Holocene Paleoenvironmental Reconstruction of the Buor Khaya Peninsula - A Multiproxy Approach

• Main Author: Eichhorn, Luise
• Format: Thesis
• Language: unknown
• Published: 2012
• Subjects: Arctic; Laptev Sea; Talik; Khaya; Buor-Khaya; Ice; Ice Sheet; laptev; permafrost; Thermokarst; wedge*
• Online Access: https://epic.awi.de/id/eprint/32294/; https://hdl.handle.net/10013/epic.40901

In order to make reliable predictions for future climatic trends in the Arctic, recent processes and former ecosystem dynamics during periods of climatic changes must be intensely studied. Even, because the high latitudes react fast to temperature changes, a warming could, amongst others, release the stored organic carbon from the permafrost deposits and become an essential source for greenhouse gas emissions. The study site is situated on the Buor Khaya Peninsula (71°34’N, 132°12’E) in the north east Siberian lowlands which were non-glaciated during the Pleistocene. Due to the missing isolation of an ice sheet, the ground is riddled with deep, continuous permafrost. Therefore, this region maintains typical continuous permafrost features. Focus of this study was the Holocene periglacial landscape dynamic of the peninsula which is strongly influenced by thermokarst processes until today. During the expedition “Laptev Sea – Buor Khaya 2010” in August 2010, two permafrost sediment profiles from a thermokarst depression (Alas) and an ice-rich Pleistocene remnant hill (Yedoma) were sampled. Today, the investigated profiles are exposed by the Laptev Sea as a result of coastal erosion. The Holocene paleoenvironmental reconstruction of the Buor Khaya Peninsula was accomplished using a multiproxy approach of geochronological, sedimentological, biogeochemical, and cryolithological parameters as well as macro fossils. Sediments were analysed for radiocarbon age, grain-size determination, mass specific susceptibility, gravimetric ice content, biogeochemistry (total organic carbon, total carbon, total nitrogen, stable carbon isotopes) and density fractionation. On ground-ice (texture ice, fissure ice and an ice wedge), stable hydrogen isotopes were measured as well as on recent lake water and precipitation. The profiles exposed three different stages of a periglacial landscape: a subaquatic stage of a thermokarst lake and a subaerial stage after the lake’s existence. The third stage refers to the initiation phase of a thermokarst development. Geochronological results based on six radiocarbon ages using macro plant remains, dated the profiles of Holocene age (8.000 ± 80 a BP and 4.760 ± 40 a BP). Both profiles show very poorly sorted sediment with a small peak in fine silt and a large peak in the coarse silt to fine sand fraction. The resulting bi- to polymodal grain-size distribution reveals polygenetic depositional regimes. The similarity of the grain-size distribution of both profiles suggests that Alas sediment was relocated due to thermokarst processes and originates from a Yedoma. Thermokarst lake sediments formed due to thermokarst processes with stable deposition conditions. Ostracods verified lake environments and plant macro fossils describe wet to boggy conditions. The disappearance of the lake is visible in the sedimentological and biogeochemical data. The freezing processes of the lake’s talik could be reconstructed from stable water isotopes of texture ice. An increasing mean grain size during the terrestrial deposition indicates higher transportation energies. The initiation stage of a thermokarst development shows sediment characterized by alternating peat-rich and silty layers. These different layers indicate changes from boggy to drier conditions which were caused by periodic active layer deepening and/or drainage. The density fractionation revealed the mechanical degradation of organic matter due to the steady freeze-thaw-cycles around the permafrost table. The frozen and thawed samples of the same material revealed no significant changes e.g. in the TOC content within approximately one year of incubation time.