Weathering, Rainwater and Atmosphere Chemistry: Example and Modelling of Granite Weathering in Present Conditions in a CO2-Rich, and in an Anoxic Palaeoatmosphere
- Médard Thiry and
- Régine Simon-Coinçon
Published Online: 14 APR 2009
Copyright © 1999 The International Association of Sedimentologists
Palaeoweathering, Palaeosurfaces and Related Continental Deposits
How to Cite
Schmitt, J.-M. (1995) Weathering, Rainwater and Atmosphere Chemistry: Example and Modelling of Granite Weathering in Present Conditions in a CO2-Rich, and in an Anoxic Palaeoatmosphere, in Palaeoweathering, Palaeosurfaces and Related Continental Deposits (eds M. Thiry and R. Simon-Coinçon), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304190.ch1
- Published Online: 14 APR 2009
- Published Print: 26 MAY 1995
Print ISBN: 9780632053117
Online ISBN: 9781444304190
- chemical weathering, referring to chemical and mineral transformations;
- weathering of granitic rocks, resulting in development of lateritic profiles;
- O2 and CO2 fugacities, characterizing rainwater chemistry;
- chemical reactions affecting rock-forming minerals - carbonation, oxidation and hydration;
- thermodynamic equilibrium;
- general circulation models (GCMs);
- Huronian rainwater and atmospheric composition
Chemical weathering refers to the chemical and mineral transformations induced by rainwater percolation through rocks at Earth's surface. The main types of weathering reactions are oxidation and carbonation, which are caused by the oxygen and carbon dioxide dissolved in rainwater. The fugacities of these two gases in rainwater are imposed by their abundance in the atmosphere, which in this way governs the chemistry and intensity of weathering. There is very strong evidence, however, that atmospheric abundance of both O2 and CO2 have varied significantly during Earth's history.
In this paper two striking examples are provided of palaeoprofiles formed, respectively, in a CO2-rich atmosphere and in an anoxic atmosphere, and the conditions of their development are discussed with the aid of a geochemical modelling code. Recent data and models of the variation of atmospheric CO2 show that the Middle Cretaceous corresponds to a well-marked high, with values approaching 10 times that observed at present. Modelling shows that such CO2 abundance highly accelerates the formation of deep kaolinitic profiles, without the need for weathering duration as long, nor climate as hot and humid, as thought previously.
Palaeoweathering profiles fossilized below the lower Huronian strata in Canada are characterized by an unusual depletion in total iron, and the preservation of pyrite. These characters are considered as strong indicators of reducing atmospheric conditions c. 2.4 Ga. Geochemical modelling enables us to reproduce the major features of these profiles for O2 and CO2 partial pressures compatible with current estimates for this period. These results also are used to discuss the anoxic character as well as the potential H2S content of the Huronian atmosphere.
The main conclusion that can be drawn from this review and from these examples is that palaeoweathering profiles have registered the fluctuations in the atmosphere's chemistry and climatic conditions, and that consequently they are one of the main indicators of Earth's past environments.