Shallow-water carbonate sediments are one of the most extensive and well-studied records of the chemistry and temperature of ancient oceans (Kasting et al., 2006; Veizer et al., 1999; Veizer and Hoefs, 1976). One of the major limitations in the utilization of this archive is the potential for changes in the chemical composition of the sediments at any time after they were initially precipitated. Using a large data set of Ca and Mg isotope measurements in Neogene shallow-water carbonate sediments and associated pore-fluids from the platform to the slope in the Bahamas we have shown that stratigraphic variability in these isotopic systems is due to variations in both mineralogy and style of diagenetic alteration (fluid-buffered vs. sediment-buffered). This interpretation is rather counterintuitive given that these elements, and Ca in particular, are major components of the carbonate sediment and should be relatively robust – almost as robust as C – to diagenetic alteration.

The observation that sediment δ44Ca values in Neogene shallow-water carbonate sediments from the platform top, margin, and slope are largely controlled by mineralogy and the extent of fluid-buffered early marine diagenesis and that temporal variations in fluid-buffered diagenesis can generate stratigraphically coherent co-variation between many carbonate-bound geochemical proxies (δ13C, δ18O, Sr/Ca, etc.) has significant implications for the interpretation of both the major and trace element chemistry of shallow-water carbonate sediments in the geologic record. In particular, it suggests that stratigraphic co-variation between carbonate-bound geochemical proxies need not reflect changes in the global geochemical cycles of these elements but rather changes in the composition of bank-top waters and/or the extent of fluid-buffered vs. sediment-buffered early marine diagenesis. Thus, records of secular change and extreme variability in shallow-water carbonate sediments might be better interpreted as records of the effects of global environmental change and evolution on shallow-water carbonate-producing environments and not archives of global geochemical fluxes (e.g. the relative rates of organic carbon and carbonate burial from the δ13C of CaCO3).