Biogenic silica

  Biogenic silica (BSi) is the equivalent to the terms opal, biogenic opal, and amorphous opaline silica. BSi is essential to many plants and animals. Chemically, BSi is hydrated silica (SiO₂•nH₂O).

Diatoms in both fresh and salt water extract silica from the water to use as a component of their cell walls. Likewise, some holoplanktonic protozoa (Radiolaria), some sponges, and some plants (leaf phytoliths) use silicon as a structural material. Silicon is known to be required by chicks and rats for growth and skeletal development. Silicon is in human connective tissues, bones, teeth, skin, eyes, glands and organs. It is a major constituent of collagen which helps keep our skin elastic, and it helps calcium in maintaining bone strength.

BSi is silica that originates from the production out of dissolved silica. BSi can either be accumulated "directly" in marine sediments (via export) or be transferred back into dissolved silica in the water column.

Increasingly, isotope ratios of oxygen (O¹⁸:O¹⁶) and silicon (Si³⁰:Si²⁸) are analysed from BSi preserved in lake and marine sediments to derive records of past climate change and nutrient cycling (De La Rocha, 2006; Leng and Barker, 2006). This is a particularly valuable approach considering the role of diatoms in global carbon cycling. In addition, isotope analyses from BSi are useful for tracing past climate changes in regions such as in the Southern Ocean, where few biogenic carbonates are preserved.

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BSi production

The mean daily BSi rate strongly depends on the region:

• Coastal upwelling: 46 mmol m^-2 d^-1
• Sub-arctic Pacific]]: 18 mmol m^-2 d^-1
• Southern Ocean: 3–38 mmol m^-2 d^-1
• mid-ocean gyres: 0.2–1.6 mmol m^-2 d^-1

Likewise, the integrated annual BSi production strongly depends on the region:

• Coastal upwelling: 3 ^. 10¹² mol yr^-1
• Subarctic Pacific: 8 ^. 10¹² mol yr^-1
• Southern Ocean: 17–37 ^. 10¹² mol yr^-1
• mid-ocean gyres: 26 ^. 10¹² mol yr^-1

BSi production is controlled by:

• Dissolved silica availability, however, half saturation constant Kµ for silicon-limited growth is lower than Ks for silicon uptake.
• Light availability: There is no direct light requirement; silicon uptake at 2x depth of photosynthesis; silicon uptake continues at night but cells must be actively growing.
• Micronutrient availability.

BSi dissolution

BSi dissolution is controlled by:

• Thermodynamics of solubility: Temperature (0 to 25 °C - 50x increase).
• Sinking rate: Food web structure—grazers, fecal pellets, discarded feeding structures, Aggregation - rapid transport.
• Bacterial degradation of organic matrix (Bidle and Azam, 1999).

BSi preservation

BSi preservation is measured by:

• Sedimentation rates, mainly sediment traps (Honjo);
• Benthic remineralization rates ("recycling"), benthic flux chamber (Berelson);
• BSi concentration in sediments, chemical leaching in alkaline solution, site specific, need to differentiate lithogenic vs. biogenic Si, X-ray diffraction.

BSi preservation is controlled by:

• Sedimentation rate;
• Porewater dissolved silica concentration: saturation at 1.100 µmol/L;
• Surface coatings: dissolved Al modifies solubility of deposited biogenic silica particles, dissolved silica can also precipitate with Al as clay or Al-Si coatings.

References

• Brzezinski, M. A. (1985). “The Si:C:N ratio of marine diatoms: Interspecific variability and the effect of some environmental variables.” Journal of Phycology 21(3): 347-357.
• De La Rocha, C.L. (2006). "Opal based proxies of paleoenvironmental conditions." Global Biogeochemical Cycles 20. doi:10.1029/2005GB002664.
• Dugdale, R. C. and F. P. Wilkerson (1998). “Silicate regulation of new production in the equatorial Pacific upwelling.” Nature 391(6664): 270.
• Dugdale, R. C., F. P. Wilkerson, et al. (1995). “The role of the silicate pump in driving new production.” Deep-Sea Research I 42(5): 697-719.
• Leng, M.J. and Barker, P.A. (2006). "A review of the oxygen isotope composition of lacustrine diatom silica for palaeoclimate reconstruction." Earth Science Reviews 75:5-27.
• Ragueneau, O., P. Treguer, et al. (2000). “A review of the Si cycle in the modern ocean: recent progress and missing gaps in the application of biogenic opal as a paleoproductivity proxy.” Global and Planetary Change 26: 317-365.
• Takeda, S. (1998). “Influence of iron availability on nutrient consumption ratio of diatoms in oceanic waters.” Nature 393: 774-777.
• Werner, D. (1977). The Biology of Diatoms. Berkeley and Los Angeles, University of California Press.