Southward Flowing North Atlantic Deep Research Articles

Strato-structural evolution of the deep-water Orange Basin: constraints from 3D reflection seismic data

Abstract. Deep-water fold-and-thrust belt (DWFTB) systems are gravity-driven collapse structures often found in passive margin settings and are comprised of a linked up-dip extensional domain, central transitional/translational domain, and down-dip compressional domain. Many Late Cretaceous DWFTB systems occur along the SW African passive margin with multiple, over-pressurized, seaward-dipping shale detachment surfaces accommodating gravitational slip. In this study we use 3D reflection seismic data to constrain the strato-structural evolution of the translational and compressional domains of a Late Cretaceous DWFTB system and the overlying Cenozoic deposits in the Orange Basin, South Africa. The stratigraphy and structure of the Late Cretaceous DWFTB system is shown to have controlled fundamental sedimentary processes and the stability of the evolving margin. The compressional domain exhibits large-scale landward-dipping DWFTBs with thrust faults detaching the main Turonian shale detachment surface at depth and terminating at the early Campanian surface. A major ∼ 7 km wide seafloor slump scar reflecting margin instability occurs directly above a syncline of the same width from the buried DWFTB system's compressional domain. The translational domain is imaged as a complex region displaying overprinted features of both extensional and compressional tectonics with the downslope translation of sediment comprising listric normal and then thrust and oblique-slip faults distally. Thrust sheets are segmented along strike by extensive oblique-slip faults which extend from the translational domain into the down-dip compressional domain. Smaller, localized fold-and-thrust belts are found directly below the kilometre-scale DWFTB system in the down-dip compressional domain detaching a lower, Albian shale detachment surface which corresponds to an older gravitational collapse. The upward propagation of normal and oblique-slip faults with progressive sedimentation is hindered by the Oligocene or Miocene stratigraphic markers corresponding to mass erosional processes in the Cenozoic. A large (∼ 2.3 km wide), roughly slope-perpendicular Oligocene submarine canyon formed by turbidity currents is attributed to a major sea-level fall at ∼ 30 Ma. Oceanographic circulation is shown to have held a significant control on the deposition of mid-Miocene to present-day sedimentary sequences. Between 1200 to 1500 m water depths along the upper continental slope well-preserved extensive slope-parallel, sinusoidal channel-like features occur on the Miocene stratigraphic marker. The channels are confined within a ∼ 14 km wide zone at the interface of the upper northward-flowing Antarctic Intermediate Water (AAIW) and deeper southward-flowing North Atlantic Deep Water (NADW) currents. The erosive interaction of these oppositely flowing bottom currents combined with the effects of the Benguela Upwelling System (BUS), all of which formed or intensified at ∼ 11 Ma, are responsible for the creation and preservation of the extensive slope-parallel channels. This study shows the difference in structural styles of the translational and compressional domains of a Late Cretaceous DWFTB system and the processes responsible for mass-scale erosion in the Cenozoic.

Late Pleistocene sediment accumulation in the lower slope off the Rio Grande terrace, southern Brazilian Continental Margin

Here are described the analyses of sediments from the lower slope of the Pelotas Basin in the southern Brazilian continental margin, southwestern Atlantic. The samples were obtained from the core SIS-249, collected adjacent to the Rio Grande Terrace (RGT) at a water depth of 2091 m, an area poorly known in terms of sedimentary characteristics compared to the oil and gas-rich basins to the north. The age model indicates that the sampled sediments were accumulated during the last glacial period, between 112.5 and 34.47 ka (MIS 5 to MIS 3). The fine (<63 μm) fraction is dominated by silt with subordinate clay, whereas the coarse (sandy) fraction is chiefly composed of biogenic carbonate (tests of foraminifers) with subordinate very fine (63–125 μm) lithogenic sand. The composition of the lithogenic sand is essentially uniform downcore, dominated by quartz with glaucony as the dominant accessory mineral, and the presence of augite, derived from Mesozoic volcanic rocks, points to a source from the adjacent coastal plain. The uniform mineral composition indicates continuous input of sediments removed from the RGT by action of the Brazil Current (BC) and transferred to the lower slope by gravity flows. The higher proportion of lithogenic sand from the upper RGT along the intervals corresponding to MIS 5 and MIS 3 points to erosion of the terrace due to a more landward position of the BC, related to sea-level highstand and falling stage tracts. The higher amount of silt along the MIS 4 interval suggests increased transfer of sediments from the flanks of the RGT as the result of a more downslope position of the BC driven by the sea-level lowstand. Winnowing of the sortable silt fraction during MIS 5 and 3 points to influence of the southward-flowing North Atlantic Deep Water (NADW). The higher estimated accumulation rates and less negative peaks of δ13C in benthic foraminifers Uvigerina peregrina during MIS 4 could have been caused by oscillations of water masses with distinct isotopic compositions, but also by increased burial and decomposition of organic matter as a result of high ocean surface primary productivity, indicated by the correspondence between the δ13C oscillations and higher accumulation rates and proportion of biogenic sand in the core. Higher productivity was apparently driven by episodes of increased eolian transport of dust from Patagonia and central-western Argentina in milenial timescales, coeval with oscillations in strength and position of the southern westerly winds (SWW) system.

Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

Abstract. Dissolved manganese (Mn) is a biologically essential element. Moreover, its oxidised form is involved in removing itself and several other trace elements from ocean waters. Here we report the longest thus far (17 500 km length) full-depth ocean section of dissolved Mn in the west Atlantic Ocean, comprising 1320 data values of high accuracy. This is the GA02 transect that is part of the GEOTRACES programme, which aims to understand trace element distributions. The goal of this study is to combine these new observations with new, state-of-the-art, modelling to give a first assessment of the main sources and redistribution of Mn throughout the ocean. To this end, we simulate the distribution of dissolved Mn using a global-scale circulation model. This first model includes simple parameterisations to account for the sources, processes and sinks of Mn in the ocean. Oxidation and (photo)reduction, aggregation and settling, as well as biological uptake and remineralisation by plankton are included in the model. Our model provides, together with the observations, the following insights: – The high surface concentrations of manganese are caused by the combination of photoreduction and sources contributing to the upper ocean. The most important sources are sediments, dust, and, more locally, rivers. – Observations and model simulations suggest that surface Mn in the Atlantic Ocean moves downwards into the southward-flowing North Atlantic Deep Water (NADW), but because of strong removal rates there is no elevated concentration of Mn visible any more in the NADW south of 40° N. – The model predicts lower dissolved Mn in surface waters of the Pacific Ocean than the observed concentrations. The intense oxygen minimum zone (OMZ) in subsurface waters is deemed to be a major source of dissolved Mn also mixing upwards into surface waters, but the OMZ is not well represented by the model. Improved high-resolution simulation of the OMZ may solve this problem. – There is a mainly homogeneous background concentration of dissolved Mn of about 0.10–0.15 nM throughout most of the deep ocean. The model reproduces this by means of a threshold on particulate manganese oxides of 25 pM, suggesting that a minimal concentration of particulate Mn is needed before aggregation and removal become efficient. – The observed distinct hydrothermal signals are produced by assuming both a strong source and a strong removal of Mn near hydrothermal vents.

Nitrate isotope distributions on the US GEOTRACES North Atlantic cross-basin section: Signals of polar nitrate sources and low latitude nitrogen cycling

We report and interpret the nitrate δ15N and δ18O distributions in the cross-basin transect of the US GEOTRACES North Atlantic expedition. Low nitrate δ15N observed in the shallow thermocline of the central western portion of the basin (reaching as low as 2.5‰ vs. air; ~2.3‰ lower than deep Atlantic nitrate) reflects the remineralization of nitrogen with a low δ15N, presumably mostly from nitrogen fixation. A strong correlation between the δ15N and concentration of nitrate argues that the focusing of the low δ15N signal in the upper thermocline of the North Atlantic subtropical gyre is at least partly due to the low concentration of nitrate in those waters, such that the east-to-west δ15N decrease in shallow thermocline nitrate does not require an east-to-west increase in nitrogen fixation rate. In underlying intermediate-depth water, which enters the section largely from the South, the nitrate δ15N is ~1‰ higher than in deep water while the δ18O is similar to that in deep water. The δ15N elevation derives from the partial assimilation of nitrate in Southern Ocean surface waters prior to Antarctic Intermediate and Subantarctic Mode Water formation. The δ18O elevation of preformed nitrate in southern-sourced intermediate waters has been diluted by the addition of nitrate from regeneration/nitrification in the low latitudes, which does not similarly weaken the δ15N elevation. In deep water, the nitrate δ15N in samples with a high relative contribution of North Atlantic Deep Water is almost indistinguishable from that of samples with a higher contribution of Antarctic Bottom Water (4.8–4.9‰ vs. air). Thus, the high nitrate δ15N in northward flowing intermediate waters is almost entirely erased by the input of low-δ15N newly fixed nitrogen to the Atlantic (including that from the Mediterranean) prior to the incorporation of those intermediate depth waters into southward flowing North Atlantic Deep Water. The data appear consistent with a nitrogen isotopic balance between northward and southward nitrate transports across the GEOTRACES section, suggesting that most Atlantic nitrogen fixation occurs south of the section.

Meridional overturning circulation in the South Atlantic at the last glacial maximum

The geostrophic shear associated with the meridional overturning circulation is reflected in the difference in density between the eastern and western margins of the ocean basin. Here we examine how the density difference across 30°S in the upper 2 km of the Atlantic Ocean (and thus the magnitude of the shear associated with the overturning circulation) has changed between the last glacial maximum and the present. We use oxygen isotope measurements on benthic foraminifera to reconstruct density. Today, the density in upper and intermediate waters along the eastern margin in the South Atlantic is greater than along the western margin, reflecting the vertical shear associated with the northward flow of surface and intermediate waters and the southward flowing North Atlantic Deep Waters below. The greater density along the eastern margin is reflected in the higher δ18O values for surface sediment benthic foraminifera than those found on the western margin for the upper 2 km. For the last glacial maximum the available data indicate that the eastern margin foraminifera had similar δ18O to those on the western margin between 1 and 2 km and that the gradient was reversed relative to today with the higher δ18O values in the western margin benthic foraminifera above 1 km. If this reversal in benthic foraminifera δ18O gradient reflects a reversal in seawater density gradient, these data are not consistent with a vigorous but shallower overturning cell in which surface waters entering the Atlantic basin are balanced by the southward export of Glacial North Atlantic Intermediate Water.

Deep-marine seabed erosion and gravel lags in the northwestern Rockall Trough, North Atlantic Ocean

A zone of active seabed erosion has been identified in the northwestern Rockall Trough using seismic reflection profiles and cores. The region from George Bligh Bank to Rockall Bank has been subject to vigorous bottom-current activity, for at least the last 35 Ma. Bottom currents originate from southward flowing North Atlantic Deep Water, in water depths of 500–2000 m. Acoustic character mapping reveals a zone of erosion extending over 8500 km 2 along the northwestern margins of the trough. The erosion surface is characterized by an acoustically reflective seabed, with occasional parallel to transparent reflectors. These are the result of strong bottom-currents exposing the underlying volcanic basement, drift sequences and fan sediments. The erosion surface is covered by a &lt;10 m veneer of Quaternary sediment. The majority of the basin consists of well-stratified, parallel Quaternary drift and hemipelagite sequences. Along the western margin of the trough, these sediments form broad-sheeted drifts. Eocene sediments adjacent to George Bligh Bank have been exposed by strong bottom-currents for the last 35 Ma, compared with the flanks of Rockall Bank, where sedimentation was intermittent. Core sampling from George Bligh Bank and Rockall Bank recovered Quaternary gravelly–sandy muds interpreted as gravel lags, and muddy sandy contourites, overlying early–mid-Eocene aged sediments. The gravels represent the influence of a strongest North Atlantic Deep Water flow, winnowing coarse sediment into ‘lags’, commonly preserved within sandy contourite sequences.

The Meridional Oceanic Transports of Heat and Nutrients in the South Atlantic

Meridional transports of mass, heat, nutrients, and carbon across coast-to-coast WOCE and pre-WOCE sections between 11°S and 45°S in the South Atlantic are calculated using an inverse model. Usually salt preservation is used as a condition in the inverse model, and only in the case of heat transport the condition of zero total mass transport is taken instead. Other constraints include silica conservation, prescribed southward fluxes of salt and phosphate, and transports in the southward Brazil Current and in the northward Antarctic Bottom Water flow obtained from WOCE moored current meter arrays. The constraints set the underdetermined system of linear equations of the inverse model whose solutions depend on weights, scales, and matrix ranks. The discussion emphasizes the sensitivity of the fluxes to changes in the model input. The transports given in the following are obtained as the means of “reasonable” solutions at 30°S. The error numbers in parentheses include uncertainties due to wind stress and temporal variability, the numbers without parentheses do not contain these terms:0.53 ± 0.03 (0.09) Tg s−1 mass to the south, 0.29 ± 0.05 (0.24) PW heat to the north, 15 ± 120 (500) kmol s−1 oxygen to the south, 121 ± 22 (75) kmol s−1 nitrate to the south, 64 ± 110 (300) silica to the north, and 1997 ± 215 (600) kmol s−1 dissolved inorganic carbon to the south. The above errors in transports are obviously dominated by uncertainties in wind stress and temporal variability. The divergence in meridional heat and mass transport is consistent with integral surface flux changes between corresponding zonal bands. The mass compensation of southward flowing North Atlantic Deep Water occurs to a greater extent in the warm surface waters than in the Antarctic Intermediate Water below. If one follows the arguments of earlier authors on the relation between meridional fluxes and the significance of the two possible pathways for the global thermohaline circulation, the warm water path south of Africa seems to be somewhat more important than the cold water path through Drake Passage.

Anomalously low alkenone temperatures caused by lateral particle and sediment transport in the Malvinas Current region, western Argentine Basin

We analysed the alkenone unsaturation ratio ( U K′ 37) in 87 surface sediment samples from the western South Atlantic (5°N–50°S) in order to evaluate its applicability as a paleotemperature tool for this part of the ocean. The measured U K′ 37 ratios were converted into temperature using the global core-top calibration of Müller et al. (1998) and compared with annual mean atlas sea-surface temperatures (SSTs) of overlying surface waters. The results reveal a close correspondence (<1.5°C) between atlas and alkenone temperatures for the Western Tropical Atlantic and the Brazil Current region north of 32°S, but deviating low alkenone temperatures by −2° to −6°C are found in the regions of the Brazil–Malvinas Confluence (35–39°S) and the Malvinas Current (41–48°S). From the oceanographic evidence these low U K′ 37 values cannot be explained by preferential alkenone production below the mixed layer or during the cold season. Higher nutrient availability and algal growth rates are also unlikely causes. Instead, our results imply that lateral displacement of suspended particles and sediments, caused by strong surface and bottom currents, benthic storms, and downslope processes is responsible for the deviating U K′ 37 temperatures. In this way, particles and sediments carrying a cold water U K′ 37 signal of coastal or southern origin are transported northward and offshore into areas with warmer surface waters. In the northern Argentine Basin the depth between displaced and unaffected sediments appears to coincide with the boundary between the northward flowing Lower Circumpolar Deep Water (LCDW) and the southward flowing North Atlantic Deep Water (NADW) at about 4000 m.

A Model of early to middle Miocene Deep Ocean circulation for the Atlantic and Indian Oceans

Abstract Tethyan Outflow Water (TOW), characterized by high benthic δ 13 C values, was a component of the early Miocene deep water mass of the Atlantic ( c. 20 and c. 13 Ma) and Indian ( c. 20 and c. 14.5 Ma) Oceans. Fluctuations in temperature, salinity and quantity of dissolved oxygen coincided with high abundances of infaunal, smooth-walled bolivinids within the Atlantic and Indian Ocean TOW between 19.5 and 16.5 Ma. High δO values recorded in the Atlantic water mass may have originated, in part, from evaporation and the formation of warm, saline water within the Tethys. The southward flowing Atlantic TOW at depths of 2–4.5 km enhanced meridional heat transport. It was overlain by Northern Component Water (NCW) with a lower δC signal. TOW and NCW were replaced by southward flowing North Atlantic Deep Water at c. 13 Ma when cold dense Norwegian—Greenland Sea water flowed across the Greenland—Scotland Ridge. Indian Ocean TOW was restricted laterally by the Chagos—Lacadive Ridge, and vertically (1.3 to 3 km) by overlying intermediate water, characterized by the occurrence of coarse-walled bolivinids, and underlying deep water characterized by high abundances (&gt;10%) of Nuttallides umboniferus . The coarse-walled bolivinid and ‘ N. umboniferus ’ faunas were associated with lower temperature, salinity and higher oxygen concentrations. Tethyan outflow to the northwest Indian Ocean ended at c. 14.5 Ma, probably in response to the closure of a gateway, and resulted in a two-component deep water mass structure. The coeval increase in the relative abundances of N. umboniferus at Indian Ocean Sites 237 ( c. 2 km) and 710 ( c. 3.5 km), at c. 13 Ma, probably records the impact of this change on the nature of Indian Ocean deep water at depths between 2 and 4 km. The termination of the Atlantic and, to a lesser extent, Indian Ocean TOW contributed to global cooling and to the expansion of the Antarctic Ice Sheet by limiting the meridional heat transport.

Sr-Nd isotopes as tracers of fine-grained detrital sediments: the South-Barbados accretionary prism during the last 150 kyr

We studied a core retrieved from the deformation area of the South-Barbados accretionary prism. Sr and Nd isotopic ratios were used in conjunction with the clay mineral assemblage to determine the origin and fluxes of fined-grained detrital material and the sedimentological processes in this area during the last 150,000 years. During the high sea-level stands of events 1.1, 3.3 and stage 5, sedimentation was mainly fed by Amazon material conveyed by the Guiana Surface Current and by volcanogenic material transported by the southward flowing North Atlantic Deep Water (NADW); the flux of clays were very low (< 0.2 g cm −2 10 −3 yr), except during stage 5 (0.7 g cm −2 10 −3 yr). During the low sea-level stands of events 2.2, 4.2, 5.2, 5.4 and 6.4 and stage 5, the appearance of Orinoco material (1.5 g cm −2 10 −3 yr at event 2.2), primarily derived from granitic batholiths of the Guiana Shield and transported directly by turbidity currents, indicate an important change in sedimentary processes. The high flux of volcanogenic materials (0.9 g cm −2 10 −3 yr at event 2.2) indicates the importance of air-fall ashes from Dominica, Guadeloupe and/or Martinique islands and of the NADW as the transport agent. The importance of allochthonous fine-grained material originating from the South-American continent (Andes or Guiana Shield) and the northern part of the Lesser Antilles arc clearly stresses the major role that climate and eustacy had on detrital sedimentation during the last climatic cycle.

On the North Atlantic Circulation

A new, speculative, and, we hope, provocative summary of the North Atlantic circulation is described, including both horizontal currents (wind‐driven) and the primarily (thermohaline) meridional flows that involve the transformation of warm to cold water at high latitudes. Our picture is based on a synthesis of a variety of independent investigations that are contained in the literature as opposed to a presentation of the results of one technique or the point of view of one author. We describe a thermohaline cell (the so‐called thermohaline conveyor belt) that is concentrated within the Atlantic and Southern oceans (rather than essentially global), with the most important upwelling sites being in the circumpolar and the equatorial current regimes. We concentrate on deep water formation and its replacement relative to intermediate‐water formation. It has been pointed out recently that the formation of 13 Sv (1 Sv = 106m³ s−1) of southward flowing North Atlantic Deep Water is compensated for in the upper ocean by northward cross‐equatorial transport. We suggest that this thermocline layer flow passes through the Straits of Florida, transits the Gulf Stream system on its inshore side, and exits through the North Atlantic Current system after recirculation and modification. There is now a clear observational basis for the structure of recirculating gyres on the southern and northern sides of the Gulf Stream. We suggest a recirculation for the North Atlantic Current as well. We also describe a C‐shaped component to the southern Gulf Stream recirculation and identify a roughly 10‐Sv circulation in the eastern North Atlantic associated with the Azores Current. Recirculations play an important role in deep boundary current regimes and in water mass formation and modification. The transport of the deep western and northern boundary currents in the North Atlantic Ocean may be boosted (roughly doubled or tripled) by counterclockwise recirculating gyres and by additions of modified bottom or intermediate water. While the North Atlantic is the most completely observed ocean, there are still significant gaps in our knowledge of its circulation.

A comparison of atmospheric inputs and deep‐ocean particle fluxes for the Sargasso Sea

Trace metal distributions in ocean waters are dominated by internal recycling processes. However, fluxes out of the water column must balance inputs if steady state is to be maintained. We use deep‐sea particle fluxes at 3200 m in the Sargasso Sea as measured by sediment traps to define the flux to the sediments beneath the Sargasso Sea and compare this flux to the atmospheric input for a group of trace metals. Despite the uncertainties associated with the comparison, several important features are revealed. For elements predominantly associated with crustal particles (Al, Mn, Fe, and V), lateral advection provides a significant part of the deep‐sea particle flux. For Cd, Cu, Ni, Pb, and Zn this lateral component is minor, and atmospheric inputs to the Sargasso Sea exceed deep‐sea particle fluxes. These excesses appear to be consistent with calculations of the export of these metals from the Sargasso Sea arising from ocean boundary scavenging (Pb) and release from decomposing organic matter in the southward flowing North Atlantic Deep Water (Cd, Cu, Ni, and Zn). In the atmospheric deposition itself, wet deposition exceeds dry deposition for Cd, Cu, Mn, Ni, Pb, and Zn, while wet and dry deposition are of similar magnitudes for V, Fe, and Al.

Major Change in Atlantic Deep and Bottom Waters 700,000 yr Ago: Benthonic Foraminiferal Evidence from the South Atlantic

AbstractIn the modern South Atlantic the transition between deep water and bottom water is marked by a clear change in the associated benthonic foraminiferal fauna. Uvigerina and Globocassidulina characterize oxygen-poor Circumpolar Deep Water which has long been isolated from the surface. Planulina and miliolids are found associated with the more newly formed, oxygen-rich North Atlantic Deep Water. Antarctic Bottom Water is characterized by “Epistominella” umbonifera. Analysis of the benthonic foraminiferal faunas in two sediment cores recovered from the Vema and Hunter Channels in the western South Atlantic shows that the level of the transition between deep and bottom waters shallowed sharply about 700,000 yr ago. This rise indicates a sharp, sustained increase in the volume of bottom water flowing through the South Atlantic after this time. Prior to about 700,000 yr ago, the amount of Antarctic Bottom Water entering the western South Atlantic was greatly reduced and Circumpolar Deep Water apparently accounted for the bulk of northward flow. Production of southward-flowing North Atlantic Deep Water seems not to have been affected. The timing of this change in circulation regime suggests a possible causal link to similar changes in records of terrestrial and sea-surface climate.

Sedimentation processes on the continental rise of northeastern South America

A section of the continental rise of northeastern South America northeast of the Orinoco delta contains physiographic features built by the interaction of southward-flowing North Atlantic Deep Water and turbidity currents generated in the Orinoco region during the last Pleistocene glacials. A sedimentary outer ridge of low relief (Demerara Outer Ridge) trends northeast along the rise and a field of westward-migrating sediment waves trending north-northwest is superimposed on the outer ridge. The sediment waves have a maximum amplitude and wavelength of 20 m and 4 km, respectively. Seismic profiler records indicate that the outer ridge was probably built during the Pleistocene. A major turbidity-current pathway adjacent to the outer ridge on the north supplied sediment to the southward-flowing North Atlantic Deep Water which then deposited this sediment down-stream on the outer ridge and formed the sediment waves. Piston cores from the outer ridge contain numerous silt—sand beds and appear to be contourites. The cores consist primarily of gray hemipelagic clay of a Late Wisconsin age and have high ( > 10 cm 1000 yrs ) sedimentation rates. In contrast, cores from the continental rise north of the turbidite channel are brown clays with relatively low sedimentation rates ( 3.0 cm 1000 yrs ) and do not contain silt—sand contourites.

Antarctic bottom water fluctuations in the Vema Channel: Effects of velocity changes on particle alignment and size

The anisotropy of magnetic susceptibility (AMS) and mean particle size in the silt fraction have been measured in surface sediments and down selected cores from the Vema Channel. Results show that a function ( F s ) which represents magnetic grain long axis alignment is highly correlated with variation in mean size of the silt fraction. An increase in alignment is caused by an increase in bottom water velocity which corresponds to an increase in mean particle size. Best alignments and coarsest particle sizes are found in the axis of the Vema Channel. In contrast, poor particle alignment and fine silt mean characteristics the shallower region east of the channel axis, consistent with lower current velocities. Core CH 115-61 was collected near the Level of Least Motion at the transition between northward-flowing Antarctic Bottom Water (AABW) and southward-flowing North Atlantic Deep Water. A positive relationship exists between silt size and AMS in the core. Fluctuations in particle alignment and mean silt size indicate three or four periods of increased velocity during the past 150,000 years. Geographic orientation of the maximum AMS axes in cores CH 115-60 and -61 was accomplished using the declination of remanent magnetism and an axial dipole as the reference azimuth. The data show that the larger magnetic grains in these sediments are aligned normal to the bathymetric trend of the channel which suggests that these grains are aligned by traction transport. The result indicates that AABW in the Vema Channel has flowed along an azimuth of approximately N30°E for the past 150,000 years.