East Coast Continental Shelf Research Articles

The effect of a mining dam failure on the genetic diversity and population resilience of marine fishes along the eastern Brazilian coast

We used microsatellite DNA markers to examine the impact of the Fundão Dam failure, one of the world’s worst mining-related environmental disasters, on the genetic diversity of four marine and estuarine fishes across the Brazilian east coast's continental shelf and estuarine ecosystems. The strong mud flow along the Doce River and nearby areas resulted in changes in genetic diversity, with all species exhibiting low genetic diversity indices, primarily in estuarine species, and particularly those with a greater association with the substrate. Furthermore, the low M-ratio values in all species tested indicated a recent decrease in the effective population size. Estuarine species, particularly the slipper sole Trinectes paulistanus, were more directly impacted by the mud plume than marine species. The loss of genetic variability due to population decline is concerning because it is ultimately an important component in determining the ability of marine fishes to adapt to environmental changes. Combining genetic data from four fish species with different behaviours, habitats, life histories, and economic values revealed that genetic diversity was a valuable tool for assessing the effects of the Fundão Dam failure.

The Recent Cypraeidae of Northern New Zealand from the Kermadec Islands to the Poor Knights Islands, Southwest Pacific Ocean (Mollusca: Gastropoda: Cypraeidae)

For researchers, isolated regions at the periphery of species’ distributions hold a peculiar fascination. The causes of their remoteness vary based on: distance (e.g. the Tropical Eastern Pacific), distance and countervailing currents (e.g. the Marquesas), location in a present day gyre (e.g. the Pitcairn Group) or the absence of present day means of veliger transport (e.g. the Vema Seamount). (Daughenbaugh & Beals 2013; Daughenbaugh 2015a & b, 2017). The northern New Zealand Region from the Kermadec Islands (Kermadecs) to the coastal and shelf areas in the northernmost part of New Zealand’s North Island (Northland), including the Poor Knights Islands (PKI), constitute the distributional boundaries for a number of Cypraeidae species. The boundaries are the result of the absence of coastal shelves along the east side of the Kermadec Ridge (Ridge) and precipitous drops to abyssal depths along Northland’s east coast continental shelf. Tropical waters, with their potential to transport Cypraeidae larvae, flow eastward from southern Queensland, Australia, entrained in the Tasman Front which terminates when reaching North Cape, the northernmost tip of Northland. There, the North Cape Eddy captures most of this flow while the remainder, the East Auckland Current (EAUC), flows intermittently southward along the eastern coastal, shelf and offshore areas of Northland into waters incapable of supporting Cypraeidae populations.

Characteristics of storms driving wave-induced seafloor mobility on the U.S. East Coast continental shelf

This study investigates the relationship between spatial and temporal patterns of wave-driven sediment mobility events on the U.S. East Coast continental shelf and the characteristics of the storms responsible for them. Mobility events, defined as seafloor wave stress exceedance of the critical stress of 0.35mm diameter sand (0.2160Pa) for 12 or more hours, were identified from surface wave observations at National Data Buoy Center buoys in the Middle Atlantic Bight (MAB) and South Atlantic Bight (SAB) over the period of 1997–2007. In water depths ranging from 36–48m, there were 4–9 mobility events/year of 1–2 days duration. Integrated wave stress during events (IWAVES) was used as a combined metric of wave-driven mobility intensity and duration. In the MAB, over 67% of IWAVES was caused by extratropical storms, while in the SAB, greater than 66% of IWAVES was caused by tropical storms. On average, mobility events were caused by waves generated by storms located 800+km away. Far-field hurricanes generated swell 2–4 days before the waves caused mobility on the shelf. Throughout most of the SAB, mobility events were driven by storms to the south, east, and west. In the MAB and near Cape Hatteras, winds from more northerly storms and low-pressure extratropical systems in the mid-western U.S. also drove mobility events. Waves generated by storms off the SAB generated mobility events along the entire U.S. East Coast shelf north to Cape Cod, while Cape Hatteras shielded the SAB area from swell originating to the north offshore of the MAB.

Copper distribution and speciation across the International GEOTRACES Section GA03

Copper (Cu) distribution and speciation were characterized along a zonal section in the North Atlantic Ocean from Lisbon, Portugal, to Woods Hole, Massachusetts as part of the U.S. GEOTRACES program. Dissolved Cu profiles displayed many of the same features identified by other researchers, including sub-surface scavenging and a linear increase with depth, but many also exhibited unique properties and geographic trends. Concentrations ranged from 0.43nM at the surface to 3.07nM near the seafloor. The highest concentrations were measured in deep waters to the west of Cape Verde and northwest of the Canary Islands while the lowest concentrations were measured in upper waters, mostly between Mauritania and Cape Verde. The westernmost sampling sites overlying or adjacent to the U.S. east coast continental shelf featured surface maxima that decreased in magnitude moving east toward Bermuda, reflecting declining inputs from Cu-enriched coastal waters and North American aerosols. Free Cu (Cu2+) concentrations were tightly controlled by organic complexation and scavenging across the section with values varying between 1.54fM and 1.07pM. These results provide the first evidence that Cu2+ concentrations are strongly complexed throughout the water column, even in boundary zones where dissolved Cu concentrations are elevated because of local sources. Strong organic ligands (L) acted as a buffer for Cu2+, restricting concentrations to a narrow range (10–100fM) throughout most of the water column. Cu2+ and dissolved Cu were strongly scavenged by suspended particulate matter within several benthic nepheloid layers and a hydrothermal plume above the Trans-Atlantic Geotraverse (TAG) vent field on the Mid-Atlantic Ridge (MAR).

Nitrogen cycling in the Middle Atlantic Bight: Results from a three‐dimensional model and implications for the North Atlantic nitrogen budget

The biogeochemistry of continental shelf systems plays an important role in the global elemental cycling of nitrogen and carbon, but remains poorly quantified. We have developed a high‐resolution physical‐biological model for the U.S. east coast continental shelf and adjacent deep ocean that is nested within a basin‐wide North Atlantic circulation model in order to estimate nitrogen fluxes in the shelf area of the Middle Atlantic Bight (MAB). Our biological model is a relatively simple representation of nitrogen cycling processes in the water column and organic matter remineralization at the water‐sediment interface that explicitly accounts for sediment denitrification. Climatological and regionally integrated means of nitrate, ammonium, and surface chlorophyll are compared with its model equivalents and were found to agree within 1 standard deviation. We also present regional means of primary production and denitrification, and statistical measures of chlorophyll pattern variability. A nitrogen budget for the MAB shows that the sediment denitrification flux is quantitatively important in determining the availability of fixed nitrogen and shelf primary production (it was found to remove 90% of all the nitrogen entering the MAB). Extrapolation of nitrogen fluxes estimated for the MAB to the North Atlantic basin suggests that shelf denitrification removes 2.3 × 1012 mol N annually; this estimate exceeds estimates of N2 fixation by up to an order of magnitude. Our results emphasize the importance of representing shelf processes in biogeochemical models.

Observations of wave‐generated vortex ripples on the North Carolina continental shelf

Sand ripples with wavelengths between 0.5 and 3 m were observed on the bottom across the U.S. east coast continental shelf off North Carolina during three side‐scan sonar surveys in September and December 1999. Ripples were present in about 75% of the survey images, in particular, in regions with coarser sediments. Analysis of surficial sediment samples shows that median grain diameters range from 0.1 to 4.7 mm with large variations on the inner shelf over distances <1 km. The observed ripple properties are consistent with wave‐generated vortex ripples. Analysis of concurrent wave observations indicates that the ripple crests were aligned perpendicular to the average direction of near‐bottom wave‐induced motions during preceding events that were sufficiently energetic to mobilize surficial sediments. Furthermore, the ripple wavelengths proportionality to near‐bottom wave orbital excursions is consistent with wave‐formed vortex ripples. These findings support the hypothesis that the observed strong attenuation of waves across the shelf resulted from form drag over large vortex ripples.

Interaction between surface wind and ocean circulation in the Carolina Capes in a coupled low-order model

Interactions between surface winds and ocean currents over an east-coast continental shelf are studied using a simple mathematical model. The model physics includes cross-shelf advection of sea surface temperature (SST) by Ekman drift, upwelling due to Ekman transport divergence, differential heating of the low-level atmosphere by a cross-shelf SST gradient, and the Coriolis effect. Additionally, the effects of diabatic cooling of surface waters due to air-sea heat exchange and of the vertical density stratification on the thickness of the upper ocean Ekman layer are considered. The model results are qualitatively consistent with observed wind-driven coastal ocean circulation and surface wind signatures induced by SST. This simple model also demonstrates that two-way air-sea interaction plays a significant role in the subtidal frequency variability of coastal ocean circulation and mesoscale variability of surface wind fields over coastal waters.

A Review Of Canadian Frontier Technology Advances In The Search For Oil And Gas

Abstract Exploration activity in Canada's frontier areas started in the early 1960s and has resulted in the discovery of several large oil and gas reservoirs. This paper gives an overview of recent technology advancements in the areas of exploration and drilling. Production and transportation facilities for three crude oil offshore development projects are described to illustrate the innovative technologies being developed to cope with the harsh physical environments in which the developments are located. Introduction Canada's frontier regions include the MacKenzie Delta - Beaufort Sea, the Arctic Islands and the east coast continental shelf, offshore of Newfoundland and Nova Scotia as shown in Figure 1. Environmental conditions are severe with ice being the major feature that differentiates Canada's Northern and Grand Banks offshore areas from other offshore regions. While the Scotian Shelf area is subject to severe winter storms, ice intrusions are rare. In spite of the environmental conditions, over 500 exploration wells have been successfully drilled over the past 25 years. This activity has resulted in the discovery of numerous oil and gas reservoirs, several of which are in various stages of evaluation and development. Significant technological advancements have been made in recent years in the areas of exploration and drilling. Innovative features must be incorporated into the design, construction and operation of production and transportation facilities in order to accommodate the harsh environmental conditions. This paper discusses the technology which has been developed for the Northern and Grand Banks frontier areas, where sea ice and icebergs present a unique challenge to hydrocarbon exploration and production activities. Environmental Conditions The primary environmental constraint in the Beaufort Sea is sea ice, as the area is usually ice covered for at least nine months of the year. Open water conditions exist for two or three months per year but heavy, multi-year ice intrusions are possible at any time. In winter, pressure-ridged landfast ice forms to the 20 m water depth contour. Sporadically moving pack ice is present in deeper water. Sea bottom scouring by ice features is common in water depths to 40 m. The long winter nights are characterized by bitter cold and poor visibility. Sea ice is also the most severe environmental problem in the Arctic Islands region. Thick, ridged, multi-year ice is present year round. The ice in the inter-island area is confined and landfast for most of the year with some small sporadic movements. Break-up and ice drifting can occur in some years. There is the potential for seabed scouring by ice ridge keels in shallow water areas. Activities in this region must contend with extremely cold temperatures, wind chill and poor visibility for much of the year. The environment of the Grand Banks region, offshore Newfoundland, is similar to the North Sea in terms of sea state, winds and currents. The unique environmental characteristic of the region is the seasonal occurrence of icebergs and sea ice. Structural icing, fog and poor visibility are additional adverse environmental conditions. Icebergs pose a collision hazard with fixed or floating systems.

Sediment movement along the U.S. east coast continental shelf—I. Estimates of bottom stress using the Grant-Madsen model and near-bottom wave and current measurements

Bottom stress is calculated for several long-term time-series observations, made on the U.S. east coast continental shelf during winter, using the wave-current interaction and moveable bed models of Grant and Madsen (1979, Journal of Geophysical Research, 84, 1797–1808; 1982, Journal of Geophysical Research, 87, 469–482). The wave and current measurements were obtained by means of a bottom tripod system which measured current using a Savonius rotor and vane and waves by means of a pressure sensor. The variables were burst sampled about 10% of the time. Wave energy was reasonably resolved, although aliased by wave groupiness, and wave period was accurate to 1–2 s during large storms. Errors in current speed and direction depend on the speed of the mean current relative to the wave current. In general, errors in bottom stress caused by uncertainties in measured current speed and wave characteristics were 10–20%. During storms, the bottom stress calculated using the Grant-Madsen models exceeded stress computed from conventional drag laws by a factor of about 1.5 on average and 3 or more during storm peaks. Thus, even in water as deep as 80 m, oscillatory near-bottom currents associated with surface gravity waves of period 12 s or longer will contribute substantially to bottom stress. Given that the Grant-Madsen model is correct, parameterizations of bottom stress that do not incorporate wave effects will substantially underestimate stress and sediment transport in this region of the continental shelf.

Sediment movement along the U.S. east coast continental shelf—II. Modelling suspended sediment concentration and transport rate during storms

Long-term near-bottom wave and current observations and a one-dimensional sediment transport model are used to calculate the concentration and transport of sediment during winter storms at 60–80 m water depth along the southern flank of Georges Bank and in the Mid-Atlantic Bight. Calculations are presented for five stations, separated by more than 600 km alongshelf, that have different bottom sediment texture, bedforms and current conditions. A modified version of the sediment transport model presented by Grant and Glenn (1983, Technical Report to the American Gas Association), Glenn (1983, D.Sc. Thesis, M.I.T.), and Glenn and Grant (1987, Journal of Geophysical Research, 92, 8244–8264) is used to examine the influence of wave-current interaction, sediment stratification, and limitations on the erodibility of the bottom sediments on the concentration of sediment in the water column and on transport. Predicted suspended sediment concentrations are higher than observed, based on beam transmissometer measurements, unless an erosion limit of order a few millimeters for sediments finer than 94 μm is imposed. The agreement between predicted and measured beam attenuation is better at stations that have significant amounts of silt plus clay in the surficial sediments than for stations with sandy sediments. Sediment concentrations during storms estimated by Moody et al. (1987, Continental Shelf Research, 7, 609–628) are within 50% of the model predictions. Sediment transport rates for sediments 94 μm and finer are determined largely by the concentrations in the surficial sediment and the erosion depth limit. Large alongshelf transports in the direction of storm-driven currents are inferred for stations in the Mid-Atlantic Bight. During a 115-day period in winter 1979–1980, the net transport of sediment along the shelf was westward; benthic storms (defined as periods when the bottom wave stress exceeded the current stress by 2 dyn cm −2) occurred between 23 and 73% of the time, and greater than 91% of the net alongshelf transport was during storms.

Shallow water acoustics on the Canadian East Coast Continental Shelf

The Canadian East Coast Continental Shelf is a large area (roughly 600 000 km2) of complex bottom types geologically related to the glacial cover and low water level of the recent Ice Age. The bottom types include thickly sedimented sand banks under less than 100 m of water, thickly sedimented silt/clay basins under more than 200 m of water, thinly covered or bare glacial till, and outcropped rock. This paper describes DREA measurements of underwater acoustic propagation and ambient noise in the 1‐ to 1000‐Hz frequency band, and attempts to explain these measurements. Acoustic propagation over thickly sedimented seabeds can be explained by models that assume “fluid” sediment layers (i.e., no shear waves). However, over thinly sedimented rocky seabeds the acoustic models need to include shear waves to explain the measured acoustic propagation, in particular the high losses between 10 and 100 Hz. Ambient noise is affected by the seabed, mainly by the difference in acoustic propagation over unlike seabed types, from distant sources.

Near-bottom suspended matter concentration on the Continental Shelf during storms: estimates based on in situ observations of light transmission and a particle size dependent transmissometer calibration

A laboratory calibration of Sea Tech and Montedoro-Whitney beam transmissometers shows a linear relation between light attenuation coefficient ( c p ) and suspended matter concentration (SMC) for natural sediments and for glass beads. However the proportionality constant between c p and SMC depends on the particle diameter and particle type. Thus, to measure SMC, observations of light attenuation must be used with a time-variable calibration when suspended particle characteristics change with time. Because of this variable calibration, time series of light attenuation alone may not directly reflect SMC and must be interpreted with care. The near-bottom concentration of suspended matter during winter storms on the U.S. East Coast Continental Shelf is estimated from light transmission measurements made 2 m above the bottom and from the size distribution of suspended material collected simultaneously in sediment traps 3 m above the bottom. The average concentrations during six storms between December 1979 and February 1980 in the Middle Atlantic Bight ranged from 2 to 4 mg l 1 (maximum concentration of 7 mg l 1) and 8 to 12 mg l 1 (maximum concentration of 22 mg l 1) on the south flank of Georges Bank.

Generation of 2–3‐hour oscillations on the east coast of New Zealand

Abstract Observed sea‐surface oscillations with periods of about 2.5 hours in New Zealand east coast harbours have previously been regarded as a response to edge waves on the continental shelf and slope, generated by atmospheric forcing and occasionally by tsunamis (Heath 1976, 1979). Calculations of the longshore wavelengths of the edge waves associated with the bathymetry of the east coast continental shelf and slope, and the Chatham Rise (a rise extending 1000 km perpendicular to the east coast of South Island), suggests that the Chatham Rise also takes part in the generation mechanism by acting as a three‐quarter wavelength aerial to double‐sided edge waves.

An instrument system for long‐term sediment transport studies on the continental shelf

A bottom‐mounted instrument system has been designed and built to monitor processes of bottom sediment movement on the continental shelf. The system measures bottom current speed and direction, pressure, temperature, and light transmission and photographs the bottom. The system can be deployed for periods of 2–6 months to monitor intermitent processes of sediment movement such as storms and to assess seasonal variability. Deployments of the system on the U.S. east coast continental shelf show sediment resuspension and changes in bottom microtopography due to surface waves, tidal currents, and storms.

Circulation on the Continental Shelf

A simple analysis is presented of a hypothetical shelf-circulation that could apply to the eastcoast continental shelf of North America under winter conditions.

CYCLONIC MICROSEISMS GENERATED IN THE WESTERN NORTH ATLANTIC OCEAN

Nearly all the microseism storms generated by extra-tropical cyclones in the western North Atlantic during the fall and winter seasons of 1948–49 and 1949–50 have been studied, together with some storms that occurred during the 1950–51 fall-winter. Seismograms from a number of east-coast stations and Bermuda were examined for each storm. Each of the microseism storms has been correlated with a particular marine atmospheric disturbance. The period of the microseisms appears to be a function of the depth of water in the generating area, and the regularity of the microseisms appears to be a function of the uniformity of depth in the generating area. Consequently, the position and nature of marine disturbances often can be estimated from the nature of the recorded microseisms. All the evidence on origin tends to negate ocean swell, standing waves produced by interference of ocean swell, and surf as mechanisms producing the observed microseisms. The study suggests atmospheric pulsations, resulting from turbulence or instability, to be at the root of microseism origin. Anomalies of propagation of hurricane microseims suggest the existence of a zone or surface of discontinuity along at least a portion of the outer margin of the east-coast continental shelf.