Regional and Global Biotic Effects

Except for the role ofatmospheric oxygen, so far we have focused on direct and local biotically mediated effects. On the larger scale of regions and the globe, other phenomena occur that probably act to increase the overall biotic enhancement ofweathering.

The diversity of effects on different scales entailed by biotic enhancement ofweathering is shown in figure 4-7. The local and direct effects are perhaps better understood than the global effects. The latter may include the effects of global cooling resulting from the evolution of land biota, a topic to be discussed in chapter 8. Thus, frost wedging, a major mechanism of physical weathering, may be biotic in the sense of arising from biotically mediated global cooling, triggering significant ice formation in mountains. The problematic role ofatmospheric oxygen includes the possible increase ofproduc-tivity of land biota by virtue of an ozone shield, to be discussed in more depth later in chapter 8. The emergence ofnew microbial soil consortia with the rise of atmospheric oxygen in the Proterozoic could have promoted the acceleration of dissolution of CaMgFe silicates by the oxidation of ferrous iron and the organic chelation ofits product, ferric iron, thus driving forward the weathering process and intensifying the carbon sink.

FIGURE 4-7.

A summary of the influences contributing to the biotic enhancement of weathering.

FIGURE 4-7.

A summary of the influences contributing to the biotic enhancement of weathering.

Another important regional effect is likely, the role of evapotranspiration, as Lovelock (1987) emphasized in his discussion of the geophysiology of Amazonia. Evapotranspiration coupled with precipitation accounts for an efficient recycling of water through the forested ecosystem. In the absence of evapotranspiration from a vegetative cover on land, chemical denudation should be reduced from the lack of water flow-through alone (Berner 1992; Drever 1994). It is interesting that sphagnum moss peatlands also recycle their water efficiently by the same mechanism as forested ecosystems (Klinger 1991). One can therefore speculate that the evapotranspiration/precipitation cycle amplified chemical denudation in terrestrial microbial and bryophyte mat ecosystems in the Precambrian and early Paleozoic.

We will return to the evolutionary aspects of this discussion in chapter 8. Attempts to quantify the combined effects, particularly in relation to the regulation ofatmospheric carbon dioxide levels and surface temperature, are discussed in the next chapter.

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