‘Clumped isotopes’ thermometry in land gastropod carbonate shells

The carbonate ‘clumped isotope’ (Δ47) thermometer is based on the dependence of the abundance of 13C-18O bonds in carbonates on the carbonate formation temperature. We repeated at higher analytical precision the original thermometer calibration experiments of Ghosh et al., (2006) using carbonates precipitated synthetically by slow degassing of CO2 from saturated Ca(HCO3)2 solutions at a temperature range of 8°C to 70°C. Our data fall within error of the original calibration and generally confirms it. However, the samples precipitated at lower temperatures are slightly shifted, forming a line with a lower slope, fitting better the theoretical calibration thermometer predicted by Guo et al., (in press). We tested the Δ47 thermometer in land gastropods (snails) by analyzing modern gastropod shells from various geographical locations in comparison to the gastropods’ growing season temperatures. No significant inhomogeneity was observed within an individual shell. Segments growing at different stages in the gastropods’ life (e.g., inner part of the helix reflecting young gastropod growth and outer shell that grew at older gastropod age) revealed no significant variations within a specimen in either Δ47 or δ18O, whereas δ13C varied significantly (0.5 ‰) implying a change in the gastropods’ food source. Inter-species comparison revealed significant δ13C and δ18O variations among specimens collected at the same location (Sphincterochila zonata and Trochoidea simulate, south Israel, and Pleurodonte acuta and Orthalicus undutus, Jamaica) but no significant interspecies variations in Δ47, providing a strong indication for carbonate precipitation in isotopic equilibrium. Gastropod shells do not grow year-round, with most gastropods being dormant during dry cold seasons. ‘Clumped isotopes’ values are therefore expected to record the temperature of the gastropods’ growth season. Hence gastropod ecology and preferred growth conditions has to be taken into consideration in thermometer calibration. Considering this we conclude that land gastropod shells, like marine organisms, precipitate under isotopic equilibrium and fall on the same ‘clumped isotopes’ thermometer calibration. References: [1] Ghosh et. al. (2006) GCA, 70: 1439-1456. [2] Guo et al. GCA, in press