[M-IS03] Evolution and variability of the Asian Monsoon and Indo-Pacific climate during the Cenozoic Era

[MIS03-07] A critical assessment of multiproxy-derived zonal and vertical upper ocean thermal gradients in the southern South China Sea over the last glacial cycle

*Ai-Lin Chen^1,2、Ren Yi Ooi²、Raul Tapia²、Kuo-Fang Huang¹、Stephan Steinke³、Li Lo²、Min-Te Chen⁴、Sze Ling Ho² (1.Academia Sinica、2.National Taiwan Univ.、3.Xiamen Univ.、4.National Taiwan Ocean Univ.)

キーワード：multi-proxy paleotemperature, foraminifera Mg/Ca, UK’37, TEX86, thermal gradient, East Asian Winter Monsoon

The southern South China Sea (sSCS), located at the margin of the Western Pacific Warm Pool, is influenced by the East Asian Monsoon. During the East Asian Winter Monsoon (EAWM) season, cold surface water is transported from the northern to the southern SCS along the coast of China and Vietnam, forming a west-east thermal gradient in the surface ocean of the sSCS. This gradient forms a “cold tongue” extending offshore Vietnam. The modern spatial extent of this west-east gradient increases with the intensity of the EAWM. At the same time, strong monsoonal winds cause vertical mixing in the upper water column, which in turn affects the thermal gradient of the upper water column. Thus, reconstructed zonal (west-east) and vertical thermal gradients have been used to investigate past changes in the EAWM. However, reconstructions of the thermal gradients using different proxy approaches reveal significant differences, which may reflect proxy-specific biases for this region. Addressing this, we reconstruct the upper ocean temperatures at two sites (MD97-2151 and MD01-2390) in the sSCS using planktic foraminiferal Mg/Ca and biomarker-based proxies (algae-based U^K’₃₇ and archaea-based TEX₈₆). We focus on the temporal evolution of (1) the west-east thermal gradient and (2) the upper water column vertical thermal gradient over the last glacial cycle. Sea surface temperature records based on U^K’₃₇ and Mg/Ca of surface dwelling foraminifera indicate a comparable decrease in the west-east thermal gradient evolution since the last glacial maximum (LGM), suggesting a general decrease in the intensity of the EAWM since the LGM. However, the fluctuations across the deglaciation derived from these two proxies are anti-correlated. Regarding vertical thermal gradients we noted distinct patterns based on the thermocline dwelling species (Pulleniatina obliquiloculata and Neogloboguadrina dutertrei) during the last deglaciation. In contrast, the vertical thermal gradients obtained from biomarkers exhibit consistent patterns between the two sites, which suggest a reduction in the intensity of the EAWM since the LGM and a temporary intensification during deglacial cooling events. These events match well with reconstructions based on terrestrial EAWM proxy records. These proxy-specific differences highlight the importance of understanding the relationship between the hydrography and ecology of the proxy carriers in the SCS, particularly for foraminiferal species dwelling in the thermocline. Further research into the modern spatial and temporal variability of these paleotemperature proxies and their proxy carriers can offer more comprehensive insights into the interpretations derived from different proxies.