Siliceous biogenic silicate

Biogenic A. Calcareous B. Siliceous >30 >30 48 Foraminifera, coccoliths, calcareous algae, molluscs, bryozoa, and corals 14 Diatoms and radiolaria... 

From a geochemical perspective, sinking POM is an important mechanism by which carbon and other elements are transferred from the sea surfece into the deep sea and onto the sediments. This transport is termed the biological pump and includes the sinking of inorganic particles that are of biogenic origin, namely calcium carbonate and silicate shells. 

A global map of quartz abundance is given in Figure 14.12. In this case, the contribution of quartz is presented as the contribution to the bulk sediment from which biogenic carbonate and silica have been removed. This map is very similar to the global distribution of dust presented in Figure 11.4, reflecting the importance of aeolian transport for this detrital silicate. 

Changes in phosphate, nitrate, ammonia, and silicate concentrations associated with the biogenic production and destruction of POM can alter seawater alkalinities. These effects are usually so small in scale that they can be ignored. Since the largest biotic impact on alkalinity in oxic seawater is exerted by the formation and dissolution of... 

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 chemical weathering of crustal rock was discussed in Chapter 14 from the perspective of clay mineral formation. It was shown that acid attack of igneous silicates produces dissolved ions and a weathered solid residue, called a clay mineral. Examples of these weathering reactions were shown in Table 14.1 using CO2 + H2O as the acid (carbonic acid). Other minerals that undergo terrestrial weathering include the evaporites, biogenic carbonates, and sulfides. Their contributions to the major ion content of river water are shown in Table 21.1. 

For example, organic matter deeply embedded in the mineral matrix of biogenous hard parts would not be exposed to exoenzyme attack. This embedding could occur during deposition of the minerals or through adsorption of the organic matter from seawater. Most of the ballasting effect exerted on POM is conferred by calcareous and siliceous hard parts and by clay minerals. 

This reaction is similar to Eq. 21.8. Wollastonite is an example of a calcium silicate mineral produced by the metamorphosis of detrital biogenic calcium carbonate and BSi. 

Opal (or opaline silica) An amorphous silicate formed through the polymerization of silicic acid molecules. Though most is biogenic in origin, some forms as a result of diagenesis. 

Silicon isotope variations in the ocean are caused by biological Si-uptake through siliceous organisms like diatoms. Insofar strong similarities exist with C-isotope variations. Diatoms preferentially incorporate Si as they form biogenic silica. Thus, high 5 °Si values in surface waters go parallel with low Si-concentrations and depend on differences in silicon surface water productivity. In deeper waters dissolution of sinking silica particles causes an increase in Si concentration and a decrease of 5 °Si-values. 

Conley, D. (1988) Biogenic silica as an estimate of siliceous microfossil abundance in Great Lake sediments. Biogeochemistry 6, 161-179. 

Svensen C, Egge JK, Stiansen JE (2001) Can silicate and turbulence regulate the vertical flux of biogenic matter A mesocosm study. Mar Ecol Prog Ser 217 67-80... 

Callot, G., Maurette, M., Pottier, L. Dubois, A. (1987). Biogenic etching of microfeatures in amorphous and crystalline silicates. Nature, 328, 147-9. 

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