Topographic controls on asynchronous glacial maxima in the European Alps during the Late Pleistocene

Glacial records worldwide increasingly indicate that mountain glaciers and parts of continental ice sheets reached their maximum extents asynchronously throughout the Late Pleistocene, often before the global Last Glacial Maximum (LGM; 26−19 ka) during Marine Isotope Stage 2 (MIS 2). Local glacial maxima throughout MIS 4 and MIS 3 are usually attributed to regional paleoclimate dynamics. The role of topography in controlling pre-LGM ice culminations remains less explored, despite its recognized influence on glacier dynamics. To address this gap, we applied a combined geochronological and numerical modeling approach to two paleoglaciers of the western European Alps: the Dora Baltea (DB) and Ticino-Toce (TT) glaciers. Our dating results show that the DB and TT glaciers reached their maximum extents asynchronously, during MIS 3 and MIS 2, respectively. Because the DB and TT glaciers occupied neighboring catchments, their millennial-scale asynchrony in Late Pleistocene dynamics is unlikely to reflect climatic differences, but more probably derives from topographic controls. Our modeling shows that the higher hypsometry, steeper relief, and shorter valley length of the DB catchment lowered the climate-forcing threshold for extensive ice advance, allowing the DB glacier to build its piedmont lobe under moderate MIS 3 cooling, unlike the TT glacier. Moreover, preexisting prominent moraines likely limited the MIS 2 DB glacier advance. Our results highlight the key role of topography in driving asynchronous ice maxima and call for caution when using ice-marginal moraines and related glacial reconstructions as paleoclimate proxies.