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The spatial separation of suspended Fe and Mn in plumes, recorded at Mid-Atlantic Ridge (MAR) sites, some regions on the East Pacific Rise (EPR) and in the western Pacific, is reflected in the Fe and Mn distribution in metalliferous sediments of hydrothermal origin. Investigations at 21.5–23.5°S EPR show that at a downstream distance of 10 km from the vents the total Fe and Mn contents are lower than those on the flanks. Primary suspended Mn precipitates more than 20 km from the high-temperature vents, whereas the main suspended Fe precipitates within 10 km of the active vent area. In the vicinity of the vents a zone of ‘geochemical minimum’ was observed. Bacterial masses were observed in plumes and sediments in hydrothermal sites of the Pacific. Microbial coenoses were studied to show the increased activity of heterotrophic and Fe-Mn bacteria at several horizons of MAR hydrothermal plumes. Near-surface plumes studied in Matupi Harbour (New Britain) were characterized by both the separation of suspended Fe and Mn horizontally and a corresponding increased presence of micro-organisms. The activity of micro-organisms accompanied differentiation of the suspended and dissolved Mn at the periphery of the plume. Evidence suggests a significant role for bacteria in removing Mn from plumes in different regions of the ocean.

The paper analyses features of the species composition and diversity of biotic communities living within the ferromanganese nodule fields (the Clarion-Clipperton field), cobalt-manganese crusts (the Magellan Seamounts) and deep-sea polymetallic sulphides (the Ashadze-1, Ashadze-2, Logatchev and Krasnov fields) in the Russian exploration areas of the Pacific and Atlantic Oceans. Prospects of mining solid minerals of the world’s oceans with the least possible damage to the marine ecosystems are considered that cover formation of the sediment plumes and roiling of significant volumes of water as a result of collecting the minerals as well as conservation of the hydrothermal fauna and microbiota, including in the impact zone of high temperature hydrothermal vents. Different concepts and layout options for deep-water mining complexes (the Indian and Japanese concepts as well as those of the Nautilus Minerals and Saint Petersburg Mining University) are examined with respect to their operational efficiency. The main types of mechanisms that are part of the complexes are identified and assessed based on the defined priorities that include the ecological aspect, i.e. the impact on the seabed environment; manufacturing and operating costs; and specific energy consumption, i.e. the technical and economic indicators. The presented morphological analysis gave grounds to justify the layout of a deep-sea minerals collecting unit, i.e. a device with suction chambers and a grip arm walking gear, selected based on the environmental key priority. Pilot experimental studies of physical and mechanical properties of cobalt-manganese crust samples were performed through application of bilateral axial force using spherical balls (indenters) and producing a rock strength passport to assess further results of the experimental studies. Experimental destructive tests of the cobalt-manganese crust by impact and cutting were carried out to determine the impact load and axial cutting force required for implementation of the collecting system that uses a clamshell-type effector with a built-in impactor.

Observations made during a cruise of the R/V Professor Logachev at the Snake Pit hydrothermal vent field provide a large database with which to examine the relationships between the geochemical characteristics of the hydrothermal sediments and the fauna that inhabit the area. Correlations between the nature and geochemistry of sediments, the distribution of hydrothermai and peripheral fauna and the hydrothermal activity are established. Three geochemical zones in the Snake Pit area are distinguished. (i) The Central Zone, located within 10-100 m of black smoker chimneys, consisting mainly of hydroxide-sulphide sediments with Fe sulphides predominant. Barite is found only in this zone and vent-associated organisms distinctly predominate over non-vent fauna. (ii) The Intermediate Zone encompasses high-temperature springs at a distance from 50 to 150 m. Sediments are chiefly sulphide-hydroxide composition and are dominated by Fe hydroxide and sulphide minerals in an oxide envelope. The non-vent specific fauna show maximum concentrations. (iii) The Outer Zone is located within a radius of 150-500m from the smokers. The sediment composition is characterized by a predominance of sulphidehydroxide transitional differences with notably varying compositions of oxide and sulphide minerals. Minor sestonophages prevail in the fauna, which is typical of communities with a pronounced oligotrophic structure. From the Central Zone towards the Outer Zone, the frequency of the bacterial mats seen on photographs decreases and the hydrothermal sediments show a similar trend. Analysis of the hydrothermal sediment obtained from the cores shows: (i) Mn is absent; (ii) barite is found only in the Central Zone; and (iii) chalcopyrite and pyrrhotite concentrations decrease and opal and quartz concentrations increase with increasing distance from the nearest smoker. Comparisons between the Mid-Atlantic Ridge hydrothermal fields of TAG, Snake Pit and 15~ indicate a decrease in the hydrothermal and biological activities with latitude (TAG, Snake Pit, 15~ which might be due to their evolutionary history.