Ice wedge polygon development on different temporal and spatial scales in the northern Yukon, Canada

Ice wedge polygons (IWP) are amongst the most typical permafrost phenomena in Arctic lowlands. Within the northern hemisphere, IWP are thought to occupy between 250,000 km² (Minke et al., 2007) and 2,600,000 km² (Mackay 1972) of the tundra and boreal forest, which accounts for 3 to 31% of the arctic land mass including glaciated regions. Besides the wide spatial distribution, IWP have stored large quantities of organic carbon and nitrogen on geological timescales and are therefore regarded as greenhouse gas sinks. Continuous organic matter accumulation and preservation due to syngenetic permafrost aggradation make arctic polygon mires an excellent climate and environmental archive. \n \nHere we present the results of a multidisciplinary palaeoenvironmental study on IWP in the northern Yukon, Canada. High-resolution laboratory analyses were carried on a permafrost core together with the overlying active layer (233 cm length) which was drilled in 2012. Based on 14 AMS radiocarbon dates spanning the last 5,000 years, we report high-resolution ground ice stratigraphy, stable water isotopes (δ18O, δD), sedimentary data including grain size distribution and biogeochemical parameters (OC, N, C/N ratio, δ13C), as well as pollen and diatom assemblages. This is accompanied by high-resolution remote sensing data based on airborne LIDAR and on underground investigations using electrical resistivity tomography in different resolutions. \n \nThe studied low-centered IWP indicates that the whole IWP field was established after a shallow lake had drained at about 3200 cal BP. The diatom assemblage in the lower part of the sedimentary record is dominated by planktonic and pioneer species and by those preferring alkaline conditions. Ice-wedge cracking in water-saturated sediments started immediately after lake drainage and led to the formation of a polygon mire. Downward closed-system freezing of the talik is indicated by continuously decreasing δ18O (δD) values, a δ18O/δD-regression-slope below the Global Meteoric Water Line and a negative relationship between δD and D excess. \n \nOn the one hand, pollen assemblages in lake sediments have captured a regional signal of vegetation composition and climate. On the other hand, we assume that after lake drainage the pollen record represents a very local signal as it is dominated by the local plant communities growing in the IWP. Therefore, we suggest that the ability to infer regional climate information on temperature and precipitation is good for lake sediments but weak for the overlying peat record. This is indicated by a sudden dominance of Cyperaceae pollen after the transition from lake sediments into terrestrial peat of the IWP. \n \nOther IWP along the mainland coast of the Yukon suggest a high temporal diversity in polygon mire origin and behavior. IWP beyond the late Wisconsin glacial limit are mostly high-centered with strong signs of degradation. Coastal cliff exposures with deeply thawed ice wedge surfaces and secondary or even tertiary IW generations support this view. Ice wedge casts dating back until 8,400 cal BP (Fritz et al., 2012) indicate previous periods of ice wedge degradation and meltout. Intermediate forms of IWP (neither low-centered nor high-centered) dominate the mainland coast of the Yukon on the rolling ground moraines. Glacial outwash plains at the former glacial border host mostly low-centered IWP. On Herschel Island, we find many generations of IW and corresponding sedimentary records in the centers although IWP are not as frequent on the high-relief endmoraine, which is Herschel Island, than on the relatively flat mainland coast of the Yukon. Holocene IWP have mostly a surficial expression whereas the older late Wisconsin/Pleistocene/glacial IWP are often buried under a 0.7 to 1.5 m thick sediment cover. Higher than modern summer air temperatures, presumable during the Holocene thermal maximum, caused deeper thaw and led to a truncation of late Wisconsin/Pleistocene/glacial IW. This suggests that remote-sensing based estimations of arctic-wide IWP coverage will give conservative numbers as buried IWP systems will remain invisible. \n \nReferences \nFritz, M., Wetterich, S., Schirrmeister, L., Meyer, H., Lantuit, H., Preusser, F., Pollard, W.H., 2012. Eastern Beringia and beyond: Late Wisconsinan and Holocene landscape dynamics along the Yukon Coastal Plain, Canada. Palaeogeography, Palaeoclimatology, Palaeoecology 319–320, 28-45. doi:10.1016/j.palaeo.2011.12.015. \n \nMackay, J.R., 1972. The world of underground ice. Annals of the Association of American Geographers 62, 1-22. \n \nMinke, M., Donner, N., Karpov, N.S., de Klerk, P., Joosten, H. (2007). Distribution, diversity, development and dynamics of polygon mires: examples from Northeast Yakutia (Siberia), Peatlands International 1/2007, 36-40.

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