Past Permafrost and Landscape Dynamics in Central Beringia: Two Case Studies from Drained Thermokarst Lake Basin Cores

Josefine.Lenz [ at ]


Thermokarst is a commonly observed process in the Arctic and an indicator of permafrost degradation. The formation of thermokarst landforms may indicate a localized disturbance to the ground thermal regime or be indicative of widespread permafrost degradation driven by climate-induced top-down permafrost thaw. Thermokarst lakes are one of the most prominent landforms that develop in ice-rich arctic and boreal lowlands. In our study, we investigate two drained thermokarst lake basins on the northern Seward Peninsula in Central Beringia to gain insights into site-specific landscape development and past permafrost dynamics during glacial and interglacial periods. A 350 cm permafrost core (core ID: Kit-43) and a 400 cm permafrost core (core ID: Kit-64) were acquired from two drained lake basins that represent contrasting geological settings and cover a range of time periods (Mid-Wisconsin to Holocene). Kit-64 was a first generation lake in yedoma upland; Kit-43 was a later generation lake in a thermokarst-shaped lowland. The cores were analyzed using a multi-proxy approach including sedimentological (magnetic susceptibility, grain size), biogeochemical (TN, TC, TOC, δ13C), and micropaleontological methods (ostracods, testaceans), tephra analyses, radiocarbon dating on sediments, as well as isotope geochemical methods (δD and δ18O) on intra-sedimentary ground ice. The Kit-64 core was acquired from a first generation lake basin on a yedoma upland that preserved a depositional environment of more than 45,000 years (Lenz et al., 2015) including: (1) Mid-Wisconsin yedoma accumulation, (2) intermediate wetland development between 41,500 and 44,500 yr BP, (3) South Killeak Maar tephra deposition that interrupted the wetland development, (4) continued terrestrial yedoma accumulation probably until the Late Glacial when a significant gap in the sedimentary record indicates formation of thermokarst lakes in the surroundings of this site that prevented further accumulation, and (5) finally a 300 cal yr BP thermokarst lake initiated and rapidly grew at the site which then drained in 2005 AD. Modern permafrost aggradation is indicated in core Kit-64 however, unfrozen talik sediments were still present below a depth of 266 cm four years after drainage. The Kit-43 core archived 12,100 cal yrs BP of predominately lacustrine deposition (Lenz et al., submitted) including: (1) Late Glacial thermokarst initiation and development of a deep thermokarst lake, (2) Early Holocene transition to a second, large and more dynamic lake generation by 9,500 cal yr BP with water level changes and intermediate wetland phases and tephra re-deposition from the catchment and (3) Late Holocene complete drainage at 1,060 cal yr BP with subsequent peatland development. Interestingly, the isotopic composition of the intrasedimentary ice does not only capture the filling of the modern active layer by summer precipitation but also preserve similar patterns for the paleo-active layer 42-82 cm below modern surface, as well as permafrost aggradation in the refreezing paleo-lake talik from the surface down to 154 cm. The presence of an adjacent lake as a uniform water source presumably explains constant isotopic values below 154 cm. The presence of multiple lake cycles in the Kit-43 core, which was also corroborated from other basins by landscape-scale analysis of remote sensing images (Jones et al., 2012), emphasizes the complexity of organic carbon trajectories in thermokarst lake environments. Present TOC storage in lake sediments of the first lake generation of the Kit-43 basin during the Late Glacial period appears to be reduced due to originally lower organic matter inputs from late Pleistocene sediments being eroded at the shores as well as their long-term position within the talik of the following lake generation, allowing ongoing decomposition and release of carbon for several thousands of years. The later lake generation in turn had higher TOC contents most likely due to higher input from lake expansion into neighboring basins with peat cover. Hence, later generation lakes have high potential to sequester organic matter from erosion of organic-rich Holocene deposits. These two case studies reveal the complex nature of Arctic landscapes that are affected by permafrost degradation and aggradation during a time when first humans migrated from Eurasia through North-America. They also highlight the interaction of global climate change, regional environmental dynamics, and local disturbance processes on different temporal scales. The maturation of landscapes by thermokarst dynamics on local level can have a significant influence on regional to global biogeochemical cycles. References: Jones MC, Grosse G, Jones BM, Walter Anthony KM (2012). Peat accumulation in drained thermokarst lake basins in continuous, ice-rich permafrost, northern Seward Peninsula, Alaska, Journal of Geophysical Research 117: G00M07. DOI: 10.1029/2011JG001766 Lenz J, Grosse G, Jones BM, Walter Anthony KM, Bobrov A, Wulf S and Wetterich S (2015). Mid-Wisconsin to Holocene Permafrost and Landscape Dynamics based on a Drained Lake Basin Core from the Northern Seward Peninsula, Northwest Alaska, Permafrost and Periglacial Processes. DOI: 10.1002/ppp.1848 Lenz J, Wetterich S, Jones BM, Meyer H, Bobrov A, Grosse G (submitted). Evidence of multiple thermokarst lake generations from a 12,100 years permafrost core on Northern Seward Peninsula, NW Alaska.

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11th International Conference on Permafrost, 20 Jun 2016 - 24 Jun 2016, Potsdam, Germany.
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Lenz, J. , Wetterich, S. , Jones, B. M. , Walter Anthony, K. M. , Meyer, H. , Bobrov, A. , Wulf, S. and Grosse, G. (2016): Past Permafrost and Landscape Dynamics in Central Beringia: Two Case Studies from Drained Thermokarst Lake Basin Cores , 11th International Conference on Permafrost, Potsdam, Germany, 20 June 2016 - 24 June 2016 .

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