Page C. Valentine

The impacts of mobile fishing gear on seafloor habitats in the gulf of Maine (Northwest Atlantic): Implications for conservation of fish populations

Abstract Fishing gear alters seafloor habitats, but the extent of these alterations, and their effects, have not been quantified extensively in the northwest Atlantic. Understanding the extent of these impacts, and their effects on populations of living marine resources, is needed to properly manage current and future levels of fishing effort and fishing power. For example, the entire U.S. side of the Gulf of Maine was impacted annually by mobile fishing gear between 1984 and 1990, based on calculations of area swept by trawl and dredge gear. Georges Bank was imparted three to nearly four times annually during the same period. Studies at three sites in the Gulf of Maine (off Swans Island, Jeffreys Bank, and Stellwagen Bank) showed that mobile fishing gear altered the physical structure (=complexity) of benthic habitats. Complexity was reduced by direct removal of biogenic (e.g., sponges, hydrozoans, bryozoans, amphipod tubes, holothurians, shell aggregates) and‐ sedimentary (e.g., sand waves, depressions)...

U.S. Geological Survey Core Drilling on the Atlantic Shelf

The first broad program of scientific shallow drilling on the U.S. Atlantic continental shelf has delineated rocks of Pleistocene to Late Cretaceous age, including phosphoritic Miocene strata, widespread Eocene carbonate deposits that serve as reflective seismic markers, and several regional unconformities. Two sites, off Maryland and New Jersey, showed light hydrocarbon gases having affinity to mature petroleum. Pore fluid studies showed that relatively fresh to brackish water occurs beneath much of the Atlantic continental shelf, whereas increases in salinity off Georgla and beneath the Florida-Hatteras slope suggest buried evaporitic strata. The sediment cores showed engineering properties that range from good foundation strength to a potential for severe loss of strength through interaction between sediments and man-made structures.

Geology and biology of Oceanographer submarine Canyon

Abstract We investigated Oceanographer Canyon, which is on the southeastern margin of Georges Bank, during a series of fourteen dives in the “Alvin” and “Nekton Gamma” submersibles. We have integrated our observations with the results of previous geological and biological studies of Georges Bank and its submarine canyons. Fossiliferous sedimentary rocks collected from outcrops in Oceanographer Canyon indicate that the Cretaceous—Tertiary boundary is at 950 m below sea level at about 40°16′N where at least 300 m of Upper Cretaceous strata are exposed; Santonian beds are more than 100 m thick and are the oldest rocks collected from the canyon. Quaternary silty clay, deposited most probably during the late Wisconsin Glaciation, veneers the canyon walls in many places, and lithologically similar strata are present beneath the adjacent outer shelf and slope. Where exposed, the Quaternary clay is commonly burrowed by benthic organisms that cause extensive erosion of the canyon walls, especially in the depth zone (100–1300 m) inhabited by red crabs (Geryon) and/or jonah crabs (Cancer). Bioerosion is minimal on high, near-vertical cliffs of sedimentary rock, in areas of continual sediment movement, and where the sea floor is paved by gravel. A thin layer of rippled, unconsolidated silt and sand is commonly present on the canyon walls and in the axis; ripple orientation is most commonly transverse to the canyon axis and slip-faces point downcanyon. Shelf sediments are transported from Georges Bank over the eastern rim and into Oceanographer Canyon by the southwest drift and storm currents; tidal currents and internal waves move the sediment downcanyon along the walls and axis. Large erratic boulders and gravel pavements on the eastern rim are ice-rafted glacial debris of probable late Wisconsinan age; modern submarine currents prevent burial of the gravel deposits. The dominant canyon megafauna segregates naturally into three faunal depth zones (133–299 m; 300–1099 m; 1100–1860 m) that correlate with similar zones previously established for the continental slope epibenthos. Faunal diversity is highest on gravelly sea floors at shallow and middle depths. The benthic fauna and the fishes derive both food and shelter by burrowing into the sea floor. In contrast to the nearby outer shelf and upper slope, Oceanographer Canyon has not been extensively exploited by the fishing industry, and the canyon ecosystem probably is relatively unaltered.

Stratigraphy, structure, absolute age, and paleontology of the upper Pleistocene deposits at Sankaty Head, Nantucket Island, Massachusetts

The Sankaty Head cliff of Nantucket Island, Massachusetts, exposes drift of at least two glaciations and interglacial marine deposits. Radiocarbon, amino-acid-racemization, and uranium-thorium analyses were used to determine the absolute ages of the beds. The results indicate that (1) the Sankaty Sand correlates with oxygen-isotope stage 5 (Sangamonian), (2) the underlying drift is older than stage 5 (Illinoian or older), and (3) the overlying drift is Wisconsinan in age. Ostracodes and molluscs within the Sankaty Sand indicate that the marine climate during deposition of the lower part was somewhat warmer than the present climate off Sankaty Head and that the marine climate during the deposition of the upper part was as cold as or somewhat colder than the present climate. The paleoenvironmental data support a stage 5 (Sangamonian) age for the marine deposits.

Fish species and community distributions as proxies for seafloor habitat distributions: The Stellwagen Bank National Marine Sanctuary example (Northwest Atlantic, Gulf of Maine)

Defining the habitats of fishes and associated fauna on outer continental shelves is problematic given the paucity of data on the actual types and distributions of seafloor habitats. However many regions have good data on the distributions of fishes from resource surveys or catch statistics because of the economic importance of the fisheries. Fish distribution data (species or communities) have been used as a proxy for the distribution of habitats to develop precautionary conservation strategies for habitat protection (e.g., marine protected areas, fishing gear restrictions). In this study we assessed the relationships between the distributions of fish communities and species derived from trawl survey data with the spatial distribution of sediment types determined by sampling and acoustic reflectance derived from multibeam sonar surveys in Stellwagen Bank National Marine Sanctuary. Fish communities were correlated with reflectance values but all communities did not occur in unique sediment types. This suggests that use of community distributions as proxies for habitats should include the caveat that a greater number of communities within an area could indicate a greater range of habitat types. Single species distributions showed relationships between abundance and reflectance values. Trawl catches with low abundances had wide variations in reflectance values while those with high abundances had narrower ranges indicating habitat affinities. Significant non-random frequency-dependent relationships were observed for 17 of 20 species although only 12 of 20 species had significant relationships based on rank correlation. These results suggest that species distributions based on trawl survey data can be used as proxies for the distribution of seafloor habitats. Species with known habitat associations can be used to infer habitat requirements of co-occurring species and can be used to identify a range of habitat types.

Semidiurnal Temperature Changes Caused by Tidal Front Movements in the Warm Season in Seabed Habitats on the Georges Bank Northern Margin and Their Ecological Implications

Georges Bank is a large, shallow feature separating the Gulf of Maine from the Atlantic Ocean. Previous studies demonstrated a strong tidal-mixing front during the warm season on the northern bank margin between thermally stratified water in the Gulf of Maine and mixed water on the bank. Tides transport warm water off the bank during flood tide and cool gulf water onto the bank during ebb tide. During 10 days in August 2009, we mapped frontal temperatures in five study areas along ∼100 km of the bank margin. The seabed “frontal zone”, where temperature changed with frontal movment, experienced semidiurnal temperature maxima and minima. The tidal excursion of the frontal boundary between stratified and mixed water ranged 6 to 10 km. This “frontal boundary zone” was narrower than the frontal zone. Along transects perpendicular to the bank margin, seabed temperature change at individual sites ranged from 7.0°C in the frontal zone to 0.0°C in mixed bank water. At time series in frontal zone stations, changes during tidal cycles ranged from 1.2 to 6.1°C. The greatest rate of change (−2.48°C hr−1) occurred at mid-ebb. Geographic plots of seabed temperature change allowed the mapping of up to 8 subareas in each study area. The magnitude of temperature change in a subarea depended on its location in the frontal zone. Frontal movement had the greatest effect on seabed temperature in the 40 to 80 m depth interval. Subareas experiencing maximum temperature change in the frontal zone were not in the frontal boundary zone, but rather several km gulfward (off-bank) of the frontal boundary zone. These results provide a new ecological framework for examining the effect of tidally-driven temperature variability on the distribution, food resources, and reproductive success of benthic invertebrate and demersal fish species living in tidal front habitats.

Submarine glacial landforms on the Bay of Fundy–northern Gulf of Maine continental shelf

The Bay of Fundy–northern Gulf of Maine region surrounds the southern part of Nova Scotia, encompassing, from west to east, the Bay of Fundy, Grand Manan Basin, German Bank, Browns Bank, Northeast Channel and northeastern Georges Bank (Fig. 1a, b). During the last glacial maximum ( c. 24–20 14C ka BP), the SE margin of the Laurentide Ice Sheet (LIS) occupied the study area, the rest of the Gulf of Maine and the continental Scotian Shelf off Atlantic Canada (see Dyke et al. 2002, fig. 1; Shaw et al. 2006, fig. 8; Hundert & Piper 2008, fig. 16). Early mapping of the glaciated region on the Scotian Shelf using side-scan sonar imagery and seismic-reflection profiles revealed topographic features interpreted to be recessional moraines indicative of retreat of the LIS (King et al. 1972; King 1996). Subsequently, multibeam sonar seafloor mapping of local-scale glacial landforms on the inner Scotian Shelf off Halifax, Nova Scotia (Fig. 1b) provided further information on the dynamics of the advance and retreat of the ice sheet (Loncarevic et al. 1994). Interpretation of seismic-reflection profiles across Georges Bank revealed that the surficial sediment is a veneer of glacial debris transported to Georges Bank by the LIS during the late Pleistocene from continental areas to the north (Shepard et al. 1934; Knott & Hoskins 1968; Schlee 1973; Twichell et al. 1987; Fader et al. 1988). Recent high-resolution multibeam sonar surveys of German Bank and the Bay of Fundy mapped a complex of ice-advance and ice-retreat features attributed to the activity of the LIS (Todd et al. 2007; Todd & Shaw 2012). Fig. 1. Regional topography and seafloor features of the Bay of Fundy–northern Gulf of Maine shelf system. ( a ) Location of study area (red box; map from GEBCO_08). …

Large Submarine Sand Waves and Gravel Lag Substrates on Georges Bank Off Atlantic Canada

Publisher Summary Georges Bank is a large, shallow, continental shelf feature offshore of New England and Atlantic Canada. The bank is mantled with a veneer of glacial debris transported during the late Pleistocene from continental areas lying to the north. These sediments were reworked by marine processes during postglacial sea-level transgression and continue to be modified by the modern oceanic regime. The surficial geology of the Canadian portion of the bank is a widespread gravel lag overlain in places by well-sorted sand occurring as bedforms. Georges Bank is dominated by energetic tidal currents associated with the M2 semidiurnal tidal resonance in the Bay of Fundy–Gulf of Maine system. The major axis of the semidiurnal tidal current ellipse is oriented northwest–southeast across the bank, and the amplitude of this current at the seabed increases from 100 cm/s on the relatively shallow northern edge of the bank. The most widespread bedforms are large, mobile, asymmetrical sand waves up to 19 m in height formed through sediment transport by strong tidal-driven and possibly storm-driven currents. Well-defined curvilinear bedform crests up to 15 km long form a complex bifurcating pattern having an overall southwest–northeast strike, which is normal to the direction of the major axis of the semidiurnal tidal current ellipse. Minor fields of immobile, symmetrical sand waves are situated in bathymetric lows. Rare mobile, asymmetrical barchan dunes are lying on the gravel lag in areas of low sand supply. On Georges Bank, the management of resources and habitats requires an understanding of the distribution of substrate types, their surface dynamics and susceptibility to movement, and their associated fauna.

Shaded relief and sea floor topography of Quadrangle 2 in western Massachusetts Bay offshore of Boston, Massachusetts

Location map showing mapped Quadrangle 1 shaded in blue. The western Massachusetts Bay map series comprises quadrangles 1–3 (outlined in black); the maps for Quadrangle 1 include this map and Butman and others (2003a,b). The shaded relief and sea floor topography of Quadrangle 2 are shown at scale 1:25,000 in Butman and others (2003c). Quadrangles 1–18 (outlined in gray) compose the companion Stellwagen Bank National Marine Sanctuary (SBNMS) map series. The shaded relief and sea floor topography of the entire area of quadrangles 1–18 is shown at scale 1:60,000 in Valentine and others (2001, 2003b); it is also shown by quadrangle at scale 1:25,000 as U.S. Geological Survey Geologic Investigations Series Maps I–2701 (for quadrangle 1) through I–2718 (for quadrangle 18). The SBNMS boundary is shown as a dashed line. Selected bathymetric contours are labeled in meters. 25

Blake Escarpment Carbonate Platform Edge: Conclusions Based on Observations and Sampling from Research Submersible: ABSTRACT

Three continuous transects of the Blake Escarpment, east of Florida, were made during 10 dives in the submersible Alvin at depths from 1,400 to 4,000 m. We observed and sampled outcrops of horizontal strata known, from multichannel profiles across the dive sites, to extend westward beneath the Blake Plateau carbonate platform. The northern end of the Blake Escarpment, at the salient of the Blake Spur, is a nearly vertical limestone cliff, which is pitted and commonly fluted by vertical borings, coated by ferromanganese oxide, and heavily encrusted by organisms. Presumably, the cliff face is maintained by bioerosion and corrosion, and debris is removed by the strong turbulent currents (2 kn). Average slopes were less steep at transects 130 and 200 km south of the Blake Spu , but vertical cliffs as much as 450 m high exist. Talus slopes are common, and the large blocks and landward dips of beds suggest collapse of fragments at least several hundred meters across. On the southern transect are broad slopes of rippled pteropod sand between near-vertical outcrops; a vertical 160-m cliff of massive limestone at the top of the escarpment and rudists in talus blocks suggest the presence of a Mesozoic reef. Preliminary analysis of calcarenous nannofossils shows rocks as old as Early Cretaceous; identification of older rocks is anticipated. Sedimentary structures and components indicate deposition in shallow water. Thousands of meters of subsidence and extensive erosional retreat were required to create the escarpments present configuration. End_of_Article - Last_Page 918------------