ABSTRACT: Ancient permafrost and a future, warmer Arcti.c

CGRG Bibliography of Canadian Geomorphology
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Abstract

An understanding of the history of permafrost, and in particular the impacts of past climate on terrestrial permafrost, provides an important analogue for future changes. Numerical models predict its degradation over extensive areas of the Arctic and Subarctic in the near future, with potential for concomitant release of large volumes of stored carbon. In a general sense, little is known of the past record of permafrost from which to evaluate these models. Here we present recent research on the history of permafrost in unglaciated Yukon and Alaska highlighting the role cryostratigraphy. This region provides a unique archive of paleoenvironmental change by virtue of the numerous distal tephra beds interbedded with long, rich sedimentary records and their association with relict ground ice. The earliest appearance of permafrost is marked by well-developed frost cracks in the early Pliocene of northern Yukon, and well-developed ice wedge casts by the middle Pliocene in central Yukon and Alaska. Permafrost was ephemeral through the middle Pliocene and early Pleistocene, and likely absent during interglacials prior to 780,000 years ago. The oldest relict ground ice is associated with ice wedges overlain by the ca. 740,000 yr old Gold Run tephra in the discontinuous permafrost zone of central Yukon. Surprisingly these ice wedges are within several metres of the surface, demonstrating the survival of permafrost through several interglaciations. Additional sites associated with the last interglacial (ca. 120,000 years ago) indicate that widespread ground thaw and thermokarst development occurred with past warming. However, thaw was limited to the uppermost several metres of permafrost, and relict ice wedges that pre-date the last interglaciation are present at several sites separated by over 700 km. The relict ice wedges indicate that the antiquity and resilience of discontinuous permafrost is regional in nature. However, the ubiquity and magnitude of last interglacial thermokarst suggests that terrain effects associated with current permafrost degradation foreshadow more widespread and severe thaw under even modest future warming scenarios.

Bibliography of Canadian Geomorphology