Quantification of bubble methane reaching atmosphere in the Arctic

Relevance. Gas flares or seeps consisting of bubbles continuously rising from the seabed have been recorded throughout the World Ocean at depths ranging from several meters to three kilometers or more. Measurements of the gas composition of the rising bubbles shown that they are dominated by methane (CH4). The East Siberian Arctic Shelf contains more than 30% of the world CH4 and carbon dioxide reserves, preserved in bottom sediments by underwater permafrost. In the shallow seas of the East Siberian Arctic Shelf, the main mechanism for transporting CH4 from bottom sediments into water is bubble transport. Therefore, it is extremely important to estimate the amount of CH4 transported by rising bubbles from bottom sediments into the water column and the atmosphere. Aim. Estimation of CH4 quantity transported by chains of rising bubbles from the seabed to the atmospheric surface layers. Methods. The manuscript presents a study of the mechanism of gas exchange between rising bubbles and a liquid column, carried out using a specially designed stand that allows simulating local upwelling. Results and conclusions. The paper shows that chains of bubbles coming from the seabed with intensities of ~40 ml∙min–1 and ~110 ml∙min–1, taking into account the hydrostatic pressure, deliver 206 mg∙min–1 and 616 mg∙min–1 of CH4 to the bottom layer of the water column, respectively. The results obtained during laboratory studies allowed us to specify the flux of CH4 from bottom sediments to the atmospheric surface layers. Taking into account the quantity of gas exchange and local upwelling, the amount of methane delivered to the atmospheric surface layers was 69 mg∙min–1 and 286 mg∙min–1. The paper presents an acoustic estimate of the amount of CH4 transported by the seep, which includes the considered chains of rising bubbles. According to calculations, the flux of CH4 into water from this area in 2012 was ~40 g∙min–1. At the same time, the amount of CH4 transported by this seep into the atmospheric surface layers, taking into account local upwelling and gas exchange occurring between the rising bubbles and the liquid column, is ~24.5 kg per day or ~9 tons per year.