Continental Rise

Particulate matter that reaches the seafloor becomes part of the blanket of sediments that lie atop the crust. If bottom currents are strong, some of these particles can become resuspended and transported laterally until the currents weaken and the particles settle back out onto the seafloor. The sedimentary blanket ranges in thickness from 500 m at the foot of the continental rise to 0 m at the top of the mid-ocean ridges and rises. Marine scientists refer to this blanket as the sedimentary column. Like the water column, the sediments contain vertical gradients in their physical and chemical characteristics. Similar to the vertical profile convention used in the water column, depth in the sediments is expressed as an increasing distance beneath the seafloor. 

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. 

A second mechanism by which CO2 is regenerated as part of the crustal rock cycle is thought to occur imder high pressures and temperatures such as found in subduction zones and under thick sedimentary prisms in the continental rise. This decarbonation... 

The resulting CO2 gas is returned to the atmosphere by two means (1) volcanic emissions associated with eruptions near subduction zones, i.e., back-arc volcanoes or (2) diffusion through the sediments of the continental rise into the ocean, followed by gas exchange across the air-sea interfece. The combined production of CO2 from these two settings is thought to exceed that from the high-temperature hydrothermal reaction zones. 

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°. 

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. 

Cai W.-J., Reimers C. E., and Shaw T. (1995) Microelectrode studies of organic carbon degradation and calcite dissolution at a Cahfornia continental rise site. Geochim. Cosmochim. Acta 59, 497-511. 

Figure 8 Summary of the ratio of CaCOs dissolved and organic material oxidized for bottom chamber deployments in the northeastern Pacific, Ontong-Java Plateau, Ceara Rise, Cape Verde Plateau, northwestern Atlantic continental rise and California borderland basins (R. A. Jahnke and D. B. Jahnke, 2002). The absence of measurable alkalinity fluxes from high-CaCOs sites bathed in supersaturated bottom water appears to be inconsistent with observations...

Pauli, C.K., Spiess, F.N., Ussier, W. and Borowski, W.A., 1995. Methane-rich plumes on the Carolina continental rise association with gas hydrates. Geology, 23 89-92. 

The results of measurements of major ions in sediment porewaters collected by in situ filtration. Values are presented as the difference in the concentration between bottom water and porewater samples. Stations are from the continental margin and deep sea in the South Atlantic (A) on the mid-Atlantic Ridge between Rio de Janeiro and Capetown at c.2000 m (B) from the continental rise off Rio de Janeiro at 2000 m (C) from the continental slope off Rio de Janeiro at c. 1000 m and (D) from the Venezuela Basin east of Barbados at c.3000 m. Note that the concentrations of both K" " and Mg + decrease with depth in almost all cases. From Sa/les, (1979). 

With eustatic sea level transgression and flooding of continental lowlands, transport of terrigenous organic matter into the North American basin may have increased. Analyses of Upper Cretaceous sediments from DSDP Site 6O3B, lower continental rise east of Cape Hatteras, indicate that, the organic matter was continentally derived. d C values of -23.5 to 27.1°/oo, C/N ratios of 32 to 72, and lipid class maxima of unbound alkanes (C-. and Cp-), unbound fatty acids (C and C2g) and bound fatty acids (C-g. 

Table I. Site description of DSDP Site 6O3B lower continental rise, U.S. east coast (10).

Rullkotter, J., Mukhopadhyay, P.K. and Welte, D.H., 1987. Geochemistry and petrography of organic matter from Deep Sea Drilling Project Site 603, lower continental rise off Cape Hatteras. In van Hinte, J.E., Wise, S.E. Jr et al. (eds). Initial Reports DSDP, 92, US Government Printing Office, Washington DC, pp. 1163-1176. 

Fig. 14.4 Left Stability field of pure methane hydrate at normal seawater salinity, as defined by temperature and pressure expressed as water depth. Intersections of the temperature profiles (stippled lines) with the phase boundary (heavy line) define the area of the gas hydrate stability zone (GHSZ). Right Inferred thickness of the gas hydrate zone in sediments at a schematic continental margin assuming a typical geothermal gradient of 28°C km. Typical bottom water temperatures are marked, and range from 18°C on shallow shelf regions to 2°C at the bottom of the continental rise (after Kvenvolden and McMenamin 1980).

Claypool G. E., Lorenson T. D. and Johnson C. A. (2(X)3) Authigenic Carbonates, Methane Generation and Oxidation in Continental Rise and shelf sediments, ODP Leg 188 Sites 1165 and 1166, Offshore Antarctica (Prydz Bay). In Proceedings of ODP, Scientific Results (eds. P. E. O Brien, A. K. COOPER, C. Richter and Shipboard Scientific Party), Vol. 188, pp. 1 -15. Available online from World Wide Web E-mail http //www-odp.tamu.edu/ publications/188 SR/VOLUME/CHAPTERS/004.PDF [cited 2003-02-10]. 

Continental Slope and Continental Rise 2.5.3.1 Siliciclastic Slopes and Submarine Fans... 

The continental slope is the "relatively steep (usually 3-6°) portion of the seafloor which lies at the seaward border of the continental shelf" (Heezen et al., 1959) in water depths ranging from 100-200 m to 1,400-3,200 m, and locally to much greater depths (Bouma, 1979). This area is located just above the transition between continental and oceanic crust. The continental slopes are narrow (20-100 km), covering an area approximately 2.87 X 10E6 km, 5.6% of the Earth s surface (Drake and Burke, 1974). About half of the world s continental slopes terminate in deep-water trenches or shallower depressions, the rest gradually merging with slopes of 1 100 to 1 700 into continental rises and their deep-sea fans (Curray, 1966). 

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