Iceland-Scotland Overflow Water Research Articles

230Th in the eastern North Atlantic: the importance of water mass ventilation in the balance of 230Th

The 230Th distribution in the water column on a north-south transect in the eastern North Atlantic was measured using thermal ionization mass spectrometry (TIMS). Water samples at three locations (L1, 33°N 21.6°W; L2, 47.4°N 19.5W; and L3, 54.4°N 21.1°W) were filtered and the dissolved and particulate fractions were analysed for their 230Th and 232Th contents. In the upper water column (< 1000 m water depth), the total 230Th ex distribution at the three locations could be explained by a reversible scavenging model. In the deep water column, however, 230Th ex concentrations decrease from south to north with relatively low and constant concentrations at stations L2 and L3. The major water mass below 2000 m is North East Atlantic Deep Water (NEADW), filling the deep water column at L1. The water column at L2 and L3 is influenced by Labrador Sea Water (LSW) down to 2000 m, whereas at L3, the deep water is strongly influenced by Iceland-Scotland Overflow Water (ISOW). The anomalous distribution of 230Th ex in the deep water column of stations L2 and L3 could be described after a ventilation term was added to the equations of the reversible scavenging model. For L3, a ventilation time (3–12 years) was found comparable to results derived from other tracers. However, the ventilation of the deep water (>2000 m) at L2 suggested by our model (8–25 years) is probably too rapid in comparison with previous findings based on chlorofluorocarbon data. Enhanced scavenging of 230Th would be an alternative explanation for the low concentration in NEADW at L2. With the scavenging-mixing model, we calculated an advective export flux of 230Th of up to 50% of the production in the water column in the eastern North Atlantic. Advective loss of 230Th has to be taken into account when using the 230Th flux into sediments for paleoceanographic studies in ocean basins where deep water residence time is comparable to the 230Th scavenging residence time.

Changes in the strength of the Iceland–Scotland Overflow Water in the last 200,000 years: Evidence from magnetic anisotropy analysis of core SU90-33

Results from a high resolution study of magnetic anisotropy combined with mineral magnetic analysis, clay analysis and oxygen isotopes stratigraphy are reported for core SU90-33 located (60°34'4N, 22°05'1W) at 2400 m water depth along the Iceland–Scotland overflow water (ISOW), a branch of the North Atlantic Deep Water (NADW). This core covers the last 6 climatic stages. The main magnetic mineral is low Ti-content magnetite with very slight changes in the grain size. The susceptibility record, corrected for the carbonate content, indicates down-core variations in the amount of magnetite which largely co-vary with the changes in the proportion of smectite in the clay fraction. Both parameters are climatically controlled, with lower values during glacial than during interglacial periods. The measurements of the anisotropy of magnetic susceptibility document down-core changes in the degree of anisotropy, with significantly higher values during interglacial periods than during glacial times. These changes appear to be related to differences in the degree of alignment of the magnetic particles and not to changes in grain shape, providing evidence that they arise from changes in strength of the bottom water circulation. Therefore, the strength of the contour current associated to the transport of the ISOW appears to have been significantly larger during climatic stages 5, 3 and 1 than during stages 6, 4 and 2.

Evidence for topography- and current-controlled deposition on the Reykianes Ridge between 59°N and 60°N

Sediment patterns derived from sediment sampling and acoustic subbottom profiling were mapped on the Reykjanes Ridge (North Atlantic) between 59°N and 60°N. Five discrete sediment echo patterns were distinguished and mapped on a regional scale. The prolonged and layered echo facies, which mainly reflect sediment filled basins on the ridge flanks, indicate deposition of predominantly fine-grained sediments deposited by the Iceland-Scotland Overflow Water. A combination of westward flowing currents spilling over the ridge crest due to the Coriolis force together with the existing morphology probably caused the N-S trending facies distribution pattern on the northwest flank. Furthermore, the modern surface sediment distribution is controlled by biological productivity, which is closely related to the mixing zone of cold subpolar surface water masses and the warm North Atlantic Current, and bottom water transport processes. The effect of bottom current transport is reflected in the pattern of settling velocity and sediment grain size. The clay mineral composition indicates that most of the fine-grained material is supplied predominantly from the Icelandic province by the Iceland-Scotland Overflow Water. Erosional processes are concentrated on narrow zones on top of the axial ridges and on the steep flanks of the Catalonia Seamount. Well-sorted foraminiferal sands on these exposed regions are assumed to represent residual sediments.

Late Quaternary sedimentation on the Mid-Atlantic Reykjanes Ridge: clay mineral assemblages and depositional environment

Sediment samples from the Mid-Atlantic Reykjanes Ridge (59°N) were taken to get information about sediment genesis and to identify different sources during the late Quaternary. Samples were investigated by X-ray diffraction and grain-size analyses. The clay mineral assemblages in sediments of the Reykjanes Ridge reflect paleoceanographic changes during the late Quaternary. Holocene sediments are characterized by high contents of smectite, mainly of less developed crystallinity. In the spatial distribution of clay minerals high smectite concentrations on the eastern flank and slightly decreasing concentrations on the western flank of the Reykjanes Ridge indicate the action of bottom-water transport. The smectite originates mainly from the volcanogenous Icelandic shelf and reflects the influence of Iceland-Scotland Overflow Water (ISOW). Stratigraphic variability in the clay mineral composition reflects predominantly the influence of different sources, resulting from oceanographic and glacial transport processes. During glacial time sediment transport is due mainly to input by icebergs. Increasing amounts of illite, chlorite, and kaolinite characterize ice-rafted sediments of the “Heinrich layers”. In these sediments smectite crystallinity is well developed. In contrast, several other ice-rafted layers contain smectite with low crystallographic order, similar to smectites of Holocene age. The icelandic source was proved by distinct amounts of basaltic glass in the coarse-grained sediment. At approximately 55 ka increasing amounts of chlorite and kaolinite suggest an enhanced influx of warm North Atlantic surface waters. This hypothesis is supported by a high carbonate shell production at this time. Relative low concentrations and the well-developed crystallinity of smectite minerals characterize the Last Glacial Maximum (LGM; 18–16 ka), indicating a reduced supply of fine icelandic material. Shortly after the LGM, at the beginning of termination IA, a distinct increase in fine-grained quartz (<2µm) and smectite are visible, which are proposed to reflect a supply of fine-grained ice-rafted material. At 13 ka linear increasing smectite concentrations of lower crystallographic order indicate increasing supply of fine-grained material from Iceland, linked to reinitiation of bottom currents of the ISOW. Full reinitiation is indicated at around 10 ka, where a strong increase in smectite of low crystallographic order is detected.

On the circulation of the northeastern North Atlantic

Along a section from Cape Farvel at the southern tip of Greenland to the Porcupine Bank off the Irish coast (WOCE section A1E/AR7E), obtained in September 1991, geostrophic velocities were calculated from CTD measurements and referenced to velocities recorded by an Acoustic Doppler Current Profiler (ADCP) in the upper 500 m. The mean accuracy of the absolute geostrophic velocities is estimated at±3.6 cm/s, which is comparable to that resulting from the assumption of a layer of no motion. The derived flow field is essentially columnar without a pronounced layer of no motion. Maximum velocities of 20–30 cm/s occur in the upper 1000 m of the East Greenland Current and the Porcupine slope current and at 2000 m depth in the Denmark Strait overflow. The meandering of the Irminger Current crossing the Reykjanes Ridge and an enhanced mesoscale variability along the Rockall Plateau and in the Rockall Trough are indicated. Compared to Schmitz and McCartney (1993) the estimated volume transports suggest an intensified meridional circulation with an increased supply from the subtropics to the Subpolar Mode Water (SPMW) and an increased entrainment of SPMW into the deeper layers. In the bottom layer with densities σ 0 ⩾ 27.80 kg/m 3 a reduced circulation of Iceland-Scotland Overflow Water (ISOW) in the Irminger Basin and a recirculation of ISOW in the Iceland and West European basins are found. About 12 Sv of bottom water are transported southwards and join the Deep Western Boundary Current.

Hydrographic Variability in the Bottom Layer of the Iceland Basin

Abstract The variability of the lowest 400 dbar in the Deep Iceland Basin has been studied for the years 1988–1991. It appears that hydrographic parameters of the Iceland–Scotland Overflow Water in the lowest 100 dbar mainly vary on longer timescales (interannual). Centered around 140 dbar from the bottom, at a more or less constant density level, salinity, dissolved silica, oxygen, and potential vorticity show a maximum in variability. This is attributed to the isopycnal advection of lenses of water with distinct, coherent hydrographic properties. These lenses are assumed to form a mechanism for lateral isopycnal advection of deep water, enriched with either Lower Deep Water or Subpolar Mode Water. The vertical extent of the lenses is determined by a density window between Labrador Sea Water and Iceland–Scotland Overflow Water, centered around σ2.5 = 39.27 kg m−3. The lateral, isopycnal advection, followed by subsequent diapycnal mixing, is proposed as the mechanism to incorporate Lower Deep Water into t...

Mean currents and current variability in the iceland basin

Long-term (>6 months) current measurements from five moorings in the Iceland Basin have been analysed for the mean currents and the structure of the variable current components. The time-averaged flow at all five moorings had a strong baroclinic character. The mean circulation in the upper layers with relatively warm Sub-Polar Mode Water appears to have a general north-eastward direction with maximum mean velocities of 6 to 7 cm·s −1. In the bottom layer south of Iceland, where the cold Iceland-Scotland Overflow Water flows westwards along the topography in a Deep Northern Boundary Current, mean velocities of the order of 10 to 20 cm·s −1 have been observed. Over the deep slope of the Hatton Bank, water enters the Iceland Basin in a branch of the Deep Northern Boundary Current which has a cyclonic rotation sense in the Iceland Basin. The variable part of the current has been analysed by means of principal-component analysis. The current variations in the central Iceland Basin appear to have a mainly barotropic character while variations in the baroclinic flow of Iceland-Scotland Overflow Water contributed 10% or less to the total energy of the variable deep flow. Over the slope of the Hatton Bank the variable currents had a mainly baroclinic character with shear in both current speed and direction. Comparison of the geostrophic velocity with the mean Eulerian velocity has revealed that the σ θ =27.725 kg·m −3 surface can be used adequately as level of non-motion for the geostrophic modelling of the flow along the Iceland and Hatton slopes. The mean westward geostrophic transport of ISOW south of Iceland relative to this reference surface amounted to 3.5 Sv, in agreement with existing independent estimates.

On the synoptic hydrography of intermediate and deep water masses in the Iceland Basin

The hydrography of intermediate deep water masses in the Iceland Basin is studied from quasi-synoptic surveys carried out in 1990 and 1991. The general water mass structure was identical for both years. The interaction and mixing of the different water types present in the basin is reviewed by means of property-property plots, vertical tracer sections and isopycnal analyses. It appears that overflow waters from the Norwegian Sea are modified in successive stages during their descent into the deep Iceland Basin. They mix with Sub-Polar Mode Water at short distances from the sills in the Faroe Bank Channel and on the Iceland-Faroe Ridge, thereby forming Iceland-Scotland Overflow Water. This water type entrains Labrador Sea water during the descent into the deep Iceland Basin, where Iceland-Scotland Overflow Water is further modified mainly by diapycnal mixing with overlying Lower Deep Water, which contains a large fraction of Antarctic Bottom Water. At intermediate levels Labrador Sea Water and Intermediate Water appear to mix laterally with a slope water mass flowing along the Icelandic and Reykjanes slopes. This slope water is formed by the direct mixing of Iceland-Scotland Overflow Water with Sub-Polar Mode Water and differs from the water mass, encountered in the central Iceland Basin. The intermediate and deep circulation in the Iceland Basin has a cyclonic character with smaller-scale variations due to topographic steering along ridges on the Icelandic slope.

The Deep Western Boundary Current: tracers and velocities

In the Deep Western Boundary Current (DWBC) mean velocities obtained by the F11/F12 dating method are far smaller (1–2 cm s −1) than direct velocity measurements (5–20 cm s −1). To resolve this discrepancy, a simple box model is presented that uses the ideas of Pickart et al. (1989, Physical Oceanography, 19, 940–951) to parametrize turbulent diffusion of the current with its surroundings. In contrast to previous models, however, the boundary conditions include all water masses forming the lower part of the DWBC (Denmark Strait Overflow Water, Iceland Scotland Overflow Water and Northeast Atlantic Water). The model-derived mean velocity of the DWBC leads to tracer concentrations that have to fit the observed F11 and F12 distributions, the F11/F12 ratios, and the tritium distributions. Moreover, the model area is extended from south of the Faroe bank along the continental margin of the American continent to 10°S. The model assumes uniform velocity and uniform turbulent mixing along the flow path of the DWBC, and enhanced turbulent mixing in the vicinity of the current compared to the ocean's interior allows the surrounding waters, which remain motionless, to accumulate tracers. The highest mean velocity of the DWBC, which results in model F12, F11, and 3H distributions as well as F11/F12 ratios, compatible to measurements of these tracers along the western boundary, are 4.8 cm s −1. Variations in the composition of the DWBC as well as changes in the time history of the source water masses do not increase the range of the model velocities.

A chlorofluorocarbon section in the eastern North Atlantic

We present the distributions of two chlorofluorocarbons (CFCs) CFC-11 and CFC-12, measured as part of a hydrographic section between Iceland and the equator during July and August 1988. CFC-tagged water has filled the entire subpolar water column and subtropical thermocline in the eastern North Atlantic. Measurable CFC concentrations are observed at the ocean bottom as far south as 35°N, and the CFC penetration depth shoals to ≈750 m in the tropics. Specific features in the CFC distributions include a clear signal of Labrador Sea mid-depth ventilation, a CFC-enriched overflow water boundary current along the Iceland slope, and a mid-depth, equatorial plume of upper North Atlantic Water. The CFC data are used, in conjuction with the hydrographic data from the cruise, to illustrate the ventilation time-scales and pathways for the water masses in the eastern basin. A subsurface CFC maximum at about 100–200 m depth in the subtropics is shown to be a by-product of the heating and degassing of the seasonal thermocline and of the temperature sensitivity of CFC solubility. The CFC concentrations in the subpolar mode water are undersaturated by 15–18% relative to the atmosphere, reflecting the age of the mode waters and the very deep winter mixed layers in the eastern subpolar gyre. The CFC concentrations in the oxygen minimum off tropical Africa are much lower than the concentrations in the subtropical gyre, supporting previous work that suggests that isolation and enhanced productivity both contribute to the formation of the tropical oxygen minimum. In addition, the CFC inventories at the tropical stations have increased between 1982 and 1983 (TTO/TAS) and the summer of 1988 at a slower rate relative to the subtropical inventories over the same period. Thermocline oxygen utilization rates calculated from the CFC concentration data range from 5 to 10 μmol kg −1y −1 and are in line with previous estimates for the eastern subtropical thermocline. The low CFC concentrations in Mediterranean Water, about one-quarter those in the Labrador Sea Water, are shown to result from entrainment near the Straits of Gibraltar of a large component of low CFC, lower Atlantic thermocline water. Based on the CFC and other transient tracer distributions, the deep eastern basin can be divided into two regions: the Iceland Basin and surrounding area influenced by Iceland-Scotland Overflow Water that is ventilated on a decadal time-scale, and the area south of ≈50°N that has little or no CFC and is ventilated from a southern source on a much longer time-scale. A northward flowing boundary current of low CFC, modified Eastern Basin Bottom Water is also found along the Rockall Plateau and in the Iceland Basin.

Renewal rates of East Atlantic deep water estimated by inversion of 14C data

The renewal of the deep water of the East Atlantic and its large‐scale internal circulation are studied on the basis of the distributions of potential temperature, silicate, ΣCO2, and 14C. An isopycnal multibox model including advection, mixing, and sources and sinks is set up and described. Tracer data are input for the model, and balance equations for the various properties for the boxes of the model serve as constraints for the determination of water fluxes, mixing coefficients, and source parameters. Extremal values for various model parameters that are consistent with the tracer data (satisfy the balance equations within the estimated tolerances) are calculated by linear programming techniques. 14C data are seen to be valuable in determining absolute flow rates. Model results confirm the importance of the Romanche Facture Zone for the renewal of east Atlantic deep water. Eastward flows through the Romanche Fracture Zone were found to be between 2.6 and 5.1 Sv. Flows through the Vema Fracture Zone amount to at most 20% of the Romanche Fracture Zone inflow. Contributions of Antarctic Bottom Water at the southern end of the East Atlantic (Walvis Ridge) and of Iceland Scotland Overflow Water at the northern end are very small (&lt; 5% of equatorial inflow). Diapycnal mixing coefficients are between 1 and 10 cm2/s, and values for the dissolution rates of silicate and carbon are in the expected range.

Facies and sediment dynamics in Charlie-Gibbs fracture zone during the Late Quaternary

Analysis of five cores from the Charlie-Gibbs fracture zone enables us to define the sedimentary characteristics in four different types of environments: a sill disrupting the southern valley, open channels in the northern and southern valleys, a flanking plateau of the northern side wall in the northern valley, and an enclosed depression in the northern valley. Five lithofacies have been recognized: clayey-silty mud (40%), clayey-silty calcareous mud (25%), calcareous sands (20%), sedimentary breccia (10%) and silty-sandy calcareous mud (5%). The predominance of terrigenous clayey material is evidence for a mainly continental origin of the fracture-zone sediments. Yet another striking feature is the abundance of foraminiferal sands rich in pelagic calcareous and siliceous components associated with coarser ice-rafted elements. Despite the pelagic origin of components, a uniform draping of sediments — which kind of sedimentation we call “ubiquiste” (Faugères et al., 1979) — has not been noted in the survey area, due to the reworking processes of already deposited materials by bottom geostrophic currents or gravity flows. The influence of the bottom-current processes on sedimentation is confirmed by the following characteristics: (1) the presence of sharp or erosional contacts between two facies; (2) low clay fraction in certain levels, caused by winnowing of sedimentary materials; (3) high clay fraction in other levels, considered to be the result of dilution of pelagic material by clay particles transported in the nepheloid layer and trapped in a depression downstream of a sill; and (4) Icelandic smectites transported by Iceland—Scotland Overflow Water (I.S.O.W.). The influence of gravity flow dynamic processes is indicated by sandy or silty-sandy centimetric turbidites, very thick mega-sequences (8 m) transported by a single mass flow, and important sedimentary gaps, whence gravity flows originated.

θ-S relationships and water masses in the eastern North Atlantic

Observations of temperature and salinity from IGY and Erika Dan sections in the eastern North Atlantic have been re-examined to identify the θ-S relationships of the water masses and the fronts which separate them, using charts of salinity at potential temperature surfaces and sections of salinity at potential temperature levels. In the Deep Water ( θ ⪕ 4.0°C) four water masses are identified: Mediterranean Water, Labrador Sea Water, Iceland-Scotland Overflow Water, and Bottom Water. In the Intermediate Water (4.0°C < θ < 12.0°C) ee water masses are identified: Sub-Arctic Intermediate Water, Eastern North Atlantic Water (ENAW), and Mediterranean Water; the first two are separated by the Polar Front, whilst a fairly marked sub-surface front separates the latter two. A standard θ-S relationship is presented for ENAW, together with salinity anomalies from this for four zonal sections to illustrate the distributions of the intermediate water masses. The formation of ENAW is considered in relation to winter temperatures and salinities at the surface in the eastern North Atlantic.

Dissolved hydrogen in the Norwegian Sea: Mesoscale surface variability and deep-water distribution

The distribution of dissolved hydrogen (H 2) was studied during April 1979 in waters along the Iceland-Faroe Islands Ridge and in the Norwegian Sea. Surface water concentrations were well below air equilibrium throughout the region. Three areas (40 × 65 km) were sampled extensively by a surface pumping system allowing contour maps to be prepared with analysis spacings of 4 km. In general, H 2 concentrations were 6 to 7 nl l −1 on the ridge and decreased to 4 nl l −1 on the Voring Plateau. The results for the Norwegian Sea obtained before the spring plankton bloom suggest active consumption of H 2, presumably by microorganisms, at the sea surface. Apparent net production of H 2, which predominates in near-surface tropical North Atlantic waters, was not observed anywhere in the region. Vertical distributions showed that H 2 undersaturation persisted at all depths, averaging 4 nl l −1 below 300 m. Iceland-Scotland Overflow Water contained 4.5 nl l −1. The apparent absence of net H 2 production at high latitudes is unexplained, but may reflect the presence and high activity of chemolithotrophic H 2 bacteria and the relative absence or low activity at high latitudes of H 2-producing microorganisms such as nitrogen-fixers.

Charlie-Gibbs fracture zone: bottom-water transport and its geological effects

Hydrographic sections of the northern valley of the Charlie-Gibbs fracture zone show that below 2000 m this passage through the Mid-Atlantic Ridge is filled mainly with Iceland-Scotland Overflow Water (ISOW). Two current meters moored within this water mass for 9 months, at sites just downstream from the 3675-m sill of the passage, recorded mean westerly currents of 3.8 and 4.4 cm s −1 These averages include mean westerly velocities of 7 to 8 cm s −1 during fall and spring and almost zero during the winter. Another long-termmeter and an array of meters deployed within 100 m of the sea floor showed steering of the Overflow Water current by the complex topography near the intersection of the transform fault and the spreading center. West of 35°W a different, denser water mass fills the deepest parts of the valley below about 3400 m; it also flows westward, as shown by meters moored 10 and 20 m above the valley floor for 1.5 to 4 days. However, its properties (potential temperature <2.3°C; salinity < 34.93% oxygen 6.4 ml 1 −1; silica <16 μ mol 1 −1) indicate an origin in the Newfoundland Basin west of the Mid-Atlantic Ridge. It probably enters the northern valley from the south, via a leak in the bounding transform ridge near 34°30′W, and is entrained westward, back into the western Atlantic, by the overlying ISOW current. The total westward transport across the sill, below 2000 m, is estimated at 2.4 × 10 6 m 3 s −1. The ISOW carries a significant load of suspended sediment (25 μg 1 −1), and there is an intense 100-m-thick bottom nepheloid layer. The seabed, which was examined with a deeply towed instrument package, has been locally scoured downstream from the passage sill, and there are patches of ripples and moated rock fragments. However, most of the floor of the transform valley is a depositional environment.

Sediment Transport Over the Hatton and Gardar Contourite Drifts

ABSTRACT The contrasted modes of transport and deposition of sandy and muddy contourites were examined by hydrographic, photographic, lithologic, and seismic data from Hatton and Gardar sediment drifts in the Iceland Basin, northeastern Atlantic Ocean. Gardar Drift is made of muddy contourites deposited from Iceland-Scotland Overflow Water with a thick and relatively intense nepheloid layer whose particles originate from Iceland. Hatton Drift is also mainly mud, possibly from Rockall Bank, but at its foot a strong northeast-flowing boundary current moves winnowed foraminiferal sand as ripples and low sand waves. The nepheloid layer on Hatton Drift is locally generated and is mainly in a stratum of North Atlantic Deep Water that is contaminated with Antarctic Bottom Water. This stratum, with it entrained particles, spreads across the basin as far as the crest of Gardar Drift. The bottom mixed layer over Gardar Drift is mainly thin (30-40 m) and indicative of depositional shear-stresses (u* 0.85 cm/sec over sands at the foot of Hatton Drift, the mixed layer is 90 m thick, but this thins to less than 40 m higher up on the drift as the environment changes from winnowing towards deposition.

Influences of Norwegian Sea overflow water on sedimentation in the northern North Atlantic and Labrador Sea

Reflection profiles and hydrographic data indicate that sedimentation in parts of the northernmost Atlantic and in the Labrador basin has been largely governed by the movement of bottom water derived from the Norwegian Sea. Three areas are considered in detail. The first is the channel separating Rockall bank from the Irish continental shelf, where a ridge, nearly 600 km long, composed of sediment deposited on a gently dipping reflector of early Tertiary age, has been found. Hydrographic observations suggest that the feature, for which the name Feni ridge is proposed, is the result of deposition close to the boundary of a component of the Iceland-Scotland overflow water from the Norwegian Sea, which enters the channel from the north and in flowing southward is constrained by Coriolis force to the Rockall scarp. Farther north, reflection profiles show a marked asymmetry of sediment distribution across the volcanic Faeroe-Iceland ridge, the sediment cover being much thicker and more continuous on the north-facing slope than on the crest and southern flank. This difference is attributed to fast bottom currents in the south, caused by sinking of the Norwegian Sea overflow, and the persistence of tranquil bottom conditions in the north. Seismic profiles from the Labrador basin reveal a well-defined body of thick sediments trending NNW on a portion of the continental rise of southern Greenland where turbidity currents cannot reach. The thick sediments lie in a region in which the flow of deep water is weak and in which rapid deposition from sediment-laden bottom waters is allowed. Appreciably stronger bottom currents associated with the counterclockwise circulation of dense water derived from the Norwegian Sea appear to have greatly inhibited deposition of pelagic material in the area surrounding the sediment body. Sedimentation under the influence of bottom currents in this region, in the Rockall area, and on the Blake-Bahama outer ridge took place throughout a large part of Tertiary time but did not occur in the Upper Cretaceous. This is attributed to a major alteration in the circulation pattern of the North Atlantic caused by continental drift, polar wandering, and climatic deterioration between late Cretaceous and late Eocene times.