A late Pleistocene glacial chronology for the southern Brooks Range: Stratigraphic record and regional significance

Abstract

Radiocarbon dates from 11 measured sections in the Koyukuk region provide a chronology of the last Pleistocene glaciation. Glaciers were advancing strongly by 24,000 yr ago; they built moraines near the south flank of the Brooks Range, retreated briefly about 22,000 to 20,000 yr B.P., then readvanced at least one more time into their terminal zones. Glaciation was accompanied by alluviation of the Koyukuk and Kobuk drainage systems and by periglacial processes that resemble those taking place today farther north and at higher altitudes. Moraines near the south flank of the range were being revegetated by 13,500 yr B.P. A strong final readvance into end-moraine belts of some northern valleys occurred about 13,000 to 12,500 yr ago, and this event may be synchronous with less extensive glacier readvances in upper valleys of the Koyukuk region. Upper valleys were largely deglaciated by 11,800 yr B.P. Dated Stratigraphic sections from the northern Alaska Range show similar ages for initiation and close of glaciation and also suggest a possible interstadial episode about 20,000 yr ago. Scanty records of fluctuations during ice wastage probably reflect the general scarcity of datable wood, peat, and organic soils between about 19,500 and 13,500 yr ago. The Brooks Range and Alaska Range chronologies closely approximate glacial successions determined else-where in eastern Beringia and in Siberia. Advance and retreat of glaciers throughout this region evidently were associated with widespread climatic changes that also controlled the late Wisconsin history of the Laurentide ice sheet. The Itkillik II and late Itkillik phases of former usage are part of a single glaciation that was entirely separate from the preceding Itkillik I ice advance. For this reason, the local term "Walker Lake Glaciation" is here extended to the last major glaciation of the entire southern Brooks Range, and use of Itkillik phases should be discontinued. The term "Itkillik Glaciation" is hereby restricted to the next older ice advance, in accord with its original definition. Cold and dry conditions during the last glaciation of eastern Beringia are indicated by (1) relatively small mountain glaciers, (2) slight (200-m) depression of glaciation limits below modern values, (3) periglacial features indicating severe frost action on slopes with little protective plant cover, (4) widespread accretion of eolian sand, (5) low pollen influx rates, (6) scarcity of radiocarbon-datable organic remains, and (7) general absence of carbonaceous paleosols. Plant growth may have been much more restricted than generally believed, with relatively low capacity to support grazing animals and human hunting bands.