Gas-Geochemical Features of Bottom Sediments in the Linear Depression Zone of the West Kara Stage

During cruise 89 of the R/V Akademik Mstislav Keldysh in 2022, sediments cores were sampled at stations 7441 and 7444 in the southwestern Kara Sea. Station 7444 was located on a large submeridional depression, under the bottom of which gas-saturated sedimentary strata were detected. Background station 7441 was located 68 km from station 7444. For the sediments of the background station 7441, the ratio of hydrocarbon gases C1/C2+ < 100 indicated their thermogenic nature. In the sediments at station 7441, the formation of the gas component in the sediments was due to degradation of OM and influx of thermogenic gases, while in the sediments of station 7444, there was an influx of biogenic gas, apparently from permafrost. The average CH4 concentration in the sediments of station 7444 exceeded the average concentration in the sediments of core 7441 by 700 times, and the average CO2 concentrations in the sediments of stations 7444 and 7441 were comparable. A sulfate–methane transition zone (SMTZ) was detected at the 541–545 cm horizon of the sediments of station 7444, where the sulfate concentration decreased to the minimum values, CH4 and CO2 concentrations reached maximum values. The sulfur isotopic composition of δ34S in this region was +20.8‰. The biogenic nature of gas in the sediments of station 7444 was evidenced by low values of the carbon isotopic composition of CH4 (mean value δ13C(CH4) = –99.7‰) and high C1/C2+ >10 000 ratio near the SMTZ.