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

Abstract

A 45 m outcrop of Quaternary sediments on Birch Creek, near Circle, Alaska, reveals a record of fluctuating environmental conditions that probably spans several glacial–interglacial cycles. From base to top the deposits are forested floodplain (warm), colluvium with ice wedges (cold), forest soil (warm), loess (cold), paleosol containing Old Crow Tephra (OCt) (cool-to-warm), loess (cold), lacustrine (very warm), loess (cold), and modern forest soil (warm). Resolution of the paleoclimatic history associated with the OCt event is critical to understanding the nature of stage 5 in the western North 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 developed glacial interval occurred within stage 5, and the overlying very warm interval occurred in 5.3 or 5.1, or (ii) the tephra was deposited during a “non-Milankovitch” warming event late in stage 6. A paleoclimate chronology provides an alternative interpretation, (iii), in which the tephra/paleosol corresponds 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 stage 5, consistent with paleoclimate model simulations and data from other regions.