Deep Western Boundary Undercurrent Research Articles

Distribution of deep-sea benthic foraminifera in the Neogene of Blake Ridge, NW Atlantic Ocean

Abstract. This study describes and illustrates the evolution of deep-sea benthic foraminifera from the Blake Ridge during the late Neogene. In total, 305 species of benthic foraminifera belonging to 107 genera were identified. The Blake Ridge receives fine-grained nannofossil-bearing hemipelagic sediments, transported from the Canadian continental margin by the Deep Western Boundary Undercurrent (DWBUC). We thus presume that changes in benthic foraminifera at Ocean Drilling Program (ODP) sites 991A, 994C, 995A and B and 997A reflect mainly changes in the intensity of the DWBUC, which is closely related to North Atlantic Deep Water (NADW) production. However, the dominance of Uvigerina peregrina, U. proboscidea and Cassidulina carinata during the late Miocene in all the holes suggests an increased influence of Southern Component Waters in the Blake Ridge region. During the early Pliocene (4.8–2.8 Ma) in all the sites benthic faunal assemblages suggest that there was an increased transport of organic-rich sediments by the DWBUC from the Canadian margin to the Blake Ridge, driven by increased production of NADW. During this time the species diversity (Sanders' rarefied values) was low. In the younger interval (since 2.8 Ma), the faunal data suggest less transport of organic-rich sediments to the Blake Ridge, which appears to be related to weakening of the DWBUC during cold intervals. An increase in species diversity at 3 Ma probably resulted from decreased population of bacteria due to low organic matter and/or less competition. In the late Pleistocene (c. 0.6 Ma), Stilostomella lepidula became extinct in all the studied holes, suggesting that this species may have possessed a mode of feeding which no longer existed in the cold, well-oxygenated oceans of the present.

Long‐term observations of transport, eddies, and Rossby waves in the Mozambique Channel

Data from an array of current meter moorings covering a period of two and a half years are used to estimate the varying transport through the Mozambique Channel. The total transport during this period is small (8.6 · 106 m3 s−1 or 8.6 Sv southward). Below 1200 m the transport is weak but a prominent deep western boundary undercurrent with cores at 1700 and 2200 m is found that transports 1.5 Sv to the north. The transport shows a large temporal variability, and neither a continuous upper layer western boundary current nor a continuous deep undercurrent is found. The variability in the upper layer is dominated by a period of 68 days and results mainly from eddies that migrate southward through the Mozambique Channel. In addition to this southward propagation, a westward‐propagating signal is evident from a space‐time diagram of the throughflow. The signal is interpreted as a Mozambique Channel Rossby normal mode. This interpretation is consistent with results from a Principal Oscillation Pattern Analysis (that estimates normal modes from the data) and a quasi‐geostrophic channel model. A detailed inspection of a single “eddy event” shows that a precursor of an anticyclone is a strong southward current along the Madagascar coast that propagates westward to the center of the Channel. During the westward propagation, the current becomes unstable inducing an anticyclone. This scenario connects the westward‐propagating mode with the eddy growth and explains the coincidence of the eddy and Rossby mode frequency. Still, the type of instability that leads to eddy growth could not be determined yet.

On The Meridional Flow Of Source-driven AbyssalCurrents Along A Continental Slope

A hybrid3-layer quasi-geostrophic/planetarygeostrophic(QG/PG) model is introduced to examine the baroclinic evolution and meridional flow of abyssal currents in a wind-driven, stratified and differentially-rotating basin with variable bottom topography.The model resolves mesoscale processes and allows for the formation of a wind driven surface intensified ocean circulation, with a poleward western boundarycurrent,and asourcedrivenequatorwardflowing deep western boundary undercurrent. Baroclinic and barotropic instability within the surface intensified western boundary current, and baroclinic instability between the abyssal current and the overlying wind driven circulation, is resolved. The model allows for finite amplitude variations in the height field of the abyssal current so that groundings in the thickness or isopycnal field associated with the undercurrent are resolved. Downwelling and upwelling between the abyssal water mass and the overlying ocean conserves vertical mass exchange. A southern boundary upwelling scheme is introduced, within the context of a closed basin with no-slip boundary conditions, to balance the northern source of abyssal water thereby allowing the meridional transport of abyssal water to evolve toward a steady state.

Modelling of Pliocene–Pleistocene abyssal mudwaves using synthetic seismograms

The geological significance of seismic reflectors within Pliocene–Pleistocene mudwaves at DSDP Site 603 has been examined using synthetic seismograms. The seismograms generated from physical properties in Hole 603C show good agreement with field seismic profiles. The travel-time to depth conversion permits accurate location of the reflectors in the borehole. Impedance contrasts in the Late Pliocene to Early Pleistocene part of the section are controlled predominantly by density which are related to mean grain size variations. Grain size is inferred to have been controlled by fluctuations in the velocity of the Deep Western Boundary Undercurrent.

Paleogene-Neogene depositional history of the middle U.S. Atlantic continental rise: mixed turbidite and contourite depositional systems

The construction of the middle U.S. Atlantic continental rise from late Paleogene to late Pliocene time included downslope sediment gravity flows and alongslope contourite deposition. Some previous studies have suggested that contour-following bottom currents were the dominant control on depositional patterns. Depositional relief from two large contourite drifts, largely built in late Miocene to late Pliocene time, continue to dominate present-day lower rise morphology. Detailed seismic sequence mapping presented here suggests that significant turbidite (fan) systems also were active and are preserved in the stratigraphic section. A single thick depositional sequence is mapped for Oligocene to middle Miocene sediments deposited when inferred fan systems off the Norfolk-Washington canyons were reworked by bottom currents along the lower rise to form the proto-Hatteras Outer Ridge. Detailed mapping of three late Miocene to late Pliocene depositional sequences indicates that fan-channel systems were active at the same time large contourite drifts were being constructed. Sediment delivered to the lower rise by these elongate turbidite systems undoubtedly was an important local source for construction of the Hatteras Outer Ridge and, through entrainment and resuspension, was an important source for regional contourite deposition controlled by the deep Western Boundary Undercurrent. We suggest that submarine fans and current-controlled sediment drifts may develop simultaneously as companion systems. Types of fan/drift interaction include current-only modification of fans, transitional fan-to-drift, and adjacent or overlapping submarine fan and sediment drift deposition.

The planetary boundary layer of the deep western boundary undercurrent

Current meter observations of the deep western boundary undercurrent of the North Atlantic at 4800 m show a near-bottom turning and speed defect characteristic of a planetary boundary layer.

Observations of a fast burst of the deep western boundary undercurrent and sediment transport in South Wilmington Canyon from DSRVAlvin

On October 3 and 4, 1986, DSRVAlvin dives encountered a strong current at 2,300 m in South Wilmington Canyon. The current, estimated at 1 knot, transported surficial sediment and constructed and modified bedforms. It appears to have been constant in its direction of flow from 30 to 40°. The observed current was probably a burst of fast flow in a region of slow average currents in the Deep Western Boundary Undercurrent. Such episodic events may have a greater influence on the stratigraphic record than the temporally longer more tranquil flow conditions.

Paleoceanography of the Deep Western Boundary Undercurrent on the North American continental margin for the past 25 000 yr

Temporal fluctuations of silt mean particle size and foraminiferal indices were used to examine fluctuations in velocity and position of the axis of the Deep Western Boundary Undercurrent (DWBUC) on the North American continental margin. From 25 to 17 ka the DWBUC axis was 500 m shallower and farther inshore than its present position on the lower rise; current speed was high from 25 to 20 ka and then waned until 17 ka. Between 17 and 7 ka the major axis of the DWBUC shallowed to 4000 m; current speed increased from 17 to 11 ka but slowed from H to 7 ka. Since 7 ka the DWBUC axis has deepened to its modern position at 4900 m. The zone of maximum carbonate dissolution remained below the DWBUC; the most intense dissolution was from 22 to 9 ka and 5 to 2 ka. Highest abundance of Uvigerina and highest percent benthic foraminifera were immediately above the DWBUC from 22 to 7 ka, after which values of both parameters fell to near zero.

Long-range sonar observations of sedimentary facies patterns in and around the Amirante Trench, western Indian Ocean

Long-range side-scan sonographs are used to map two distinct acoustic facies in the Amirante Trench, western Indian Ocean. A structural barrier formed by a seamount within the trench separates the two facies types. The acoustic facies developed S of the seamount is characterized by weak reflectivity returned from sediments deposited by the flow of the Deep Western Boundary Undercurrent through the channels of the Amirante Passage into the southern part of the Amirante Trench. N of the seamount the acoustic facies is characterized by high reflectivity and is confined to the trench axis: it arises from sediments episodically deposited by turbidity currents or debris flows possibly originated by slumping of reef talus from the atolls surmounting the adjacent Amirante Ridge.

Magnetic fabric and remanence analyses of cores from the U.S. continental margin and the Vema Channel

Paleomagnetic analyses have been performed on 8 cc cubic samples from 20 gravity and piston cores (172 samples) taken at the base of the U.S. continental margin in the region of a Deep Western Boundary Undercurrent (DWBUC) and from 28 gravity and piston cores (484 samples) from the eastern flank on the Vema Channel. Downcore remanent magnetic (RM) and magnetic fabric measurements have been performed on these samples to examine the effects of depositional mechanisms on the magnetization of sediment at both sites. Vector diagrams after stepwise a.f. demagnetization to 50 mT for both suites of samples indicate variable RM stability above 8–10 cm depth. Below this depth, samples exhibit a stable, apparently primary RM after demagnetization to fields of 10 mT. These results indicate that if the RM is to be used to orient these deep-sea cores, samples should be chosen from greater than 10 cm depth in core and demagnetized to at least 10 mT. Fabric analyses, based on anisotropy of magnetic susceptibility (AMS) measurements, reveal primary sedimentary fabric in most cores from the U.S. continental margin, but anomalous fabric below 40–80 cm depth in some Vema Channel cores. This anomalous fabric is believed to be the result of soft-sediment deformation at the site. AMS long-axis alignments are essentially random for these Vema Channel cores. Under the DWBUC however, the dominant fabric appears to be normal to the current flow direction while cores from outside the DWBUC exhibit a long-axis AMS alignment which parallels bottom contours. The magnetic fabric parameter F s for core-top samples on the eastern flank of the Vema Channel exhibits high values directly under fast flowing Antarctic Bottom Water (AABW) and relatively low values under more sluggish water masses above AABW. These F s results are consistent with those reported for earlier Vema Channel studies and for results from DWBUC core-top samples.

Deep Western Boundary Undercurrent delineated by sediment texture at base of North American continental rise

A Deep Western Boundary Undercurrent (DWBUC) flows south along the North American continental slope and rise between 1,000 m and 5,000 m with the exact location differing between investigators. The silt-mean particle-size of seafloor samples, however, defines a ∼300-km wide highvelocity zone within the DWBUC below ∼4,500 m. Analysis of silt subfractions shows that the DWBUC high-velocity zone winnows both fine and very fine silt, with preferential deposition of medium and coarse silt. Silt textural analyses indicate that velocity increases with depth in the DWBUC, reaching a maximum at 4,900 to 5,000 m consistent with results of the High Energy Benthic Boundary Layer Experiment (HEBBLE).

Linear 'lower continental rise hills' off Cape Hatteras

A field of continental rise situated on the seaward flank of the Hatteras Outer Ridge was investigated with a grid of bathymetric profiles, a continuous seismic reflection profile, and short (1.2 m) gravity cores. Throughout the field investigated the lower continental rise are not isolated hills but rather are linear waveforms with fairly regular distribution and orientation. The waveform parameters are as follows: trough-to-trough wavelength between 3 and 12 km, trough-to-peak amplitude between 10 and 100 m, crestal lengths of tens of kilometers trending NW-SE, asymmetric with steeper limb facing SW. Core samples recovered from the contain lutite and clay. The NW-SE crestal orientation and morphological similarity of these linear lower continental rise to megaripples or dunes are evidence that their development is related to deposition controlled by the deep Western Boundary Undercurrent, which flows southwest over the continental rise. The waveforms may extend down to reflection horizon A (Late Cretaceous-Eocene). The similarity of the waveforms to megaripples that extend continuously from horizon A to the present ocean bottom in the Argentine Basin implies that the North and South Atlantic have been open and intercommunicating at least since the initiation of the waveforms