Bottom Current Flow Research Articles

Seismic expression of glaciomarine deposits in the eastern Riiser Larsen Sea, Antarctica

By studying multichannel seismic data across the continental slope and rise of the eastern Riiser Larsen Sea and through a comparison with other East Antarctic continental margins, the base of the glaciomarine deposits has been traced in this area. The seismic data reveal the presence of large channel-levee complexes as well as multiple types of contourite accumulations. Downslope and alonglope processes thus interacted in forming the glaciomarine deposits. The deposits are attributed to the advances of ice sheet, delivering huge amounts of sediment to the shelf edge and upper slope during glacial maxima. Oversteepening and instability generated down-slope turbidity currents forming channel–levee complexes whereas the contourite accumulations were probably mostly formed during interglacials. The spatial distribution of the current controlled deposits indicates that bottom currents flow along the western slope of the Gunnerus Ridge.

Chronostratigraphy of ODP 181, Site 1121 sediment core (Southwest Pacific Ocean), using 10Be/ 9Be dating of entrapped ferromanganese nodules

A 10Be/ 9Be-based chronostratigraphy has been determined for ODP 181, Site 1121 sediment core, recovered from the foot of the Campbell Plateau, Southwest Pacific Ocean. This core was drilled through the Campbell ‘skin drift’ in ca. 4500 m water depth on the mid-western margin of the extensive Campbell Nodule Field, beneath the flow of the major cold-water Deep Western Boundary Current (DWBC). In the absence of detailed biostratigraphy, beryllium isotopes have provided essential time information to allow palaeo-environmental interpretation to be undertaken on the upper 7 m of the core. Measured 10Be/ 9Be ratios of sediment, and of ferromanganese nodules entrapped in the sediment, decrease systematically with depth in the core, in accordance with radioactive decay. However, the 10Be/ 9Be data diverge from ca. 3 m below the seafloor (mbsf) to the top of the core, giving rise to several possible geochronological models. The preferred model assumes that the measured 10Be/ 9Be ratios of the nodule rims reflect initial 10Be/ 9Be ratios equivalent to contemporary seawater, and that these can be used to derive the true age of the sediment where the nodules occur. The nodule rim ages can be then used to interpret the sediment 10Be/ 9Be data, which indicate an overall age to ca. 7 mbsf of ca. 17.5 Ma. The derived chronology is consistent with diatom biostratigraphy, which indicates an age of 2.2–3.6 Ma at 1 mbsf. Calculated sedimentation rates range from 8 to 95 cm m.y. −1, with an overall rate to 7 mbsf of ca. 39 cm m.y. −1. The lowest rates generally coincide with the occurrence of entrapped nodules, and reflect periods of increased bottom current flow causing net sediment loss. Growth rates of individual nodules decrease towards the top of the sediment core, similar to the observed decrease in growth rate from core to rim of seafloor nodules from the Campbell Nodule Field. This may be related to an overall increase in the vigour of the DWBC from ca. 10 Ma to the present.

Variability of silt grain size and planktonic foraminiferal preservation in Plio/Pleistocene sediments from the western equatorial Atlantic and Caribbean

Records of mean sortable silt and planktonic foraminiferal preservation from the Ceará Rise (western equatorial Atlantic) and from the Caribbean are presented to analyze the Pliocene (3.5–2.2 Ma) to Pleistocene (1.6–0.3 Ma) evolution of near-bottom current strength and the carbonate corrosiveness of deep water. During the mid-Pleistocene climate transition (∼1 Ma) a drastic decrease in glacial bottom current strength and an increase in carbonate corrosiveness is registered, demonstrating a substantial decrease in the glacial contribution of the Lower North Atlantic Deep Water (LNADW) to the Atlantic Ocean. Also, an increased sensitivity to eccentricity orbital forcing is registered after the MPT. By contrast, carbonate preservation increases considerably in the deep Caribbean in response to a strong and persistent stable contribution of Upper North Atlantic Deep Water (UNADW). We found evidence for the strongest and most stable circulation within the LNADW cell during the Northern Hemisphere cooling period between ∼3.2 and 2.75 Ma. This is in agreement with the ‘superconveyor model’ which postulates that the highest NADW production took place prior to ∼2.7 Ma. A considerable decrease in bottom current strength and planktonic foraminiferal preservation is observed synchronous with the first occurrence of large-scale continental ice sheets in the Northern Hemisphere. This documents the final termination of the ‘superconveyor’ at ca. 2.75 Ma. However, our data do not support a ‘superconveyor’ in the interval between 3.5 and 3.2 Ma when high-amplitude fluctuations in bottom current flow and preservation in planktonic foraminifera are observed. Because of the great sensitivity of NADW production to changes in surface water salinity, we assume that the high-amplitude fluctuations of LNADW circulation prior to ∼3.2 Ma are linked to changes in the Atlantic salinity budget. After 2.75 Ma they are primarily controlled by ice-sheet forcing. In contrast to the stepwise deterioration of planktonic foraminiferal preservation in the western deep Atlantic, a trend toward better preservation from the Pliocene to Pleistocene is observed in the deep Caribbean. This indicates a long-term increase in the contribution of UNADW to the Atlantic Ocean.

Contourite deposits in the central Scotia Sea: the importance of the Antarctic Circumpolar Current and the Weddell Gyre flows

New swath bathymetry with multichannel and high resolution seismic profiles shows a variety of contourite drift, sediment wave morphologies, and seismic facies in the central Scotia Sea. The deposits are to be found at the confluence between the two most important bottom current flows in the southern ocean: the eastward flowing Antarctic Circumpolar Current (ACC) and the northward outflow of the Weddell Sea Deep Water (WSDW). The contourite drifts are wedge-like deposits up to 1 km thick, that exhibit aggradational reflectors along axis thinning towards the margins. The contourite drifts occur in areas of weaker flows along the margins of contourite channels and in areas protected by obstacles. The elongate-mounded drifts are best developed along the left-hand margins of channelized bottom current flows, due to the Coriolis force. A contourite fan has a main channel and two distributary channels that expand over a gentle seafloor. The proximal fan exhibits sediment waves with the distal fan incised by furrows. Sediment wave fields are well developed in areas of intensified bottom flows without channels, particularly at the confluence of the ACC and the WSDW. Small sediment waves occur where unidirectional bottom current flows predominate. Sediment waves may develop under the influence of internal waves produced by the interaction of the flows and sea-bottom relief. The stratigraphic sequence above the oceanic crust of Early to Middle Miocene age contains six seismic units separated by major reflectors. All the units were shaped under the influence of strong bottom current flows, although they exhibit distinct seismic facies changes that record the variations of the bottom current pathways over time. The age of the units was calculated based on the age of the oceanic crust and sedimentation rates of deep-sea deposits in the region. The oldest, Units VI–IV, are of Early to Middle Miocene age and developed under the influence of the ACC. They are characterized by a southward progradational pattern of the seismic units and sedimentation rates of 5–8 cm/ky. Unit III, with an estimated Middle Miocene age, evidences the first incursion of WSDW into the central Scotia Sea, when plate movement caused openings in the South Scotia Ridge and allowed the connection with the northern Weddell Sea through Jane Basin and gaps in the ridge. Unit II, estimated to be of Late Miocene to Early Pliocene age, extends over the area and is characterized by internal unconformities. A major unconformity at the base of Unit II records an important reorganization of bottom current flows that may predate the onset of grounded ice sheets on the Antarctic Peninsula shelf. Unit I, of Late Pliocene to Quaternary age, shows intensified bottom currents. The unconformity at the base of Unit I probably predates the onset of major Northern Hemisphere glaciations and the greater expansion of Antarctic ice sheets during the Late Pliocene. The extensive distribution of contourite deposits above the oceanic crust testifies to the long-term production of Antarctic Bottom Water. Cold, deep water was swept northward from the Weddell Gyre, interacting with the ACC, and possibly exerting profound influences on the global circulation system and the onset of major glaciations.

Deep‐water sediment wave fields, bottom current sand channels and gravity flow channel‐lobe systems: Gulf of Cadiz, NE Atlantic

Abstract A study of the seafloor of the Gulf of Cadiz west of the Strait of Gibraltar, using an integrated geophysical and sedimentological data set, gives new insights into sediment deposition from downslope thermohaline bottom currents. In this area, the Mediterranean Outflow (MO) begins to mix with North Atlantic waters and separates into alongslope geostrophic and downslope ageostrophic components. Changes in bedform morphology across the study area indicate a decrease in the peak velocity of the MO from >1 m s−1 to <0·5 m s−1. The associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves. A series of downslope‐oriented channels, formed by the MO, are found where the MO starts to descend the continental slope at a water depth of ≈700 m. These channels are up to 40 km long, have gradients of <0·5°, a fairly constant width of ≈2 km and a depth of ≈75 m. Sand waves move down the channels that have mud wave‐covered levees similar to those seen in turbidite channel–levee systems, although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems. The channels terminate abruptly where the MO lifts off the seafloor. Gravity flow channels with lobes on the basin floor exist downslope from several of the bottom current channels. Each gravity flow system has a narrow, slightly sinuous channel, up to 20 m deep, feeding a depositional lobe up to 7 km long. Cores from the lobes recovered up to 8·5 m of massive, well‐sorted, fine sand, with occasional mud clasts. This work provides an insight into the complex facies patterns associated with strong bottom currents and highlights key differences between bottom current and gravity flow channel–levee systems. The distribution of sand within these systems is of particular interest, with applications in understanding the architecture of hydrocarbon reservoirs formed in continental slope settings.

Migration history of a fine-grained abyssal sediment wave on the Bahama Outer Ridge

Abyssal mud waves (or fine-grained sediment waves) are often cited as evidence for deep current activity because subbottom profiles show that the wave form has migrated with time. The migration history of a fine-grained sediment wave on the Blake-Bahama Outer Ridge (ODP Site 1062) has been studied through the analysis of multiple ODP holes spaced across the wave. Additional information about wave migration patterns comes from 3.5-kHz records and watergun seismic profiles. These data suggest that wave migration has varied during the last ∼10 Myr, although the only sediments sampled are younger than 4.8 Ma. Seismic profiles suggest wave migration was initiated about 8–10 Ma, and wave migration was pronounced from about 5 Ma to about 1 Ma (with an episode of wave reorganization about 4.5 Ma). Analysis of ODP cores suggests that migration rates have been somewhat lower and more variable during the last 1 Myr. Intervals of no wave migration are observed for several time intervals and appear to characterize deglaciations, especially during the last 500 kyr. Comparisons between seismic profiles and the core record show that most of the seismic horizons correlate closely with time horizons, and thus that the seismic profiles give a reasonable representation of sediment wave migration. Models suggest that wave migration is more pronounced during periods of higher bottom current flow and less pronounced during periods of lower current flow. Thus the migration record is consistent with generally higher bottom flow speeds at this site prior to 1 Ma and lower bottom flow speeds after 1 Ma. The Mid-Pleistocene Transition from a dominant climatic periodicity of 40 kyr to a dominant climatic periodicity of 100 kyr starts at about this time, suggesting an overall reduction in bottom flow speed at this site coincident with changing climate patterns. These changes in flow speed could be related to changes in the depth of the Western Boundary Undercurrent as well as to changes in the speed of thermohaline circulation.

Hydrodynamic significance of variable ripple morphology across deep-water barchan dunes in the Faroe–Shetland Channel

A combination of high-resolution (30 kHz) sidescan sonar and seafloor photography has been used to image barchan dunes and sand ripples in the Faroe–Shetland Channel. The barchan dunes display body lengths and horn-to-horn widths of up to 120 m, and the horns point downcurrent to the southwest. The seafloor surrounding the dunes is variable, with gravel patches, sand with gravel streaks, and uniform rippled sand all observed. The pattern of sand ripples across the dune surface is strongly controlled by the dune morphology. On the lower stoss slopes of the dunes, straight-crested transverse ripples gradually merge upslope into sinuous-crested transverse ripples. Most of the upper dune surface is dominated by short-crested linguoid ripples. This sequence represents an increase in height above seafloor and increasing flow velocity. The overall pattern of ripples across the dune surface indicates that the bottom current flow is modified by the dune morphology, with flow concentrated over the dune crest and along the inner edge of the horns. It is also possible to estimate flow velocity across the dune field, using published analogues and short-term current meter observations. Generally, published examples of submarine barchans suggest a flow velocity of 40–80 cm/s. This range of values is in agreement with near-bed current meter measurements in the study area, which indicate peak flow velocities of 50–60 cm/s. However, in the longer term higher flow speeds may be possible, especially considering the presence of erosional furrows adjacent to the dune field.

Bottom-current sediment waves, sediment drifts and contourites in the northern Rockall Trough

Four types of large-scale sediment bedform occur in the northeast Rockall Trough: broad sheeted drifts, elongate drifts, sediment waves and thin contourite sheets. A large sheeted drift occupies the northern basin; its crest can be traced for 40 km perpendicular to the bottom-current flow direction. Elongate drifts occur adjacent to steep areas of continental slope, separated from the break of slope by a moat. Drift crests are oriented alongslope, parallel to bottom-current flow. Elongate drifts are asymmetric, with the steeper flank facing upslope; sedimentation rates are higher on the downslope flank. Sediment waves, with a wavelength of 1–2 km and wave height up to 20 m, occur locally on the flanks of sheeted and elongate drifts; larger waves (wavelength of 3–4 km and wave height of 50–100 m) occur on the flank of the Wyville–Thomson Ridge. Wave crests are oblique to bathymetric contours, to drift crests and to bottom-current flow. The association of sediment waves and drifts, their orientation oblique to regional slopes, and the presence of contourite sediments in cores, indicate that these are bottom-current rather than turbidity-current waves. A contourite sheet, typically 10–25 cm in thickness and composed of well sorted sand, covers the present-day seafloor along the lower slope. Bedform evolution through time has involved a complex interaction between bottom currents of variable intensity, sea level change and sediment input. The present sea level highstand is characterised by minimal sediment input and redistribution of sediments by strong bottom currents, giving active bedform growth and contourite development. Variability of sediment input is the main factor affecting sedimentation during sea level lowstands. Where input was high, as when ice sheets reached the shelf edge, downslope sediment transport dominated, overwhelming any bottom currents that were active. When input was low, such as during the last lowstand, some evidence for active currents can be observed, although these were less energetic than those during highstands.

Muddy contourites in the Baltic Sea: an example of a shallow-water contourite system

Abstract Bottom currents in the Baltic Sea have had a pronounced effect on the nature and distribution of sediments throughout the Holocene. Due to the intermittent nature of water exchange between the North and the Baltic Seas, the flow of bottom currents is impermanent. Well-developed nepheloid layers are commonly associated with these bottom currents, and provide evidence that active re-suspension and sediment transport is taking place. Periodically intensified inflow from the North Sea through the Baltic Sea gateway has led to erosion of narrow elongate channels, and associated deposition of small elongate patch drifts and contourite levees. The latter two are best developed on the left flanks of the channels. As a result of strong lateral sediment transport, the overall thickness of muddy contourite layers increases towards the steeper slopes of the sea which are situated on the right-hand side of the inflows. Muddy contourites of the Baltic Sea are predominantly terrigenous in composition, made up of soft, black sulphidic muds that are enriched in organic carbon and manganese. Both enrichments are caused by high biological productivity and periodic stagnation of near-bottom waters. Atypically for most oceanic contourites, these shallow-water organic-rich contourites are finely laminated and generally unbioturbated.

Contourite sedimentation in the Falkland Trough, western South Atlantic

Abstract The Falkland Trough is a west-east bathymetric deep that separates the Falkland Plateau from the North Scotia Ridge in the western South Atlantic. It lies in the path of Circumpolar Deep Water flowing within the Antarctic Circumpolar Current (ACC), and Weddell Sea Deep Water flowing beneath the ACC east of Shag Rocks passage. Marine geophysical and sediment core data demonstrate the influence of ambient bottom currents on deposition in this area, and reveal two styles of contourite sedimentation: (1) deposition of glauconite-rich sandy contourites in exposed areas of the Falkland Plateau and Falkland Trough, where vigorous ACC bottom currents control sedimentation, and (2) deposition of biogenic sandy contourites, muddy contourites and hemipelagites (western Falkland Trough), and muddy diatom ooze (eastern Falkland Trough), in the form of two elongate sediment drifts, which have developed in the presence of more sluggish bottom currents. The drift sediments contain a depositional record of bottom current flow through the glacial cycle (southern-origin bottom water flow in the east, and probably ACC flow in the west); analyses of core data from the western Falkland Trough suggest a reduction in bottom current strength during the Last Glacial Maximum at present depths of > 2500 m below sea level.

Holocene contourite sand sheet on the Barra Fan slope, NW Hebridean margin

Abstract Surficial sediment analyses, bottom photographs and current–meter data, coupled with studies of sidescan and 3.5 kHz echodata clearly show the imprint of a strong, seasonally-affected, slope current on the Hebrides shelf and slope. The present day shelf sediments are in fact relict, coarse-grained material (gravel with boulders) of Pleistocene glacial deri–vation, which have been modified during the Holocene by winnowing, sea-floor polishing and transport of the finer fraction across the shelf to the northwest. On the outer shelf and upper slope the sharp change from gravel to sand is marked by a physiographic-textural boundary, herein termed the sandline , which occurs at a depth of between 170 m and 300 m. Further down the continental margin on the lower slope, the lower limit of the sand-rich facies is marked by the mudline . This typically occurs at a depth of around 1200 m, and marks the depth of substantially increased clay-sized material. Above the mudline the long-term (Holocene to present), time-integrated signature of bottom current flow has resulted in the relatively slow accumulation of a mid-slope sandy contourite deposit, the Barra contourite sand sheet. This covers an area of 1000-1500 km 2 with an estimated sand volume of 30 000 m 3 . Below the mudline the clay-rich deposits represent a hemipelagic drape with only minor bottom current influence and intense reworking by benthic organisms.

Patterns of bottom current flow deduced from dune asymmetries over the Gulf of Cadiz shelf (southwest Spain)

Abstract The analysis of dune-like morphologies on the Gulf of Cadiz continental shelf, using high-resolution seismic reflection profiles and sediment samples, reveals a well-defined distribution pattern controlled by the area's hydrodynamics. These bedforms are considered to be modern features from the Holocene, and therefore they provide information about the bottom circulatory patterns established over the continental shelf. Most of the submarine dunes have been identified over a shallow physiographic feature named the Barbate High. The current flows that generate the bedform fields can be attributed to a complex interaction of several hydrodynamic agents, in which the process of current direction reversal related to the tidal cycle in the Straits of Gibraltar seems to be involved. Eastward-oriented submarine dunes located on the onshore zones of the inner shelf are generated by the Atlantic inflow, which over these shallow zones is mainly orientated towards the east during high-tide conditions in the Straits of Gibraltar. Bedforms oriented westwards and west–northwestwards in the offshore zones indicate that a previously undescribed flow occurs on the Gulf of Cadiz continental shelf. This flow is attributed to the influence of ebb tidal currents, which move preferentially towards the west over these zones. As a result of this complex bottom flow pattern, a clockwise sand transport pattern over the Barbate High is established.

Antarctic Circumpolar Deep Water; a Quaternary paleoflow record from the northern Scotia Sea, South Atlantic Ocean

In the northern Scotia Sea, the main pathway of Circumpolar Deep Water (CPDW) flows north to pass through a deep gap in the North Scotia Ridge before turning east into the Falkland Trough. A sediment drift has developed on the seabed since the early-middle Miocene, coincident with the opening of Drake Passage and the inception of deep-water flow. Seismic and acoustic surveys show that the drift covers an area of 10,500 km 2 and forms a broadly asymmetrical mound up to 800 m thick. There is a zone of sediment thinning along the northwestern margin, the result of accentuated CPDW flow around rough ocean floor topography. Small debris flows originating around the margins of the drift suggest localized instability and high sediment supply. Four cores 3-9 m long have been recovered from the crest and margins of the drift in water depths of 3900-4300 m. Biostratigraphy and chemostratigraphy reveal that the longest core extends down to oxygen isotope stage 10 (approx. 370 ka). The sediments are predominantly fine-grained contourites and diatom-rich hemipelagites, capped by sandy-silty contourites rich in the planktonic foraminifer Neogloboquadrina pachyderma. Grain-size analysis of the fine fraction, finer than 4 phi (63 mm), combined with radiocarbon (AMS) dating and magnetic susceptibility, provide an indication of relative CPDW strength over the last 18 ka. Shortly after the last glacial maximum (LGM), at approximately 17 ka, silt modes fluctuated from 5.5 phi to up to 6.25 phi; this increased current winnowing is indicative of an unstable CPDW, with stormier glacial benthic conditions producing sporadic, high-energy currents across the drift crest and flanks. At approximately 12,280 ka, an increase in sediment sorting is noted, indicative of a strong flow of CPDW over the drift crest, suggesting an unstable and fluctuating deep-water flow. During deglaciation and into the Holocene, at approximately 10 ka, CPDW flow stabilized, becoming less vigorous across the drift crest and flanks with silt modes from 6 phi to 5.5 phi accompanied by increased sorting of the sediments. The gross average sedimentation rate from the crest of the drift is 11.2 cm/ky compared to 2.3 cm/ky on the southeastern flank. The unsteadiness of CPDW during glacials compared to interglacial periods may be the result of stronger wind forcing and a northward shift in the Polar Front. Older CPDW flow records from the cores suggest variable and cyclic bottom-current flow corresponding to glacial-interglacial episodes. Modern CPDW flow across the crest of the drift averages 11.6 cm s (super -1) but with intermittent benthic storm activity resuspending the fines.

Growth of diagenetic ferromanganese nodules in an oxic deep-sea sedimentary environment, northeast equatorial Pacific

To study the accretion mechanism of diagenetic ferromanganese nodules formed in an oxic deep-sea sedimentary environment, manganese nodules, sediments, and pore waters were sampled in the northeast equatorial Pacific. Diagenetic ferromanganese nodules are ellipsoidal to discoidal in shape, rough in surface texture, enriched in Cu, Ni and 10 Å-manganite, and are ubiquitous on the abyssal plain; hydrogenetic nodules are polynucleated, irregular in shape, smooth in surface texture, enriched in Co and δ-MnO 2, and occur dominantly on seamounts. Numerous microlayers with diagenetic or hydrogenetic characteristics occur successively in the nodules. Concentration of nitrate in sediment pore waters indicates that overall redox conditions are oxic. However, Mn-oxide grains on the walls of burrows and worm tubes are interpreted to be reprecipitated metal ions supplied from oxygen-depleted microenvironments that formed in the oxic sediment column by decomposition of organic matter. Active mixing of bottom sediments by benthic fauna and bottom currents is indicated by ubiquitous burrows, worm tubes, and tracks and trails, high values in sediment mixing coefficients (>1 cm 2/yr), and scattering of data points in depth profiles of pore water nitrate concentrations. We suggest that diagenetic ferromanganese nodules in an oxic deep-sea sedimentary environment grew from remobilized metal ions as well as reprecipitated Mn-oxide grains, which were supplied to the nodules episodically during the stirring of bottom sediments by benthic fauna and intermittent strong bottom current flow.

Quaternary history of the Antarctic Circumpolar Current: evidence from the Scotia Sea

Within the Scotia Sea, the axis of the Antarctic Circumpolar Current (ACC) is geographically confined, and sediments therefore contain a record of palaeo-flow speed uncomplicated by ACC axis migration. We outline Holocene and Last Glacial Maximum (LGM) current-controlled sedimentation using data from 3.5-kHz profiles, cores and current meter moorings. Geophysical surveys show areas of erosion and deposition controlled by Neogene basement topography. Deposition occurs in mounded sediment drifts or flatter areas, where 500–1000 m of sediment overlies acoustic basement. 3.5-kHz profiles show parallel, continuous sub-bottom reflectors with highest sedimentation rates in the centre of the drifts, and reflectors converging towards marginal zones of non-deposition. Locally, on the flanks of continental blocks (e.g. South Georgia), downslope processes are dominant. The absence of mudwaves on the sediment drifts may result from the unsteadiness of ACC flow. A core transect from the ACC axis south to the boundary with the Weddell Gyre shows a southward decrease in biogenic content, controlled by the Polar Front and the spring sea-ice edge. Both these features lay farther north at LGM. The cores have been dated by relative abundance of the radiolarian Cycladophora davisiana, and by changes in the biogenic Ba content, a palaeoproductivity indicator. Sedimentation rates range from 3 to 17 cm/ka. The grain size of Holocene sediments shows a coarsening trend from south to north, consistent with strongest bottom-current flow near the ACC axis, though interpretation is complicated by the presence of biogenic grains. Year-long current meter records indicate mean speeds from 7 cm/s in the south to 12 cm/s in the north, with benthic storm frequency increasing northwards. LGM sediments are predominantly terrigenous and show a clearer northward-coarsening trend, with well-sorted silts in the northern Scotia Sea. Assuming a constant terrigenous source, this implies stronger ACC flow at the LGM, contrasting with weaker Weddell Gyre flow deduced from earlier work.

Sediment drifts and contourites on the continental margin off northwest Britain

Seismic reflection profiles and short cores from the continental margin off northwest Britain have revealed a variety of sediment-drift styles and contourite deposits preserved in the northeast Rockall Trough and Faeroe-Shetland Channel. The sediment drifts include: (1) distinctly mounded elongate drifts, both single- and multi-crested; (2) broad sheeted drift forms, varying from gently domed to flat-lying; and (3) isolated patch drifts, including moat-related drifts. Fields of sediment waves are locally developed in association with the elongate and gently domed, broad sheeted drifts. The contrasting styles of the sediment drifts most probably reflect the interaction between a variable bottom-current regime and the complex bathymetry of the continental margin. The bulk of the mounded/gently domed drifts occur in the northeast Rockall Trough, whereas the flat-lying, sheet-form deposits occur in the Faeroe-Shetland Channel, a much narrower basin which appears to have been an area more of sediment export than drift accumulation. Patch drifts are present in both basins. In the northeast Rockall Trough, the along-strike variation from single- to multi-crested elongate drifts may be a response to bottom-current changes influenced by developing drift topography. Muddy, silty muddy and sandy contourites have been recovered in sediment cores from the uppermost parts of the drift sequences. On the basis of their glaciomarine origin, these mid- to high-latitude contourites can be referred to, collectively, as glacigenic contourites. Both partial and complete contourite sequences are preserved; the former consist largely of sandy (mid-only) and top-only contourites. Sandy contourites, by their coarse-grained nature and their formation under strongest bottom-current flows, are the most likely to be preserved in the rock record. However, the very large scale of sediment drifts should be borne in mind with regard to the recognition of fossil contourites in ancient successions.

Evidence for westward-flowing Weddell Sea Deep Water in the Falkland Trough, western South Atlantic

The North Scotia Ridge controls the eastward and northward flow of the Antarctic Circumpolar Current (ACC) emerging from Drake Passage. Existing physical oceanographic data in this region are sparse and do not define the flow pattern of Circumpolar Deep Water (CDW) within the ACC, or of Weddell Sea Deep Water (WSDW) heading northward beneath it, in the region of the North Scotia Ridge and Falkland Trough. 3.5-kHz reflection profiles show mudwaves at the surface of a sediment drift along the axis of the eastern Falkland Trough that have a consistent NE-SW alignment and are migrating SE, indicating persistent westward bottom-current flow along the trough axis. Sediment thinning and non-deposition at the southern drift margin indicate intensified westward flow, considered to be Weddell Sea Deep Water from the Malvinas Outer Basin to the east. This flow probably continues to 48°W, but beyond there its fate is unknown. Similar non-deposition along the northern margin of the drift is considered to result from intensified eastward return flow of WSDW, or from CDW. The mudwave geometry appears to extend to at least 400-m depth within the drift, which therefore most probably contains a record of southern-origin bottom water (presently WSDW) extending back for several million years.

Abyssal bedforms as indicators of changing bottom current flow: Examples from the U.S. East Coast continental rise

Direct indications of circulation patterns in the deep sea (especially along continental rises and sediment drifts) can be obtained through the analysis of abyssal bedforms such as furrows and mud waves. Such direct evidence of current flow will supplement indirect indicators of circulation provide by paleochemical indicators preserved in ocean sediments. Water that sinks in the northern North Atlantic forms part of the Western Boundary Undercurrent along the continental rise of eastern North America and the understanding of temporal changes in the speed and depth of flow along this continental rise can provide important information on how the speed at which the global conveyor system is operating changes with time. Studies of furrows and mud waves along the Blake‐Bahama Outer Ridges show that they are active sedimentary features today, and that bottom flow speeds near the time of the Younger Dryas were increased over later intervals. A more detailed analysis of bedform structure and sediments is needed to more clearly show the timing and reason for this change in sedimentation pattern.

Modeling of Rectangular Settling Tanks

A mathematical model for predicting the velocity field and suspended solids transport in secondary rectangular settling tanks is presented. The model is applied to investigate the effects of density stratification on the hydrodynamics in the settling tank, such as the behavior of the bottom current and surface return flow and solids concentration distribution. The model can be used to simulate the so‐called density waterfall phenomenon at the front end of the settling tank. With the application of a double exponential relationship to describe the settling process of suspended solids with nonuniform particle sizes, the simulations of the solids concentration distribution in the low‐concentration environment can be significantly improved. By comparing the predictions with field measurements, this model is verified and generally good agreement is obtained.

Selection of Positions by Drift-Feeding Salmonids in Dominance Hierarchies: Model and Test for Arctic Grayling (Thymallus arcticus) in Subarctic Mountain Streams, Interior Alaska

In this work I describe a model to predict position choice by each individual in a dominance hierarchy of drift-feeding stream salmonids. This is an adaptation of Hughes and Dill's model (1990. Can. J. Fish. Aquat. Sci. 47: 2039–2048) of position choice by solitary fish. I have included the effect that prey consumption, lateral diffusion of drifting invertebrates, and entry of invertebrates into the drift have on the density of prey downstream of feeding fish and the restrictions that dominant fish place on freedom of choice by their subordinates. l assume that each fish chooses the most profitable position that its rank in the hierarchy will allow. There was an encouraging match between the distribution patterns predicted by the model and the distribution patterns actually adopted by Arctic grayling (Thymallus arcticus) in two pools of a mountain stream. This result suggests that Arctic grayling locate and rank positions based on their profitability. The predictions of reduced models, and the location of positions in relation to bottom topography and current flow, suggest that the physical habitat forms the template for distribution patterns by determining the location and ranking of the most profitable positions.