Hydrological and Vegetation Dynamics in Central Indonesia since the Last Glacial Maximum

The Indonesian archipelago sits within the Indo-Pacific Warm Pool and plays a crucial role in today's global water vapor and heat transport. Despite the region's importance in the global climate system, we understand relatively little of long-term patterns of convection and precipitation across Indonesia. To better characterize the regional precipitation response in Indonesia since the Last Glacial Maximum (LGM), we developed continuous high-resolution records of the carbon and hydrogen isotopic composition of terrestrial leaf waxes (long-chain n-alkanoic acids; δ13Cwax and δDwax respectively) from marine and lacustrine cores retrieved offshore and in central Sulawesi, in the heart of Indonesia. Present-day rainfall variability in central Sulawesi is strongly influenced by variations in topography and wind pattern, including land-sea breezes, orographically-forced winds, and monsoonal winds related to the seasonal migration of the Intertropical Convergence Zone. Our terrestrial and marine records show excellent agreement. Together, our δ13Cwax records suggest that rainforests contracted and grasslands became more abundant during the LGM, indicating that the aridification and the likely increase of water stress during the dry season in central Indonesia are linked to high-latitude climate forcings (i.e. global cooling and Northern Hemisphere ice sheet expansion). However, our data also indicate that the highlands of Sulawesi remained relatively wet and might have become a refuge for rainforests during the LGM, suggesting that altitudinal gradients within the archipelago strongly influence local manifestations of hydrological changes at glacial-interglacial timescales. Despite evidence for regional drying, we observe depleted δDwax values during the LGM, demonstrating that the amount effect may not always be the most important factor controlling the δD of precipitation (δDprecip) in this region at orbital timescales. Instead, the presence of strong glacial boundary conditions might have resulted in "upstream" processes, such as adjustments in the Siberian High and the East Asian Winter Monsoon, modulating δDprecip over central Indonesia during the LGM.