New Estimates of Ancient Glacier Changes

Scientists are learning more about how big the glaciers were in Alaska during the last ice age and when they began to shrink. They study things glaciers left behind—like piles of rocks, smooth landforms, large boulders moved by ice, and lines on mountainsides. That said, the traces of older glaciations are buried, and so cannot be as easily studied. A recent study shows what the land looked like before the last big glaciation, more than 20,000 years ago. Researchers looked underground in Wrangell–St. Elias National Park and Preserve to better understand how far the glaciers had pulled back during a warm period between 50,000 and 20,000 years ago.

The modern valley surface

A satellite model of today’s Chitina Valley shows that the shape of the valley floor still reflects what was happening under the glacier during the last glaciation (see Figure 1). When you drive into the park on McCarthy Road, you actually cross landforms shaped by a huge glacier that once covered the entire valley and reached all the way to the town of Gakona. Just as warmer temperatures over the last 20,000 years caused the glaciers to melt and retreat to where they are now, similar warm conditions created a similar landscape 50,000 to 40,000 years ago.

Figure 1: The modern surface of the Chitina River Valley, including McCarthy and the snout of the Kennicott Glacier, showing the streamlined subglacial record of the sliding of the Last Glacial Maximum ice stream.

The study

Scientists studied and dated the layers of sediment in tall river bluffs near McCarthy, Alaska (Figure 2). They discovered that the Kennicott Glacier, which once fed into a massive ice stream, had already shrunk back close to its modern position before about 40,800 years ago.

Figure 2: Researchers (Bruno Belotti and Elizabeth Cassel) studying the sedimentary layers exposed on a river bluff along the Kennicott River, south of McCarthy, Wrangell St-Elias National Park (location KN1 in Figure 1). The Kennicott glacier is in the background.

Photo by Timothy Bartholomaus

Near the bottom of the valley, they found sediment that forms in front of a glacier where streams spread out and braid together. They figured out the minimum age of these deposits (40,800 years old) by dating the sediments on top of them, which were dumped by the glacier in a large lake. Since the earlier major glaciation in this area (the Early Wisconsinan) ended around 50,000 years ago, the glaciers must have melted back to near-modern levels between about 50,000 and 40,800 years ago.

The scientists noticed that differences in the sediment layers from different glacial periods were linked to whether Paleolake Atna (a lake that existed in ancient times) was present and how high its water level was. The lake may have helped preserve many of the sediments in the Copper Basin and contributed to the tall, continuous river bluffs found there today.These findings help us understand the period of milder climate that occurred between the Early and Late Wisconsinan glacial peaks. This time, called “Marine Isotope Stage 3” (57,000–29,000 years ago), saw big and frequent climate shifts. Scientists still don’t know exactly what these changes looked like in Alaska, but studies like this bring us closer to the answer.

Stratigraphic evidence for modern-like glacier extents in south-central Alaska within the last glacial period (MIS 3)

Abstract

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.

Belotti, B., E. J. Cassel, T. C. Cartholomaus, M. G. Loso, M. S. Christoffersen, and J. W. Holt. 2025. Stratigraphic evidence for modern-like glacier extents in south-central Alaska within the last glacial period (MIS 3). Boreas.