Suspended Particles and Marine Sediments

The sediments represent the major sink for material in the oceans. The main pathway to the sediments is the deposition of suspended particles. Such particles may be only in transit through the ocean from a continental origin or be formed in situ by chemical and biological processes. Sinking particles can scavenge material from solution. Accordingly, this section introduces the components found in marine sediments, but emphasises processes that occur within the water column that lead to the formation and alteration of the deposited material. 

Marine sediments cover the ocean floor to a thickness averaging 500 m. The deposition rates vary with topography. The rate may be several millimetres per year in nearshore shelf regions, but is only from 0.2 to 7.5 mm per 1000 years on the abyssal plains. Oceanic crustal material is formed along spreading ridges and moves outwards eventually to be lost in subduction zones, the major trenches in the ocean. Because of this continual movement, the sediments on the seafloor are no older than Jurassic in age, about 166 million years. 

The formation of marine sediments depends upon chemical, biological, geological and physical influences. There are four distinct processes that can be readily identified. Firstly, the source of the material obviously is important. This is usually the basis for classifying sediment components and will be considered below in more detail. Secondly, the material and its distribution on the ocean floor are influenced by its transportation history, both to and within the ocean. Thirdly, there is the deposition process that must include particle formation and alteration in the water column. Finally, the sediments may be altered after deposition, a process known as diagenesis. Of particular importance are reactions leading to changes in the redox state of the sediments. 

The components in marine sediments are classified according to origin. Examples are given in Table 6. Lithogenous (or terrigenous) material comes from the continents as a result of weathering processes. 

Cosmogenous Aragonite Opaline silica Apatite Barite Organic matter Cosmic spherules Meteoric dusts CaC03 SiOz nHzO CajfF.Cl) (P04)3 BaS04 

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