Seabed fluid flow cases in paleovalleys of the Kara sea shelf

Expeditionary research during 52 and 56 cruises of the R/V “Akademik Nikolaj Strakhov” in 2021‒2023, was aimed at surveying a part of the Kara Sea water area northwest of the Bely Island. Based on seismoacoustic data, signs of degassing were identified in the relief, sedimentary cover and water column at several sites covering fragments of relict fluvial forms, i. e. paleovalleys. Three sites with different mesorelief and structure of the upper part of the loose sediment section were studied, covering fragments of paleovalleys at depths from 40 to 250 m. It was established that there are special degassing conditions in the paleovalleys of the southwestern part of the Kara Sea shelf, which are predetermined by the relief and geocryological structure of the bottom. Vertical migration of fluids from deep horizons leads to their accumulation at the bottom surface, where their uneven unloading occurs, depending on the combination of geomorphologic and geocryological conditions. Permafrost layer, which are a powerful fluid seal, are developed fragmentarily in the valleys. The concentration of gas shows increases in areas of their wedging out. Based on seismoacoustic and bathymetric data, pockmarks, pingo-like forms, dome-shaped uplifts, and mud volcanic formations were identified resulting from the fluidogenic discharge. The spectrum of fluidogenic forms developed in paleovalleys is determined by the morphology of the relief and their depth: pockmarks and mud volcanic structures are more typical for valley bottoms, where frozen rocks are distributed fragmentarily or are completely absent. Pingo-like forms (PLF) are found on the slopes of valleys where the thickness and cohesion of permafrost decreases. Mud diapirsdome-shaped uplifts are developed in the areas with almost complete absence of permafrost. Outside the valleys, gas shows and fluidogenic forms are isolated, which is associated with the presence of a fluid seal in the form of a permafrost layer. The results of the study allowed to establish that paleovalleys are the main fluid discharge zones on the shelf of the Kara Sea, and the permafrost degradation, including under the influence of fluid migration, determines the modern microrelief and dynamics of the processes. The data obtained are consistent with studies in other Arctic regions (the Beaufort Sea shelf, the Mackenzie River delta), thus confirming the universality of the mechanisms of interaction between permafrost and fluid flow within the shelf zone of the Arctic.

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