Kane Gap Research Articles

Water Mass Structure and Variability in the Kane Gap (Equatorial Atlantic Ocean)

Water Mass Structure and Variability in the Kane Gap (Equatorial Atlantic Ocean)

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.

Multibeam bathymetry data from the Kane Gap and south-eastern part of the Canary Basin (Eastern tropical Atlantic).

We present a multibeam bathymetry dataset from eastern part of the Canary Basin and from the Kane Gap located north and south of the Cape Verde Islands, respectively. Special attention was paid to the deep-water Kane Gap between Sierra Leone Rise and Guinea Plateau. The Kane Gap is the only possible gateway that allows the exchange of Antarctic Bottom Water between the Sierra Leone and Gambia Basins. Bathymetry surveys were carried out during the 44th cruise of the Research Vessel (R/V) “Akademik Nikolaj Strakhov” in October 2019. The data were collected using the system RESON SeaBat 7150 (12.5 kHz) and processed using QINSy software. The multibeam bathymetry data are presented in tabular format (ASCII (*.txt)) and as digital elevation models in ESRI ASCII grid (*.asc) and GeoTIFF raster (*.tif) formats with a resolution of 100 m. The dataset is available with the article.

Complex Oceanological Research during the 44th Cruise of the Research Vessel Akademik Nikolaj Strakhov in the Eastern Tropical Atlantic

Geological, hydrological, hydrochemical, and biological research has been carried out in the eastern tropical Atlantic. Based on the multibeam echo-sounding and seismic profiling north of the Kane Gap (Sierra Leone Rise), an extensive sedimentary body (contourite fan) has been identified. Data of hydrophysical sounding allows assuming the existence of an overflow of the Antarctic Bottom Water (AABW) through the Kane Gap and its northward penetration. In the sediment cores (Middle Pleistocene to Holocene age), an influence of the AABW is reflected in the alternation of the high and medium carbonate dissolution rates.

Study of the Contourite Drift north of the Kane Gap (eastern equatorial Atlantic)

Study of the Contourite Drift north of the Kane Gap (eastern equatorial Atlantic)

Atypical anticlockwise internal tidal motions in the deep ocean

In the ocean, horizontal motions associated with freely propagating semidiurnal tidal inertia-gravity waves mainly describe an ellipse that is traversed in a clockwise direction in the Northern Hemisphere. In this article, rare observations of anticlockwise polarised semidiurnal motions are presented from deep North-Atlantic mooring sites. Anticlockwise motions are found to occur 1–20% of the time, irrespective of latitude and (weak) vertical density stratification. In a deep narrow channel like Kane Gap however, anticlockwise motions dominate over clockwise. The statistics of turbulence match the occurrence of polarisation change. It is unclear whether these observations represent theoretical predictions of reflected baroclinic Poincaré waves, or those involving the effects of the horizontal Coriolis parameter on internal wave propagation in weak stratification. The observed switching between clockwise and anticlockwise polarisation with time at a fixed position cannot be explained by the internal wave theory using the traditional approximation. Instead, the polarisation switching points at baroclinic effects involving varying background conditions like stratification and shear-induced mixing. It is suggested the polarisation statistics may be used as a diagnostic tool for such varying conditions.

Convective and shear-induced turbulence in the deep Kane Gap

[1] The boundary layer above a 4569 m deep slope in the near-equatorial N-Atlantic Ocean Kane Gap, a throughflow for Antarctic Bottom Water (AABW), is characterized by two distinct turbulent regimes that differ by an order of magnitude in intensity depending on the direction of throughflow. During southward and downward flow, vertical mixing is vigorous. This is inferred from high-resolution temperature observations between 6 and 132 m above the bottom. For a representative case study of 2 days, average values are found for dissipation rate of e?=?2.1?±?1 × 10-9 W kg-1 and eddy diffusivity of Kz?=?7?±?4 × 10-4 m2 s-1. The mixing is across relatively large vertical overturns. During northward and upward flow, smaller overturns are more horizontal as in stratified shear flow (with representative 2 day mean e?=?6?±?3 × 10-11 W kg-1, Kz?=?4?±?2 × 10-5 m2 s-1). Stratification is approximately the same during both flow directions. Although the different turbulence regimes are partially associated with frictional boundary layers of large-scale flows above sloping topography, but not with those over flat bottoms, and partially with flow across a hill-promontory, internal waves are a dominant process in promoting turbulence. In addition, internal waves are observed to push stratification toward the bottom thereby importantly contributing to the mixing of AABW.

Transport of Antarctic bottom water through the Kane Gap, tropical NE Atlantic Ocean

Abstract. We study low-frequency properties of the Antarctic Bottom Water (AABW) flow through the Kane Gap (9° N) in the Atlantic Ocean. The measurements in the Kane Gap include five visits with CTD (Conductivity-Temperature-Depth) sections in 2009–2012 and a year-long record of currents on a mooring using three AquaDopp current meters. We found an alternating regime of flow, which changes direction several times during a year. The seasonal signal seems to dominate. The maximum daily average values of southerly velocities reach 0.20 m s−1, while the greatest north-northwesterly velocity is as high as 0.15 m s−1. The velocity and transport at the bottom are aligned along the slope of a local hill near the southwestern side of the gap. The distribution of velocity directions at the upper boundary of AABW is wider. The transport of AABW (Θ < 1.9 °C) based on the mooring and LADCP (Lowered Acoustic Doppler Current Profiler) data varies approximately within ±0.35 Sv in the northern and southern directions. The annual mean AABW transport through the Kane Gap is almost zero.

Strong magnetic field effect on over-the-barrier transport in Pb-p-Hg1 − xCdxTe Schottky barriers

It is usually believed that the over-the-barrier current in Schottky barriers (SB) on p-type semiconductor is controlled by heavy holes. However, there is an additional potential barrier caused by an oxide layer inevitably existing at the interface in real SB. For typical parameters of the barrier, its tunnelling transparency for light holes can be higher by three orders of magnitude than that for heavy holes. Thus, one can expect that the current is mainly due to the contribution of light holes. To clarify this problem, the investigation of carrier transport in a magnetic field is used as a key experiment in this work. The notable magnetic field effect for heavy holes in Pb-p-Hg1 − xCdxTe SB investigated is expected only at magnetic fields B > 10 T. However, experimentally more than twofold decrease in the saturation current is observed even at B ∼ 0.5 T. The studies performed on Hg1 − xCdxTe with different Kane's gap, at numerous magnetic field orientation and at various temperatures show that the magnitude of the magnetic field effect is uniquely determined by the ratio of the light hole cyclotron energy to the thermal energy, θ = ℏωclh/kT. However, the experimental dependences of the saturation current do not follow the exponential decay (∝exp ( − θ/2)) predicted by a simple theory. A more sophisticated theoretical model is needed for data interpretation.

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

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.

Transport of bottom waters in the Kane Gap

Transport of bottom waters in the Kane Gap

Rashba Splitting in MIS Structures Based on Narrow Gap Semiconductors: Positive and Negative Gap

The measured and calculated “Rashba polarizations” Δn/n ≃ (0.1 ÷ 0.5) in metal–insulator–semiconductor (MIS) structures based on semiconductors with small Kane gap |E g| (HgCdTe, HgTe, HgMnTe) exceed the ones in the other systems. If we neglect the Rashba splitting, the subband parameters calculated in 8 × 8, 6 × 6, and 2 × 2 Kane models are practically the same, and they are the same for materials with different sign and value of E g in agreement with experimental data for low-doped samples. In contrast, the Rashba splitting is critically sensitive to taking into account both heavy-hole and spin-off bands. Calculated and measured values of Δn/n are higher in materials with E g > 0. The Rashba splitting is essentially nonlinear in the wave vector. This restricts the applicability of the α parameter as the measure of the magnitude of the Rashba effect. The linear-in-k approximations can underestimate α up to several times. The theory underestimates the Rashba splitting by 20–40%. Allowing for electron–electron interaction reduces but does not eliminate the discrepancies. In narrow- and zero-gap semiconductors, the two-dimensional spectrum, including the Rashba splitting and resonant shift, can be adequately described by the simplest treatment based on a WKB approach.

Rashba splitting in MIS structures HgCdTe

The measured parameters of spin-orbit spectral splitting in HgCdTe-based MIS structures with positive and negative Kane gap Eg are compared with the parameters calculated using the three-and four-band Kane model. The disregard of the finite spin-orbit splitting Δ of the valence band in calculations leads to exaggerated values of Rashba splitting (especially for Eg < 0) even for small ratios |Eg|/Δ, although the subband parameters averaged over two spin branches of the spectrum in the two-, three-, and four-band Kane approximations for the same concentrations are practically identical. In the zero-gap HgCdTe, the measured as well as calculated values are noticeably higher, but the four-band approximation leads to values of splitting for both materials which are 20–40% lower than the experimental value. The inclusion of the interband interaction reduces these discrepancies, but does not eliminate them completely. It is shown that the approximations of the 2D spectrum with spin-orbit splitting linear in quasimomentum, which are conventionally used in the analysis, may lower the effective Rashba parameter by a factor of 2–4.

Radiolarian contourites record Eocene AABW circulation in the equatorial East Atlantic

50 m of Middle Eocene pure radiolarian ooze were drilled at ODP Site 660 in the equatorial East Atlantic, 80 km northeast of the Kane Gap. The oozes comprise a 10 m high and 2 km broad mound of seismic reverberations, covered by manganese-rich sediment, and contain trace amounts of sponge spicules and diatoms, negligible organic carbon (0.15%), clay, and variable amounts of pyrite. The yellow to pale brown silty sediments are relatively coarse-grained (30–45% coarser than 6 μm), little bioturbated, and commonly massive or laminated on a cm-scale. The unlithified radiolarian ooze may indicate an interval of high oceanic productivity, probably linked to a palaeoposition of Site 660 close to the equatorial upwelling belt during Middle Eocene time. The absence of organic matter, however, and both the laminated bedding and the mound-like structure of the deposit on the lower slope of a continental rise indicate deposition by relatively intense contour currents of oxygen-rich deep water, which passed through the Kane Gap, winnowed the fine clay fraction, and prevented the preservation of organic carbon. The ooze may be either a contourite-lag deposit, or a contourite accumulation of displaced radiolarians, originating south of the Kane Gap and being deposited in its northern lee, thus documenting the passage of a strong cross-equatorial bottom-water current formed near Antarctica. These Eocene contourites may be an analogue for ancient radiolarites in the Tethyan Ocean.

The manganese nodules of the Kane Gap (East Atlantic). Indicators of sedimentation — erosion changes

Manganese nodules from the Kane Gap (a deep sea channel connecting the Sierra Leone and Gambia Basins off West Africa) were investigated chemically and dated by10Be along cross-sections. Comparing the nodule structure with the stratigraphy of the related sediments, the following conclusions are drawn concerning the sedimentation processes during the last 4 to 6×106 years: The growth of the nodules started about 4 to 4.5 Ma before present during or near to the end of a period of erosion on a fossil-free, probably Miocene sediment. During the first phase of only a few hundred thousandyears, the nodules grew very quickly (7 to >18 mm/Ma). Fe-rich hydrogenetic material formed the internal sections of the nodules during this time. Slowing down of the bottom currents, resulting in deposition of thin sediment covers for short intervals, caused the character of the nodules to change to a more diagenetic composition. The growth rates were reduced to about 1 to 2 mm/Ma. The time of slowing is roughly 3 to 4 Ma BP.

Calibration of unsaturation patterns in long-chain ketone compositions for palaeotemperature assessment

A series of long-chain (C37, C38, C39), primarily di- and tri-unsaturated methyl and ethyl ketones, first identified in sediments from Walvis Ridge off West Africa and from the Black Sea1, has been found in marine sediments throughout the world2. The marine coccolithophorid Emiliania huxleyi and members of the class Prymnesiophyceae are now the recognized sources of these compounds3,4. Experiments with laboratory cultures of algae showed the degree of unsaturation in the ketone series biosynthesized depends on growth temperature2,5, a physiological response observed for classical membrane lipids6. Brassell and co-workers2,7 thus proposed that systematic fluctuations in the unsaturation of these alkenones noted down-core in sediments from the Kane Gap region of the north-east tropical Atlantic Ocean and correlated with glacial-interglacial cycles provide an organic geochemical measure of past sea-surface water temperatures. Using laboratory cultures of E. huxleyi, we have calibrated changes in the unsaturation pattern of the long-chain ketone series versus growth temperature. The calibration curve is linear and accurately predicts unsaturation patterns observed in natural particulate materials collected from oceanic waters of known temperature. We present evidence supporting the proposed palaeotemperature hypothesis2,7 and suggesting absolute 'sea-surface temperatures' for a given oceanic location can be estimated from an analysis of long-chain ketone compositions preserved in glacial and interglacial horizons of deep-sea sediment cores.

Palaeoclimatic signals recognized by chemometric treatment of molecular stratigraphic data

The high-resolution stratigraphy of various marker compounds has been studied, using GC, HPLC and GC-MS, in a 13 m gravity core recovered from the Kane Gap region, eastern equatorial Atlantic, which provides a record of the glacial/interglacial episodes over the last million years. Downhole variations in many presumed source-specific components are observed (e.g. in n-alkanes from terrigenous land plants and dinosterol from dinoflagellates), which may be due to perturbations or cyclicities resulting from climatic change. Fluctuations in the unsaturation of alkenones attributable to variations in water temperatures show correlations with the glacial/interglacial cycles recorded in the δ 18O values for planktonic foraminifera, thereby providing a potential organic geochemical measure of past climates. These molecular abundance data can be linked to the palaeotemperature record, following computer treatments using principal component and spectral analyses. Molecular stratigraphy shows promise as a new chemostratigraphical tool where other means of stratigraphy fail, for example, through calcium carbonate dissolution.

Bottom water flow through the Kane Gap, Sierra Leone Rise, Atlantic Ocean

Continuous seismic profiling, detailed temperature versus depth measurements, heat flow measurements, and a bottom photograph in the Kane Gap on the Sierra Leone Rise in the Atlantic Ocean suggest that a bottom current of cold water flows along the floor of the gap. The layer of cold water is 320 m thick and is marked by a large temperature gradient at the top. The layer is nearly isothermal at 2.28°C, which is 0.12°C colder than the water 500 m off the bottom. A channel exists in flat-lying sediments, which in turn overlay thicker, dipping sediments which exhibit internal evidence for erosion or selective nondeposition. This suggests a three-step depositional history: (1) erosion of a sequence of thick, dipping sediments, (2) deposition of flat-lying sediments, and (3) erosion of small channels in flat-lying sediments. It is not possible to determine conclusively the velocity or direction of flow of the bottom current from our data. The bottom photograph suggests flow to the north. This current probably reflects the effects on eastern basin circulation of the crossing of Antarctic bottom water through the Mid-Atlantic Ridge at either the Romanche fracture zone or the Vema fracture zone.