Silica cycle
They include micro-organisms such as diatoms, rhizarians, silicoflagellates and several species of choanoflagellates, as well as macro-organisms such as siliceous sponges.
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.
[2] Silica release from phytolith degradation or dissolution is estimated to occur at a rate double that of global silicate mineral weathering.
Silicate minerals are abundant in rock formations all over the planet, comprising approximately 90% of the Earth's crust.
[16] Conversion of this dissolved silica into authigenic silicate clays through the process of reverse weathering constitutes a removal of 20-25% of silicon input.
[17] Reverse weathering is often found in river deltas as these systems have high sediment accumulation rates and are observed to undergo rapid diagenesis.
[12] 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.
Delivery of amorphous silica to the fluvial system has been reviewed by Frings and others in 2016,[24] who suggested a value of 1.9(±1.0) Tmol Si yr−1.
[1] No progress has been made regarding aeolian dust deposition into the ocean [25] 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.
[1] A 2019 study has proposed that, in the surf zone of beaches, wave action disturbed abiotic sand grains and dissolved them over time.
[3] 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.
This undersaturation promotes rapid dissolution as a result of constant recycling and long residence times.
[1] 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.
When opal silica accumulates faster than it dissolves, it is buried and can provide a diagenetic environment for marine chert formation.
[37] The processes leading to chert formation have been observed in the Southern Ocean, where siliceous ooze accumulation is the fastest.
[39] The only other major sink of silica in the ocean is burial along continental margins (3.6 ± 3.7 Tmol Si year −1), primarily in the form of siliceous sponges.
[15] Due to the high degrees of uncertainty in source and sink estimations, it's difficult to conclude if the marine silica cycle is in equilibrium.
The dominance of non-siliceous phytoplankton due to anthropogenic nitrogen and phosphorus loading and enhanced silica dissolution in warmer waters has the potential to limit silicon ocean sediment export in the future.
[15] In 2019 a group of scientists suggested acidification is reducing diatom silica production in the Southern Ocean.
The silica cycle plays an important role in long term global climate regulation.
[43] The process of silicate mineral weathering transfers atmospheric CO2 to the hydrologic cycle through the chemical reaction displayed above.
This balance of weathering and volcanoes is part of what controls the greenhouse effect and ocean pH over geologic time scales.
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.
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.
[44][45] 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).
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.
The silicon isotope compositions in fossil sponge spicules (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.
