Geochemistry and biogeochemistry of anaerobic habitats.

Abstract

On a global average, the environment is, with regard to a proton and electron balance, in a stationary situation corresponding to a present-day atmosphere of 20.9% O<sub>2</sub> , 0.03% CO<sub>2</sub>, 79.1% N<sub>2</sub>, a world ocean with pH ≈ 8 and a redox potential of <em>E<sub>H</sub></em> = 0.75 V. This situation is the result of an interplay of different types of reactions, namely geochemical and biochemical processes.<br />The geochemical processes can best be understood considering the schematic reaction proposed by Sillén:<br /><br />igneous rock + volatile substances ⇔ air + seawater + sediments<br /><br />According to this equation, the volatiles (H<sub>2</sub>O, CO<sub>2</sub>, N<sub>2</sub>, HCl, HF, SO<sub>2</sub>, CH<sub>4</sub>, H<sub>2</sub>S, H<sub>2</sub>, NH<sub>3</sub>) that have leaked from the interior of the earth through different "cracks" (volcaneous, volcanic activities at the oceanic ridges, etc.) have reacted in a giant acid-base and redox reaction with the rocks (silicates, oxides, and carbonates). The result of what is called weathering was a certain kind of atmosphere, seawater, and sediment. Their composition is, over geological time, still changing slightly as a function of volcanic activities. If these activities were to cease, the surface of the earth would reach an equilibrium and all processes would stop. In other words, the earth would be geochemically dead. [...]