Carbonate clumped isotope compositions of modern marine mollusk and brachiopod shells

Abstract

We present an empirical calibration of the carbonate clumped isotope thermometer based on mollusk and brachiopod shells from natural and controlled environments spanning water temperatures of −1.0 to 29.5°C. The clumped isotope data (Δ47) are normalized to CO2 gases with equilibrium distributions of clumped isotopologues at high temperature (1000°C) and low temperature (27 or 30°C), and thus the calibration is unique in being directly referenced to a carbon dioxide equilibrium reference frame (Dennis et al., 2011, Defining an absolute reference frame for clumped isotope studies of CO2, Geochimica et Cosmochimica Acta, 75, 7117–7131). The shell clumped isotope data define the following relation as a function of temperature (in kelvin):Δ47=0.0327×106/T2+0.3286(r2=0.84).The temperature sensitivity (slope) of this relation is lower than those based on corals, fish otoliths, foraminifera, and coccoliths, but is similar to theoretical predictions for calcite based on lattice dynamics calculations. We find no convincing methodological or biological explanations for the difference in temperature sensitivity between this calibration and the previous calibrations, and suggest that the discrepancy might represent real but unknown differences in mineral–DIC clumped isotope fractionation between mollusks/brachiopods and other taxa. Nevertheless, revised analytical methods similar to those used in this study are now in wide use, and it will be important to develop calibrations for other taxonomic groups using these updated methods, with analyses directly referenced to the carbon dioxide equilibrium reference frame.