The silica cycle is the biogeochemical cycle in which biogenic silica is transported between the Earth's systems. Silicon is one of the most abundant elements on Earth, and is considered necessary for life.
Overview
Silicon is the eighth most abundant element in the universe and the second most abundant element in the Earth's crust (the most abundant is oxygen). The weathering of the Earth's crust by rainwater rich in carbon dioxide is a key process in the control of atmospheric carbon dioxide.
[Garrels, R.M. (1983) "The carbonate-silicate geochemical cycle and its effect on atmospheric carbon dioxide over the past 100 million years". American Journal of Science, 283: 641-683.] It results in the generation of
silicic acid in aqueous environments. Silicic acid, Si(OH)
4, is a
hydrated form of silica found only as an unstable solution in water, yet it plays a central role in the silica cycle.
Silicifiers are organisms that use silicic acid to precipitate biogenic silica, SiO2. Biogenic silica, also referred to as opal, is precipitated by silicifiers as internal structures
The diatoms dominate the fixation and export of Suspended solids in the contemporary marine silica cycle. This includes the export of organic carbon from the euphotic zone to the deep ocean via the biological carbon pump. As a result, diatoms, and other silica-secreting organisms play crucial roles in the global carbon cycle by sequestering carbon in the ocean. The connection between biogenic silica and organic carbon, together with the significantly higher preservation potential of biogenic siliceous compounds compared to organic carbon makes opal accumulation records of interest in paleoceanography and paleoclimatology.
Understanding the silica cycle is important for understanding the functioning of marine food webs, biogeochemical cycles, and the biological pump. Silicic acid is delivered to the ocean through six pathways as illustrated in the diagram above, which all ultimately derive from the weathering of the Earth's crust.
Terrestrial silica cycling
Silica is an important nutrient utilized by plants, trees, and grasses in the terrestrial
biosphere. Silicate is transported by rivers and can be deposited in soils in the form of various siliceous polymorphs. Plants can readily uptake silicate in the form of H
4SiO
4 for the formation of
. Phytoliths are tiny rigid structures found within plant cells that aid in the structural integrity of the plant.
Phytoliths also serve to protect the plants from consumption by
who are unable to consume and digest silica-rich plants efficiently.
Silica release from phytolith degradation or dissolution is estimated to occur at a rate double that of global silicate mineral
weathering.
Considering biogeochemical cycling within ecosystems, the import and export of silica to and from terrestrial ecosystems is small.
Weathering
Silicate minerals are abundant in rock formations all over the planet, comprising approximately 90% of the Earth's crust.
The primary source of silicate to the terrestrial biosphere is
weathering. The process and rate of weathering is variable, depending on rainfall, runoff, vegetation, lithology, and topography.
Given sufficient time, rainwater can dissolve even a highly resistant silicate-based mineral such as quartz.
The overall reaction for the dissolution of quartz results in silicic acid
Another example of a silicate-based mineral is enstatite (MgSiO3). Rainwater weathers this to silicic acid as follows:
MgSiO3(s) + 2CO2(g) + H2O(l) = Mg2+(aq) + 2HCO3- (aq) + SiO2(aq)
Reverse weathering
In recent years, the effect of reverse weathering on
biogenic silica has been of interest in quantifying the silica cycle. During weathering, dissolved silica is delivered to oceans through
Meltwater and riverine inputs.
This dissolved silica is taken up by a multitude of marine organisms, such as
, and is used to create protective shells.
When these organisms die, they sink through the water column.
Without active production of biogenic SiO
2, the mineral begins
diagenesis.
Conversion of this dissolved silica into
authigenic silicate
through the process of reverse weathering constitutes a removal of 20-25% of silicon input.
Reverse weathering is often found in as these systems have high sediment accumulation rates and are observed to undergo rapid diagenesis.
Silicate weathering also appears to be a dominant process in deeper methanogenic sediments, whereas reverse weathering is more common in surface sediments, but still occurs at a lower rate.
Sinks
The major sink of the terrestrial silica cycle is export to the ocean by rivers. Silica that is stored in plant matter or dissolved can be exported to the ocean by rivers. The rate of this transport is approximately 6 Tmol Si yr
−1.
This is the major sink of the terrestrial silica cycle, as well as the largest source of the marine silica cycle.
A minor sink for terrestrial silica is silicate that is deposited in terrestrial sediments and eventually exported to the Earth's crust.
Marine inputs
File:Marine Silica Cycle.png| Inputs to the marine silica cycle
adapted from Treguer et al., 1995
Riverine
As of 2021, the best estimate of the total riverine input of silicic acid is 6.2 (±1.8) Tmol Si yr
−1.
This is based on data representing 60% of the world river discharge and a discharge-weighted average silicic acid riverine concentration of 158 μM−Si.
However, silicic acid is not the only way silicon can be transferred from terrestrial to riverine systems, since particulate silicon can also be mobilised in crystallised or amorphous forms.
According to Saccone and others in 2007,
the term "amorphous silica" includes biogenic silica (from
, freshwater
,
), altered biogenic silica, and
pedogenic silicates, the three of which can have similar high solubilities and reactivities. Delivery of amorphous silica to the
Fluvial has been reviewed by Frings and others in 2016,
who suggested a value of 1.9(±1.0) Tmol Si yr
−1. Therefore, the total riverine input is 8.1(±2.0) Tmol Si yr
−1.
Aeolian
No progress has been made regarding
aeolian dust deposition into the ocean
[Tegen, I. and Kohfeld, K. E. (2006) "Atmospheric Transport of Silicon". In: The Silicon Cycle: Human Perturbations and Impacts on Aquatic Systems, edited by: Ittekot, V., Unger, D., Humborg, C., and Tac An, N. T., 7: 81–91, Island Press.] and subsequent release of silicic acid via dust dissolution in seawater since 2013, when Tréguer and De La Rocha summed the flux of particulate dissolvable silica and wet deposition of silicic acid through precipitation.
Thus, the best estimate for the aeolian flux of silicic acid, FA, remains 0.5(±0.5) Tmol Si yr
−1.
Sandy beaches
A 2019 study has proposed that, in the
surf zone of
beaches,
wind wave action disturbed
abiotic sand grains and dissolved them over time.
To test this, the researchers placed sand samples in closed containers with different kinds of water and rotated the containers to simulate wave action. They discovered that the higher the rock/water ratio within the container, and the faster the container spun, the more silica dissolved into solution. After analyzing and upscaling their results, they estimated that anywhere from 3.2 ± 1.0 – 5.0 ± 2.0 Tmol Si yr
−1 of lithogenic DSi could enter the ocean from sandy beaches, a massive increase from a previous estimate of 0.3 Tmol Si yr
−1.
[Wollast, R., & Mackenzie, F.T. (1983). "Global Cycle of Silica". In S.R. Aston (Ed.), Silicon Geochemistry and Biogeochemistry, Academic Press, pages 39–76.] If confirmed, this represents a significant input of dissolved LSi that was previously ignored.
Marine silica cycling
Siliceous organisms in the ocean, such as
and
radiolaria, are the primary sink of dissolved silicic acid into opal silica.
Only 3% of the Si molecules dissolved in the ocean are exported and permanently deposited in
marine sediments on the seafloor each year, demonstrating that silicon recycling is a dominant process in the oceans.
This rapid recycling is dependent on the dissolution of silica in organic matter in the water column, followed by biological uptake in the
photic zone. The estimated residence time of the silica biological reservoir is about 400 years.
[ Opal silica is predominately undersaturated in the world's oceans. This undersaturation promotes rapid dissolution as a result of constant recycling and long residence times. The estimated turnover time of Si is 1.5x104 years.]
Biogenic silica production in the photic zone is estimated to be 240 ± 40 Tmol Si year −1.
Sources
The major sources of marine silica include rivers, groundwater flux, seafloor weathering inputs, hydrothermal vents, and atmospheric deposition (aeolian flux).
The diagram on low-temperature processes shows how these can control the dissolution of (either amorphous or crystallized) siliceous minerals in seawater in and to the coastal zone and in the deep ocean, feeding submarine groundwater (FGW) and dissolved silicon in seawater and sediments (FW).
Sinks
Rapid dissolution in the surface removes roughly 135 Tmol opal Si year−1, converting it back to soluble silicic acid that can be used again for biomineralization.
Deep seafloor deposition is the largest long-term sink of the marine silica cycle (6.3 ± 3.6 Tmol Si year−1), and is roughly balanced by the sources of silica to the ocean.
The siliceous ooze is eventually subducted under the crust and metamorphosed in the upper mantle.
Anthropogenic influences
The rise in agriculture of the past 400 years has increased the exposure rocks and soils, which has resulted in increased rates of silicate weathering. In turn, the leaching of Amorphous solid silica stocks from soils has also increased, delivering higher concentrations of dissolved silica in rivers.
In 2019 a group of scientists suggested acidification is reducing diatom silica production in the Southern Ocean.[ New threat from ocean acidification emerges in the Southern Ocean, Phys.org, 26 August 2019.][Petrou, K., Baker, K.G., Nielsen, D.A. et al. (2019) "Acidification diminishes diatom silica production in the Southern Ocean". Nature: Climate Change, 9: 781–786. ]
File:Annual mean sea surface silicic acid (World Ocean Atlas 2009).png| Concentration of silicic acid in the upper pelagic zone,
Role in climate regulation
The silica cycle plays an important role in long term global climate regulation. The global silica cycle also has large effects on the global carbon cycle through the carbonate-silicate cycle.
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. The mean oceanic residence time for silicate is approximately 10,000–15,000 yr. This relative short residence time, makes oceanic silicate concentrations and fluxes sensitive to glacial/interglacial perturbations, and thus an excellent proxy for evaluating climate changes.[DeMaster, D.J. (1981)."The supply and accumulation of silica in the marine environment". Geochimica et Cosmochimica Acta 45: 1715-1732.][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.]
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 cycle (De La Rocha, 2006; Leng and Barker, 2006). This is a particularly valuable approach considering the role of 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 are preserved.
The silicon isotopes compositions in fossil (δ30Si) are being increasingly often used to estimate the level of silicic acid in marine settings throughout the geological history, which enables the reconstruction of past silica cycles.[ Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.]
See also
