Geology topics

Research about Middle Atlantic Bight

Source-linked reports with geographic coverage including Middle Atlantic Bight.

3 recordsLinked to original sources

USGS1994

210Pb balance and implications for particle transport on the continental shelf, U.S. Middle Atlantic Bight

Supply of 210 Pb to the continental shelf off the northeastern United States is dominated by the deposition from the atmosphere, the rate of which is reliably known from previously published work. Excess 210 Pb inventories in the shelf sediments show accumulations that are nearly in balance with the supply, even in areas of relict sands where it is believed that no net accumulation of sediment presently occurs. The 210 Pb distributions in shelf and slope water indicate that the two-way fluid exchange at the shelf-slope front and the net transport in the alongshore flow make comparatively small contributions to the shelf 210 Pb budget. The near balance between supply and decay of 210 Pb on the shelf implies a limit to the particle export flux. It is concluded that the export of particulate organic carbon does not exceed 60 g m −2 y −1 (&sim;25% of primary production) and is probably lower. The hypothesis is advanced that fine particulate matter introduced to the continental shelf is detained in its transit of the shelf because of bioturbational trapping in the sediment due to benthic animals. Distributions of 210 Pb in suspended particulate matter and in the fine fraction of shelf sediments suggest that the average fine particle must undergo several cycles of deposition-bioturbation-resuspension-redeposition and requires a number of decades for its transit and ultimate export from the shelf. Thus, only the most refractory organic matter is likely to be exported.

Middle Atlantic Bight↗

USGS1991

Modes of cross-shore sediment transport on the shoreface of the Middle Atlantic Bight

The mechanisms responsible for onshore and offshore sediment fluxes across the shoreface zone seaward of the surf zone were examined in a 3-year field study. The study was conducted in the southern part of the Middle Atlantic Bight in the depth region 7–17 m using instrumented tripods supporting electromagnetic current meters, pressure sensors, suspended sediment concentration sensors, and sonar altimeters. The observations embraced fairweather, moderate energy, swell-dominated, and storm conditions. Cross-shore mean flows ranged from near zero during fairweather to > 20 cm s −1 during the storm; oscillatory flows were on the order of 10 cm s −1 during fairweather and 100 cm s −1 during the storm. Suspended sediment concentrations at about 10 cm above the bed were < 0.1 kg m −3 under fairweather conditions, 1–2 kg m −3 under moderate swell conditions, and > 5 kg m −3 during the storm. Three methods were applied to evaluate the relative importance of incident waves, long-period oscillations, mean flows and gravity in effecting shoreward or seaward sediment flux: (1) an energetics transport model was applied to instantaneous near-bottom velocity data, (2) higher moments of near-bottom flows were estimated and compared, and (3) suspended sediment fluxes were estimated directly from the instantaneous products of cross-shore velocity and suspended sediment concentration. The results show that measurable contributions were made by all four of the processes. Most significantly, mean flows were seen to dominate and cause offshore fluxes during the storm and to contribute significantly to onshore and offshore flux during fairweather and moderate energy. Incident waves were, in all cases, the major source of bed shear stress but also caused shoreward as well as seaward net sediment advection. Low-frequency effects involving wave groups and long-period waves made secondary contributions to cross-shore sediment flux. Contrary to expectations, low-frequency fluxes were just as often shoreward as seaward. Whereas cross-correlations between suspended sediment concentration and the instantaneous near-bottom current speed were high and in phase under storm conditions, they were weak and out of phase during fairweather conditions. This suggests that simple energetics models are probably inadequate for predicting fairweather transport of suspended sediment.

Middle Atlantic Bight↗

USGS1979

Small-scale slump deposits, Middle Atlantic Continental Slope, off eastern United States

Analyses of 24 high-resolution seismic-reflection profiles that were collected during local and regional surveys show that small-scale slump deposite are ubiquitous whthin the intercanyon areas of the Continental Slope of the Middle Atlantic Bight. The deposits involve the upper 10-90 m of sediments, extend downslops for 1.8-7.2 km, and are present at water depths ranging from 545 to 1500 m. The characteristics of the deposits vary from thin, homogeneous or fairly regularly bedded lenses of sediment, to masses of intermediate thickness with contorted bedding, to relatively large slump blocks. A detailed survey of one slump mass just south of Hudson Canyon (by means of close-spaced Minisparker profiles and sediment cores) showed that it had a thickness of about 30 m and a volume of at least 0.4 km3 and consisted of homogeneous clay which accumulated rapidly during the late Pleistocene or Holocene. Although some of the slump deposits undoubtedly are relict, stemming from sediment instability porduced by rapid deposition during Pleistocene sea-level regressions, others were formed relatively recently. Possible causes of modern slumps include gas generation in the sediments, bottom-water turbulence on the upper slope, and shallow faulting. This study indicates that small-scale slumping in the intercanyon areas may be an important process in transporting sediments to the deep sea and suggests that recent mass movements may constitute a geologic hazard to future economic development of this part of the Continental Slope.

Middle Atlantic Bight↗