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Atmospheric oxygenation and volcanism Gaillard et al. reply

Abstract : Kasting et al.1 question the model of ref. 2, in which we suggest that the oxygenation of the atmosphere, around 2.45 Gyr ago, was promoted by the emergence of subaerial volcanism, producing volcanic gases with much more elevated SO2/H2S ratios than submarine volcanism. Kasting et al.1 claim that the enhanced SO2/H2S ratio in subaerial volcanic gases was accompanied by enhanced H2 production, which may limit the oxidative capacity of emitted gases. This is only partly correct, because enhanced SO2 also derives from the reaction2,3: S22(melt) 1 3Fe2O3 (melt) R SO2 1 6FeO (melt) 1 O22 (melt) which implies that subaerial degassing extracts more oxygen from the melt than submarine degassing. The oxygen reservoir of the melt, a fundamental aspect of our model3 that has so far not been taken into account, implies that more oxygen was therefore degassed as subaerial volcanism became abundant at about 2.7 Gyr ago. Also, Kasting et al.1 argue that the amount of outgassed CO2 decreases by a factor of 3 as venting pressure decreases from 100 bar to 1 bar, which should limit production of organic carbon (CH2O) and thereby limit the associated consumption4 of atmospheric H2. However, although the molar fraction of CO2 in the gas decreases, the flux of CO2 into the atmosphere is unchanged between 100 and 1 bar venting pressures2, owing to the exceedingly low solubility of CO2 in silicate melt in this pressure range (unlike the case for sulphur). The f parameter of Holland4 is used by Kasting et al.1 to evaluate how much H2 is consumed to reduce volcanic CO2 into organic matter and SO2 to pyrite. According to Kasting et al.1, as pressure decreases, the f values of our calculated gas compositions indeed decrease (that is, their reducing power decreases, as required), but do not reach low enough values to drive the atmosphere to oxidizing conditions. However, the calculation of f is based on the way H2S is produced or consumed in volcanic gases: Holland4 first considered decomposition of H2S during cooling, which is equivalent to production of H2 (hence the 13m(H2S) term in the f equation). The more recent analysis5 by Holland considers instead that H2S is the product of reaction between SO2 and H2 during cooling, a H2-consuming reaction (23m(H2S) in the f equation). Conventionally, about 20% of volcanic CO2 is consumed to produce organic matter4. Any variation of the amount severely affects the results of calculations made using the f equation, highlighting the difficulties in using it as to determine the oxidative capacity of volcanic gases. Holland's more recent analysis5 of the causes of oxygenation suggests that oxidation was due to an increase in CO2 and SO2 volcanic fluxes, which is what our model predicts as volcanism changed from quasi-exclusively-submarine to partially subaerial. At this point, we stress that our model2 not only describes an increase in the oxidative capacity of volcanic gas but also a chain reaction likely to facilitate atmospheric oxygenation. Of prime importance are the sulphate reduction processes, which should have been exacerbated by elevated volcanic SO2 emissions. Biological sulphate reduction transforms sedimentary organic carbon into CO2, which results in oxygen production6. In parallel, hydrothermal sulphate reduction, which decreases the reducing potential of hydrothermal fluids and fixes hydrothermal ferrous iron as pyrite, also contributed to atmospheric oxygenation4. All these reaction paths are not included in Holland's f factor, whereas they were certainly involved in the Great Oxidation Event. We agree with the final recommendation of Kasting et al.1 that both volcanic gases and hydrothermal fluids should be considered in models of the Great Oxidation Event. However, whereas we accept that thick Archaean oceanic crust was on average more mafic than younger crust, the uppermost layers--those most susceptible to hydrothermal alteration--would have consisted of olivine-poor basalt. In both modern oceanic plateaus and presumably in Archaean oceanic crust, parental picritic magma differentiates, leaving olivine cumulates at the Moho and erupting relatively evolved lava7. Basalt with little to no olivine is the dominant component of the upper parts of both modern oceanic plateaus and Archaean greenstone belts7,8. These rocks are not susceptible to serpentinization; therefore little H2 would have been produced during their hydrothermal alteration, and its impact on the atmospheric oxygenation should not have been as important as claimed by Kasting et al.1. Last, Kasting et al.1 expressed concern about low-temperature re-equilibration processes between volcanic gases and basalts that were not considered by us2. We answer that this comment seems to re-introduce confusion between volcanic gas inputs (from mantle to exosphere) and hydrothermal recycling (seawater that reacts with basalts) that may arise from a misinterpretation of ref. 9. Volcanic degassing and hydrothermal emissions are two fundamentally distinct processes, which not only differ in temperature, but chiefly differ in their source (igneous input versus surficial recyling).
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Fabrice Gaillard, Bruno Scaillet, Nicholas Arndt. Atmospheric oxygenation and volcanism Gaillard et al. reply. Nature, Nature Publishing Group, 2012, 487 (26 juillet 2012), pp.E2. ⟨10.1038/nature11275⟩. ⟨insu-00723549⟩

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