ИСТИНА

Recent studies do change our view of the vulnerability of the large sub-sea permafrost carbon reservoir on the east Siberian Arctic Shelf (ESAS); the permafrost ��lid�� is clearly perforated, and sedimentary CH4 is escaping to the atmosphere. To discern whether this extensive CH4 venting over the ESAS is a steadily ongoing phenomenon or signals the start of a more massive CH4 release period, there is an urgent need for expanded multifaceted investigations into these inaccessible but climate-sensitive shelf seas north of Siberia. Subsea permafrost stability is the key to whether preformed methane sequestered in hydrate deposits or deep geological methane pools escapes to the overlying strata. The present-day coverage, thickness and thermal state of the ice bonded permafrost layer in the East Siberian Arctic Shelf (ESAS) is a subject to active discussion, and its intact nature is argued due to existence of methane release evidence pointing to numerous gas pathways. The objective of this study is to both establish time-domain electromagnetic method (TDEM) as a powerful technique to upscale the observational window of subsea permafrost and to apply it to several long (10s-100s km) transects in the ESAS region to contribute the much-needed observational constraints on the actual thermal and physical state of the subsea cryogenic domain. Based on TDEM data collected in the fall of 2020 onboard research vessel Akademik Mstislav Keldysh (AMK) in ESAS, we construct subbottom resistivity images for area with earlier-discovered powerful methane seeps east of the Lena delta as well as for the arbitrary-chosen W-E line stretching across the shallow-water shelf from Buor-Khaya Gulf of the Laptev Sea to Dmitry Laptev Strait, interpreted assuming 4-layer structure. In the methane-seep area, a conductive region was imaged which displays pronounced correlation with seep-activity profile, thus interpreted as thawed sediments�� zone. High spatial correlation between massive methane release and TDEM pinpointed locations of thawing zones support our hypothesis about existence of through taliks which are functioning as gas pathways to connect huge subsea geological methane potential with atmosphere. In case of a long transect in the southern part of Laptev Sea, the most clear features of the generally-resistive permafrost layer include high-resistivity region in the western part of the line and a series of conductive regions associated with tectonic faults, elevated heat flux and taliks, characterized by massive methane ebullition from sediments into the atmosphere and pointing substantial heterogeneity in the state of the permafrost, possibly linked to complex tectonic structure and heat flux heterogeneity. Discovery of deep gas conduits in the nearshore zone despite the analyzed transect is located close to the shore, where permafrost temperature is about 10 C colder and thicker, i.e. higher degree of stability of the permafrost is expected. This experimentally proofed result is indicative of substantial permafrost degradation even in such setting. Thus, TDEM technique can be further used for the study of subsea permafrost dynamics and its mapping and validation of process-oriented permafrost models.