Shallow gas seeps offshore Crimea (Black Sea): Mapping, bubble flow quantification and geochemical study

This paper is the first focused study of gas bubble emissions at the Crimean shelf in the northern Black Sea. During R/V Professor Vodianickiy 125/1 Сruise in November 2022 hydroacoustic and geochemical studies were carried out to detect gas seepage along Crimean coastline to assess methane-related processes taking place in the local hydrosphere. More than 200 bubble-induced hydroacoustic anomalies in the water column were found at 7.1 km². We allocated 4 seepage areas which are arranged in the west of the study area and included most of the identified flares located primarily along 35 – 45 m isobaths. Vertical profiles of temperature, salinity, dissolved oxygen content were measured, and a video survey of the seafloor was conducted. The spatial and temporal variability of the flares was examined at Martyan seep site by multiple hydroacoustic surveys on a hours- and days-scales. The observations of the flare appearance showed that most flares are temporally variable. Calculated gas flow rate from individual flare ranged from 4.1 (0.04) to 88.6 (0.101) L·day⁻¹ (t·year⁻¹), while the average value was 38 (0.07) L·day⁻¹ (t·year⁻¹). Distinct methane transport from the seep sites into the hydrosphere was observed only for Martyan seep area by relative enrichments of dissolved methane in near-bottom waters. Sediments from the active seep area were significantly enriched in mercury (87 ± 21 ng g⁻¹) compared to the reference sites (26 ± 7 ng g⁻¹). Our analyses provide one of the first interdisciplinary descriptions of shallow gas seeps sites within the Black Sea seep system.