Shelf Edge Off Research Articles

Roughhead grenadier (Macrourus berglax) on the shelf edge of the northeastern Norwegian Sea, 1997–2020: Distribution, abundance, size and age structure, growth

Records of roughhead grenadier (Macrourus berglax) in a time series of bottom trawl surveys along the Norwegian Sea shelf edge off Norway and Spitsbergen (between 68 and 80 °N in ICES subareas 1 and 2) confirmed that the species is widely distributed in the depth range 500−800 m. Abundance consistently declined to low levels towards northern areas along the Spitsbergen shelf. A corresponding decline was not observed towards the south, reflecting that the distribution area extends southwards beyond the sampling area. Throughout the bottom trawl survey time series 1997–2020, abundance and biomass varied without any apparent temporal trend. Size distributions comprised the entire size range and were usually bimodal. The mode of small fish varied and was comparatively high in some years (e.g. 2009 and 2015), suggesting some temporal variation in recruitment. Age distributions and growth data are presented based on data collected in the period 2009−2017.

The winter western boundary current of the South China Sea: physical structure and volume transport in December 1998

The unique survey in December 1998 mapped the entire western boundary area of the South China Sea (SCS), which reveals the three-dimensional structure and huge volume transport of the swift and narrow winter western boundary current of the SCS (SCSwwbc) in full scale. The current is found to flow all the way from the shelf edge off Hong Kong to the Sunda Shelf with a width around 100 km and a vertical scale of about 400 m. It appears to be the strongest off the Indo-China Peninsula, where its volume transport reached over 20×106 m3/s. The current is weaker upstream in the northern SCS to the west of Hong Kong. A Kuroshio loop or detached eddy intruded through the Luzon Strait is observed farther east where the SCSwwbc no more exists. The results suggest that during the survey the SCSwwbc was fed primarily by the interior recirculation of the SCS rather than by the “branching” of the Kuroshio from the Luzon Strait as indicated by surface drifters, which is likely a near-surface phenomenon and only contributes a minor part to the total transport of the SCSwwbc. Several topics related to the SCSwwbc are also discussed.

Differential habitat use of reef fishes on a shelf-edge reef off La Parguera, Puerto Rico

The biotic community at a shelf-edge reef, which abruptly delineates a continuous fringing reef from the adjacent mesophotic reef off southwestern Puerto Rico, USA, was used to identify patterns of differential habitat use in reef fishes. Our study used multivariate analyses to identify groupings within the fish community associated with this area. These groups were suggestive of differential habitat use by the fish community, in particular with respect to the reef-terrace vs slope locations. The benthic composition among these two location stratifications differed while remaining surprisingly consistent within each location strata (i.e., reef-terrace vs slope). The variability in both the fish and benthic communities was best explained in our model by the location stratifications within each site as opposed to the physical position of the site itself along the shelf edge. Food resources, structure (i.e., habitat complexity), or both may contribute to differential habitat use within the reef-fish communities observed. A lesser division was apparent within the reef-terrace locations that grouped the western samples separately from the eastern. The results presented in the present study suggest that the fish communities along the shelf-edge reef adjacent to La Parguera, Puerto Rico, are divided by habitat type and location as determined by food and shelter availability. Here, we present an analysis whereby community structure can be identified at varying spatial scales within a single depth zone to inform managers prior to strategy implementation.

Movement, depth distribution and survival of spinetail devilrays (Mobula japanica) tagged and released from purse‐seine catches in New Zealand

Abstract Mobulid rays are protected in New Zealand, but the spinetail devilray Mobula japanica is caught as bycatch in skipjack tuna purse seine fisheries. Between 2005 and 2014, rays were recorded in 8.2% of observed purse seine sets. Rays were caught during summer, with a ‘hotspot’ (24.3% of sets) near the shelf edge off North Island over seabed depths of 150–350 m. Rays were usually brailed aboard with the tuna catch from successful sets, but were often entangled in the bunt of the net during unsuccessful sets. Observers tagged nine rays with popup archival tags to obtain preliminary information on their post‐release survival, and spatial and vertical movements. Seven of the nine tags reported data, and four of those rays died within 2–4 days of release. All four rays that died had been brought aboard entangled in the bunt. The three surviving rays were all brailed aboard with the tuna catch. One surviving ray remained near New Zealand for 2.7 months during summer, and the other two migrated 1400–1800 km northward to tropical waters near Vanuatu and Fiji at minimum speeds of 47 and 63 km day−1 at the end of summer. Archive data from one ray showed that it made regular vertical movements of 25–100 m amplitude, but spent most of its time shallower than 50 m, more so during the night (89.6%) than the day (76.6%), and mainly experienced temperatures of 18−22 °C. Dives deeper than 200 m were usually made during the day or twilight. All three surviving rays typically moved between the surface and 200–300 m daily, and reached greatest depths of 649 m, 1000 m and 1112 m, respectively, substantially exceeding the previous depth record for this species of 445 m. Recommendations are made for reducing purse seine mortality of mobulid rays by avoiding areas of high ray abundance, avoiding setting on ray‐associated tuna schools, and adopting best‐practice methods of returning rays to the sea from the net or vessel. Copyright © 2016 John Wiley & Sons, Ltd.

The summer foraging ranges of adult spectacled petrels Procellaria conspicillata

AbstractSatellite transmitters were attached to eight adult spectacled petrels Procellaria conspicillata Gould captured during the early incubation period at their breeding grounds on Inaccessible Island, one of the Tristan da Cunha Islands in the central South Atlantic Ocean. Data on their at-sea distribution was obtained for up to six months. All birds remained within the South Atlantic from 24–44°S, with most between 25 and 40°S. Breeding birds mainly foraged in oceanic waters, but failed breeders or non-breeders concentrated their foraging activity over the Rio Grande Rise and the Walvis Ridge and along the shelf break off the east coast of South America. Little foraging occurred along the Benguela shelf break off southern Africa. Non-breeders favoured relatively warm water with low chlorophyll concentrations, reducing the risk of bycatch in fisheries. Tracked birds spent 16% of their time in areas with high levels of tuna longline fishing activity, with overlap greater for non-breeding birds (22%) than breeding birds (3%). Birds in this study foraged in shallower waters along the continental shelf edge off South America than spectacled petrels tracked in this area in winter, potentially increasing their risk of exposure to demersal longline fisheries in this area in summer.

Bryozoan faunal composition and community structure from the continental shelf off Cap de Creus (Northwestern Mediterranean)

Bryozoan specimens obtained in 2009–2010 from the continental shelf off Cap de Creus (Northwestern Mediterranean) were studied. Samples were collected using a Rauschert sled at depths ranging from 61 to 225m. Bryozoans were present in all 26 samples examined, although they were only abundant in 20 of them. A total of 113 species of Bryozoa were identified (2 Ctenostomata, 90 Cheilostomata and 21 Cyclostomata), most of them are well known to science, although a few of the species have barely or never been cited in the Mediterranean Sea (Hincksinoflustra octodon, Alderina imbellis, Escharella immersa, Neolagenipora collaris and Escharina johnstoni), or are currently poorly described (Lagenipora lepralioides). The species Palmicellaria aff. aviculifera (sensu Gautier, 1957) is redescribed, for which the new name of Palmiskenea gautieri is proposed. Species richness, abundance and biomass were linked to the availability of suitable substrates. Multivariate analysis in relation to environmental data showed that the spatial distribution of the bryozoan species was related to the sediment type. Samples from areas dominated by silt and sandy sediments showed few or no bryozoans, whereas coarse sands and gravels presented higher diversity, abundance and biomass. Within the depth range studied, the faunistic composition of the bryozoan assemblages was similar for the whole continental shelf off Cap de Creus. The bulk of bryozoans was found near the canyon rim. This is related to the proximity of the submarine canyon and its associated hydrological processes. The high diversity and abundance of the bryozoan community located on the circalittoral and shelf-edge off Cap de Creus reflect the presence of critical habitats that are essential for the design of marine protected areas.

Forced regressive and lowstand Hudson paleo-Delta system: Latest Pliocene growth of the outer New Jersey shelf

Abstract Interpretation of the latest Pleistocene stratigraphy of the outer New Jersey shelf has evolved over the last three decades with addition of new geological and geophysical data. During the last decade, new results have described and interpreted various depositional elements within the latest Pleistocene that overlie the regional reflector known as the R-horizon. On the basis of observations made from detailed mapping of the R-horizon and the overlying sediment wedge(s) using high resolution shallow seismic (CHIRP) data acquired in several phases between 2001 and 2006, we revise the depositional model for the offlapping, multi-clinothem sediment wedge on the latest Pleistocene outer New Jersey Shelf as strike-elongated lobes of a falling-stage, wave-influenced delta system fed by a pre-LGM paleo-Hudson River Channel(s) that probably reached the late Pleistocene shelf-edge off New Jersey. This sediment wedge is composed of a series of prograding and offlapping clinothems of varying thickness. Within our data, which does not extend to the basinward termination of the wedge, we seldom observe preserved topsets of these clinothems. Parts of the foresets of some clinothems are also not preserved, particularly in the proximal part of the wedge. The sediment wedge is elongated in strike direction (NNE–SSW), which suggests along-strike sediment movement and possible redistribution of sediment in that direction, by ambient shore-parallel currents. Our data do not record any time-equivalent fluvial channels at the proximal end of the sediment wedge, though there are channel features that cap and cut down into several of the clinothems that make up the wedge farther seaward. We suggest that this can be explained in two possible ways: 1) relatively large spatial separation between the prograding front of the delta system and the delta-plain topset deposits, as is very common under a falling-stage sea-level condition; and 2) lack of preservation of the stratigraphic record of the delta-plain channels because of the likely vigorous falling-stage erosion of topsets during the fall of sea level, and possibly also during the subsequent transgressive ravinement and flooding after the LGM. The clinothems within the sediment wedge show systematic variation in thickness, indicating a northward shift of the main depocenter. This shift might also be an indication of gradual northward migration of the principal fluvial feeder. Such a possibility is supported by previously reported late Pleistocene paleo-Hudson Channel deposits south of the modern Hudson Shelf Valley.

Temporal and alongshore distribution of decapod larvae in the oceanic island of Gran Canaria (NW Africa)

Larval transport strongly influences neritic populations of oceanic islands. We investigated the spatial-temporal variability of the assemblage of decapod larvae at the shelf edge off Gran Canaria Island. Environmental and planktonic monitoring were conducted weekly from January to July 2001, at three stations located at different positions in relation to the Trade winds and Canary Current. A total of 5969 decapod larvae of 112 different taxa were recorded. Temporal distribution of decapod larvae abundance displayed two main peaks: the first one took place during February–March associated with the mesozooplankton increase and the second in June–July related to a rise in temperature. The spatial distribution of larvae had a windward–leeward pattern. Larval abundances were much higher in the area of the weak flow around the stagnation point upstream of the island and the warm lee region downstream and were dominated by the benthic species: Galathea intermedia, Philocheras bispinosus, Anapagurus spp., Nanocassiope melanodactyla and Xantho spp. In contrast, on the flank of the island where the current and winds increase, abundances decreased notably with a higher contribution of holopelagic species, such as Gennadas spp. and Deosergestes henseni, suggesting a greater influence of the open ocean waters. Oceanographic conditions related to the island mass effect seems to control the retention zones for decapod larvae. The stagnation point and the lee of the island clearly favour the maintenance of the larval pool nearshore and the self-recruitment in the neritic populations. Consequently, these

Genetic connectivity in scleractinian corals across the Northern Gulf of Mexico: oil/gas platforms, and relationship to the Flower Garden Banks.

The 3,000 oil/gas structures currently deployed in the northern Gulf of Mexico (GOM) provide hard substratum for marine organisms in a region where such has been rare since the Holocene. The major exception to this are the Flower Garden Banks (FGB). Corals are known to have colonized oil/gas platforms around the FGB, facilitating biogeographic expansion. We ask the question, what are the patterns of genetic affinity in these coral populations. We sampled coral tissue from populations of two species occurring on oil and gas platforms: Madracis decactis (hermatype) and Tubastraea coccinea (invasive ahermatype). We sampled 28 platforms along four transects from 20 km offshore to the continental shelf edge off 1) Matagorda Island, TX; 2) Lake Sabine, TX; 3) Terrebonne Bay, LA; and 4) Mobile, AL. The entire population of M. decactis was sampled between depths of 5 m and 37 m. T. coccinea populations were sub-sampled. Genetic variation was assessed using the PCR-based Amplified Fragment Length Polymorphisms (AFLPs). Data were analyzed via AFLPOP and STRUCTURE. Genetic connectivity among M. decactis platform populations was highest near the FGB and decreased to the east. Connectivity increased again in the eastern sector, indicating isolation between the populations from different sides of the Mississippi River (Transects 3 and 4). A point-drop in genetic affinity (relatedness) at the shelf edge south of Terrebonne Bay, LA indicated a population differing from all others in the northern GOM. Genetic affinities among T. coccinea were highest in the west and decreased to the east. Very low genetic affinities off Mobile, AL indicated a dramatic difference between those populations and those west of the Mississippi River, apparently a formidable barrier to larval dispersal.

Pliocene to Recent shallow-water contourite deposits on the shelf and shelf edge off south-western Mallorca, Spain

This paper presents evidence for the presence of shallow-water contourite drifts on the south-western shelf and shelf edge off Mallorca in water depths between 150 and 275 m. These are called the Mallorca contourite depositional system (CDS). The elongate-mounded shallow-water CDS in this area is ascribed to an offshoot of the Balearic Current, which flows north to south through the Mallorca Channel as part of the overall thermohaline circulation in the Mediterranean Sea. Drift geometry suggests that the north–south current is deflected into an east–west flow pattern by interaction with a marked seafloor bathymetry, associated with major fault displacement. Four seismic units separated by three prominent discontinuities can be identified. The three internal discontinuities are correlated to large-scale basin-wide events: the lower Pliocene revolution (4.2 Ma), the upper Pliocene revolution (2.4 Ma) and the mid-Pleistocene revolution (0.9 Ma). The Plio-Quaternary succession has been deposited on top of a Miocene reef, which serves as an acoustic basement and is affected by a large fault, offsetting the basement on average by 150 m. Marked erosional features throughout and further incision of the Sant Jordi Channel along the basement fault in the Pleistocene deposits indicate stronger currents in this period. The Pleistocene deposits also show a pronounced cyclicity, which is tentatively ascribed to climatic variations and the effects of eustatic sea-level fluctuation over the south-western Mallorca shelf at that time.

Seasonal variation in Tsushima Warm Current paths over the shelf off the San’in coast, Japan

Using long-term sea surface temperature (SST) and acoustic Doppler current profiler (ADCP) data, we examined variations in the current axis of the Tsushima Warm Current (TWC) off the San’in coast of Japan, near the entrance to the Japan Sea. There were large horizontal temperature gradients along the shelf edge in the southwestern Japan Sea from October to May, suggesting that the second branch of the TWC appears not only in spring and autumn but also in winter. From the ADCP data analysis, we found that currents with speeds of approximately 20 cm s −1 and greater appeared around the shelf edge off San’in coast in all seasons. The SST and ADCP data analyses suggested that the second branch of the TWC exists around the shelf edge off the San’in coast throughout the year. This finding differed from those of previous studies. A relatively strong current (speed greater than 15 cm s −1) appeared on the shore side in all seasons, except at line W in winter. This current might be the first branch of the TWC. The first branch seemed to occur around in 100 m isobaths, but shifted northward and southward because the bottom topography around lines W and M was relatively flat and the shelf was broad. The first branch was very obscure, and it was difficult to define the two branches of the TWC off the San’in coast from the seasonally averaged vectors. However, snapshots of current distribution derived from the ADCP data clearly showed these branches. Hence, both the first and second branches might occur throughout the year off the San’in coast.

Contrasting modes of shelf sediment dispersal off a high-yield river: Waiapu River, New Zealand

Recent studies of continental margins suggest that small, high-yield rivers are capable of generating shelf sediment-gravity flows, an idea that fundamentally alters our understanding of material flux from the continents to the ocean. Discharge measurements indicate that the Waiapu River, North Island, New Zealand reaches hyperpycnal concentrations (> 36 kg m − 2 ) on a yearly basis. This study contrasts shelf-edge basins with a broad trough along the shelf-edge off the Waiapu River, testing whether there is evidence that shelf sediment-gravity flows propagate to topographic lows. Observations and measurements through geochemical and sedimentological analyses of sediment cores, EM1002 swath bathymetry, and Chirp sub-bottom profiles suggest differing transport modalities on the outer shelf. In general a southern trough-shaped region exhibits high terrigenous inputs and non-steady-state 210Pb profiles, whereas the northern basins contain steady-state 210Pb profiles and increased marine influence. Sediment-gravity flows dominate accumulation in the southern region, whereas within the northern portion, surface plume sedimentation is indicated. Overall this study suggests that sediment-gravity flows could be bypassing the northern basins, perhaps a result of oceanographic influences and bathymetric steering as they seek a more direct route across the shelf.

Alongshore currents and mesoscale variability near the shelf edge off northwestern Australia

Shelf break conditions and alongshore flow off northwestern Australia are studied during the strongly evaporative conditions of austral winter 2003. Present results, along with those of previous authors, confirm that a poleward, fresh Leeuwin current core is normally found near the shelf break. Salinity increases alongshore toward the southwest. Although there is no obvious shelf break front, there is a persistent offshore upward tilting of isopycnals in the depth range of 100–150 m. Repeated mesoscale surveys were made at the shelf edge during 8 days when the shelf break flow was equatorward. Waters offshore of the shelf break contain a rich baroclinic ageostrophic eddy field, with typically 10 km length scales, and the eddy patterns in the upper 60 m are uncorrelated with those below 90 m. The two depth horizons yield energy transfers from mean to eddy potential energy and appear to represent distinct finite amplitude instabilities on upper ocean and 180 m mean velocity cores, respectively. A linear stability model supports the existence of the two vertically separated instability types.

Modeling of sequence geometry north of Gargano Peninsula by changing sediment pathways in the Adriatic Sea

Sea level changes can reorganize sediment pathways on continental shelves in ways that can alter sediment supply and the resulting sedimentary deposits. The Adriatic Sea is one place where changing sediment pathways and along-strike currents have a major impact on sequence architecture. The Adriatic Sea, the marine portion of the Apenninic and Dinaric–Hellenic foreland basins, is being filled longitudinally, similar to other active forelands, and sediment transport patterns dramatically reorganized during Quaternary sea level cycles. We investigate the dynamics of sequence formation in the central Adriatic near the Gargano Promontory and the Mid-Adriatic Deep (MAD), where four depositional sequences each recording 100-kyr glacio-eustatic cycles have been mapped. These sequences are composed primarily of progradational units separated by regional unconformities. The geometry of the units is such that the constitutive clinoforms flatten out at their seaward termination into relatively planar strata, particularly in the upper parts of each sequence. Attempts to numerically simulate the sequences using the modeling software Sequence4 were frustrated by the difficulty of flattening the clinoforms seaward of the rollover (or depositional shelf break). The clinoform flattening observed in the Adriatic sequences contrasts with clinoform and depositional shelf break development that is characteristic of both conceptual and numerical models of sequences, including the one used here. We, therefore, modified the numerical model to account for the changes in sediment pathways that occur in the Adriatic Sea. During times with high sea level, such as the present, sediment from the Po and smaller Apennine Rivers is transported southwards along the coast by marine coastal currents and storms. At times of low sea level, fluvial transport of an enlarged Po River, in which the Apennine Rivers are likely captured as tributaries, discharges directly into the MAD basin. This produces a reciprocal sediment supply pattern where the coastal dispersal at high sea level produces prograding clinoforms, but cuts off as the exposure of the northern shelf at low sea level switches supply to the fluvial system, which discharges into the 260-m deep MAD. When the model is adapted for the shift in supply, the clinoforms flatten as nearshore sediment supply decreases and is replaced by deposition in the MAD during sea level lowstands. Including these sediment supply changes as a function of sea level enabled us to obtain a good fit to the overall stratigraphic architecture, supporting conceptual depositional models based on seismic stratigraphy. Similar dramatic shifts in sediment supply and sequence architecture may also occur at other settings, such as where canyons capture fluvial systems and bypass the terrigenous sediment supply to the deep basin. The paucity of observations of the unusual geometry seen at the Adriatic margin suggests that only rarely does the shore and fluvial sediment discharge overreach the shelf edge and cut off along-strike sediment transport to continental margins.

Gridded isopach maps from the South Pacific and their use in interpreting the sedimentation history of the West Antarctic continental margin

We model sediment isopach grids for the southern Pacific margin of West Antarctica on the basis of a compilation of more than 10,000 km of single‐channel and multichannel seismic reflection data and correlations with ocean drilling sites. Following recent seismic stratigraphic models, we differentiate two main sequences, the upper of which alone is defined by seismostratigraphic indications for frequent grounded ice advances to the shelf edge off West Antarctica. The subsequent modeling of sediment thickness grids allows us to compare the pre‐glacially dominated and glacially dominated sedimentary development of the study area. On the basis of available age constraints from drilling sites, we assume the onset of accumulation of sediments on the continental rise that were supplied by frequent advances of grounding ice on the continental shelf to have occurred at about 10 Ma. The thickest glacial sediment accumulations occur in front of major glacial drainage outlets, i.e., Marguerite Trough on the western Antarctic Peninsula margin, Belgica Trough in the Bellingshausen Sea, and a depression on the inner and middle shelves off Pine Island Bay in the Amundsen Sea. Glacially dominated sedimentation rates of between 140 and 170 m/m.y. are calculated for these sites.

The tributary valley systems of the Almeria Canyon (Alboran Sea, SW Mediterranean): Sedimentary architecture

Three tributary valley systems of the Almeria Canyon (Dalias, Andarax and Gata) located on the Almeria Margin (Alboran Sea, SW Mediterranean) were studied using high-resolution techniques including swath bathymetry and TOPAS seismic reflection profiles. A detailed morphological and seismic stratigraphy study shows that the tributary valley systems (TVS) are formed by two types of valleys: gullies and leveed channels. Dalias TVS is the best developed system, covering an area of more than 300 km 2 from the shelf-edge off Cabo de Dalias to the Almeria Canyon (100–1500 m depth). Andarax TVS covers about 65 km 2, from the shelf off the Andarax River mouth to the westernmost branch of the Almeria Canyon (40–300 m depth). Gata TVS covers more than 100 km 2 from the shelf-edge off Cabo de Gata to the Almeria Canyon (200–650 m depth). Tributary valley systems can be divided into two models based on the detailed morpho-seismic study: the sea level-controlled model and the sediment supply-controlled model. The sea level-controlled model includes the Dalias and Gata TVS. This model is mainly governed by sea level fluctuations, and the valleys grow as a result of the activity of turbidity currents and hemipelagic sedimentation, displaying a retrogressive upslope pattern. The valleys' development would have been favoured by glacio-eustatic oscillations during the Late Quaternary. A tentative chronostratigraphic correlation has been made, relating the evolution of Dalias and Gata TVS with the Marine Isotopic Stages and sea level curves. Tectonics and margin physiography play a secondary role in the development of this type of valley. The sediment supply-controlled model is applicable to the Andarax TVS. The growth of the valleys is controlled by the ephemeral sediment discharges from the Andarax River. The valleys are small-scale gullies located on the shelf and slope, which would grow as a result of the turbidity currents which channel the fluvial sediment supplied in form of hyperpycnal currents. This model would have been developed during the last highstand of sea level, once the coast-line reached its present situation.

Aeromagnetic mapping and reconnaissance geochemistry of the Early Cretaceous Henties Bay-Outjo dike swarm, Etendeka Igneous Province, Namibia

An interpretation of high-resolution aeromagnetic data, backed up by Landsat ETM+ images and field observations, reveals a major NE-trending regional dike swarm in west-central Namibia which we name the Henties Bay-Outjo dike swarm (HOD). The HOD is some 100 km wide and extends at least 500 km from the continental margin, thus ranking among the regionally important dike swarms on the South Atlantic margins. Field relations and radiometric dates indicate Early Cretaceous emplacement ages for the dikes, contemporary with Etendeka Group flood basalts and with the Damaraland intrusive complexes that occur in the same area. The orientation and distribution of dikes within the HOD suggest a strong influence by Damara Belt structures within the first 100 km from the coast. Farther inland, the dikes are more discordant to the Damara Belt and finally the swarm leaves the Damara Belt entirely and crosses into the Angola craton, where dikes fan out to the north and extend for at least another 200 km. Geochemical analysis of about 100 dikes distributed throughout the HOD reveals a compositional spectrum ranging from basalt to rhyolite, with the dominant composition being tholeiitic, low-Ti basalt. The basaltic dikes show some compositional diversity, but most can be assigned to known compositional subtypes of the Etendeka Group and are thus likely to represent feeder dikes to now-eroded lava fields. The silicic dikes have compositional variations (metaluminous to peraluminous, 64–76 wt% SiO 2) matching the range found in the Early Cretaceous Damaraland intrusive complexes, and they only marginally overlap with felsic volcanic units of the Etendeka. These dikes are probably related to the silicic magma systems of the Damaraland complexes. We interpret the HOD as the failed arm of a triple junction centered at the shelf edge off Walvis Bay. Late Cretaceous magmatism in Namibia is plume-related, but we believe the triple junction did not result from domal uplift above a plume. The triple junction coincides with the intersection of three Pan-African orogenic belts: the inland Damara Belt and the coast-parallel Gariep/Dom Feliciano Belt and Kaoko/Ribeira Belt. Mesozoic opening of the South Atlantic propagated northward from the Cape, and when rifting reached the inherited Proterozoic triple junction, extension and magmatism affected all three belts initially but the Damara Belt became inactive shortly thereafter and continental separation followed the coast-parallel belts.

Assemblages of vertical migratory mesopelagic fish in the transitional region of the western North Pacific

Assemblages of vertical migratory mesopelagic fish are described for the nighttime upper 20 m layer of the transitional and adjacent waters in the western North Pacific. Twenty‐three mesopelagic fish species belonging to six families and 16 genera were collected during June 1996. Family Myctophidae was most speciose, represented by 17 species, followed by Sternoptychidae (two spp.) and Bathylagidae (one spp.). Six assemblages are recognized based on their species composition: Subarctic, Northern Transition, Southern Transition, Subtropical, Slope Water and Seamount Assemblages. These assemblages are also characterized by size composition and biomass of the catches. The distributions of the Subarctic, Northern Transition, Southern Transition and Subtropical Assemblages are approximately defined by the Kuroshio and Oyashio fronts and the Subarctic Boundary. Slope Water and Seamount Assemblages are closely related to the slope water around the shelf edge off Japan and waters around the Shatsky Rise and the Emperor Seamount chain, respectively. The importance of the Southern Transition Assemblage area between the Kuroshio front and Subarctic Boundary as spawning and nursery grounds of many subarctic, transitional and subtropical fishes is elucidated, based on the geographical distribution patterns of their larvae and juveniles.

Comparison of deep-water coral reefs and lithoherms off southeastern USA

Two types of deep-water coral bioherms occur off the coast of southeastern United States: Oculina and Lophelia/Enallopsammia. The deep-water Oculina bioherms form an extensive reef system at depths of 70–100 m along the shelf edge off central eastern Florida. These reefs are comprised of numerous pinnacles and ridges, 3–35 m in height. Each pinnacle is a bank of unconsolidated sediment and coral debris that is capped on the slopes and crest with living and dead colonies of Oculina varicosa, the ivory tree coral. In comparison, deep-water reefs of Lophelia pertusa and Enallopsammia profunda corals occur at depths of 500–850 m (maximum 150-m relief) along the base of the Florida-Hatteras slope in the Straits of Florida. On the western edge of the Blake Plateau off South Carolina and Georgia, 54-m high banks of Enallopsammia and Lophelia occur at depths of 490–550 m, whereas on the eastern edge of the plateau the reefs form structures 146 m in height and at depths of 640–869 m. The geomorphology and functional structure of both the Oculina and Lophelia reefs are similar. North of Little Bahama Bank, at depths of 1000–1300 m, a region of bioherms is dominated by the coral Solenosmilia sp.; Lophelia is reportedly absent. This paper summarizes 25 years of submersible studies on the deep-water Oculina reefs, describes submersible reconnaissance of deep-water Lophelia reefs off the southeastern United States, and contrasts these types of bioherms with the deep-water lithoherms in the Straits of Florida west of the Bahamas.

Influence of Sedimentation on the Geomorphology of the Northwestern Continental Margin of India

Based upon the analysis of about 10,000 line km of echosounding and bathymetric data and variations in mass accumulation rate along the NW continental margin of India (between Kori creek and Mumbai), we have deduced that in the northern region (in the vicinity of the River Indus) the shelf-break occurs at a shallower depth and the slope is shallower, has the steepest gradient (<1:20), is smoother with no major features, and has reduced width (slope edge at 1450 m; width 19 km than off Mumbai). The width and depth of the slope edge gradually increases southwards, and is at maximum off DamanMumbai (slope edge depth 2900 m; width 84 km). The intensity of the occurrence of physiographic features also increases southwards. The shelf edge off Saurashtra is undulating and on the slope, regional notches and benches (the most prominent at 560 m) are observed. Further southwards off Khambhat-Mumbai, the slope is characterized by the presence of bathymetric highs and lows. We have also observed numerous features on the shelf, with a variable depth of the shelf-break. The gradient of the continental slope is also reduced from the northern region to the southern region. The variations in the gradient of the slope and the presence of distinct physiographic features in this area are examined vis-a-vis fluvial supply of the sediments into the region.