Landf orms and Coastal Retreat in Relation to Thawing of ^|^ldquo;Edoma^|^rdquo; on Southern Coast of Bolshoi Lyakhovsky Island, Northeastern Siberia

Abstract

The characteristics of landforms and coastal retreat resulting from thawing of permafrost called edoma, which is widely distributed along the Arctic coast in northeastern Siberia, are discussed. Edoma is composed of massive ground ice containing evenly scattered frozen soil pillars, and underlain by ice-poor silty sediments, The meteorological data at some adjacent locations indicate that thawing is active from mid June to mid September. Four landform units: upper flat, upper cliff, middle gentle slope, and lower cliff, are recognized from inland to seaside near the coastline on which edoma is exposed. Upper flat roughly corresponds with the top of the edoma, while many mounds formed by thermokarst subsidence are scattered on it. Thawing of edoma is extremely active on the upper cliff, and consequently the cliff itself retreats. Although much microtopography indicating active thawing of edoma is widespread on the middle gentle slope, downward thermal erosion seems to become ineffective in the lower horizon of edoma because the underlayer of edoma consists of ice-poor silt which does not suffer considerable morphological change by thawing. The lower cliff which directly faces the sea is exposed to thermal erosion by waves, and a notch is successively formed at the base. Sudden coastal retreat amounting to several meters is expected after the notch becomes deep enough to collapse. These processes are controlled by the characteristics of material, structure, and stratigraphy of the permafrost in this area.