. Positive thermal ionization mass spectrometry (PTIMS) method was utilized in order to enable the highest precision and most accurate measurements of δ¹¹B values. The internal precision and reproducibility for δ¹¹B of our measurements are better than ±0.2‰ (2σ), which translates to a precision of better than ±0.02 pH units. Our results indicate that the long-term pre-industrial variation of seawater pH in this region is partially related to the decadal-interdecadal variability of atmospheric and oceanic anomalies in the Pacific. In the periods around 1940 and 1998 there are also rapid oscillations in δ¹¹B compositions equivalent changes in pH of almost 0.5 U. The 1998 oscillation is co-incident with a major coral bleaching event indicating the sensitivity of skeletal δ¹¹B compositions to loss of zooxanthellate symbionts. Importantly, from the 1940s to the present-day, there is a general overall trend of ocean acidification with pH decreasing by about 0.2-0.3 U, the range being dependent on the value assumed for the fractionation factor α_(B3-B4) of the boric acid and borate species in seawater. Correlations of δ¹¹B with δ¹³C during this interval indicate that the increasing trend towards ocean acidification over the past 60 years in this region is the result of enhanced dissolution of CO₂ in surface waters from the rapidly increasing levels of atmospheric CO₂, mainly from fossil fuel burning. This suggests that the increased levels of anthropogenic CO₂ in atmosphere has already caused a significant trend towards acidification in the oceans during the past decades. Observations of surprisingly large decreases in pH across important carbonate producing regions, such as the Great Barrier Reef of Australia, raise serious concerns about the impact of Greenhouse gas emissions on coral calcification.     

Sr, C, and O isotope geochemistry of Ordovician brachiopods: a major isotopic event around the Middle-Late Ordovician transition

Here we present Sr, C, and O isotope curves for Ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide. These are the first Ordovician-wide isotope curves that can be placed within the newly emerging global biostratigraphic framework. A total of 182 brachiopods were selected for C and O isotope analysis, and 122 were selected for Sr isotope analysis. Seawater ⁸⁷Sr/⁸⁶Sr decreased from 0.7090 to 0.7078 during the Ordovician, with a major, quite rapid fall around the Middle-Late Ordovician transition, most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates. Mean δ¹⁸O values increase from −10‰ to −3‰ through the Ordovician with an additional short-lived increase of 2 to 3‰ during the latest Ordovician due to glaciation. Although diagenetic alteration may have lowered δ¹⁸O in some samples, particularly those from the Lower Ordovician, maximum δ¹⁸O values, which are less likely to be altered, increase by more than 3‰ through the Ordovician in both our data and literature data. We consider that this long-term rise in calcite δ¹⁸O records the effect of decreasing tropical seawater temperatures across the Middle-Late Ordovician transition superimposed on seawater δ¹⁸O that was steadily increasing from ≤−3‰ standard mean ocean water (SMOW). By contrast, δ¹³C variation seems to have been relatively modest during most of the Ordovician with the exception of the globally documented, but short-lived, latest Ordovician δ¹³C excursion up to +7‰. Nevertheless, an underlying trend in mean δ¹³C can be discerned, changing from moderately negative values in the Early Ordovician to moderately positive values by the latest Ordovician. These new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 Ma. The juxtaposition of the greatest recorded swings in Phanerozoic seawater ⁸⁷Sr/⁸⁶Sr and δ¹⁸O at the same time as one of the largest marine transgressions in Phanerozoic Earth history suggests a causal link between tectonic and climatic change, and emphasizes an endogenic control on the O isotope budget during the Early Paleozoic. Better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes. We recommend that future work on Ordovician isotope stratigraphy focus on this outstanding Middle-Late Ordovician event.     

Isotopic and element exchange during serpentinization and metasomatism at the Atlantis Massif (MAR 30°N): Insights from B and Sr isotope data

The Lost City hydrothermal system at the southern Atlantis Massif (Mid-Atlantic Ridge, 30°N) provides a natural laboratory for studying serpentinization processes, the temporal evolution of ultramafic-hosted hydrothermal systems, and alteration conditions during formation and emplacement of an oceanic core complex. Here we present B, O, and Sr isotope data to investigate fluid/rock interaction and mass transfer during detachment faulting and exhumation of lithospheric sequences within the Atlantis Massif. Our data indicate that extensive serpentinization was a seawater-dominated process that occurred predominately at temperatures of 150-250 °C and at high integrated W/R ratios that led to a marked boron enrichment (34-91 ppm). Boron removal from seawater during serpentinization is positively correlated with changes in δ¹¹B (11-16‰) but shows no correlation with O-isotope composition. Modeling indicates that B concentrations and isotope values of the serpentinites are controlled by transient temperature-pH conditions. In contrast to prior studies, we conclude that low-temperature marine weathering processes are insignificant for boron geochemistry of the Atlantis Massif serpentinites. Talc- and amphibole-rich fault rocks formed within a zone of detachment faulting at temperatures of approximately 270-350 °C and at low W/R ratios. Talc formation in ultramafic domains in the massif was subsequent to an early stage of serpentinization and was controlled by the access of Si-rich fluids derived through seawater-gabbro interactions. Replacement of serpentine by talc resulted in boron loss and significant lowering of δ¹¹B values (9-10‰), which we model as the product of progressive extraction of boron. Our study provides new constraints on the boron geochemical cycle at oceanic spreading ridges and suggests that serpentinization associated with ultramafic-hosted hydrothermal systems may have important implications for the behavior of boron in subduction zone settings.     

Sulfur and oxygen isotope study of sulfate reduction in experiments with natural populations from Fællestrand, Denmark

This study investigates the sulfur and oxygen isotope fractionations of dissimilatory sulfate reduction and works to reconcile the relationships between the oxygen and sulfur isotopic and elemental systems. We report results of experiments with natural populations of sulfate-reducing bacteria using sediment and seawater from a marine lagoon at Fællestrand on the northern shore of the island of Fyn, Denmark. The experiments yielded relatively large magnitude sulfur isotope fractionations for dissimilatory sulfate reduction (up to approximately 45‰ for ³⁴S/³²S) with higher δ¹⁸O accompanying higher δ³⁴S, similar to that observed in previous studies. The seawater used in the experiments was spiked by addition of ¹⁷O-labeled water and the ¹⁷O content of residual sulfate was found to depend on the fraction of sulfate reduced in the experiments. The ¹⁷O data provides evidence for recycling of sulfur from metabolic intermediates and for an ¹⁸O/¹⁶O fractionation of ∼25-30‰ for dissimilatory sulfate reduction. The close correlation between the ¹⁷O data and the sulfur isotope data suggests that isotopic exchange between cell water and external water (reactor water) was rapid under experimental conditions. The molar ratio of oxygen exchange to sulfate reduction was found to be about 2.5. This value is slightly lower than observed in studies of natural ecosystems [e.g., Wortmann U. G., Chernyavsky B., Bernasconi S. M., Brunner B., Böttcher M. E. and Swart P. K. (2007) Oxygen isotope biogeochemistry of pore water sulfate in the deep biosphere: dominance of isotope exchange reactions with ambient water during microbial sulfate reduction (ODP Site 1130). Geochim. Cosmochim. Acta71, 4221-4232]. Using recent models of sulfur isotope fractionations we find that our combined sulfur and oxygen isotopic data places constraints on the proportion of sulfate recycled to the medium (78-96%), the proportion of sulfur intermediate sulfite that was recycled by way of APS to sulfate and released back to the external sulfate pool (∼70%), and also that a fraction of the sulfur intermediates between sulfite and sulfide were recycled to sulfate. These parameters can be constrained because of the independent information provided by δ¹⁸O, δ³⁴S, δ¹⁷O labels, and Δ³³S.     

Molybdenum isotope evidence for mild environmental oxygenation before the Great Oxidation Event

We use the molybdenum isotope paleoredox proxy to look for evidence of small amounts of O₂ in the environment ∼50 Ma before the Great Oxidation Event (GOE) in a high resolution profile from the ∼2.5 Ga Mt. McRae Shale. The molybdenum isotope compositions (δ^98/95Mo) from samples throughout the sequence span a range from 0.99‰ to 1.86‰. All samples have heavier δ^98/95Mo compared to average upper continental crust. In addition, the upper (S1) and lower (S2) black shale units within the Mt. McRae Shale exhibit systematic differences in average isotopic compositions and distinct patterns of δ^98/95Mo variation. Heavier δ^98/95Mo values occur in the S1 unit, where δ^98/95Mo correlates with Mo enrichments. In the S2 unit, δ^98/95Mo is not as heavy and is relatively invariant.Based on sedimentary Fe proxies we infer that S1 sediments were deposited under euxinic conditions, so that Mo removal from bottom waters was likely quantitative. Thus, δ^98/95Mo in this interval likely records coeval seawater. The lighter δ^98/95Mo values in the S2 unit may indicate a less fractionated ocean Mo inventory relative to the S1 unit. However, sedimentary Fe proxies suggest that S2 sediments accumulated under a water column that was ferruginous rather than euxinic, raising the possibility of non-quantitative Mo scavenging and hence an expressed δ^98/95Mo fractionation relative to coeval seawater. Because any associated fractionations during this process would have favored the light isotope in sediments, the lighter δ^98/95Mo values in the S2 unit represent a lower limit on the value in contemporaneous seawater.After evaluating a range of hypotheses, we conclude that the isotopically heavy δ^98/95Mo values seen throughout the Mt. McRae Shale likely reflect the effects of oxidative weathering and adsorption of Mo to oxide mineral surfaces on land or in surface oceans. The extent of environmental oxygenation in either unit is difficult to assess due to uncertainties in the global Mo isotope budget. Because of the small ocean Mo inventory in the Late Archean, documented by low concentrations of Mo and low Mo/TOC, the extent of oxygenation required to account for the observed fractionations is much smaller than in modern oceans. However, when juxtaposed against the record of δ^98/95Mo through time, our findings provide further evidence of the onset of environmental oxygenation before the GOE.