Deglacial and Holocene Hydrologic Shifts in Southeast Asia Inferred from Speleothem Trace Elements and 14C

Speleothem δ¹⁸O variations have provided significant insight into past behavior of the Asian Monsoon system, but related variations in regional precipitation remain unclear due to the complex controls on precipitation δ¹⁸O. To address this, we developed a new hydroclimate record from Mainland Southeast Asia based on trace elements (Mg/Ca and Sr/Ca), stable isotopes (δ¹⁸O and δ¹³C), and ¹⁴C measurements in a precisely-dated speleothem (TM-17) from Tham Doun Mai cave, Laos that spans the last 38,000 years. To investigate the hydrologic controls on these proxies, we also analyzed cave drip waters, modern calcite, CO₂, bedrock, and soil samples. We find that a large fraction of speleothem C (24-60%) is obtained from bedrock (the dead carbon proportion or DCP) and that DCP correlates with δ¹³C (R=0.62), suggesting the potential influence of sulfuric acid-enhanced bedrock dissolution via pyrite oxidation in the vadose zone. A simple mixing model shows that TM-17 δ¹³C variations cannot be fully explained by DCP variability, however, and that other hydrologically sensitive processes in the soil, epikarst, and/or cave must contribute to shifts observed during Heinrich Stadial 1 (HS1) and the 8.2 kyr event. A variable HS1 response in trace elements complicates interpretations, yet a positive correlation between Mg/Ca and Sr/Ca over the full record (R=0.67), as well as cave monitoring results, suggest that TM-17 trace element ratios are likely influenced by a combination of prior calcite precipitation (PCP) and/or extended water-rock interaction. In the Holocene, similar variations in the δ¹³C and trace element records also suggest influence from PCP. To further investigate the hydrologic influence on our multi-proxy TM-17 data, we also conducted principal components analyses. Altogether, the examined proxies point to a dynamic HS1 at our site, including a dry period prior to the event that is potentially linked to low-latitude monsoon changes seen in other regions. In the Holocene, the 8.2 kyr event is abrupt and dry, and it may mark the beginning of a major regime shift in SE Asian hydroclimate towards wetter conditions. Our record then implies a rainfall maximum that is decoupled from and much later than the peak in monsoon intensity suggested by δ¹⁸O.