Effect of early marine diagenesis on coral reconstructions of surface-ocean 13C/12C and carbonate saturation state

Recent research suggests that future decreases in the carbonate saturation state of surface seawater associated with the projected build-up of atmospheric CO2 could cause a global decline in coral reef-building capacity. Whether significant reductions in coral calcification are underway is a matter of considerable debate. Multicentury records of skeletal calcification extracted from massive corals have the potential to reconstruct the progressive effect of anthropogenic changes in carbonate saturation on coral reefs. However, early marine aragonite cements are commonly precipitated from pore waters in the basal portions of massive coral skeletons and, if undetected, could result in apparent nonlinear reductions in coral calcification toward the present. To address this issue, we present records of coral skeletal density, extension rate, calcification rate, (delta)13C, and (delta)18O for well preserved and diagenetically altered coral cores spanning 1830–1994 A.D. at Ningaloo Reef Marine Park, Western Australia. The record for the pristine coral shows no significant decrease in skeletal density or (delta)13C indicative of anthropogenic changes in carbonate saturation state or (delta)13C of surface seawater (oceanic Suess effect). In contrast, progressive addition of early marine inorganic aragonite toward the base of the altered coral produces an apparent 25% decrease in skeletal density toward the present, which misleadingly matches the nonlinear twentieth century decrease in coral calcification predicted by recent modeling and experimental studies. In addition, the diagenetic aragonite is enriched in 13C, relative to coral aragonite, resulting in a nonlinear decrease in (delta)13C toward the present that mimics the decrease in (delta)13C expected from the oceanic Suess effect. Taken together, these diagenetic changes in skeletal density and (delta)13C could be misinterpreted to reflect changes in surface-ocean carbonate saturation state driven by the twentieth century build-up of atmospheric CO2.