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A section of 46 CTD/Lowered Acoustic Doppler Current Profiler stations along the Vema Fracture Zone (VFZ) was made for the first time with a repeat of the transverse section on the main sill (at 41° W) for the sixth time. Potential temperature of Antarctic Bottom Water when it flows into the fracture increases from 1.3°С to 1.6°C. This temperature increase is much smaller than in the Romanche Fracture Zone because in the VFZ there are not many transverse sills that force the flow to become turbulent. A zone with minimal velocities of the bottom flows exists in the western part of the VFZ up to the main sill. A current with velocities of 0.25 m/s flows above the low velocity zone at a depth of about 4,000 m. Underwater spillways with hydraulically controlled flow of bottom water down the slope from 4,500 m to 5,000 m at speeds up to 0.40 m/s were found east of the sill (in the central and southern channels). The bottom current accelerates as it flows down, but then its kinetic energy decreases. The bottom current slows down and mixes with surrounding waters. A thin (∼30 m) bottom flow descends further to the deep depression. In the southern channel of the VFZ, bottom water flows into a deep (5,400 m) basin. The coldest and densest water reaches this depression during rare inflows. Plain Language Summary: Antarctic Bottom Water formed on Antarctic slopes slowly flows northward. The flow passes many deep basins and abyssal channels between them until it reaches the Vema Fracture Zone (VFZ). The VFZ is the main conduit in the Mid‐Atlantic Ridge for the bottom water flow to the Northeast Atlantic; this flow fills the abyssal basins of the Northeast Atlantic. Unlike the Romanche and Chain fracture zones near the equator, Antarctic Bottom Water only slightly warms here while flowing through the fracture because there are not many transverse sills that can prevent the bottom flow and induce mixing with North Atlantic Deep Water occupying the overlying water. The fastest current flows over a low velocity zone with low friction. After overflowing the main sill, the water descends from 4,500 m to 5,000 m as a spillway, in which the flow partly mixes with the surrounding waters. Key Points: Antarctic Bottom Water flows in the Vema Fracture Zone (VFZ) without strong warming because it flows over a low velocity zone with less frictionTwo underwater spillways were found in the VFZ in which bottom water flows down from 4,500 m to 5,000 mThe downflow of water in the spillways is governed by hydraulic control [ABSTRACT FROM AUTHOR]

Transport of Antarctic Bottom Water in the deep-water channels of the Atlantic based on the author's measurements in 2002–2022 is considered. Bottom water is formed in the Weddell Sea and spreads north into the Atlantic Ocean. Near the equator, the flow splits into the equatorial branch through the Romanche and Chain fracture zones and the branch to the northwest into the North American Basin. Further, the flow is directed to the Northeast Atlantic through the Vema Fracture Zone (11° N). This flow fills the deep Cape Verde and Canary basins. The flow through the Romanche Fracture Zone does not penetrate beyond the Kane Gap (9° N) due to strong mixing caused by internal tidal waves. [ABSTRACT FROM AUTHOR]

Field research has been carried out to study the flow of Antarctic Bottom Water along the entire length of the Vema Fracture Zone in the tropical part of the North Mid-Atlantic Ridge. Russian–Brazilian field studies were carried out in the largest river plume of the World Ocean, in the region of the Amazon River mouth on the northern shelf of Brazil. The expedition was an important step in the development of international scientific cooperation between BRICS countries: Russia and Brazil. [ABSTRACT FROM AUTHOR]

This paper addresses changes of Antarctic Bottom Water (AABW) transport through the Vema Fracture Zone based on direct measurements and Glorys12v1 reanalysis. Instrumental measurements of currents and other principal hydrological characteristics were carried out during the 45th cruise of the R/V Akademik Nikolaj Strakhov in November 2019. According to the current velocity measurements in 2019, the transport in the Vema Fracture Zone is 0.9 Sv. Moreover, based on the distribution of dissolved oxygen concentration, it is more correct to consider the isotherm 1.7°С rather than 2°С as the upper boundary of the AABW. The reanalysis showed very good agreement with direct measurements. The average transport of bottom water during the reanalysis period is 0.66 Sv. In general, there is a tendency towards an increase in both transport and temperature over the 25-year period covered by reanalysis. The measurements of the bottom water temperature during twenty successive samplings showed changes from 1.36 to 1.41°С within a day. This interval almost completely covers the entire range of values observed in previous years. [ABSTRACT FROM AUTHOR]

We present here a scenario of magmatic and tectonic processes associated with the interaction between the Reunion mantle plume and the mantle beneath Central Indian Ridge (CIR) at the Vema Trench (VT) along the Vema Fracture Zone (VMFZ) in the Indian Ocean. The VMFZ offsets the CIR by about 300 km between 8°S and 10°S latitudes and holds probably the deepest point in the Indian Ocean, the Vema Trench, reaching to a depth in excess of 6,500 m. The trench has a width of about 5 km and extends for about 12 km in a NE–SW direction. Petrography and major and trace element concentrations collectively suggest that the rocks recovered from the VT are largely basaltic (pillow, columnar, dikes), composed of plagioclase and clinopyroxene, much evolved, and tholeiitic to calc‐alkaline in composition. The REE signatures ([Sm/Yb vs. Sm] and [Ce/Sm]N vs. [Yb/Sm]N) hint its source to a spinel lherzolite mantle with additional input from enriched mantle (EM‐1). This EM‐1 signature generally indicates presence of recycled ancient subducted oceanic crust in the mantle domain wherefrom the plume originates. The detailed petrological and geochemical studies of these rocks suggest dynamic processes involved in transfer of deep mantle‐sourced material to mid‐ocean ridges leading to geochemical heterogeneity. Trace element ratios (Hf/3–Th–Ta and Thn vs. Nbn) also suggest that the VMFZ basalt preserves the signature of mixing of ridge melt and plume melt. [ABSTRACT FROM AUTHOR]

Abstract—: We study the Antarctic Bottom Water (AABW) flow in the Vema Fracture Zone of the Mid-Atlantic Ridge using the high-resolution Institute of Numerical Mathematics Ocean Model (INMOM) and data from field measurements. The key feature of this numerical modeling is high horizontal and vertical resolution in the bottom layer for the simulation of the flow in the narrow deepwater fracture, as well as the use of high-quality topography based on multibeam echo sounder measurements. Direct CTD and LADCP measurements performed onboard the R/V Akademik Sergey Vavilov in 2006 and 2014-2016 were used to verify the model. In this work, we analyze both the thermohaline structure of the bottom layer in the Vema Fracture Zone and kinematics of the flow over its entire length. [ABSTRACT FROM AUTHOR]

We hypothesized that mixing zones of deep-water masses act as ecotones leading to alterations in microbial diversity and activity due to changes in the biogeochemical characteristics of these boundary systems. We determined the changes in prokaryotic and viral abundance and production in the Vema Fracture Zone (VFZ) of the subtropical North Atlantic Ocean, where North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) are funneled through this narrow canyon and therefore, are subjected to intense vertical mixing. Consequently, salinity, potential temperature, oxygen, PO4, SiO4, NO3 were altered in the NADW inside the VFZ as compared to the NADW outside of the VFZ. Also, viral abundance, lytic viral production (VP) and the virus-to-prokaryote ratio (VPR) were elevated in the NADW in the VFZ as compared to the NADW outside the VFZ. In contrast to lytic VP, lysogenic VP and both the frequency of lytically (FIC) and lysogenically infected cells (FLC) did not significantly differ between in- and outside the VFZ. Generally, FIC was higher than FLC throughout the water column. Prokaryotic (determined by T-RFLP) and viral (determined by RAPD-PCR) community composition was depth-stratified inside and outside the VFZ. The viral community was more modified both with depth and over distance inside the VFZ as compared to the northern section and to the prokaryotic communities. However, no clusters of prokaryotic and viral communities characteristic for the VFZ were identified. Based on our observations, we conclude that turbulent mixing of the deep water masses impacts not only the physico-chemical parameters of the mixing zone but also the interaction between viruses and prokaryotes due to a stimulation of the overall activity. However, only minor effects of deep water mixing were observed on the community composition of the dominant prokaryotes and viruses. [ABSTRACT FROM AUTHOR]

Abstract: Mantle peridotites from an exposed lithospheric section (Vema Lithospheric Section, VLS), generated during ~26Ma at a ~80km long Mid Atlantic Ridge segment (11° N), have been sampled and studied to understand the evolution of the serpentinization process. The VLS was uplifted due to a 10Ma transtensional event along the Vema transform. Before the uplift residual mantle rocks were lying beneath a 0.8–1.3km thick basaltic crustal layer. The major and trace element compositions of the serpentinites, as well as their H, O, Sr, Cl and B isotopic compositions were interpreted based on thermal models of lithospheric spreading from ridge axis. The results suggest that serpentinization occurred mostly near the ridge axis. Serpentinization temperatures, estimated from stable isotopes, are consistent with resetting of the closure temperatures during the tectonic uplift of the lithospheric sliver, reflected by decreasing δ18O and increasing δ11B values. Modeling shows that the thermal influence of the transtensional event affected mainly the region close to the RTI (ridge–transform intersection). Petrological, elemental and isotopic data suggest that, when the ultramafic basal unit of the VLS was uplifted and exposed on the ocean floor, serpentinization became superseded by low temperature water–rock reactions, with Fe–Mn crust formation, which is still progressing, as recorded by δD. Ultramafic mylonites, prevalent in a short stretch of the VLS, show only a partial serpentinization process, together with pervasive contamination by low-temperature Fe–Mn crust. [Copyright &y& Elsevier]

Melting processes beneath the Mid-Atlantic Ridge were studied in residual mantle peridotites sampled from a lithospheric section exposed near the Vema Fracture Zone at 11°N along the Mid-Atlantic Ridge. Fractional and dynamic melting models were tested based on clinopyroxene rare earth element and high field strength element data. Pure fractional melting (non-modal) cannot account for the observed trends, whereas dynamic melting with critical mass porosity <0·01 fits better the measured values. Observed microtextures suggest weak refertilization with 0·1–1% quasi-instantaneous or partially aggregated melts trapped during percolation. The composition of the melts is evaluated, together with their provenance, with respect to the garnet–spinel transition. Partial melts appear to be aggregated over short but variable intervals of the melting column. Deep melts (generated within the garnet stability field at the base of the melting column) escape detection, being separated from the residues by transport inside conduits or fractures. The temporal evolution of the melting process along the exposed section shows a steady increase of mantle temperature from 20 Ma to present. [ABSTRACT FROM AUTHOR]

Given that oceanic basalts form by partial melting of mantle peridotites that rise below mid-ocean ridges, peridotite and basalt should have identical Sr and Nd isotope ratios. We tested this concept on parallel sets of peridotites and basalts sampled from an exposed section of lithosphere representing 20 m.y. of accretion at the Mid-Atlantic Ridge. The 143Nd/144Nd ratios of the basaltic glasses stay constant, whereas those of the peridotitic clinopyroxenes extend both higher and lower than the basalt ratios, suggesting that the constant isotopic composition of the basalts results from mixing of melts released by peridotites from a broad region of the subridge mantle. The degree of melting undergone by the peridotites correlates inversely with their 143Nd/144Nd ratios. Small-scale isotopic heterogeneity of the peridotites may result from variable premelting metasomatism in the mantle, mostly during periodic, dynamic, subridge upwelling, possibly superimposed upon time-integrated radioactive decay of ancient heterogeneities. [ABSTRACT FROM AUTHOR]