Biogenous sediments definition

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

Whereas positive proof is lacking that humic substances participate directly in weathering processes, it has been definitely established that humic substances play a role in the migration and enrichment of commercially important elements (Ag, Au, Cu, Ni, Zn, Pb, U, and V) in sediments and biogenic deposits. This topic is beyond the scope of the present chapter and the reader is directed to the reviews mentioned earlier for additional information. 

There is no consistent definition for the use of the term phosphorites. Suggestions reach from a limiting P content of 6 wt.% (van Cappellen and Berner 1988) to a threshold value of 18 wt.% P O, as representative for authigenic and biogenic phosphate minerals (Jarvis et al. 1994). However, the formation of secondary P phases from initially more labile-P in marine sediments (see above) is one major sink for phosphoms on Earth, because this general process results in sequestration of P from the nutrient cycle in the water column (e.g. Compton et al. 2000). 

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