Environmental controls on coral skeletal δ13C in the northern South China Sea

In this paper, we investigate the relationship between seasonal climatic and environmental variables, and the skeletal C-13 of modern and mid-Holocene Porites lutea corals from the southern coast of Hainan Island in the northern South China Sea. No significant correlations were observed between C-13 in the modern coral and solar insolation or sea surface temperature. However, seasonal variability of C-13 in the modern coral covaries with rainfall on Hainan Island. Furthermore, the seasonal variations of C-13 in both the modern and mid-Holocene coral are synchronous with those of the coral O-18, which is a proxy for seawater O-18 and, in turn, largely controlled by local rainfall. These observations suggest that coral C-13 variations are closely associated with rainfall in this region. Given that river runoff contains dissolved inorganic carbon (DIC) with a negative C-13, we suggest that periods of high rainfall on Hainan Island deliver increased amounts of C-13-depleted DIC to coastal seawater, resulting in an enhanced negative C-13 in the corals. Our findings, together with previous studies, appear to demonstrate that in coastal environments, coral skeletal C-13 levels are controlled mainly by terrestrial carbon input and are significantly influenced by terrestrial river runoff. Consequently, the geochemical interpretation of coral C-13 records may differ between coastal areas and offshore areas or the open ocean.