The Neoglacial was a period of cold that lasted more than 2000 years during the mid-Holocene, from approximately 4700 to 2500 years ago. Although proxy data from a number of sources document the regional onset and duration of cold conditions in the Northern Hemisphere during this period, none have suggested an expansion of sea ice in the Bering Sea. Here we provide new evidence that Neoglacial sea ice expansion in the Bering Sea was substantial enough to have altered the distribution of North Pacific pinnipeds and cetaceans, using prehistoric skeletal remains recovered from an archaeological site on the island of Unalaska in the eastern Aleutians (Amaknak Bridge, occupied from c. 3500—2500 yr BP (radiocarbon years before present, uncalibrated)). Comprehensive archaeozoological analysis of the Amaknak Bridge fauna indicates that sea ice in the Bering Sea must have reached a more southerly position at the height of the Neoglacial and persisted longer than it does today. We infer from this evidence that for most of the Neoglacial period, sea ice must have surrounded the Pribilof Islands until early summer and blocked the Bering Strait until late summer. Such an expansion and seasonal persistence of sea ice would have prevented fur seals from using the Pribilofs as a summer breeding rookery and whales from making summer migrations into arctic waters to feed, as they do today. We suggest this expansion of sea ice in the Bering Sea during the Neoglacial may explain several unresolved phenomena of mammalian distributions, genetic partitioning and extinctions in the North Pacific.
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