Thermokarst lake dynamics and its influence on biogeochemical sediment characteristics: A case study from the discontinuous permafrost zone in Interior Alaska

Josefine.Lenz [ at ]


Under the currently projected scenarios of a warming climate, discontinuous and warm permafrost in Interior Alaska is expected to experience dramatic thinning. Thermokarst ponds and lakes give evidence for permafrost thaw and, vice versa, amplify deep thaw by talik development. During the thawing process, previously preserved organic matter is decomposed and potentially released as greenhouse gases carbon dioxide and methane. In the course of lake development and shoreline expansion, both, younger near-surface and older organic matter from slumping shores are potentially deposited in the lake basin. Lake internal bioproductivity is complementing carbon accumulation in lacustrine deposits and provides an additional source of young carbon transformed into greenhouse gases. This study presents results of two intersecting, limnolithological transects of 5 sediment cores from Goldstream Lake, a typical small, boreal thermokarst lake in Interior Alaska. With the aim to distinguish external terrestrial and internal aquatic carbon contributions to sediments, sediment samples are analyzed for the total organic carbon/total nitrogen ratio (C/N) as well as stable carbon isotopes. Selected samples are analyzed for their grain size distribution in order to reconstruct the depositional environment and accumulation conditions. The littoral zone with actively eroding shorelines is characterized by methane bubbles produced from anaerobic microbial decomposition but near-shore sediments have surprisingly low total organic carbon contents of mean 1.5 wt%; the low C/N ratio of 8.7 indicate a dominance of lacustrine plant material. Very similar results are found for sediments in the central basin but a clear shift to a terrestrial carbon signal (C/N of 22) with total organic carbon content of almost 30 wt% is presumably indicating the trash layer of the initial lake phase. The talik sediments seem to have carbon storage as low as the lake sediments but are not as well layered. Subarctic aquatic environments like Goldstream Lake demonstrate a relatively low aquatic productivity and a high biogeochemical turn-over over short periods of time. In addition, the ongoing decomposition of organic matter in talik sediments proves to be crucial to assess the contribution of thermokarst lakes to future climate change by mobilizing Ice Age soil carbon previously frozen in permafrost.

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Conference (Poster)
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AGU Fall Meeting, 11 Dec 2017 - 15 Dec 2017, New Orleans, Louisiana, USA.
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Lenz, J. , Walter Anthony, K. M. , Maio, C. , Matuszewski, F. and Grosse, G. (2017): Thermokarst lake dynamics and its influence on biogeochemical sediment characteristics: A case study from the discontinuous permafrost zone in Interior Alaska , AGU Fall Meeting, New Orleans, Louisiana, USA, 11 December 2017 - 15 December 2017 .

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