The last interglacial high sea-level stand, the Pelukian transgression of isotope substage 5e, is recorded along the western and northern coasts of Alaska by discontinuous but clearly traceable marine terraces and coastal landforms up to about 10 m altitude. The stratigraphy indicates that sea level reached this altitude only once during the last interglacial cycle. From the type area at Nome, to St. Lawrence Island in the Bering Sea, to the eastern limit of the Beaufort Sea, Pelukian deposits contain extralimital faunas indicating that coastal waters were warmer than present. Amino acid ratios in molluscs from these deposits decrease to the north toward Barrow, consistent with the modern regional temperature gradient. Fossil assemblages at Nome and St. Lawrence Island suggest that the winter sea-ice limit was north of Bering Strait, at least 800 km north of its present position, and the Bering Sea was perennially ice-free. Microfauna in Pelukian sediments recovered from boreholes indicate that Atlantic water may have been present on the shallow Beaufort Shelf, suggesting that the Arctic Ocean was not stratified and the Arctic sea-ice cover was not perennial for some period. In coastal regions of western Alaska, spruce woodlands extended westward beyond their modern range and in northern Alaska, on the Arctic Coastal Plain, spruce groves may have entered the upper Colville River basin. The Flaxman Member of the Gubik Formation on the Alaskan Arctic Coastal Plain was deposited during marine isotope substage 5a and records the breakup of an intra-stage 5 ice sheet over northwestern Keewatin.
Postulations on the existence of forest refugia in parts of Beringia during the last glacial have been, in large part, based on ambiguous evidence. Existing data on radiocarbon-dated and identified fossil wood and macrofossils from Alaska and northwest Canada are synthesized here and are augmented by results of palynological studies in an effort to show the persistence of some, and total extinction of other, tree and large shrub species. Possible dispersal routes taken by species that reinvaded Beringia in postglacial times are also reconstructed from the fossil record. Macrofossil and pollen evidence, when combined with climatic factors, makes cottonwood a good candidate for survival during the last glacial. Larch and aspen are also candidates, though the evidence for them is less positive. Pollen and macrofossils of alder are very scarce in deposits of the last glacial age, and if it survived at all, it was probably in very isolated vegetatively reproducing clones. Shrub birch may have been present in Beringia, but tree birch probably was reintroduced during the Holocene. Spruce also appears to have been absent in Alaska from about 30,000 to 11,500 yr ago and probably reinvaded Beringia from a refugium south of the Laurentide ice sheet.
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