The Yedoma cliff of Sobo Sise Island, eastern Lena Delta - insights into past and modern permafrost dynamics and related organic matter stock and release

• Main Authors: Wetterich, Sebastian, Fuchs, Matthias, Kizyakov, Alexander I., Strauss, Jens, Nitze, Ingmar, Fritz, Michael, Wolter, Juliane, Opel, Thomas, Meyer, Hanno, Aksenov, Aleksei, Schirrmeister, Lutz, Grosse, Guido
• Format: Still Image
• Language: unknown
• Published: Zenodo 2020
• Subjects: Arctic; Ice; lena delta; lena river; permafrost; wedge*; Siberia
• Online Access: https://dx.doi.org/10.5281/zenodo.4268104; https://zenodo.org/record/4268104

The Lena Delta in eastern Siberia is the largest Arctic delta. Its terrestrial surface is shaped by four geomorphologic units. The oldest unit is built of remnants of late Pleistocene Yedoma Ice Complex (IC) and its degradation features. The studied Yedoma cliff on Sobo Sise Island in the South-Eastern part of the delta ranges from the river level to about 28 m height and is about 1.7 km long. During a field campaign in 2018, the entire permafrost sequence of the Sobo Sise Yedoma cliff has been sampled in 0.5-m vertical intervals. The geochronological record of the Sobo Sise Yedoma spans the last 52 ka cal BP based on radiocarbon dating and age-height modelling. The sequence differentiates into three cryostratigraphic units that are MIS3 Yedoma IC (52–28 ka cal BP), MIS2 Yedoma IC (28–15 ka cal BP) and MIS1 Holocene cover (7–0 ka cal BP). The cryostratigraphic sequence is not continuous, but has chronological gaps at 36–32.5 ka cal BP, at 20.5–18 ka cal BP and at 12.5–9 cal ka BP. These gaps represent traces of past changes in permafrost deposition and/or erosion regimes and climatic conditions. The cryostratigraphic units of the Yedoma cliff are characterized by differing properties of their clastic, organic and ice components. All units are built of poorly sorted sandy silt but differ in prevalent grain-size fractions ranging from fine silt to middle sand. The organic matter (OM) content is highest in the thin MIS1 Holocene cover (TOC of 11.3±9.9 wt%, TN of 0.6±0.3 wt%), but still substantial in MIS3 Yedoma IC (TOC of 4.5±2.5 wt%, TN of 0.3±0.1 wt%) and in MIS2 Yedoma IC (TOC of 2.1±1.3 wt%, TN of 0.2±0.1 wt%). The presence of syngenetic ice wedges in all units and the high content of intrasedimentary ice amount to a total volumetric ice content of 88.4 vol%. The high ice content in combination with the exposition of the cliff towards the main river channel results in a very high susceptibility to thaw and thermo-erosion. The high mean cliff erosion rate of 10.3 m yr−1 (1975-2018) results in large OM quantities entering the Lena River (3.2±2.1 kt organic carbon per year, 0.3±0.1 kt nitrogen per year along the 1.7 km long Yedoma cliff). Ongoing fluvial dynamics and changing runoff regimes with extended ice-free seasons and warmer water will most likely maintain high permafrost cliff erosion rates in the future and further facilitate high fluxes of terrestrial fossil OM into the riverine and eventually marine ecosystems.