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 (Delta(47)) is a novel paleotemperature proxy based on the temperature dependent clumping of C-13-O-18 bonds. Similar Delta(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 Delta(47) vital effects. Here, we present the first systematic survey of Delta(47) in shallow water corals. Sub-annual Red Sea Delta(47) in two Porites corals shows a temperature dependence similar to inorganic precipitation experiments, but with a systematic offset toward higher Delta(47) values that consistently underestimate temperature by similar to 8 degrees C. Additional analyses of Porites, Siderastrea, Astrangia and Caryophyllia corals argue against a number of potential mechanisms as the leading cause for this apparent Delta(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 Delta(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 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 Delta(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. (C) 2012 Elsevier Ltd. All rights reserved.