Cryogenic structure and ice content of lacustrine sediments in the Yukon River Basin, Alaska

Lacustrine sediments often present a significant part of the upper permafrost of plains and lowlands of Alaska. Lacustrine sediments in their contemporary state vary from ice-poor to extremely ice-rich. The ice content depends on initial conditions of freezing and further history of permafrost development, which can include partial degradation and recovery of permafrost. In Alaska, the primary mechanism of freezing and cryogenic structure formation of lacustrine sediments is para-syngenetic (term by E.M. Katasonov, 1978), typical of sediments accumulated in lakes surrounded by permafrost. Though the freezing of such sediments occurs before or immediately after the termination of sedimentation (similar to syngenetic permafrost), the freezing conditions and cryogenic structure of para-syngenetic permafrost are similar to epigenetic permafrost. The ice lenses in para-syngenetic sediments are often inclined, and the ice content is relatively small in the central parts of refrozen taliks. This type of ice distribution is generally governed by the migration of water to multidirectional fronts of freezing. We studied the cryogenic structure of lacustrine sediments across different regions of the Yukon River Basin, Alaska, including Koyukuk Flats (Hozatka Lake area), Innoko Lowlands, and Tanana-Kuskokwim Lowlands (Lake Minchumina area). Study sites are located in the discontinuous permafrost zone, where permafrost was encountered mainly within uplifted peat plateaus. Field work included study of natural exposures and drilling. The upper part of studied sections is formed by frozen organic soils up to 2-3 -m -thick underlain by lacustrine silt, which is mostly ice-rich. Volume of visible ice in silt reaches at places 40% and more. A combination of layered and reticulate cryostructures is the most typical and common cryostructure assemblage. The thickness of ice lenses generally varies from 1 to 5 cm and occasionally reaches 10 cm. Aggradation of ice during the freezing of lacustrine silt caused a sufficient heave of the ground surface. Remnants of peat plateaus are surrounded by unfrozen bogs and fens, a result of thawing and settling of ice-rich lacustrine silt. Thermokarst scars initially form at places where ice-rich silt is not protected by a thick layer of organic soil. Further development of thermokarst bogs is related mostly to lateral enlargement of thaw bulbs and collapsing of the margins of peat plateaus. Lacustrine silt within taliks is covered by woody peat accumulated under forests during the permafrost plateau stage and then by aquatic sphagnum peat accumulated after collapse.