"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.Our theoretical calculations for carbonate minerals indicate equilibrium lattice calcite values for δ47 and δ18O are intermediate between HCO3- and CO32-. We analyzed synthetic calcites grown at temperatures ranging from 0.5 to 50°C with and without the enzyme carbonic anhydrase present. This enzyme catalyzes oxygen isotopic exchange between DIC species and is present in many natural systems. The two types of experiments yielded statistically indistinguishable results, and these measurements yield a calibration that overlaps with our theoretical predictions for calcite at equilibrium. The slow-growing Devils Hole calcite exhibits δ47 and δ18O values consistent with lattice equilibrium.Factors influencing DIC speciation (pH, salinity) and the timescale for DIC equilibration, as well as reactions at the mineral-solution interface, have the potential to influence clumped-isotope signatures and the δ18O of carbonate minerals. In fast-growing carbonate minerals, solution chemistry may be an important factor, particularly over extremes of pH and salinity. If a crystal grows too rapidly to reach an internal equilibrium (i.e., achieve the value for the temperature-dependent mineral lattice equilibrium), it may record the clumped-isotope signature of a DIC species (e.g., the temperature-dependent equilibrium of HCO3-) or a mixture of DIC species, and hence record a disequilibrium mineral composition. For extremely slow-growing crystals, and for rapidly-grown samples grown at a pH where HCO3- dominates the DIC pool at equilibrium, effects of solution chemistry are likely to be relatively small or negligible. In summary, growth environment, solution chemistry, surface equilibria, and precipitation rate may all play a role in dictating whether a crystal achieves equilibrium or disequilibrium clumped-isotope signatures.
Bibliographical noteFunding Information:
AKT thanks the reviewers and editor for their comments, as well as Kate Ledger, Hagit Affek, Tobias Kluge, James Watkins, Jim Rustad, William Casey, Oleg Pokrovsky, Ian Fairchild, Bruce Watson, Henry Teng, Philippe Van Cappellen, Adrian Villegas-Jimenez, Andreas Luttge, Bernhardt Trout, Michael Reddy, Dan Breecker, Andrew Dickson, Gideon Henderson, Frank Millero, Justine Kimball, and Chris Roberts for discussions relevant to this work. AKT and PSH thank Ben Elliott, Anastassia Alexandrova, and Ben Schwartz for input, and Fernando R. Clemente of Gaussian, Inc., for helpful suggestions with Gaussian09. AKT acknowledges support from the Department of Energy through BES grant DE-FG02-13ER16402 , a UCLA Career Development Award, a Hellman Fellowship, NSF grants EAR-0949191 , EAR-1325054 , ARC-1215551 , and ACS grant # 51182-DNI2 . RAE and JBR acknowledge support from NSF grant OCE-1437166 . REZ was supported by NSF grant OCE09-27089 . JBR acknowledges support from NSF grant OCE-1357665 and NOAA grant NA13OAR4310186 . The support of the U.S. Geological Survey National Research Program made this article possible. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
© 2015 Elsevier Ltd.