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In 2018–2020 the research vessel (R/V) Professor Logachev (cruises 39 and 41) carried out geological and geochemical studies in the bottom waters of the Mid-Atlantic Ridge hydrothermal fields at 14°45’ N, 13°07’ N, and 13°09’ N. Two new hydrothermal fields were discovered—the Molodezhnoye and Koralovoye. Standard conductivity, temperature, and depth (CTD) sounding with a methane sensor was accompanied by video surveillance and sampling of rocks and water. The rocks were characterized by a zonal composition with opal and sulfides of copper and zinc. An increase in methane concentration values was accompanied by CTD anomalies in the bottom waters. The methane anomaly was formed within the hydrothermal plume of both high-temperature and low-temperature systems. Methane was almost absent in the plume of neutral buoyancy and was associated in all the studied manifestations with the ascending flow of hot waters over the hydrothermal vents. The hydrothermal plumes were characterized by increased Cu, Zn, and Fe concentrations at background Mn concentrations. Signs of low-temperature hydrothermal activity were also observed. Different sources and mechanisms are required to explain the elevated concentrations of base metals and methane in the hydrothermal plumes.

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This paper presents in situ methods used in the search for areas of modern hydrothermal activity, as well as the results of their long-term use during cruises within the Russian exploration area of the Mid-Atlantic Ridge (MAR). In this study, the following methods were used: CTD-sounding, methane sounding, teleprofiling and measurements with Eh, pS, pNa, pH and EF sensors. During profiling by towed complexes (RIFT, MAK-1M), various geophysical and geochemical anomalies near high-temperature, low-temperature and inactive fields were detected. Geophysical anomalies are more distinct when profiling near the bottom, and geochemical anomalies are located at a distance from the bottom (~150–200 m). Direct signs of high-temperature discharging (black smoker′s smoke, hydrothermal buildings) and indirect signs of low-temperature discharging (lithified carbonate sediments and accumulations of endemic hydrothermal fauna) were detected during teleprofiling. We have described 6 types of complex geophysical anomalies determined by CTD-sounding that allow the detection of plumes at different stages of formation and from different sources. The use of a methane sensor during sounding makes it more likely to identify a low-temperature discharge. Methane has a highly variable nature of distribution—over distances from the first hundreds of meters to tens of kilometers and a seabed height of ~50–500 m. The use of these methods together makes it more possible to detect low- and high-temperature hydrothermal discharges within mid-oceanic ridges (MOR).

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