TIBETAN LAKE LEVEL CHRONOLOGIES AND HYDROLOGIC MODELING SUGGEST 50% INCREASE IN PRECIPITATION DURING THE EARLY HOLOCENE IN THE SOUTHWESTERN TIBETAN PLATEAU

Numerous paleoclimate records from the Indian Summer Monsoon region suggest higher, but variable summer precipitation amounts during the Holocene. These studies provide a basis for projecting future precipitation change in the region, which serves a water source for over half of Earth’s population. However, few records provide quantitative constraints on past climate, making detailed model-data comparisons difficult. We address this using lake level reconstructions and hydrologic modeling at two adjacent closed-basin lake systems in western Tibet, Ngangla Ring Tso (NRT) and Baqan Tso (BT). This work leverages the advantages presented by the two lake basins. The larger NRT system has abundant shorezone deposits, which were dated with ¹⁴C and U/Th series, to provide a robust lake level reconstruction. In contrast, BT is a small lake with limited shorezone deposits and an uncertain ¹⁴C reservoir effect, but it is in a watershed with no evidence for glaciation. BT is therefore conducive to hydrologic modeling, but provides a shorter, poorer ¹⁴C-based lake level record. The NRT record shows the paleolake exceeded modern lake level between ≈12.8 and 2.3 ka, with a highstand from 10.3 to 8.6 ka that is coincident with peak Northern Hemisphere summer insolation and Indian Summer Monsoon intensity. Lake level generally declined through the Holocene, with several prominent oscillations in monsoon strength at 11.5, 10.8, 8.3, 5.9 and 3.6 ka. This is similar to other proxy records from Tibet. The BT record mimics that of NRT, recording deep lake conditions from 11.0 to 5.0 ka, suggesting a minimal reservoir effect. A combined physically-based runoff and lake surface energy budget model was used to estimate the climate conditions required to sustain these lakes at their early Holocene highstands. Modern calibration accurately reproduced the current BT lake level. The highstand paleolake, covering approximately 5x modern surface area, required only a 55% increase in annual precipitation under early Holocene conditions, with an estimated change in mean annual temperature of +2 °C and relative humidity of +37%. This suggests modest changes in hydroclimate can result in outsized hydrologic changes, highlighting the high sensitivity of runoff in arid western Tibetan watersheds to future change.