5)CO2 removal from the Atmosphere

The most interesting part of Arrhenius (1896) p. 271, is when Arrhenius quotes Högbom (1894) «... as it proves that the most important of all the processes by means of which carbonic acid has been removed from the atmosphere in all times, namely the chemical weathering of siliceous minerals, is of the same order of magnitude as a process of contrary effect, which is caused by the industrial development of our time, and which must be conceived of as being of a temporary nature.» who develops his views in «Högbom’s klassiska arbete om kolsyrans kretslopp i naturen (1894)», i.e. The classic works of A. G. Högbom on carbon cycles in nature (1894).

Högbom further states «In comparison with the quantity of carbonic acid which is fixed in limestone (and other carbonates), the carbonic acid of the air vanishes. With regard to the thickness of sedimentary formations and the great part of them that is formed by limestone and other carbonates, it seems not improbable that the total quantity of carbonates would cover the whole earth's surface to a height of hundreds of meters. If we assume 100 meters,-a number that may be inexact in a high degree, but probably is underestimated,- we find that about 25,000 times as much carbonic acid is fixed to lime in the sedimentary formations as exists free in the air. Every molecule of carbonic acid in this mass of limestone has, however, existed in and passed through the atmosphere in the course of time .» again quote by Arrhenius (1896).

Högbom (1894) concludes that because the weathering processes might have had very different intensities at different geological times and because it has consumed quantities of CO2 many thousand times greater than the amount now disposable in the air, the probability of important variations of CO2 seems large, especially as the supply is related to volcanic exhalations and other intermittent phenomenons that do not necessarily remain balanced over geological times with other mechanisms withdrawing CO2 from the air. This is summarized by «If we pass the above-mentioned processes for consuming and producing carbonic acid under review, we find that they evidently do not stand in such a relation to or dependence on one another that any probability exists for the permanence of an equilibrium of the carbonic acid in the atmosphere». Furthermore Högbom (1894) does not underestimate "the consumption of carbonic acid by vegetative processes. The ocean, too, plays an important rôle as a regulator of the quantity of carbonic acid in the air by means of the absorptive power of its water, which gives off carbonic acid as its temperature rises and absorbs it a it cools"

In the end, the most interesting part of Arrhenius’ paper (1896) are the quotes from Högbom (1894) who clearly shows a deep understanding of the carbon cycle and of the relative importance of the several processes involved and who acknowledges that the atmospheric content of CO2 might have changed significantly, but who as a good geologist refrains himself from establishing any causal relationship with the temperature that may have been observed at the corresponding geological times.

As Galvez and Gaillardet (2012) reminds us, by 1845 Jacques-Joseph Ébelmen (1845; 1847) had brilliantly contributed to the emerging question of atmospheric composition by proposing that the alteration of silicates on continents and the precipitation of carbonates in the ocean should be considered as a sink of atmospheric CO2. As reminded by Berner (2012) «The fundamental principles of the factors affecting the global carbon cycle, the global sulfur cycle and the levels of atmospheric CO2 and O2 over long-term (multi-million year) time scales were first elucidated by Jacques-Joseph Ébelmen in 1845. He covered all major processes in such a correct manner that no appreciable changes in them have been elucidated since then. Unfortunately, his ideas were forgotten and were independently deduced by others only 100 to 150 years later». From what we have seen above, and the quotes of Högbom (1894) in Arrhenius’ paper (1896) it could be that Ébelmen had left a legacy at least in Högbom’s thinking.

From thereof, one should note that the weathering of carbonates has surprisingly attracted little attention in the geochemical community both in terms of its mechanisms and global budget, to the notable exception of Walker et al. (1981). This lack of interest most likely relates to the widely accepted idea that, over long periods of time (0.5 to 1 Myr timescale), carbonate weathering is not a significant contributor to changes in the amount of atmospheric CO2 (Berner and Berner, 2012). This understanding is based on the following reaction showing that the weathering of carbonate on land is exactly compensated by the opposite precipitation reaction in the ocean56 :

56 The origin of the protons can be traced back to the following reactions CO2 + H2O  H2CO3 and H2CO3  H+ + HCO3

and (Calcite) CaCO 3+ H 2 O+CO 2 <=>Ca 2++ 2 HCO 3

(Forstérite) Mg 2 SiO 4 +4 H 2 O +4 CO 2 <=> 2 Mg 2++ H 4 SiO 4+4 HCO 3 (66)

(67)

On the left side of the equation the carbonate rock is weathered and once ions reach the oceans they precipitate mainly thanks to the biota, i.e. corals, shell animals and in the pelagic environment thanks to plankton species. More generally, the reversible equation can be written as: Ca x , Mg 1-xCO 3+CO2 +H 2 O <=> x Ca 2++(1− x) Mg 2++2 HCO 3 − (68)

But, as suggested by Gaillardet et al. (2018) there could be some reasons why carbonate weathering may be important for the Earth's surface regulation as the fast kinetics of carbonate weathering allows it to respond to changes at human timescales making it consequently one of the components of climate equilibrium. Nevertheless, the focus will be given here on the weathering of silicates (e.g.Anorthite, Albite)57 : (Anorthite) CaAl 2 Si 2 O8 +2 CO2 +3 H 2 O ⇒Ca 2++2 HCO3 − + Al 2 Si 2 O5 (OH )4 (Kaolinite) (69) In this mono-directional reaction we use 2 moles of CO2 whereas the precipitation of calcite (using Ca2+ + 2 HCO3 - ) will just liberate one (therefore the ocean operate as a CO2 sink);

and: (Albite) 2 NaAlSi 3 O8 +11CO 2+2 H 2 O ⇒ 2 Na 2++ 2 HCO 3 − + 4 H 4 SiO4 + Al 2 Si2 O5 (OH )4 (Kaolinite) (70)

So, we’re going to try to put figures on the processes aforementioned and to give an indication to the reader of how much CO2 can be trapped in calcareous rocks and limestones by the weathering of silicates. The weathering of silicate minerals makes a big difference to atmospheric CO2, by-products of hydrolysis reactions affecting silicate minerals are bicarbonates (i.e. HCO3 -) anions and calcium cations, further metabolized by marine plankton in the oceans and converted to calcium carbonate. The calcite skeletal remains of the marine organisms are deposited as deep-sea sediments (as long as they do not deposit below the carbonate compensation depth) and are subtracted definitely from the biogeochemical cycle until they are recycled into subduction zones and produce calc-alcaline series and associated volcanism.

We forget temporary reversible reactions and just focus on one, irreversible, the alteration of rocks containing calcium silicates (e.g. basalts, andesites, granites, etc.):

Calcium_silicates + CO2 + H2O -> Clays + Limestones

Calcium silicates (e.g. plagioclases, pyroxenes, amphiboles) are a good source from which to originate the weathering, and if we take a calcium plagioclase we get: 2 CaAl 2 Si2 O 8+2 CO 2+4 H 2 O⇒ 2 CaCO3 +Si 4 O10 Al 4 (OH )8 i.e. Kaolinite (71)

Simplified we retain:

CO2+CaSiO 3 ⇒ SiO 2 +CaCO 3 (Urey, 1952 p. 35458, 1956) (72)

So now, let's make some simple calculation, since the 20 Myr that Himalaya has surged, it has been eroded and the sediments (e.g. clays, sandstones, etc.) have deposited in the Gange and Indus deltas. As a first conservative estimate, 2.106 km3 of rocks have been eroded, with an average density of 2.7 g/cm3, one gets an estimate of 5.4.1018 Kg of rocks

57 Most of the common minerals found in igneous rocks are solid-solution phases. These include olivine, pyroxene, amphibole, biotite, and plagioclase feldspars. Such a crystallization behavior is often illustrated by using the NaAlSi3O8 (albite or Ab) -

CaAl2Si2O8 (anorthite or An) plagioclase system. 58 Urey (1952) states “Of course the silicates may have been a variety of minerals but the pressure of CO 2 was always kept at a low level by this reaction or similar reactions just as it is now. Plutonic activities reverse the reaction from time to time, but on the average the reaction probably proceeds to the right as carbon compounds come from the earth's interior, and in fact no evidence for the deposition of calcium silicate in sediments seems to exist”, therefore unless metamorphism occurs, the reaction is mainly to the right, see also (Duff and Morel, 1980).

which have produced 1.62.1017 kg of altered calcium silicates. Knowing that N CaSiO3 have a mass of 116g and N CO2 a mass of 44g (N Avogadro), 1kg of altered calcium silicate rocks absorbs (definitely) 0.38Kg of CO2.

So far, we can easily compute that the erosion of Himalaya removed over 20 Myr 6.2.10 16 Kg of CO2, i.e. 6.2.1013 tonnes of CO2 or 6.2.104 GT of CO2 (Giga Tons). If we consider that we have 407.4 ppm of [CO2] by volume in 2018, then the concentration by weight is: 0.04074*(44.0095/28.97)=0.06189% of weight with CO2 molar mass=44.0095 g/mole and molar mass of air=28.97 g/mole, knowing that the total mean mass of the atmosphere is 5.148 1015 tonnes we get the total weight of CO2 = 0.06189 % x 5.1480 x 1015 tonnes=3.186 1012 tonnes or 3.186.103 GT of CO2 (1 GT=109 tonnes). As one knows that the entire atmosphere has 3.186.103 GT of CO2, one can conclude that the erosion of Himalaya has removed from the atmosphere (62/3.186)=19.46, i.e. nearly 20 times the total actual atmospheric CO2. One can easily understand that if [CO2] atmospheric concentrations react fast to changes of temperature adjusting to the solubility changes as per Henry's law (see p.32), a longer term steady state between all carbonate reservoirs depends on many more parameters, the weathering rate being just one of them, which itself depends on extremely intricate factors such as the distribution of mountain belts, of plates, of the atmospheric circulation, etc.

As a side note, whatever the cause of their formation, the reliefs immediately formed are prey to erosion which destroys them at the rate of a few millimeters or even centimeters per century. The crust thus thins by superficial ablation. But the erosion of a 1km thick of mountain does not lower the relief as much. According to the principle of isostasis this slice of density 2.8 is necessarily replaced in depth by a slice of mantle terrain (d=3.3) of equivalent mass, ie of thickness 2.8 / 3.3 * 1 = 0.85km. A large part of the destroyed relief is therefore reconstituted by a regional uprising (the altitude has only dropped by 150m in the previous example) and by the migration of the Moho (i.e. Mohorovičić discontinuity) upwards. In fact, zero altitude is only reached when the continental crust has regained its thickness of 30km59. Thus, the complete erosion of a mountain range lead to far more CO 2, removed than what could be evaluated from the simple height of the chain.

In summary, the alteration of a calcium silicate (e.g. the alteration reaction of Kaolinite or Anorthite) consumes 2 moles of CO2 on the continent, but only one is released by the precipitation of calcite in the ocean (notice that this CO2 is immediately converted back into carbonate and bicarbonate ions and does not remain as CO2). The alteration of Ca silicates (and also of Mg, because magnesium limestones are precipitated) is therefore a mechanism capable in the long term of efficiently pumping atmospheric CO2.

Let’s remember that the CO2 cycle is fueled by carbon dioxide of mantle origin which leaves at the level of volcanoes. The surface C reservoir seems to keep a constant mass because what happens in the system through volcanism is equal to what plunges back into the mantle in subduction. The fluxes involved in the geological carbon cycle are very low compared to the carbon exchange fluxes between the atmosphere and the biomass on the one hand and the atmosphere and the ocean on the other hand (1000 times larger). However, even if these enormous fluxes play a large role on a small time scale on the regulation of atmospheric CO2, they cannot play an influence on the geological scale. Indeed, the flux of photosynthesis is almost instantaneously compensated by the flow of respiration60 and the atmosphere and the ocean are in dynamic equilibrium on a geological scale.

The carbonate reservoir (limestones and sediments) is the largest carbon reservoir at the surface of the Earth along with the fossil organic carbon reservoir. Accumulating during the Earth's geological history (essentially during the Proterozoic era), its size is estimated as > 50,000 000 Gt-C (carbonate), in the range [66,000,000 Gt-C – 100,000,000 GtC] and the fossil organic is > 13,000,000 Gt-C61 (kerogen) (Berner and Berner, 2012). The amounts of C stored in the atmosphere and in the ocean are dwarfed in comparison at respectively 875 Gt-C (2019) and [36,000-38,000] Gt-C. As a consequence, any imbalance, even small, in the carbonate reservoir between the two processes, carbonate precipitation by oceanic organisms on the one hand and chemical weathering and metamorphism on the other hand could have important transient consequences on the atmospheric CO2 level.

In response to global change, carbonate weathering is an interesting atmospheric CO2 sink and a source of alkalinity to the ocean that is able to play a key role at the 100 years to 10,000 years timescales (Beaulieu et al., 2012). Chemical

59 This leads to the denudation of very deep areas at the base of mountain ranges and gives geologists access to petrological facies of very high pressures / temperatures. 60 Cellular respiration is the biochemical process in which the cells of an organism (e.g. bacteria) obtain energy, i.e. Adenosine 5'triphosphate or ATP by breaking down Glucose into carbon dioxide and water using using oxygen in aerobic cellular respiration, and other molecules such as nitrate (NO3) in anaerobic cellular respiration. 61 Note that 1 petagrams of carbon equals 1 Giga tonnes (1 PgC = 1 Gt-C).

weathering of continental surfaces consumes 0.3 Gt yr-1 of atmospheric carbon. This flux is of the same importance as the net uptake of CO2 by the terrestrial biosphere in LIA-type conditions (0.4 Gt C yr-1). The 0.3 Gt C yr-1 weathering flux encompasses the dissolution of the outcropping silicate and carbonate minerals under the corrosive action of dissolved atmospheric or biologically respired CO2 in continental waters. Both processes can be summarized by two generic equations :

CaSiO3 (silicate mineral) + 2 CO2 (atmosphere) + H2O → Ca2+ (river) + 2HCO3 - (river) + SiO2

CaCO3 (carbonate mineral) + CO2 (atmosphere) + H2O → Ca2+ (river) + 2HCO3 - (river)

In both cases, atmospheric carbon is captured in rivers and transferred to the ocean. This atmospheric CO2 consumption is balanced at the million-year timescale by the supply of volcanic CO2 and at the millennial (or much less) timescale (ocean mixing time) by the release of one mole of CO2 to the atmosphere for each mole of carbonate deposited on the sea floor (the reverse of the second reaction) as reminded by Beaulieu et al. (2012).