UC Irvine

The Southeast Asian autumn monsoon affects millions of people along the eastern coast of Mainland Southeast Asia. Relatively small changes to autumn rainfall amount can cause floods and prolonged droughts, which impact regional agriculture, water availability, and infrastructure. Thus, understanding Southeast Asian autumn monsoon variability and how it might change in the future is crucial for the livelihood of those living in the region. However, instrumental records are short and future projections are unclear, leaving the variability of the autumn monsoon poorly constrained. Records of past climate from reliable hydroclimate archives, such as cave deposits, would improve our understanding of the Southeast Asian autumn monsoon, particularly on longer timescales. My dissertation uses a stalagmite from central Vietnam to generate a multiproxy record of the Southeast Asian autumn monsoon from 45 to 4 ka. I use stable carbon isotopes and trace elements to generate a record of local rainfall change, which shows substantial, and at times abrupt, change over the last 45,000 years. Using the record in conjunction with climate model output, I identify ice sheet-driven sea-level change as the primary driver of Southeast Asian autumn monsoon variability on millennial to orbital timescales. During periods of low sea level, the exposure of nearby landmasses (the Gulf of Tonkin and the South China Shelf) decreases autumn moisture delivery to central Vietnam. The opposite occurs during periods of high sea level. I next semi-quantitatively estimate this sea-level-driven rainfall change using calcium isotopes, a relatively new proxy. The calcium isotope record reveals that when sea level was low during the termination of the last glacial period, rainfall amount was up to 50 % less than modern levels. Lastly, I use stable oxygen isotopes to investigate large-scale changes in hydroclimate. I find that the stalagmite oxygen isotope record reflects changes in regional precipitation as well as an in-cave process called “prior calcite precipitation”. After removing this in-cave effect using a geochemical model, I find that the oxygen isotope record follows the expected pattern from regional stalagmite records and climate model output. The collective results of my dissertation provide new insights to the past variability and main drivers of the Southeast Asian autumn monsoon over the last 45,000 years. By employing cutting-edge proxies and innovative geochemical models, I have also contributed significant advancements to the field, with far-reaching implications for speleothem science. Moreover, my results contribute to the validation of climate model output, which improve projections of future hydroclimate change.