Segregated-intrusive ice of subglacial meltwater origin in retrogressive thaw flow headwalls, Richardson Mountains, NWT, Canada

Extensive beds of icy sediments are exposed in the headwalls of active retrogressive thaw flows, which developed on the Aklavik Plateau, Richardson Mountains, NWT. Sedimentary and isotopic characteristics defined two units: debris-rich ice overlain by icy diamicton. These units originated from the same glacial sediment deposited during the Late Wisconsin westward expansion of the Laurentide Ice Sheet. The icy diamicton is a result of the development of thaw flows and mass movement processes during the early Holocene warm interval, which reworked the sediments. The ice in the diamicton is characterized by average delta(18)O and deltaD values of -22.6 +/- 1.8parts per thousand and -181 +/- 15parts per thousand, respectively, which are similar to the modern isotropic composition of precipitation for that area. The debris-rich ice is more depleted in (18)O and D, with delta(18)O ranging from -30parts per thousand to -27parts per thousand and deltaD ranging from -230parts per thousand to 210parts per thousand. Air bubbles enclosed in the ice have delta(13)C values ranging between -24parts per thousand and -19parts per thousand and a CO(2) concentration nine times higher than the expected concentration of air entrapped in modern glacier ice but similar to that of soil CO(2). Both physical and isotopic properties of the debris-rich ice suggest that it is segregated-intrusive ice that formed after the retreat of the Laurentide ice sheet during permafrost aggradation as sediment-laden subglacial meltwater moved through a proglacial talik. This article discusses the mechanism involved in the formation of massive segregated-intrusive ice and how it differs from both buried glacier ice and Holocene massive intrasedimental ice. (C) 2003 Elsevier Ltd. All rights reserved.