Describe at least three types of evidence that supports the changing levels of atmospheric oxygen in the Proterozoic Banded iron formations show that O2 levels were low from about 3.5 to 1.9 by. Uranium and iron minerals from early Proterozoan rocks are evidence that oxygen was being more rapidly absorbed by chemical sinks at this time, about 2.3 billion years ago. Rocks like uranite and pyrite readily dissolve when exposed to atmospheric oxygen and these rocks are very rare in sandstones younger than 2.3 Ba. Oxygen sinks were becoming used up as levels in the atmosphere exceeded previous amounts. Their absence from the record after 1.9 by indicates that O2 levels had built up enough in the ocean to where banded iron formations couldn't form readily.
Carbon isotopes are also very valuble for determining paleo-oxygen levels.
The burial of large volumes of organic carbon leaves isotopically heavy carbon behind in the ocean. The heavy carbon ends up incorporated into limestones which precipitate from the sea water. Thus, a marked shift toward heavier carbon isotope ratios suggests the burial of organic carbon and the increase of atmospheric oxygen. Two types of skeletons-calcium phosphate and calcium carbonate are common. The variety of skeletons is believed to be the result of the evolution of advanced predators and changes in the chemistry of the ocean, in addition to rising level of free oxygen available for aerobic respiration.