Biogenic silica preservation

Preservation versus Dissolution of Sinking Detrital Biogenic Silica... 

PRESERVATION VERSUS DISSOLUTION OF SINKING DETRITAL BIOGENIC SILICA... 

The geographic distribution of opal in the surfece sediments is controlled by (1) the local rain rate of biogenic silica, (2) the degree of its preservation in the sediments, and (3) the relative rate of accumulation of other types of particles. Preservation is promoted by rapid burial as this isolates BSi from seawater. But if the BSi is buried by other particle types, the relative contribution of BSi to the sediment is diluted. This dilution effect causes the BSi content of most continental margin sediments to be low despite high rain rates. Preservation efficiency is also dependent on (1) the intensity of bioturbation and suspension feeding and (2) the various factors that control... 

Figure 3 Seabed silica-preservation efficiency plotted as a function of sediment accumulation rate in Antarctic Ross Sea deposits. As sediment accumulation rates increase, the amount of time that siliceous material is exposed to the highly undersaturated bottom waters at the sediment-water interface decreases, which enhances the preservation of biogenic silica in the seabed.

DeMaster D. J., Ragueneau O., and Nittrouer C. A. (1996) Preservation efficiencies and accumulation rates for biogenic silica and organic C, N, and P in high-latitude sediments the Ross Sea. J. Geophys. Res. 101, 18501-18518. 

Diatoms have been used to look at the history of changing productivity, and hence nutrient inputs, in lake systems. The use of diatoms as palaeoenvironmental indicators in coastal environments is limited because of the preservation problem (Barker et al., 1994) and the paucity of areas which are continuously accreting sediment. Brown (1994) studied a number of sediment cores from the Wash for preservation of diatom material. The biogenic silica content of the sediments decreases with depth (Figure 5.6) which may indicate progressive corrosion with depth and/or increased input of silica to the sediments with time. Gross changes in the type of diatom present in the sediment may support the latter conclusion but the work is of a preliminary nature. Further examination of siliceous palaeoenvironmental indicators in intertidal sediments will be useful. 

In the OC BSi ratio, the maximum value is 0.37 in Jiaozhou Bay sediment, which is much smaller than the Redfield ratio, indicating that OC decomposed much faster than biogenic silica did in the same environment. Most OC would be decomposed and released to seawater and then participate in the carbon recycle in seawater. The biogenic silica is preserved in sediments and the silicate concentration in seawater is low, which may explain why Si becomes a limiting factor for ph3doplankton in Jiaozhou Bay. 

Alexandre et al. (1997) found that the biogenic sihca input into the biogeochemical silica cycle from the dissolution of phytoliths is twice as large as silica input from primary silicate mineral weathering in the tropical Congo rainforest. Biogenic (opaline) silica dissolves faster than sihcate minerals. While most of the phytoliths dissolve rapidly with a mean residence time of 6 months (Alexandre et al., 1994), and the sihca is recycled by the forest, a small part (7.5%) does not dissolve and is preserved in the soil. 

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