Isotopic disequilibrium in brachiopods disentangled with dual clumped isotope thermometry

The stable oxygen and clumped isotope composition of brachiopod calcite are important proxies for the reconstruction of Phanerozoic seawater temperatures and delta O-18 values. The utility of brachiopods as a temperature archive is nonetheless challenged by indications that their shells precipitate out of isotopic equilibrium with ambient seawater, though the origin of disequilibrium effects has remained elusive. Here, we apply the recently developed dual clumped isotope thermometer (i.e., the simultaneous measurement of Delta(47) and Delta(48) values in CO2 derived from the acid digestion of carbonates) to modern and fossil brachiopods to investigate disequilibrium signatures recorded in brachiopod calcite. We present evidence that disequilibrium signatures are derived from the combination of two rate-limiting processes: the hydration/hydroxylation of CO2, and, potentially, the diffusion of HCO3-/CO32- in water. An empirical correction for clumped isotope disequilibrium is presented, based on correlation between measured disequilibrium offsets in the Delta(47) and Delta(48) values of modern brachiopods (Delta(47)(disequilibrium) = -0.88 x Delta(48) (disequilibrium) + 0.02). Dual clumped isotope thermometry of Eocene age brachiopods from Seymour Island (similar to 65 degrees S) yields temperatures in agreement with previously reported Delta(47) derived temperatures from coeval bivalves. In contrast, uncorrected Delta(47) derived temperatures from the same brachiopods are 6-11 degrees C colder. Measurement of dual clumped isotope composition alongside delta O-18 values of brachiopod carbonate should also enable more accurate reconstruction of seawater-delta O-18. In comparison with group-specific Delta(47)-temperature calibrations, which correct for an average kinetic bias in Delta(47), dual clumped isotope thermometry accounts for the magnitude of disequilibrium inherent to an individual sample, facilitating more reliable paleotemperature and seawater-delta O-18 reconstructions.