Siliceous ooze

Minerals South Pacific North Pacific West Indian Atlantic Red Clay Siliceous Oozes... 

While rapid burial enhances preservation, the type of sediment produced is determined by the relative particle composition of the deposit. For example, rapid burial of biogenic silicate by clay minerals helps protect the shells against dissolution, but the resulting deposit is classified as an abyssal clay, rather than a siliceous ooze, if the sediment is less than 30% by mass BSi. Thus, prediction of the sediment type likely to be found at a given location requires knowledge of the relative magnitudes of the accumulation rates of all particle types. 

The model provided in Figure 20.1 is for an ocean basin whose abyssal plains all lie below the CCD. This most closely resembles the conditions in the North Pacific, whereas the rest of the ocean basins have a significant portion of their abyssal plains lying above the CCD, and, hence, contain some calcareous oozes. From a global perspective, calcareous oozes are more abundant than siliceous oozes. This is caused by two phenomena (1) all seawater is undersaturated with respect to opal, whereas all surface waters and 20% of the deep waters are saturated with respect to calcite, and (2) siliceous plankton are dominant only in upwelling areas. 

On the eastern margin, a small deposit of siliceous ooze is located slightly south of the equator. This deposit is associated with the coastal upwelling area near Walvis Bay (23°S). The geographic spread of this deposit is limited because the seafloor in this area lies above the CCD, so calcite dilutes the BSi. This effect increases with increasing distance from the upwelling area. 

Composition Red clay Calcareous ooze Siliceous ooze... 

Siliceous oozes are accumulations of opaline silica (opal-A, an amorphous phase of high water content and porosity) in the tests of diatoms, radiolarians, and/or silicoflagellates. Opal-A solubility at 25 °C is 60-130 ppm Si02(aq) (e.g., Williams etal., 1985), and solubility increases with increasing temperature and pressure (Walther and Helgeson, 1977). Adsorption of aluminum and iron on the surfaces of siliceous tests decreases their solubility (Her, 1955 Lewin, 1961). Opal-A is a metastable phase that with burial eventually recrystallizes to quartz, often with another metastable intermediary phase, opal-CT (e.g., Hein et ai, 1978 Williams et ah, 1985 Williams and Crerar, 1985). Opal-CT structurally resembles an inter-layering of the two silica phases, cristobalite... 

Results of factor analysis (varimax used in this chapter throughout) on chemical composition data for the surface siliceous oozes from the Wahine survey area (8°2(y N, 153° W 34 samples Calvert et al. (1978)) are shown in Figures 3(a) (factor... 

O pelagic clay siliceous ooze X topographic high... 

Figure 3 Plots of (a) factor loadings and (b) factor scores of factors FI and F2 for the surface siliceous ooze samples from Wahine survey area near Hawaii (Calvert et al, 1978). (c) and (d) are similar plots for surface pelagic sediment samples from the equatorial Pacific (Calvert and Price, 1977).

Figure 4 X-ray diffraction patterns contrasting various crystallinities of silica (a) radiolarian silica, Porcelanite (opal-CT) and a-Cristobalite (made by heating silica gel at 1,350 °C for 4 h) from Calvert (1983) (b) diatom assemblage from Antarctic plankton tow, deep-sea siliceous ooze (Holocene in age) from beneath the Antarctic Polar Front, and two chert deposits from state of New York. The sharpness of the silica peak(s) between 20° and 26° two theta increases as silica undergoes diagenetic transformation from a fresh-diatom assemblage to buried sediment for...

Kastner M., Keene J. B., and Gieskes J. M. (1977) Diagenesis of siliceous oozes 1. Chemical controls on the rate of opal-A to opal-CT transformation—experimental study. Geochim. Cosmochim. Acta 41, 1041-1058. 

Bohrmann G., Abehnann A., Gersonde R., Hubberten H., and Kuhn G. (1994) Pure siliceous ooze, a diagenetic environment for early chert formation. Geology 22(3), 207—210. 

Seawater Pore water Siliceous oozes Carbonate oozes Pelagic days Clastic sediments Altered basalts Fresh basalts Layer-3 gabbros Continental sediments Sandstones Limestones Shales... 

Deep-sea sediments cover more than 50% of the earth s surface and consist of carbonates, red clay and siliceous ooze (cf. Chap. 1). On average, red clay covers about 31% of the world s ocean basins but its abundance is much higher in the Pacific (49%) than in the Atlantic (26%) and Indian (25%) Oceans (Glasby 1991). Carbonates act as a diluent for the transition elements in deep-sea sediments because of the low contents of these elements in them and the composition of deep-sea sediments is therefore often presented on a carbonate-free basis. 

C) and the low productivity SW Pacific subtropical anticyclonic gyre (Area K), Stoffers et al. (1981) showed that the siliceous oozes from the equatorial North Pacific have much higher contents of Mn, Ni, Cu and Ba but lower contents of Fe and Co than the red clays from the SW Pacific (Table 11.3). Sedimentation rates on a carbonate-free basis for the two areas are of the same order (1-3 mm ka for the equatorial North Pacific and 0.5-1 mm ka" for the SW Pacific). Differences in the transition metal contents of these sediments were therefore considered to be eontrolledby sediment type rather than sedimentation rate. Caleulations based on the equations of Bischoff et al. (1979) confirmed that the hydrogenous (authi-genic) eomponent was much higher in Area C (7.9%) than in Area K (3.2%) sediments. 

Micronodules from the C-CF.Z. occur mainly within siliceous ooze. They consist of 10 A manganate with traces of quartz and sometimes phillipsite. They have Mn/Fe ratios of 4.7 andNi+Cu contents of 1.7%. Again, the Mn/Fe ratios are somewhat higher than for the associated nodules but the Ni+Cu contents somewhat lower. The micronodules are dominantly spheroidal or have rod-like stractures. Under the SEM, the surfaces of the micronodules consist of plates. 

Radiometric dating has revealed wide variations (by four orders of magnitude) for the growth rates of marine manganese deposits Co-rich Mn cmsts (0.8 mm Ma Puteanus and Halbach 1988), deep-sea manganese nodules on red clay substrates (1-2 mm Ma Hu and Ku 1984), deep-sea manganese nodules on siliceous ooze substrates (3 -8 mm Ma" Hu and Ku... 

Calcareous and siliceous sediment differ significantly by their sediment grain density. It amounts to about 2.7 g cm for calcareous ooze and to about 2.1 g cm for siliceous ooze, so that a downcore grain density model has to be developed from the carbonate (and opal) content, in order to calculate wet bulk densities from porosities. 

Pacific Ocean As shown in, nodules are abundant in the Pacific Ocean in a broad area, called the Clarion-Clipperton Zone, between about 6°N and 20°N, extending from approximately 120°W to 160°W. The limits of the area are largely determined by sedimentation rates. Nodules are also locally abundant further west in the Central Pacific Basin. Sediments in the northern part of the areas of abundant nodules in the North Pacific are red clays with accumulation rates of around 1 mm per thousand years whereas in the south they are siliceous oozes with accumulation rates of 3 mm per thousand years, or more. 

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