Ancient Anoxic Soils

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Science  02 Feb 2001:
Vol. 291, Issue 5505, pp. 791
DOI: 10.1126/science.291.5505.791e

The rise of oxygen in Earth's early atmosphere was a critical event both for the evolution of life and for geologic and atmospheric processes, although whether this occurred early in Earth's history or about 2.2 billion years ago has been contentious. Much of the evidence for a late increase, corresponding to oxygen production from photosynthesis, has come from assessing the oxidation state of iron in ancient soils (for example, the presence of hematite). One problem is that it can be difficult to separate effects of early atmospheric-dependent weathering from those of later metamorphism in these old rocks.

Murakami et al. identified rhabdophane, a mineral that readily forms from apatite early in weathering, in a soil horizon that formed about 2.5 billion years ago on an exposed granite. This mineral contains the rare-earth element cerium in abundance and in a relatively reduced state, consistent with a late rise of atmospheric oxygen. — BH

Earth Planet. Sci. Lett.184, 523 (2001).

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