Seawater properties in two intense rings in the South Atlantic are considered. One ring separated from the Brazil Current and the other from the Malvinas Current. The analysis is based on the CTD casts and SADCP measurements from the onboard velocity profiler. The optical properties, chemical parameters, methane concentration, and biological properties such as primary production, plankton, and fish were also analyzed. Analysis of strong differences between the eddies is supplemented by observations of whales and birds in the region.

Dissolved methane coming from its various sources is an important component of seawater. Finding these probable sources allows for the determination of potential oil and/or gas deposit areas. From an ecological point of view, methane transport studies can reveal probable pollution areas on the one hand and biological communities, being the lower part of the food chain commercial species, on the other hand. Moreover, the methane transport mechanism can help to obtain a better understanding of the contribution of the World’s oceans to global greenhouse gas emissions. Our research combines gas geochemistry and oceanography. In comparing the research results of both branches, we show the mechanism of methane transport. The features of the dissolved methane on oceanographic sections in the southern part of the Tatar Strait are discussed. The CH4 intake from the bottom sediment and the transport of dissolved methane by the currents in the Tatar Strait are shown. The absolute maximum concentration of CH4 (155.6 nM/L) was observed on the western Sakhalin Island shelf at the near-bottom layer at a depth of 65 m. The local maximum, 84.4 nM/L, was found north of the absolute maximum in the jet current under the seasonal pycnocline. A comparison of the simulated surface seawater origin and dissolved methane in the 4 m depth distribution shows methane transport with the currents in the Tatar Strait. Another studied section is along 134° E in the Japan Basin of the Japan (East) Sea. Here, the East Korean Warm Current close to the Yamato Rise slope and a quasi-stationary mesoscale anticyclonic eddy centered at 41° N intersect. The local maximum methane concentration of 8.2 nM/L is also observed under the seasonal pycnocline. In a mesoscale anticyclonic eddy at 134° E in the deep part of the Japan Basin, a local methane maximum of 5.2 nM/L is detected under the seasonal pycnocline as well.

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The paper presents the results of a complex joint Russian-Vietnamese geological, geophysical and oceanographic expedition in the South-China Sea (R/V “Akademik M.A. Lavrentyev”, cruise 88, 2019), as well as related joint Russian-Vietnamese marine and land researches in the area of north and south Vietnam under a series of local FEB RAS – VAST grants. The organizers of the marine expedition are the V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences (POI FEB RAS) and the Institute of Marine Geology and Geophysics of the Vietnam Academy of Science and Technology (IMGG VAST). In comparison with the Sea of Japan and the Sea of Okhotsk, it can be noted that the active bottom degassing on the on the Vietnamese shelf and slope have is a local, although the intensity of gas-geochemical anomalies is comparable to similar zones in the Far Eastern Seas. For the first time, anomalous methane fields (up to 5000 nl/l) were found in the water column of the South-China Sea, which are comparable to anomalies on the oil and gas shelf and the gas-hydrate-bearing slope of Sakhalin Island. Metamorphosed sedimentary and volcanic rocks were discovered for the first time in the southern part of Catba Island (Gulf of Tonkin), which indicates the introduction of an endogenous body into the sedimentary strata and its further transformation. In connection with the discovery in 2020 of the large Ken Bau gas field at the southern end of the sedimentary basin of the Red River, the forecast of POI scientists about the presence of significant hydrocarbon reserves in this area was confirmed. The work was carried out within the framework of the joint Vietnam-Russia Laboratory for Marine Sciences and Technology (V. I. Ilyichev Pacific Oceanological Institute of the Far Eastern Branch of the Russian Academy of Sciences and the Institute of Marine Geology and Geophysics Vietnam Academy of Science and Technology). The expedition of the R/V “Akademik M.A. Lavrentyev” (cruise 88) is part of a series of expeditions in accordance with the UN Decade dedicated to the Ocean Science for Sustainable Development.