Biogenic Silica in the Sediments

Sihcate, or silicic acid (H4Si04), 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 requirement only for certain biota such as diatoms, radiolarian, silicoffageUates, and siliceous sponges. But siliceous phytoplankton contributes significantly to the primary production in the world s oceans. More than 40% of the entire primary production is contributed by diatoms, which reveals a close coupling of silica and carbon in the ocean. Therefore, silicate cycling has received sig- 

In Fig. 3.58, the correlation coefficient between biogenic sihca and organic carbon at B3 (inner bay) is obviously greater than that at D7 (outer bay), and the ratios of OC BSi in the inner bay and bay mouth are close to but higher than that in the outer bay, which might result from different hydrodynamics. 

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. 

According to Ma et al. (2002), the decomposition rate of the biogenic element is N P OC Si, which is consistent with the phenomenon that the 

Region Water content (%) Sedimentation rate (cm/yr) Sedimentation flux (mmol/(m -d)) 

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Schelske, C., Stoermer, E., Conley, D., Robbins,., and Glover, R. (1983). Early eutrophication in the lower Great Lakes New evidence from biogenic silica in the sediments. Science 222, 320—322. 

Koning E., Brummer G.-J., van Raaphorst W., van Bennekom J., Helder W., and van Iperen J. (1997) Settling dissolution and burial of biogenic silica in the sediments off Somalia (northwestern Indian Ocean). Deep-Sea Res. II 44, 1341-1360. 

Zhao YF, Liu SM, Ye YW, Zhang J (2005) The analysis of biogenic silica in the sediments of the East China Sea and the Yellow Sea. Period Ocean Univ Chin 35(3) 423-428 (in Chinese with English abstract)... 

Biogenic silica, in the form of opal, makes up an important part of marine sediments, particularly in the southern and eastern equatorial oceans (Figure 15). These deposits are formed primarily from tests of diatoms that lived in the surface oceans. More than half of the opal formed in the... 

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.

Input of silicon to intertidal sediments comes from two sources, the sedimentation of the tests of pelagic diatoms and in situ uptake by benthic diatoms from the water. Both processes result in the input of particulate biogenic silica to the sediment which corrodes in alkaline sea water (Barker et al., 1994). Silicon accumulates in the interstitial water which results in a flux of silicon out of the sediments. The flux may have a significant seasonal variation due to the growth of benthic diatoms during the spring/summer period and due to bioirrigation (Marinelli, 1994). 

Figure 5.6 Distribution of biogenic silica in intertidal sediments of the Wash. Biogenic silica was determined by a wet chemical method.

The sediments containing the highest fraction of biogenic silica in the world occur in a 1000 km-wide... 

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... 

Accumulation of silica sediments on the bottom can be expected only in certain zones of the ocean with intensive accumulation of biogenic silica in suspension and a unique hydrodynamic regime (Lisitsyn et al., 1966). 

Gallinari M., Ragueneau O., Corrin L., DeMaster D., and Treguer P. (2002) The importance of water column processes on the dissolution properties of biogenic silica in deep-sea sediments I. Solubility. Geochim. Cosmochim. Acta 66, 2701-2717. 

Van Cappellen P. (1996) Reactive surface area control of the dissolution kinetics of biogenic silica in deep-sea sediments. 

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