ACOUSTIC INVESTIGATIONS OF THE DEEPEST METHANE SEEPS IN THE OKHOTSK SEA

Abstract

The relevance. Seeps – areas of bubble emanation of natural gases from the seabed into the water column and the atmosphere – were found in different World Ocean locations at depths from tens meters to several kilometers. Direct measurements of gas samples carried by rising bubbles have shown that they contain more than 80 % of the second most important greenhouse gas methane. The emissions of which, due to the re-preservation of deposits of natural hydrocarbons (for example, arctic or oceanic gas hydrates), can acquire a large-scale character and cause irreversible climatic changes. Detection of seeps in the water column may be the first indication of the presence of gas hydrates in the sediment. Therefore, assessment of the methane fluxes carried by seeps into the water column, detection of their variability, and identification of new regions of seepage are in the forefront of the marine and climate-related sciences. Moreover, understanding of deep seepage mechanism from disturbed gas hydrates observed in the Sea of Okhotsk is crucially important for study of methane ebullition from the shelf slope hydrates in the East Siberian Arctic seas – source of atmospheric methane of global significance. The main aim: to assess the quantity of methane transported by methane seeps from a depth of 2220 m into the water for the period from 2012 to 2018; to identify the relationship between the flux of methane from this area and deep earthquakes that occurred in the Sea of Okhotsk; based on the available acoustic data, estimate the speed of deep-water currents in the seepage areas. Objects: seeps, which forms hot spots of anomalously high dissolved methane concentrations in the water column-atmosphere. Methods. Acoustic observations of the seeps were carried out using a complex unit installed on board of the RV «Academic M.A. Lavrentiev», which is based on modernized ship echosounders Sargan-EM, ELAC LAZ-72, Sargan-GM sonars and a multichannel system for digital recording of acoustic signals. Methane flux from the seabed into the water column was estimated using methods based on measuring the profiles of the sound backscattering at frequencies above the resonant frequency of the escaping bubbles. Results. In 15 expeditions accomplished in the Sea of Okhotsk, 1168 individual seeps were recorded in the depth interval from 46 to 3330. To identify the patterns of the spatial distribution of seeps, a graph was plotted as their number vs the depth of their detection. The resulting graph shows five local maximums associated with the features of the bottom relief and the zone of stability of methane gas hydrate. The deepest known seeps in the Sea of Okhotsk were found in the northern and eastern slopes of the Kuril Basin at depths of 3330 and 2220, respectively. The seepage data were characterized by strong temporal variability, confined to tectonic activity in this region, causing a change in the intensity of the methane transported by them in the form of rising bubbles. It is shown that the flow of methane released by one of the seeps varied three orders in the range from 5 mmol/s to 5 mol/s. Based on the available acoustic data, the speed of the deep-water current in the Kuril Basin, ~7 cm/s, was calculated in the depth interval from 1100 to 2200 m.

Keywords

• sea of okhotsk
• deep and shallow arctic hydrates
• acoustic research
• seeps
• methane
• deep-water currents
• methane release from sea bottom to water column