Carbonate clumped isotope thermometry of molluscs and its applications to Pleistocene gastropod fossils from South Dakota

Abstract

Ghosh et al., (2006) [1] present a carbonate clumped isotope palaeo-thermometer. Otolith aragonite from fishes collected over a range of known temperatures [2] validates the original calibration of this thermometer for inorganic calcite precipitate (ICP) and coralline aragonite [1]. Here we present a calibration for molluscs, including gastropods collected from nature and inferred to have grown at widely different temperatures (2-25°C). The growth temperatures for these samples were estimated based on mean annual temperature (MAT) at the site of collection. We speculate that these growth temperature estimates could be biased by unknown and possibly variable amounts to due seasonality of growth and/or other factors. Despite these uncertainties, the mean trend in the relationship between inferred growth temperature and ^(13)C^(18)O^(16)O content of CO_2 extracted from mollusks is indistinguishable from that previously documented for ICP. The proportionality for molluscs can be described between 1 and 25°C by the function: Δ_(47) = 0.0603.10^6•T^(-2) -0.0496, where Δ_(47) is the enrichment, in per mil, of ^(13)C^(18)O^(16)O in CO_2 relative to the amount expected for a stochastic (random) distribution of isotopes among all CO_2 isotopologues [3,4], and T is the temperature in Kelvin. However, due to scatter in these new data, we suggest that the statistically indistinguishable and more precisely known trend for ICP should be used for molluscs.

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