What Are Deep Sea Clay? Deep Sea Clay Information

The clay materials formed in large parts of the deep sea and oceanic basins are, generally, quite distinct from terrigenous clays. This is because many such areas are so far removed from land that detrital terrigenous material becomes a minor, even insignificant, source of sediment.

As a result, the products of other processes make a more important contribution to the fine grained sediments that accumulate in these environments (Berger 1974). Globally, the most important of these are the minute skeletal components of microfossils which form a continuous pelagic rain from surface to deeper waters.

Their contribution to the fine grained sediment accumulating at the ocean floor depends upon the dynamic balance between the processes of their production in surface waters and their destruction by dissolution on their journey down through the water column following death of the organisms. The two most important biogenic components are calcareous and siliceous microfossils.

The calcareous microfossils include foraminifera and coccoliths composed of calcium carbonate (CaCO3) mainly in the form of calcite, whilst the silceous microfossils include diatoms and radiolaria composed of opaline amorphous silica (SiO2), in the form known as opal-A. The rate of production of these organisms in surface ocean waters depends on biological fertility.

Diatoms dominate in more fertile nutrient rich water whereas coccoliths dominate in less fertile regions. Since seawater is universally under saturated with respect to amorphous silica, most silica is dissolved and recycled
as the skeletons of opaline microfossils descend through the water column.

A further fraction arrives at the sediment water interface where more is dissolved but in regions of high productivity some is preserved and may accumulate. Thus the distribution of siliceous pelagic sediments mirrors the patterns of the most highly productive ocean waters such as in regions of oceanic divergence and upwelling where nutrient-rich deep ocean waters rise to the surface.

The fate of calcareous pelagic sediment is similar except that the degree of undersaturation of seawater with respect to carbonates increases with depth. This gives rise to a depth in the oceans, known as the Calcite Compensation Depth (CCD), below which calcite does not accumulate.

In the deepest parts of the Ocean basins (> 3500 m) below the CCD, sedimentation rates may be extremely slow and hydrogenous processes involving iron and manganese oxides take on an important role. Such areas accumulate deposits know as deep sea red clays (Glasby 1991).

Red clays are extremely fine grained with often more than 80% < 2 um in size. They cover about 30% of the ocean basins and are most prevalent in the Pacific Ocean. Most of the components of Pacific red clays are allogenic, the most important being aeolian dust.

Red clays accumulate very slowly with the highest rates of sedimentation coeval with Pleistocene glacial periods when aeolian dust production was at a maximum (Glasby 1991). Because of their fine grain-size and long term stability serious consideration has been given to using red clays as sites for radioactive waste disposal (Burkett et al. 1991).

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