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Research about Georges Bank

Source-linked reports with geographic coverage including Georges Bank.

26 records · Page 2Linked to original sources
USGS1984

The Georges Bank monitoring program 1983: Analysis of trace metals in bottom sediments

From July 1981, when drilling began on Georges Bank, to May 1982, the concentration of barium, a major element in drilling mud, has increased by a factor of 3.5 in bulk sediments 200 m from the drill site in block 410 and by a factor of 2.3 at the drill site in block 312. The postdrilling barium concentrations are within the range of predriiling concentrations measured at other locations on Georges Bank. We estimate that no more than 21 percent of the barite (principal barium-bearing mineral) discharged during drilling remains within 6 km of the drill site in block 312. No drilling-related changes in the concentration of chromium or other metals have been observed in bulk sediments from blocks 410 or 312. The chemical signal of drilling mud in surficial sediments has been enhanced by separating and analyzing the sediment fraction finer than 60 ?m. Barium concentrations in the fine fraction have increased by factors of 36 and 22 at the drill sites in blocks 410 and 312, respectively. At the drill site in block 410, aluminum, chromium, copper, and mercury concentrations in this fraction temporarily increased by approximately a factor of 2 and then decreased to background. The concentrations of aluminum, barium, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, organic carbon, vanadium, and zinc in surface sediments of Georges Bank both before and after drilling are low compared to concentrations in average crustal rocks and are characteristic of unpolluted, coarse-grained sediments as found in other Continental Shelf areas.

Georges Bank↗
USGS1984

The Georges Bank monitoring program, 1984: Analysis of trace metals in bottom sediments during the second year of monitoring

Of the 12 elements analyzed in bulk (undifferentiated) sediments collected adjacent to drilling rigs on Georges Bank, only barium was found to increase in concentration during the drilling period (July 1981 until September 1982). The maximum postdrilling concentration of barium (a major element in drilling mud) reached 172 ppm in bulk sediments near the drill site in block 410. This concentration is a factor of 5.9 times higher than the predrilling concentration at that location. This maximum postdrilling barium concentration is within the range of predrilling concentrations {28-300 ppm} measured in various sediment types from the regional stations of this program. No drilling-related changes in the concentrations of chromium or other metals have been observed in bulk sediments at any of the locations sampled in this program to date. We estimate that between 21 percent and 31 percent of the barite (principal barium-bearing mineral} discharged at block 312 was present in the sediments within 6 km of the rig, 4 weeks after drilling was completed. The barite deposited near this well was found to decrease in concentration with a half-life of 0.4year. At this rate, the average barium concentration in sediments within 6 km of the drilling rig in block 312 is expected to be only 10 percent higher than the predrilling concentration within approximately 1.5 years. Although the inventory of the barite discharged on Georges Bank is based on only a few data points, most (approximately 69 percent) of the barite discharged by the eight exploratory wells apparently can be found in sediments west of the drilling locations. The increase in barium concentration above background can be measured in the fine fraction of sediment at a distance of 65 km to the west of block 312. Analysis of sediment-trap samples collected 25 m above the bottom in block 312 indicates that the dispersion of barmm-rich fine sediment is enhanced by resuspension from the sea floor and transport to the west with the mean current flow. Evidence exists of small accumulations of barium near the heads of Lydonia and Oceanographer Canyons. However, the increased concentrations can be defined only by analyzing the fine fraction of sediment.

Georges Bank↗
USGS1983

Characteristics of resuspended sediment from Georges Bank collected with a sediment trap

A sediment trap was deployed 3 m from the bottom at a water depth of 62 m on the southern flank of Georges Bank (41°02·2′N, 67°33·5′W) from 30 September 1978 to 10 March 1979 to qualitatively determine the size of sediments resuspended from the bottom by winter storms and to determine if seasonal changes in the phytoplankton could be observed in the trapped sediment. Bulk X-ray analyses of the trapped sediment showed layers of distinctly different textures preserved in the collection vessel. The median grain size of sampled layers ranged from 2·7 to 6·5 φ (fine sand to silt), but all layers contained a pronounced mode in the 3 φ (fine sand) range. Nine layers containing relatively large amounts of sand were present. The sand content was 75% in the coarest layers and about 32% in the fine layers. The median grain size of bottom sediments at the deployment site was considerably coarser than the trap samples, although the dominant grain size was also 3 φ. Average bottom-current speeds during the deployment period were about 30 cm s −1 with a range of 10 to 50 cm s −1 . Bottom stress, computed from the observed currents and waves, suggest that 11 storms caused sufficient stress to resuspend 3 φ-sized sediments, in good agreement with the nine layers of relatively coarse sediments collected in the trap. Surface waves had to be included in the calculation of bottom stress because the bottom currents alone were insufficient to cause the resuspension of 3 φ-sized sediment. The trapped sediments contain numerous diatoms and coccoliths that are typical of late fall and winter assemblages. No clear seasonal difference in the flora was noted among sampled layers, probably due to the large influx of resuspended material and a reduced primary flux during this period. An undescribed species of Thalassiosira (G. Fryxell, personal communication), and siliceous scales of unknown systematic position were observed at all levels.

Georges Bank↗
USGS1981

Temporal and spatial variations in suspended matter in continental shelf and slope waters off the north-eastern United States

Seston in waters of Georges Bank originates primarily from biological production and from resuspension of bottom sediments. The concentrations of suspended matter observed on the central shoals are more influenced by storms than by seasonal changes. Winter storms produce highest concentrations of non-combustible material throughout the water column, and summer storms appear to increase biological production by mixing additional nutrients into the photic zone. On the south-east flank of the bank, in water depths between 80 and 200 in, the concentrations of total suspended matter and non-combustible material show little variation compared with the central shoals, and storm effects are far less noticeable. Highest concentrations (>15 mg 1 −1 ) of suspended matter occur in bottom waters south of Nantucket Island after winter storms and appear to be primarily resuspended bottom sediment. Resuspended sediment is also common in near-bottom waters of the south-western Gulf of Maine, and occasionally near the intersection of the shelf/slope water mass front and the bottom. Seasonal variations were observed in the distribution and species composition of phytoplankton. Coccoliths are predominant on the central bank during the winter, but during the spring and summer they are concentrated on the eastern flank at deeper depths.

Georges Bank↗
USGS1981

Single-channel seismic-reflection profiles and sidescan-sonar records collected on Georges Bank

Subbottom seismic-reflection, echo-sounding, and sidescan-sonar records were collected by the U.S. Geological Survey during two cruises over Georges Bank aboard the R/V OCEANUS (cruise 056, March 6-15, 1979 and cruise 067, August 6-13, 1979) (fig. 1). The data collected during these two cruises contributed to a study of bed-form types and distribution and of sediment transport paths on Georges Bank. The equipment used on these cruises consisted of an EG&G Uniboom 12-kHz echo-sounder, and Klein sidescan-sonar system. The Uniboom data were collected on the OCEANUS 067 cruise along 185 km of trackline (fig. 1), and the 12-kHz echo-sounder and sidescan-sonar data were collected during both cruises along 465 km of trackline. Navigation during both cruises consisted of Loran-C fixes logged at least every 15 minutes. After the cruise, the navigation data were digitized and stored on magnetic tape. The original records can be studied at the U.S. Geological Survey Data Library at Woods Hole, MA 02543. Microfilm copies of the subbottom, echo-sounding, and sidescan-sonar records can be purchased only from the National Geophysical and Solar-Terrestrial Data Center, NOAA/EDIS/NGSDC, Code D621, 325 Broadway, Boulder, CO 80303 (303-497-6338).

Georges Bank↗
USGS1981

Bed form distribution and inferred sand transport on Georges Bank

Four bed-form provinces have been identified on Georges Bank using sidescan sonar and echo-sounding techniques: large sand waves, small sand waves, megaripples, and featureless sea floor. The large sand waves are found on the bank crest where the surface tidal currents are strongest, and are bordered, north and south, by areas of small sand waves and/or megaripples, formed where tidal currents are moderate in strength. Featureless sea floor is found farthest from the bank crest where surface tidal currents are weakest. Bed-form asymmetry and surface-sediment texture have been used to infer bedload transport paths on the bank. In the large sand-wave area, bed forms converge on linear northwest-striking ridges from both sides implying erosion from the troughs and growth of the ridges. The asymmetry of the small sand waves along the south side of the bank indicates that sand is also transported southward away from the bank. Though the bed-form asymmetry of megaripples could not be determined in this study, the occurrence of megaripples between the sand-wave provinces and areas of featureless sea floor suggests a decreasing effectiveness of sand transport away from the bank crest. This sand transport pattern is further supported by surface-sediment texture which becomes progressively finer both to the north and southwest away from the crest of Georges Bank.

Connecticut, Maine, Massachusetts, New Hampshire, ↗
USGS1973

Gravity measurements in the vicinity of Georges Bank

A total of 97 new bottom gravity measurements on the continental shelf in the vicinity of Georges Bank was reduced to the simple Bouguer anomaly, using a density of 2.80 gm per cm 3 for the correction. Results help substantiate the presence of mafic and felsic intrusive bodies along the northern edge of the bank. A gravity low near the center of the bank, trending northeast, corresponds to the Georges Bank trough. An abrupt change in the gravity gradient near the southeast edge of the bank probably represents the thinning of the crust from continental to oceanic thicknesses. Differences in gravity gradient support the suggestion of a fault along the northern edge of the bank.

Georges Bank↗