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.

The carbon dioxide concentration in the atmosphere has progressively risen since pre-industrial times. About one-third of the anthropogenically generated CO₂ is absorbed by the waters of the World Ocean, whereas the waters of the Southern Ocean take up about 40% of this CO₂. The concentrations of oxygen and carbon dioxide dissolved in seawater are sensitive to climate changes, transferring anthropogenic pressures with consequences for the biogeochemical cycles in the World Ocean. The Southern Ocean is a key region for the exchange of oxygen and carbon between the surface water and the atmosphere and for their transfer with cold water masses to the deep layers of the Ocean. In this paper, we discuss the dynamics of the carbon dioxide partial pressure (pCO₂) and dissolved oxygen (O₂) in the surface waters of the Atlantic Southern Ocean based on data collected during the 87th cruise of the R/V “Academik Mstislav Keldysh”. The study area includes the Bransfield Strait, Antarctic Sound, the Powell Basin, the Weddell, and Scotia Seas. We have analyzed the spatial distribution of pCO₂ and oxygen for the areas of transformation of water masses and changes in biogeochemical processes. In the zone of Scotia and Weddell Seas, we have observed an increase in pCO₂ and a decrease in oxygen concentrations at the transect from the Weddell Sea at 56° W to the Powell Basin. From the Antarctic Sound to the Bransfield Strait, a decrease in oxygen saturation and an increase in pCO₂ has been traced. The surface waters of the Bransfield Strait have revealed the greatest variability of hydrochemical characteristics due to a complex structure of currents and intrusions of different water masses. In general, this area has been characterized by the maximum pCO₂, while the surface waters are undersaturated with oxygen. The variability of the AOU/ΔpCO₂ (w-a) ratio has revealed a pCO₂ oversaturation and an O₂ undersaturation in the waters of the Bransfield Strait. It is evidence of active organic carbon decomposition as the major controlling process. Yet, photosynthesis is the major biogeochemical process in the studied areas of the Weddell and Scotia seas, and their waters have been undersaturated with pCO₂ and oversaturated with O₂. As it comes from the analysis of the distribution and correlation coefficients of AOU and the sea-air gradient of pCO₂ with other physical and biogeochemical properties, the predominance of the biotic processes to the dynamics of O₂ and pCO₂ in the surface water layer has been demonstrated for the studied areas. Yet, there is evidence of additional sources of CO₂ not associated with the production and destruction processes of organic matter.

The Vema Fracture Zone is located in the North Atlantic Ridge and extends along 11°N from 38 to 46°W. It is the main pathway for the spreading of Antarctic Bottom Water to the Northeast Atlantic. Due to its location and structure, the Vema Fracture Zone is an excellent object for studying the properties of the bottom gravity flow. An oceanographic section along the entire Vema Fracture Zone was carried out during cruise 52 of the R/V “Akademik Boris Petrov” in November–December 2022. In our work, we analyzed 25 oceanographic stations; at 15 stations, dissolved oxygen and nutrients were also determined. Such studies of the structure of the bottom gravity flow of Antarctic Bottom Water in the central channel of the Vema Fracture Zone based on high spatial resolution in situ data were made for the first time. A supercritical flow accompanied by a hydraulic jump was detected behind the main sill of the fracture zone. Simultaneous measurements of dissolved oxygen, silicate, and phosphate allowed us to examine the hydrochemical structure along the entire Vema Fracture Zone. Its analysis revealed high correlation between the distribution of hydrochemical and oceanographic parameters in both the stable flow and turbulent regimes of the current.

—The article studies the abiotic characteristics in waters of the Bransfield Strait in January 2022. The cross-section of January 2020 was repeated from the Antarctic Peninsula to the Southern Shetland Islands; new data were also obtained: two cross-sections in the eastern and western parts of the strait. The scheme of currents in the Strait reflects modern knowledge; absolute values have been measured up to 50 cm/s in the northeast direction and up to 35 cm/s in the southwest. In the studied area, modified waters of the Bellingshausen Sea are clearly distinguished (warm and least saline, with reduced values of total alkalinity and all nutrients); modified Weddell Sea water (colder and more saline, with reduced values of silicates and nitrates), as well as Circumpolar Deep Water in the 200–450 m layer (warm and salty, with low values of dissolved oxygen and pH and increased content of phosphates, silicates. and nitrates). In bottom waters of the strait, there were no serious changes in structure. The amplitude of the interannual variability of the abiotic characteristics of Bransfield Strait waters is poorly expressed, nevertheless it can serve as a climate change marker in the Southern Ocean.