The problems arise, says Fike, because scientists don't have dense enough data to recognize spatial variations in Earth's geochemical past and because the geochemical proxies are so devilishly hard to interpret.
The world beach The story started to fall apart in 1998, says Fike, when Don Canfield of Odense University in Denmark suggested that sulfur compounds had also played a role in the transformation of Earth's chemistry.
Canfield argued that that the Great Oxygenation Event actually took place in two steps and that it was sulfides rather than oxygen that removed the iron from deep ocean water.
The first rise in oxygen caused oxidative weathering of rocks on land that delivered sulfates to the ocean through rivers and streams. In the ocean, sulfate-reducing bacteria converted the sulfates to sulfide to gain the energy they needed for daily housekeeping. The dissolved iron combined with the sulfides to form iron sulfide minerals such as pyrite that dropped out of solution.
During the second, much later stage, enough oxygen was generated to sweep the deep ocean of the toxic sulfides, ushering in the era of biological innovation, a.k.a. the mud sacks and quilted mattresses.
These transitions were still discussed as changes in bulk ocean chemistry just from one anoxic chemistry to another anoxic chemistry.
However, in the July issue of Nature Geoscience, Simon Poulton of the University of Newcastle in England showed that sulfidic water protruded into the ocean only in a narrow wedge along the shorelines of ancient continents. This meant that the water column, instead of being homogeneous, was stratified, with different chemistries in different layers.
|Contact: Diana Lutz|
Washington University in St. Louis