ONSET OF THE YOUNGER DRYAS CHRONOZONE IN THE AMERICAN WEST AND ITS TEMPORAL RELATIONSHIP TO CLOVIS EXPANSION

In North America, the climatic and environmental effects of the Younger Dryas stadial are most evident in regions closest to the North Atlantic, where abrupt cessation of thermohaline circulation probably initiated the rapid drop in temperature. Greenland ice cores date the YD onset either to 12,700 cal BP (GRIP) or 12,940 cal BP (GISP2). A sudden jump in radiocarbon ages, from ~11,200 to 10,700 rcbp in less than a century of real time, is coincident with the initial atmospheric changes and appears to be a pan-regional phenomenon. In the western US, important questions include: (1) are there unambiguous YD signatures in regional environmental records; (2) how rapidly were climatic signals transmitted and how quickly were various biota affected; (3) was the YD onset strictly coeval across North America, or significantly delayed in the interior. To better understand the initial peopling of the western states, and how human arrival was linked to the extinction of Pleistocene megafauna, it is crucial to establish the precise temporal relationship of Clovis sites to the Younger Dryas onset and slightly earlier Alleröd climate fluctuations. The Clovis ¹⁴C date range of 11,200-10,800 rcbp can be fitted to a brief interval on the YD radiocarbon "cliff," 12,900-12,850 cal BP. Alternatively, Clovis may be assigned to a period of ¹⁴C inversion in the late Alleröd, ~13,200-13,000 cal BP. Resolution of this issue will require more precise dating of black mats and the Glacier Peak tephra, which provide stratigraphic constraints for the Clovis horizon.