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Boike, Julia; Georgi, C; Kirilin, G; Muster, Sina; Abramova, Katya; Fedorova, Irina; Chetverova, Antonina; Grigoriev, Mikhail N; Bornemann, Niko; Langer, Moritz (2015): Temperature, water level and bathymetry of thermokarst lakes in the continuous permafrost zone of northern Siberia - Lena River Delta, Siberia. PANGAEA,, Supplement to: Boike, J et al. (2015): Thermal processes of thermokarst lakes in the continuous permafrost zone of northern Siberia - observations and modeling (Lena River Delta, Siberia). Biogeosciences, 12(20), 5941-5965,

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Thermokarst lakes are typical features of the northern permafrost ecosystems, and play an important role in the thermal exchange between atmosphere and subsurface. The objective of this study is to describe the main thermal processes of the lakes and to quantify the heat exchange with the underlying sediments. The thermal regimes of five lakes located within the continuous permafrost zone of northern Siberia (Lena River Delta) were investigated using hourly water temperature and water level records covering a 3-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. Lakes were covered by ice up to 2 m thick that persisted for more than 7 months of the year, from October until about mid-June. Lake-bottom temperatures increased at the start of the ice-covered period due to upward-directed heat flux from the underlying thawed sediment. Prior to ice break-up, solar radiation effectively warmed the water beneath the ice cover and induced convective mixing. Ice break-up started at the beginning of June and lasted until the middle or end of June. Mixing occurred within the entire water column from the start of ice break-up and continued during the ice-free periods, as confirmed by the Wedderburn numbers, a quantitative measure of the balance between wind mixing and stratification that is important for describing the biogeochemical cycles of lakes. The lake thermal regime was modeled numerically using the FLake model. The model demonstrated good agreement with observations with regard to the mean lake temperature, with a good reproduction of the summer stratification during the ice-free period, but poor agreement during the ice-covered period. Modeled sensitivity to lake depth demonstrated that lakes in this climatic zone with mean depths > 5 m develop continuous stratification in summer for at least 1 month. The modeled 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.
Median Latitude: 72.365420 * Median Longitude: 126.438892 * South-bound Latitude: 72.328690 * West-bound Longitude: 126.177830 * North-bound Latitude: 72.378175 * East-bound Longitude: 126.512292
Date/Time Start: 2009-07-04T09:00:00 * Date/Time End: 2012-08-14T16:00:00
In July 2009, water level and temperature sensors (HOBO Temp Pro v2, HOBO U20, Onset, ± 0.2°C across a temperature range from 0 °C to 70 °C, and ± 0.4°C across a temperature range from -40 °C to 0 °C) were installed within the water columns of the investigated lakes on Samoylov and Kurungnakh Island. Figure 1 shows the locations of the lakes (labelled Sa_Lake_1-4 for Samoylov and Ku_Lake_1 for Kurungnakh) and the location of the long term weather station. Gaps in the climate data record (air temperature, radiation, humidity, wind speed and direction, and snow depth) were filled whenever possible with data from temporary climate and eddy covariance stations located in close proximity to the weather station (Boike et al., 2013). Temperature and water depth sensors were placed directly above the sediment-water interface and then temperature sensors at 2 m intervals up to 2 m below the water surface (Figure 2). The sensors were suspended in the water column from a buoy and anchored in the sediment below. The sensor at the bottom of the lake (just above the sediment) was labelled as "0 m", the sensor 2 m above the sediment as "2 m", and so on. The uppermost sensors were usually about 2 m below the water surface since we were concerned about the formation of ice and the potential drift of sensors with the shifting of ice cover. End-of-winter ice thickness (obtained by drilling) was measured in 2014; it ranged between 1.9 and 2 m in lakes Sa_Lake_1-4 on Samoylov Island. During some winters the uppermost sensors became enclosed within the ice cover (for example, Sa_Lake_1 in 2012), but they were not moved out of position. One sensor was installed in the Lena River during August 2009 (Figure 1) and recorded data from July 2009 to August 2010 but was lost during the following year.
Sensors were usually retrieved once a year (in August) and then re-launched in approximately the same position. The temperature record was therefore briefly interrupted during the period when the sensors were retrieved and read. The water depth ("sensor depth") recorded by the bottom sensor sometimes changed following retrieval due to a change in the sensor position, although the actual water level of the lake remained the same. For example, for Sa_La_4 (a perched lake), sensors that were deployed at a water depth of about 8.5 m in 2009 were and re-installed at a depth of about 9.5 m in August 2010 Water level variations due to water balance changes (when the sensor position had not changed), for example during the summer period, were usually less than 0.5 m.
Data is only available over a one-year period for the lake on Kurungnakh Island (2009-2010) as the loggers were subsequently displaced, presumably during ice break-up. For the lakes on Samoylov Island, however we obtained continuous temperature and water level data over a period of 3 years from 2009 to 2012.
Bathymetric surveys were carried out in 2009 and 2010 on all of the investigated thermokarst lakes, using a GPSMAP 178 C echo sounder, a GPSMAP 421S plotter and a GPS 60 navigator, all from Garmin. The shorelines were mapped either by GPS field survey or by manually digitizing the shoreline from high resolution aerial images. The accuracy of the echo sounder equipment was about 0.1 m and was regularly checked using manual profiling. Depth measurements were taken along the longest lake axis as well as along a zigzag track in order to cover most of the lake surface and to locate any local "holes" that might exist as a result of thermokarst processes.
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