| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 105 === Deciphering ocean carbon system and hydrologic variability is a key to further our understanding of global carbon cycle and climate dynamics within the glacial-interglacial cycles. During the last deglaciation, oceanic carbon dioxide (CO2) outgassed from upwelling regions in the open ocean has been considered to be the main source of CO2 in the atmosphere. However, marginal seas receive less attention despite they are potential CO2 sources based on the modern observations and more sensitive to abrupt climate changes through ocean-atmosphere teleconnection. In this study, we focus on reconstructing surface ocean pH and hydrologic conditions (sea surface temperature and upwelling intensity) in the western South China Sea (W-SCS) during the last deglaciation.
A new analytical protocol is established for measuring the isotopic composition of boron (δ11B) and trace element ratios (TEs) in small sample sizes (< 2 mg) of foraminifera using MC-ICP-MS and HR-ICP-MS, respectively. By applying micro-sublimation technique, the external precision for δ11B determination is better than ± 0.30 ‰ (2SD), while for TEs (e.g. Mg/Ca, B/Ca, Li/Ca, Ba/Ca and U/Ca) is better than ± 2.0 %, (2SD). Multi-proxies approach is applied for planktonic foraminifera (Globigerinoides sacculifer, mixed-layer dwelling species) collected from sediment core MD05-2901 (water depth 1454 m, located at the summer upwelling region off middle Vietnam) in the W-SCS
The reconstructed surface water pH values derived from the δ11B values suggest that the W-SCS was a CO2 source throughout the last deglaciation, and the flux of CO2 was greater than that of the modern condition. This is most likely influenced by the degree of the basin-wide vertical mixing within the entire SCS basin during the last deglaciation. The Mg/Ca-SST record indicates a latitudinal control since the Last Glacial Maximum, and the late Holocene SST was ~3°C higher than the glacial period. Combining with the upwelling intensity derived from the foraminifera-based Ba/Ca record, the deglacial hydrologic variability in the W-SCS is mainly controlled by the East Asian Summer Monsoon.
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