Sediment-biogeochemical characterization of a thermokarst lake basin in Central Alaska

When permafrost is thawing, the excess water forms so called thermokarst lakes which are very dynamic systems. In this thesis, the depositional environment of a thermokarst lake in discontinuous permafrost is characterized sedimentological and biogeochemical using four sediment cores from Goldstream Lake in Central Alaska to test the hypotheses: 1) Near-shore deposits are characterized by a coarser grain size distribution, larger minerogenic input from terrestrial origin and a mixed source of lacustrine and terrestrial organic matter whereas central lake deposits are finer grained and contain less organic matter from terrestrial origin than near-shore sediments. 2) A longer sediment record from the lake center potentially preserves pre-lake deposits which are indicated by terrestrial organic matter and low organic carbon content due to microbial decomposition in the talik. Low decomposed, terrestrial peat indicates the lake on-set and is followed by a lacustrine phase. Grain size analysis, magnetic susceptibility, total carbon, total nitrogen, total organic carbon and stable carbon isotopes were used to describe the differences between near-shore and central sediments. Furthermore, the biogeochemical methods were used to describe the sedimentary succession by distinguishing between present lake sediments with lacustrine organics and wetland peat underlaid by pre-lake sediments with terrestrial organics. The results show that central lake sediments are characterized by smaller grain sizes, less minerogenic input of terrestrial origin and a more organic matter from lacustrine origin than near-shore sediments. While present lake sediments and the wetland peat layer show the expected origin of organic matter, the pre-lake sediments indicate an older lake generation preserved below the terrestrial drainage peat