Tropical South China Sea Surface 14C Record in an Annually-Banded Coral

Abstract

A surface-water 14C record of AD 1948-1999 in the tropical South China Sea (SCS) has been reconstructed from accelerator mass spectrometric radiocarbon measurements of annual bands of a Porites coral collected from Con Dao Island, Vietnam. Results gave the following 14C time series: a steady state of 47.8 +/- 2.8 (mean SD, n = 8) during 1948-1955 (i.e. in the pre-bomb period); a sharp increase during 1956-1966; a gradual increase during 1967-1973; a relatively high maximum value of ~174 in 1973; and a gradual decrease for the following period to ~86 in 1999. This 14C record having a sharp increase and a relatively high peak is similar to the records of subtropical corals (latitudes 21-27) and is distinctly different from the records of equatorial/tropical corals (latitudes 10), although our coral sample was collected from an equatorial/tropical region (8 degrees 39'N, 106 degrees 33'E). This can be explained by the geographic, oceanographic, and climatic setting of our study site. The SCS is a semi-enclosed marginal sea in the far western tropical Pacific and is little influenced by equatorial upwelling or related ocean currents. Our study site is located in the southwestern SCS, where an enormous submerged plain (the Sunda Shelf) spreads out with very shallow waters (mean depth 100 m). Furthermore, in the SCS, the East Asian monsoon (a strong, seasonally reversing wind system) enhances air-sea gas exchange especially in the mainland coastal waters, including our study site. Such semi-enclosed shallow waters with enhanced ventilation were probably very sensitive to the atmospheric nuclear explosions in the late 1950s and early 1960s and caused the sharp increase and high peak in the coral 14C record. Our coral 14C values in the southwestern SCS are significantly higher than the values in the northwestern SCS (Xisha Islands), which seems to suggest that meridional mixing of surface waters is not active in the SCS and that the open-ocean water intruding into the northern SCS (i.e. the Kuroshio intrusion) has only a limited influence on the southern SCS.