Modeling ground thermal conditions and the limit of permafrost within the nearshore zone of the Mackenzie Delta, Canada

This study examines the interrelated effects of snow and ice on ground thermal conditions beneath regions of shallow water within the nearshore zone of the Mackenzie Delta. Field-and model-based data were used to determine the thermal boundary conditions at the sediment bed surface and to define the contemporary limit of permafrost. Over two consecutive winters, mean sediment bed temperatures deviated up to 9.8 degrees C beneath bottom-fast ice that ranged from 10 cm to 100 cm thick, with intrasite variability as much as 4.7 degrees C. Measured and modeled temperatures were found to exponentially relate to the duration of time ice is bottom-fast with the sediment bed. Mean winter ground temperatures at this boundary were predicted within +/- 0.25 degrees C of the observed measurements using numerical thermal modeling. As on-ice snow depth decreased, the limit of equilibrium permafrost shifted toward progressively deeper water because of longer durations of ice contact and greater heat loss from the ground. The critical water depth for permafrost under equilibrium conditions was 84 cm (calculated from an ice thickness of 93 cm), which is equivalent to an ice contact time of 142 days. Equilibrium permafrost was mapped beneath 393.8 km(2) of bottom-fast ice. An additional 387.9 km(2) exhibited seasonal ground freezing in the winter of 2006-2007. Areas affected by bottom-fast ice represent locations that are actively receiving sediment from distributary channels. These results provide the first estimates of contemporary permafrost distribution for shallow water regions of the outer Mackenzie Delta.