240-Year Record of Sr/Ca and Ba/Ca from a Coral off Central Vietnam Reveals Impacts of Monsoonal Upwelling and La Nina Rains

The South China Sea (SCS) is a semi-enclosed sea with an area of about 3.5 x 10⁶ km² that links the Indian Ocean and Pacific Ocean. The climate of Southeast Asia is predominantly influenced by the East Asian Monsoonal system, which is modulated by the SCS and the El-Nino Southern Oscillation (ENSO). In the recent years, climate extremes in this region have been increasing, resulting in flooding and droughts. However, our understanding of regional teleconnections and climate history is limited by temporally short and geographically sparse data. To improve our understanding, long, continuous proxy records are required. Here, we present a seasonal sea surface temperature (SST) record reconstructed from a Porites lutea spp. coral cored from Hon Tre Island, off the eastern coast of Central Vietnam (12°12'49.90''N, 109°18'17.51''E). The reconstruction spans approximately 240 years, reflecting climate variability and trends from the late 1700s, during the Little Ice Age, to the present. The first 33 years (1977-2010) of this core was previously calibrated with a robust correlation between Sr/Ca and HADISST at monthly (r²=0.75, p<0.0001) timescales. In addition, to understand upwelling occurring along the coast of Vietnam, we present a long-term Ba/Ca record. Ba/Ca is a proxy for upwelling as deep water has higher Ba/Ca ration and also, for runoff which occurs at this site during periods of heavy rainfall. High intensity Ba/Ca peaks have been found to correlate to periods of heavy rainfall during La Nina while smaller and more consistent seasonal cycles have been found to correlate to the upwelling events during the East Asian Summer Monsoon (EASM).