Evidence of Quaternary climatic variations in a sequence of loess and related deposits at Birch Creek, Alaska: implications for the Stage 5 climatic chronology

A 45 m outcrop of Quaternary sediments on Birch Creek, near Circle, Alaska, reveals a record of fluctuating environmental conditions that probably spa ns several glacial-interglacial cycles. From base to top the deposits are f orested floodplain (warm), colluvium with ice wedges (cold), forest soil (w arm)? loess (cold), paleosol containing Old Crow Tephra (OCt) (cool-to-warm ), loess (cold), lacustrine (very warm), loess (cold), and modern forest so il (warm). Resolution of the paleoclimatic history associated with the OCt event is critical to understanding the nature of stage 5 in the western Nor th American Arctic. Application of recent age estimates for the OCt tephra (ca. 140,000 yr BP) to the Birch Creek section would indicate that either ( i) the tephra/paleosol dates from the 6/5.5 transition, a strongly develope d glacial interval occurred within stage 5, and the overlying very warm int erval occurred in 5.3 or 5.1, or (ii) the tephra was deposited during a "no n-Milankovitch" warming event late in stage 6. A paleoclimate chronology pr ovides an alternative interpretation, (iii), in which the tephra/paleosol c orresponds to stage 6 or even stage 7, the overlying loess to stage 6, and the lake sediments to all or part of stage 5, but the OCt is older than 140 ,000 yr BP. Chronologies (ii) and (iii) imply a very warm beginning to stag e 5, consistent with paleoclimate model simulations and data from other reg ions. (C) 2000 Elsevier Science Ltd. All rights reserved.