Retrogressive Thaw Slumps

Retrogressive Thaw Slumps
Retrogressive thaw slump near Holmes Creek. Photo by Trevor Lantz.	Slumps grow as melting ground ice in the headwall turns materials into a muddy slurry that falls downslope to the base of the exposure. Melting of the exposed ground ice causes the headwall to move upslope and the disturbance grows. A slump headwall may retreat by several metres in a single summer, and over time a slump can impact several hectares of terrain. The rate of headwall movement is linked to several environmental factors, but generally, rates of slump growth are most rapid when the temperature is warmest. Over longer time periods the growth and the development of thaw slumps may respond to changes in regional climate. For example, in tundra uplands around the Mackenzie Delta, the growth rate of slumps has nearly doubled with rapidly increasing air temperatures during the past 40 years.
Slumping liberates soluble materials previously trapped in the permafrost so that disturbed soils typically possess elevatedconcentrations of soluble ions and higher pH than the adjacent tundra. Sediments and soluble materials are transported into adjacent lakes by runoff, which can influence the ionic concentrations and optical properties of the water. Recent studies have shown that environmental factors typically evoked to explain variation in tundra lake water quality, including surficial geology and proximity to the treeline or to the coast were subordinate to the main driver which is permafrost degradation. These studies improve the understanding of natural variation in baseline water quality conditions in the Mackenzie Delta region, which is necessary for assessing the impacts of ongoing hydrocarbon exploration and potential pipeline development in the region.		Massive retrogressive thaw slump on the Peel Plateau. Photo by Trevor Lantz.
Retrogressive thaw slump in the Mackenzie Delta Uplands. Photo by Trevor Lantz.	Elevated ionic concentrations and water clarity in disturbed lakes, and the abrupt nature of thaw slumping suggests that water quality in small tundra lakes may be rapidly modified and thus, thawing permafrost has the potential to dramatically alter the aquatic food webs of tundra lakes. Archives of such changes may be held in the sedimentary records of many northern lakes and could provide insight into the timing, magnitudes and effects of past thawing events. With respect to future warming of the Arctic, thermokarst processes can be expected accelerate and increase in importance as a driver of ionic chemistry and optical properties of northern lakes and ponds.
The environmental conditions in thaw slumps are significantly different than the undisturbed tundra. Ground temperatures (near surface and at 10 m) are higher, snow pack is deeper, and the active layers are thicker. The hospitable environment on these exposed surfaces facilitates the proliferation of tall shrub communities that persist for at least a century. Combined with the concave morphology of the slump scar, the dense vegetation at these sites also helps to trap the deep snow that maintains high ground temperatures. Modeling studies indicate this thermal disturbance can cause stable slumps to re-activate in time. Over centuries it is likely that slumps cycle through periods of stability and activity. The modeling also implies that as permafrost or lake temperatures increase in response to climate warming, unfrozen ground beneath tundra lakes may expand and cause slump activity to accelerate.		Retrogressive thaw slump in the Mackenzie Delta Uplands. Photo by Trevor Lantz
References Kokelj S.V., Jenkins R.E.L., Milburn D., Burn C.R., Snow N. (2005). The influence of thermokarst disturbance on the water quality of small upland lakes, Mackenzie Delta Region, Northwest Territories, Canada. Permafrost and Periglacial Processes 16: 343-353. PDF Kokelj S.V., Lantz T.C., Kanigan J., Smith S.S., Coutts R. (2009). Origin and polycyclic behavior of tundra thaw slumps, Mackenzie Delta Region, Northwest Territories, Canada. Permafrost and Periglacial Processes, 20(2):173-184. doi:10.1002/ppp.642. PDF Kokelj S.V., Smith C.A.S., Burn C.R. (2002). Physical and chemical characteristics of the active layer and permafrost, Herschel Island, western arctic coast, Canada. Permafrost and Periglacial Processes 13: 171-185. PDF Kokelj S.V., Zajdlik B., Thompson M.S., Jenkins R.E.L. (2008). Thawing permafrost and temporal variation in the electrical conductivity of small tundra lakes, Mackenzie Delta region, N.W.T., Canada. In Proceedings of the Ninth International Conference on Permafrost. Institute of Northern Engineering, University of Alaska Fairbanks (Vol 1). 965-970. PDF Lantz T.C., Kokelj S.V. (2008). Increasing rates of retrogressive thaw slump activity in the Mackenzie Delta Region, N.W.T., Canada. Geophysical Research Letters, 35, L06502, doi:10.1029/2007GL032433. PDF Lantz, T.C., Kokelj, S.V., Gergel, S.E. and Henry, G.H.R. (2009). Relative impacts of disturbance and temperature: persistent long-term changes in microenvironment and vegetation in retrogressive thaw slumps. Global Change Biology. 15: 1664-1675. PDF Mesquita, P.S. (2008). Effects of Retrogressive Permafrost Thaw Slumping on Benthic Macrophyte and Invertebrate Communities of Upland Tundra Lakes. MSc. Thesis, University of Victoria. PDF Mesquita P.S., Wrona F.J., Prowse T.D. (2008). Effects of retrogressive thaw slumps on sediment chemistry, submerged macrophyte biomass, and invertebrate abundance of upland tundra lakes. In Proceedings of the Ninth International Conference on Permafrost. Institute of Northern Engineering, University of Alaska Fairbanks (Vol 2). 1185-1190. PDF Thompson M.S., Kokelj S.V., Wrona F.J., Prowse T.D. (2008). The impact of sediments derived from thawing permafrost on tundra lake water chemistry: An experimental approach. In Proceedings of the Ninth International Conference on Permafrost. Institute of Northern Engineering, University of Alaska Fairbanks (Vol 2). 1763-1768. PDF