ScienceDaily (2010-08-19) — Recent work with geochemical proxies for oxygen levels suggests that oxygen levels continued to fluctuate long after the Great Oxygenation Event 2.7 billion years ago, and that the oceans were many different flavors of anoxic right up until the Edicaran period, 600 million years ago. What happened in the intervening 2 billion years will be contested until scientists have more data, says a geochemist. http://bit.ly/bWrhAX
The original source is a News & Views article by David Fike: Nature Geoscience 3, 453 – 454 (2010) doi:10.1038/ngeo903
The short version is that changes in the redox potential in the oceans was heterogeneous and much more complicated than the narrative in the textbooks. Paleogeochemists use proxies that are tricky to interpret, and limited in their spatial distribution. More samples are needed to construct a global model of redox changes. Such a model should lead to a better understanding of evolution of multicellularity, and the scope and distribution of microbial metabolism during the 2 billion years after the Great Oxygenation Event and before the appearance of the Ediacaran fauna.