Biogenic silica

Biogenic silica (bSi), also referred to as

minerals. For example, microscopic particles of silica called phytoliths

can be found in grasses and other plants.

Silica is an amorphous metalloid

salts) whose precipitation is dictated by solubility equilibria.^[1] Chemically, bSi is hydrated silica

(SiO₂·nH₂O), which is essential to many plants and animals.

Diatoms in both fresh and salt water extract

organs

.

Silica in marine environments

silicoflagellates, and siliceous sponges. These organisms extract dissolved silicate from open ocean surface waters for the buildup of their particulate silica (SiO₂), or opaline, skeletal structures (i.e. the biota's hard parts).^[2]^[3] Some of the most common siliceous structures observed at the cell surface of silica-secreting organisms include: spicules, scales, solid plates, granules, frustules, and other elaborate geometric forms, depending on the species considered.^[4]

Marine sources of silica

Five major sources of

dissolved silica to the marine environment can be distinguished:^[3]

Riverine

influx of dissolved silica to the oceans: 4.2 ± 0.8 × 10¹⁴ g SiO₂ yr⁻¹

Submarine volcanism and associated

hydrothermal

emanations: 1.9 ± 1.0 × 10¹⁴ g SiO₂ yr⁻¹

Glacial weathering: 2 × 10¹² g SiO₂ yr⁻¹
Low temperature submarine weathering of oceanic basalts
Some silica may also escape from silica-enriched pore waters of
seafloor

Once the organism has perished, part of the siliceous skeletal material

protists communities. This biologic process has operated, since at least early Paleozoic time, to regulate the balance of silica in the ocean.^[4]

Radiolarians (

Pacific waters, for example, about 16,000 specimens per cubic meter can be observed.^[4]

Silica cycle

The silicon cycle has gained increasingly in scientific attention the past decade for several reasons:

Firstly, the modern

preservation potential of biogenic siliceous compounds, compared to organic carbon, makes opal accumulation records very interesting for paleoceanography and paleoclimatology

.

Secondly, biogenic silica accumulation on the sea floor contains lot of information about where in the ocean

export production

has occurred on time scales ranging from hundreds to millions of years. For this reason, opal deposition records provide valuable information regarding large-scale oceanographic reorganizations in the geological past, as well as paleoproductivity.

Thirdly, the mean oceanic

glacial/interglacial perturbations, and thus an excellent proxy for evaluating climate changes.^[3]^[5]

Increasingly, isotope ratios of oxygen (O¹⁸:O¹⁶) and silicon (Si³⁰:Si²⁸) are analysed from biogenic silica 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.

Marine silica sinks

Siliceous ooze

The remains of diatoms and other silica-utilizing organisms are found, as opal sediments within pelagic deep-sea deposits.

nutrients are abundant and productivity is high, are also characterized by local siliceous ooze.^[2]

Siliceous oozes are composed primarily of the remains of diatoms and radiolarians, but may also include other siliceous organisms, such as silicoflagellates and

spicules. Diatom ooze occurs mainly in high-latitude areas and along some continental margins, whereas radiolarian ooze are more characteristic of equatorial areas. Siliceous ooze are modified and transformed during burial into bedded cherts.^[2]

Southern Ocean sediments

Southern Ocean sediments are a major sink for biogenic silica (50-75% of the oceanic total of 4.5 × 10¹⁴ g SiO₂ yr⁻¹; DeMaster, 1981), but only a minor sink for organic carbon (<1% of the oceanic 2 × 10¹⁴ g of organic C yr⁻¹). These relatively high rates of biogenic silica accumulation in the Southern Ocean sediments (predominantly beneath the Polar Front) relative to organic carbon (60:1 on a weight basis) results from the preferential preservation of biogenic silica in the Antarctic water column.

In contrast to what was previously thought, these high rates of biogenic silica accumulation are not the result from high rates of

micronutrients, such as iron.^[6]

This preferential preservation of biogenic silica relative to organic carbon is evident in the steadily increasing ratio of silica/organic C as function of depth in the water column. About thirty-five percent of the biogenic silica produced in the

microbial

degradation in these near-surface waters.

Consequently, considerable decoupling of organic C and silica occurs during settling through the water column. The accumulation of biogenic silica in the seabed represents 12% of the surface production, whereas the seabed organic-carbon accumulation rate accounts for solely <0.5% of the surface production. As a result, polar sediments account for most of the ocean's biogenic silica accumulation, but only a small amount of the sedimentary organic-carbon flux.[6]

Effect of oceanic circulation on silica sinks

Large-scale oceanic circulation has a direct impact on opal deposition. The Pacific (characterized by nutrient poor surface waters, and deep nutrient rich waters) and Atlantic Ocean circulations favor the production/preservation of silica and carbonate respectively. For instance, Si/N and Si/P ratios increase from the Atlantic to the Pacific and Southern Ocean, favoring opal versus carbonate producers. Consequently, the modern configuration of large-scale oceanic circulation resulted in the localization of major opal burial zones in the Equatorial Pacific, in the eastern boundary current upwelling systems, and by far the most important, the Southern Ocean.^[5]

Pacific and Southern Oceans

Waters from the modern Pacific and Southern ocean, typically observe an increase in Si/N ratio at intermediate depth, which results in an increase in opal export (~ increase in opal production). In the Southern Ocean and North Pacific, this relationship between opal export and Si/N ratio switches from linear to exponential for Si/N ratios greater than 2. This gradual increase in the importance of silicate (Si) relative to nitrogen (N) has tremendous consequences for the ocean biological production. The change in nutrient ratios contributes to select

microcosm experiments have demonstrated that diatoms are DSi supercompetitors and dominate other producers above 2 μM DSi. Consequently, opal vs. carbonate export will be favored, resulting in increasing opal production. The Southern Ocean and the North Pacific also display maximum biogenic silicate/C_organic flux ratios, and consist thus in an enrichment in biogenic silicate, compared to C_organic export flux. This combined increase in opal preservation and export makes the Southern Ocean the most important sink for DSi today.^[5]

Atlantic Ocean

In the Atlantic Ocean, intermediate and deep waters are characterized by a lower content in DSi, compared to the modern Pacific and Southern Ocean. This lower interbasin difference in DSi has the effect of decreasing the

dissolution/production ratio is much higher in the Atlantic upwelling than in the Pacific upwelling. This is due to the fact that coastal upwelling source waters are much richer in DSi off Peru, than off NW Africa.^[5]

Marine biogenic silica budget

Rivers and submarine

North Pacific

. Total biogenic silica accumulation rates in these regions amounts nearly 0.6 × 10¹⁴ g SiO₂ yr⁻¹, which is equivalent to 10% of the dissolved silica input to the oceans.

Continental margin upwelling areas, such as the Gulf of California, the Peru and Chile coast, are characteristic for some of the highest biogenic silica accumulation rates in the world. For example, biogenic silica accumulation rates of 69 g SiO₂/cm²/kyr have been reported for the Gulf of California. Due to the laterally confined character of these rapid biogenic silica accumulation zones, upwelling areas solely account for approximately 5% of the dissolved silica supplied to the oceans. At last, extremely low biogenic silica accumulation rates have been observed in the extensive deep-sea deposits of the Atlantic, Indian and Pacific Oceans, rendering these oceans insignificant for the global marine silica budget.^[7]

Biogenic silica production

μm

, the entire image covers a region of approximately 1.13 by 0.69 mm.

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.

Biogenic silica 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).

Biogenic silica 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
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.

Opaline silica on Mars

In the

Spirit inadvertently discovered opaline silica. One of its wheels had earlier become immobilized and thus was effectively trenching the Martian regolith as it dragged behind the traversing rover. Later analysis showed that the silica was evidence for hydrothermal conditions.^[8]

References

^ Coradin, T., Lopez, P.J. (2003). "Biogenic Silica Patterning: Simple Chemistry or Subtle Biology?" ChemBioChem 3: 1-9.
^ ^a ^b ^c ^d Boggs, S. (2005). "Principles of Sedimentology and Stratigraphy (4th Edition)". Pearson Education, Inc, 662p.
^ ^a ^b ^c ^d DeMaster, D.J. (1981)."The supply and accumulation of silica in the marine environment". Geochimica et Cosmochimica Acta 45: 1715-1732.
^ ^a ^b ^c Ehrlich et al. (2010). "Modern Views on Desilicification: Biosilica and Abiotic Silica Dissolution in Natural and Artificial Environments ". Chem. Rev. 110: 4656-4689.
^ ^a ^b ^c ^d Cortese, G., Gersonde, R. (2004). "Opal sedimentation shifts in the World Ocean over the last 15 Myr". Earth and Planetary Science Letters 224: 509-527.
^ ^a ^b DeMaster, D. (1992)."Cycling and Accumulation of Biogenic Silica and Organic Matter in High-Latitude Environments: The Ross Sea". Oceanography 5(3): 147-153
^ DeMaster, D.J. (2002). "The accumulation and cycling of biogenic silica in the Southern Ocean: revisiting the marine silica budget". Deep-Sea Research Part II 49: 3155-3167
^ [1] Ruff, S. W., et al. (2011). "Characteristics, distribution, origin, and significance of opaline silica observed by the Spirit rover in Gusev crater, Mars". J. Geophys. Res., 116, E00F23.