Assessing bulk carbonates as archives for seawater sulfur isotopic composition using shallow water cores from the South China Sea

The sulfur isotopic composition (δ34S) of Carbonate Associated Sulfate (CAS) is widely used to track changes in the isotopic composition of ancient seawater sulfate which signify the global sulfur cycle, Earth's surface redox evolution, and biological activity. However, our understanding of to what extent the isotopic composition of CAS records ambient seawater sulfate is primarily deduced from modern calcifying species. A few studies have examined modern bulk carbonate sediments which are important analogs for sedimentary carbonate rocks in the geologic record. To evaluate the fidelity of bulk carbonate sediments in recording ambient seawater δ34S values, we extracted CAS component and provided δ34SCAS results of five modern corals from the Hainan Island and 74 core-top unlithified carbonate sediments from the Nansha Islands in the South China Sea. Modern coral samples show a uniform δ34SCAS range from 21.5‰ to 21.7‰ close to previously published δ34S values for modern coral and seawater in other open oceans, consistent with globally homogeneous seawater δ34S compositions (i.e., δ34Ssw = 21.2 ± 0.2‰). The δ34SCAS values of core-top carbonate sediments vary within a narrow range from 21.1‰ to 22.1‰, which is fractionated from the overlying seawater by 0.4‰ ± 0.2‰. We suggest that these irregularly positive but small S-isotope fractionations (<1‰) are potentially due to the varying proportions of various calcifying organism granules with species-dependent sulfur isotopic fractionation. Overall, our data indicate that the CAS of bulk carbonate sediments from the open ocean settings is a reliable archive of seawater δ34S composition, provided the isotopic compositions have not been altered by post-depositional diagenesis.