Isotropic Thaw Subsidence in Natural Landscapes of Northern Alaska
Abstract
Recent research documents warming of permafrost, increased emissions of greenhouse gases in permafrost regions, and damage to civil infrastructure induced by melting of ground ice. Particular attention has been focused on 'thermokarst terrain,' localized systems of irregular pits, mounds, and ponds caused by differential subsidence accompanying thaw of ice-rich permafrost. Development of thermokarst terrain is often triggered by discrete, geographically constrained disturbance of vegetative cover or hydrological patterns. Here, we describe landscape-scale, thaw-induced subsidence in northern Alaska lacking the topographic contrasts associated with thermokarst terrain. Observations in some regions of the Arctic reveal little correlation between increasing air temperature and active-layer thickness (ALT) above permafrost in undisturbed terrain. The apparent stability of ALT in many Arctic landscapes may, however, be illusory if thaw penetrates into an ice-rich layer underlying the long-term base of the active layer. The apparent stability in ALT is attributable to the presence in many permafrost regions of an ice-rich 'transition layer' that resists thaw owing to the large amounts of latent heat involved in melting it. During warm summers, this layer protects underlying permafrost from thaw and creates nonlinearities in the response of the permafrost system to climatic forcing. We sought to determine whether widespread, relatively homogeneous, decadal-scale thaw subsidence, possibly attributable to climatic change, is occurring in natural, undisturbed landscapes and, if so, to estimate its magnitude and evaluate its role in the response of permafrost to atmospheric forcing. Field investigations designed to track interannual vertical movements associated with formation and ablation of ice near the permafrost table were begun in the summer of 2001 and continued annually at two 1 ha Circumpolar Active Layer Monitoring (CALM) sites representative of landscapes in the Arctic Coastal Plain and Arctic Foothills physiographic provinces of northern Alaska. Observations were conducted at the end of the thawing season with high-resolution differential GPS equipment, using a four-stage nested sampling design that provides full geographic representation of surface cover types and microtopographic elements within each sampling area. Both sampling areas experienced net subsidence of the ground surface over the period of observation. The record of temperature and vertical movement at the ground surface indicates that penetration of thaw into the transition layer has produced relatively uniform subsidence extending over entire landscapes. Without specialized observation techniques the subsidence is not apparent to observers at the surface. Integrated over extensive regions, this 'isotropic thaw subsidence' may be responsible for thawing large volumes of carbon-rich substrate, and could have negative impacts on infrastructure.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.C53C..07S
- Keywords:
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- 0702 CRYOSPHERE Permafrost;
- 0706 CRYOSPHERE Active layer;
- 0708 CRYOSPHERE Thermokarst;
- 0794 CRYOSPHERE Instruments and techniques