UC Irvine

Paleoclimate proxy records from U-Th dated speleothems (cave deposits) provide evidence of past climate change on a range of timescales. The oxygen isotope composition (δ18O) of speleothems from the East Asian Summer Monsoon (EASM) region have been critical in confirming large-scale changes in monsoon intensity linked to Earth’s orbital variations, which influence the spatial and temporal patterns of incoming solar radiation, and to abrupt millennial-scale changes in ocean circulation, which influence the meridional temperature gradient. However, multiple controls affect speleothem δ18O, and significant debate continues about the precise interpretation of these records in regard to monsoon precipitation patterns. Furthermore, a scarcity of records from Mainland Southeast Asia (MSEA) has thus far prevented confident reconstructions of past monsoon and precipitation changes across the entire Asian Monsoon region. The research presented in this dissertation addresses these issues through: (1) extending the spatiotemporal coverage of speleothem paleoclimate records in MSEA, and (2) developing more robust records of past monsoon history and local water balance through the use of multiple proxies in a ~38,000 year old stalagmite from Tham Doun Mai Cave, Laos.

Through the application of cave monitoring data and geochemical modeling of the local environment, trace element ratios (Mg/Ca and Sr/Ca) and carbon isotopes (δ13C and 14C) in stalagmite TM-17 are determined to be responsive to local hydrology through various controls: prior calcite precipitation (PCP) increases trace elements and δ13C during drier periods, while the dead carbon proportion (DCP), calculated from 14C data, reveals a significant bedrock dissolution control on TM-17 carbon. DCP data also reveals alternate δ13C controls as dominant during disparate abrupt climate events during the last deglaciation and Holocene. During the Holocene, when PCP is likely a shared control, trace element ratios and δ13C provide evidence for a mid-Holocene peak in rainfall amounts, which is at odds with interpretations of EASM δ18O records that suggest an earlier peak in monsoon intensity during the Northern Hemisphere summer insolation maximum. The results of these studies emphasize the importance of multiproxy approaches to speleothem-based climate reconstruction and provide new data for comparison with other records and climate model simulations.