Abstract
Arising from: H. Ohmoto, Y. Watanabe & K. Kumazawa Nature 429, 395–399 (2004); see also communication from J. F. Kasting; H. Ohmoto & Y. Watanabe reply Ferrous carbonate, as the mineral siderite, occurs in Archaean palaeosols (ancient soils). Ohmoto et al.1 contend that siderite was not in equilibrium with the oxygen in Archaean air and that its presence in palaeosols provides little constraint on the partial pressure of carbon dioxide in Archaean air. But their argument is invalid because it fails to distinguish the different behaviours of the trivial component oxygen and the significant component carbon dioxide in the partly closed system of soil waters. The presence or absence of siderite in ancient soils is a valid constraint on the carbon dioxide partial pressure (pCO2) in ancient atmospheres.
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To illustrate the problem using the approach of Ohmoto et al., I use their lowest oxygen partial pressure (pO2). Solar ultraviolet and lightning would dynamically maintain pO2 at 10−13 atm (ref. 2). Ohmoto et al. correctly state that this is far above the critical pO2 for siderite stability, namely 10−65 ± 5 atm. The partial pressure at siderite stability has only mathematical significance; there is no actual molecule of oxygen in any realizable volume of water at the pO2. Rather, the dissolved oxygen at Archaean pO2 in water, which is about 10−16 mol kg−1, is too small to affect the chemistry of a soil observably, as illustrated by a simple mass balance. Basalt contains about 10% by mass of ferrous oxide, FeO, so it would take 2 × 1015 kg of oxygen-saturated water to oxidize the ferrous oxide in a kilogram of rock to magnetite, Fe3O4. With a typical rainfall of 1 m yr−1, it would require 1015 years of rain to oxidize a metre of section. This conclusion applies as long as oxygen is a trivial component. Even at a partial pressure of 10−6 atm, it would take 108 years to oxidize a metre of section.
Rather, the trivial oxygen dissolved in soil water at pO2 = 10−13 atm reacts with the rock, producing an undetectable amount of ferric iron. This leaves a solution that is quantitatively depleted in oxygen and buffered by ferrous silicates, where siderite is stable if there is enough pCO2. At a higher pO2 of 10−6 atm, trace oxygen in soil water would produce observable ferric iron before it was exhausted, again leaving a buffer with ferrous silicates. Moreover, if the oxidation reaction is kinetically inhibited at trivial concentrations, the presence of oxygen in the air is irrelevant to the presence of siderite.
References
Ohmoto, H., Watanabe, Y. & Kumazawa, K. Nature 429, 395–399 (2004).
Kasting, J. F. Precambrian Res. 34, 205–229 (1987).
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Sleep, N. Archaean palaeosols and Archaean air. Nature 432, 1 (2004). https://doi.org/10.1038/nature03167
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DOI: https://doi.org/10.1038/nature03167
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