Controls on Seasonal Ground Freezing and Permafrost in the Near-shore Zone of the Mackenzie Delta, NWT, Canada.

摘要

The controls on permafrost were investigated beneath shallow water in the near-shore zone of the Mackenzie Delta, NWT, Canada. A multi-scale approach to investigating subsurface conditions was conducted using ground temperature and drill measurements, ground thermal modeling, ground-based geophysics and satellite radar imagery.;These results provide the first estimates of contemporary permafrost distribution for shallow water regions of the outer Mackenzie Delta. The results also demonstrate that shallow water environments represent an important link in the thermal evolution of permafrost within Arctic ice-dominated deltas. In the context of a prograding delta, a significant thermal imprint is placed on the ground prior to the emergence of the delta plain. The term "shallow-water permafrost" is proposed to address this distinct form of permafrost that aggrades and persists in response to cooling of the ground, which takes place through the presence of BFI in aggrading freshwater sediment deposits.;The controls on ground temperatures were determined to relate to water bathymetry and interannual changes in winter surface layer conditions, which impacted the duration of ice contact and heat loss from the underlying sediments. Sedimentation was found to exert a long-term control on ground temperatures. Under an average on-ice snow depth, temperatures at the sediment bed and top of permafrost exponentially related to the ice contact time (ICT) as a function of surface temperature, water depth, snow depth and the rate of ice growth. The critical bottom-fast ice (BFI) thickness for permafrost under equilibrium conditions was 93 cm, which is equivalent to an ICT of 142 days. A near-shore distribution map of contemporary permafrost and seasonal frost beneath shallow water was developed by applying the critical conditions for permafrost to a time series of satellite radar images. Permafrost was mapped beneath 393.8 km2 of BFI. An additional 387.9 km2 of BFI was affected by seasonal ground freezing in the winter of 2006-2007. The distribution of frozen ice-bonded permafrost and unfrozen suprapermafrost taliks beneath zones of bottom-fast ice were found to be spatially variable over tens of metres to kilometres in distance. Areas affected by BFI represent locations that are actively receiving sediment from distributary channels.