Most bivalves and gastropods calcify indistinguishably from dual clumped isotope equilibrium

Molluscan shell-carbonates are extensively used to reconstruct paleo-temperatures at sub-annual resolution. The accurate application of two widely used temperature proxies, the shell carbonate oxygen isotope (ẟ 18 O) and carbonate clumped isotope (Δ 47 ) composition, is based on the assumption that kinetics in the DIC-H 2 O-CaCO 3 system were either absent or invariant during shell formation, and thus can be corrected for using empirical calibrations. Here, we analysed the dual clumped isotope composition, Δ 47 and Δ 48 , of a wide range of modern and Eocene molluscs (bivalves and gastropods) to investigate the potential importance of kinetics during molluscan biomineralisation. We show that Δ 47 and Δ 48 values of most of our modern samples are indistinguishable from equilibrium. For these samples, Δ 47 -derived temperatures conform to corresponding growth temperatures within their fully propagated 95 % uncertainties of ≤ +/-2.3 °C. Significant departures from equilibrium values are only noted for a single gastropod specimen characterised by a growth temperature < 10 °C. Together, these results strongly imply that bivalve and gastropod shell carbonates represent key archives for accurate and highly precise reconstructions of sea surface temperatures by means of clumped isotope thermometry. Kinetic biases on this thermometer, if relevant at all, may only become important at relatively low temperatures. Δ 47 -derived temperatures for our Eocene samples (∼39 Ma) from the Hampshire Basin (paleo-latitude ∼ 40°N) range from 17.3 to 23.2 °C. These paleo-temperatures are in agreement with sea surface temperatures for mid-Eocene mid latitude regions based on foraminifera clumped isotopes; adding confidence to both datasets. In order to aid the accurate reconstruction of seawater ẟ 18 O values, we compiled published oxygen isotope fractionation data for molluscs and established relationships that describe the temperature dependence of oxygen isotope fractionation between water and molluscan calcite and aragonite, respectively. Applying the equation for aragonite to the Eocene samples, we obtain reconstructed seawater δ 18 O values for the Hampshire Basin between −2.4 and −3.4 ‰ (VSMOW).