Biogenic silicate

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. 

When silica-depositing organisms die, the oi anic constituents of their cells decompose, and the polymeric silica originally deposited within and around these cells is released, usually in a particulate form. The sources, nature, and ultimate fate of this biogenic silica are the subjects of this chapter. The first section deals with biogenic siliceous deposits on land and the second with such deposits in the sea. 

Silicate is a very important nutrient in the ocean. Unlike other major nutrients such as phosphate and nitrate or ammonium, which are needed by almost all marine plankton, silicate is an essential chemical only for certain biota such as diatoms, radiolarian, sihcoflageUates, and siliceous sponges. However, this biology is one of the most important producers in marine. The estimation shows that diatoms contribute more than 40% of the entire primary production. Therefore, silicate cycling has received significant scientific attention in recent years and many scientists have studied silicate behavior in marine environments. Biogenic silicate is the amorphous content extracted by chemical methods, which is named as biogenic opal or opal in brief. The concentration of dissolved silicate in the world ocean is about 70.6 pmol/L and the net input of dissolved silicate from land to ocean is (6.1 2.0)x 10 mol (calculated by Si) every year, and the primary contribution (about 80%) comes from river. 

Li XG, Song JM, Yuan HM, Li FY, Sun S (2005) High contents of biogenic silicate in Jiaozhou Bay sediments-evidence of Si-hmitation to phytoplankton primary production. Oceanol Limnol Sin 36(6) 92-98 (in Chinese with English abstract) Li XJ, Chen F, Liu J, Huang XH (2004) Distribution and its dissolution of carbonate in seafloor surface sediment in the western South China Sea. Geochimica 33(3) 254-260 (in Chinese with English abstract)... 

In Jiaozhou Bay sediment, OC, N, and P decompose much faster than BSi does in similar conditions, which means that most biogenic silicate will be buried and separated from silicate recycling. 

Only 15.5% of biogenic silicate is hydrolyzed during the journey from the surface to the bottom in seawater, thus ca. 84.5% can be deposited on the bottom. The silicate release rate at the sediment-seawater interface is quite a bit lower than its accumulation rate from water to sediment. They are the main reasons for a constantly low silicate concentration in sea water and Si-limitation in phytoplankton primary production. In one... 

Hesse, R.. Origin of Chert Diagenesis of Biogenic Siliceous Sediments. In Diagenesis . Mcllreath, I.A. Morrow, D.W., Eds. Geological Association of Canada, Geoscience Canada Reprint Series 4,1990 pp 227-251. 

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... 

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. 

---