Orbital and Millennial Scale Variability of the Southeast Asian Monsoon Since 45 ka

Abstract:

Despite significant advances in our understanding of tropical Indo-Pacific and monsoon climate variability on orbital to millennial timescales, we still know very little about the range and mechanisms of variability in the Southeast Asian monsoon region. To address this need, we have developed a new, decadally-resolved speleothem δ¹⁸O and δ¹³C record from two overlapping stalagmites (TM-17 and TM-8), collected from Tham Mai Cave in Northern Laos. The TM-17 stalagmite was dated with 25 U-Th measurements, which indicate nearly continuous growth since 37.8 ka at ~20 microns/year. Based on 16 U-Th dates, the TM-8 stalagmite grew continuously between 33.7 and 45.6 ka at ~35 microns/year. Both samples were microdrilled at ~0.5 mm resolution and >2000 samples were analysed for stable isotope composition (δ¹⁸O and δ¹³C). Based on cave monitoring work conducted since 2010 and the strong correlation between the overlapping segments of the two records, these two speleothems faithfully record the mean δ¹⁸O of rainfall at this site, which reflects an integrated signal of upstream rainout over the Bay of Bengal and tropical Indian Ocean. The composite TM record clearly shows orbital and millennial scale variability over the last 45 kyr, with a strong precessional signal during the Holocene and clear δ¹⁸O increases during Heinrich Stadials 1-5, the Younger Dryas, and the 8.2 kyr event. The strong similarity between the Tham Mai record and the Chinese speleothem records supports recent interpretations of these records as reflecting large-scale Indian monsoon intensity rather than local precipitation over East Asia. In contrast to δ¹⁸O, speleothem δ¹³C from Tham Mai Cave may be more reflective of local water balance than large-scale monsoon intensity. The composite δ¹³C record shows increased values during the Heinrich stadials, especially HS1, potentially reflecting dry conditions with increased prior calcite precipitation and/or decreased soil respiration. Interestingly, the δ¹³C record shows no change during the Younger Dryas and decreased values during the 8.2 kyr event, suggesting no change and wetter conditions during these periods, respectively. Preliminary trace element data and ¹⁴C data will be presented to provide additional evidence for local water balance changes at this site.