Vema Fracture Zone Research Articles

An influence of the Antarctic bottom water currents on the formation of the upper sediment cover in the Vema Fracture Zone, Central Atlantic

An influence of the Antarctic bottom water currents on the formation of the upper sediment cover in the Vema Fracture Zone, Central Atlantic

Mineralogy of Quaternary Sediments from the Valley of Vema Fracture Zone (Central Atlantic)

The content of sediment forming minerals in two cores from the eastern (ANS45-37) and western (ANS45-48) parts of the Vema transform fault valley is studied using the semi-quantitative XRD analysis of bulk powder sediment samples. The mineral composition of deep-sea sediments from the Amazone cone is also analyzed for comparison. It appeared that the average composition of the terrigenous component of both cores (according to prevailing quartz, secondary mica, plagioclase and potassium feldspar, as well as smectite, chlorite, kaolinite, illite) is quite similar and approximately corresponds to the composition of sediments from the Amazon cone. The ratio of four clay minerals suggests the supply of terrigenous material to the Amazon and Orinoco due to the erosion of the Andes and humid tropical weathering in the lower course of the rivers with further transportation of the suspended load to the ocean. This material was transported to the Vema transform valley due to the interplay between the gravity flows from the South American continental slope and the current of the Antarctic Bottom Water. Data on biogenic calcite (planktic foraminiferal tests, nannofossils) and opal A (radiolarians, sponge spicules) are obtained in addition. In the study area, several authigenic (diagenetic) minerals are identified. In particular, siderite and greigite are first found in the sediments from the Vema valley and Amazon cone, respectively.

Mineralogy of Quaternary Sediments from the Vema Fracture Zone Valley (Central Atlantic)

Mineralogy of Quaternary Sediments from the Vema Fracture Zone Valley (Central Atlantic)

Unveiling the contourite depositional system in the Vema Fracture Zone (Central Atlantic)

A combination of a high sediment input and intense bottom currents often leads to the formation of contourites (sediments deposited or significantly reworked by bottom currents). Both of these components are present in the Vema Fracture Zone valley which is the most important passageway for the distribution of the Antarctic Bottom Water from the West to the North-East of the Atlantic. However, no contourite drifts, moats or contourite channels have been found in this region in more than half a century of research. The prevailing sedimentation paradigm postulates that turbidity currents have predominantly governed sedimentation in this region during the Pleistocene. This work describes the first example of contourite depositional system identified in the Vema Fracture Zone. The discovery was made through detailed high-resolution sub-bottom profiling, as well as numerical modeling and direct measurements of bottom current velocities. Such systems are exceptionally uncommon in fracture zones. This study highlights the importance of further research of contourites along the Vema Fracture Zone based on modern concepts of contourite and mixed depositional systems. The work also emphasizes the need to reevaluate the impact of bottom currents on sedimentation in this region, and particularly in the narrow segments of the fracture zone valley.

Antarctic Bottom Water in the Vema Fracture Zone

AbstractA 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.

Oceanographic Investigations in the Tropical Atlantic (Cruise 52 of the Research Vessel “Akademik Boris Petrov”)

Information is presented on the work performed to study the flow of Antarctic Bottom Water along the entire length of the Vema Fracture Zone in the tropical part of the North Atlantic Ridge and Russian-Brazilian field studies in the largest river plume of the World Ocean, on the shelf of the northern part of Brazil near the mouth of the Amazon River. A total of 46 stations were completed in the Vema Fracture Zone and 28 stations offshore Brazil. The new expedition that took place was an important step in the development of international scientific cooperation between the BRICS countries: Russia and Brazil.

Water bioproductivity estimate of the equatorial Atlantic under the influence hydrological, hydrochemical and hydrooptical factors in winter 2022

Based on contact observation data obtained in the 60th cruise of the RV “Akademik Ioffe”, water bioproductivity was estimated based on the distribution of chlorophyll a (CCl) and microplankton adenosine triphosphate (CATP) concentrations under changing environmental conditions. These parameters were studied in the western equatorial Atlantic (Vema Fracture Zone, 11°N; Guiana Basin–Guiana section, 4°N–7°N) in the winter of 2022. For the first time, the stages of production–destruction succession were estimated based on the heterotrophic–photoautotrophic index (HPI), and water trophicity was investigated according to ATP content in the study area. According to the data obtained on CCl, CATP, and HPI, in the Vema Fracture Zone, waters were mesotrophic, with prevalence of communities of mixed nutrition — heterotrophs and autotrophs (an equilibrium community; HPI 15–25). In the Guiana section area, waters were oligotrophic, with prevalence of heterotrophic nutrition (developed and mature communities; HPI: 25–112 and 112–163). At the first bioproductivity depth maximum (10 m), significant relationships were obtained between spatial distribution of concentrations of chlorophyll a and phosphates (r > 0.5) and oxygen (r > 0.68), as well as between microplankton ATP and phosphates and pH (r > 0.5); negative relationships were recorded microplankton CATP, CCl with temperature (r = -0.42, -0.85) and salinity (r = -0.68, -0.41). At the second bioproductivity depth maximum (50 m), significant relationships were established between HPI and temperature (r=0.64); a negative relationship was found with pH (r = -0.44). The continuity of ATP and chlorophyll a concentrations distribution with depth was shown in the 30-m layer; the heterogeneity of HPI distribution with depth was revealed as well.Finally, the research based on our own instrumental measurements shows a low productivity in the western part of the Equatorial Atlantic in winter 2022. This is caused by the features of water circulation and chemical parameters distribution.

Generation and evolution of the oceanic lithosphere in the North Atlantic

Half a century ago, our view of the Earth shifted from that of a Planet with fixed continents and ancient stable ocean basins to one with wandering continents and young, active ocean basins, reviving Wegener’s Continental Drift that had rested dormant for years. The lithosphere is the external, mostly solid and relatively rigid layer of the Earth, with thickness and composition different below the oceans and within the continents. We will review the processes leading to the generation and evolution of the Earth’s lithosphere that lies beneath the oceans. We will discuss how the oceanic lithosphere is generated along mid-ocean ridges due to upwelling of convecting hot mantle. We will consider in particular lithosphere generation occurring along the northern Mid Atlantic Ridge (MAR) from Iceland to the equator, including the formation of transform offsets. We will then focus on the Vema fracture zone at 10°–11° N, where a ~ 300 km long uplifted and exposed sliver of lithosphere allows to reconstruct the evolution of lithosphere generation at a segment of the MAR from 25 million years ago to the Present. This axial ridge segment formed 50 million years ago, and reaches today 80 km in length. The degree of melting of the subridge mantle increased from 16 million years ago to today, although with some oscillations. The mantle presently upwelling beneath the MAR becomes colder and/or less fertile going from Iceland to the Equator, with “waves” of hot/fertile mantle migrating southwards from the Azores plume. Scientific revolutions seem to occur periodically in the history of Science; we wonder when the next revolution will take place in the Earth Science, and to what extent our present views will have to be modified.

Transport of Deep and Bottom Waters through the Mid-Atlantic Ridge in the Vema Fracture Zone

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.

Residual Sediments of the Vema Fracture Zone, Central Atlantic

The Vema Fracture Zone orthogonally intersecting the Mid-Atlantic Ridge along 11° N is a complex tectono-sedimentary system, through which bottom water is transferred from the western to the eastern Atlantic. The southern surrounding of the fault valley is an extended (about 320 km) transverse ridge. At the early stages of the Vema System evolution, the ridge was located at shallow depths in the photosynthesis zone, which led to the formation of extensive biogenic carbonate buildups on its surface. Spreading, general sinking of the system, and associated movements along faults caused disintegration of ultramafic basement and bioherms with subsequent fractionation of colluvial–pelagic sediments and their eastward transport by bottom currents. The formation of coarse-grained residual sands completely devoid of pelitic material is associated with the high flow rates of Antarctic bottom waters. The presence of rounded pebbles of organogenic limestones and ultramafic rocks with traces of bioproductivity indicates that clastic material was transferred over a few hundred kilometers.

Discovery of widely available abyssal rock patches reveals overlooked habitat type and prompts rethinking deep-sea biodiversity

Habitat heterogeneity and species diversity are often linked. On the deep seafloor, sediment variability and hard-substrate availability influence geographic patterns of species richness and turnover. The assumption of a generally homogeneous, sedimented abyssal seafloor is at odds with the fact that the faunal diversity in some abyssal regions exceeds that of shallow-water environments. Here we show, using a ground-truthed analysis of multibeam sonar data, that the deep seafloor may be much rockier than previously assumed. A combination of bathymetry data, ruggedness, and backscatter from a trans-Atlantic corridor along the Vema Fracture Zone, covering crustal ages from 0 to 100 Ma, show rock exposures occurring at all crustal ages. Extrapolating to the whole Atlantic, over 260,000 km2 of rock habitats potentially occur along Atlantic fracture zones alone, significantly increasing our knowledge about abyssal habitat heterogeneity. This implies that sampling campaigns need to be considerably more sophisticated than at present to capture the full deep-sea habitat heterogeneity and biodiversity.

Geochemical heterogeneity along the Vema Fracture Zone, Indian Ocean: Mixing of melts from the Reunion plume and the Central Indian Ridge

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.

Spatial Structure of the Antarctic Water Flow in the Vema Fracture Zone of the Mid-Atlantic Ridge

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.

Molecular species delimitation and its implications for species descriptions using desmosomatid and nannoniscid isopods from the VEMA fracture zone as example taxa

We found 72 species for COI and 45 for 16 S by species delimitation among 186 (from 195 extracted) desmosomatid (144) and nannoniscid (42) sequenced specimens of a total of > 400 specimens for both families. Multiple “discovery”-type species delimitation methods were used, so that consistency across methods could be assessed: The ABGD analysis detected a barcode gap of 3–6% for COI and 4–6% for 16 S, in the whole dataset. Most putative species have a horizontally limited distribution along the Vema fracture zone, although the details depend in part on the interpretation of species delimitation analyses. Putative species were mostly restricted to the eastern or western Vema fracture zone, with only eight crossing the complete Vema fracture zone. Our data suggest that even robustly-sampled species exhibited small ranges; the range estimates calculable from present data were around 500 km, and three were on the order of 1000–2500 km. We chose an abundant, but geographically restricted species (Eugerdella egoni Tschesche and Brix sp. nov.) collected at a single site in the Vema transform fault, and two species (Prochelator barnacki Bober and Brix sp. nov. and Whoia sockei Brix and Kihara sp. nov.) with a broad, but disjunct distribution in the Vema fracture zone for taxonomic description.

Habitat characterization of the Vema Fracture Zone and Puerto Rico Trench

Although many of the regions on and close to the mid-ocean ridges have been extensively mapped and sampled, the abyssal intraplate regions remain essentially unsampled and unmapped, leaving huge gaps in our understanding of their geologic history and present activity. Prominent bathymetric features in these intraplate regions are fracture zones. Here we present bathymetric and sampling information from a transatlantic transect along the Vema Fracture Zone (ca. 11°N), covering crustal ages from 109 − 0 Ma on the African plate and 0–62 Ma on the South American plate. The Vema Fracture Zone is the intraplate trace of the active Vema Transform plate boundary, which offsets the present-day Mid-Atlantic Ridge by ca. 300 km left-laterally, juxtaposing zero-age crust with crust of 20 million years age. Our results show clear evidence of tectonic activity along most of the Fracture Zone, in most places likely associated with active fluid flow. Within the active Vema Transform at crustal ages of ca. 10 Ma we found clear indications of fluid flow both in the sediments and the overlying water column. This region is > 120 km from the nearest spreading axis and increases by almost an order of magnitude the maximum off-axis distance that active hydrothermal discharge has been found on the oceanic crust. Sampling of the igneous seafloor was possible at all crustal ages and the accretionary fabric imprinted on the plate during its production was prominent everywhere. Seafloor sediments show signs of extensive bioturbation. In one area, high concentrations of spherical Mn-nodules were also found and sampled.At the end of the transect we also mapped and sampled the Puerto Rico Trough, a > 8000 m-deep basin north of the Caribbean arc. Here the seafloor morphology is more complicated and strongly influenced by transpressive tectonics.

Does the Mid-Atlantic Ridge affect the distribution of abyssal benthic crustaceans across the Atlantic Ocean?

A trans-Atlantic transect along the Vema Fracture Zone was sampled during the Vema-TRANSIT expedition in 2014/15. The aim of the cruise was to investigate whether the Mid-Atlantic Ridge (MAR) isolates the abyssal fauna of the western and eastern abyssal basins.Based on two genetic datasets of Macrostylidae and Desmosomatidae/Nannoniscidae studied by Riehl et al. and Brix et al. in this issue we found that most of the therein-delimitated species were found at only one side of the MAR. We analysed those species of Macrostylidae and Desmosomatidae that were sampled across the MAR and complemented these with one species of a third family: Munnopsidae. With these datasets we were further able to consider the effect of different niche adaptations: Macrostylidae are infaunal (burrowing), Munnopsidae are considered epifaunal with pronounced swimming capabilities and Desmosomatidae and Nannoniscidae are partly able to swim, but are not as well adapted to swimming as Munnopsidae. We concluded that the MAR seems to be a dispersal barrier for the non-swimming Macrostylidae as well as weakly-swimming Desmosomatidae and Nannoniscidae. However, four species of Macrostylidae and Desmosomatidae did cross the MAR, but evidence for regular unrestricted gene flow is still lacking. For the swimming Munnopsidae we were able to detect persistent gene flow across the MAR.

Age of Plutonic Rocks from the Vema Fracture Zone (Central Atlantic) and Nature of Their Mantle Sources

This study presents the results of petrographic, geochemical, and isotope geochronological analyses of rock samples from the Southern flank of the Vema transform fault (Atlantic), which were dredged on cruises 19-th and 22-nd of the R/V Akademik Nikolai Strakhov. The sample suite includes both fresh and metamorphosed gabbros, dolerites, serpentinites, metapyroxenites. Zircons separated from three gabbro samples recovered at three different stations were used for in situ U–Pb dating by LA-ICP-MS. The ages reveal a strong linear relationship with a distance from the axis of the Mid-Atlantic ridge, which allowed us to estimate the rate of spreading in this segment of the Mid-Atlantic Ridge. It can be concluded that the estimated spreading rate of 16.2 ± 0.8 mm/yr was constant over the past 15 Myr. The mutual consistency of all U-Pb zircon and 39Ar–40Ar amphibole ages (Cipriani et al., 2009) obtained from the sampled transect suggests the temporal continuity of magmatic events that led to the formation of the original gabbroic rocks and their transformation during subsequent metamorphism. Rb—Sr isotope data show that hydrothermal activity took place in the presence of seawater between 14.7 and 9 Ma in the spreading axis region. Variations in the Nd isotopic composition in the time sequence of magmatic events indicate a high degree of chemical and isotopic heterogeneity of the ascending mantle material which became later entrained in the melting region beneath a spreading zone. Melting of the sources with primitive mantle composition (eNd ~ + 8 to +9) as well as enriched sources took place in the time interval between ~ 17 and 14.7 Ma and at about 8 Ma. The enriched source material is most likely represented by ancient mafic substratum.

Harpacticoida (Crustacea, Copepoda) across a longitudinal transect of the Vema Fracture Zone and along a depth gradient in the Puerto Rico trench

The aim of this study was the investigation of abundance, composition and biodiversity of benthic deep-sea Harpacticoida (Crustacea, Copepoda) in the Vema Fracture Zone (VFZ) and Puerto Rico trench. The study revealed a clear East-West gradient in total abundance of Harpacticoida with a westward decrease in abundances in the VFZ and significant differences in the community composition in the Eastern (East Vema) and Western Atlantic basin (West Vema) on family and genus level. The Puerto Rico trench and its upper slope did not only differ in abundance, but were distinct with respect to community composition on family and genus level. Thus, the upper slope might be considered as an ecotone, a transition zone where a rapid distinction of species composition occurs. In our study fiarea, 837 adult harpacticoid specimens could be assigned to 16 families and 1 subfamily. The most abundant families found were Ameiridae Boeck, 1865, Pseudotachidiidae Lang, 1936 and Ectinosomatidae Sars, 1903. Genera and species were investigated within selected families (Argestidae Por, 1986, Cletodidae T. Scott, 1905, Canthocamptidae Brady, 1880 and Zosimeidae Seifried, 2003) where 11 genera, and 73 species could be discriminated. Within the selected families, the genera Zosime Boeck, 1873 and Mesocletodes Sars, 1909 were dominant. In the study area, a high number of singletons was detected, which might be endemic to the respective region. Furthermore, a low total number of species in the trench was observed which was attributed to frequent disturbances in the dynamic environment of the Puerto Rico trench (e.g. turbidites or seismic activity) and high adaptability of specialists and opportunists to these disturbances.

Meiofauna abundance and community patterns along a transatlantic transect in the Vema Fracture Zone and in the hadal zone of the Puerto Rico trench

Despite the increasing sampling effort that occurred in the deep-sea environment during the last decades, knowledge about meiofauna ecology in trenches and Fracture Zones is still scarce. Based on the lack of this information, a longitudinal transect across the Vema Fracture Zone in the North Atlantic was sampled to test whether meiofauna abundances differ between Northeast and Northwest Atlantic basins, separated by the Mid-Atlantic Ridge. Also, for examination of meiofauna depth pattern, the Puerto Rico trench floor, its upper trench slope and the Western North Atlantic abyssal were investigated.In this study, meiofauna communities were dominated by Nematoda (93%) and Copepoda (4%). The highest total abundance of meiofauna was found in the Puerto Rico trench and the lowest in the Western basin. We found significant differences between the Eastern and Western Atlantic basins, which were potentially caused by differences in current regimes. Stronger currents observed in the Western basin possibly led to the coarser sediment grain size observed in this region, and consequently to the lower abundances of the major groups found there. Besides grain size, the total abundance of meiofauna was significantly correlated with total nitrogen, total organic carbon, and water depth. Moreover, our study reveals a trend of increasing abundance of total meiofauna with increasing water depth in the Puerto Rico trench. Also, significant differences between the Western abyssal and the Puerto Rico trench were discovered. Generally, the meiofauna abundance in the investigated area decreased from East to West but increased with increasing water depth in the Puerto Rico trench. Due to funnelling of organic sediments increased food availability towards deeper regions in trenches could occur and promote higher abundance.

The effects of depth, distance, and the Mid-Atlantic Ridge on genetic differentiation of abyssal and hadal isopods (Macrostylidae)

The largest habitat on Earth, the abyssal oceans below 3500m depth, is commonly assumed to represent a continuous environment due to homogeneity of environmental factors and the lack of physical barriers. Yet, the presence of bathymetric features, such as Mid-Ocean Ridges, and hadal trenches provide a discontinuation. During the Vema-TRANSIT expedition in 2014/2015 to the tropical North Atlantic, a transatlantic transect was studied following the full extent of the Vema Fracture Zone in an east-west direction and including the Puerto Rico Trench (PRT).The aim of this study was to test whether large bathymetric features represent barriers to dispersal and may lead to differentiation and eventually speciation. In this study, these potential barriers included the Mid-Atlantic Ridge (MAR) and the transition (~ 3000m) from the hadal PRT to the adjacent abyss. Genetic differentiation and differences in community structure (species composition) from east and west of the MAR, as well as abyssal and hadal depth zones were tested for using the poor dispersers Macrostylidae (Crustacea, Isopoda) as a modelDistribution patterns showed that certain macrostylid species have ranges extending more than 2000km, in some cases across oceanic ridges and trench-abyss transitions. Contrastingly, there was a clear signal for geographic population structure coinciding with the east-west division of the Atlantic by the MAR as well as with the abyss-hadal zonation. These results support the hypotheses that depth gradients as well as oceanic ridges reduce dispersal even though barriers may not be absolute. Additionally, positive correlation between genetic- and geographic distances showed that the vast size of the deep sea itself is a factor responsible for creating diversity.