Are molluscs a reliable archive for stable isotope paleo-temperature reconstructions? Implications from dual clumped isotope thermometry

Shell carbonates of marine molluscs are a widely used archive for paleo-environmental reconstructions. However, their use for temperature reconstructions may be impeded by species-specific vital effects and/or a lack of knowledge of the chemical and isotopic composition of paleo-seawater. Clumped isotope (∆₄₇) thermometry of marine carbonates enables temperature reconstructions independent of seawater composition provided the carbonate formed in thermodynamic equilibrium. Dual clumped isotope thermometry, i.e. simultaneous analysis of ∆₄₈ alongside ∆₄₇, offers the opportunity to determine if a carbonate formed in isotopic equilibrium and to account for kinetic effects taking place prior to and/or during precipitation. Here, we present dual clumped isotope data for several modern mollusc specimens (including bivalves and gastropods) with average growth temperatures ranging from 5-27°C. We find that most specimens analysed in this study exhibit dual clumped isotope compositions which are indistinguishable from equilibrium. Moreover, their ∆₄₇-derived temperatures agree within errors with their growth temperatures. We interpret the apparent equilibrium calcification of mollusc shell carbonates to be a possible consequence of a relatively low pH at the site of calcification. The absence of any resolvable kinetic isotope effects makes molluscs a reliable archive for highly precise (95CI of <2.2°C) temperature reconstructions via ∆₄₇-analysis. Based on previous ∆₄₇ investigations of molluscs grown at known seawater oxygen isotope compositions (δ¹⁸O_SW), we also determine the temperature dependencies of the oxygen isotope fractionation between seawater and molluscan aragonite/calcite.

∆₄₇ and δ¹⁸O values of fossil molluscs may be used in conjunction with these calibrations to reconstruct both - seawater-δ¹⁸O and temperature.