Considering the effects of diagenesis on the Ca isotope geochemical proxy

The Ca isotopic composition of marine carbonates has been proposed as an indicator of past conditions at the Earth's surface. Some have suggested that Ca isotopes are a proxy for sea surface temperature while others have used Ca isotopes to constrain relative weathering flux through geologic time. Recent results by Fantle and DePaolo (2007) GCA 71(10) suggest that bulk carbonates in some sedimentary columns may experience more reaction than previously thought. In light of this, it is critical to evaluate the fidelity of the proxy by elucidating the effect of diagenetic processes on the Ca isotopic composition of marine carbonates in various geologic settings. The current study uses reactive transport models to constrain the effects of recrystallization during diagenesis on the Ca isotopic composition of marine carbonates. The study considers a sedimentary column in which: (1) advection of a seawater-like fluid dominates the pore fluid chemistry, and (2) a minor component of the bulk sediment reacts at a rate that is different from the bulk sediment. The modeling suggests that, in the former case, the original isotopic composition of marine carbonates can be altered by as much as 0.4‰, given the sorts of recrystallization rates previously suggested by modeling Sr isotopes (e.g. Richter and Liang, 1993, GCA 117(3--4)). However, the sort of enhanced reaction suggested by Fantle and DePaolo (2007) increases the diagenetic effect substantially. In the latter case, Ca isotope variability in bulk sediments may persist even at high recrystallization rates, due to a buffering effect on pore fluid chemistry. At low rates of bulk sediment recrystallization, significant diagenetic alteration can occur when components within the bulk sediment react at faster rates. The implications of the current study are noteworthy. Current calibrations of the Ca isotope-paleotemperature proxy indicate that the relationship between fractionation and temperature of most carbonate-producing organisms in the surface ocean is ~0.02‰/°C. Thus, isotope shifts (0.2‰) due to fairly large temperature changes (10°C) may be erased during diagenesis, complicating the interpretation of paleotemperature. The key to using Ca isotopes as a proxy is selecting suitable sites and appropriate samples, while using modeling techniques to constrain interpretations.