Holocene coral reef carbonate REY geochemistry during the neomorphism from aragonite to calcite: A case study in the South China Sea

Abstract

The geochemistry of rare earth elements and yttrium (REY with Y, REE without Y) in reefal carbonates is increasingly used to investigate both palaeoceanography and modern oceans. Nevertheless, the application of these methods to elucidate climate dynamics of the geologic past is limited by their vulnerability to diagenetic alterations. Given the meteoric transformation of aragonite to calcite, which represents an extremely unfavorable scenario for preserving the original marine signature, we focused on the REY geochemistry of a Holocene coral reef, obtained from Well CK2 in the northern South China Sea, which initiated at ⁓7.8 ka BP, but ceased to grow vertically at ⁓3.9 ka BP. The Holocene reefal carbonates have undergone neomorphism, transforming aragonite into calcite in a meteoric environment and enabling a direct comparison of REY distributions between the original aragonite and neomorphic calcite. Despite the preserved REY patterns of stabilized calcite closely mirroring those initially present in surface seawater, the ΣREE contents and Ce anomalies vary significantly, reflecting mixing of REY from reefal microbialites. Despite these disturbances, the Nd_N/Yb_N and Y/Ho ratios of Holocene reefal carbonates still demonstrate a highly conservative behavior during diagenesis. Our study indicates that the initial REY parameters, such as Nd_N/Yb_N and Y/Ho ratios, are frequently preserved in Holocene reefal carbonates, thus offering significant support for employing ancient marine limestones as indicators of marine REY geochemistry. Nevertheless, prudence is advised when utilizing ΣREE contents and Ce anomaly.