Upper Continental Slope

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). 

More recently in situ microsensor measnrements of Oj, pH, pCOj, and Ca conld be obtained from the upper continental slope off Gabon (Wenzhofer et al. 2001 Adler et al. 2001 Pfeifer et al. 2002), which, to date, provide one of the most complete sets of non-conupted pore water data from deep-sea sediments. The bottom water at a water depth of abont 1300 m is slightly oversatnrated with respect to calcite (Q =... 

The thermodynamically defined gas hydrate zone in sediments of the deep sea is much thicker than in sediments underlying shallow water, such as those in the upper continental slope (see Fig. 

Slides that occur long after deposition of sediments on the slope are caused by major changes in sedimentary and erosional processes. These changes typically may be caused by eustatic sea-level changes during glacial times, which causes direct deposition of sediments on the upper continental slopes. 

Stanley, D.J., and Wear, G.M. 1978. The "mud-line" An erosion-deposition boundary on the upper continental slope. Marine Geology, 28 M19-M29. 

Coleman, J.M. 1975. Subaqueous Mass Movement of Mississippi River Delta Sediments and Adjacent Continental Shelf and Upper Continental Slope. Coastal Studies Institute Files, Louisiana State University, Baton Rouge, LA. 

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

At the outer edge of the shelf and in the upper part of the continental slope, silty and clayey coquina and lowly-carbonate oozes are mostly developed. The sediments of the continental slope (as well as those of the major part of the deep-water basin of the Black Sea) are represented by clayey-calcareous (coc-colith) oozes. 

In the upper 500-m layer, the BSGC consists (see Fig. 1) of the MRC along the entire continental slope several SBCGs in the central area, whose number and position change during the year, and a few NSAEs existing in fixed areas between the MRC and the shore over 5-9 months per year. 

The seasonal and interannual variabilities of the upper mixed layer, the seasonal pycnocline, and the CIL are caused by the corresponding variations in the heat and freshwater fluxes through the sea surface and in the riverine runoff. The ventilation of the Black Sea waters is restricted to the CIL it is renewed over the major part of the area in severe winters and in some regions (focuses of ventilation) over the shelf and continental slope of the western part of the sea in other years. The seasonal and interannual variabilities of the... 

Approximately 70 % of the continental shelf surface is covered with relict sediment, i.e. sediment deposited during the last glacial period under conditions different from today s, especially at times when the sea level was comparatively low (Emery 1968). It has to be assumed that there is a kind of textural equilibrium between these relict sediments and recent conditions. The fine-grained constituents of shelf sediments were eluted during the rise of the sea level in the Holocene and thereafter deposited, over the edge of the shelf onto the upper part of the continental slope, so that extended modem shelf surface areas became covered with sandy relict sediment (Milliman et al. 1972 Milliman and Summerhays 1975). 

No Defined by its stability field the potential volume for gas hydrate is mueh larger in the deep sea than on the upper slope however, the availability of sufficient methane is a critical requirement for gas hydrate formation. Due to high plankton productivity continental slopes have, in general, higher organic carbon concentrations than deep sea areas. Higher amounts of methane and gas hydrate are therefore restricted to continental margins. 

Fig. 1.37 shows the seasonal variation in the vertical temperature distribution at four different sites shown in Fig. 1.36. The temperature variation in the SCS mainly occurs in the upper eight layers, 0 400 m. The thermal structure to the west of Luzon, station C, and in the southern SCS, station F, shows distinct seasonal variations, cold in winter and warm in summer. The seasonal change in the temperature on the continental slope to the southwest of Taiwan, station D, has quite a different pattern. The Kuroshio intrusion across the Luzon Strait in October increases the upper layer temperature at station D. The water temperature in this slope area shows a major drop only when the cyclonic eddy to the west of Luzon extends into this area after February...

Example—Continental Slope Without shortchanging the unique environment of the continental margins, it can be generalized that the continental slope represents a transition between the shelf and the deep-sea floor. Consequently, the upper portions of the margins can be expected to be more variable than the lower portions. Areas such as submarine canyons are special cases that may be as complex as the most variable areas on the continental shelf. 

Watkins, D.J., and Kraft, L.M. 1978. Stability of continental shelf and slope off Louisiana and Texas Geochemical aspects. In Framework, Facies and Oil-Trapping Characteristics of the Upper Continental Margin, Bouma, A.H., Moore, G.T., and Coleman, J.R., eds., American Association of Petroleum Geologists, Tulsa, OK, vol. 7, pp. 267-286. 

On the continental slopes, gas hydrates typically occur below 500 m water depth and within the upper 700 m of sediment, though they can go significantly deeper. In the permafrost regions, the zones can range between 100 m (in places in Siberia) and as deep as 2000 m. The exact conditions of stability depend on the composition of the hydrate. The zones where they are found depend on the... 

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... 

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 ...

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