Upwelling of Pacific Intermediate Water in the South China Sea Revealed by Coral Radiocarbon Record

Abstract

Annual radiocarbon from a massive Porites lutea coral collected from Hon Tre Island, Vietnam, South China Sea (SCS) was analyzed over a ~100-yr-long period from AD 1900 to 1986. The pre-bomb results from 1900–1953 show a steady Δ14C value of –54.4±1.8‰ (n=60). These values are similar to coral records located in the central and southern SCS and from Indonesian waters, but are lower than those from Japan. Following the input of anthropogenic bomb 14C, our results show a sharp increase in Δ14C from 1960, reaching a peak value of 155.3‰ in 1973. The Hon Tre Island post-bomb Δ14C values are lower than those of other corals located in the SCS and Japan, but higher compared to those in the Indonesian Seas. This study infers a seasonal input of upwelled water depleted in 14C from the deeper SCS basin that originates from the tropical Pacific via the Luzon Strait. The bifurcation of the North Equatorial Current feeds the surface and intermediate currents in the SCS and Makassar Strait region. However, unlike the Makassar site, this study’s coral Δ14C does not receive lower 14C water from the South Pacific Equatorial Current. The Vietnam record therefore represents a unique oceanographic position, reflecting the seasonal influence of older, deeper SCS waters that upwell periodically in this area and have modified the surface waters locally in this region over the last 100 yr.