Romanche Fracture Zone Research Articles

Extremely long Kelvin‐Helmholtz billow trains in the Romanche Fracture Zone

AbstractIn the Atlantic Ocean, the densest water mass Antarctic Bottom Water “AABW” can only cross the Mid‐Atlantic Ridge from its southwestern to northeastern basins in limited, because deep, conduits. At the southwestern entrance of one of these, the equatorial Romanche Fracture Zone, AABW crosses a sill at 4550 m depth in a 7 km narrow channel before plunging into the deep. At the sill‐slope, the rapidly flowing AABW causes shear‐induced turbulent mixing with the overlying water masses. We present an excerpt of 1 Hz sampled, half‐yearlong moored observations from 99 high‐resolution temperature sensors that demonstrate and quantify the turbulence details. On top of quasi‐steady shear flow, an internal tide modulates the mixing. Together, they constitute a means for an extremely long train of >250 consecutive Kelvin‐Helmholtz billows in a day that vary between 5 and 100 m in vertical scale.

Comparison of deep-water viromes from the atlantic ocean and the mediterranean sea.

The aim of this study was to compare the composition of two deep-sea viral communities obtained from the Romanche Fracture Zone in the Atlantic Ocean (collected at 5200 m depth) and the southwest Mediterranean Sea (from 2400 m depth) using a pyro-sequencing approach. The results are based on 18.7% and 6.9% of the sequences obtained from the Atlantic Ocean and the Mediterranean Sea, respectively, with hits to genomes in the non-redundant viral RefSeq database. The identifiable richness and relative abundance in both viromes were dominated by archaeal and bacterial viruses accounting for 92.3% of the relative abundance in the Atlantic Ocean and for 83.6% in the Mediterranean Sea. Despite characteristic differences in hydrographic features between the sampling sites in the Atlantic Ocean and the Mediterranean Sea, 440 virus genomes were found in both viromes. An additional 431 virus genomes were identified in the Atlantic Ocean and 75 virus genomes were only found in the Mediterranean Sea. The results indicate that the rather contrasting deep-sea environments of the Atlantic Ocean and the Mediterranean Sea share a common core set of virus types constituting the majority of both virus communities in terms of relative abundance (Atlantic Ocean: 81.4%; Mediterranean Sea: 88.7%).

On the finding of a prejuvenile of the rare deepsea species Lyconus pinnatus (Merlucciidae) in central waters of the Mid-Atlantic Ridge

Among materials collected in cruise 29 of research vessel Akademik Ioffe to the southern Atlantic (Koby� liansky et al., 2010) performed in November 2009 as part of the international MidAtlantic Ridge Ecosys� tem Project (MARECO), we found a prejuvenile of the rare deepsea species Lyconus pinnatus (Merlucci� idae). The purpose of this study was to describe the specimen. The 50+ mm long prejuvenile of L. pinnatus was collected in pelagic waters of the Romanche Fracture Zone, at station 2171�3, 04°06.16' S, 12°25.50 W, during sampling of the layer between 0 and 1250 m. It was stained with alizarin and cleared in glycerol. The following abbreviations are used in this study: TL, total length; SL, standard length; c, head length; D1, D2, A, P, V, number of rays in dorsal fins 1 and 2, anal, pectoral, and ventral fins, respectively; vert.a., vert.c., number of trunk and caudal vertebrae; sp.br., number of rakers on upper and lower parts of 1th gill arch; and r.br., number of branchiostegal rays.

Antarctic bottom water flow in the western part of the Romanche Fracture Zone based on the measurements in October of 2011

The properties of the Antarctic Bottom Water flow in the region of its inflow to the channel of the Romanche Fracture Zone at 22°10′–22°30′ W are studied on the basis of CTD and LADCP profiling in the western part of the equatorial fracture zone. A deep water cataract was found at the sill over the southern wall of the fracture with a depth of approximately 4600 m, which is associated with the abyssal flow, whose potential temperature is lower than 1°C. The inflow of water into the channel of the fracture in this temperature range is fully localized over this sill. The minimum potential temperature θ recorded in 2011 near the bottom was equal to 0.51°C, which is lower approximately by 0.12°C than the minimum temperatures ever measured in the western part of the fracture. The water transport in the cataract was estimated at 0.2 Sv (1 Sv = 106 m3/s), which is approximately 30% of the known estimates of the total transport of Antarctic Bottom Water (θ 1.70°C through the Romanche Fracture Zone between the West and East Atlantic can also occur through the depressions in the southern and northern walls of the fracture in the region of the Vema Deep.

Recent seismicity of southern Ghana and re-interpretation of the 1939 Accra earthquake: implications for recurrence of major earthquake

The purpose of this research is to re-interpret the 1939 Accra Earthquake, explain the nature and cause of the recent seismicity of the Accra region, and assess the likelihood of occurrence of any major cyclic earthquake in the Accra region and southern Ghana. Combining re-interpretation of historical earthquake data of Ghana with new field data, the nature of the seismicity of the Accra region has been assessed. The results show that the 1939 Accra Earthquake was a shallow-focus tectonic event caused by high-angle submarine strike-slip faults along the eastern extension of the Romanche fracture zone 40 kilometres south of Accra. Recent seismicity in the Accra region has emanated from sources similar to those of the 1939 Accra Earthquake related to the offshore fault systems. The structural setting and mechanism of the 1939 earthquake, however, preclude the likelihood of occurrence of a major cyclic earthquake based on a regional cyclic earthquake phenomenon. The results, however, do not exclude the possibility of occurrence of any significant random earthquake striking Accra region or other parts of southern Ghana now or in the future, as there are many independent active fault systems associated with the fault systems of the region.

Spatial patterns of bacterial and archaeal communities along the Romanche Fracture Zone (tropical Atlantic)

The composition of prokaryotic communities was determined in the meso- and bathypelagic waters funneled through the Romanche Fracture Zone (RFZ, 2°7′S, 31°79′W to 0°6′N, 14°33′W) in the tropical Atlantic. Distinct water masses were identified based on their physical and chemical characteristics. The bacterial and archaeal communities were depth-stratified with a total of 116 and 25 operational taxonomic units (OTUs), respectively, distributed among the distinct water masses as revealed by terminal restriction fragment length polymorphism, and cloning and sequencing. The relative abundance of Thaumarchaeota, determined by catalyzed reporter deposition-fluorescence in situ hybridization, was significantly higher in deeper layers (Antarctic Bottom Water, AABW, > 4000 m depth), contributing up to 31% to the total prokaryotic community, than in the mesopelagic and lower euphotic layer. Although the contribution of SAR11 to bacterial abundance did not increase with depth, SAR202, SAR324, SAR406 and Alteromonas did increase with depth. Terminal restriction fragment length polymorphism analysis revealed successional changes in the bacterial and archaeal community composition of the North Atlantic Deep Water (NADW) with a passage time through the RFZ of c. 4 months but not in the under- and overlying water masses. Our results indicate that specific water masses harbor distinct bacterial and archaeal communities and that the prokaryotic community of the NADW undergoes successional changes in this conduit between the western and eastern Atlantic basin. Apparently, in the absence of major input of organic matter to specific deep-water masses, the indigenous prokaryotic community adapts to subtle physical and biogeochemical changes in the water mass within a time frame of weeks, similar to the reported seasonal changes in surface water prokaryotic communities.

Diversity and distribution of microbial eukaryotes in the deep tropical and subtropical North Atlantic Ocean

Employing a combination of 4',6-diamidino-2-phenylindole and fluorescein isothiocyanate (DAPI-FITC) staining and catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH), we distinguished a variety of taxonomic and morphological types of eukaryotic microbes in the central and deep water masses of the tropical and subtropical North Atlantic Ocean. Samples were taken along a transect across the tropical Atlantic, along the equatorial upwelling and into the West-African upwelling region. Samples were collected as deep as 7000m in the Romanche Fracture Zone within the Mid-Atlantic Ridge. Approximately 50–70% of FISH-identified eukaryotes in deep water masses belong to one of seven groups: kinetoplastids, labyrinthulomycetes, fungi, diplonemids, group II alveolates, MAST 4 (stramenopiles), and an unidentified organism with a peculiar nuclear morphology. A smaller percentage of total eukaryotes was identified in the Central Water, especially in the oxygen minimum zone, than in deep water masses. CARD-FISH probes designed to identify broad taxonomic groups revealed kinetoplastids and fungi were more abundant than noted in previous studies employing 18S rRNA gene clone libraries. Group II alveolates, in contrast, were much less prevalent than previously reported. On a second survey, eukaryotic microbes were enumerated in the deep-sea basins below the North Atlantic subtropical gyre including the Vema Fracture Zone, which is another prominent trench in the Mid-Atlantic Ridge. The abundance of eukaryotes and chlorophyll concentrations were significantly different between the two cruises, which covered very different hydrographic regimes with associated high and low levels of primary production, respectively.

Antarctic Bottom Water Flow in the West Part of Romanche Fracture Zone Based on Measurements in October 2011

Antarctic Bottom Water Flow in the West Part of Romanche Fracture Zone Based on Measurements in October 2011

Lineaments study using aeromagnetic data over parts of southern Bida basin, Nigeria and the surrounding basement rocks

Trend analysis using aeromagnetic data was carried out over parts of the southern Bida basin, Nigeria and the surrounding basement rocks. Qualitative analysis of the total magnetic intensity map was carried out. Magnetic lineaments were inferred using the Euler deconvolution and analytic signal techniques. Deductions made from the study are; the Romanche fracture zone passed through the study area and the prominent trends of lineaments are the northsouth, northeastsouthwest and northwestsoutheast directions.

Measurements of currents in abyssal channels during cruise 32 of the research vessel Akademik Ioffe and cruise 34 of the research vessel Akademik Sergey Vavilov

The investigations of the flows in the abyssal chan� nels of the Atlantic were continued during cruise 32 of the R/V Akademik Ioffe (2010) and cruise 34 of the R/V Akademik Sergey Vavilov (2011). The measure� ments were made in the Discovery Gap (37°20′ N, 15°45′ W), the Kane Gap (9°20′ N, 19°50′ W), in two study regions in the Romanche Fracture Zone (0°47′ N, 13°15′ W; 1°00′ S, 22°00′ W), and in a study region in the Northern part of the Vema Channel (26°43′ S, 34°12′ W). The measurements were carried out using an SBE�19 profiler (CTD profiles). The CTD profiles were combined with measurements of the currents using a Workhorse Sentinel lowered acoustic Doppler current profiler (LADCP). The routes of the vessels and the studied regions are shown in the figure. The cruise of the R/V Akademik Ioffe in 2010 started in Kaliningrad and ended in Ushuaia, and the cruise of the R/V Akademik Sergey Vavilov in 2011 started in Rotterdam and ended in Ushuaia. We studied the propagation of the Antarctic Bottom Water (the AABW, the potential temperature θ < 2°C) from the Southern Ocean to the North Atlantic [1, 2], into which this water spreads mainly through the basins of the West Atlantic beginning from the Argen� tine Basin. The Antarctic Bottom Water propagates from the Argentine Basin to the Brazil Basin through the Vema Channel and then to the basins of the East Atlantic through the Romanche and Chain fracture zones as well as through the Vema Fracture Zone. The Antarctic Bottom Water propagates through the Vema Fracture Zone to the Cabo Verde Basin and Canary Basin, while, through the Romanche and Chain frac� ture zones, this water propagates to the Sierra Leone and Guinea basins. The Kane Gap is the deepest pas� sage connecting the Cabo Verde and Sierra Leone basins. It is located between the Sierra Leone Rise and the Guinea Plateau near the African continent. The Discovery Gap is the terminal point of the AABW’s propagation in the sense of its definition as water with

Exploration for Cretaceous stratigraphic traps in the Gulf of Guinea, West Africa and the discovery of the Jubilee Field: a play opening discovery in the Tano Basin, Offshore Ghana

Abstract The discovery of the Jubilee Oil Field in 2007 in the Tano Basin of Ghana is a primary example of the potential for Late Cretaceous deep-water stratigraphic traps in basins previously overlooked by the exploration industry. The Tano Basin forms the eastern extension of the Deep Ivorian Basin, which resulted from Aptian–Albian trans-tension associated with the opening of the Atlantic between the St Paul and Romanche fracture zones and the subsequent post-rift subsidence. The basin was the focus for deposition of a thick Upper Cretaceous, deep-water clastic sequence, which, in combination with a modest Tertiary section, provided sufficient thickness to mature a Cretaceous source rock in the central part of the Tano Basin. This well-defined reservoir and charge fairway forms the play, which, when draped over a large plunging nose (the South Tano high), resulted in the formation of a stratigraphic–structural combination trapping geometry that is characteristic of the Jubilee Field and subsequent discoveries in this play.

SPREADING OF ANTARCTIC BOTTOM WATER IN THE ATLANTIC OCEAN

This paper describes the transport of bottom water from its source region in the Weddell Sea through the abyssal channels of the Atlantic Ocean. The research brings together the recent observations and historical data. A strong flow of Antarctic Bottom Water through the Vema Channel is analyzed. The mean speed of the flow is 30 cm/s. A temperature increase was found in the deep Vema Channel, which has been observed for 30 years already. The flow of bottom water in the northern part of the Brazil Basin splits. Part of the water flows through the Romanche and Chain fracture zones. The other part flows to the North American Basin. Part of the latter flow propagates through the Vema Fracture Zone into the Northeast Atlantic. The properties of bottom water in the Kane Gap and Discovery Gap are also analyzed.

10.1007/s11479-008-1003-2

The structure of anomalously uplifted areas in transverse ridges of the Vema, São Paulo, and Romanche fracture zones is considered. It is concluded that their formation and eventual development in the present-day structure of the central Atlantic bottom proceeded during two stages. The first stage that corresponds to a short period at the Tortonian-Messinian transition (10 Ma ago) was marked by transportation of deep-seated rocks into the upper part of the lithosphere along thrust faults with mass motion in the meridional direction along the axis of the Mid-Atlantic Ridge. The second stage was characterized by contrasting highamplitude vertical movements from 10 to 3 Ma ago. It is suggested that near-meridional compression in the domains surrounding the Western Tethys in the Tortonian-Messinian resulted in deformation of the upper lithosphere within large transform fracture zones of the central Atlantic. The deformation that occurred 10 Ma ago was a manifestation of the global neotectonic epoch of the Earth.

Transport of antarctic waters in the deep channels of the Atlantic Ocean

Starting from 2002, the Shirshov Institute of Oceanology has been conducting investigations of Antarctic water transport in deep channels of the Atlantic Ocean, namely, the Vema Channel (31 ° S), Romanche Fracture Zone (0 ° N), and Vema Fracture Zone (11 ° N). The flow of bottom water in the Vema Channel is estimated as 4 Sv (1 Sv = 10 6 m 3 /s). Velocities in the channel reach 60 cm/s. A strong flow with velocities up to 30 cm/s was recorded in the Vema Fracture Zone. This flow includes the upper part of bottom waters of Antarctic origin and the lower part of North Atlantic Deep waters. The easterly transport of Antarctic waters is 0.1–0.7 Sv. In the Romanche Fracture Zone, maximum velocities reach 10 cm/s, while the entire easterly water transport is estimated as 0.1–0.8 Sv. The dominating propagation of Antarctic waters into the deep basins of the Northeast Atlantic occurs through the Vema Fracture Zone but not through the Romanche Fracture Zone due to strong mixing of deep waters in the latter channel caused by internal tidal waves. Bottom waters are formed at polar latitudes of the World Ocean. In the classic work by Wust [15], all waters of Antarctic origin are called Antarctic Bottom Waters (AABW). They propagate near the bottom in the Atlantic, being formed mainly in the Weddell Sea near the Antarctic slope as a result of mixing of cold and heavy Antarctic Shelf Water with lighter and warmer more saline Circumpolar Deep Waters. The pathways of Antarctic water propagation between the basins of the Atlantic are confined to depressions in the bottom topography. Antarctic Bottom Water from the Weddell Sea propagates through four passages in the South Scotia Ridge and through the South Sandwich Trench. The further propagation of AABW to the north into the Argentine Basin occurs through the Falkland Gap in the Falkland Ridge [14]. Part of this flow propagates along the southern and western margins of the Argentine Basin. The other part is trapped by the Subantarctic Front and flows to the east in the field of the Antarctic Circumpolar Current [14]. The waters of Antarctic origin are later transported to the Brazil Basin along three pathways: in the Vema Channel, in the Hunter Channel, and over the Santos Transport of Antarctic Waters in the Deep Channels of the Atlantic Ocean

Transtension and alkaline magmatism of the Romanche Fracture Zone

New magnetometric, petrological, and geochemical data on basalts from the central Romanche Fracture Zone (FZ) allow us to classify these rocks into two groups. The igneous rocks from the active part of the fracture zone that experienced transtension are referred to as alkaline rocks. According to some indications, they are younger that the oceanic tholeiites of the southern fault-line ridge, which were affected by elevated pressure in the past. These data indicate with a high probability that the Romanche FZ belongs to a rare group of magmatically active demarcation transform lines that separate large oceanic domains different in structural and geochemical features.

Rift–shear architecture and tectonic development of the Ghana margin deduced from multichannel seismic reflection and potential field data

The Ghana margin displays one of the best-known transform margins. Studies of the margin have provided the framework for a number of conceptual models aimed at understanding transform margin development worldwide. However, the deep structure of the margin is poorly known as knowledge is based only on wide-angle refraction measurements obtained from two separate localities on the margin. Consequently, complexities in the rift–shear margin architecture have been overlooked by current interpretations of margin development. Based on combined analysis of a detailed grid of ∼2710 km multichannel (MCS) lines and potential field data, we provide new insights into the structural architecture and tectonic development of the Ghana margin. In particular, we outline the deep structure of the entire margin using a series of 2D gravity modelled transects constrained by MCS and published wide-angle data. Our study reveals more complex rift–shear margin architecture than previously envisaged. We demonstrate that the main transform boundary representing the continental extension of the Romanche Fracture Zone, is actually composed of two distinct margin segments, i.e., the ENE–WSW trending sheared margin segment of the Cote d’Ivoire-Ghana Ridge and the NE–SW trending rift-influenced sheared margin segment of the Ghana Platform. These segments evolved under varying stress regimes, and during different time intervals. West of the transform margin, divergent rifting during the Early Cretaceous initiated the development of the Deep Ivorian Basin, essentially, as a single major pull-apart structure. However, east of the shear zone, oblique rifting resulted in the development of the Eastern Ghana Slope Basin as a composite of at least two coalescing pull-apart basins displaced along strike-slip faults. Our structural interpretation of the transform boundary geometry shows that the ridge and platform margin segments were each subjected to separate thermal influences from two different migrating spreading centres. Tectonic uplift of the ridge began through transpression during mid-Albian time following a change in relative direction of plate motion from NE–SW to ENE–WSW. However, the ridge uplift was amplified by thermal heating from a previously undocumented spreading centre whose progressive westward migration along the ridge followed closely after the Albian transpressional phase. The structural architecture of the Ghana margin resulted from a combination of factors, notably, pre-existing basement structure, plate boundary geometry, the relative direction of plate motion and thermal heating.

Synchronism in compression of the lithosphere in the central Atlantic and Western Tethys at the Tortonian-Messinian transition

The structure of anomalously uplifted areas in transverse ridges of the Vema, Sao Paulo, and Romanche fracture zones is considered. It is concluded that their formation and eventual development in the present-day structure of the central Atlantic bottom proceeded during two stages. The first stage that corresponds to a short period at the Tortonian-Messinian transition (10 Ma ago) was marked by transportation of deep-seated rocks into the upper part of the lithosphere along thrust faults with mass motion in the meridional direction along the axis of the Mid-Atlantic Ridge. The second stage was characterized by contrasting highamplitude vertical movements from 10 to 3 Ma ago. It is suggested that near-meridional compression in the domains surrounding the Western Tethys in the Tortonian-Messinian resulted in deformation of the upper lithosphere within large transform fracture zones of the central Atlantic. The deformation that occurred 10 Ma ago was a manifestation of the global neotectonic epoch of the Earth.

Intense mixing of lower thermocline water on the crest of the Mid-Atlantic Ridge

Buoyancy exchange between the deep and the upper ocean, which is essential for maintaining global ocean circulation, mainly occurs through turbulent mixing. This mixing is thought to result primarily from instability of the oceanic internal wave field, but internal waves tend to radiate energy away from the regions in which they are generated rather than dissipate it locally as turbulence and the resulting distribution of turbulent mixing remains unknown. Another, more direct, mixing mechanism involves the generation of turbulence as strong flows pass through narrow passages in topography, but the amount of turbulence generated at such locations remains poorly quantified owing to a lack of direct measurements. Here we present observations from the crest of the Mid-Atlantic Ridge in the subtropical North Atlantic Ocean that suggest that passages in rift valleys and ridge-flank canyons provide the most energetic sites for oceanic turbulence. Our measurements show that diffusivities as large as 0.03 m2 s(-1) characterize the mixing downstream of a sill in a well-stratified boundary layer, with mixing levels remaining of the order of 10(-4) m2 s(-1) at the base of the main thermocline. These mixing rates are significantly higher than the diffusivities of the order of 10(-5) m2 s(-1) that characterize much of the global thermocline and the abyssal ocean. Our estimates suggest that overflows associated with narrow passages on the Mid-Atlantic Ridge in the North Atlantic Ocean produce as much buoyancy flux as has previously been estimated for the entire Romanche fracture zone, a large strait in the Mid-Atlantic Ridge that connects the North and South Atlantic basins. This flux is equivalent to the interior mixing that occurs in the entire North Atlantic basin at the depth of the passages, suggesting that turbulence generated in narrow passages on mid-ocean ridges may be important for buoyancy flux at the global scale.

Structure and variability of deep waters in the Romanche Fracture Zone

Structure and variability of deep waters in the Romanche Fracture Zone

Mantle process beneath Philippine Sea back‐arc spreading ridges: A synthesis of peridotite petrology and tectonics

Abstract In order to obtain a general view of the mantle process beneath a back‐arc basin spreading ridge, the diversity of peridotite petrology and tectonic occurrences in two back‐arc basin spreading ridges from the Philippine Sea were examined: the Parece Vela Rift and the Mariana Trough. The Parece Vela Basin spreading ridge (Parece Vela Rift) was a physically fast/intermediate‐spreading ridge, although many tectono‐magmatic features resemble those of slow‐ to ultraslow‐spreading ridges. Two unusual features of the Parece Vela Rift further demonstrate the uniqueness of the ridge: full‐axial development of oceanic core complexes and exposure of mantle peridotite at segment midpoints. The Parece Vela Rift yields a lithological assemblage of residual but still fertile lherzolite/harzburgite, plagioclase‐bearing harzburgite and dunite; similar assemblages are reported from the equatorial Mid‐Atlantic Ridge at the Romanche Fracture Zone and the ultraslow‐spreading ridges from the Indian and Arctic Oceans. The tectono‐magmatic characteristics of the Parece Vela Rift suggest that diffuse porous melt flow and pervasive melt–mantle interaction were the important mantle processes there. Globally, this ‘porous melt flow‐type’ mantle process is likely to occur beneath a segment midpoint of the ridge having a thick lithosphere, typically an ultraslow‐spreading ridge. In contrast, the Mariana Trough is a typical slow‐spreading ridge, exposing mantle peridotite at segment ends. The Mariana Trough yields a lithological assemblage of residual harzburgite and veined harzburgite, a common assemblage among the global abyssal peridotite suite. The tectono‐magmatic characteristics of the Mariana Trough suggest that channeled melt/fluid flow and limited melt–mantle interaction are the important mantle processes there, because of the colder wall‐rock peridotite in the segment end. This ‘channeled melt flow‐type’ mantle process is likely to occur in the shallow lithospheric mantle at the segment ends of any spreading ridges.