The dual-clumped isotope thermometer as a tool to identify and correct for isotopic disequilibrium in the clumped isotope composition of biogenic carbonate

The clumped isotope composition (∆47) of marine biogenic carbonates is an increasingly applied proxy for temperature reconstruction. The clumped isotope measurement is particularly valuable as ∆47 is independent of the δ18O of seawater. However, kinetic effects during biomineralization can impact the ∆47 of some marine archives, impeding accurate paleotemperature reconstruction.

Dual carbonate clumped isotope thermometry (i.e., simultaneous ∆47 and ∆­48 measurements on a single carbonate) has the potential to identify kinetically driven isotopic disequilibrium in biogenic carbonate and facilitate the reconstruction of carbonate formation temperature independent of kinetic biases. We present the dual-clumped isotope compositions of several key marine archives, historically used to reconstruct changes in sea surface temperature and δ18O across the Phanerozoic. These include brachiopods, cold and warm-water corals, and belemnites.

Corals and brachiopods exhibit disequilibrium ∆47 and ∆48, corresponding kinetic effects relating to mixing of an equilibrium DIC pool with kinetically derived HCO3- produced by hydration and hydroxylation of CO2. We demonstrate how measurement of the dual clumped isotope composition of corals and brachiopods can be used to correct for kinetic bias in ∆47, yielding more accurate temperature reconstructions (<±3°C at the 95% confidence interval). Dual clumped isotope thermometry, therefore, (re-)opens brachiopod and coral archives for reconstruction of shallow and intermediate water mass temperatures on geological timescales. Furthermore, we confirm that belemnites precipitate calcite in dual-clumped isotope equilibrium yielding precise growth temperatures, thus belemnite ∆47 may be reliably used as a proxy for Mesozoic sea surface temperatures (<±2.5°C at the 95% confidence interval).