Carbonate clumped isotope variability in shallow water corals: Temperature dependence and growth-related vital effects

Abstract

Geochemical variations in shallow water corals provide a valuable archive of paleoclimatic information. However, biological effects can complicate the interpretation of these proxies, forcing their application to rely on empirical calibrations. Carbonate clumped isotope thermometry (Δ_(47)) is a novel paleotemperature proxy based on the temperature dependent "clumping" of ^(13)C–^(18)O bonds. Similar Δ_(47)-temperature relationships in inorganically precipitated calcite and a suite of biogenic carbonates provide evidence that carbonate clumped isotope variability may record absolute temperature without a biological influence. However, large departures from expected values in the winter growth of a hermatypic coral provided early evidence for possible Δ_(47) vital effects. Here, we present the first systematic survey of Δ_(47) in shallow water corals. Sub-annual Red Sea Δ_(47) in two Porites corals shows a temperature dependence similar to inorganic precipitation experiments, but with a systematic offset toward higher Δ_(47) values that consistently underestimate temperature by ∼8 °C. Additional analyses of Porites, Siderastrea, Astrangia and Caryophyllia corals argue against a number of potential mechanisms as the leading cause for this apparent Δ_(47) vital effect including: salinity, organic matter contamination, alteration during sampling, the presence or absence of symbionts, and interlaboratory differences in analytical protocols. However, intra- and inter-coral comparisons suggest that the deviation from expected Δ_(47) increases with calcification rate. Theoretical calculations suggest this apparent link with calcification rate is inconsistent with pH-dependent changes in dissolved inorganic carbon speciation and with kinetic effects associated with CO_2 diffusion into the calcifying space. However, the link with calcification rate may be related to fractionation during the hydration/hydroxylation of CO_2 within the calcifying space. Although the vital effects we describe will complicate the interpretation of Δ_(47) as a paleothermometer in shallow water corals, it may still be a valuable paleoclimate proxy, particularly when applied as part of a multi-proxy approach.

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