Weathering, Rainwater and Atmosphere Chemistry: Example and Modelling of Granite Weathering in Present Conditions in a CO2-Rich, and in an Anoxic Palaeoatmosphere

  1. Médard Thiry and
  2. Régine Simon-Coinçon
  1. J.-M. Schmitt1,2,3

Published Online: 14 APR 2009

DOI: 10.1002/9781444304190.ch1

Palaeoweathering, Palaeosurfaces and Related Continental Deposits

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

Author Information

  1. 1

    C.I.G., École Nationale Supérieure des Mines de Paris, 35 rue St-Honoré, 77305 Fontainebleau Cedex, France

  2. 2

    IGCP 317 Palaeoweathering records and palaeosurfaces, France

  3. 3

    CNRS–UMR SISYPHE C 7619 Structure et fonctionnement des systèmes hydriques continentaux, France

Publication History

  1. Published Online: 14 APR 2009
  2. Published Print: 26 MAY 1995

ISBN Information

Print ISBN: 9780632053117

Online ISBN: 9781444304190

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Keywords:

  • 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

Summary

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.