Late Pleistocene marine resources from the Bering Glacier Foreland and human coastal migration in the northern Gulf of Alaska region

Recent research on the Bering Glacier forelands in the northern Gulf of Alaska provides new insights into late Pleistocene/early Holocene shorelines, providing a favorable route for human migration as early as ~16,000 cal yr BP. This route included an irregular coastline with embayments and islands offering protection from the open ocean; edible marine invertebrates dating from 15,000 to 5,500 cal yr BP; and marine vertebrates dating as early as 16,000 cal yr BP. The latter included walrus (Odobenus rosmarus), bearded seal (Erignathus barbatus), and ringed seal (Phoca cf. hispida), all associated with pack ice conditions unlike those present today. While this ecosystem could have supported humans migrating along the coastline, and coastal refugia may have existed elsewhere in the region, coastal archaeological sites in the northern Gulf of Alaska and southwest Alaska are no older than ~9,500 cal yr BP. This suggests that the earliest sites have been eroded or destroyed, that the earliest migrants ignored available marine resources, and/or that these migrants did not use a coastal route. In contrast, the earliest archaeological sites in southeast Alaska date to ~12,500 cal yr BP, suggesting migration from interior Alaska to the coast somewhere east of the Copper River delta.

Lacuna band (surface depressions) occurrence and conditions of formation, Bering Glacier, Alaska

Bering Glacier lacunas are steep-sided flat-floored depressions ranging from 40 to 60 m wide, 80 to 120 m long and 35 to 50 m deep. They are confined within a band of debris-free ice (1.5 km wide, 5 km long) parallel to the eastern margin of the Bering piedmont lobe. After the 1993–95 surge displaced the lacuna band several kilometers onto the foreland, a new band of lacunas began to form 5–6 years later in the same location as occupied by the displaced band. Conditions essential to lacuna formation were initiated during the surge, as overriding ice was thrust into position across the trend of a subglacial trough, leading to stagnation deep within the trough. It is proposed that stagnation combined with englacial water movement altered ice crystal fabric and resistance to ablation. Exposure of this ice through normal ablation led to differential ablation and the formation of lacunas.

Surface depressions (Lacunas) on Bering Glacier, Alaska: a product of downwasting through differential ablation

Bering Glacier lacunas are steep-sided, flat-floored hollows ranging in size from 40 to 60 m wide, 80 to 120 m long, and 35 to 50 m in deep. They are confined within a band of clean ice (1.5 km wide, 5 km long) paralleling the eastern margin of the Bering piedmont lobe. The 1993–1995 surge displaced the lacuna band several kilometers onto the foreland. Specifically significant is the formation of a new band of lacunas 5–6 years later in the same location occupied by the displaced band prior to the surge. Conditions essential to lacuna formation were initiated during the surge as overriding ice was thrust into position across the trend of a subglacial trough, leading to stagnation of ice within the trough. Stagnation combined with saturation at depth altered ice texture and density. Exposure of this ice through normal ablation led to areas of differential ablation and the formation of lacunas.

Mass balance, runoff and surges of Bering Glacier, Alaska

The historical net, ablation and accumulation daily balances, as well as runoff of Bering Glacier, Alaska are determined for the 1951–2011 period with the PTAA (precipitation-temperature-area-altitude) model, using daily precipitation and temperature observations collected at the Cordova and Yakutat weather stations, together with the area-altitude distribution of the glacier. The model mean annual balance for this 61 yr period is −0.6 m w.e., the accumulation balance is +1.4 and the ablation balance is −2.0 m w.e. Average annual runoff is 2.5 m w.e. Periodic surges of this glacier transport large volumes of ice to lower elevations where the ablation rate is higher, producing more negative balances and increasing runoff. Runoff from Bering Glacier (derived from simulated ablation and precipitation as rain) is highly correlated with four of the glacier surges that have occurred since 1951. Ice volume loss for the 1972–2003 period measured with the PTAA model is 2.7 km3 w.e. a−1 and closely agrees with losses for the same period measured with the geodetic method. It is proposed that the timing and magnitude of daily snow accumulation and runoff, both of which are controlled by the glacier's area-altitude distribution and are calculated with the PTAA model, can be used to determine the probability that a glacier will surge.