Kinetics of CO_2(g)-H_2O(l) isotopic exchange, including ^(13)C^(18)O^(16)O

The analysis of mass 47 isotopologues of CO_2 (mainly ^(13)C^(18)O^(16)O) is established as a constraint on sources and sinks of environmental CO_2, complementary to δ^(13)C and δ^(18)O constraints, and forms the basis of the carbonate ‘clumped isotope’ thermometer. This measurement is commonly reported using the Δ_(47) value — a measure of the enrichment of doubly substituted CO_2 relative to a stochastic isotopic distribution. Values of Δ_(47) approach 0 (a random distribution) at high temperatures (≥ several hundred degrees C), and increase with decreasing temperature, to ~1 ‰ at 25 °C

The hydrogen isotopic composition and water content of southern Pacific MORB: A reassessment of the D/H ratio of the depleted mantle reservoir

In this paper, we re-investigate the isotopic composition of hydrogen in MORB and the possible effects of contamination on δD and water content. A suite of 40 N-MORB from the Pacific–Antarctic ridge, far from any hotspot, was analyzed for chlorine content by electron microprobe and for water content and δD with silica tubes. Cl concentrations (from 29 to 2400 ppm) indicate widespread contamination, more intense with faster spreading rates, while water contents (from 840 to 7800 ppm) are mainly controlled by igneous processes. δD values range from −76 to −48‰−48‰, with an average value of −61‰−61‰. The lack of correlation between Cl content and either H_2O/Ce or δD indicate that contamination has a negligible effect on δD for our samples, which is therefore characteristic of the mantle below the Pacific–Antarctic ridge. We suggest that the 20‰ lower δD value reported for the North Pacific and North Atlantic is highly unlikely from geodynamical arguments. We propose that the convecting mantle is characterized by a δD of −60±5‰−60±5‰, as supported by the most recent data from North Atlantic N-MORB

The MAT-253 Ultra — a novel high-resolution, multi-collector gas source mass spectrometer

We present the design, performance and representative applications of the MAT 253 Ultra – the first prototype of a new class of high-resolution gas source isotope ratio mass spectrometers

B(OH)4- and CO32- do not compete for incorporation into aragonite in synthetic precipitations at pHtotal 8.20 and 8.41 but do compete at pHtotal 8.59

This work was supported by the UK Natural Environment Research Council (NE/S001417/1) to NA, KP, RK, MC and AF. We thank Gavin Peters, University of St Andrews, for assistance with BET analyses and Adam Kerrigan, University of York, for support with scanning electron microscopy.Coral skeletal B/Ca (effectively B/CO32–), in combination with boron isotopic composition (δ11B), has been used to reconstruct the dissolved inorganic carbon chemistry of coral calcification media and to explore the biomineralisation process and its response to ocean acidification. This approach assumes that B(OH)4−, the B species incorporated into aragonite, competes with dissolved inorganic carbon species for inclusion in the mineral lattice. In this study we precipitated aragonite from seawater in vitro under conditions that simulate the compositions of the calcification media used to build tropical coral skeletons. To deconvolve the effects of pH and [CO32–] on boron incorporation we conducted multiple experiments at constant [CO32–] but variable pH and at constant pH but variable [CO32–], both in the absence and presence of common coral skeletal amino acids. Large changes in solution [CO32–], from 1000 µmol kg−1, or in precipitation rate, have no significant effect on aragonite B/Ca at pHtotal of 8.20 and 8.41. A significant inverse relationship is observed between solution [CO32–] and aragonite B/Ca at pHtotal = 8.59. Aragonite B/Ca is positively correlated with seawater pH across precipitations conducted at multiple pH but this relationship is driven by the effect of pH on the abundance of B(OH)4– in seawater. Glutamic acid and glycine enhance the incorporation of B in aragonite but aspartic acid has no measurable effect. Normalising aragonite B/Ca to solution [B(OH)4–] creates KDB(OH)4− which do not vary significantly between pH treatments. This implies that B(OH)4– and CO32– do not compete with each other for inclusion in the aragonite lattice at pHtotal 8.20 and 8.41. Only at high pH (8.59), when [B(OH)4–] is high, do we observe evidence to suggest that the 2 anions compete to be incorporated into the lattice. These high pH conditions represent the uppermost limits reliably measured in the calcification media of tropical corals cultured under present day conditions, suggesting that skeletal B/Ca may not reflect the calcification media dissolved inorganic carbon chemistry in all modern day corals.Peer reviewe

Insights into the response of coral biomineralisation to environmental change from aragonite precipitations in vitro

This work was supported by the UK Natural Environment Research Council (NE/S001417/1) to NA, KP, RK, MC and AF. We thank Gavin Peters, University of St Andrews, for assistance with BET analyses. Electron microscopy was carried out in the Aberdeen Centre for Electron Microscopy, Analysis and Characterisation (ACEMAC).Precipitation of marine biogenic CaCO3 minerals occurs at specialist sites, typically with elevated pH and dissolved inorganic carbon, and in the presence of biomolecules which control the nucleation, growth, and morphology of the calcium carbonate structure. Here we explore aragonite precipitation in vitro under conditions inferred to occur in tropical coral calcification media under present and future atmospheric CO2 scenarios. We vary pH, ΩAr and pCO2 between experiments to explore how both HCO3- and CO32- influence precipitation rate and we identify the effects of the three most common amino acids in coral skeletons (aspartic acid, glutamic acid and glycine) on precipitation rate and aragonite morphology. We find that fluid ΩAr or [CO32-] is the main control on precipitation rate at 25°C, with no significant contribution from HCO3- or pH. All amino acids inhibit aragonite precipitation at 0.2-5 mM and the degree of inhibition is inversely correlated with ΩAr and, in the case of aspartic acid, also inversely correlated with seawater temperature. Aspartic acid inhibits precipitation the most, of the tested amino acids (and generates changes in aragonite morphology) and glycine inhibits precipitation the least. Previous work shows that ocean acidification increases the amino acid content of coral skeletons and probably reduces calcification media ΩAr, both of which can inhibit aragonite precipitation. This study and previous work shows aragonite precipitation rate is exponentially related to temperature from 10-30°C and small anthropogenic increases in seawater temperature will likely offset the inhibition in precipitation rate predicted to occur due to increased skeletal aspartic acid and reduced calcification media ΩAr under ocean acidification.Publisher PDFPeer reviewe

Contrasting the effects of aspartic acid and glycine in free amino acid and peptide form on the growth rate, morphology, composition and structure of synthetic aragonites

Funding: This work was supported by the UK Natural Environment Research Council (NE/S001417/1) to NA, KP, RK, MC, and AF. Raman analyses were supported by the EPSRC Light Element Analysis Facility Grant EP/T019298/1 and EPSRC Strategic Equipment Resource Grant EP/R023751/1 at the University of St Andrews.Corals and mollusks produce aragonite skeletons and shells containing highly acidic proteins, rich in aspartic acid (Asp) and glycine (Gly). These biomolecules are pivotal in controlling biomineral formation. We explore the effects of l-Asp, Gly, and two peptides: glycyl-l-aspartic acid (Gly-Asp) and tetra-aspartic acid (Asp4) on the precipitation rate, crystal morphology, and CO3 group rotational disorder (inferred from Raman spectroscopy) in aragonite precipitated in vitro at the approximate pH, [Ca2+], and Ωar occurring in coral calcification media. All of the biomolecules, except Gly, inhibit aragonite precipitation. Biomolecules are incorporated into the aragonite and create CO3 group rotational disorder in the following order: Asp4 > Asp = Gly-Asp > Gly. Asp4 inhibits aragonite precipitation more than Asp at comparable solution concentrations, but Asp reduces aragonite precipitation more effectively than Asp4 for each Asp residue incorporated into the aragonite. At the highest solution concentration, the molar ratio of Asp4:CaCO3 in the aragonite is 1:690. We observe a significant inverse relationship between the aragonite precipitation rate and aragonite Raman spectrum ν1 peak fwhm across the entire data set. Tetra-aspartic acid inhibits aragonite precipitation at all concentrations, suggesting that the aspartic acid-rich domains of coral skeletal proteins influence biomineralization by suppressing mineral formation, thereby shaping skeletal morphology and preventing uncontrolled precipitation.Peer reviewe

Insights into the response of coral biomineralisation to environmental change from aragonite precipitations in vitro

Funding: This work was supported by the UK Natural Environment Research Council (NE/S001417/1) to NA, KP, RK, MC and AF. We thank Gavin Peters, University of St Andrews, for assistance with BET analyses. Electron microscopy was carried out in the Aberdeen Centre for Electron Microscopy, Analysis and Characterisation (ACEMAC).Precipitation of marine biogenic CaCO3 minerals occurs at specialist sites, typically with elevated pH and dissolved inorganic carbon, and in the presence of biomolecules which control the nucleation, growth, and morphology of the calcium carbonate structure. Here we explore aragonite precipitation in vitro under conditions inferred to occur in tropical coral calcification media under present and future atmospheric CO2 scenarios. We vary pH, ΩAr and pCO2 between experiments to explore how both HCO3- and CO32- influence precipitation rate and we identify the effects of the three most common amino acids in coral skeletons (aspartic acid, glutamic acid and glycine) on precipitation rate and aragonite morphology. We find that fluid ΩAr or [CO32-] is the main control on precipitation rate at 25°C, with no significant contribution from HCO3- or pH. All amino acids inhibit aragonite precipitation at 0.2-5 mM and the degree of inhibition is inversely correlated with ΩAr and, in the case of aspartic acid, also inversely correlated with seawater temperature. Aspartic acid inhibits precipitation the most, of the tested amino acids (and generates changes in aragonite morphology) and glycine inhibits precipitation the least. Previous work shows that ocean acidification increases the amino acid content of coral skeletons and probably reduces calcification media ΩAr, both of which can inhibit aragonite precipitation. This study and previous work shows aragonite precipitation rate is exponentially related to temperature from 10-30°C and small anthropogenic increases in seawater temperature will likely offset the inhibition in precipitation rate predicted to occur due to increased skeletal aspartic acid and reduced calcification media ΩAr under ocean acidification.Publisher PDFPeer reviewe

Methane Clumped Isotopes: Progress and Potential for a New Isotopic Tracer

The isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric greenhouse gas concentrations, the global carbon cycle, and life in extreme environments. Recent analytical developments focusing on multiply substituted isotopologues (‘clumped isotopes’) are opening a valuable new window into methane geochemistry. When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions. We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism

The influence of seawater pCO2 and temperature on the amino acid composition and aragonite CO3 disorder of coral skeletons

Funding: This work was supported by the Leverhulme Trust (Research project Grant 2015-268 to NA, RK, and KP) and the UK Natural Environment Research Council (NE/G015791/1 to NA and AAF; NE/S001417/1 to NA, KP, RK, MC and AAF). The Raman microscope at the University of St. Andrews is supported by the EPSRC Light Element Analysis Facility Grant EP/T019298/1 and the EPSRC Strategic Equipment Resource Grant EP/R023751/1.Coral skeletons are composites of aragonite and biomolecules. We report the concentrations of 11 amino acids in massive Porites spp. coral skeletons cultured at two temperatures (25°C and 28°C) and three seawater pCO2 (180, 400 and 750 µatm). Coral skeletal aspartic acid/asparagine (Asx), glutamic acid/glutamine (Glx), glycine, serine and total amino acid concentrations are significantly higher at 28°C than at 25°C. Skeletal Asx, Glx, Gly, Ser, Ala, L-Thr and total amino acid are significantly lower at 180 µatm seawater pCO2 compared to 400 µatm and Ser is reduced at 180 µatm compared to 750 µatm. Concentrations of all skeletal amino acids are significantly inversely related to coral calcification rate but not to calcification media pH. Raman spectroscopy of these and additional specimens indicates that CO3 disorder in the skeletal aragonite lattice is not affected by seawater pCO2 but decreases at the higher temperature. This is contrary to observations in synthetic aragonite where disorder is positively related to the aragonite precipitation rate mediated by either increasing temperature (this study) or increasing Ω (this study and a previous report) and to the concentration of amino acid in the precipitation media (a previous report). We observe no significant relationship between structural disorder and coral calcification rate or skeletal [amino acid]. Both temperature and seawater pCO2 can significantly affect skeletal amino acid composition and further work is required to clarify how environmental change mediates disorder.Peer reviewe

Clumped isotope and Δ17O measurements of carbonates in CM carbonaceous chondrites: new insights into parent body thermal and fluid evolution

The CM carbonaceous chondrites are key archives for understanding the earliest history of the solar system. Their C-complex asteroid parent body(ies) underwent aqueous alteration, among the products of which are carbonate minerals that can faithfully record the conditions of their formation. In this study we report carbon, triple oxygen and clumped isotope compositions of carbonates in six CM chondrites which span a range in degrees of aqueous alteration (Allan Hills 83100, Cold Bokkeveld, LaPaz Icefield 031166, Lonewolf Nunataks 94101, Murchison, Scott Glacier 06043). Δ¹⁷O values range from −2.6 to −1.0 ‰ (±0.1), and where calcite and dolomite co-exist their Δ¹⁷O differ by 0.6 permil, suggesting precipitation from distinct fluids. Calculated crystallization temperatures range from 5 to 51 °C for calcite (typically ± 10 °C) and 75 to 101(±15) °C for dolomite. The δ¹⁸ᴼVSMOW of the aqueous fluids from which they formed ranges from −6.6 to 2.3 ‰, with no relationship to the δ¹³C of carbonates. As the population of carbonates in any one CM chondrite can include multiple generations of grains that formed at different conditions, these values represent the mode of the temperature of carbonate formation for each meteorite. We observe that in the more altered meteorites carbonate Δ¹⁷O values are lower and formation temperatures are higher. These correlations are consistent with aqueous alteration of the CM chondrites being a prograde reaction whereby the hotter fluids had undergone greater isotope exchange with the anhydrous matrix. Our data are broadly consistent with the closed system model for water/rock interaction, but carbonate mineral formation in the latter stages of aqueous alteration may be linked to fluid movement via fractures