The last interglaciation in Alaska: Stratigraphy and paleoecology of potential sites

Abstract At least 20 localities in Alaska contain deposits that may provide information on the last interglaciation (Oxygen-Isotope Substage 5e). These widely dispersed localities include river bluffs, coastal bluffs and terraces, elevated marine shorelines, lake basins, and artificial excavations. Most of the inferred interglacial deposits contain macrofossils or pollen that are older than the range of radiocarbon dating and commonly indicate climate as warm as or warmer than the present. At a few localities, evidence for deep thaw of permafrost also indicates a warm paleoclimate. At eight localities, the Old Crow tephra occurs at or below organic deposits that may represent Substage 5e. The tephra occurs beneath conspicuous organic deposits at Fairbanks, the Yukon Palisades, and Holitna lowland, and directly above a peat bed at Hogatza Mine. At Birch Creek, Halfway House, Ky-11, and Imuruk Lake, the tephra occurs within a paleosol or organic deposit, but other organic horizons that more likely indicate interglacial conditions occur at higher stratigraphic levels. The varied stratigraphic relations of the Old Crow tephra suggest that it may have been deposited close to the boundary between Isotope Substages 6 and 5, which is dated at about 130 ka in the marine record and between 132 and 140 ka on land. These age relations suggests that the tephra may have been deposited about 135 ± 5 ka, validating the recent fission-track age determination of 140 ± 10 ka for this deposit. Six coastal localities contain deposits of probable interglacial age, and these commonly are associated with evidence for eustatic sea levels higher than those of the present. Beach and sublittoral sediments of the Pelukian transgression occur up to 12 m asl along the northwest coast of Alaska, and are correlative with barrier island and lagoonal sediments on the Alaskan Arctic Coastal Plain. Both sets of deposits commonly contain extralimital mollusks and microfauna that indicate marine water slightly warmer than present and suggest that seasonal sea ice did not extend south of Bering Strait during the last interglacial as it does today. Farther south, elevated marine-terrace deposits on Amchitka Island contain marine invertebrates that indicate a climate warmer than at present. Peat horizons in coastal exposure at Goose Bay and coastal terraces at Lituya Bay contain pollen spectra that suggest forests like those of the present day, and spruce macrofossils exposed on Baldwin Peninsula indicate boreal forest more extensive than at present. Sediments from several lakes in northwestern Alaska may contain continuous records of the last interglaciation. A major warm interval, possibly Isotope Substage 5e, has been identified in a core from Squirrel Lake by a peak in Picea pollen that indicates forest extension beyond present limits. Similar pollen records are potentially available from two maars which formed in the Cape Espenberg area more than 125 ka. Terrestrial organic deposits thought to record the last interglaciation occur interstratified with marine and glaciogenic sediments in the Nushagak Lowland of southwest Alaska and on Baldwin Peninsula in Kotzebue Sound. Extensive exposures along the Copper and Nenana Rivers may also contain organic deposits that record the last interglaciation.