Paleoclimatic reconstruction during Marine Isotope Stages 2 and 6 in Southeast Tibet using numerical modelling of glacial flow constrained by imbricated moraines

Abstract

In the field at Cuopu in Southeast Tibet, the two largest imbricated moraines have been dated using terrestrial cosmogenic nuclides as Marine oxygen Isotope Stage 6 (MIS-6, ∼180 ka) and MIS-2 (or Last Glacial Maximum, LGM, ∼20 ka), consistent with the two largest last glaciations. We conducted a coupled mass balance-glacier flow model, calibrated with our field observations (moraine geometry and trimline locations) and in-situ age measurements at Cuopu, to quantitatively estimate the paleotemperature and paleoprecipitation during these two glaciations. Simulations for MIS-6 suggest that only a small cooling range (∼6–6.35 °C colder than the present) reproduces field observations for documented precipitation similar to those at present (−10 to +10 %). For the LGM, simulations require a broader cooling range (∼6.0–7.5 °C) to reproduce the field observations for a documented precipitation decrease of 30–70 %. Considering similar cooling for the LGM and MIS-6, consistent with the δ¹⁸O curves, we conclude that at Cuopu the most probable cooling range during the LGM was thus ∼6.0–6.35 °C, corresponding to a precipitation decrease of 30–45 %. The MIS-6 paleoclimate we constrain remains distinctly separate from that of the LGM, even when accounting for a wide range of modelling parameters, and thus provides a robust reference for estimating the age of such older, poorly preserved moraines throughout SE Tibet.