Continental slope

The first reports of plastic in the North Atlantic indicated the presence of 50-12,000 particles/km in the Sargasso Sea in 1972 (52) and from 0-14.1 particles per m in coastal waters of southern New England (42), where the main source was river-borne effluents from plastic fabrication plants (44). Plastic objects discarded from boats and from recreational activities on beaches were the main sources of debris in Narragansett Bay, being deposited at a rate of 9.6 g m of beach front per month (53). During a detailed survey off the southeast coast of the United States (43, 54), fragments of plastic were present in about 70% of the samples collected from the waters of the continental shelf, the continental slope and the Gulf Stream between Florida and Cape Cod, 50% of those from the Caribbean Sea, and 60% of those from the Antilles Current. Since unprocessed plastic was more prevalent in continental shelf waters and fabricated objects were common offshore but rare near land, the authors surmised... 

Carpenter R, Beasley TM, Zahnle D, et al. 1987. Cycling of fallout (Pu,241 Am, 137Cs) and natural (U, Th, 210Pb) radionuclides in Washington continental slope sediments. Geochim Cosmochim Acta 51 1897-1921. 

In the case of the turbidity currents, this redistribution usually occurs along the foot of the continental slope and is largely responsible for the accumulation of sediments in the continental rise. The resuspension of particles by contour currents can also maintain permanent nepheloid layers as shown in Figure 13.10. 

Continental rise The large sedimentary deposit that lies at the foot of the continental slope. 

Continental slope A declivity that extends from the outer edge of the continental shelf to the continental rise. The angle is approximately 4 to 5°. 

Hemipelagic sediments Sediments that lie in water depths of 200 to 3000 m (roughly encompassing the continental slope and upper part of the rise). 

Turbidite The sedimentary deposit created by turbidity currents. The latter are underwater mudslides common to the continental slope and rise. 

Turbidity current An underwater mudslide common to the continental slope. The particles deposit at the foot of the slope to form the continental rise. The resulting deposit is often referred to as a turbidite. 

Vanden Heuvel, 1966 Paquet, 1970), continental lakes (Millot, 1964 McLean, et al., 1972 Parry and Reeves, 1968) and they appear in shallow seas, continental slope and deep-sea sediments (Latouche, 1971 Bonatti and Joenesu, 1968 Hathaway and Sachs, 1965 MUller, 1967 Chamley, et al... 1962 Millot, 1964 Bowles, et al., 1971 Blanc-Vernet and Chamley, 1971 Fleischer, 1972). They are present in carbonates and salt deposits (Bartholome, 1966b Braitsch, 1971 Millot, 1964 Peters and von Salis, 1965). There seems to be no exclusion of either species from any of these environments. There is little conclusive evidence for their diagenetic formation during burial and lithification processes but the possibility should be considered (Millot, 1964). 

Our study was based on samples from 11 (Figure 1) 1000-foot core holes drilled in the Gulf of Mexico by four oil companies Humble, Chevron, Gulf, and Mobil. All cores are from the present continental slope within three morphological areas the Upper Continental Slope off Texas and Louisiana, the Upper Continental Slope off west Florida, and the upper reaches of the Mississippi Cone—a mass of sediment derived from drainage of the Mississippi River which has locally buried the continental-slope morphology. 

For comparison, a core (K) in the Mississippi Canyon was studied this site on the upper continental slope has likely had the maximum contribution of terrestrial sediments. The n-paraffin chromatograms showed here, as expected, a marked odd-carbon preference (Figure 6). 

Sassen, R., MacDonald, I.R., Thermogenic Gas Hydrates, Gulf of Mexico Continental Slope, in Pmc. 213th ACS National Meeting, San Francisco, CA, April 13-17, 42(2), 472 (1997b). 

Boetius, A., and E. Damm. 1998. Benthic oxygen uptake, hydrolytic potentials and microbial biomass at the Arctic continental slope. Deep-Sea Research I 45 239—275. 

Poremba, K., and H.-G. Hoppe. 1995. Spatial variation of benthic microbial production and hydrolytic enzymatic activity down the continental slope of the Celtic Sea. Marine Ecology Progress Series 118 237-245. 

Sassen R, Joye S, Sweet ST, DeFreitas DA, Milkov AV, MacDonald IR (1999) Thermogenic gas hydrates and hydrocarbon gases in complex chemosyn-thetic communities, Gulf of Mexico continental slope. Org Geochem 30 485-497... 

It is convenient to divide the deposition of carbonates in marine sediments into those being deposited in shallow (shoal) water (water depths of a few hundred meters or less) and those being deposited in deep sea sediments, where the water depth is on the order of kilometers. The primary reasons for this division are the differing sources, dominant mineralogies, and accumulation processes operative in these environments. Shoal water carbonates are the topic of Chapter 5. Naturally, there are "grey" areas of intermediate characteristics between these two extremes, such as continental slopes and the flanks of carbonate banks and atolls. 

More recent calculations such as those in this book indicate substantially lower saturation depths. Those calculated here are plotted in Figure 4.21. The SD is generally about 1 km deeper than that presented by Berger (1977). Clearly the new SD is much deeper than the R0 and appears only loosely related to the FL. Indeed, in the equatorial eastern Atlantic Ocean, the FL is about 600 m shallower than the SD. If these new calculations are even close to correct, the long cherished idea of a "tight" relation between seawater chemistry and carbonate depositional facies must be reconsidered. However, the major control of calcium carbonate accumulation in deep sea sediments, with the exceptions of high latitude and continental slope sediments, generally remains the chemistry of the water. This fact is clearly shown by the differences between the accumulation of calcium carbonate in Atlantic and Pacific ocean sediments, and the major differences in the saturation states of their deep waters. 

Other reactions of probable less importance than those above leading to undersaturated conditions with respect to calcium carbonate near the sediment-water interface include nitrate reduction and fermentation (e.g., Aller, 1980). Such reactions may also be important near the sediment-water interface of continental shelf and slope sediments, where bioturbation and bioirrigation can result in enhanced transport of reactants. Generally, as water depth increases over continental slope sediments, the depth within the sediment at which significant sulfate reduction commences also increases. It is probable that the influence of reactions other than sulfate reduction on carbonate chemistry may increase with increasing water depth. 

Morse J.W. and Cook N. (1978) The distribution of phosphorous in North Atlantic deep sea and continental slope sediments. Limnol. Oceanogr. 23, 825-830. 

Active margin a margin consisting of a continental shelf, a continental slope, and an oceanic trench. 

Alperin, M.J., Martens, C.S., Albert, D.B., Suayah, I.B., Benninger, L.K., Blair, N.E., and Jahnke, R.A. (1999) Benthic fluxes and porewater concentration profiles of dissolved organic carbon in sediments from the North Carolina continental slope. Geochim. Cosmochim. Acta 63, 427 148. 

DeMaster, D.J., Thomas, C.J., Blair, N.E., Fornes, W.L., Plaia, G., and Levin, L.A. (2002) Deposition of bomb 14C in continental slope sediments of the Mid-Atlantic Bight assessing organic matter sources and burial rates. Deep-Sea Res. II. 49, 4667-4685. 

Ransom, B., Kim, D., Kastner, M., and Wainwright, S. (1998) Organic matter preservation on continental slopes importance of mineralogy and surface area. Geochim. Cosmochim. Acta 62, 1329-1345. 

Walsh, J.J., Premuzic, E.T., Gaffney, J.S., Rowe, GT., Harbottle, G, Stoenner, R.W., Balsam, W.L., Betzer, P.R., and Macko, S.A. (1985) Organic storage of carbon dioxide on the continental slope off the Mid-Atlantic Bight, the southeastern Bering... 

In the bottom topography of the sea, one can clearly distinguish three principal structures the shelf, the continental slope, and the deep-water basin. The shelf occupies up to 25% of the total area of the seafloor and, on average, is restricted to sea depths of 100-200 m. It reaches its greatest width (more than 200 km) in the northwestern part of the sea, which is entirely located within the shelf zone. Almost over the entire extension of the eastern and southern coasts of the sea, the shelf is very narrow (only a few kilometers wide) in the western part of the sea, it is wider (a few tens of kilometers). 

The continental slope includes up to 40% of the seafloor area it descends down to depths of 2000 m. It is steep and cut by submarine valleys and canyons. Its lower part, located at depths of 1500-2000 m, is referred to as the continental foot in selected cases, the boundary between the slope and the foot is poorly expressed. The floor of the basin (35% of the total area) represents a flat accumulative plain with a surface that gradually declines toward the center of the sea. 

Keywords Coasts Shelf Continental slope Continental footstep Underwater canyons Deep-sea floor Bottom sediments... 

Fig. 1 Topography of the coasts and floor of the Black Sea. Bottom relief 1 shelf a accumulative, b abrasive 2 continental slope a accumulative, b stepwise 3 floor of the basin 4 continental footstep 5 underwater canyons 6 bars a sandy, b marginal 7 morphological boundaries a distinct, b fuzzy. Coast types 1 landslide 2 abrasive 3 abrasive-accumulative 4 accumulative 5 lagoonal 6 deltaic...

The submarine topography of the Black Sea can be naturally subdivided into the zones of the shelf, continental slope, continental footstep, and the floor of the deep-sea depression (Fig. 1). 

The continental slope of the Black Sea basin is located below the outer edge of the shelf. It has a complicated heterogeneous structure caused by the particular features of the tectonics of the adjacent plains and mountain ridges -the Crimean, Caucasian, Stara Planina, and West and East Pontian ridges. The depth of the edge of the continental slope ranges from 100 to 200 m. Its lower boundary is marked by a topographic bend at sea depths of 1100-1500 m. 

In tectonically active areas of the continental slope, structural topographic features dominate and, in the transverse profiles of the slope, relatively gentle (1-3°) accumulative surfaces are sharply replaced by steep almost vertical (10-30°) escarpments, often featuring a stepwise profile and cut by systems of faults. Over the steep slopes, landslide processes develop [9]. 

For example, these kinds of processes actively proceed on the Caucasian continental slope off Dzhugba and Arkhipo-Osipovka. Here, the underwater relief is characterized by extreme complicacy and irregularity. Landslide formations are encountered at depths of about 850 m at a distance of 6 km from the coast. The thickness of the sliding units is 20-25 m at a length of 350-400 m. The landslides descend to depths of 1200-1500 m at the foot of the continental slope. 

The steepest and narrowest portions of the continental slope are confined to the Crimean coast, the Adler segment of the Caucasian coast, and to the regions off Trabzon and Zonguldak of the Anatolian coast. These parts of the continental slope are dissected by series of underwater canyons. 

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