Stratigraphic evidence for modern‐like glacier extents in south‐central Alaska within the last glacial period ( <scp>MIS</scp> 3)

The last (Wisconsinan) glacial period was punctuated in North America by two glacial maxima, known as the Early and Late Wisconsinan glaciations. In Alaska, these maxima and their subsequent retreats have been the object of dating efforts to reconstruct local climatic events and compare them to global trends. Little is known, however, about the period of milder climate and reduced glacier extents that separated the Early and Late Wisconsinan maxima, likely spanning a significant portion of Marine Isotope Stage 3 (MIS 3). Here we provide a detailed sedimentological analysis for an area at the northern margin of the Chitina River Valley in the Copper River Basin, Alaska. There, a buried proglacial sequence composed of glaciofluvial stream deposits, glaciolacustrine muds, and ice‐proximal subaqueous fan deposits reveals ice‐free conditions at ∼39 ka. Our palaeodepositional reconstructions show that the local glacier, a tributary to one of the largest ancient south‐central Alaskan ice streams, had retreated to near its modern terminus position during early to mid MIS 3, indicative of a stadial–interstadial transition with modern‐like glacier termini. We attribute differences in facies stacking patterns to drastic fluctuations in local base level, driven by the formation and drainage of a regional ice‐dammed lake (Palaeolake Atna). Our results offer new constraints on Late‐Pleistocene glacial fluctuations in south‐central Alaska, including new MIS 3 glacial minima, and provide insights into the relationships between climatic fluctuations, ice extents, and associated landscape evolution in the Copper River Basin.