Eastern Canadian Continental Shelf Research Articles

Transport of fine-grained sediment in oceanic currents: Holocene supply to sediment drifts around Flemish Cap by the Labrador Current

Hemipelagic sediment on the upper continental slope is commonly transported from the adjacent continental shelf, but along-slope transport of sediment by thermohaline currents may be an important source of sediment. One such case is Flemish Cap, a promontory of the eastern Canadian continental shelf protected from direct continental sediment supply by the deep-water Flemish Pass. Pleistocene sediment drifts around Flemish Cap accumulated glacially derived sediment from the north, transported by the Labrador Current. This study determines the changing sources and controls on the accumulation of bioturbated hemipelagic sediment through the Holocene. From a set of >150 box cores in water depths of 600–1200 m, some 30 push cores, up to 55 cm long, were studied in detail for stratigraphic variation in grain size, bulk geochemistry (XRF) and detrital petrology (granule petrology and qXRD). Criteria were developed to identify trawling disturbance. Two Holocene stratigraphic units, A) almost lacking ice-rafted detritus and B) with some coarse IRD and higher detrital carbonate, correlate with similar units farther north on the Labrador Shelf and Slope. Sedimentation rates based on 26 radiocarbon dates were ~ 0.02–0.03 m/ka over much of the region below the 800 m isobath, locally reaching 0.1 m/ka in some sediment drifts, but as low as 0.01 m/ka on southeastern Flemish Cap. Where sedimentation rate was >0.03 m/ka, the record of flow vigor from the sortable silt proxy since 6 ka shows three different zones of Labrador Current flow with different timing of changes in flow vigor. The mid to early Holocene record shows that the meltwater-driven inner Labrador Current extended across the entire study area. Where sedimentation rates are lower, periods of sediment bypassing are inferred and the sortable silt record varies little. Correlation with external drivers is hampered by century-scale core resolution and sparse dating control. The distinctive abundance of dolomite rock flour in hemipelagic sediments around Flemish Cap indicates important supply from Baffin Bay icebergs during the entire Holocene, diluted prior to 6 ka by sediment from Hudson Strait and some Labrador ice streams. Baffin Bay icebergs melt preferentially in the southern Labrador Sea, dropping their load, with the fine grained component transported to the south by the Labrador Current. Similar partitioning of coarse- from fine-grained components of IRD may take place on other high-latitude outer continental margins.

Bottom water methane sources along the high latitude eastern Canadian continental shelf and their effects on the marine carbonate system

Measurements of dissolved methane (CH4) and carbonate system parameters were conducted in shelf and slope waters off Baffin Island and northern Labrador during September 2012 and August 2016 in order to investigate potential cold seeps and their influence on the marine carbonate system. Evidence of a strong near-bottom methane source was found at two sites along the Baffin Island Shelf (Scott Inlet and Cape Dyer) and above a pair of reported sea floor mounds in the Saglek Basin between Baffin Island and northern Labrador. Stable carbon isotope measurements of dissolved methane made at the Cape Dyer site give an estimated δ13C value of −71.2‰ for the source, consistent with microbial methanogenesis. The distributions of pCO2, pH and aragonite and calcite saturation states showed no evidence of enhanced ocean acidification in the vicinity of these sites, so it would seem that these seeps were not significant local sources of carbon dioxide, either as a component in the seep fluid or produced subsequent to release by water column microbial methane oxidation, nonetheless, aragonite saturation states of one or less were seen along the Baffin Island Shelf suggesting that ecosystems here may be vulnerable to further acidification. Spectrophotometric pH measurements of fresh samples in 2012 revealed a bottom water spike in pH at two seep sites that were not replicated by measurements of preserved seawater samples. We hypothesise that this was due to an unstable anionic compound present in seep fluid that can locally increase total alkalinity, pH and calcium carbonate saturation with HS− being a likely candidate, possibly to the benefit of calcifying organisms in an environment bordering on aragonite under-saturation.

Mean circulation and high-frequency flow amplification in the Sable Gully

The Sable Gully, a broad, shelf break submarine canyon approximately 40km east of Sable Island on the eastern Scotian Shelf, separates Banquereau and Sable Island Banks. Unique among canyons on the eastern Canadian continental shelf because of its depth, steep slopes and extension far onto the shelf, its ecological significance and increasing human pressures led to its designation in 2004 under Canada's Oceans Act as the first Marine Protected Area (MPA) in the Atlantic Region. To improve the state of knowledge of the Gully MPA, a multi-disciplinary field program was carried out in 2006–07; the physical oceanographic component consisted of the deployment (April 2006) and recovery (August 2007) of four current meter moorings and CTD surveys. Analysis of this 16-month mooring deployment demonstrates that the mean circulation above the canyon rim (~200m) is characterized by a southwestward flow that appears unaffected by the canyon topography. There is also some indication of the existence of an eddy at rim depth. Below 500m, the circulation is dominated by an upcanyon flow (of order 0.02ms−1) at the mooring array (halfway between the canyon head and mouth). The mean, 200m-bottom transport towards the head of the Gully was estimated as 35,500m3s−1, implying an upwelling velocity of 1.7×10−4ms−1 (14md−1) over the area. Results also show bottom-intensified tidal flows and non-linear constituents due to the interaction of K1, O1, M2 and S2 components along the thalweg of the canyon; the strong overtides and compound tides observed in the Gully make it unique among canyons. Further analyses provide evidence of enhanced mixing in the Gully (Kv~180×10−4m2s−1), which is approximately 20 times that observed on the adjoining Scotian Shelf. Total variance of the currents in the Gully is about 2.5 times greater than that observed on the nearby continental slope with an equivalent water depth.

Interannual Variability of the Circulation over the Eastern Canadian Shelf

This study examines the main physical processes affecting the interannual variability of circulation over the eastern Canadian continental shelf (ECS) based on numerical circulation results for the period 1988–2004 produced by a regional ocean–ice model. The regional circulation model is applied to the northwest Atlantic and uses a horizontal curvilinear grid with a horizontal resolution of approximately 1/4°. The model is forced by atmospheric reanalysis fields produced by Large and Yeager (2004) and boundary forcing based on the ocean reanalysis data produced by Smith et al. (2010). In comparison with previous observational and numerical results in the literature, the regional circulation model has reasonable skill in simulating the large-scale circulation and associated seasonal and interannual variability over the ECS. Complex Empirical Orthogonal Function (CEOF) analysis is used to examine the interannual variability in the monthly mean temperature and salinity anomalies in the model. The CEOF analysis of model results demonstrates that the interannual variability over the Labrador and northern Newfoundland shelves is significantly affected by the variability at high latitudes which propagates onto these shelves through the northern open boundary. Over the eastern Newfoundland Shelf, the interannual variability is significantly affected by the non-linear interaction of the Labrador Current with the North Atlantic Current and by the variability propagating from the southern Labrador Shelf. The interannual variability of circulation and hydrography over the Slope Water region off the Scotian Shelf is significantly affected by anomalies advected by the Gulf Stream and by the non-linear dynamics taking place in the deep waters to the south of the Tail of the Grand Banks. RÉSUMÉ [Traduit par la rédaction] La présente étude examine les principaux processus physiques ayant une influence sur la variabilité interannuelle de la circulation dans l'est du plateau continental canadien (ECS) d'après les résultats numériques de circulation pour la période 1988–2004 obtenus d'un modèle régional océan–glace. Le modèle de circulation régional est appliqué au nord-ouest de l'Atlantique et utilise une grille curvilinéaire horizontale ayant une résolution horizontale d'approximativement 1/4°. Le modèle est forcé par des champs de réanalyse atmosphériques produits par Large et Yeager (2004) et subit un forçage aux limites basé sur des données de réanalyse océaniques produites par Smith et coll. (2010). Comparativement aux résultats observationnels et numériques précédents que l'on trouve dans la littérature, le modèle de circulation régional possède une habileté raisonnable pour la simulation de la circulation à grande échelle et de la variabilité saisonnière et interannuelle associée sur l'ECS. Nous utilisons l'analyse par fonctions orthogonales empiriques complexes (CEOF) pour examiner la variabilité interannuelle des anomalies mensuelles moyennes de température et de salinité dans le modèle. L'analyse par CEOF des résultats du modèle montre que la variabilité interannuelle sur les plateaux du Labrador et du nord de Terre–Neuve est significativement influencée par la variabilité aux hautes latitudes qui se propage sur ces plateaux à travers la limite nord ouverte. Sur le plateau de l'est de Terre–Neuve, la variabilité interannuelle est significativement influencée par l'interaction non linéaire du courant du Labrador avec le courant de l'Atlantique Nord et par la variabilité se propageant depuis le plateau du sud du Labrador. La variabilité interannuelle de la circulation et de l'hydrographie dans la région du talus continental au large du plateau néo-écossais est significativement influencée par les anomalies advectées par le Gulf Stream et par la dynamique non linéaire se produisant dans les eaux profondes au sud de la queue des Grands Bancs.

Lake Agassiz outburst age and routing by Labrador Current and the 8.2 cal ka cold event

Catastrophic drainage of glacial Lake Agassiz, impounded by the Laurentide Ice Sheet (LIS), into Hudson Bay and Strait is widely thought to have caused the 8.2 cal ka cold event, though the path of its drainage in the North Atlantic is poorly known. Analysis and compilation of 10 sediment cores from 8 sites distributed on the eastern Canadian continental shelf and upper slope south of Hudson Strait revealed distinct beds with elevated detrital carbonate (DC) concentrations, known by their distinct composition to originate from erosion of carbonate bedrock and carbonate-rich glacial sediments in the Hudson Bay-Hudson Strait region. At all but one site, radiocarbon ages, bracketing and within the thickest and most widespread of these beds, are coeval with the age constraints for the Agassiz floods in Hudson Strait and southeastern Hudson Bay. The DC distribution and concentration of this bed are consistent with southward transport in the Labrador Current. Transfer function analysis of dinoflagellate assemblage data from cores on the central Labrador Shelf, in Hudson Bay, and on the Northeast Newfoundland Shelf reveal that early Holocene seasonal ice cover was 5–6 months longer than at present, requiring the application of a 200-year reduction to the radiocarbon age constraints on the Lake Agassiz outburst floods. The age of the floods is reduced to about 8.33 cal ka (7.5 14C ka) from the previous determination of 8.47 cal ka (7.7 14C ka), with a similar reduction in their error ranges to 8.04–8.49 cal ka from 8.16–8.74 cal ka. Based on present current routing, the Agassiz floodwaters and icebergs are inferred to have travelled south in the Labrador Current, and to have possibly become incorporated into the North Atlantic Current, and transported to the Nordic seas. There the freshwater additions would have lessened deepwater production with a concomitant reduction in northward heat transport in the upper limb of the Atlantic meridional overturning circulation, thereby contributing to the 8.2 cal ka cold event. Variation in the calcite and dolomite content of the Agassiz DC bed, two salinity reductions during Agassiz bed deposition, and low-level shorelines of lakes Agassiz and Ojibway suggest that more than one outburst was delivered during the final Agassiz drainage. The 8.2 cal ka cold event registered in Greenland was coeval with a second or later flood, which possibly accompanied the final collapse of the LIS in Hudson Bay.

Does the north Atlantic oscillation affect hydrographic properties on the Canadian Atlantic continental shelf?

An analysis of hydrographic data from the eastern Canadian continental shelf indicates that large‐scale spatial patterns of bottom temperature and salinity respond to sustained periods of weak and strong meteorological forcing represented by the winter North Atlantic Oscillation (NAO) index. Warm, salty (cold, fresh) conditions prevail on the Newfoundland‐Labrador Shelf, the eastern Scotian Shelf and the Gulf of St. Lawrence during periods of negative (positive) NAO anomalies. The opposite response is seen on the central and western Scotian Shelf and in the Gulf of Maine. Comparison of years when the NAO anomaly was positive and had the same sign for at least the two preceding years with those years when the NAO anomaly was negative and had the same sign for at least the two preceding years, shows differences in bottom temperature and salinity, at the same location, of up to approximately 2°C and 0.4. A plausible explanation of the pattern lies in a combination of local forcing and the highly advective nature of the oceanography that responds to NAO forcing. Greater westward transport of Labrador Slope Water along the shelf edge and subsequent on‐shelf penetration of hydrographic anomalies during periods of negative NAO anomalies give rise to the dipole nature of the temperature and salinity patterns. The effects on hydrographic properties appear to be integrated over several years of meteorological forcing, again likely related to advection in the region.

A dynamical model for wind‐driven ice motion: Application to ice drift on the Labrador Shelf

A three‐dimensional coupled ice‐ocean model has been developed to study short‐term ice motion over the eastern Canadian continental shelf. The model consists of a Hibler ice model and a diagnostic ocean model. Ice is coupled to the ocean through a surface Ekman layer. The model is implemented for the Labrador Sea using 6‐hourly winds and atmospheric pressures as input forcings. The results show that the model is able to produce many desired features of ice motion and ocean currents including wind‐generated coastal currents, an ice velocity field reflecting the influence of permanent and transient currents, and an increased sea surface tilt and ice internal stress at the coast. The model is used to simulate ice drift trajectories from six ice beacons deployed over the Labrador and Newfoundland Shelves in 1992. In the subtidal frequency range the modeled and observed ice velocities are in excellent agreement. Model errors, measured by the rms separation between the modeled and observed positions, increase with time. The increase is 10 km d−1 in the first 2 days and slower after 2 days. The model results are compared with a model run without ice‐ocean coupling and a calculation using an empirical relationship and parameters (speed ratio and turning angle). The errors are 20% (no ice‐ocean coupling) and 70% (empirical relationship) larger than the errors in the coupled model in the first 2 days and much larger after several days.

Mean circulation and variability on the eastern Canadian continental shelf

The circulation and variability on eastern Canadian continental shelves is reviewed with emphasis on horizontal and vertical structure in the water column. Mean and seasonal currents, sheared by stratification and friction, generally follow bathymetry and are driven primarily by surface wind stress and buoyancy fluxes. In certain near-resonant embayments, nonlinear interactions of the tide also make significant contributions to the mean and low-frequency currents. Direct forcing of low-frequency shelf circulation by offshore currents and pressure fields is of lesser importance according to recent observations and models. At higher frequencies, the wintertime response to atmospheric forcing occurs in three important frequency bands: surface wave (periods of 2–20 s), inertial (17–22 h) and synoptic or subtidal (2–10 days). Most of the subtidal energy in the subsurface pressure (SSP) field is attributable to direct local forcing by alongshore wind and to remote forcing by coastal-trapped waves (CTW). The subtidal current variability is less coherent with these sources primarily because of small scale circulations created by vortex stretching, topographic steering and scattering by the rugged shelf bathymetry. Bottom friction and scattering control the ratio of locally and remotely forced subtidal energy as a function of position on the shelf. Intermittent inertial currents in stratified shelf waters, often associated with mesoscale atmospheric forcing, exhibit a 180° phase change across the pycnocline and are inhibited near the coast. Surface waves generated by storm winds are strongly dissipated in shallow water, where they enhance bottom stresses that: (1) balance the surface wind stress; and (2) may contribute to the damping of inertial waves. Specific examples of these phenomena are drawn from observations on the Scotian Shelf and in the Gulf of Maine.

Sediment transport on the eastern Canadian continental shelf

A synthesis is presented of the sediment transport pathways resulting from hydrodynamic processes over the eastern Canadian continental shelf. This is based on interpretation of bedform type and orientation, grain size trends, sediment stratigraphy, visual observation and direct measurements. These pathways are then compared with results of numerical simulations of the transport of 0.35 mm (medium) sand for the Grand Banks of Newfoundland, the Scotian Shelf and the Gulf of Maine. The abundance of hydrodynamically reworked sediment increases from north to south. There is no evidence for sediment transport on the Baffin Shelf or Arctic Island Channels due largely to ice cover and deep (200 m) water. Sediment transport on the Labrador Shelf, by contrast, is apparent though restricted to bank tops in depths less than 120 m. This transport is predominantly to the southeast under the influence of strong wind-driven and geostrophic flows. The Grand Banks of Newfoundland is subject to storm-induced sand transport in depths less than 120 m. Transport here is to the south and southeast over much of Grand Bank and Avalon Channel. Above approximately 60 m the seabed is devoid of sand, suggesting scouring; between 60 and 120 m sand transport takes place principally as bedload. The shelf immediately south of Newfoundland is subject to strong easterly sediment transport that has scoured the banktops of all but gravel-sized sediment. The Scotian Shelf and Georges Bank also exhibit storm-induced sediment transport that is tidally-modulated in depths less than 100 m. The predicted and observed sand transport pathways are remarkably consistent. Predicted zones of sand convergence and divergence correspond to known regions of sand accumulation and depletion, respectively. Quantitative predictions are, by contrast, only within an order of magnitude of observations. Percentage exceedence predictions of sand transport for the Hibernia (Grand Banks of Newfoundland) and Venture (Sable Island Bank) discovery sites show strongly linear trends on a Gumbel distribution. These plots show that intense storms (greater than 20 year return interval) dominate net transport at Hibernia, whereas short-term storms (annual or shorter return interval) dominate at Venture.

Gas-related sedimentary features from the eastern Canadian continental shelf

The first discovery of pockmarks (gas-escape craters) was in muds of the basins of the central Scotian Shelf off eastern Canada. Since then, pockmarks have been found in many continental shelf environments of the world. They may be used as a hydrocarbon exploration tool and recently their role as foci of intense biological chemosynthetic activity has attracted considerable attention. Pockmarks occur in the Gulf of Maine, Passamaquoddy Bay, eastern Scotian Shelf, Scotian Slope, Laurentian Channel, Gulf of St Lawrence, Halibut Channel and on Labrador Shelf. Their formation in Passamaquoddy Bay may be enhanced through earthquake activity as their greatest density occurs in association with a large fault zone. Pockmarks on Labrador Shelf and in Laurentian Channel may have been formed preferentially as a consequence of disturbance of the seabed by grounded moving icebergs. Pockmarks in Placentia Bay, Newfoundland occur in an area of large megaflutes at the seabed and the pockmarks may have provided an initial seabed roughness for subsequent formation of the megaflutes by a tsunami-generated turbidity current. Subsurface zones of gas-charged Quaternary sediments are often associated with areas of pockmarks. Gas-charged sediments in the nearshore frequently appear as thick Holocene deposits occurring in areas overlying buried channel systems. In the nearshore, biogenic methane is the probable source. In other areas, such as Downing Basin on the Grand Banks of Newfoundland, the gas-charged sediments contain ethane, butane and methane, suggesting a deeper hydrocarbon source. Active gas venting has been observed at the seabed in Downing Basin without the formation of pockmarks. Seabed observations from research submersibles and bottom photography have identified areas of white filamentous bacteria on sandy sediments sometimes associated wtih circular patches of dense benthic communities. Seismic reflection profiles from these areas also indicate the presence of shallow gas (bright spots) within Tertiary age sediments. Taken in their entirely, the broad distribution of gas-related features suggests that the major basins of the eastern Canadian continental shelf vent gas, but pockmarks or other morphologic venting features are only produced where soft cohesive sediments are present at the seabed. The chronic release of gas from the seabed may contribute to the biological productivity of the offshore areas as well as provide a significant, but as yet unquantified, volume of methane and other gases to the ocean and atmosphere.

Interaction of internal waves with the seabed on continental shelves

Large-amplitude internal waves generated by tidal fluctuations at the outer edge of the continental shelf propagate shoreward and interact with seabed sediment. A crude mathematical model describing the propagation of such internal waves is used to predict wave-induced sediment flux and thereby to provide a possible explanation of sand-ridge formation on continental shelves. The ocean over the shelf is modelled as a two-layer system and it is assumed that the internal waves are generated at the shelf edge. Field measurements indicate that the very low frequency tidal fluctuations create a train of internal oscillations of periods between 15 and 20 min. Because of nonlinear effects these wave trains generate differential sediment fluxes and, in the model, this leads to the creation of seabed topography. The predictions of the model are tested against field data collected on the eastern Canadian continental shelf and in particular on Sable Island Bank and the Grand Banks of Newfoundland. The comparisons between predictions and measurements are encouraging especially in light of the fact that the model has no free parameters.

The effects of trawling, dredging and ocean dumping on the eastern Canadian continental shelf seabed

This paper presents an overview of current knowledge on the effects of trawling, dredging and ocean dumping on the eastern Canadian continental shelf seabed. The impact of trawling and dredging for fish and shellfish on marine habitats has recently attracted international attention among fisheries and environmental scientists. In Atlantic Canada, trawling and dredging are the principal methods of harvesting groundfish and scallops and ocean clams, respectively. It is estimated that fish trawlers and scallop dredges have swept tracks, cris-crossing the Canadian continental shelf, approximately 4.3 million km in length in 1985. In the past few years several studies were carried out by scientists from Canada, the United States and Europe to assess the impacts of trawling and dredging but results were inconclusive. Some studies showed physical damage as well as biological effects, whereas others indicated that the adverse effects were not considered to be serious. Fishermen are not the only potential users of the resources of the continental shelf. There is an increasing demand for good-quality sand and gravel aggregate and the ocean seabed is being seen as a possible source. The eastern Canadian continental shelf also exhibits hydrocarbon potential and operational and accidental discharges are an environmental concern. Increased marine transportation and expansion of the fishing fleet have resulted in a greater need for harbour dredging. Dredging and dredge spoil disposal were controlled by the Ocean Dumping Control Act and now the Canadian Environmental Protection Act which places restrictions on the composition of material that can be disposed of in the sea. Nevertheless some harbours contain contaminant concentrations exceeding the maximum allowable limits. It is concluded that the impacts of human activities on the continental shelf seabed environment are inevitable and the long-term effects, while difficult to determine, must be assessed. The sub-lethal effects of increased suspended sediment loads on benthic organisms and potential changes to benthic community structure are major concerns and should be the focus of further research.

Underwater acoustic ambient noise levels on the eastern Canadian continental shelf

This paper reports the analysis of shallow-water ambient noise levels collected by Defence Research Establishment Atlantic during 14 cruises over the period 1972 to 1985. A weighted average is formed to de-bias the samples, with the aim of answering the question: ‘‘If one were to pick a site randomly on the eastern Canadian continental shelf at a random time, and perform an ambient noise measurement, what would be the expected noise level?’’ The samples are also grouped according to whether they were taken on the Scotian Shelf, the Grand Banks, or the Flemish Cap, and according to season. The frequency range covered is 30 to 900 Hz. The weighted mean and standard deviation of the noise levels are presented, as well as the correlation coefficient between the noise levels and wind speed. The results show that the eastern Canadian continental shelf as a whole presents levels that are characteristic of areas with high shipping density and good acoustic propagation, with the Scotian Shelf showing generally higher ambient noise levels. Finally, a comparison with Piggott’s ambient noise measurements [C. L. Piggott, ‘‘Ambient sea noise at low frequency in shallow water of the Scotian Shelf,’’ J. Acoust. Soc. Am. 36, 2152–2163 (1964)] on the Scotian Shelf is made, and an explanation for the observed difference is suggested.

Iceberg grounding and scouring on the Labrador Continental Shelf

Icebergs drifting in seas of the eastern Canadian Continental Shelf present serious hazards to offshore drilling operations. Damage to offshore structures may be caused by direct collisions with a floating or gravity-based structure. Icebergs whose keels touch and plough through or scour the soft sediments of the seabed may crush and rupture seabed installations such as wellheads, anchoring/mooring systems, pipelines and telecommunication cables. Observations made at exploration wells on the Labrador Shelf from 1973 to 1981 are used to delineate areas of active iceberg scouring and to quantify the incidence of grounding and scouring icebergs. The term grounded is used to describe icebergs whose keels have contacted the seabed and which have thus been halted. Scouring icebergs are those whose keels have contacted the seafloor, but which continue to move forward. Observations of more than 1500 icebergs from twenty-two well sites have been analyzed and criteria for identifying grounded and scouring bergs have been established. Over fifty icebergs have been observed to ground and scour in eleven areas. Over the observation periods, the average grounding frequency for Makkovik and Saglek Banks were 3.3% (data collected in seven years) and 4.3% (data collected in six years), respectively. It appears likely, however, that these frequencies are an underestimate and that many more “possible” grounding and scouring icebergs will eventually be included in the data set. We show that many scouring icebergs can move over large (60 km) distances and are able to traverse significant (up to 45 m) ranges in bathymetry. We suggest that they may accomplish this through increases and decreases in draft by continual gradual rotation about a horizontal axis normal to the movement direction.