Abstract
For as long as it has had both water and rock, the Earth has had a silica cycle. This cycle begins with the creation of the silicate rock of new mountains, mountain belts, and ocean floor. Weathering wears these materials down, transferring their silica slowly to the sea. Thanks to plate tectonics, solid silica that ends up at the bottom of the ocean returns to the interior of the Earth to replenish the reservoir of material from which new silicate rock is made. These days, along the way from rock to sea to deep sea sediment, silica cycles through the fields and forests, diatom blooms, and glass sponges of the biosphere. But these enormous diversions are a new thing. For most of Earth history, there was no such thing as silica biomineralization and there were no biological loops in the silica cycle. Thus the silica cycle that we have today, the one that we tend to think of as normal, default, and everlasting, has not just features but major features it did not possess for the majority of its existence. On the other hand, that shouldn’t surprise us. That’s what happens with systems that progressively evolve.
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- 1.
Yes, we know—Don’t quit the day job.
- 2.
Primary silicate minerals are the minerals that formed during the initial solidification of the molten silicate from the interior of the Earth. In contrast, secondary minerals are those which form due to the chemical or mechanical alteration of primary minerals.
- 3.
We were almost sad to discover that wollastonite was not named for Roland Wollast, one of the great recent heroes of biogeochemical research, but for William Hyde Wollaston (1776–1828), a wide-ranging scientific genius whose insights, inventions, and discoveries in just one of the many fruitful areas of his work laid the groundwork for the modern field of crystallography.
- 4.
Although proximally most of the dissolved calcium has come from the dissolution of secondary calcium carbonate minerals formed from calcium that ultimately came from primary silicate minerals.
- 5.
The dissociation reaction is: H2O ↔ H+ + OH−
- 6.
Meaning it is either already dissolved silica, which can participate in all sorts of chemical reactions and be taken up biologically, or it is in the form of some easily dissolvable solid silica, generally a noncrystalline variety such as biogenic silica or a non-biogenic amorphous silica.
- 7.
Net means that the number doesn’t include the amount of silica trapped permanently in secondary minerals such as clays, a quantity that could be up to two-thirds of the gross amount weathered out of primary silicate minerals.
- 8.
During subduction, the slab of oceanic crust, the oceanic lithosphere just beneath it, and the sediments that have collected on top of it, having met an obstacle such as a continent, gets pushed down and begins to descend into the mantle, as show in Fig. 9.1. By the way, because oceanic crust is richer in magnesium and iron and poorer in aluminum than continental crust, it is the denser of the two. When the twain collide, the lighter ends up on top and the heavier is forced down into the mantle.
- 9.
How much is coming in or being lost through lower temperature, more diffuse flow through ridge flanks, however, is a serious issue that remains to be solved.
- 10.
To put it somewhat bluntly, the critters are either photosynthetic or eating things that are.
- 11.
Most instead of all because some of the sediments get scraped off against the continental mass and accumulate at that intersection (or even get pushed up onto the continent itself).
- 12.
People have been cultivating and consuming Spirulina (now more correctly known as Arthrospira), a genus of freshwater cyanobacteria, since at least Aztec times. Which goes to show how desperate people can get when they’re hungry.
Further Reading
Dürr HH, Meybeck M, Hartmann J, Laruelle GG, Roubeix V (2011) Global spatial distribution of natural riverine silica inputs to the coastal zone. Biogeosci 8:597–620
Fontorbe G, Frings PF, De La Rocha CL, Hendry KR, Conley DJ (2016) A silicon depleted North Atlantic since the Palaeogene: Evidence from sponge and radiolarian silicon isotopes. Earth Planet Sci Lett 453:67–77
Siever R (1992) The silica cycle in the Precambrian. Geochim Cosmochim Acta 56:3265–3272
Struyf E, Smis A, Van Damme S, Meire P, Conley DJ (2009) The global biogeochemical silicon cycle. Silicon 1:207–213
Tréguer P and De La Rocha CL (2013) The world ocean silica cycle. Annu Rev Mar Sci 5:477–501
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De La Rocha, C., Conley, D.J. (2017). The Venerable Silica Cycle. In: Silica Stories. Springer, Cham. https://doi.org/10.1007/978-3-319-54054-2_9
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DOI: https://doi.org/10.1007/978-3-319-54054-2_9
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