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

Abstract

Abstract. The thermal regimes of five lakes located within the continuous permafrost zone of northern Siberia (Lena River Delta) have been investigated using hourly water temperature and water level records covering a three year period (2009–2012), together with bathymetric survey data. The lakes included thermokarst lakes located on Holocene river terraces that may be connected to Lena River water during spring flooding, and a thermokarst lake located on deposits of the Pleistocene Ice Complex. The data were used for numerical modeling with FLake software, and also to determine the physical indices of the lakes. The lakes vary in area, depths and volumes. The winter thermal regime is characterized by an ice cover up to 2 m thick that survives for more than 7 months of the year, from October until about mid-June. Lake-bottom temperatures increase at the start of the ice-covered period due to upward-directed heat flux from the underlying thawed sediment. The effects of solar radiation return prior to ice break-up, effectively warming the water beneath the ice cover and inducing convective mixing. Ice break-up starts the beginning of June and takes until the middle or end of June for completion. Mixing occurs within the entire water column from the start of ice break-up and continues during the ice-free periods, as confirmed by the Wedderburn numbers. Some of the lakes located closest to the Lena River are subjected to varying levels of spring flooding with river water, on an annual basis. Numerical modeling using FLake software indicates that the vertical heat flux across the bottom sediment tends towards an annual mean of zero, with maximum downward fluxes of about 5 W m−2 in summer and with heat released back into the water column at a~rate of less than 1 W m−2 during the ice-covered period. The lakes are shown to be efficient heat absorbers and effectively distribute the heat through mixing. Monthly bottom water temperatures during the ice-free period range up to 15 °C and are therefore higher than the associated monthly air or ground temperatures in the surrounding frozen permafrost landscape. The investigated lakes remain unfrozen at depth, with mean annual lake-bottom temperatures of between 2.7 and 4 °C. The data are available in the Supplement for this paper and through the PANGAEA website (http://www.pangaea.de/).

Observation-based modelling of permafrost carbon fluxes with accounting for deep carbon deposits and thermokarst activity

“…Walter et al (2007a) suggest an annual CH 4 release of 30 to 40 Tg-CH 4 from thermokarst lakes to partially explain CH 4 excursions of early Holocene atmospheric CH 4 levels. Brosius et al (2012) discuss a yearly contribution from thermokarst lakes of 15 ± 4 Tg-CH 4 during the Younger Dryas and 25 ± 5 Tg-CH 4 during the Preboreal period.…”

“…g The potential for increases in wetland extent in mineral soils is considered larger than for the other soil pools because the initial assumed wetland fraction in mineral soils is rather small. h Early Holocene warming by a few degrees Celsius in Northern Hemisphere land areas (Kaufman et al, 2004;Velichko et al, 2002;Marcott et al, 2013) resulted in rapid and intensive thermokarst activity (Walter et al, 2007a;Brosius et al, 2012). zova et al, 2012;Laurion, 2010).…”

Observation-based modelling of permafrost carbon fluxes with accounting for deep carbon deposits and thermokarst activity

“…This is because the degradation in thermokarst-affected sediments is driven by lake-bottom temperatures. Averaged over a full year, lakebottom temperatures do not strongly differ between moderate and strong surface air warming (see also Boike et al (2015) and the Supplement).…”

Ratio of In Situ CO2 to CH4 Production and Its Environmental Controls in Polygonal Tundra Soils of Samoylov Island, Northeastern Siberia

“… The study site on Samoylov Island, Lena River Delta, Northeastern Siberia (72°22′N, 126°28′E) from the perspective of the Eurasian continent. Panel (a): Eurasia (Google, n.d.); (b): Orthorectified aerial picture of Samoylov Island, Lena River Delta, Northeastern Siberia, Russia (Boike et al., 2012); (c): Studied polygon in Samoylov Island (Boike et al., 2015). …”