Identiﬁcation of Two New Hydrothermal Fields and Sulﬁde Deposits on the Mid-Atlantic Ridge as a Result of the Combined Use of Exploration Methods: Methane Detection, Water Column Chemistry, Ore Sample Analysis, and Camera Surveys

: 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 (cid:48) N, 13 ◦ 07 (cid:48) N, and 13 ◦ 09 (cid:48) 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.


Introduction
Since the end of the 1970s, numerous studies have been carried out on polymetallic sulfides and metalliferous sediments across the ocean floor.As a result, more than 500 hydrothermal fields have been discovered globally [1,2], and research has begun on the feasibility of developing ocean mineral resources [3].
The known manifestations of active hydrothermal discharge on the ocean floor belong to rift zones associated with mid-oceanic ridges and island arc systems, at divergent and convergent boundaries, respectively, and to a lesser extent, areas of intraplate volcanism.Hydrothermal systems, located at great depths (ranging from tens to thousands of meters), are challenging to detect.However, new opportunities for detecting deep-sea hydrothermal activity are emerging with the use of methane sensors in the process of sounding the water column.
In the 39th (2018) and 41st (2020) cruises of the R/V Professor Logachev, new data on the formation of methane anomalies in areas of submarine hydrothermal activity on the Mid-Atlantic Ridge were obtained.Instrumental CTD and geochemical observations were carried out in the bottom waters of the Logachev field at 14 • 45 N (cruise 39 of the R/V Professor Logachev).The Koralovoe (~2800-2900 m) and Molodezhnoe (~3500 m) hydrothermal fields, at 13 • 07 N and 13 • 09 N, respectively, were discovered on the MAR (Mid-Atlantic Ridge) during the 41st cruise of the R/V Professor Logachev.Standard CTD sounding was accompanied by the measurement of methane concentrations with a Franatech METS methane sensor (with a methane calibration range of 1 nM-500 nM) to study in detail hydrothermal plumes near high-temperature sources.New data were obtained on the CTD structure of hydrothermal plumes and methane anomalies.

Methane in Submarine Hydrothermal Solutions
The elements of the hydrothermal plume can be divided into conservative and nonconservative.Conservative elements (for example, 3 He) change their concentration in the plume exclusively due to dissolution.At the same time, non-conservative elements (including CH 4 ) can also change their concentration in the plume due to precipitation or transformation during various chemical and biochemical reactions (for example, microbial oxidation, etc.) [4].As a result, their residence time in the plume can vary significantly.For CH 4 , this period can range from several days to months [5].The distance from the source within which methane anomalies can be detected can reach 10-15 km [6,7].
The background concentrations of methane in seawater average about 0.0003 mmol/kg [8].However, within zones of modern hydrothermal activity, these values can increase by more than four orders of magnitude.Two main groups of hydrothermal fields (based on the composition of the host rocks) can be distinguished-those associated with basalts (often represented by axial-type systems) [9] and with widely serpentinized ultramafic rocks (edge type) [8,10,11].
High methane concentrations characterize hydrothermal fields associated with serpentinized ultramafic rocks.The content of dissolved methane within such systems can reach 2.6 mmol/kg [21].This is due to mafic silicate minerals (especially olivine and orthopyroxene) which are subjected to serpentinization.The most widespread theory on the appearance of methane in hydrothermal solutions is based on the concept of the abiotic nature of methane anomalies.Thus, the appearance of methane is explained by the serpentinization of ultrabasic rocks by the hydrothermal fluid [22].In this process, oxidation of ferrous Fe (FeII) from the reactant minerals to ferric Fe (FeIII) in the products is coupled with the reduction of water to produce H 2 [23].The reaction of the H 2 with dissolved inorganic carbon can then lead to the formation of CH 4 .
Serpentinite-hosted hydrothermal systems are thought to be ideal places for abiotic organic synthesis, which consists of the catalytic hydrogenation of dissolved inorganic carbon species.The di-hydrogen necessary for the aforementioned synthesis in hydrothermal conditions is mostly related to the anaerobic oxidation of ferrous iron by the protons of water.In the case of serpentinization, the ferric iron is in the fayalite and ferrosilite end-members of the olivine and orthopyroxene, respectively, whereas the Mg-end members (forsterite and enstatite) give rise to serpentine minerals [24].However, laboratory studies [23,25] have shown that, although such a system can exist, it does not yield methane concentrations comparable to those found in natural conditions.
According to the theory of abiotic methane synthesis, ocean water entering the upper mantle or lower oceanic crust is retained in the olivine as secondary fluid inclusions at temperatures above 400 • C. When temperatures drop below 340 • C, the serpentinization of the olivine lining the walls of the liquid inclusions leads to an almost quantitative consumption of the captured liquid H 2 O.The formation of molecular H 2 through precipitation of daughter minerals rich in Fe (III) leads to the creation of conditions conducive to reducing inorganic carbon and the formation of CH 4 .Once formed, CH 4 (g) and H 2 (g) can be stored for a geological period until extracted by dissolving or breaking up the olivine host [26].Succinctly, such a hypothesis can be described as "crack and seal".This hypothesis was put forward by several microscopic studies of deep rocks obtained within the Southeast Indian Ridge [27,28] and within the Mid-Cayman Rise [25,29].
However, under the conditions of the Guaymas Basin (Gulf of California), the concentration of methane reaches 16 mmol/kg [30], while within the hydrothermal fields of MOR, confined to basalts and ultrabasic rocks, concentrations are about 120 µmol/kg and 2.5 mmol/kg, respectively [17,19].The conditions of this basin are characterized by the simultaneous presence of an active hydrothermal system and a thick layer of sediments overlying igneous and metamorphic rocks.As a result, high-temperature hydrothermal fluid circulates through the sediment column.Under such conditions, the thermal decomposition of organic matter occurs, resulting in thermogenic methane and other hydrocarbons.During the subsequent oxidation of hydrocarbons by various microorganisms in the upper layers of the sedimentary layer, the introduction of the biogenic component of methane is also possible [30].
On the other hand, it was shown that within the limits of low-temperature hydrothermal fields (up to 122 • C) [31], the formation of methane is possible through the activity of organisms [21,[32][33][34].These are representatives of methanogenic microorganisms that produce methane from CO 2 and H 2 [35][36][37][38].
None of the presented theories gives a complete picture of the processes taking place deep in the oceanic crust and leading to the appearance of methane in the bottom part of the ocean.Instead, we may discuss a set of processes leading to the appearance of methane under different initial conditions [17,39].However, the combination of these processes can vary.

Materials and Methods
A variety of techniques were used to characterize the hydrothermal systems along the MAR: CTD sounding, the use of a methane sensor and nephelometry, bottom side-scan sonar use, electrical exploration works (the natural electrical field method), television profiling, and geological sampling with a box sampler and a rock dredge.The detection ranges of the sensors used in the cruise are shown in Table 1.Previously, increased concentrations of methane were obtained by analyzing samples of fluids circulating through the hydrothermal system.At the same time, such studies were carried out in on-board or stationary laboratories [40][41][42].
In our work, an increased methane content (relative to background concentrations) was recorded directly in the hydrothermal plume as part of sounding at point stations using the methane sensor.Carrying out such complex observations in situ with the simultaneous measurement of several water column parameters allows us to obtain a more complete picture of the nature of the formation of both hydrothermal plumes and dissolved methane anomalies.
CTD sounding was carried out by the SBE 911 plus probe at point stations from the ocean surface to the bottom and back, with a discreteness of 24 measurements per second to identify anomalous structures of ocean waters associated with modern hydrothermal activity and geoecological monitoring [43].The coordinates of the hydrological sounding complex were determined using the low-frequency ultra-deep water positioning system Kongsberg HiPAP 101.The Kongsberg CNode Mini transponder was attached to the frame of the sounding complex.In the process of sounding, water samples were taken for chemical analysis.
The analysis of geochemical samples was carried out in the laboratory of VNIIOkeangeologiya (St.Petersburg) using flame atomic absorption spectrophotometry (JSC "Aquilon", Podolsk, Russia) (for micro-components).The chemical analysis of macro components was carried out using the titrimetric method (for Ca, Mg, CO 3 , and HCO 3 ), argentometric method (for Cl), potentiometric method (for pH), gravimetric method (for SO 4 ), and flame emission spectrometry (for K and Na).
Correlation and regression analyses were used to determine the relationship between the obtained parameters based on hydrophysical sounding and chemical analysis.
The small amount of sampling resulted insufficient for the statistical analysis of the data.Therefore, for comparison with the results of previous studies, we used the data of the analysis of 180 samples taken during the mass sampling of bottom waters in other hydrothermal fields of the MAR [44][45][46].Elevated, high, and anomalous concentrations of dissolved and suspended trace elements were determined by an excess by 1, 2, and 3 standard deviations, respectively (Table 2).
where Me is the geometric mean and ε is the standard factor (the anti-logarithm of the standard deviation).

Results and Discussion
4.1.The Logachev Hydrothermal Field (14 During the 39th cruise of the R/V Professor Logachev, data were obtained on the formation of methane anomalies in the zone of discharge of hydrothermal solutions.The observations were carried out in the near-bottom waters near the Irina 1 spring located on the eastern side of the MAR rift valley (Figure 1).The host rocks are of ultrabasic composition, and are overlain by foraminiferal sediments [44,47,48].The methane sensor used made it possible to record methane anomalies not in solutions circulating through hydrothermal systems, but in hydrothermal plumes.
Earlier, detailed measurements as well as photo and video surveillance in the SERPEN-TINE expedition on the R/V Pourquoi pas? [8] from the remotely controlled multifunctional apparatus ROV Victor 6000 (Ifremer, Plouzané, France) showed that the Irina 1 source is a combination of a "smoking" crater and short pipes of black smokers that are 0.2-1.0m high surrounding the crater (Figure 2).The temperature of the hydrothermal solution measured in situ in the vents of smokers was 190-350 • C. The formation of a plume of neutral buoyancy is observed at a depth of 2700-2800 m.The plume is characterized by increased turbidity and a decrease in temperature and salinity.This combination is a standard version of the so-called "Atlantic" model of hydrothermal plume formation [49,50].In the deep Atlantic, temperature and salinity decrease with depth.Through entrainment during plume rise, the neutrally buoyant plume acquires a lower temperature and salinity than the surrounding water.When density equilibrium is finally achieved in the Atlantic, the neutrally buoyant plume is stabilized at a lower temperature and salinity than in the surrounding water [50].
Increased methane concentrations were observed in the bottom strata at the depth interval of 2840-2930 m (Figure 3).There are sharp fluctuations in turbidity, temperature, salinity, and density on the CTD plots of sounding at this depth interval.This indicates the confinement of the methane anomaly to the ascending turbulent plume, with a distinctly uneven, pulsating character of the discharging fluid (due to phase differentiation) [51-54].Almost no methane anomaly is observed in the neutral plume.Only a slight increase in concentration can be noted, confined to the lower part of the turbidity anomaly.Its maximum concentration (0.019 µmol/L) is confined to 2910-2900 m.With further ascent up the section, its concentration decreases from 2900 to 2774 m to 0.0022 µmol/L.This is due to the high scattering rate of methane as it rises from the source.
Detailed plots of the distribution of CTD parameters for an ascending hydrothermal plume show three separate layers of CTD anomalies, reflecting the discrete nature of the hydrothermal discharge (Figure 4).Correlation and regression analyses were carried out to identify the nature of the relationship between CTD parameters.The results of the correlation analysis for the three selected layers are shown in Table 3.For example, Figures 5 and 6 show the regression plots of salinity/temperature and turbidity/salinity for the anomalous upper layer.The analysis performed indicates a pronounced positive relationship between turbidity and temperature for three anomalous horizons of the ascending hydrothermal plume.This reflects the high-temperature nature of the discharge.The negative relationship between temperature and turbidity, on the one hand, and salinity, on the other, indicates the possible involvement of the cold, relatively low salinity waters of the rift valley into the ascending flow [45,46,50].7).
The Ashadze-2 hydrothermal field is located to the south of the work area, within the same oceanic core complex [55][56][57].The rocks that make up this complex were repeatedly studied earlier during the cruises of the R/V Professor Logachev.They are serpentinized peridotites and gabbros.
Video surveillance in the Molodezhnoe hydrothermal field revealed complexes of sulfide ores, represented mainly by inactive smokers (Figure 8).The chimney structures of the Molodezhnoye field are distinctly zoned.These include an outer opal-sulfide zone and an inner chalcopyrite-bornite zone.The channels are often inlaid with opal and are usually not clogged (Figure 9).Chalcopyrite in the inner zones forms rather large crystals (up to 2 mm), and sometimes forms aggregates.Nests of fine-grained sphalerite are observed.The amount of bornite gradually increases towards the outer zone.The outer part is represented by opal-sulfide aggregates dominated by chalcopyrite and bornite.Opal overgrows crystals of sulfides.Vein-disseminated ores of the Molodezhnoye field are represented by nested and vein pyrite mineralization in calcified serpentinites.Observations during video surveillance revealed transparent "flickering" solutions formed during diffuse diffusion unloading of hydrothermal solutions.When analyzing the obtained CTD section, it is noted that a decrease in temperature and salinity begins from a depth of about 3520 m, indicating the presence of the cold freshened waters of the rift valley here.According to the results of CTD sounding, a turbidity anomaly, weak negative temperatures, salinity anomalies, and a distinct positive density anomaly were observed in the bottom layer at the depth interval of ~3300-3450 m (Figure 10).These observations suggest hydrothermal activity within the Molodezhnoe hydrothermal field and the formation of a plume of neutral buoyancy at this horizon.In addition, a weak but distinct methane anomaly was recorded.The maximum methane concen- These observations suggest hydrothermal activity within the Molodezhnoe hydrothermal field and the formation of a plume of neutral buoyancy at this horizon.In addition, a weak but distinct methane anomaly was recorded.The maximum methane concentrations were observed below the turbidity anomaly, as in the Logachev field.
At the Korallovoe vent field, video surveillance revealed sulfide chimneys, black hydrothermal smoke, a hydrothermal fauna of crabs and mussels (Figure 11), and hydroxideferruginous crusts.Most of the collected samples of hydrothermal formations are veindisseminated ores.Pipe fragments and hydrothermal crusts are rare.A meridional fault was revealed within the field, with a depth of about 30 m with steeply dipping, almost vertical sides.Outcrops of hydrothermal formations and metalliferous sediments were observed in the walls of the fault.
Sedimentary rocks were sampled 300 m northwest of the field.They are represented by foraminiferal ooze among the clastic material of bedrocks.Sulfide minerals in sedimentary rocks are represented by pyrite (both oxidized and unoxidized) with minor marcasite and pyrrhotite.Quartz and opal can indirectly be considered relative indicators of the temperature of hydrothermal activity.However, their distribution over the area is different and has a zonal character.Opal is mainly found near the field, and quartz on the periphery.This may indicate that a later (low-temperature) stage of hydrothermal activity exists within the study area [58].
Hydrothermal crusts raised in the field are represented by ferromanganese and ferrocopper-hydrochloride compositions.The crusts are mostly flattened, with an average size of 15 × 10 cm and a maximum of 35 × 20 cm (Figure 12).
The chimneys of the Koralovoye vent field differ in their zonal structure (Figure 13).The inner zone (from 3 mm to 5 cm) is composed of loose copper sulfides, sometimes impregnated with tiny crystals of chalcopyrite and sphalerite.The chalcopyrite-bornite zone is usually about 0.5-1 cm.In places, there is a thin black coating on chalcopyrite crystals (probably tenorite).One of the most massive zones of the structure is bornite with a blue tempering.The width of the bornite zone is usually 1-3 cm.In this zone, nests of fine-medium-grained chalcopyrite and opal smears are often observed.
The outer zone of the chimney structure is very dense, represented by sulfides (chalcopyrite and bornite with a small number of copper sulfides) and opal.Sulfide crystals are covered with an opal shell.The opal content increases towards the surface.In the outer part of the opal zone (last 5 mm), many pores are observed.The concentration of sulfides is sharply reduced.Globular or filamentary opal is observed inside the pores and cavities.In some samples, this zone is thin and barely reaches 0.5 mm, while in others, it reaches 4 cm.The mineral composition of the vein-disseminated ores of the Koralovoye field is diverse.The chalcopyrite-bornite type represents the richest disseminated ores, sometimes with pyrrhotite.Pyrite mineralization is rare.The presence of opal and opal-carbonate hydrothermal structures was noted, and may indicate low-temperature hydrothermal activity.In addition, a chalcopyrite-sphalerite veinlet with smears of talc-chlorite rocks was observed.This is considered evidence of lower-temperature hydrothermal activity during the late stages of ore formation.
CTD sounding was carried out near the alleged source.At the bottom, from 2756 to 2803 m, numerous micro-inversions of temperature, salinity, and density were observed, reflecting the influence of an ascending flow of warm solutions.Unlike observations at the Logachev field, the temperature and salinity jumps were not accompanied by turbidity anomalies, which usually indicates the unloading of high-temperature solutions.The CTD plots show no signs of cold rift water inflow.At a depth of about 2550 m, the beginning of forming a plume of neutral buoyancy can be assumed based on the presence of negative temperature and salinity anomalies, positive density anomalies, and a slight increase in turbidity (Figure 14).An apparent methane anomaly is confined to the micro inversions of CTD parameters in a thin bottom layer.The maximum concentration is 0.0167 µmol/L, which exceeds the values of the overlying stratum (0.0026 µmol/L) by about 10 times.All these facts indicate weak hydrothermal activity at present.
The second CTD sounding station at the Koralovoe hydrothermal field is located 350 m southeast of the previous one.The CTD plots show negative temperature and salinity anomalies at the same depth (2550 m) as at the first sounding station (Figure 15).A slight turbidity anomaly was recorded at a depth of 2640-2790 m, accompanied by a slight increase in temperature and salinity.In the lower part of this interval, increased methane concentrations were observed.The maximum methane concentrations at this station were four times less than at the previous one, indicating the scattering of methane during lateral drift under the influence of bottom currents.An apparent methane anomaly is confined to the micro inversions of CTD parameters in a thin bottom layer.The maximum concentration is 0.0167 µmol/L, which exceeds the values of the overlying stratum (0.0026 µmol/L) by about 10 times.All these facts indicate weak hydrothermal activity at present.
The second CTD sounding station at the Koralovoe hydrothermal field is located 350 m southeast of the previous one.The CTD plots show negative temperature and salinity anomalies at the same depth (2550 m) as at the first sounding station (Figure 15).A slight turbidity anomaly was recorded at a depth of 2640-2790 m, accompanied by a slight increase in temperature and salinity.In the lower part of this interval, increased methane concentrations were observed.The maximum methane concentrations at this station were four times less than at the previous one, indicating the scattering of methane during lateral drift under the influence of bottom currents.
Based on the results of CTD sounding at anomalous horizons, water samples were taken and analyzed (3 samples at the 41L173 sounding station in the interval of 2785-2787 m and 2 samples at the 41LP station in the interval of 2728-2758 m).
The results of checking the concentrations of microelements for exceeding anomalous values (Table 2) are shown in Table 4. Increased concentrations of dissolved/suspended microelements are highlighted in italics, while high concentrations are in bold and abnormal italics and bold.
In all samples, elevated and abnormal concentrations of both dissolved and suspended iron are observed.This observation suggests that at this point the formation of iron oxides has already begun.However, not all dissolved iron has yet managed to pass into a suspended form.This indicates proximity to the source of the fluid.
The concentrations of both dissolved and suspended forms of manganese do not exceed the background values for seawater that are usual for this element.The absence of elevated values of the suspended form of manganese can be explained by the proximity to the hydrothermal source, since manganese often transforms into the suspended form in the form of hydroxides at a distance from the source in the lateral part of the hydrothermal plume, mainly under the influence of bacteria.The results of checking the concentrations of microelements for exceeding anomalous values (Table 2) are shown in Table 4. Increased concentrations dissolved/suspended microelements are highlighted in italics, while high concentrations are in bold and abnormal italics and bold.In all samples, elevated and abnormal concentrations of both dissolved and suspended iron are observed.This observation suggests that at this point the formation of iron oxides has already begun.However, not all dissolved iron has yet managed to pass into a suspended form.This indicates proximity to the source of the fluid.
The concentrations of both dissolved and suspended forms of manganese do not exceed the background values for seawater that are usual for this element.The absence of  The presence of elevated concentrations of suspended forms of zinc and copper can also indicate the proximity of the sounding point to the source.These elements precipitate rapidly as sulfides when the hydrothermal fluid contacts seawater.The irregularity in the vertical distribution of zinc and copper concentrations may be associated with subsurface phase differentiation and the pulsating nature of the hydrothermal fluid.This is evidenced by fluctuations in salinity in the turbulent part of the plume.
The composition and concentration of macro components in the studied samples are typical for seawater.However, magnesium concentrations are of interest.This element is absent in the end member hydrothermal solutions.This is a typical geochemical feature for ocean hydrothermal fluids.In the studied samples, the magnesium concentration is lower than the values typical for seawater.This may be due to the inflow of hydrothermal solutions into the bottom waters.
To search for possible relationships between variations in the concentrations of ore elements, magnesium, and methane in the bottom waters, a correlation analysis was carried out (Table 5).According to the results of the correlation analysis, there is no connection or a negative correlation between Mg and ore elements in solution and suspensions (Table 5).This indicates their different origins.Magnesium comes from seawater, and Cu, Zn, Mn, Fe from hydrothermal solutions.
Suspended and dissolved Mn has a solid mathematical connection with suspended and dissolved Fe.High correlation coefficients between Mn and Fe can explain the coprecipitation of Mn with Fe oxides at the initial stage of the formation of a hydrothermal plume near the source.As a result, there is a rapid decrease in the concentrations of both dissolved and suspended Mn even before forming a plume of neutral buoyancy.
There is no correlation between Cu, Zn, Mn, Fe, and CH 4 .This indicates different input sources and mechanisms for the formation of increased concentrations of ore elements and methane.

Conclusions
Our studies allowed us to obtain new data on the position, structure, and intensity of methane anomalies associated with hydrothermal activity in the Mid-Atlantic Ridge.Brief conclusions from the research results are as follows: (1) Two new hydrothermal fields with high-temperature and low-temperature activity were discovered along the MAR (Koralovoe, 13 • 07 N, and Molodezhnoe, 13 • 09 N). (2) In the 39th and 41st cruises of the R/V Professor Logachev, the presence of methane anomalies in the bottom waters near the hydrothermal vents of the Koralovoe and Molodezhnoe fields, as well as the well-known Logachev field, was instrumentally established.
(3) The composition of ores at new deposits is characterized by a zonal structure with the presence of an opal-sulfide zone and zones of Zn and Cu sulfides.This mineralogy indicates the possibility of both high-and low-temperature hydrothermal activity.(4) Methane anomalies were noted at the bottom in the upward flow of the plume near the sources.In a plume of neutral buoyancy, high concentrations of methane are absent.(5) The results of the correlation analysis indicate the hydrothermal nature of the increased concentrations of suspended and dissolved Cu, Zn, Mn, and Fe in the plume.(6) The relationship between suspended Cu, Zn, Mn, and Fe in the hydrothermal plume and CH 4 was not observed.This indicates different sources of input and mecha-nisms for the formation of increased concentrations of ore elements and methane in hydrothermal plumes.(7) The use of a methane sensor for CTD sounding increases the efficiency of detecting active hydrothermal vents in rift zones of mid-ocean ridges.

Figure 1 .
Figure 1.Position of the CTD sounding station (star) at the Irina 1 crater, Logachev field.The bathymetric map was obtained by the ROV Victor 6000 near-bottom multibeam survey during the SERPENTINE cruise [8].

Figure 4 .
Figure 4. Horizons with anomalous CTD parameters in the ascending plume of the hydrothermal plume near the Irina 1 crater, Logachev field.Red dotted line: scattering horizons; black dotted line: layers of relatively cold waters.

Figure 5 .
Figure 5. Salinity/temperature regression line for the bottom water layer at a depth of 2835-2850 m.Source: Irina 1, Logachev field.

Figure 6 .
Figure 6.Turbidity/temperature regression line for the bottom water layer at a depth of 2835-2850 m.Source: Irina 1, Logachev field.

4. 2 .
The Molodezhnoe (13 • 09 N 44 • 52 W) and Koralovoe (13 • 07 N 44 • 54 W) Hydrothermal Fields During cruise 41 of the R/V Professor Logachev, two new hydrothermal fields were discovered and studied-the Koralovoe and Molodezhnoe.The hydrothermal fields are of a marginal (flank) type, located on the gentle western side of the rift valley (Figure

Figure 13 .
Figure 13.Fragment of the wall from the chimney structure.Koralovoye field.

Figure 15 .
Figure 15.Results of CTD sounding at the Koralovoye field, station 41LP.Left: brown-turbidity, FTU; blue-salinity, ‰; red-temperature, °C; green-density, kg/m3; right graph: methane concentration, µmol/L.Based on the results of CTD sounding at anomalous horizons, water samples were taken and analyzed (3 samples at the 41L173 sounding station in the interval of 2785-2787 m and 2 samples at the 41LP station in the interval of 2728-2758 m).The results of checking the concentrations of microelements for exceeding anomalous values (Table2) are shown in Table4.Increased concentrations dissolved/suspended microelements are highlighted in italics, while high concentrations are in bold and abnormal italics and bold.

Table 1 .
Detection ranges of the sensors.

Table 2 .
Statistical characteristics of the concentrations of dissolved and suspended forms of trace elements in the search area (µg/L).

Table 4 .
Elevated (in italics), high (in bold), and abnormal (in italics and bold) concentrations of dissolved/solid microelements.

Table 4 .
Elevated (in italics), high (in bold), and abnormal (in italics and bold) concentrations of dissolved/solid microelements.

Table 5 .
Correlation between Mg, CH 4 , and dissolved (d) and suspended (s) ore elements in the hydrothermal plume.Hydrothermal fields: Korallovoe and Molodezhnoe.