The Silicic Acid Cycle

Earth's thermostat

Recently I promised to write more about the bare necessities of silicon.

Herein I am going to make a case for the silicic acid cycle as Earth’s thermostat. I am going to identify the major driving force behind so-called climate change.

Silicic acid plays a primary role in mineral weathering. As the significant soluble product of carbon dioxide driven dissolution of aluminosilicate rocks (mountains), silicic acid influences the kinetics or rate of mineral weathering. Through equilibrium dynamics, the concentration of silicic acid in any given environment is potentially a brake on mineral weathering.

The advent of biological silicification (see for example my post on horsetail)

and primarily silicification by diatoms had (and has) the effect of lowering the concentration of silicic acid in the natural environment and releasing the brake on mineral weathering. Biological silicification (of which more to come in future posts) is a mechanism by which silicic acid harvested from environments in which it is significantly under-saturated is concentrated and subsequently deposited as amorphous biogenic silica. The advent of biological silicification in the evolution of life on Earth, through the removal of silicic acid from solution, acted as an accelerant of carbon dioxide fuelled mineral weathering and in consequence lowered atmospheric levels of carbon dioxide and contributed significantly to global cooling. The biological availability of silicon as silicic acid is acting as a cooling system for Earth so why is contemporary Earth warming.

An often-overlooked consequence of the burning of fossil fuels is a shift in the major acidifying anion in both wet (rain) and dry deposition. In many regions of the planet, the pH of rainfall is no longer governed by carbonic acid (pH 5.2-5.5), but by much stronger mineral acids and primarily sulphates (acid rain, pH 4.0-4.5). In these regions, there has been a switch in the major acidifying anion in rainwater brought about by release of sulphur and nitrous oxides in anthropogenic (coal, oil and gas) emissions. The consequences for mineral weathering and silicic acid as a global coolant are primarily two-fold.

The first is that sulphuric and not carbonic acid is the primary reactant in aluminosilicate weathering. Consequently, less atmospheric carbon dioxide is drawn down directly through mineral weathering.

The second is that the products of mineral weathering, where sulphate is the major anion, are different to those where bicarbonate is dominant (See equations 1&2 below.). This shift in pattern of mineral weathering is sometimes evident in regions heavily impacted by acid deposition as changes in clay mineral profiles. A further possible consequence of such a change in acid fuelled mineral weathering is a two-fold reduction in the stoichiometric release of silicic acid (See equations 1&2 below.).

The weathering of potassium feldspar (rock) where carbonic acid is the principal weathering acid in rainfall has a relatively slow reaction rate;

10^–15.5 μmol m^{−2 s−1}

2KAlSi₃O₈ + 2CO₂ + 11H₂O→Al₂Si₂O₅(OH)₄ + 2 K⁺ + 2HCO₃⁻ + 4Si(OH)₄

K‐Feldspar                                Kaolinite                                        Silicic Acid (1)

The weathering of potassium feldspar where sulphuric acid is the principal weathering acid in rainfall has a relatively fast reaction rate;

10^–14.6 μmol m^{−2 s−1}

2KAlSi₃O₈ + H₂SO₄ + 4H₂O→Al₂Si₄O₁₀(OH)₂ + 2K⁺ + SO₄²⁻ + 2Si(OH)₄

K‐Feldspar                              Montmorillonite                          Silicic Acid (2)

Carbon dioxide fuelled aluminosilicate mineral dissolution acts as a slow-release system for silicic acid, while sulphuric acid fuelled dissolution is more rapid, but less productive per mole of aluminosilicate mineral.

I have argued that the industrial revolution and the concomitant acidification of the atmosphere primarily through burning fossil fuels resulted in an accelerated diminution of biologically available silicic acid. One possible consequence of such would be a fall in primary productivity, predominantly attributable to diatom populations in the oceans, and a concomitant reduction in the drawdown of carbon dioxide from the atmosphere.

Is this one of the (main) reasons why the Earth is warming?

If you would like to read more about this and access all the relevant references to my musings then please see my recent paper. Unfortunately it is not open access so if you cannot access the full paper online then do feel free to email me for a copy.

Finally, a friendly reminder that my posts are available free to all though anyone wishing to become a paid subscriber is helping me to maintain our website and through this and this substack the profile of aluminium and silicon in life.