Vema Channel Research Articles

Morphology and sedimentary processes in Santa Catarina Plateau and Vema Channel, Brazil

Morphology and sedimentary processes in Santa Catarina Plateau and Vema Channel, Brazil

Structure of Bottom Flows in Abyssal Channels of the Atlantic: Comparison of Autonomous and Vessel Observations in the Vema Channel, Romanche Fracture Zone, and Kane Gap

Structure of Bottom Flows in Abyssal Channels of the Atlantic: Comparison of Autonomous and Vessel Observations in the Vema Channel, Romanche Fracture Zone, and Kane Gap

Structure of Bottom Flows in Abyssal Channels of the Atlantic: Comparison of Autonomous and Vessel Observations in the Vema Channel, Romanche Fracture Zone, and Kane Gap

Oceanographic data from measurements in deep-water channels of the Atlantic Ocean were analyzed. Data from two different methods were compared: CTD/LADCP measurements from research vessels and data from autonomous mooring stations. The comparison showed that discrepancies between the data obtained by two different methods can significantly exceed the instrumental accuracy of the instruments. This result highlights the importance of correct selection of mooring station locations in conditions of significant spatial gradients in measured characteristics. The results of the analysis showed that throughout the Antarctic waters pathway, the measurement differences are of the same orders of magnitude. Therefore, the conclusions can be valid for all abyssal channels with intense AABW flows.

Pleistocene Paleoceanographic Conditions in the Ioffe Drift Area (South Atlantic) Based on Benthic Foraminiferal Assemblages

The Ioffe Drift located in the western South Atlantic to the north-east to the Vema Channel was discovered in the 32nd cruise of the R/V “Akademik Ioffe” in 2010. The analysis of the seismoacoustic, biostratigraphic, lithological, grain-size, geochemical and X-ray fluorescence data allowed to estimate the Pleistocene age of the upper part of the drift as well as to confirm contourite genesis of the sediments. Quantitative analyses of benthic foraminiferal assemblages are carried out in the sediment core AI-2436 (26°51.6′ S, 34°01.40′ W, 3800 m water depth) collected near the drift summit. The obtained results allowed reconstructing relative changes of the organic matter flux to the seafloor, its periodicity and bottom current intensity as well as suggesting interplay of the deep-water masses during the Pleistocene. Visual evaluation of benthic foraminiferal tests allowed distinguishing three groups of damages of presumably variable genesis. First group includes damages possibly resulted from impacts of particles transporting by bottom currents. Second group consists of damages that appeared due to movement of the tests by strong bottom currents. Third group contains deformations caused by dissolution in carbonate-aggressive bottom-water environment. Thus, specific features of the tests in combination with traditional methods may serve as a supporting technique for diagnostics of contourites. However, this method demands a development of application criteria and should be verified on reliable material.

Antarctic Bottom Water Jets Flowing from the Vema Channel

Properties of the abyssal current of Antarctic Bottom Water (AABW) from the Vema Channel are studied based on temperature, salinity, and velocity profiler (CTD/LADCP) data. Previous studies over a period of almost 30 years revealed that very intense current of AABW exists in the Vema Channel. Later, it was found that this current consists of two branches. One branch spreads over the bottom of the channel; the other branch is elevated over the western wall of the channel. The deepest branch decays after it passes approximately 100 km while the upper one continues further to the North Atlantic and is the source of abyssal waters in the Canary and Cabo Verde basins of the North Atlantic. Data analysis suggested that the upper jet splits into two. One of these descends down a canyon at 24°30′ S, while the other (the third one) remains on the continental slope, and indications of its existence are also found at 24°00′ S. This research analyzes the existence and pathway of this third branch that can be traced up to latitude 24° S. Velocity measurements in 2022 allowed us to confirm the existence of this third branch.

Modeling of bottom currents for estimating their erosional-depositional potential in the Southwest Atlantic

Contourites are sediments deposited or substantially reworked by the persistent action of bottom currents which are capable of forming a wide range of erosional and depositional features on the seafloor. The study of contourites is a multidisciplinary topic linking physical oceanography and marine geology. While near-bottom circulation is an important subject itself due to different dynamic aspects and climatic estimates of abyssal water transports, it becomes especially important for studies of lateral sedimentation. The precise direct data on bottom currents on the South American continental rise and adjacent abyssal plains remain extremely rare. Numerical modeling can appear helpful for filling the gaps in our knowledge on bottom circulation in the region. This work describes an approach for mapping erosion- and deposition-dominated areas based on the modeled circulation in the bottom layers. The model domain covers the regions with different seabed topography and velocities of bottom currents: the Vema Channel, the Santa Catarina Plateau, the Rio Grande Rise, and parts of the Argentine and Brazil basins. The high vertical resolution used in the model allowed us to study the near-bottom circulation in the deep ocean, while high horizontal resolution enabled modeling currents in narrow underwater passages such as the Vema Channel. The equations that encompass the bottom current velocities and data on surface sediment grain-size are well-known; in this study, we estimate the erosional-depositional potential of bottom currents using these equations. This approach allowed identification of the regions of dominant erosion (to the northeast of the northern exit from the Vema Channel), deposition (northern part of the Santa Catarina Plateau), and non-deposition under the influence of bottom currents. The results were validated by the analysis of echo-facies distribution in the study area and variations in the surface sediment grain-size. This study embraces the transition zone between the Argentine and Brazil basins, which is crucial for understanding the deep and bottom circulation in the western South Atlantic under modern conditions and in the geological past. The presented technique can improve our knowledge on how bottom currents shape the seafloor and control sediment distribution in the Southwest Atlantic.

Contourite systems around the northern exit from the Vema Channel

Several drifts of different types and sizes are identified near the northern exit of the Vema Channel, within the projects Neogene-Quaternary contourites of the Central and South Atlantic and Lateral sedimentation in the deep ocean (on the examples from the Central and South-Western Atlantic). Herein, we discuss new results of a multidisciplinary study of sixteen sediment cores and high-resolution sub-bottom (seismoacoustic) profiles, to address the impact of Lower Circumpolar Deep Water (LCDW) and Antarctic Bottom Water (AABW) passing through the Vema gateway on contourite accumulation and erosion. To the west of the northern exit from the Vema Channel, an anticyclonic gyre of LCDW is instrumental in the development of contourite drifts and sediment waves. The contourite origin of generally silty terrigenous sediments in the Santa Catarina Plateau – São Paulo Plateau area is ascertained by both morpho-seismic and sedimentary characteristics. Sedimentary features include: a lack of primary sedimentary structures and pervasive bioturbation; sharp erosional contacts, local hiatuses and stiff mud horizons; some sandy/silty layers and indistinct bedding; mostly fine grain-size, very poor sorting and distinctive bi-gradational sequences; a high degree of correlation between the content of sortable silt (SS) in the total < 63 μm size fraction and SS mean sizes in all eight cores studied. Biostratigraphy, oxygen isotope records and ten new accelerator mass-spectrometry (AMS) 14C dates reveal the mid to late Quaternary age of this mainly terrigenous contourite depositional system (CDS).By contrast, the dominantly calcareous Ioffe Drift, overlying the Florianopolis Fracture Zone (FFZ) ridge to the northeast of the Vema Channel, is far from any source of terrigenous material, and accumulated in an area of low biological productivity. The overall asymmetric geometry, mainly lenticular, upward-convex seismic units separated by erosional unconformities, reflection truncation, small-scale moats dissecting the drift surface collectively indicate its contourite origin. The interpretation of the drift as the CDS is supported by sediment characteristics including common hiatuses corresponding in some cases to erosional contacts, pervasive bioturbation, generally poor sediment sorting, and more or less well-developed bi-gradational sequences. The erosion and deposition in the drift area are mostly controlled by the main LCDW branch following northeastward along the FFZ. Extensive erosion by bottom currents has created numerous hiatuses and markedly reduced the thickness of drift's sediments. The stratigraphic record from the Ioffe Drift sediment cores reveals an Upper Pliocene – Quaternary succession. The results of detailed analyses of six cores from the Ioffe Drift area, compared with two cores and the upper part of DSDP Site 516 from the Rio Grande Rise, notably the use of a SS analogue, provide new information on calcareous biogenic contourites deposited in the pelagic realm of the South Atlantic.

Warming Trend in Antarctic Bottom Water in the Vema Channel in the South Atlantic

AbstractThe excess heat absorbed from the atmosphere has increased the temperature in the upper layers of the ocean (&lt;2,000 m). In the abyss, infrequently repeated ship sections, deep Argo float measurements, and sparse moored observations have found signs of warming in the Southwest Atlantic, possibly linked to changes in the Weddell Sea. We present a new moored temperature time series sampled near the bottom in the Vema Channel, from February 2019 to August 2020. Together with historical data, the combined record confirms the warming of the abyssal waters, with an increase of 0.059°C in potential temperature between January 1991 and August 2020, embedded within intense high‐frequency variability. Moreover, the data suggest the possibility of an accelerated warming, with a change in the temperature trend from 0.0016°C yr−1, between the early 1990s and 2005, to 0.0026°C yr−1 afterwards.

Structure of the bottom water flow in the Vema Channel based on the measurements from the R/V Akademik Sergey Vavilov

Structure of the bottom water flow in the Vema Channel based on the measurements from the R/V Akademik Sergey Vavilov

Hydraulic Continuation of the Abyssal Flow From the Vema Channel in the Southwestern Part of the Brazil Basin

AbstractIn 2013 and 2018, Shirshov Institute of Oceanology performed 12 stations with conductivity‐temperature‐depth and lowered acoustic Doppler current profiler (LADCP) profiling in the southwestern part of the Brazil Basin in order to find continuations of hydraulic controlled flow of the coldest Antarctic bottom water that propagates along the deepest bed of the Vema Channel. Comparison of these measurements with the historical database and our previous measurements in the Vema Extension region (27°S, 34°W) showed that such a continuation of the flow does not exist in the valley directed to the east, which seemed to be a topographic extension of the Vema Channel. Continuation of the flow was found over the section at 25°34′S across the meridionally oriented channel (depth up to 4,950 m) approximately along 33°30′W north of the Vema Extension region. Northward velocity speeds exceeding 35 cm/s were measured in the bottom flow (100–150 m thick above the bottom), which is displaced to the western slope of the meridional channel. Transport in the high‐speed core (velocity speed greater than 10 cm/s, potential temperature less than 0.62°C) in this channel was estimated at 0.150 ± 0.007 Sv on the basis of the LADCP measurements. There are several indirect indications of the formation of a local spillway (or a cascade of spillways) here in the regime of hydraulic control overflow.

Hiatuses in the late Pliocene–Pleistocene stratigraphy of the Ioffe calcareous contourite drift, western South Atlantic

Abstract The Ioffe Drift located in the Antarctic Bottom Water pathway from the Vema Channel to the Brazil Basin provides a suitable site to study past variations in bottom contour currents and their contribution to erosion and accumulation of deep-sea sediments. Our previous study of the reference core AI-2436 from the drift summit (Ivanova et al., 2016a) documented the stratigraphic sequence of the uppermost part of the sedimentary cover ranging from the recent through upper Pliocene, the reduced thickness and/or absence of biostratigraphic zones, and the occurrence of several hiatuses. Here, we provide the multi-proxy biostratigraphic, magnetic susceptibility (MS), color reflectance and X-ray fluorescence (XRF) data in the sediment cores AI-3318 from the summit and AI-3316 from the NE slope of the Ioffe Drift. The new data report numerous long- and short-term stratigraphic gaps over the last ~3 Ma. An interval of specific high-amplitude peaks representing abrupt changes in volume MS and XRF variability is identified from 2.51/2.59 to ~1.9 Ma which can likely serve as a regional stratigraphic benchmark in the future studies of deep-sea contourites. The correlation of three sediment records from the drift suggests that the most pronounced series of hiatuses, associated with enhanced AABW flow intensity occurred from 1.6 to ~0.81 Ma (i.e. roughly, during the Mid-Pleistocene Transition), and from 2.51/2.59 to ~1.9 Ma (i.e. covers the onset of the modern-type deep-water circulation in the South Atlantic (Turnau and Ledbetter (1989)). Comparison of the studied sediment records with DSDP Site 516 reveals reduced thickness of all recovered biostratigraphic zones and more often occurrence of hiatuses in the Ioffe Drift than on the Rio Grande Rise suggesting more vigorous contour currents in the former area.

Extreme transport velocities of Antarctic Bottom Water flow through the deep Vema Channel

In April 2017 and in October 2018, a Russian expedition on the R/V “Akademik Sergey Vavilov” in the South-West Atlantic carried out measurements of temperature, salinity, and velocity over a standard section across the deep Vema Channel at 31(градус) 12(минут) S. Extremely high and extremely low velocities and transports of Atlantic Bottom Water (AABW) in the entire history of our observations over this section were found. In 2017, the maximum speed over the section reached 55 cm/s, and in 2018, it did not exceed 30 cm/s. In 2018, a core of high velocities in the bottom layers of the channel characteristic of the AABW flow was not found.

Extreme Transport Velocities of Antarctic Bottom Water in the Deep-Water Vema Channel

In April 2017 and October 2018, a Russian expedition on the R/V Akademik Sergei Vavilov in the Southwest Atlantic carried out measurements of the temperature, salinity, and velocity over a standard section across the deep Vema Channel at 31°12′ S. Extremely high and extremely low velocities and transports of Antarctic Bottom Water for the entire history of our observations over this section were found. In 2017, the maximum speed over the section reached 55 cm/s, and in 2018, it did not exceed 30 cm/s. In 2018, no core of high velocities in the bottom layers of the channel characteristic of the Antarctic Bottom Water flow was found.

Currents and water structure north of the Vema Channel

Currents and water structure north of the Vema Channel

Glacial–interglacial interplay of southern- and northern-origin deep waters in the São Paulo Plateau – Vema Channel area of the western South Atlantic

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the global climate system due to its role in oceanic heat transport, CO2 exchange between the ocean and atmosphere, and carbon storage in the ocean interior. Previous studies have proposed dramatic changes in the global thermohaline circulation during glacial–interglacial cycles. Here, we present isotopic, geochemical and foraminiferal records from the core AI-3152 (water depth 3435 m). These data demonstrate shifts in the depth of northern- and southern-sourced deep water masses, as well as of foraminiferal lysocline in the São Paulo Plateau area, north-westward of the Vema Channel, in the western South Atlantic over the last 167 kyr. That the study area was bathed by southern-origin deep water masses during MIS 6, 4–2 and MIS 5.4–5.1, whereas a prevalence of northern-sourced waters is reconstructed for interglacials and MIS 5.5 and 1. Our data suggest that a carbonate saturation state associated with deep water ventilation, Antarctic air temperatures, ocean–atmosphere gas exchange, Southern Ocean upwelling rates and likely productivity changes significantly affected the deep sea oceanographic conditions in the São Paulo Plateau area during glacials and MIS 5.4–5.1. A comparison of δ18OCw and δ13СCw recorded from core AI-3152 to other stable isotope records from the Rio Grande Rise, to the east of the Vema Channel clearly indicated the presence of a southern-derived analogue of dense Weddell Sea Deep Water at 3.4–4 km depth in the Vema Channel area during the MIS 3–2 interval. An approximately 500 m steeper W-E slope on the boundary of two southern-origin deep water masses was inferred on both sides of the Vema Channel during MIS 3–2. This boundary shift likely reflects a northward expansion of the densest water mass in the Atlantic, thereby impacting geostrophic balance.

ВНУТРЕННИЙ ПРИЛИВ НА ПЛАТО САНТОС

We analyze internal tides over the Santos Plateau in the South Atlantic. The study is based on the measurements of temperature and currents on the line of moorings normal to the South America coast in the latitudinal band between 28°S and 31°S. The moorings were deployed at distances between 25 and 888 km (from the 500 m isobath) southeast of the continental slope. The instruments were set at 900 m. Numerical modeling revealed the properties of internal tides (amplitudes, wavelength, and decay of amplitudes) along the line of their propagation from the continental slope. The amplitudes of internal tide decrease from the continental slope in the direction to the Vema Channel from 36 m to 15 m. The densities of the energy of tidal internal waves were calculated from the semidiurnal tidal components of current and temperature time series taking into account the vertical gradients of temperature and the Brunt-Vaisala frequency. Decay of the energy of internal tide occurs according to an inverse power law. Numerical modeling of the generation and propagation of internal tides reveals that beams of internal tides are formed near the continental slope. The energy decay based on modeling is similar to the measured data.

Grain Size Constraints on Glacial Circulation in the Southwest Atlantic

AbstractKnowledge of past deep‐ocean current speeds has the potential to inform our understanding of changes in the climate system on glacial‐interglacial timescales, because they may be used to help constrain changes in deep‐ocean circulation rates and pathways. Of particular interest is the paleo‐flow speed of southern‐sourced deep water, which may have acted as a carbon store during the last glacial period. A location of importance in the northward transport of southern‐sourced bottom water is the Vema Channel, which divides the Argentine and Brazil basins in the South Atlantic. We revisit previous studies of paleo‐flow in Vema Channel using updated techniques in grain size analysis (i.e., mean sortable silt grain size), in Vema Channel cores and cores from the Brazil margin. Furthermore, we update the interpretation of the previous grain size studies in the light of many years further research into the glacial circulation of the deep Atlantic. Our results are broadly consistent with the existing data and suggest that during the last glacial period there was slightly more vigorous intermediate to middepth (shallower than 2,600 m) circulation in the South Atlantic Ocean than during the Holocene, whereas around 3,500 m the circulation was generally more sluggish. Increased glacial flow speed on the eastern side of the Vema Channel was likely related to an increase in northward velocity of AABW in the channel. An increase in Antarctic Bottom Water flow through the Vema Channel may have helped to sustain the large volume of southern‐sourced deep water in the Atlantic during the glacial period.

Moho depth and equivalent elastic thickness of the lithosphere over the Vema Channel: A new evidence of an aborted ridge

We investigate the Vema Channel in terms of spatial variations of the elastic thickness (Te) in the frame of the thin plate flexure model using the convolutive method. The modeling of the Moho in terms of the thin plate flexure model is done by a least squares approximation of the Moho obtained from gravity inversion. The flexure is calculated by the convolution of the crustal load with the point-load flexure response curves. The RMS difference between the gravity and flexure Moho surfaces is minimized by varying the Te by inverse modeling. The result is a solution of the flexed crust that is in best agreement with the long-wavelength component of the gravity field.The flexure Moho depths vary between 12 and 18 km and agree well with those obtained from gravity inversion. The spatial variations of Te range from 2 to 30 km and have a good correlation with the geological interpretation for an aborted ridge near Vema Channel, called in this paper as the Vema Aborted Ridge (VAR). The occurring of seamounts appears to be correlated to a weak and deformed region. Attempts of crustal breakup are marked by high Te values (30 km) while lower values (3–12 km) are found for the suggested aborted ridge. The VAR is on Isochron of 93 Ma and shows symmetrical older along both sides of its axis. Asymmetric magnetic anomalies are found over the ridge and may be related to upper-extended continental crust broken by the Vema.

A Late Cretaceous mixed (turbidite-contourite) system along the Uruguayan Margin: Sedimentary and palaeoceanographic implications

The Late Cretaceous is a critical period in the evolution of the South Atlantic. Significant changes in the tectonic configuration of continental landmasses and marine basins during this time lead to major shifts in the sedimentary and oceanographic regimes. Using a combination of 2D and 3D seismic data with well data from the Uruguayan Margin, this work provides a new, highly detailed description of the Late Cretaceous sedimentary evolution of the margin and its regional and global palaeoceanographic implications.The Polonio Drift System (PDS), occupies the middle to lower-slope of the Uruguayan Margin. The PDS is characterised by four channel-drifts, which propagate obliquely (120–170°) from the palaeoslope and exhibit over 800m of local relief relative to the surrounding seafloor. Seismic interpretation identified four seismic units (SU.1–SU.4), separated by five major discontinuities (R.1–R.5). These record the onset (Albian–upper Turonian, 113–89.8Ma), growth (Coniacian–upper Maastrichtian, 89.8–66Ma), and burial (Paleocene, 66–56Ma) phases for the PDS. During the onset stage, down-slope processes dominate but, three major shifts in the sedimentary stacking pattern occur. These coincide with palaeoceanographic changes in the South Atlantic during the Cenomanian–Turonian (100.5–89.9Ma), Campanian-Maastrichtian (83.6–66Ma) and Paleocene (66–56Ma), following the opening of the Falkland/Malvinas-Agulhas Seaway (FAS), the subsidence of the Walvis-Ridge and the opening of the Vema channel. Bottom current influence first occurs during the growth stage (Coniacian–Campanian) when mounded deposits shift from aggradational and slightly progradational towards the N (SU.2), to clear lateral migration towards the S (SU.3). This sedimentary pattern indicates a regional palaeoceanographic shift in bottom current flow from northerly, to a subsequent southerly direction. Bottom currents do not appear to have actively eroded sediment, although they apparently had the potential to pirate and entrain fine-grained material from turbidity plumes, which build on underlying bathymetric features such as basement horsts and channel-levees. These produce large mounded morphologies adjacent to channels. Drift growth ceases following the termination of shelf progradation (R.4), enabling sluggish bottom currents contributed to a homogeneous sedimentary drape. Bottom currents intensify in the beginning of the Paleocene, forming contourite depositional systems with well-developed contourite terrace along the middle slope. Further research can elucidate the nature and mechanics of turbidite–contourites interactions along continental margins. This work offers new insights for the implications and consequences of the interaction of turbidites and contourites, which can improve the characterisation of deep-water deposits and understanding palaeoceanographic interpretations.

New model and field data on estimates of Antarctic Bottom Water flow through the deep Vema Channel

We used a numerical model of the ocean circulation with a high spatial resolution to obtain estimates of the kinematic characteristics of Antarctic Bottom Water flow through the abyssal Vema Channel in the southwestern part of the Atlantic Ocean. The results of simulations correspond to the data of direct velocity measurements made at several locations in the channel. The high horizontal and vertical resolution of the model in the bottom layer allowed us to study in detail the hydrodynamics of this flow over its entire length.