PhD Oral Defense of Liu Guangxin: Tracking Hydro-Climate Changes In Mainland Southeast Asia Through The Past 180,000 Years
- Oral Defense
About the Event:
The Asian Monsoon (AM) and its changes affect livelihoods of about half of the global population. Understanding AM variability and its impacts, therefore, is critical for local stakeholders to prepare appropriate climate mitigation and adaptation. In recent years, speleothems have been used as an important archive to reconstruct paleo-hydroclimate variability at different timescales. However, the climatic interpretation of speleothem proxies, in particular oxygen isotope ratio (δ18O), is highly debatable. In my thesis, I used speleothems from mainland Southeast Asia to reconstruct the annual to glacial-interglacial variability of the AM, and essentially reconciled the long-lasting debate on the interpretations of Chinese speleothems δ18O.
Three key findings have been achieved in my study. Firstly, I presented a 450-yr long record of trace element variations in an annually laminated stalagmite retrieved from a cave in central Myanmar. The record suggests that monsoon regime shifted from relatively wet to frequently dry conditions around the ninth century. Its timing coincides with the mysterious demise of the ancient Pyu Kingdom that flourished in central Myanmar over a thousand years. Secondly, I obtained a speleothem δ18O record from the eastern coastal region of the Bay of Bengal, which covers both the Last Glacial Maximum (LGM) and the present day. The dramatic difference in δ18O values between the two periods indicates that forced mainly by glacial boundary conditions, regional rain amount was substantially low during the LGM, ~56% of modern values. Lastly, I established three sets of speleothem δ18O records along a SW-NE transect of monsoon trajectory from southeastern coastal Myanmar to southwestern China. I found that speleothem δ18O values vary significantly between glacial and interglacial periods at sites closer to monsoon moisture source, but much less further inland. This spatial variability suggests a stronger oxygen isotope fractionation in precipitation associated with progressive rainout of AM moisture during glacial times. The stronger fractionation during glacial periods is probably resulted from a larger temperature gradient between the sites and suppressed plant transpiration effect. I further propose that the muted glacial-interglacial variations shown in speleothem δ18O records from southern China, located in the downwind region of the AM, were probably due to enhanced (weakened) isotope fractionation of transported moisture during glacial (interglacial) periods. This so-called transport pathway effect on water isotope fractionation counteracts the forcing of glacial boundary conditions.
These findings in my thesis not only contribute to our understanding of AM variability in the recent 180,000 years, but also provide an evaluation to climate models, and therefore, help to better predict hydroclimate change in the region.
Members of the Examination Panel:
Chair: Associate Professor Richard Webster
Oral examiner: Associate Professor Adam Switzer
Main supervisor: Assistant Professor Wang Xianfeng
IGS Co-supervisor: Associate Professor Wen Yonggang
Mentor: Associate Professor Fidel Costa
About the Speaker:
Liu Guangxin is a PhD candidate at the Interdisciplinary Graduate School (IGS)-NTU. He received a BS degree in Geochemistry from Nanjing University, Nanjing, China. At IGS/ASE, he has been conducting his research under the supervision of Assistant Professor Wang Xianfeng. His research focuses on Asian Monsoon variability from annual to orbital timescales. In his PhD work, he used speleothems as an archive to investigate hydroclimate changes in Southeast Asia. More specifically, he examined the spatial-temporal variability of speleothem oxygen isotope ratios across Asia. His study sheds light on the long-lasting debate on the orbital-timescale Asian Monsoon variabilities.