Atmosphere Archaean

This review will first examine the controls on Earth s modern atmosphere, to give a uniformi-tarian basis then consider parallel worlds. Next, models of Earth evolution are built, on unifor-mitarian grounds by looking at the early abiotic Earth and by considering biogeochemical processes in the Archaean. To provide experimental constraint, the Belingwe area is examined in detail. The operation of the Archaean atmosphere is discussed then the links between air and ocean, and the links with the mantle and interior. Lastly, the overall controls are considered. 

In the modem atmosphere, free methane is oxidized within about a decade by OH, derived from water vapour attacked by O which in turn is derived from ozone and dioxygen. If the Archaean atmosphere were reducing and if it had lower O2, biologically released methane would have lasted far longer in the air. If abundant, atmospheric methane would have been a powerful greenhouse gas. 

There is no cold trap for methane, and thus it would have been abundant in the high Archaean atmosphere. Here it would have been decomposed by UV radiation, releasing hydrogen. Some of this hydrogen would have diffused to the top of the atmosphere and would have been lost to space. There would also have been significant... 

Phillips et al. (2001) discussed the central evidence on which the reduced model of the Archaean atmosphere is based. This evidence includes the composition of detrital gold grains, inferred detrital uraninite, inferred detrital pyrite, and inferred palaeosols, especially in the Witwaters-rand succession. In a careful detailed review. Philips et al. concluded that the geological evidence for a reducing atmosphere remains ambiguous. In particular, post-depositional processes may need far more examination. Phillips et al. pointed out that some of the mineralogical and field evidence can be interpreted as supporting an oxidized Archaean atmosphere. 

Nevertheless, although some arguments for Archaean redbeds can be questioned, Phillips et al. (2001) have a powerful case. The question was the Archaean atmosphere reducing must be regarded as still unsettled. 

Links between the Archaean atmosphere and the ocean biology and temperature... 

The Archaean atmosphere-ocean system was probably less oxic than today, but supply of oxidant would have occurred nevertheless as, in an Archaean C02-based atmosphere, volcanic sulphur and nitrogen sources would have provided SO t and NO, or precursors that would have been oxidized in the atmosphere to and NO. Together with nitrogen-fixing by lightning, this would have supplied sulphate and nitrate for use by microbes living in mud that contained picoplankton debris, to sustain mats, possibly of colourless sulphur bacteria. The REE record a distant hydrothermal input, which, depending on water currents, supplied metals and perhaps episodes of reduction. 

If N, O and C fluxes in the biosphere were all biologically managed then, excepting the noble gases, the late Archaean atmosphere, as today, was a biological construct. 

There are two reasons why it is likely that the Archaean sulfur cycle might have been very different from that which is observed in the modern. First, as will be discussed later in this chapter, the Archaean atmosphere had very low levels of oxygen, so that there was no oxidative weathering of sulfide in crustal rocks at this time. This means that no sulfate was delivered to the Archaean oceans through weathering and consequently the Archaean ocean was probably very low in sulfate. Second, the removal of sulfate from the ocean is bacterially mediated. The operation of this... 

Paleosols. Paleosols formed before 2.2 Ga tend to contain iron silicates, rather than siderite-iron carbonate. Rye et al. (1995) used carbonate-silicate mineral equilibria in the 2.75 Ga Mount Roe paleosol to estimate the partitioning of COa between soil and air in the late Archaean. Their calculations suggest a maximum partial pressure of 10 14 (0.04) atmospheres COa in the late Archaean atmosphere, significantly lower than the estimate of Lowe and Tice (2004) based on nahcolite. A lower limit for atmospheric COa comes from the study of siderite-clay mineral equilibria for weathering rinds on clasts in river gravels from the 3.2 Ga Moodies Group of the Barberton greenstone belt, in south Africa (Hessler et al., 2004). In this study the minimum partial pressure for atmospheric COa at 3.2 Ga was calculated to be 2.5 X 10 3 bars at 25°C. 

Archaean weathering profiles. Further evidence for a C02-rich Archaean atmosphere comes from indications of an aggressive weathering regime in the late Archaean sedimentary record, driven by a high content of carbonic acid in the weathering environment. Two different arguments are used to support this claim. First, first-cycle sediments are much more refractory than their modern... 

If the Zahnle and Sleep (2002) model for COa-drawdown is correct, and C02 was principally stored in the oceanic crust during the Archaean, this does not necessarily negate the calculations of Kramers (2002), but it does shift the time of C02-drawdown back to perhaps the mid-Archaean. However, a problem with the Zahnle and Sleep (2002) model is that, unlike the Urey cycle, the Archaean oceanic weathering cycle has no inbuilt temperature feedback and needs a "thermostat" to maintain equable surface temperatures on the early Earth. This problem could be solved (just) if there were very high levels of COa in the atmosphere, but the better solution is to include methane as a significant component of the Archaean atmosphere. 

Frimmel, H.E., 2005. Archaean atmospheric evolution evidence from the Witwatersrand gold fields, South Africa. Earth Sci. Rev., 70, 1-46. 

Lowe, D.R. and Tice, M.M., 2004. Geologic evidence for Archaean atmospheric and climatic evolution fluctuating levels of C02, CH4 and 02 with an overriding tectonic control. Geology, 32, 493-6. 

Ohmoto, H., 2004. The Archaean atmosphere, hydrosphere and biosphere. In Eriksson, P.G., Altermann, W., Nelson, D.R., and Cataneanu, O. (eds) The Precambrian Earth Tempos and Events. Elsevier, Amsterdam, pp. 361-88. 

Pavlov, A.A. and Kasting, J.F., 2002. Mass-independent fractionation of sulphur isotopes in Archaean sediments strong evidence for an anoxic Archaean atmosphere. Astrobiology, 1, 27-41. 

Rasmussen, B. and Buick, R., 1999. Redox state of the Archaean atmosphere evidence from detrital heavy minerals in ca. 3,250-2,750 Ma sandstones from the Pilbara Craton. Geology, 27, 115-18. 

Archaean supracrustal rocks are lacking in uranium occurrences, except for minor occurrences associated with late Archaean granitic rocks. Their absence within Archaean supracrustal rocks may appear enigmatic in view of the reducing nature of the Archaean atmosphere-hydrosphere and the many... 

Schidlowski M. Archaean atmosphere and evolution of the terrestrial oxygen budget. In The early history of the earth Windley B. F. ed. (New York Wiley, 1976), 525-35. 

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