Abstract Optical and acoustic backscatter sensors, more sensitive to fine and sandy sediment respectively, were used to measure the mud and sand components of a mixed suspension at a site seaward of the Wash embayment, in the southern North Sea. Data were acquired from a free-standing instrument frame during a five-week deployment in 12 m water depth about 6 km offshore. Suspended mud at this site was characterized by tidal advection of fine sediment along the coast resulting in semi-diurnal peaks in concentration near slack water. Suspended sand concentrations correlated well with tidal current speeds indicating local resuspension behaviour. Predicted sand flux direction followed the residual current while mud fluxes at the site were different in direction to both the residual current and sand flux. Residual fluxes may be biased by cumulative errors resulting from instrument calibration and inferred vertical concentration profiles. These factors are assessed in relation to both predicted flux magnitudes and directions.

Abstract Understanding of Earth’s transition from a warm, ice-free Cretaceous to today’s bipolar glaciation is hotly debated. The Oligocene–Miocene boundary is marked by a brief glacial event followed by an interval of colder temperatures. Changes are small compared to the major Antarctic ice build-up at the Eocene–Oligocene boundary and establishment of a permanent Antarctic ice-sheet in the mid-Miocene. However, fossil evidence from low latitudes, including the faunal turnover which originally defined the Oligocene–Miocene boundary, indicates a reversal in trans-Atlantic flow, i.e. from westward to eastward, at this time. Modelling results suggest that a combined narrowing of the Tethys Seaway and deep opening of Drake Passage, and hence inception of Antarctic circumpolar circulation, drove reorganization of the patterns of ocean circulation. Despite evidence for a shallow Drake Passage opening in earliest Eocene time and subsequent deepening, a comparison of Southern Ocean isotopic records suggests that circumpolar circulation did not exist prior to c. 26 Ma. In fact, sedimentary records of a grain-size current-speed indicator from the Tasman Gateway reveal a singular, marked increase immediately preceding the initial Miocene event. The likely driver of this increase is inception of the full Antarctic Circumpolar Current. Among possible causes of early Cenozoic climate deterioration, the opening of seaways appears to play the major role. Extreme orbital configurations and p CO 2 -drawdown may act as reinforcing factors.

Abstract The Nova Scotian continental rise is swept by a Deep Western Boundary Current system comprising layers of Labrador Sea Water overlying a core of Norwegian Sea Overflow Water at depths of 3100-3900 m, and below about 4600 m a cold stream of southern-source water. Seismic-reflection data show that the rise contains sediments transported downslope in channels and debris lobes, but there is also evidence of current-controlled deposition and erosion in the post-Eocene sequence. The rise is now mantled by Holocene contourites that have accumulated at a rate of c . 6 cm ka −1 and are <1 m thick. Bottom photographs show a zonation in current effects and bedform types, with longitudinal ripples and strong currents prevalent at 4800-5000 m, smaller bedforms and progressively weaker currents up to c . 4000 m, and mostly tranquil seafloor above 4000 m. Bedform scales and orientations also suggest significant short-term (hours to weeks) variability in current velocity but a mean contour-following flow to the southwest at longer time scales (months to years). These structures are not preserved in the sediment because of pervasive bioturbation and the uppermost layers have negligible preservation potential. The sediments display clear current controlled effects in their grain-size structure involving both percentage of (foraminiferal) sand, and size and percentage in the 10-63 µm range, the ‘sortable silt’. There is a sand-rich zone at 4800–4900 m and below 5000 m, and a decreasing silt/clay ratio from 5100 m up to 4000 m. Although much of the sedimentary sequence probably has been emplaced by downslope processes, it has been significantly modified by the Deep Western Boundary Current. Particularly strong and variable currents which rework sediments below c . 4800 m probably are engendered by interaction of Gulf Stream eddies with the DWBC. Although strong currents and upstream input from turbidites and debris flows might be thought to favour a coarse-grained deposit, the facies at the HEBBLE (High Energy Benthic Boundary Layer Experiment) site is muddy contourite with ≤ 12% sand.

Abstract The distribution of organic carbon in marine sediments is commonly characterized by cyclicity at different time scales. A detailed analysis of such cyclicity in three case studies of Liassic and Kimmeridgian age in England and of Cenomanian age in the northwestern Atlantic Ocean shows that specific processes playing at different time scales control the storage of organic matter. Two scales are distinguished: (1) large-scale trends (>3 m.y., 2nd- and 3rd-order cycles) are caused by plate tectonics affecting paleogeography and topography, long-term eustatic sea level, and climatic changes (“ice-house” and “green-house”); they define the storage of organic matter worldwide by influencing productivity and ventilation of deep water; and (2) small-scale trends (<3 m.y, 4th- and 5th-order cycles) are caused by orbitally induced high-frequency glacio-eustatic and other océanographic and/or climatic changes. If general conditions are favorable, the impact of these changes is a high-frequency signal of oxygenation/dilution cycles, whose particular expression strongly depends on the local sedimentary environment. A consequence of the orbitally induced climatic/océanographic control of high-frequency sedimentary cycles is that it has a regional (to worldwide) expression, and is thus a powerful tool to reconstruct basin in-fill patterns and to establish detailed correlations between basinal (source rock) and margin (reservoir) successions. Once established, a high-resolution framework provides the necessary stratigraphic control for detailed geochemical studies and allows a quantitative approach of the geochemical sediment budget, as well as interpolation and extrapolation to time-equivalent sequences.

Abstract With its multiple sources of sediment and bottom water, the North Atlantic experiences more active redistribution of sediments than most other ocean basins. North Atlantic bottom waters originate around Antarctica, in the Norwegian Sea, the Mediterranean, and the Labrador Sea. Sediment is supplied from continental land-masses, oceanic islands (notably Iceland) and from surface biological production. The water movements are controlled principally by differing densities of the water masses, and the currents tend to follow the contours of the sea floor. Where interfaces of steep density gradient intersect the seafloor, internal tides and high-frequency internal waves may also resuspend sediments. The Gulf Stream, and warm-core and cold-core mesoscale eddies with clockwise (anti-cyclonic) and anticlockwise (cyclonic) circulation, apparently contribute to the variability of current velocity at great depths and help to erode and redistribute sediments. The most important depositional products of this current activity are the great “sediment drifts” of the region (see Plate 2). These features probably contain a detailed but as yet poorly known record of the fluctuations in bottom-current activity of the North Atlantic. They have formed principally since the beginning of the Oligocene, when strong abyssal circulation began in the North Atlantic (Ewing and Hollister 1972; Tucholke and Mountain, 1979; Miller and Tucholke, 1983). The precise relationship of the drifts to the present abyssal current regime is not clear because details of that regime are poorly known. Few long-term, deep current-meter records have been taken and most of these are not adjacent to drifts.