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

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 (Δ₄₇) is a novel paleotemperature proxy based on the temperature dependent " clumping" of ¹³C-¹⁸O bonds. Similar Δ₄₇-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 Δ₄₇ vital effects. Here, we present the first systematic survey of Δ₄₇ in shallow water corals. Sub-annual Red Sea Δ₄₇ in two Porites corals shows a temperature dependence similar to inorganic precipitation experiments, but with a systematic offset toward higher Δ₄₇ 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 Δ₄₇ 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 Δ₄₇ 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 CO₂ diffusion into the calcifying space. However, the link with calcification rate may be related to fractionation during the hydration/hydroxylation of CO₂ within the calcifying space. Although the vital effects we describe will complicate the interpretation of Δ₄₇ 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.