NOAA/WDS Paleoclimatology - Tham Doun Mai Cave, Laos 1900 Year Speleothem d18O and d13C Data

The spatiotemporal variability of the Asian Monsoon (AM) over the last two millennia has been attributed to a combination of external solar and volcanic forcing and/or internal coupled atmosphere-ocean dynamics, but the relative importance of these mechanisms remains unresolved. The present knowledge of multidecadal to centennial-scale AM variability over Mainland Southeast Asia is not well-constrained, despite substantial progress in understanding seasonal to decadal variability from tree ring records. Here we present the first high-resolution stable isotope (d13C and d18O) speleothem record from northern Laos spanning the Common Era (~50 BCE to 1880 CE). The d13C record reveals substantial centennial-scale fluctuations primarily driven by local water balance. Notably, the driest period at our site occurred from ~1280 to 1430 CE, during the time of the Angkor droughts, supporting previous findings that this megadrought likely impacted much of Mainland Southeast Asia. In contrast, variations in stalagmite d18O reflect changes in rainfall upstream from our study site. Interestingly, the d18O record exhibits a positive correlation with solar activity that persists after 1200 CE, contrary to the findings in previous studies. Solar-forced climate model simulations reveal that these d18O variations may be driven by solar-forced changes in upstream rainout over the tropical Indian Ocean, which modify the d18O of moisture transported to our study site without necessarily affecting local rainfall amount. We conclude that future rainfall changes in Mainland Southeast Asia are likely to be superimposed on multidecadal to centennial-scale variations in background climate driven primarily by internal climate variability, whereas solar forcing may impact upstream rainout over the Indian Ocean.