Interannual changes in seasonal ground freezing and near‐surface heat flow beneath bottom‐fast ice in the near‐shore zone, Mackenzie Delta, NWT, Canada

Abstract

Interannual changes in seasonal ground freezing and near‐surface heat flow beneath zones of bottom‐fast ice (BFI) were examined over the winters of 2005–06 and 2006–07 within the near‐shore zone of the Mackenzie Delta, Canada. Winter variability in ground thermal conditions was determined at three monitoring sites. Ground‐penetrating radar surveys were conducted in late winter to determine spatial variability in landfast ice conditions and the extent of ice‐bonded sediments. Shallow water sites (<0.5 m‐water depth) were characterised by early onset of BFI, freezeback of the active layer and mean winter sediment bed temperatures ranging between −3°C and −10°C. In contrast, deep water sites (>1 m of water) experienced prolonged periods of floating ice, which limited the duration of ice contact with the sediment bed and the depth of seasonal frost, and resulted in warmer winter ground temperatures (between −0.5°C and −2.6°C). Under similar water depths, interannual changes in ice growth altered the timing of BFI and winter heat loss from the ground. When comparing conditions over the two winters, 2005–06 was characterised by a decrease in ice thickness that limited the extent of BFI and seasonal cooling of the ground. These changes in ice conditions had a greater effect on the thermal conditions at sites where water depths were close to the maximum ice thickness. The short ice contact times at these sites are important to the thermal state of permafrost, as only minimal heat exchange contributing to permafrost cooling occurs prior to freezeback of the active layer. Copyright © 2010 John Wiley & Sons, Ltd.

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

“…Intrasite variability in freezing season temperatures over the two winters was as much as 4.7°C. This variability in ground temperatures throughout the freezing season can be explained by differences in the timing of BFI for each water depth and thermal insulation caused by on‐ice snow [ Stevens et al , 2010]. Such differences alter the duration of ice contact with the sediment bed surface and subsequent heat loss from the ground.…”

“…The ground temperature records used in this study have been presented elsewhere [ Solomon et al , 2008; Stevens et al , 2010]. Herein, we utilize this data set to establish the measured relation between air temperatures ( T a ) and SBT and to independently verify thermal modeling results.…”

“…This term was substituted in place of the thawing n factor and TDD a, which do not apply to aquatic environments where water temperatures moderate SBT during the period of open water. The consistency of average annual water temperature in this region [ Stevens et al , 2010] allows for such simplification. The r k parameter accounts for seasonal differences in heat transfer caused by changes in the thermal conductivity of the ground.…”

“…Where ice is bottom‐fast, heat is readily conducted from the underlying sediments, which contributes to sustaining and aggrading permafrost below shallow water [e.g., Dyke , 1991, 2000; Solomon et al , 2008]. Key factors controlling heat loss from the ground include the duration of time ice is bottom‐fast and the thermal insulation of the overlying snowpack [ Stevens et al , 2010]. These two factors determine the thermal conditions at the sediment bed and whether permafrost is within thermal equilibrium or disequilibrium with surface conditions.…”

“…For example, Stevens et al [2008] showed over short distances (tens of meters) permafrost may transition to disequilibrium conditions beneath seemingly similar zones of BFI. These locations represent sites where water depth or surface conditions have changed; whereby, incomplete freezeback of the active layer occurs and permafrost is degrading [ Stevens et al , 2010]. This is in contrast to locations where permafrost is in thermal equilibrium and complete freezeback of the active layer and little interannual change in ground temperatures beneath the depth of annual variation occurs.…”