Steep Continental Slope Research Articles

Origin of the deep Bering Sea nitrate deficit: Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes

On the basis of the normalization to phosphate, a significant amount of nitrate is missing from the deep Bering Sea (BS). Benthic denitrification has been suggested previously to be the dominant cause for the BS nitrate deficit. We measured water column nitrate 15N/14N and 18O/16O as integrative tracers of microbial denitrification, together with pore water‐derived benthic nitrate fluxes in the deep BS basin, in order to gain new constraints on the mechanism of fixed nitrogen loss in the BS. The lack of any nitrate isotope enrichment into the deep part of the BS supports the benthic denitrification hypothesis. On the basis of the nitrate deficit in the water column with respect to the adjacent North Pacific and a radiocarbon‐derived ventilation age of ∼50 years, we calculate an average deep BS (>2000 m water depth) sedimentary denitrification rate of ∼230 μmol N m−2 d−1 (or 1.27 Tg N yr−1), more than 3 times higher than high‐end estimates of the average global sedimentary denitrification rate for the same depth interval. Pore water‐derived estimates of benthic denitrification were variable, and uncertainties in estimates were large. A very high denitrification rate measured from the base of the steep northern slope of the basin suggests that the elevated average sedimentary denitrification rate of the deep Bering calculated from the nitrate deficit is driven by organic matter supply to the base of the continental slope, owing to a combination of high primary productivity in the surface waters along the shelf break and efficient down‐slope sediment focusing along the steep continental slopes that characterize the BS.

IndoTROPICS studies on the plume of the Mamberamo river into the Bismarck Sea, West Papua, Indonesia

The tropical river ocean processes in coastal settings (TROPICS) program in Indonesia (Indotropics) was carried out in the Mamberamo estuary on May–June 1999 and August 2000. The Mamberamo River flows northward from the high mountains of West Papua to the narrow and steep continental slope of the Pacific Ocean. The data for the 1999 Mamberamo estuary cruises show the dispersal of fresh water coming out of the Mamberamo River into the Bismarck Sea, and the plume is clearly defined by the pattern of salinity, turbidity, and nutrient distribution. The Mamberamo River and other nearby rivers supply high concentrations of phosphate and nitrate to the New Guinea Coastal Current, so that nutrient concentrations are higher in the surface estuarine plume, compared to offshore areas. The distribution of fishes and benthic animals were also sampled over this area.

Continental slope morphology and sedimentary processes at the mouth of an Antarctic palaeo-ice stream

Continental-slope and shelf-edge morphology off Marguerite Bay, western Antarctic Peninsula, is investigated using swath-bathymetric data and parametric sub-bottom profiler records, together with sediment cores. Marguerite Bay has a well-defined cross-shelf trough, and a relatively steep continental slope. The slope beyond the trough mouth is convex in longitudinal profile, whereas to the north and south it is concave and reaches a maximum of 12°. There are no deep canyons cutting into the prograding outer shelf and slope. Instead, a series of gullies runs down the upper slope, reaching depths of >200 m south of the trough mouth but <120 m deep beyond the trough. The mid and lower slope appears to be relatively smooth and downslope sediment transfer is probably by small-scale slides, slumps and debris flows. The continental rise contains dendritic channels related to turbidity currents, and sediment drifts produced by southwest-flowing bottom currents from the fine-grained component of the turbidity currents. Elongate sedimentary bedforms indicate that a fast-flowing ice stream occupied the trough under full-glacial conditions, and transferred deforming subglacial till rapidly to the shelf edge. By contrast, on either side of the trough mouth, ice is inferred to have been slower-moving and probably cold-based, delivering little sediment to the upper slope. The steepness of the continental slope results in rapid downslope sediment transfer by debris flows, slumps and turbidity currents and accounts for the lack of a well-developed trough-mouth fan, which is typical of many lower-gradient glacier-influenced margins.

Semi‐diurnal internal wave diffraction caused by Dixon Entrance, British Columbia

To develop an understanding of the complex internal tidal phenomena observed near and inside Dixon Entrance, an idealized numerical model was developed for the area, which explores the influence of various topographic features on the scattering of internal tides. The model uses a non‐linear, two‐layered, frictionless finite difference formulation of the shallow water equations and is forced by a barotropic wave over simplified topography. It was found that the main bathymetric features responsible for the generation of semi‐diurnal internal tides are the steep continental slope together with the orientation of Dixon Entrance. The prevalent baroclinic wave pattern, which is similar to the one found by Buchwald (1971), suggests that the western end of Dixon Entrance can be considered as an internal tide generation region for the open ocean. Use of the simple model allows easy identification of the generated waves. When the model is run with a non‐flat channel it reproduces features observed inside Dixon Entrance.

Experimental multipath time spread in a bathymetrically complex coastal environment

An acoustic survey was conducted that covered both the continental shelf and slope regions in water depths from 40 m to greater than 1000 m. The types of bathymetric conditions encountered included those likely to be found in any coastal environment. At the benign end of the spectrum a relatively smooth bottom with small slopes connected the shelf and slope. In another area the shelf was relatively short and extended to water depths of approximately 100 m, followed by very steep continental slopes bordered by canyon walls. Intermediate conditions consisted of a short shelf and large continental slopes without canyon walls. Broadband (12 Hz to 4 kHz) acoustic measurements were made under strongly downward-refracting sound-speed conditions using impulsive sources at two different depths. The source at 18 m was in a deep (approximately 40 m) weak surface duct. A strong sound-speed gradient separated this source from the 86-m source depth. The impulsive data have been processed to extract a measure of the effective multipath time spread. Results will be presented showing the sensitivity to source depth, frequency range from the receiver, and bathymetric conditions.

Tidally generated shelf waves off the western Iberian coast

A high-resolution finite element model of barotropic tides in the Iberian Atlantic shelf is presented. The model accurately reproduces available tidal data, with average amplitude discrepancies at 12 Atlantic stations below 3% and 15% for semi-diurnal and diurnal tides, respectively. The accuracy of the model simulations is supported also by a qualitative comparison with velocity data. The model results are used to examine the generation of shelf waves by diurnal tides off the western Portuguese coast. Shelf waves in this area are responsible for large diurnal currents, which can exceed semi-diurnal currents by an order of magnitude. Numerical experiments show that these shelf waves can be generated by obstacles perpendicular to the coastline, and are enhanced in the presence of wide and shallow shelves and steep continental slopes. Along the Portuguese coast, the Tagus plateau is responsible for most of the shelf wave generation.

Varves, turbidites and cycles in upper Holocene sediments (Makran slope, northern Arabian Sea)

Abstract We have analysed two Late Holocene records, each about 5 ka long, consisting of varved sediments deposited in the oxygen minimum zone off Pakistan (upper continental slope off Ormara and west of Karachi). Varve counting was checked by accelerator mass spectrometry (AMS) 14 C dating. Detailed lithofacies analysis, ultra-high-resolution X-ray fluorescence scanning, flux rates from sediment traps and the lamina-by-lamina-analysis of a 5 year record (1993–1998) support our interpretation of the annual character of the laminae. Although the pelagic material is deposited throughout the year, most of it is apparently laid down during the high-productivity period of the late summer monsoon. During the winter (mainly mid-December to February-March) mainly light-coloured detrital material (siliciclastic material and reworked carbonate flour) is deposited, probably by river flood events. Event deposits include turbid-plume or suspensate deposits (light grey homogeneous to graded silty clay layers), which are being laid down at decadal or shorter intervals; they are explained by episodically strong river floods after heavy rains transporting mud-charged waters to the narrow shelf and onto the steep continental slope. Medium grey or reddish grey, graded and laminated silt turbidites originated from less frequent, unchannelized, low-density turbidity currents. In general, periods with thin varves (generally correlated with rare turbidites) are correlated with minima of detrital element ratios (TiO 2 /Al 2 O 3 , Zr/Al 2 O 3 , K 2 O/Al 2 O 3 ), especially in Period I ( c. 5600-4700 a bp), suggesting a climate with reduced precipitation and river runoff (possibly winter monsoon dominated). Period II (4700-2600 a bp) is characterized by comparatively thick varves documenting a generally wet period (with possibly summer monsoon domination). During Period III (2600-1000 a bp) a gradual thinning of varves and a decrease of turbidite abundance (thickness) per century is interpreted as a gradual decline of precipitation and river runoff leading finally to dry conditions from 1600 to 1000 a bp. The sequence of cycles detected by autocorrelation and standard Fourier analysis seems to contain a large proportion of multiples of the lunar perigee cycle (4.425 a, 8.85 a) and the lunar (half) nodal cycle (9.3 a, 18.6 a). Our test for cyclicity in the series of varve thickness (varve cycles) and of abundance of turbidites (turbidite cycles) detected prominent high-frequency cycles. Some cycles of varve thickness match the cyclicity of turbidite frequency. We also detected the presence of a 1470 a cycle, previously reported from the glacial-age Greenland ice record.

Study of tsunami propagation in the Ligurian Sea

Abstract. Tsunami propagation is analyzed for the Ligurian Sea with particular attention on the French coasts of the Mediterranean. Historical data of tsunami manifestation on the French coast are analyzed for the period 2000 B.C.–1991 A.D. Numerical simulations of potential and historical tsunamis in the Ligurian Sea are done in the context of the nonlinear shallow water theory. Tsunami wave heights as well as their distribution function is calculated for historical tsunamis and it is shown that the log-normal distribution describes reasonably the simulated data. This demonstrates the particular role of bottom irregularities for the wave height distribution function near the coastlines. Also, spectral analysis of numerical tide-gauge records is done for potential tsunamis, revealing the complex resonant interactions between the tsunami waves and the bottom oscillations. It is shown that for an earthquake magnitude of 6.8 (averaged value for the Mediterranean Sea) the tsunami phenomenon has a very local character but with long duration. For sources located near the steep continental slope in the vicinity of the French-Italian Rivera, the tsunami tide-gauge records in the vicinity of Cannes – Imperia present irregular oscillations with a characteristic period of 20–30 min and a total duration of 10–20 h. For the western French coasts the amplitudes are significantly less with characteristic low-frequency oscillations (period of 40 min–1 h).

Mixing in the Bosphorus Strait and the Black Sea continental shelf: observations and a model of the dense water outflow

Counterflowing waters of the Black Sea and Mediterranean Sea are mixed by turbulent entrainment processes along their course through the Turkish Straits. In the Bosphorus Strait, the entrainment into the upper layer from below is abruptly increased when the flow is accelerated in the narrower southern reach, where the flow passes through a contraction. In contrast, the lower layer salinity decreases towards the north first by gradual entrainment within the Strait and later at an increased rate in the wide continental shelf region upon exit into the Black Sea. After passing over the sill located north of the Strait, the flow on the continental shelf proceeds in the form of a gravity current following the local slopes. The topography of the shelf region, assembled from various sources of high-resolution surveys and maps, is reminiscent of a river delta. The water properties and thickness of the Mediterranean plume is modified by turbulent entrainment, shelf currents, stratification, bottom friction and slope. The flow first spreads out on the mid-shelf slope, follows the delta features to reach the shelf edge and, finally, cascades down the steep continental slope. Horizontal spreading by convective instabilities and eastward propagation of anomalous properties along the continental slope are characteristic features of the deeper region adjacent to the shelf. The behaviour of the density current is revealed by results obtained from a reduced gravity model, suggesting that the slope and fine-scale features of the bottom topography are crucial elements in determining plume behaviour. The model results are found to be robust to environmental changes and in good correspondence with observed flow features, especially when the topography with realistic fine scales and slope are adequately represented.

Western Hellenic subduction and Cephalonia Transform: local earthquakes and plate transport and strain

Focal parameters of local earthquakes in the region of the Ionian Islands of western Greece are constrained with a temporary dense array of three-component seismographs operated jointly offshore and onshore. Seismic deformation is documented to be confined to the east of the N20°E-striking steep continental slope west of Cephalonia island, the right-lateral Cephalonia Transform Fault, CTF, inferred from large earthquakes. The pre-Apulian continental material appears to be only deforming east of the transform fault, where it is in upper plate position to the Hellenic subduction. East of the transform fault, the transmission velocity tomography from local earthquakes, compared in depth-section with a previous marine reflection profile, provides evidence in support of a shallow landward dipping boundary around 12 km deep under the Ionian Islands along which they may override the lower plate. On either side of this interface local earthquakes occur with different focal mechanisms, in support with its interpretation as the interplate. Under Cephalonia island, reverse-faulting deforms the upper plate along NW–SE structures, which may also be affected by left-lateral bookshelf-faulting. Small earthquakes show normal faulting along the western coast of Cephalonia and its extension 20 km SSW, the trace of the CTF as inferred from the occurrence of the large strike-slip earthquakes. Another group of normal-fault earthquakes locates in the lower plate from under Cephalonia to Zante, just outboard of a possible change of interplate dip suggested from reflection seismics landward under the islands. These normal-fault earthquakes appear to coincide in position with that of the load imposed by the upper plate transported over them, rather than occurring in an outer rise, outboard the plate boundary and trench, as observed in other subductions and attributed to the control by the flexural bending of the lower plate under the pull of the sinking slab. Interpretation has to consider several peculiar features of plate interaction in western Greece with respect to a steady-state model for major subduction zones, in particular: a fast deformation of the upper plate in front of an orogenically overthickened crust and of the southwestward push of extruding Anatolia; its transport, which is the cause of the migration of the plate boundary rather than the roll-back of a slab which has been proposed to be detached; possibly a flat and ramp shape of the interplate; the geometrical complexity of the shear limit across the CTF between subduction and collision, and the nearby variation of the nature of the foreland crust.

A three-dimensional model study of the Mediterranean outflow

Evaporation over the Mediterranean basin produces a salty water mass that overflows the relatively narrow and shallow Strait of Gibraltar. The outflow is investigated with the three-dimensional Princeton Ocean Model. The model makes use of a bottom-following, sigma-coordinate system and an imbedded turbulence closure scheme to simulate the bottom boundary layer. Starting as a channel flow confined by the sidewalls of the trench of the westernmost part of the Strait, the bottom boundary current descends the steep continental slope of the eastern Gulf of Cadiz. The flow is controlled by a balance of the pressure gradient and the Coriolis acceleration, entrainment and bottom friction. Downslope Ekman fluxes are largest at the shelf break near the entrance of the Strait where we observe the most pronounced entrainment of North Atlantic Central Water. Profiles of modeled horizontal velocity, as well as scalar and turbulent quantities, show a remarkable resemblance to observed features. The model reproduces the “nose” shape of the velocity profile that is typical for a density current; below the velocity maximum, there is a well-mixed layer and a stratified shear layer exists above the nose. The evolution of water mass properties along the path of the outflow agrees favorably with observations and the sensitivity of the Mediterranean outflow (MO) to varying source water properties is investigated. The combined effect of initial stratification, differential entrainment, and routing by the topography leads to the evolution of the well-known two-core structure of the bottom layer. Whereas the upper part of the outflow continues to follow the continental slope northward in the form of a bottom boundary current after it passes Cape St. Vincent, the lower core separates from the slope and there is a lateral spreading of the saline and warm outflow water in the depth range of the lower salinity maximum (centered at about 1200 m). The outflow becomes hydrodynamically unstable and lenses of saline water shed from the core, carrying their water mass characteristic into the Atlantic. The model results confirm that the area of Cape St. Vincent is a prominent formation area for Mediterranean water (MW) eddies (Meddies).

Patterns and processes of sediment dispersal on the continental slope off Nice, SE France

The distribution of surficial sediments and sediment dispersion patterns on the steep continental slope off Nice (SE France) have been studied using side-scan sonar, 3.5 kHz profiles, short piston cores and bottom photographs. The input of terrigenous material to the Baie des Anges, a submarine embayment bounded by two prominent ridges, is dominated by fluvial input from the Var River, the Paillon River being only a minor source. The Var River provides very coarse bedload material (gravel and cobble) directly to the head of the Var Canyon. Gravel and cobble deposits are found all along the Var Canyon and the Upper Fan Valley of the Var submarine fan and have been shaped into gravel waves. The fine particles (suspension load) are separated from the coarse bedload upon entering the sea and form up to 60-m thick deposits on the uppermost continental slope of the Baie des Anges. These deposits are formed by settling out of sediment plumes. The presence of silt and fine sand laminae that decrease in thickness and frequency away from the Var River mouth indicate the influence of meso- and hyperpycnal flows on these plume deposits. Areas outside the Baie des Anges are not connected to major fluvial input and receive only hemipelagic sediments. These primary deposits are highly unstable and sediment failure due to seismic loading, sedimentary loading or undercutting is frequent. Sediment failure produces secondary sediment gravity flows that export most of the material to the basin, but also produce turbidity-current over-spill deposits on ridges bounding the slope canyons and on terraces within the Var Canyon. Slump and debris-flow deposits are also observed. At least some of these secondary flows erode the continental slope as cross-cutting chutes on the upper continental slope and erosional scours in the Upper Fan Valley demonstrate. Modern sediment dispersal patterns on the continental slope off Nice are proposed as a modern analogue to lowstand conditions on continental margins. In fact, the absence of a continental shelf together with a steep slope strongly reduces the influence of sealevel on the physiography of the margin.

Structure and tectonics of the Agulhas-Falkland fracture zone

The ∼1200-km-long Agulhas-Falkland Transform developed during the Early Cretaceous break-up of West Gondwanaland. On the African Plate, the continent bordering the Agulhas Fracture Zone extends between the Agulhas Bank and the South Tugela reentrant. It is divided into four distinct parts: Mallory Trough segment (I), Diaz Ridge segment (II), East London segment (III) and Durban segment (IV), from southwest to northeast. Each segment differs from the ohters in its physiography and in the nature of the continent-ocean crustal boundary. In segment I, a wedge-like, eastward-narrowing deep-water basin separates the steep continental slope from a marginal fracture ridge of probable volcanic origin along the transform fault trace. Along segment II the South African continental slope is deeply embayed and a great thickness of sedimentary strata has been ponded behind a buried ridge along the northern side of the fracture-zone trace, above a probable fragment of oceanic crust representing a northerly extension of the Jurassic Falkland Plateau Basin. In segment III, the continental margin is steep, especially along the middle slope; it lies directly along the transform trace, but conspicuously rugged along the continental slope, due to the effects of submarine canyoning and sediment slumping. In segment IV, the continent-ocean boundary also lies directly along the transform fault trace, and the margin shows evidence of seaward tilting above landward-dipping faults along the middle or lower slope. The segmentation is inherited from Jurassic structural fabrics formed prior to Early Cretaceous transform motion, when tectonic rotation of microplates caused rifting and partial oceanization along the future Agulhas-Falkland megafault trajectory.

Gravity current down a steeply inclined slope in a rotating fluid

Abstract. The sinking of dense water down a steep continental slope is studied using laboratory experiments, theoretical analysis and numerical simulation. The experiments were made in a rotating tank containing a solid cone mounted on the tank floor and originally filled with water of constant density. A bottom gravity current was produced by injecting more dense coloured water at the top of the cone. The dense water plume propagated from the source down the inclined cone wall and formed a bottom front separating the dense and light fluids. The location of the bottom front was measured as a function of time for various experimental parameters. In the majority of runs a stable axisymmetric flow was observed. In certain experiments, the bottom layer became unstable and was broken into a system of frontal waves which propagated down the slope. The fluid dynamics theory was developed for a strongly non-linear gravity current forming a near-bottom density front. The theory takes into account both bottom and interfacial friction as well as deviation of pressure from the hydrostatic formula in the case of noticeable vertical velocities. Analytical and numerical solutions were found for the initial (t &lt; 1/ƒ), intermediate (t ≈ 1/ƒ), and main (t » 1/ƒ) stages, where ƒ is the Coriolis parameter. The model results show that during the initial stage non-linear inertial oscillations are developed. During the main stage, the gravity current is concentrated in the bottom layer which has a thickness of the order of the Ekman scale. The numerical solutions are close to the same analytical one. Stability analysis shows that the instability threshold depends mainly on the Froude number and does not depend on the Ekman number. The results of laboratory experiments confirm the similarity properties of the bottom front propagation and agree well with the theoretical predictions.

The crustal structure across the transform continental margin off Ghana, eastern equatorial Atlantic

Forward modeling of a suite of wide‐angle seismic lines across the transform continental margin at the eastern end of the Romanche fracture zone off Ghana has shown the transition from continental to oceanic crust to be confined to a narrow, 6 to 11‐km‐wide zone located at the foot of the steep continental slope. The structure of the adjacent oceanic and continental crusts has also been resolved. These results are confirmed and enhanced by gravity and magnetic models. The crust of the ocean‐continent transition zone is characterized by high velocities (5.8–7.3 km s−1), a high density (3.10 Mg m−3), and high magnetizations (1.1–1.25 A m−1). This rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin. The oceanic crust within 70 km of the margin is abnormally thin compared to normal Atlantic oceanic crust and shows an average thickness of just 4.4 km. We suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded, on at least three sides, by cold continental lithosphere. The preferred seismic model also shows a new fracture zone, 30 km southeast of the ocean‐continent transition, which is characterized by low velocities in the upper crust, a lens‐shaped layer with velocities of 7.2–7.4 km s−1 at the base of the crust, and a crustal thickness of just 3.4 km. The continental crust appears largely unaffected by the proximity of the adjacent oceanic crust. There is no evidence for underplating of the continental crust adjacent to the ocean‐continent transition zone.

Seismic structure of the northern continental margin of Spain from ESCIN deep seismic profiles

By the end of the Carboniferous, the crust of the continental shelf in northwestern Spain was made up of deeply rooted structures related to the Variscan collision. From Permian to Triassic times the tectonic setting had changed to mainly extensional and the northern Iberian continental margin underwent rifting during Late Jurassic-Early Cretaceous times, along with sea-floor spreading and the opening of the Bay of Biscay until the Late Cretaceous. Subsequently, the northern Iberian margin was active during the north-south convergence of Eurasia and Iberia in the Tertiary. A multichannel seismic experiment, consisting of two profiles, one north-south (ESCIN-4) crossing the platform margin offshore Asturias, and another (ESCIN-3) crossing the platform margin to the northwest of Galicia, was designed to study the structure of the northern Iberian margin. The ESCIN-4 stacked section reveals inverted structures in the upper crust within the Le Danois Basin. North of the steep continental slope, ESCIN-4 shows a thick sedimentary package from 6 to 9.5 s, two-way travel time (TWT). Within this latter package, a 40-km-long, north-tapering wedge of inclined, mainly south-dipping reflections is thought to represent a buried, Alpine-age accretionary prism. In the north western part of the ESCIN-3 (ESCIN-3-1) stacked section, horizontal reflections from 6.5 to 8.5 s correspond to an undisturbed package of sediments lying above oceanic-type basement. In this part of the line, a few kilometres long, strong horizontal reflection at 11.2 s within the basement may represent an oceanic Moho reflection. Also, a band of reflections dips gently towards the southeast, from the base of the gently dipping continental slope. The part of ESCIN-3 line that runs parallel to the NW-Galicia coast (ESCIN-3-2), is characterized by bright, continuous lower crustal reflections from 8 to 10 s. Beneath the lower crustal reflectivity, a band of strong reflections dips gently toward the southwest from 10 to 13.5 s. The part of ESCIN-3 that parallels the northern margin (ESCIN 3-3), shows good reflectivity in all levels. Upper crustal reflections image the sedimentary fill of probable Mesozoic to recent basins. Mid-crustal reflectivity is characterized by dipping reflections until 8 s that are probably related to compressional Variscan features. The lower crustal level shows ‘layered’ reflections between 8 and 12 s. Dipping reflections are found below the continental Moho.

From strike-slip faulting to oblique subduction: A survey of the Alpine Fault-Puysegur Trench transition, New Zealand, results of cruise Geodynz-sud leg 2

The Geodynz-sud cruise on board the R/V l'Atalante collected bathymetric, side-scan sonar and seismic reflection data along the obliquely convergent boundary between the Australian and Pacific plates southwest of the South Island, New Zealand. The survey area extended from 44°05 S to 49°40 S, covering the transition zone between the offshore extension of the Alpine Fault and the Puysegur Trench and Puysegur Ridge. Based on variations in the nature and structure of the crust on either side of the margin, the plate boundary zone can be divided into three domains with distinctive structural and sedimentary characteristics. The northern domain involves subduction of probably thinned continental crust of the southern Challenger Plateau beneath the continental crust of Fiordland. It is characterized by thick sediments on the downgoing slab and a steep continental slope disrupted by fault scarps and canyons. The middle domain marks the transition between subduction of likely continental and oceanic crust defined by a series of en echelon ridges on the downgoing slab. This domain is characterized by a large collapse terrace on the continental slope which appears to be due to the collision of the en echelon ridges with the plate margin. The southern domain involves subduction of oceanic crust beneath continental and oceanic crust. This domain is characterized by exposed fabric of seafloor spreading on the downgoing slab, a steep inner trench wall and linear ridges and valleys on the Puysegur ridge crest. The data collected on this cruise provide insights into the nature and history of both plates, and factors influencing the distribution of strike-slip and compressive strain and the evolution of subduction processes along a highly oblique convergent margin.

Sampling the oxygen minimum zone off Pakistan: glacial-interglacial variations of anoxia and productivity (preliminary results, sonne 90 cruise)

During cruise sonne 90 the well developed oxygen minimum zone (OMZ) in the northeastern Arabian Sea was sampled in great detail to study surface ocean productivity and organic carbon accumulation/preservation during the last hundred to hundred thousands of years. Nearly anoxic conditions on and below the seafloor are indicated by the lack of macrobenthos and nekton, absence of bioturbation, high marine organic carbon accumulation and preservation of laminated sediments between about 300 and 1000 m water depth on the steep continental slope. This pattern approximately coincides with the zone of extremely low O 2 concentrations presently observed in the water column. Accumulation of organic material on the deeper slope near the lower boundary of the OMZ is enhanced by downslope transport. Downcore we observe a complex alternation of dark-colored, laminated, and light-colored, homogeneous facies. In general, laminated, organic carbon-rich intervals were deposited under suboxic bottom water and enhanced productivity conditions, mainly during warm stages (e.g., Bölling/Alleröd and Holocene from 8000 yrs B.P. to Recent). The homogeneous, bioturbated intervals indicate normal bottom water oxygen levels and reduced productivity during or just after cold stages (e.g. Terminations IA and B, and Younger Dryas). The striking facies similarities of the sedimentary record in cores collected from different areas and water depths suggest a unique pattern of late Pleistocene C org accumulation in the northeastern Arabian Sea which depends mainly on regional surface ocean productivity and intermediate water oxygenation. However, the sedimentary record is also influenced by variations in sediment supply from the Makran and Karachi shelf and from Indus River discharge during rapid sea-level changes.

Gulf Stream meanders in the South Atlantic Bight: 2. Momentum balances

Gulf Stream meanders and their associated frontal eddies have been numerically simulated using a model tuned to the southern portion of the South Atlantic Bight. A run which produced events most similar to those observed in nature was identified, and the dynamics of a particular meander event were examined in detail. A complex array of momentum balances are obtained, and they vary with position within the feature. While the along‐isobath flow is predominantly in geostrophic balance, several of the nonlinear advection terms in both horizontal momentum equations play important roles in the space‐time development of the flow. In particular, the vertical advection of horizontal momentum as well as frictional dissipation are now recognized as important contributors to the dynamical balances in the vicinity of the steep continental slope. It would appear that the semigeostrophic equations constitute the simplest description of this system.

Warm core ring dynamics derived from satellite imagery

A reinterpretation of the life history of warm core ring 82‐B is constructed from drifter trajectories, hydrographie profiles, and satellite‐derived sea surface temperature fields. An analysis of the observations strongly suggests this ring is fundamentally a dipole structure for much of its existence. The dipole has unequal strength vortices, with the cyclone being the weaker component. Quantification of the dipole indicates the anticyclone is oval shaped and rotates at approximately 7.7° per day, while the cyclone revolves around the anticyclone at about 14.5° per day, yielding an orbit period of approximately 25 days for the cyclone. The elusive part of the dipole explanation is in interpreting its surface signature as a function of time, as the cyclone appears to disappear when in proximity to the steep continental slope. Vortex modeling suggests that when the weaker cyclonic eddy is placed close to a boundary, the cyclone is drawn into a filament as it is advected through the gap between the anticyclone and the boundary. This means the cyclone is either in a filamentation state or undergoing the straining associated with filamentation approximately three fourths of the time, which is in complete agreement with the satellite imagery. In addition, the 82‐B dipole configuration is sufficiently robust to survive continuous interaction with the continental slope and the onset of a Gulf Stream interaction. Although only warm core ring 82‐B is analyzed in detail (and found to be a vortex pair), dipole configurations are found in the warm core ring 81‐F time series as well.