also great piles of the two sedimentary rocks, gypsum and anhydrate, that in similar fashion weather and release oxidized sulfur compounds into rivers and ultimately the sea.

There is a second pathway as well for the oxidation of sulfur compounds, one that takes place deeper in Earth, as pyrite is broken down during burial or subduction of pyrite-bearing rocks in the descending slabs of rock at the deep-sea trenches. Eventually this pyrite is heated to the point that it combines with oxygen, and the products of the reaction are emitted as the familiar and noxious-smelling sulfur gases found in volcanoes and hot springs, the poisonous gases called hydrogen sulfide and sulfur dioxide. When this happens, oxygen levels can drop in the atmosphere, especially if Earth is undergoing a phase of mountain building that exposes vast new reserves of sulfur-bearing rocks to erosion.

Even more important in dictating oxygen levels is the carbon cycle. Carbon makes up much of our bodies. Whether large quantities of reduced carbon compounds, such as animal and plant bodies after death, are left on the surface of the planet to react with atmospheric oxygen or are quickly buried has a major effect on oxygen levels. The rate of burial of organic carbon, along with the burial rate of sulfur-bearing compounds, is thus the major determinant of atmospheric oxygen levels.

Unfortunately, there is no direct way to measure past oxygen levels. (About a decade ago one such method was thought to be discovered: trapped air in amber was ballyhooed as a direct measure of past oxygen levels—until it was found that the small bubbles were not cut off from later changes in atmospheric levels.) Indirect methods based on an understanding of the relative ability of various minerals to undergo chemical changes in the presence or absence of oxygen have been used to infer relative oxygen levels, as have indirect methods based on biological evidence. For instance, in South Africa a very old mineral deposit was found that contains sedimentary uranium minerals. These minerals quickly change into other kinds of minerals when exposed to oxygen. But their presence in river deposits of more than 3 billion years ago yields powerful evidence that the land of the time (where rivers are) was covered with an oxygen-free atmosphere.

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