Thermal processes of thermokarst lakes in the continuous permafrost zone of northern Siberia – observations and modeling (Lena River Delta, Siberia)

[摘要] Thermokarst lakes are typical features of the northern permafrost ecosystems,and play an important role in the thermal exchange between atmosphere andsubsurface. The objective of this study is to describe the main thermalprocesses of the lakes and to quantify the heat exchange with the underlyingsediments. The thermal regimes of five lakes located within the continuouspermafrost zone of northern Siberia (Lena River Delta) were investigatedusing hourly water temperature and water level records covering a 3-yearperiod (2009–2012), together with bathymetric survey data. The lakesincluded thermokarst lakes located on Holocene river terraces that may beconnected to Lena River water during spring flooding, and a thermokarst lakelocated on deposits of the Pleistocene Ice Complex. Lakes were covered by iceup to 2 m thick that persisted for more than 7 months of the year, fromOctober until about mid-June. Lake-bottom temperatures increased at the startof the ice-covered period due to upward-directed heat flux from theunderlying thawed sediment. Prior to ice break-up, solar radiationeffectively warmed the water beneath the ice cover and induced convectivemixing. Ice break-up started at the beginning of June and lasted until themiddle or end of June. Mixing occurred within the entire water column fromthe start of ice break-up and continued during the ice-free periods, asconfirmed by the Wedderburn numbers, a quantitative measure of the balancebetween wind mixing and stratification that is important for describing thebiogeochemical cycles of lakes. The lake thermal regime was modelednumerically using the FLake model. The model demonstrated good agreement withobservations with regard to the mean lake temperature, with a goodreproduction of the summer stratification during the ice-free period, butpoor agreement during the ice-covered period. Modeled sensitivity to lake depthdemonstrated that lakes in this climatic zone with mean depths> 5 m develop continuous stratification in summer for at least1 month. The modeled vertical heat flux across the bottom sediment tendstowards an annual mean of zero, with maximum downward fluxes of about5 W m⁻² in summer and with heat released back into the water column ata rate of less than 1 W m⁻² during the ice-covered period.The lakes are shown to be efficient heat absorbers and effectively distributethe heat through mixing. Monthly bottom water temperatures during theice-free period range up to 15 °C and are therefore higher than theassociated monthly air or ground temperatures in the surrounding frozenpermafrost landscape. The investigated lakes remain unfrozen at depth, withmean annual lake-bottom temperatures of between 2.7 and 4 °C.

[学科分类] 地球化学与岩石