Norfolk Canyon Research Articles

Population structure of Desmophyllum pertusum found along the United States eastern continental margin

ObjectiveThe connectivity and genetic structuring of populations throughout a region influence a species’ resilience and probability of recovery from anthropogenic impacts. By gaining a comprehensive understanding of population connectivity, more effective management can be prioritized. To assess the connectivity and population genetic structure of a common cold-water coral species, Desmophyllum pertusum (Lophelia pertusa), we performed Restriction-site Associated DNA Sequencing (RADseq) on individuals from nine sites ranging from submarine canyons off New England to the southeastern coast of the United States (SEUS) and the Gulf of Mexico (GOM). Fifty-seven individuals and 3,180 single-nucleotide polymorphisms (SNPs) were used to assess genetic differentiation.ResultsHigh connectivity exists among populations along the SEUS, yet these populations were differentiated from those to the north off New England and in Norfolk Canyon along the North Atlantic coast of the United States, as well as those in the GOM. Interestingly, Norfolk Canyon, located just north of North Carolina, and GOM populations exhibited low levels of genetic differentiation, corroborating previous microsatellite analyses and signifying gene flow between these populations. Increasing sample sizes from existing populations and including additional sampling sites over a larger geographic range would help define potential source populations and reveal fine-scale connectivity patterns among D. pertusum populations.

Distributions of microplastics and larger anthropogenic debris in Norfolk Canyon, Baltimore Canyon, and the adjacent continental slope (Western North Atlantic Margin, U.S.A.)

Anthropogenic debris has been reported in all studied marine environments, including the deepest parts of the sea. Finding areas of accumulation and methods of transport for debris are important to determine potential impacts on marine life. This study analyzed both sediment cores and Remotely Operated Vehicle video to determine the density and distribution of debris, including both micro- and macroplastics, in Norfolk and Baltimore canyons. The average microplastic density in Norfolk Canyon sediment was 37.30 plastic particles m−2 within the canyon and 21.03 particles m−2 on the adjacent slope, suggesting that microplastics could accumulate within submarine canyons. In video transects from both Norfolk and Baltimore canyons, the largest amounts of macroplastic were recorded near the canyon heads. Our findings contribute to a growing evidence base that canyons and their associated benthic invertebrate communities are important repositories and conduits for debris to the deep sea.

Distribution of Megabenthic Communities Under Contrasting Settings in Deep-Sea Cold Seeps Near Northwest Atlantic Canyons

Cold seeps support fragile deep-sea communities of high biodiversity and are often found in areas with high commercial interest. Protecting them from encroaching human impacts (bottom trawling, oil and gas exploitation, climate change) requires an advanced understanding of the drivers shaping their spatial distribution and biodiversity. Based on the analysis of 2,075 high-quality images from six remotely operated vehicle dives, we examined cold seep megabenthic community composition, richness, density, and biodiversity at a relatively shallow (∼400 m water depth) site near Baltimore Canyon (BC) and a much deeper site (∼1,500 m) near Norfolk Canyon (NC), in the northwest Atlantic. We found sharp differences in the megabenthic composition between the sites, which were driven mostly by bathymetric gradients. At both BC and NC there were significant differences in megabenthic composition across habitats. Hard habitats in and around cold seeps had significantly higher values of species richness, density, and biodiversity than soft habitats. Depth and habitat complexity were the leading environmental variables driving megabenthic variability. The presence of microbial mats and gas bubbling sites had a statistically significant contribution to explaining megabenthic variability mainly in the shallower BC and less in the deeper NC areas examined; drivers behind this discrepancy could be related to differences between BC and NC in terms of chemical compound fluxes and megafaunal life history characteristics. Our surveys revealed marine litter, primarily from commercial fisheries. This study highlights the importance of habitat complexity for the proliferation of highly diverse cold-seep ecosystems and underscores the importance of discovery science to inform spatial management of human activities in the deep and open ocean.

The role of habitat heterogeneity and canyon processes in structuring sediment macrofaunal communities associated with hard substrate habitats in Norfolk Canyon, USA

Topographic and hydrodynamic complexity in submarine canyons promotes steep gradients in food availability and geophysical parameters which affect ecological assemblages and beta diversity. While habitat heterogeneity in submarine canyons is known to support diverse and abundant megafaunal communities, due to difficulty in sampling little is known about infaunal communities adjacent to hard substrate habitats, their contribution to canyon assemblages, and overall deep-sea diversity. Sediments were collected in three distinct habitat types: within Norfolk Canyon (western Atlantic) adjacent to hard substrate habitats including canyon walls and large boulders, along the main axis of the canyon, and on the adjacent continental slope to quantify macrofaunal (>300 μm) density, diversity and community composition, and sediment geochemical parameters including grain size, organic content, and stable isotope composition. While macrofaunal densities were similar among habitats sampled at comparable depths, diversity was higher near the hard substrate environments. Discrete communities were present in each habitat type, while annelid functional composition was similar between hard substrate adjacent and canyon axis habitats. Although diversity and abundance of hard substrate adjacent sediment communities did not change with depth, communities were driven by sediments with low organic matter content, whereas canyon and slope community assemblages were best explained by depth and higher organic content. Beta diversity among hard substrate adjacent sediments and canyon axis communities was high, with 27% of canyon taxa and 10% of regional taxa only occurring in hard substrate habitats. Given the thousands of submarine canyons worldwide, our results highlight the overall importance of substrate habitat heterogeneity within canyons in supporting deep-sea benthic diversity and suggest that both within-canyon and regional diversity are underestimated.

Physiological and behavioral sexual maturity of female red deep-sea crabs Chaceon quinquedens (Smith, 1879) (Decapoda: Brachyura: Geryonidae) in the Mid-Atlantic Bight

Abstract We describe the physiological and behavioral maturity of the red deep-sea crab, Chaceon quinquedens (Smith, 1879), using ovary and oocyte development and morphological features to estimate the size at 50% sexual maturity (SM50) for females in the Mid-Atlantic Bight. This economically important species inhabits the continental shelf and slope of the western Atlantic from Nova Scotia, Canada to the Gulf of Mexico. Samples were collected by trawling in 2011–2013 and by traps in 2014–2016. We used histological analysis to describe the stages of ovarian development. Five stages of ovarian development were identified: 1, immature; 2, early maturing; 3, late maturing; 4, mature; and 5, redeveloping. A logistic model was used to estimate the SM50 using maximum likelihood methods. Physiological SM50 varied among geographic locations and was estimated at 61.2 mm CL and 70.8 mm CL for females collected near Hudson Canyon and Baltimore and Norfolk canyons, respectively. Behavioral SM50 decreased with latitude and was estimated at 53.9 mm CL, 62.5 mm CL and 65.5 mm CL for Hudson, Baltimore, and Norfolk canyons, respectively. Results implied asynchrony between physiological and behavioral sexual maturity, suggesting that mating occurs prior to completion of ovarian development. This study provides the first evidence of a latitudinal trend in sexual maturity for C. quinquedens. Further investigation is needed to understand the cause of the latitudinal trend as our study presented limitations. Results provide useful information for fisheries management about the reproductive biology of C. quinquedens.

Submarine canyons influence macrofaunal diversity and density patterns in the deep-sea benthos

Submarine canyons are often morphologically complex features in the deep sea contributing to habitat heterogeneity. In addition, they act as major conduits of organic matter from the shallow productive shelf to the food deprived deep-sea, promoting gradients in food resources and areas of sediment resuspension and deposition. This study focuses on the Baltimore and Norfolk canyons, in the western North Atlantic Ocean, and investigates how different biogeochemical drivers influence canyon and slope macrofaunal communities. Replicated sediment cores were collected along the main axes (~180–1200 m) of Baltimore and Norfolk canyons and at comparable depths on the adjacent slopes. Cores were sorted, assessing whole community macrofaunal (>300 μm) abundance, diversity and standing stocks. Canyon communities were significantly different from slope communities in terms of diversity, abundance patterns and community assemblages, which were attributed to high levels of organic matter enrichment within canyons. There was a significant departure from the expected density-depth relationship in both canyons, driven by enhanced abundances between 800 and 900 m canyon depths, which was characterised as a deposition zone for organic matter. Bathymetric zonation, sediment dynamics, organic enrichment, and disturbance events were clear factors that structured the benthic communities in both Baltimore and Norfolk canyons. Coupling family-level community data, with sediment grain-size and biogeochemistry data explained community dynamics across depth and biogeochemical gradients, providing further evidence that canyons disrupt macrofaunal diversity and density patterns in the deep-sea benthos.

Molecular characterization of Bathymodiolus mussels and gill symbionts associated with chemosynthetic habitats from the U.S. Atlantic margin.

Mussels of the genus Bathymodiolus are among the most widespread colonizers of hydrothermal vent and cold seep environments, sustained by endosymbiosis with chemosynthetic bacteria. Presumed species of Bathymodiolus are abundant at newly discovered cold seeps on the Mid-Atlantic continental slope, however morphological taxonomy is challenging, and their phylogenetic affinities remain unestablished. Here we used mitochondrial sequence to classify species found at three seep sites (Baltimore Canyon seep (BCS; ~400m); Norfolk Canyon seep (NCS; ~1520m); and Chincoteague Island seep (CTS; ~1000m)). Mitochondrial COI (N = 162) and ND4 (N = 39) data suggest that Bathymodiolus childressi predominates at these sites, although single B. mauritanicus and B. heckerae individuals were detected. As previous work had suggested that methanotrophic and thiotrophic interactions can both occur at a site, and within an individual mussel, we investigated the symbiont communities in gill tissues of a subset of mussels from BCS and NCS. We constructed metabarcode libraries with four different primer sets spanning the 16S gene. A methanotrophic phylotype dominated all gill microbial samples from BCS, but sulfur-oxidizing Campylobacterota were represented by a notable minority of sequences from NCS. The methanotroph phylotype shared a clade with globally distributed Bathymodiolus spp. symbionts from methane seeps and hydrothermal vents. Two distinct Campylobacterota phylotypes were prevalent in NCS samples, one of which shares a clade with Campylobacterota associated with B. childressi from the Gulf of Mexico and the other with Campylobacterota associated with other deep-sea fauna. Variation in chemosynthetic symbiont communities among sites and individuals has important ecological and geochemical implications and suggests shifting reliance on methanotrophy. Continued characterization of symbionts from cold seeps will provide a greater understanding of the ecology of these unique environments as well and their geochemical footprint in elemental cycling and energy flux.

Distributions and habitat associations of deep-water corals in Norfolk and Baltimore Canyons, Mid-Atlantic Bight, USA

A multi-disciplinary study of two major submarine canyons, Baltimore Canyon and Norfolk Canyon, off the US mid-Atlantic coast focused on the ecology and biology of canyon habitats, particularly those supporting deep-sea corals. Historical data on deep-sea corals from these canyons were sparse with less than 750 records for the mid-Atlantic region, with most being soft sediment species. This study substantially increased the number of deep-sea coral records for the target canyons and the region. Large gorgonians were the dominant structure-forming coral taxa on exposed hard substrates, but several species of scleractinians were also documented, including first observations of Lophelia pertusa in the mid-Atlantic Bight region. Coral distribution varied within and between the two canyons, with greater abundance of the octocoral Paragorgia arborea in Baltimore Canyon, and higher occurrence of stony corals in Norfolk Canyon; these observations reflect the differences in environmental conditions, particularly turbidity, between the canyons. Some species have a wide distribution (e.g., P. arborea, Primnoa resedaeformis, Anthothela grandiflora), while others are limited to certain habitat types and/or depth zones (e.g., Paramuricea placomus, L. pertusa, Solenosmilia variabilis). The distribution of a species is driven by a combination of factors, which include availability of appropriate physical structure and environmental conditions. Although the diversity of the structure-forming corals (gorgonians, branching scleractinians and large anemones) was low, many areas of both canyons supported high coral abundance and a diverse coral-associated community. The canyons provide suitable habitat for the development of deep-sea coral communities that is not readily available elsewhere on the sedimented shelf and slope of the Mid-Atlantic Bight.

Macrofaunal communities associated with chemosynthetic habitats from the U.S. Atlantic margin: A comparison among depth and habitat types

Hydrocarbon seeps support distinct benthic communities capable of tolerating extreme environmental conditions and utilizing reduced chemical compounds for nutrition. In recent years, several locations of methane seepage have been mapped along the U.S. Atlantic continental slope. In 2012 and 2013, two newly discovered seeps were investigated in this region: a shallow site near Baltimore Canyon (BCS, 366–412m) and a deep site near Norfolk Canyon (NCS, 1467–1602m), with both sites containing extensive chemosynthetic mussel bed and microbial mat habitats. Sediment push cores, suction samples, and Ekman box cores were collected to quantify the abundance, diversity, and community structure of benthic macrofauna (>300μm) in mussel beds, mats, and slope habitats at both sites. Community data from the deep site were also assessed in relation to the associated sediment environment (organic carbon and nitrogen, stable carbon and nitrogen isotopes, grain size, and depth). Infaunal assemblages and densities differed both between depths and among habitat types. Macrofaunal densities in microbial mats were four times greater than those present in mussel beds and slope sediments and were dominated by the annelid families Dorvilleidae, Capitellidae, and Tubificidae, while mussel habitats had higher proportions of crustaceans. Diversity was lower in BCS microbial mat habitats, but higher in mussel and slope sediments compared to NCS habitats. Multivariate statistical analysis revealed specific sediment properties as important for distinguishing the macrofaunal communities, including larger grain sizes present within NCS microbial mat habitats and depleted stable carbon isotopes (δ13C) in sediments present at mussel beds. These results suggest that habitat differences in the quality and source of organic matter are driving the observed patterns in the infaunal assemblages, including high β diversity and high variability in the macrofaunal community composition. This study is the first investigation of seep infauna along the U.S. Atlantic slope north of the Blake Ridge Diapir and provides a baseline for future regional comparisons to other seep habitats along the Atlantic margin.

Fish species associated with shipwreck and natural hard-bottom habitats from the middle to outer continental shelf of the Middle Atlantic Bight near Norfolk Canyon

Fish species associated with shipwreck and natural hard-bottom habitats from the middle to outer continental shelf of the Middle Atlantic Bight near Norfolk Canyon

Demersal fish distribution and habitat use within and near Baltimore and Norfolk Canyons, U.S. middle Atlantic slope

Numerous submarine canyons along the United States middle Atlantic continental margin support enhanced productivity, diverse and unique habitats, active fisheries, and are vulnerable to various anthropogenic disturbances. During two cruises (15 Aug–2 Oct 2012 and 30 Apr–27 May 2013), Baltimore and Norfolk canyons and nearby areas (including two cold seeps) were intensively surveyed to determine demersal fish distributions and habitat associations. Overall, 34 ROV dives (234–1612m) resulted in 295h of bottom video observations and numerous collections. These data were supplemented by 40, 30-min bottom trawl samples. Fish observations were assigned to five general habitat designations: 1) sand-mud (flat), 2) sloping sand-mud with burrows, 3) low profile gravel, rock, boulder, 4) high profile, canyon walls, rocks or ridges, and 5) seep-mixed hard and soft substrata, the later subdivided into seven habitats based on amounts of dead mussel and rock cover. The influence of corals, sponges and live mussels (seeps only) on fish distributions was also investigated. Both canyon areas supported abundant and diverse fish communities and exhibited a wide range of habitats, including extensive areas of deep-sea corals and sponges and two nearby methane seeps (380–430m, 1455–1610m). All methods combined yielded a total of 123 species of fishes, 12 of which are either new records for this region or have new range data. Depth was a major factor that separated the fish faunas into two zones with a boundary around 1400m. Fishes defining the deeper zone included Lycodes sp., Dicrolene introniger, Gaidropsaurus ensis, Hydrolagus affinis, Antimora rostrata, and Aldrovandia sp. Fishes in the deep zone did not exhibit strong habitat affinities, despite the presence of a quite rugged, extensive methane seep. We propose that habitat specificity decreases with increasing depth. Fishes in the shallower zone, characterized by Laemonema sp., Phycis chesteri, Nezumia bairdii, Brosme brosme, and Helicolenus dactylopterus, exhibited a variety of habitat use patterns. In general, fish assemblages in the soft substrata areas (dominated by P. chesteri, N. bairdii, Glyptocephalus cynoglossus, Lophius americanus, Merluccius albidus) were different from those in more complex habitats (dominated by Laemonema spp., Hoplostethus spp., B. brosme, Benthocometes robustus, L. americanus, Dysommina rugosa). Although, when present, the dense coral and sponge cover did not statistically affect general fish assemblage patterns in hard bottom habitats, these sessile organisms markedly increased habitat complexity, and some fish species exhibited close association with them. Fish species compositions in the two canyons were not substantially different from the surrounding slopes. However, the diversity of habitats within both the canyons and seeps exerted an important influence on assemblage structure. At least for some species (e.g., B. brosme) canyon and seep environments may be a refuge from intensive bottom fishing activities. We frequently observed evidence of commercial and recreational fishing activity (lost gear on bottom and fishing boats in the area) throughout the canyons and some resulting habitat damage. Conservation measures for the unique and more vulnerable areas (seep and coral habitats) have recently been approved.

Geomorphic characterization of four shelf-sourced submarine canyons along the U.S. Mid-Atlantic continental margin

Shelf-sourced submarine canyons are common features of continental margins and are fundamental to deep-sea sedimentary systems. Despite their geomorphic and geologic significance, relatively few passive margin shelf-breaching canyons worldwide have been mapped using modern geophysical methods. Between 2007 and 2012 a series of geophysical surveys was conducted across four major canyons of the US Mid-Atlantic margin: Wilmington, Baltimore, Washington, and Norfolk canyons. More than 5700km2 of high-resolution multibeam bathymetry and 890 line-km of sub-bottom CHIRP profiles were collected along the outer shelf and uppermost slope (depths of 80-1200m). The data allowed us to compare and contrast the fine-scale morphology of each canyon system. The canyons have marked differences in the morphology and orientation of canyon heads, steepness and density of sidewall gullies, and the character of the continental shelf surrounding canyon rims. Down-canyon axial profiles for Washington, Baltimore and Wilmington canyons have linear shapes, and each canyon thalweg exhibits morphological evidence for recent, relatively small-scale sediment transport. For example, Washington Canyon displays extremely steep wall gradients and contains ~100m wide, 5–10m deep, v-shaped incisions down the canyon axis, suggesting modern or recent sediment transport. In contrast, the convex axial thalweg profile, the absence of thalweg incision, and evidence for sediment infilling at the canyon head, suggest that depositional processes strongly influence Norfolk Canyon during the current sea-level high-stand. The north walls of Wilmington, Washington and Norfolk canyons are steeper than the south walls due to differential erosion, though the underlying cause for this asymmetry is not clear. Furthermore, we speculate that most of the geomorphic features observed within the canyons (e.g., terraces, tributary canyons, gullies, and hanging valleys) were formed during the Pleistocene, and show only subtle modification by Holocene processes active during the present sea-level high-stand.

First observations of the cold-water coral Lophelia pertusa in mid-Atlantic canyons of the USA

The structure-forming, cold-water coral Lophelia pertusa is widely distributed throughout the North Atlantic Ocean and also occurs in the South Atlantic, North Pacific and Indian oceans. This species has formed extensive reefs, chiefly in deep water, along the continental margins of Europe and the United States, particularly off the southeastern U.S. coastline and in the Gulf of Mexico. There were, however, no records of L. pertusa between the continental slope off Cape Lookout, North Carolina (NC) (∼34°N, 76°W), and the rocky Lydonia and Oceanographer canyons off Cape Cod, Massachusetts (MA) (∼40°N, 68°W). During a research cruise in September 2012, L. pertusa colonies were observed on steep walls in both Baltimore and Norfolk canyons. These colonies were all approximately 2m or less in diameter, usually hemispherical in shape and consisted entirely of live polyps. The colonies were found between 381m and 434m with environmental observations of: temperature 6.4–8.6°C; salinity 35.0–35.6; and dissolved oxygen 2.06–4.41mlL−1, all of which fall within the range of known L. pertusa distributions. All colonies were observed on vertical walls or underneath overhangs in areas of high current, which differs from observations further south, where L. pertusa colonizes rocky ledges and outcroppings, often forming large bioherms. We discuss observations from Baltimore and Norfolk canyons in the context of the known distribution of this species in the North Atlantic.

Organic matter content in U.S. Atlantic continental slope sediments: Decoupling the grain-size factor

Abstract Decoupling organic content in sediments from grain size can help define spatial variations in biological productivity transport or preservation in continental margin sediments. Upper slope and upper rise sediments off the eastern United States have organic nitrogen contents similar to or lower than those predicted by grain size, whereas mid-slope sediments between Norfolk Canyon and Cape Fear have much higher organic contents. These elevated values probably result from off-shelf transport, combined with rapid burial and correspondingly reduced rates of remineralization. Low organic contents on the upper slope partly reflect higher rates of remineralization, while lower values on the upper rise presumably result from low input of organic matter and low rates of burial and increased oxic degradation.

Fluxes of particulate matter on the slope of the southern Middle Atlantic Bight: SEEP-II

An array of 17 sediment traps was moored in two cross-isobath transects during the SEEP-II experiment, one north of the Washington Canyon, and the other south of the Norfolk Canyon off the Delmarva Peninsula in the southern Middle Atlantic Bight. The overall duration of the experiment was 15 months, during which the moorings were deployed three times, and the traps collected the vertical flux of particles during a total of 30 sampling intervals of average duration 12.8 days. The 483 particulate samples successfully recovered were each analyzed for total mass flux and the abundance of major biogenic phases (organic carbon, nitrogen, calcium carbonate and opal) and for 210Pb. A smaller number were also analyzed for 234Th. The distribution of fluxes in space requires transport of particles both downslope from the shelf-slope break and out into the water column, and alongslope transport. The region of maximum in particulate-matter flux to the slope, the depocenter, is from 400 to 1000 m, and the position of this maximum coincides with the maximum in alongslope current activity as expressed by the average total kinetic energy density. The total mass flux and the flux of all components to the slope depocenter increases between the northern Middle Atlantic Bight region, studied during the SEEP-I experiment, and SEEP-II sites in the southern Middle Atlantic Bight by about a factor of three to four. The temporal variability of the fluxes measured is dominated by storm-driven resuspension events on the shelf and shelf-slope break, but this extreme, short-term variability is not seen in the depocenter traps, the fluxes to which are very constant in magnitude and in composition. Canyons are suggested to serve both as conduits for transport of sediment from the shelf to the slope, and as a reservoir that feeds shelf-derived particles to the slope depocenter on a regular basis, even when sediment resuspension on the shelf is subdued or absent.

Submarine processes of the middle Atlantic continental rise based on GLORIA imagery

Approximately 6,100 km of 3.5-kHz echo-sounding profiles was correlated with a GLORIA side-scan sonar image of the mid-Atlantic United States (34°N, 70°W) lower slope-upper continental rise. The image allows us to map the major erosional and depositional features and to identify major processes that have shaped the area. The GLORIA imagery shows three approximately triangular-shaped sediment-gather areas that cover the upper-rise and slope transition areas near Wilmington, Baltimore, and Norfolk Canyons. The gather areas, which are interspersed with hemipelagic drape areas, are composed of dendritic networks of channels that extend from the base of the slope toward major channel systems on the middle rise, such as Wilmington Valley. Seaward of Cape Hatteras, a tongue-shaped area of mottled high back scatter on the GLORIA imagery denotes the Albemarle-Currituck mass-movement complex, a large (60 km wide by >190 km long) area of the sea floor that is marked by the disruption of upper-rise sedimentary strata and by mass-flow deposits. Interpretation of GLORIA imagery and echo-sounding profiles indicates that mass movement is the predominant process affecting sediment on the United States east coast mid-Atlantic slope and upper rise and that isobath-parallel sediment movement by geostrophic currents is restricted mainly to the lower continental rise. The mass-movement processes evident on the rise probably were most active during the Pleistocene, when sea level was lower and sediment input more active.

Sediment transport in Washington and Norfolk submarine canyons

A sediment study suggests that Washington and Norfolk canyons off the Mid-Atlantic States are not inactive, but have served periodically since the Late Pleistocene as conduits of sediment originating on the adjacent shelf and upper slope. Large quantities of sand occur in the canyon heads as thin beds and laminae, and on the continental slope as mixtures of sand (to >40%), silt and clay that are extensively reworked by burrowing organisms. Sandy turbidites occur in the canyons on the rise. Basinward dispersal, from the outer shelf and uppermost slope, is recorded by heavy mineral suites and bioclastic components, primarily foraminifera of shallow marine origin, in the lower slope and upper continental rise canyon cores. The down-axis movement of material, presumably episodic, in the Holocene to recent results from offshelf spillover into canyon heads, failure on the steep walls bordering canyons on the slope, and resuspension by bottom currents.

Evolution of Veatch, Washington, and Norfolk Submarine Canyons: Inferences from strata and morphology

Investigations of Veatch, Washington, and Norfolk Submarine Canyons, on the eastern continental margin of the United States, resulted in a detailed assessment of their past and present geologic processes. Norfolk and Washington Canyons share such characteristics as similar levee geometry, stratigraphy, and mineralogy; Veatch Canyon differs from Norfolk and Washington Canyons in these respects. Seismic reflection profiles of all three submarine canyon heads indicate an erosional origin. Seismic profiles of Veatch Canyon's lower continental slope indicate a formation by depositional processes; in contrast, the continental-slope profiles of Washington and Norfolk Canyons reveal a history of alternating episodes of deposition and erosion. Cored sediments from Washington and Norfolk Canyons disclose recent, intermittent, down-canyon transport of sand to the continental rise. Bathymetric and seismic profiles suggest Veatch, Washington, and Norfolk Canyons have all been subjected to a minimum of two periods of development. Similar axial trends of Washington and Norfolk Canyons imply a common structural influence.

The gravity field of the U.S. Atlantic continental margin

Approximately 39,000 km of marine gravity data collected during 1975 and 1976 have been integrated with U.S. Navy and other available data over the U.S. Atlantic continental margin between Florida and Maine to obtain a 10 mgal contour free-air gravity anomaly map. A maximum typically ranging from 0 to +70 mgal occurs along the edge of the shelf and Blake Plateau, while a minimum typically ranging from −20 to −80 mgal occurs along the base of the continental slope, except for a −140 mgal minimum at the base of the Blake Escarpment. Although the maximum and minimum free-air gravity values are strongly influenced by continental slope topography and by the abrupt change in crustal thickness across the margin, the peaks and troughs in the anomalies terminate abruptly at discrete transverse zones along the margin. These zones appear to mark major NW—SE fractures in the subsided continental margin and adjacent deep ocean basin, which separate the margin into a series of segmented basins and platforms. Rapid differential subsidence of crustal blocks on either side of these fractures during the early stages after separation of North America and Africa (Jurassic and Early Cretaceous) is inferred to be the cause of most of the gravity transitions along the length of margin. The major transverse zones are southeast of Charleston, east of Cape Hatteras, near Norfolk Canyon, off Delaware Bay, just south of Hudson Canyon and south of Cape Cod. Local Airy isostatic anomaly profiles (two-dimensional, without sediment corrections) were computed along eight multichannel seismic profiles. The isostatic anomaly values over major basins beneath the shelf and rise are generally between −10 and −30 mgal while those over the platform areas are typically 0 to +20 mgal. While a few isostatic anomaly profiles show local 10–20 mgal increases seaward of the East Coast Magnetic Anomaly (ECMA: inferred to mark the ocean-continent boundary), the lack of a consistent correlation indicates that the relationship of isostatic gravity anomalies to the magnetic anomalies and the ocean—continent transition is variable. Two-dimensional gravity models have been computed for two profiles off Cape Cod, Massachusetts and Cape May, New Jersey, where excellent reflection, refraction and magnetic control appear to define 10 and 12 km deep sedimentary basins beneath the shelf, respectively and 10 km deep basins beneath the rise. The basins are separated by a 6–8 km deep basement ridge which underlies the ECMA and appears to mark the landward edge of oceanic crust. The gravity models suggest that the oceanic crust is between 11 and 18 km thick beneath the ECMA, but decreases to a thickness of less than 8 km within the first 20–90 km to the southeast. In both profiles, the derived crustal thickness variations support the interpretation that the ECMA occurs over the ocean-continent boundary. The crust underlying the sedimentary cover appears to be 12 to 15 km thick on the landward side of the ECMA and gradually thickens to normal continental values of greater than 25 km within the first 60 to 110 km to the northwest. Multichannel seismic profiles across platform areas, such as Cape Hatteras and Cape Cod, indicate the ocean-continent transition zones there are much narrower than profiles across major sedimentary basins, such as the one off New Jersey.

Parasitism of Galatheid Crustaceans From the Norfolk Canyon and Middle Atlantic Bight By Bopyrid Isopods 1, 2)

Abstract Among twelve species of galatheid and one species of chirostylid Crustacea collected in the vicinity of Norfolk Canyon off eastern U.S.A., four galatheids, Munida iris iris, M. longipes, M. microphthalma and Munidopsis rostrata were found to be infested with bopyrid isopod parasites Munida iris iris was the most heavily parasitized host species with incidence of infestation by Anuropodione carolinensis ranging from 2.0-5.0%. Male M. iris iris were significantly more numerous than females and also had a greater incidence of infestation by bopyrids than females. The size of female A. carolinensis was found to be related to size of the host, which suggests simultaneous molts for both species. Male A. carolinensis were found to have pleons which were either fused, completely segmented, partly segmented or with slight lateral undulations. Significant size differences between males with fused, undulate, and segmented pleons may indicate impending feminization of males with segmented pleons. Single occurrences of bopyrids were observed on the other host species. Munida microphthalina was infested with Bonnieria americana. Munidion longipedis was found on Munida longipes and an undescribed bopyrid was found on Munidopsis rostrata.