Up From the Depths

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Science  20 May 2005:
Vol. 308, Issue 5725, pp. 1088
DOI: 10.1126/science.308.5725.1088b

Recent geological and chemical evidence supports the conclusion that in the distant past, Earth's oceans were repeatedly stratified, so that an anoxic layer formed at depth, as in the Black Sea of today. During these periods, bacterial metabolism would have used sulfate (instead of oxygen) as an electron acceptor, and the deep oceans would have become enriched in hydrogen sulfide as a consequence.

Kump et al. show that the upward flux of the accumulated hydrogen sulfide would have been quenched by the mixing of atmospheric oxygen into the surface of the oceans. They go on to infer that at times when the atmospheric oxygen level was low, large-scale upwelling of hydrogen sulfide gas might have taken over and that, in extreme cases, this could have resulted in the release of significant amounts of this toxic gas into the atmosphere. Biomarkers indicative of a high abundance of nonoxygenic photosynthetic green sulfur bacteria have been found, corresponding to the times of several mass extinctions and most recently for the end-Permian extinction (see Grice et al., Reports, 4 February 2005, p. 706), which is broadly associated with low oxygen levels and extensive ocean anoxia. — BH

Geology 33, 397 (2005).

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