Narrow Continental Shelf Research Articles

Freshwater transport at Fimbulisen, Antarctica

The intricate near‐circumpolar system of fronts and currents surrounding Antarctica isolates much of Earth's freshwater from the saline oceans immediately north. The Antarctic Slope Front sustains bathymetrically steered flow at the shelf break, whereas the shallow Coastal Current travels rapidly alongside the ice front. A hydrographic survey of the southeastern Weddell Sea finds these two features to have merged near the narrow (<40 km wide) continental shelf at Fimbulisen. On the prime meridian, its Trolltunga ice tongue overshoots the shelf break northward into this slope current. Observations on either side of the ice tongue demonstrate its retarding effect on the westward‐flowing waters it overhangs and its contribution to the poorly understood freshwater budget. From oxygen isotope ratio measurements and referenced geostrophic shears, we find the combined glacial meltwater and sea ice melt transport upstream of Trolltunga at 0.6°E to account for 18.0 ± 5.8 mSv of the total 1.6 ± 0.2 Sv westward transport (Sv = 106 m3 s−1). In Trolltunga's lee and downstream at 2.8°W, we find this figure to ultimately increase to 23.8 ± 15.5 mSv of a total 2.8 ± 0.4 Sv transport. Each of these sections was impeded by sea ice cover, so these estimates of westward transport are probably lower limits. The westward glacial ice meltwater transport of 10 ± 3 mSv at 2.8°W highlights the role that Fimbulisen plays in preconditioning shelf waters before they reach broad continental shelves in the southwestern Weddell Sea where they transform to bottom waters.

Impact of ocean barriers, topography, and glaciation on the phylogeography of the catfish Trichomycterus areolatus (Teleostei: Trichomycteridae) in Chile

We examined the role of several earth history events on the phylogeographic distribution of the catfish Trichomycterus areolatus in Chile using the cytochrome b gene. We explored three biogeographic hypotheses: that sea level changes have resulted in the isolation of populations by drainages; that glaciation has impacted genetic diversity; and that ichthyological subprovince boundaries correspond to phylogeographic breaks in our focal species. We found seven well-supported clades within T. areolatus with high levels of genetic divergence. The strongest signal in our data was for an important role of sea level changes structuring populations. Five of the seven clades mapped cleanly to the geographic landscape and breaks corresponded closely to areas of narrowest continental shelf. In addition, few haplotypes were shared between rivers within clades, suggesting that only limited local movement of individuals has occurred. There was no relationship between the levels of genetic diversity and the proportion of individual drainages covered by glaciers during the last glacial maximum. Two phylogeographic breaks within T. areolatus did match the two previously identified faunal boundaries, but we found three additional breaks, which suggests that faunal breaks have only limited utility in explaining phylogeographic patterns. These results imply that the narrow continental shelf coupled with sea level changes had a strong influence on the obligate freshwater fishes in Chile.

Palaeo-drainage, incised valley fills and transgressive systems tract sedimentation of the northern KwaZulu-Natal continental shelf, South Africa, SW Indian Ocean

Two sequence boundaries are recognised in seismic data collected from the narrow northern KwaZulu-Natal continental shelf. These correspond to late Cretaceous and latest Pleistocene–Last Glacial Maximum (LGM) periods of sea-level fall where fluvial incision into the shelf resulted in several incised valleys having formed. In both examples, the ensuing transgressive valley fills document an evolution from fluvially dominant sedimentation to classic backbarrier estuarine conditions. Central estuary basin sedimentation is most prominent within each fill and is separated from the underlying fluvial sediments by bayline ravinement processes. Tidal-ravinement surfaces separate the upper estuarine sequences from barrier and shoreface deposition, most easily observed in the latest Pleistocene/Holocene valley fills where a Holocene unconsolidated sediment wedge overlies the estuarine fill. The wedge itself is characterised by two packages of sediment separated by a regionally occurring wave/shoreface transgressive ravinement surface which crops out as a semi-indurated shell hash in the mid to outer shelf. Late Cretaceous examples are capped by deposits exhibiting oblique parallel to wavy reflectors interpreted as increasingly energetic deposits analogous to flood-tide deltaic sedimentation. Valley incision and fill are linked to two onshore coastal lake systems, Lake Sibaya and Lake St Lucia and appear to be the product of barrier breaching and fluvial re-adjustment to baseline shifting of these systems during forced regression. Lake St Lucia exhibits a composite palaeo-drainage channel where both sequence boundaries merge within the lake margins. Lake Sibaya has two distinct palaeo-channels for each sequence boundary episode, a late Cretaceous channel which fed a small incised valley within which a modern day submarine canyon is located, the other an LGM channel directed in towards the head of Wright Canyon, the subsequent input of sediment causing this canyon to breach furthest into the shelf than any other canyon in the area. Offshore of Lake St Lucia, the most proximal portions of the associated Leven Canyon occupy the mid shelf and are underlain by Lake St Lucia palaeo-drainage channels of LGM age where erosion along the initial LGM palaeo-channel has occurred. Freshwater exchange between the shelf and the fringing lake system has resulted in the development of sinuous features of Leven Canyon which have eroded in a downslope-fluvial like manner, thus occupying palaeo-drainage lows as erosive flows were entrained. Models proposed here indicate that 1) the limited accommodation space offered by a narrow continental shelf may allow for a greater preservation potential of wave-dominated estuarine facies; 2) the seismic expression of these fills may vary, yet still satisfy similar sedimentological conditions and 3) antecedent valley topography increases the preservation potential of these valley fills by providing a preferred space in which to re-incise and then fill during sea-level rise.

Southeast Alaska: oceanographic habitats and linkages

AbstractWe provide an overview of the physical oceanographic and geological processes that affect marine biological habitats and production in the marine waters throughout the archipelago and continental shelf of Southeast Alaska. Given the paucity of regional data, our overview summarizes work done in adjacent regions of the Gulf of Alaska shelf and basin, and draws on research carried out in similar settings elsewhere. The geological setting, which critically influences the regional meteorology and oceanography, includes a narrow continental shelf, deep channels that permeate the archipelago, fjords, glaciers and a rugged, mountainous coast. The large‐scale meteorology is influenced primarily by seasonal variations in the intensity and position of the Aleutian Low. Winds, freshwater runoff, tides and cross‐shelf exchange control the regional oceanography. The large‐scale flow field advects mass, heat, salt, nutrients and planktonic organisms northward from British Columbia (and even further south) to the northern Gulf of Alaska along the slope, shelf, and within the channels of Southeast Alaska. The deep channels permeating the island archipelago and narrow continental shelf facilitate communication between basin and interior waters. Water properties and flow fields are subject to large annual variations in response to similarly large variations in winds and coastal freshwater discharge. The complex geological setting leads to large spatial heterogeneity in the physical processes controlling the local circulation fields and mixing, thereby creating numerous and diverse marine biological habitats. These various circulation and mixing processes modify substantially Southeast Alaska water masses and thus influence marine ecosystem processes downstream over the northern and western Gulf of Alaska shelf.

A case for deep-ocean CO2 sequestration

Abstract Carbon sequestration (CO2 disposal) may be only a temporary measure for bridging from the current situation in which carbon emissions to the atmosphere are unacceptably high and increasing, to a carbon-free economy, but it is a practical and immediate process that can be undertaken. Sequestration methods vary in effectiveness and cost, and each may have different opportunities, benefits, and drawbacks and periods of time over which the CO2 is retarded from emitting into the atmosphere. Sequestration methods need to be tested on an appropriate scale as quickly as possible because carbon sequestration may help reverse the trend of increasing carbon emissions and remediate the atmosphere for a significant period of time. Among proposed carbon sequestration technologies, temporary storage of CO2 in the deep ocean may be the most practicable for many locations, and possibly the most energy efficient and cost-effective. In addition, an important added value benefit may be derived from deep ocean sequestration. A CO2 hydrate industrial crystallization desalination/disposal process is particularly applicable to oceanic islands and coastal areas adjacent to narrow continental shelves where abyssal depths can be reached by the dense, dissolved CO2-rich water gravity mass flows composed of processed water rejected from the desalination process.

Phylogeography of Philypnodon species (Teleostei: Eleotridae) across south-eastern Australia: testing patterns of connectivity across drainage divides and among coastal rivers

Phylogeographical studies based on DNA sequences offer insights into intraspecific genetic patterns, elucidating the history and structure of populations and their habitats. We used mitochondrial DNA (cytochrome b) to study the phylogeography and population genetics in two sympatric species in the freshwater fish genus Philypnodon throughout south-eastern Australia. We sought to determine how populations were related across drainage divides, and whether transfer among adjacent coastal drainages was related to continental shelf width or intradrainage distance. Phylogeographical structure was greater in Philypnodon macrostomus Hoese & Reader, 2006 compared with Philypnodon grandiceps (Krefft, 1864), with results for P. macrostomus showing evidence for the presence of distinct groupings in different areas of south-eastern Australia. There was evidence of drainage-divide crossings in P. grandiceps in western Victoria, and in P. macrostomus between the Burnett River and the Murray-Darling Basin in south-eastern Queensland. Both species showed low levels of divergence along the narrow continental shelf of New South Wales, but as continental shelf width increased moving north in south-eastern Queensland, population divergence also generally increased. Thus, as the potential riverine connectivity during periods of low sea levels increased, genetic divergences also increased, counter to expectations. Also, population Fst measures did not correlate as predicted with continental shelf width, nor was a significant relationship detected between Fst and the distance between populations. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95, 175‐192.

Late Pleistocene and Holocene palaeogeography of the Lower Tagus Valley (Portugal): effects of relative sea level, valley morphology and sediment supply

Because of the deep glacial incision, the Lower Tagus Valley hosts a sedimentary record since ∼20 000 cal BP, making this a unique site along the European Atlantic margin with respect to palaeogeographic and sea level changes. Based on nine cross-sections and 55 radiocarbon dates together with a newly created relative sea level curve, we constructed five palaeogeographic maps of the infill of the Lower Tagus Valley since ∼20 000 cal BP. We illustrate that relative sea level rise and fluvial sediment supply were the prime forcing factors determining the depositional history and palaeogeographic changes. Around 20 000 cal BP a deeply incised braided river existed, which was directly connected to the ocean across the narrow continental shelf. After that (∼12 000 cal BP) the gradually moister and warmer climate caused a change to a single-channel river. During the following period (12 000–7000 cal BP) relative sea level rise resulted in a transgression in the Lower Tagus Valley and the establishment of extensive tidal environments. After relative sea level rise had ended (∼7000 cal BP) the valley was progressively filled by a fluvial wedge and tidally influenced bayhead delta. Since ∼1000 cal BP the valley-fill history was dominated by increased sediment input due to human-induced degradation of catchment slopes. Generally, climate was of subordinate importance during the entire studied period, merely causing a single-channel river resulting from the change from the cold Heinrich event 1 to the temperate Bölling–Allerød interstadial. Despite the tectonic activity in the region, neotectonic uplift or subsidence was limited, as supported by the horizontal relative sea level curve since ∼7000 cal BP. Neotectonics played a minor role due to the large distance from the Fennoscandian ice sheet and the narrow continental shelf, which prohibited strong glacio- and hydro-isostatic movements.

Dispersion and cycling of organic matter from the Sepik River outflow to the Papua New Guinea coast as determined from biomarkers

The Sepik River is a major contributor of water, sediment and associated organic loads to the coastal waters of northern New Guinea. With a narrow continental shelf and strong coastal currents, much of this exported material is available for long distance transport into the Bismarck Sea and beyond. CTD casts and associated instrument data showed that the river signature was visible in optical measurements of deep profiles from the Sepik Canyon. Discrete water samples were collected in clean Niskin bottles for organic analysis. Additional high volume samples for lipid classes were collected with Infiltrex samplers deployed on a floating mooring, along with two sediment trap arrays set at 100 and 260 m depth. The Infiltrex samplers were set at 55, 180, 200 and 220 m depth in an effort to target the surface layers and those of westward flowing water in the New Guinea Coastal Undercurrent (NGCUC). The samplers allowed analysis of lipid classes in both dissolved and particulate phases. Analyses of lignin phenols, hydrocarbons, fatty acids, sterols, and n-alcohols in the samples were used to estimate the cycling of the organic input from the river system to the coastal waters and possible entrainment in the NGCUC. The molecular biomarkers confirmed the Sepik River as a significant source of reduced carbon in the near coastal zone influenced by the plume. They indicated that ca. 90% of the organic carbon delivered to the coastal zone is dispersed or degraded, with only about 10% of terrestrial carbon being deposited on to the narrow shelf of coastal sediments. The dissolved lignin phenols provided evidence for desorption of organics from particles with increasing depth. However, this desorption process did not change the C/N ratio of the dissolved organic carbon (DOC) in the deep water of the Sepik Canyon. The hydrocarbons, sterols and fatty acids showed the change from lipid content dominated by phytoplankton in surface layers to zooplankton and bacterial biomarkers in deeper waters. The organic biomarkers provided evidence that some of the dissolved organic input from the Sepik River was injected into fast moving undercurrents. However, concentrations of terrestrial biomarkers were diluted and/or degraded to non-detectable levels within 100 km of the source.

Presencia de neonatos y juveniles del tiburón martillo Sphyrna lewini (Griffith & Smith, 1834) y del tiburón sedoso Carcharhinus falciformis (Müller & Henle, 1839) en la costa de Oaxaca, México

The Oaxaca coast in Mexico has a narrow continental shelf and it reaches depths greater than 500 meters near the coast; furthermore it consists of bays and sheltered areas, where high food availability is found. Each week, from March 2001 to December 2005, the unloading areas of artisanal fisheries between the bays of Huatulco and Puerto Escondido in the Oaxaca coast were visited. Data on fishing gear and zones, as well as the species composition of the captures were taken. In the zone, three fishery units operated: shark, pelagic and demersal. All three capture sharks in a direct or incidental way. At the studied region, 419 organisms of S. lewini were registered; from August to September newborn individuals (43.9%, 44 to 64 cm of total length), young of the year (9.3% 65-75 cm Tl) and juveniles (30.3%, 75 to 168 cm Tl) were observed; pregnant females were registered in March, May and June with a fecundity ranging from 18 to 24 embryos (35 to 45 cm Tl). With respect to C. falciformis, 1314 individuals were registered. Newborns (4.7%, 65-71 cm Tl) and young of the year (3.8%, 72 to 80 cm Tl) were observed from January to September; juveniles (65.4%, 80 to 170 cm Tl) were captured throughout the year; from January to November, 52 pregnant females were observed with embryos at different developmental stages. The presence of newborns, young of the year and pregnant females with embryos in advanced development condition, suggest that the coastal waters near the shoreline of the Oaxaca coast are a spawning and nursery area for these shark species.

Organic carbon and nutrient fluxes to the coastal zone from the Sepik River outflow

The Sepik River is a major contributor of water, sediment and associated organic loads to the coastal waters of northern New Guinea and the Bismarck Sea. We compare dissolved and particulate organic carbon data from September 1997 during an extremely dry El Nino year with those from 1996, 1999 and 2000 during La Nina wet season discharges. Estimated Sepik River flux of DOC is 3.2×10 10 mol yr −1 and POC is 1.1×10 11 mol yr −1. The estimates for total river nutrient fluxes to the sea are 1.1×10 10 mol yr −1 for nitrogen and 4.6×10 8 mol yr −1 for phosphorus. The Sepik DOC flux is about equal to that combined from all four major rivers that enter the Gulf of Papua on the south coast of PNG. The Sepik inorganic PIC flux is low (1.4×10 8 mol yr −1) as the river does not drain carbonate soils. With a narrow continental shelf, and strong coastal currents, much of this exported material is available for long distance transport into the coastal Bismarck Sea and beyond. With data from the water column, a sediment trap study, and analysis of a series of sediment cores, we estimate total fluxes for carbon in this relatively remote coastal system. For the mass balance of organic carbon we calculated primary production in the area as 5.6×10 10 mol yr −1. These terrestrial and marine inputs gave a total of ∼2×10 11 mol OC yr −1 entering the near coastal system. Approximately 40% of the total OC load is degraded in the coastal water column or advected away from the immediate area in coastal currents. Approximately 13% of the annual river organic carbon load of exported sediments settles on the narrow coastal fringe and upper continental slope. About 2% of the total is lost through early diagenesis and 5% preserved in the shelf and upper slope sediments. The remaining ∼53% of the sediment organic carbon apparently flows with sediments along the coastal slope or into the deep sea. The carbon/nutrient regime leaving the Sepik River has a C/N molar ratio of 12, and a C/P ratio of 284 which is in between the Atkinson ratio of 550 and the Redfield ratio of 106. The particles settling through the water column in the nearby coastal waters retain a high C/N ratio; while the dissolved organic matter changes to a lower, more marine, Redfield ratio with distance offshore and with depth.

Factors responsible for the differences in satellite-based chlorophyll a concentration between the major global upwelling areas

The aim of this study was to identify the factors responsible for the differences in chlorophyll a concentration (Chl- a) observed between the California, Canary, Humboldt and Benguela upwelling areas. Monthly climatologic values of Chl- a obtained from satellite images, covering the years 1998–2004, revealed that this pigment was higher in the Benguela system than in the other areas. Upwelling intensity, as derived from offshore Ekman transport computations, was higher in the Benguela and Humboldt regions and, for the same upwelling intensity, Chl- a was higher in Benguela than in the other regions. Upwelling intensity appears to be able to drive Chl- a densities through nutrient supply, as nutrients are correlated to offshore Ekman transport. A linear regression model including the fraction of sea surface over the shelf in each 1° × 1° box, nitrate, silicate, turbulence and variability of offshore Ekman transport explained the 84.8% of the variance in Chl- a among the areas. Differences in offshore Ekman transport explained the lower Chl- a observed in Canary and California and the higher Chl- a observed in Benguela and Peru-Humboldt. A narrow continental shelf and low water column stability also contribute to reducing phytoplankton pigment biomass in the Canary and California areas. The higher Chl- a values observed in Benguela compared to Humboldt-Peru are due to a wider extension of the continental shelf in the Benguela region.

Plankton distributions related to hydrography and circulation dynamics on a narrow continental shelf off Northern Norway

Plankton distribution, hydrography and circulation dynamics were investigated in a shelf area off Northern Norway in June 2000 and June 2001. CTD and Optical Plankton Counter data were obtained from the upper 200 m by towing an undulating underwater vehicle across the continental shelf and shelf slope. In both years the front between Atlantic Water and Norwegian Coastal Water was sharp in the region west of the shelf break. In June 2000, convergence zones with lower salinities and higher temperatures than the ambient water were detected in the upper 30 m of the water column over the shelf with a corresponding increase in concentrations of phytoplankton and zooplankton. An interpretation of the field data combined with numerical 3D model simulations indicated that the observed patchiness of phytoplankton and zooplankton in these areas possibly originated from plumes of plankton-rich fjord water trapped in eddies over the banks. Such convergence structures were not observed in June 2001. The differing levels of freshwater discharge and the differing wind conditions before and during the two cruise periods can explain the observed differences in hydrography over the shelf, and are likely to have effect on the plankton distribution.

Impact of the Eastern Weddell Ice Shelves on water masses in the eastern Weddell Sea

We use a primitive equation Ocean General Circulation Model to simulate the ocean circulation regime in the Eastern Weddell Sea. The computer model ROMBAX (Revisited Ocean Model based on Bryan And Cox) is an improved version of an earlier ocean model, which has been developed to allow the simulation of the flow regime in ice shelf covered regions. The Eastern Weddell Ice Shelf (EWIS) region is of particular importance because of its narrow continental shelf and its location at the inflow of water masses from the east into the southern Weddell Sea. We have compared the simulated flow pattern and water properties in the EWIS region with the available sparse observations. While the general observed structure of temperature and salinity is reproduced, the model tends to overestimate the on‐shore flow of warm deep waters. This discrepancy is not large enough to seriously influence the ice shelf – ocean interaction, which is in good agreement with estimates based on field observations. The mean net melt rate is found to be 0.88 m yr−1 (2.1 mSv) and has a strong seasonal cycle. Sensitivity studies with different ice shelf configurations (no melting, no ice shelf, closed cavity) show strong impacts on the water mass properties in the EWIS region, with up to 0.7°C difference in temperature and 0.05 in salinity relative to the control run. Our results suggest that the EWIS region is of substantial importance to water mass preconditioning and formation in the Weddell Sea, although no deep or bottom water formation occurs in the eastern Weddell Sea directly.

Static wave setup with emphasis on damping effects by vegetation and bottom friction

Wave setup can contribute significantly to elevated water levels during severe storms. In Florida we have found that wave setup can be 30% to 60% of the total 100-year storm surge. In areas with relatively narrow continental shelves, such as many locations along the Pacific Coast of the United States, wave setup can be an even larger proportionate contributor of anomalous water levels during major storms. Wave setup can be considered as comprising two components, with the first being the well-known static wave setup resulting from the transfer of breaking wave momentum to the water column. The second, oscillating component, is a result of nonlinear transfer of energy and momentum from the primary (linear) spectrum to waves with length and time scales on the order of the wave groups. Static wave setup is the focus of this paper with emphasis on effects due to internal or surface forces that act on the wave system and cause both dissipation of wave energy and transfer of momentum. In particular, the effects of wave damping by vegetation and bottom friction are considered. Linear wave theory is applied to illustrate these effects and, for shallow water waves, the setup is reduced by two-thirds the amount that would occur if the same amount of energy dissipation occurred in the absence of forces. Effects of nonlinear waves are then considered and it is found, for a shallow water wave of approximately one-half breaking height, that a wave setdown rather than setup occurs due to damping by vegetation and bottom friction. The problem of wave setup as waves propagate through vegetation was stimulated by studies to establish hazard zones associated with 100-year storm events along the shorelines of the United States. These storms can generate elevated water levels exceeding 4 to 6 m and can result in overland wave propagation. As these waves propagate through vegetation and damp, the question arose as to the contribution of this process to elevated mean water levels through additional wave setup.

Transport through Tavernier Creek: a primary flushing pathway for Northeast Florida Bay

Current meter data from a 411-day study are used to characterize the movement of water through Tavernier Creek, a tidal channel in the Upper Florida Keys that connects northeastern Florida Bay with the narrow continental shelf on the Atlantic Ocean side of the Keys. The record reveals active tidal and nontidal exchanges. Strongest flood and ebb current speeds commonly reach 50 cm s−1. Low-frequency exchanges are highly coherent with the across-shelf component of local wind stress over time scales in excess of 2.5 days. Bay-shelf exchanges are investigated in four ways. Current measurements made while a drogue was tracked from one end of the creek to the other provide a relationship that can be applied to the time series of current meter data. Results suggest that ocean water reaches the bay end of the creek on 92% of the floods, and bay water reaches the ocean end on 94% of the ebbs. The Eulerian tidal excursion calculated from the amplitude of the M 2 tidal constituent is 1.42 times the length of the channel, and half-tidal cycle Eulerian displacements are commonly 1.5 times the length of the channel. Salinity measurements over a 165-day period document the arrival of bay and ocean water at a study site at about the midpoint of Tavernier Creek. Results suggest that the creek becomes completely flushed after about 450,000 m3 of water have entered from either end. Histograms of ebb and flood volume transports indicate that half-tidal cycle transports are commonly between 800,000 and 1,100,000 m3. The long-term movement of water through Tavernier Creek is a net outflow from Florida Bay. Results support the idea that Tavernier Creek serves as an effective conduit for exchanging bay and ocean water, and especially for draining the northeast corner of Florida Bay.

Comparison of shelf currents off central California prior to and during the 1997–1998 El Nino

Moored current, temperature, salinity, and pressure data were collected at three sites that transect the narrow continental shelf offshore of Davenport, CA, starting in August 1996 and continuing to the spring of 1998. This data set allowed a comparison of oceanographic conditions prior to (8/96–3/97) and during (8/97–3/98) the last major El Niño. During this El Niño, mean temperatures over the 8-month time period were about 3 °C warmer than during the prior year at all of the sites. Correlations between near-surface and near-bottom temperatures, and between near-surface temperature and wind stress decreased during the El Niño compared to conditions the year before. The mean alongshore currents were more strongly poleward during El Niño at sites over the mid-shelf and near the shelf break. There was a general tendency for the energy in alongshore currents to move toward lower frequencies during the El Niño, particularly at the sites farther offshore. The processes that forced the shelf flows changed in relative importance throughout the study. The local alongshore wind stress was less important in driving shelf currents during the El Niño when much of the wind-induced upwelling was confined to less than 5 km of the coast. The observed strong poleward shelf currents on the mid- to outer-shelf were not clearly tied to local forcing, but were remotely driven, most likely by slope currents. The response of the Davenport shelf to an El Niño event may differ from other areas since the shelf is narrow, the wind forcing is weaker than areas to the north and south, and the shelf may be at times isolated by fronts that form at strong upwelling centers. In the winter, strong storm-related winds are important in driving currents at periods not only in the synoptic wind band, but also for periods on the order of 20 d and longer.

Circulation and shelf–ocean interaction off North Norway

The narrow continental shelf off Troms has a complex topography with deep trenches separating shallow banks. The front between Atlantic Water (AW) and Norwegian Coastal Water (NCW) is found near the shelf break and the currents in the area are topographically steered by the shelf slope. We have investigated the combined effects of topography, seasonal stratification, wind and tides on circulation and shelf–ocean interaction using field measurements and numerical simulations of hydrography and currents. Comparisons between simulation results and field measurements show that the model behaves realistically in terms of the distribution of NCW and general circulation. Effects of wind, tides and topography on the currents are shown to be strongly dependent on the stratification, which shows large seasonal variations in the area. Topographical effects on surface circulation are weakened when stratification increases. During stratified conditions winds have a strong impact on surface circulation and shelf–ocean exchange, and the wind direction is crucial for drift paths and residence times for particles (e.g. plankton) in the shelf area. When stratification is weak, tidal effects combined with topographic steering dominate the circulation.

Interpretation of Seabed Geomorphology Based on Spatial Analysis of High-Density Airborne Laser Bathymetry

Abstract Airborne laser bathymetric (ALB) systems rapidly acquire large, high-quality datasets via variable swath widths that are independent of water depth. Laser bathymetric survey tools have applicability in clear coastal (Case II) waters down to −70 meters depth. Deployed along the southeast Florida (Palm Beach, Broward, and Miami-Dade Counties) coast, an advanced ALB system provided a continuous dataset for 160 kilometers of coast from onshore to 6 kilometers offshore. Digital terrain models developed from this high-density bathymetric data permitted recognition of numerous seafloor features and bathymetric patterns from different image formats. Bathymetric analysis of the 600-km2 survey area on the narrow continental shelf shows inherited lithologic features that are partly covered by surficial sediments. Primary parabathic provinces include: (1) nearshore rocky zones dominated by the Anastasia Formation, (2) coral-algal reef systems (Florida Reef Tract [FRT]), and (3) marine terraces. Secondary sed...

Northern Red Sea: Nucleation of an oceanic spreading center within a continental rift

The northern Red Sea is an amagmatic continental rift in which an oceanic spreading center is beginning to develop. A new compilation of marine geophysical data permits delineation of the structure of the northern Red Sea and of the manner in which the transition from continental to oceanic extension is occurring in this rift. The margins of the northern Red Sea are formed by large, apparently active faults on the seaward edge of the narrow continental shelves. The morphology of the main trough is a series of terraces stepping down to an axial depression. The terraces are a subdued expression of the basement structure, which consists of a series of large rotated fault blocks. The axial depression is an often fault‐bounded axis of deep water that extends south from the Suez triple junction. The rift is segmented along‐strike by throughgoing accommodation zones spaced at 40–60 km intervals along the rift. In the main trough, accommodation zones truncate or offset rift‐parallel bathymetric and gravity features. The axial depression consists of a series of discrete depressions offset from each other and separated by slightly shallower areas corresponding to accommodation zones. Within each segment, the axial depression deepens away from the accommodation zones toward a small deep, a few kilometers across and a few hundred meters deeper than the surrounding seafloor. A pair of small volcanoes is perched on top of the scarps bounding the axial depression on either side of the deep within each segment. The volcanoes are all normally magnetized and interpreted as very young. The crust is uniformly thin (5–8.5 km) throughout the main trough, implying extension evenly spread across the rift through much of its development. As extension recently became concentrated at the axis, melt began to be generated and is focused to a location within the segment where it ascends along faults bounding the axial depression to form the pair of volcanoes flanking the axis. A volcano is also found on the floor of the axial depression in one segment. This isolated volcano appears to be the first step in the development of the seafloor spreading cells that are observed in the central Red Sea. The individual cells then grow and coalesce to become a continuous spreading axis.

Nature of sediment dispersal off the Sepik River, Papua New Guinea: preliminary sediment budget and implications for margin processes

A sediment budget is constructed for the slope and narrow continental shelf off the Sepik River in order to estimate the relative importance of turbid plumes versus bottom gravity transport through a near-shore submarine canyon in the dispersal of sediment across this collision margin. 210Pb geochronology and inventories of Kasten cores are consistent with the northwestward dispersal of sediment from the river mouth via hypopycnal and possible isopycnal plumes. Sediment accumulation rates are ∼5cmyr−1 on the upper slope just off of the Sepik mouth, decreasing gradually to ∼1cmyr−1 toward the northwest, and decreasing abruptly offshore (<0.2cmyr−1 at 1200m water depth). A sediment budget indicates that only about 7–15% of the Sepik River sediment discharge accumulates on the adjacent open shelf and slope. The remainder presumably escapes offshore via gravity flows through a submarine canyon, the head of which extends into the river mouth. The divergent sediment pathways observed off the Sepik River (i.e., surface and subsurface plumes versus sediment gravity flows through a canyon) may be common along high-yield collision margins of the Indo–Pacific archipelago, and perhaps are analogous to most margins during Late Quaternary low sea-level conditions.