Widespread glacier advances across the Tian Shan during Marine Isotope Stage 3 not supported by climate-glaciation simulations

Abstract

Whether there were more extensive glaciations during the Marine Isotope Stage (MIS) 3 relative to MIS 2 across the Tian Shan in Central Asia is intensely debated because of the uncertainty in chronological data and fully understanding the driving mechanisms. To help resolve the ongoing debate, we assess the climate sensitivity of the glaciers and reconstruct the extent of glaciation during MIS 2 and 3 across the Tian Shan, using a glacier-resolving (250 × 250 m) ice sheet model asynchronously coupled with a global climate model. Our results demonstrate that the equilibrium-line altitude (ELA) over the Tian Shan decreases by ∼180 m for every 1 °C cooling under a modern precipitation regime, but precipitation reduction greatly lowers the sensitivity of the glaciers to temperature decrease (e.g., the effect of 2 °C cooling is broadly offset by a 50% decrease in precipitation). Under the modeled colder/drier-than-present climate, the model predicts an ELA depression (∆ELA) of ∼75 m (162 m) over the Tian Shan during MIS 3 interstadials (stadials). The extent of MIS 3 glaciation is much smaller than that during MIS 2 (i.e., ∆ELA = ∼726 m). The more extensive glaciation during MIS 2 is largely attributed to the enhanced summer cooling. Furthermore, through a site-to-site model-data comparison, we find that the closest match between the modeled glacier margin and the locations of the glacial deposits previously argued to be MIS 3 is generally achieved under MIS 2 climatic conditions. These results suggest more extensive glacier advances over the Tian Shan during MIS 2 than MIS 3 on a regional scale, although MIS 3 glaciation may still occur in individual glacier catchments. This pattern suggests general synchronicity with the timing of maximum Northern Hemisphere ice sheets during the last glacial, which should be further tested in a multimodel framework in the future.