Continental shelf/slope

Continental margin The zone separating the emergent continents from the deep-sea bottom it generally consists of a continental shelf, slope, and rise. 

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... 

Bacon MP, Belastock RA, Tecotzky M, Turekian KK, Spencer DW (1988) Lead-210 and polonium-210 in ocean water profiles of the continental shelf and slope south of New England. Cont Shelf Res 8 841-... 

Anderson RF, Bopp RF, Buesseler KO, Biscaye PE (1988) Mixing of particles and organic constituents in sediments from the continental shelf and slope off Cape Cod SEEP-1 results. Cont Shelf Res 8(5-... 

Fitzgerald et al. [477 ] showed that the most significant quantities of mercury occurred in the waters of the Atlantic Ocean s continental shelf and slope (21 -78 ng/1), compared with open ocean samples (2 -11 ng/1). 

Position of the coastline. The position of the coastline with reference to the position of the aquifer intake area should remain constant. This is particularly critical along coastlines with gently sloping topography and shallow water on the continental shelf. In such areas, lateral shifts of the shoreline of more than 200 km have been common during the past 20,000 years. 

Trace metals are introduced to the ocean by atmospheric feUout, river runoff, and hydrothermal activity. The latter two are sources of soluble metals, which are primarily reduced species. Upon introduction into seawater, these metals react with O2 and are converted to insoluble oxides. Some of these precipitates settle to the seafloor to become part of the sediments others adsorb onto surfaces of sinking and sedimentary particles to form crusts, nodules, and thin coatings. Since reaction rates are slow, the metals can be transported considerable distances before becoming part of the sediments. In the case of the metals carried into the ocean by river runoff, a significant fraction is deposited on the outer continental shelf and slope. Hydrothermal emissions constitute most of the somce of the metals in the hydrogenous precipitates that form in the open ocean. 

Continental shelf The sea floor adjacent to a continent, extending from the low-water line to the change in slope, usually at 180 m water depth, where the continental shelf and slope join. 

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°. 

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. 

Burrows, and transport of solute in them, may contribute to dissolution by enhancing oxic degradation of organic matter near the burrow walls. However, the situation is complex, and depending on factors such as the type of burrow wall produced, cementation rather than dissolution of carbonates may be promoted. Aller s observation that the best carbonate preservation takes place in the most physically disturbed and biologically underdeveloped environments points to the need for studies of continental shelf and slope environments where carbonate dissolution could be even more intense than that observed at the sites studied in Long Island Sound. 

James N.P. and Mountjoy E.W. (1983) Shelf-slope break in fossil carbonate platforms An overview. In Shelfbreak Critical Interface on Continental Margins (eds. D.J. Stanley and G.T. Moore), pp. 189-206. SEPM Spec. Pub. No. 33, Tulsa, OK. 

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

Pore-water nitrate profiles in marine sediments typically show one of three profile shapes. In sediment with rapid rates of organic matter oxidation relative to rates of solute supply from the overlying water, both oxygen and nitrate concentrations decrease more or less exponentially from overlying water concentrations at the sediment—water interface to zero, with oxygen depletion preceding or simultaneous with nitrate depletion at shallow sediment depth (see 105 m and 440 m profiles in Fig. 6.12). These types of profiles are common in continental shelf and upper slope sediments, and are due to relatively large carbon rain to the sediments (relatively... 

Kristensen, E., Devol, A. H., and Hartnett, H. E. (1999). Organic matter diagenesis in sediments on the continental shelf and slope of the eastern tropical and temprate North Pacific. Cont. Shelf Res. 19, 1331-1351. 

Marine DOC has stable carbon isotope values between —21%o and —22%o (Dmtfel et al., 1992) consistent with a largely marine source. While these data seem to exclude a significant contribution from C3 terrestrial plants, there is increasing evidence for an important contribution from C4 plants to persistent POM in marine sediments on the continental shelf and slope (see below). Desorption of C4 plant carbon and incorporation into oceanic DOC would be difficult to detect by isotopic or molecular biomarker analyses. 

Figure 17 Generalized relationship between weight percent OC (% OC) and specific mineral surface area (SA) for marine sediments. The shaded area represents the boundaries (OC SA ratio of 0.5-1.1 mgCrn ), within which most continental shelf and upper slope sediments (outside the direct influence of rivers) fall. Sediments underlying anoxic basins and OMZs associated with high productivity (upwelUng) margins tend to exhibit OC SA ratios greater than l.lmgCm whereas deltaic and abyssal sediments exhibit OC SA ratios of less than 0.5 mg C m (after Mayer, 1994 ...

Continental shelf—A relatively shallow, gently sloping, submarine area at the edges of continents and large islands, extending from the shoreline to the continental slope. 

Bernard, B.B., Brooks, J.M. and Sackett, W.M., 1978. Light hydrocarbons in recent Texas continental shelf and slope sediments. J. Geophys. Res., 83 4053-4061. 

Plot of total organic carbon-normalized yield, A (mg of the CuO reaction products per 100 mg OC) versus 8 C for surface sediments from the Washington State continental shelf and slope (from Hedges and Mann,... 

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