Shelf Edge Off Research Articles

A high-resolution time-series analyses of particle fluxes in the Northern Benguela coastal upwelling system: carbonate record of changes in biogenic production and particle transfer processes

Fluxes of biogenic particles at the shelf edge off Walvis Bay, Namibia, are investigated from a time-series sediment trap in order to evaluate the response of biogenic production to rapid changes in the dynamics of the upwelling process, as well as the importance of particle transfer processes on the nature of sediments accumulating at a high rate on the Namibian margin. Total mass flux displays a smooth trend of lowered values from the start to the end of the experiment despite evidence, from the SST and wind records, of a variable pattern of upwelling dynamics. Contributions of the various biogenic components (carbonates, opal, organic matter) to the total mass flux were relatively constant throughout the period of trapping, suggesting that changes in upwelling dynamics have no obvious implications on the nature of biogenic particles sedimenting at the trap location. A detailed examination of the carbonate fraction indicates a partitioning of its various contributors (coccolithophores and planktonic foraminifera) in terms of sources and mechanisms of transfer to depth. From these data, it is suggested that the bulk of the biogenic particles sedimenting on the slope is resuspended material from the outer shelf, and conversely that direct input from surface waters does not contribute to a high extent to the particle flux at depth. A highly partitioned, three-dimensional flow field is proposed to explain the observed pattern of particle flux as well as the suggested transfer processes affecting the biogenic components.

Evaluation of a conservation strategy: a spawning aggregation closure for red hind, Epinephelus guttatus, in the U.S. Virgin Islands

Many tropical reef fishes spawn in large aggregations, which are readily targeted by fishers. By the 1980s, at least two grouper spawning aggregations were eliminated by intensive fishing off the island of St. Thomas, U.S. Virgin Islands, and another aggregating species, red hind, was intensively targeted. By 1988, the average length of red hind had greatly decreased to 295 mm, and the sex ratio was extremely skewed to 15 females per male, suggesting a heavily fished stock. Since this species is a protogynous hermaphrodite, the loss of large individuals (primarily males) could potentially result in sperm limitation in spawning aggregations. In 1990, a spawning aggregation closure was implemented. By 1997, average size of red hind had increased to 395 mm and sex ratio had shifted to 4 females per male. Fish were observed aggregating only in structurally complex habitat along the insular shelf edge. This habitat type is apparently not common along most of the shelf edge off St. Thomas and may provide shelter while reducing risk of predation during aggregation periods. These data suggest that protection of spawning aggregations is a sound management strategy with considerable potential for aiding the sustainable use of reef fish resources.

Profiles of ectoenzymes in the Indian Ocean: phenomena of phosphatase activity in the mesopelagic zone

Phosphatase (P-ase) activity was determined together with other extracellular enzyme activities, bacterial abundance and production rates during the 2 SW Monsoon process studies of the German JGOFS Arabian Sea Program. Water samples were collected along the cruise tracks from the equator to the upwelling region at the shelf edge off Oman. Depth profiles of P-ase activity were strikingly different from those of the other enzymes. While values of aminopeptidase and β-glucosidase generally decreased below the euphotic zone, P-ase increased by factors of 1 to 7. The relation between peptidase- and P-ase activity was from 4 to 21 at the surface and from 3 to 5 at 800 m depth. Because P-ase production (dissolved and cell-bound) in deep waters is mainly dependent on bacteria, P-ase activities per bacterial cell were calculated: these were, on average, 37 times higher at 800 m than at the surface. We also observed a positive correlation of P-ase activity with phosphate concentrations in the depth profiles below the euphotic zone, while this relationship was much more variable in the mixed surface layer. These observations suggest that C-limited bacteria in the deep strata did not primarily focus on the phosphate generated by their P-ase activity but on the organic C compounds which were simultaneously produced and which could probably not be taken up prior to the hydrolytic detachment of phosphate. It is hypothesised that a considerable part of the measured P-ase activity was dissolved (though it might have originated from bacteria). These enzymes may be important for the slow, but steady regeneration of phosphate and organic C in mesopelagic waters.

Phototrophic picoplankton community structure in three different pelagic regimes in the Arabian Sea

Cell numbers and vertical distribution of Synechococcus, eucaryotic phototrophic picoplankton and prochlorophytes as determined by epifluorescence microscopy are presented from 3 different epipelagic systems (the mesotrophic coast off Oman, the oligotrophic central Arabian Sea and the shelf edge off Pakistan) in the Arabian Sea during the inter-monsoon period 1987. The vertical distribution patterns are described in relation to the epipelagic structure of the respective study sites. Synechococcus dominated phytoplankton in the upper mixed layer and prochlorophytes at the bottom of the euphotic zone in the lower part of and below the deep chlorophyll maximum High pressure liquid chromatography analysis confirmed the presence of prochlorophytes and revealed vertical profiles of characteristic pigments of picoplankton orgamsms, chlorophyll (chl) a and b, dmnyl chl a (chl a;) and zeaxanthin. Cellular pigment concentrations of Synechococcus and prochlorophytes are presented.

The influence of advection on the spatial variability of nutrient concentrations on the Texas-Louisiana continental shelf

Water column nutrient concentrations are presented for winter 1989 over the continental margin of the northwest Gulf of Mexico. Three physical processes advect waters of different nutrient concentrations onto the Texas-Louisiana shelf. These advection processes: (1) river discharge; (2) bay discharge; and (3) shelf-edge upwelling, influence the large-scale spatial variations of nutrient concentrations observed over the shelf. On the inner shelf near Atchafalaya Bay, river discharge injects high concentrations of silicate, phosphate, nitrate and nitrite into near surface water. Farther west, near Galveston and Port Aransas, inner shelf waters have high silicate and phosphate concentrations due to nutrient enrichment in the discharges from Galveston Bay and Corpus Christi Bay. Finally, upwelling at the shelf edge can introduce high nutrient midwater near the base of the mixed layer over the outer shelf. This upwelled water usually has higher silicate, phosphate and nitrate concentrations than those in the overlying surface water. However, if upwelled from below the nitrite maximum, the upwelled water can have quite low nitrite concentrations. The shelf edge upwelling is a result of bottom Ekman upwelling caused by a northeastward current on the outer shelf. At the shelf edge off Galveston the data show that upwelling was not occurring at the time of the study but dissolved oxygen saturation levels and nutrient concentrations show that it had occurred.

The sedimentary record of late glacial changes in cores from the shelf edge off Mauritania, northwest Africa

The impact of late glacial changes on the sedimentary record was investigated in two long vibracores, collected from the shelf edge off Mauritania, northwest Africa. Lithology and radiocarbon dates indicate that the sedimentary sequences were mainly controlled by sea-level changes on the shelf. The upper Pleistocene sequence is characterized by deposition in coastal environments, while the Holocene sequence represents deposition in shelf environments. During low sea level, much sediment was supplied to the present outer shelf, and the data imply an average accumulation rate of up to 43.0 cm/1000 yrs during the late Pleistocene, which is substantially higher than the Holocene rate. Shelf sediments were continuously reworked and redistributed on a regional scale during falling and rising sea level. The presence of reworked material results in radiocarbon ages which are too old. The mollusc. Venus striatula, which presently is found north of, but not along, the Mauritanian coast, occurs in the upper Pleistocene sequence, suggesting cooler water conditions in the shelf during late glacial times. This species probably migrated to the south during late glacial times, following the southward extension of the cold Canary Current. Radiocarbon dates of the shells broadly coincide with a lowstand of sea level over this part of the continental shelf.

The character and motion of suspended particulate matter over the shelf edge and upper slope off Cape Cod

Time series of suspended particulate matter (SPM) concentration made by transmissometers deployed on the shelf edge and upper slope off New England are examined together with data from companion sensors. At the shelf-edge instrument cluster (on the 125-m isobath) the highest SPM concentrations were apparently due to sediment resuspended further onshore and advected to the transmissometer by offshore motion of the shelf-edge density front. This was due to a tendency for more frequent episodes of high bottom stress, and thus more sediment resuspention, progressing onshore from the outer shelf. The decrease of surface wave current amplitude with depth was primarily responsible for this cross-shelf gradient in bottom stress. On one occasion very turbid water was observed at the shelf edge when the bottom stress was low and the frontal motion was small. Data obtained from the U.S. National Marine Fisheries Service showed unusually intensive bottom trawling activity in the vicinity of the transmissometer during this event. This data further indicates that resuspension by trawlers may be an important source of suspended material on the shelf edge. The record of the transmissometer deployed over the upper slope indicates that transport of particulate material in suspension from the near bottom on the shelf into the slope region occurred primarily along density surfaces, and was not continuous but intermittent.

Evidence of coastal Seiche excitation by tide‐generated internal solitary waves

Coastal seiches at Magueyes Island, Puerto Rico, occur in distinct fortnightly groups with maximum seiche activity following new and full moon by 7 days. The size of the fortnightly groups depends upon the amplitude of the preceding spring tide and upon the Caribbean surface water temperature. Temperature records from the upper thermocline on the slope seaward of the shelf edge off Magueyes Island indicate the arrival of semidiurnal packets of negative internal solitary waves. We suggest that these internal waves are generated by tides in the southeastern Caribbean Sea and that they are responsible for exciting the coastal seiches. This hypothesis is supported by the characteristics of coastal seiching at Puerto Princesa on Palawan Island in the Philippines which indicate excitation by the Sulu Sea internal waves reported by Apel, Holbrook and Tsai (1980).

Seismic refraction study of the continental edge off the eastern united states

Three long, strike-parallel, seismic-refraction profiles were made on the continental shelf edge, slope and upper rise off New Jersey during 1975. The shelf edge line lies along the axis of the East Coast Magnetic Anomaly (ECMA), while the continental rise line lies 80 km seaward of the shelf edge. Below the unconsolidated sediments (1.7–3.6 km/sec), high-velocity sedimentary rocks (4.2–6.2 km/sec) were found at depths of 2.6–8.2 km and are inferred to be cemented carbonates. Although multichannel seismic-reflection profiles and magnetic depth-to-source data predicted the top of oceanic basement at 6–8 km beneath the shelf edge and 10–11 km beneath the rise, no refracted events occurred as first arrivals from either oceanic basement (layer 2, approximately 5.5 km/ sec) or the upper oceanic crust (layer 3A, approximately 6.8 km/sec). Second arrivals from 10.5 km depth beneath the shelf edge are interpreted as events from a 5.9 km/sec refractor within igneous basement. Other refracted events from either layers 2 or 3A could not be resolved within the complex second arrivals. A well-defined crustal layer with a compressional velocity of 7.1–7.2 km/sec, which can be interpreted as oceanic layer 3B, occurred at 15.8 km depth beneath the shelf and 12.9 km beneath the upper rise. A well-reversed mantle velocity of 8.3 km/sec was measured at 18–22 km depth beneath the upper continental rise. Comparison with other deep-crustal profiles along the continental edge of the Atlantic margin off the United States, specifically in the inner magnetically quiet zone, indicates that the compressional wave velocities and layer depths determined on the U.S.G.S. profiles are very similar to those of nearby profiles. This suggests that the layers are continuous and that the interpretation of the oceanic layer 3B under the shelf edge east of New Jersey implies progradation of the shelf outward over the oceanic crust in that area. This agrees with magnetic anomaly evidence which shows the East Coast Magnetic Anomaly landward of the shelf edge off New Jersey and with previous seismic reflection data which reveal extensive outbuilding of the shelf edge during the Jurassic and Lower Cretaceous, probably by carbonate bank-margin accretion.

The seismicity of the norwegian and Greenland Seas and adjacent continental shelf areas

The seismic activity of the Norwegian and Greenland Seas and adjacent areas has been examined in view of the tectonic evolution of the North Atlantic. The 529 earthquakes used covered the period 1955–1972, and for fifteen of these events fault-plane solutions were available. An analysis was made of the location precision which turned out to be better than 20 km in most cases. Expectedly, little new evidence was obtained at the midoceanic ridges and major fracture zones, with possible exceptions of the Knipovich Ridge showing a well-defined seismicity belt supporting the idea of an active spreading ridge, and the Spitsbergen Fracture Zone, which seems to be a system of en-echelon faults. Most interesting is a weak linear event pattern in the Lofoten Basin, possibly giving evidence of unknown structures parallel to the Greenland and Senja Fracture Zones, although sediment loading also may be important. Earthquakes along the shelf edge off Norway are located at or near isostatic gravity belts which may act as hinge lines for the marginal subsidence, thus implying stress release caused by differential subsidence of the continental crust. Part of the seismicity of eastern Greenland and western Norway appears to be related to zones of weakness of pre-Cenozoic age. The seismic activity along the edges of the Norwegian Channel is very limited.

Shallow Structures: Shelf Edge of Continental Margin Between Cape Hatteras and Cape Fear, North Carolina: GEOLOGICAL NOTES

Two types of structural framework are predominant on shallow seismic profiles recorded at the continental shelf edge off North Carolina between Cape Hatteras and Cape Fear. At least six prominent progradational sedimentary units, which are separated by disconformable surfaces, indicate that the continental shelf edge seaward of Cape Fear has been formed mainly by outbuilding. Uniformly thick parallel beds predominant north of Cape Fear are the result of relatively uniform upbuilding-outbuilding processes. Within this parallel bedded structure, a distinct erosional surface can be traced from the outer shelf to the upper continental slope, where it is truncated by another buried erosional surface. Surface topographic features near the shelf break off North Carolina have been formed largely by erosional processes, although locally organic accretion may have occurred. Small terraces and ledges that are exposed or buried were eroded into the outer shelf and upper slope during pauses in the fluctuating sea levels of the Pleistocene and Holocene Epochs. Notches on the upper continental slope appear to have two origins: (1) erosional, i.e., the notch was carved by the Gulf Stream, and (2) erosional or nondepositional, i.e., the notch was formed between the exposed back slope of a terrace and the sedimentary wedge occurring on the terrace platform.

Sources and dispersion of recent sediments of the Adriatic Sea

The shelf of the Adriatic basin is mostly covered by regressive relict sands deposited by the Pleistocene Po during low stands of sea level of the post-Monastirian regression-Würm Glaciation. Modern deposition over the shelf is limited to mud and sandy mud zones off the Po delta, to a nearshore strip of fine sand and to an offshore belt of mud sediments. The shelf sands belong to the Padane heavy mineral province between the Po delta and the shelf edge off Pescara, and to the Venetian province forming a broad band facing Venice. The principal source of the Padane sediments is the Po basin, which supplies unstable heavy mineral suites derived from igneous and metamorphic rocks of the central and western parts of the Alpine chain. Components belonging to the Venetian province are supplied by the Adige and the other Venetian rivers draining volcanic rocks of the Eastern Alps. In the central and southern Adriatic, mud deposition is predominant. Only an elongate zone is covered by Pleistocene sands redistributed in deeper water by displacement and reworked by wave motion. These bathyal sands belong to the southern-augite province. The principal source is to be ascribed to the volcanic Ofanto-Vulture Basin of the southern Apennine and to the Pleistocene-Holocene volcanic ash of the Vulture, Vesuvius, and the other volcanoes of central and southern Italy. The Dinaric chain in central Albania is the main source of the Albanese province. The heavy minerals of the Adriatic sediments reflect the composition of the source areas. Modification of the mineralogical composition is restricted to Pleistocene and Early Holocene weathering of relatively unstable minerals. Vector analysis of heavy mineral data has established more clearly a mainly longitudinal dispersal pattern on the Adriatic shelf and a transversal one in the central and southern portions of the basin.

Detailed sedimentary velocities from seismic refraction profiles in the Western North Atlantic : Houtz, R. E. and J. I. Ewing, 1963. J. Geophys. Res., 68 (18): 5233–5258

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.