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Beyond temperature: Clumped isotope signatures in dissolved inorganic carbon species and the influence of solution chemistry on carbonate mineral composition
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2015
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Source: Geochimica et Cosmochimica Acta, 166: 344-374
[PDF-3.18 MB]
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Journal Title:Geochimica et Cosmochimica Acta
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Description:“Clumped-isotope” thermometry is an emerging tool to probe the temperature history of surface and subsurface environments based on measurements of the proportion of 13C and 18O isotopes bound to each other within carbonate minerals in 13C18O16O22− groups (heavy isotope “clumps”). Although most clumped isotope geothermometry implicitly presumes carbonate crystals have attained lattice equilibrium (i.e., thermodynamic equilibrium for a mineral, which is independent of solution chemistry), several factors other than temperature, including dissolved inorganic carbon (DIC) speciation may influence mineral isotopic signatures. Therefore we used a combination of approaches to understand the potential influence of different variables on the clumped isotope (and oxygen isotope) composition of minerals.
We conducted witherite precipitation experiments at a single temperature and at varied pH to empirically determine 13C–18O bond ordering (Δ47) and δ18O of CO32− and HCO3− molecules at a 25 °C equilibrium. Ab initio cluster models based on density functional theory were used to predict equilibrium 13C–18O bond abundances and δ18O of different DIC species and minerals as a function of temperature. Experiments and theory indicate Δ47 and δ18O compositions of CO32− and HCO3− ions are significantly different from each other. Experiments constrain the Δ47–δ18O slope for a pH effect (0.011 ± 0.001; 12 ⩾ pH ⩾ 7). Rapidly-growing temperate corals exhibit disequilibrium mineral isotopic signatures with a Δ47–δ18O slope of 0.011 ± 0.003, consistent with a pH effect.
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Source:Geochimica et Cosmochimica Acta, 166: 344-374
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Rights Information:Accepted Manuscript
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