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The conditions of ice cover formation in the Sharapov Shar Lagoon of the Kara Sea and its role in sediment freezing at the fast ice/bottom contact zone are considered. Based on field measurements, remote sensing data, and calculations, characteristic periods of the formation and destruction of the ice cover and its thickness over the past 46 years have been analyzed. It is shown that fast ice in the lagoon is observed for an average of 252 days a year; the ice thickness by the end of winter averages 139 cm and can exceed 160 cm in some winters. The base of sea ice is in contact with the bottom within a significant part of the lagoon: 49 % of the area in mild winters and 60 % in severe winters. Numerical modeling indicates that at sea depths of more than 0.9 m, a seasonally frozen layer with a thickness of up to 0.8 m for loamy soils and 1.2 m for sandy soils is formed under the fast ice in winter and completely thaws in summer. Calculations have also shown that in the 20th century, there were conditions favorable for permafrost formation in the zone where sea ice is frozen to the bottom. The formation of permafrost was possible at depths of up to 90 cm. In the 21st century, rising air temperatures and increasing ice-free period lead to permafrost degradation. Under modern conditions of global warming, sandy sediments can remain in a frozen state at sea depths of up to 0.7 m; clayey sediments - up to 0.3 m.