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

Abstract

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 Δ 14 C 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 14 C, our results show a sharp increase in Δ 14 C from 1960, reaching a peak value of 155.3‰ in 1973. The Hon Tre Island post-bomb Δ 14 C 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 14 C 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 Δ 14 C does not receive lower 14 C 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.

Radiocarbon variability recorded in coral skeletons from the northwest of Luzon Island, Philippines

“…Hon Tre Island is located on a coastal shelf surrounded by less than 100-m-deep waters (Bolton et al 2016). Comparing with the Con Dao Δ 14 C record reported by Mitsuguchi et al (2007), Δ 14 C in Con Dao is consistently higher than that in Hon Tre Island.…”

“…Although each point of data from Hon Tre Island is represents annual average of respective years, comparisons are made for other locations in SCS in Figs. 2 and 4b, c. To estimate seasonal changes in the magnitude of Δ 14 C, we referred to the seasonal variation in 1969 in Hon Tre Island reported by Bolton et al (2016). They showed that Δ 14 C in spring/summer is lower than that in autumn/winter.…”

“…This trend is different from that observed in the 1970s. Only 1986 data are available for Hon Tre Island but it is likely that the boreal summer value in Δ 14 C was lower than that of Currimao because of upwelling reported by Bolton et al (2016). It is difficult to ascertain the origin of the water mass that reached Currimao because the difference in ).…”

“…We also compare these data with the coral data reported from Ishigaki (Hirabayashi et al 2017a), Guam (Andrews et al 2016), Con Dao (Mitsuguchi et al 2007), and Hon Tre Island (Bolton et al 2016) to understand the relationship between North Pacific western boundary currents and SCS circulation around the Luzon Strait.…”

East Asian Monsoon Variability Since the Sixteenth Century

“…During summer (May–October), southwesterly EASM winds drive surface water to the northeast and cause upwelling of deeper cold, low ∆ 14 C water off the coast of southern Vietnam due to offshore Ekman transport (Figure b). Low Δ 14 C values relative to other records in the region, during both thermonuclear bomb testing (>1950) and the prebomb baseline period, as well as seasonally high Δ 14 C values in the winter, indicate the influence of both summer upwelling and winter advection on Δ 14 C at this site (Bolton et al, ). We therefore use the Δ 14 C proxy of seawater circulation in this study to reconstruct monsoon variability from 1584 to 1950 and to understand its linkages to other aspects of the climate system.…”