Coccolithophorids precipitate carbonate in clumped isotope equilibrium with seawater

Abstract. Numerous recent studies have tested the clumped isotope (Δ₄₇) thermometer on a variety of biogenic carbonates such as foraminifera and bivalves and showed that all follow a common calibration. While the sample size requirements for a reliable Δ₄₇ measurement have decreased over the years, the availability and preservation of many biogenic carbonates is still limited and/or require substantial time to be extracted from sediments in sufficient amounts. We thus determined the Δ₄₇-temperature relationship for coccolith carbonate, which is abundant and often well-preserved in sediments. The carbon and oxygen isotopic compositions of coccolith calcite have limited use in palaeoenvironmental reconstructions due to physiological effects that cause variability in the carbon and oxygen isotopic values. However, the relatively limited data available suggest that clumped isotopes may not be influenced by these effects. We cultured three species of coccolithophores in well-constrained carbonate system conditions with a CO_2(aq) between 5 and 45 μM and temperatures between 6 °C and 27 °C.

Our results agree with a previous culture study that there are no species- or genus-specific effects on the Δ₄₇-temperature relationship in coccolithophores and find that varying environmental parameters other than temperature also do not have a significant effect. Our coccolith-specific Δ₄₇-temperature calibration agrees within ±1 °C of other biogenic carbonate calibrations yet all biogenic carbonate calibrations show a consistent offset of ≥2 °C relative to the inorganic carbonate calibrations. Thus, all biogenic specific calibrations can be used interchangeably and must be used for the reconstruction of calcification temperatures in biogenic carbonates.