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| Titel |
Thermal processes of thermokarst lakes in the continuous permafrost zone of northern Siberia – observations and modeling (Lena River Delta, Siberia) |
| VerfasserIn |
J. Boike, C. Georgi, G. Kirilin, S. Muster, K. Abramova, I. Fedorova, A. Chetverova, M. Grigoriev, N. Bornemann, M. Langer |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 20 ; Nr. 12, no. 20 (2015-10-19), S.5941-5965 |
| Datensatznummer |
250118130
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| Publikation (Nr.) |
 copernicus.org/bg-12-5941-2015.pdf |
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| Zusammenfassung |
| 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. |
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