<P> The oxygen increase had to await tectonically driven changes in the Earth, including the appearance of shelf seas, where reduced organic carbon could reach the sediments and be buried . The newly produced oxygen was first consumed in various chemical reactions in the oceans, primarily with iron . Evidence is found in older rocks that contain massive banded iron formations that were apparently laid down as this iron and oxygen first combined; most of the planet's commercial iron ore is in these deposits . Researchers found that the amount of oxygen in the air spiked each time smaller land masses collided to form a super-continent . These massive pile ups generated mountain chains and as these mountains eroded, they released nutrients into the ocean, feeding the cyanobacteria that carry out photosynthesis . </P> <P> The early chemosynthetic organisms would have been a source of methane, which is an important trap for molecular oxygen, because oxygen readily oxidizes methane to carbon dioxide (CO) and water in the presence of UV radiation . Modern methanogens require nickel as an enzyme cofactor . As the Earth's crust cooled, the supply of nickel from volcanoes was reduced and hence less methane was produced . This allowed the oxygen percentage of the atmosphere to rise as the decrease in production of methane allowed the oxygen producing algae and other such life forms to, in a sense, out - produce the methane producers . From 2.7 to 2.4 billion years ago, the rate of deposition of nickel declined steadily; it was originally 400 times today's levels . </P> <P> Another hypothesis posits a model of the atmosphere that exhibits bistability in oxygen concentrations . In this model, UV shielding decreases the rate of methane oxidation once oxygen levels are sufficient to support the formation of an ozone layer . This explanation proposes an atmospheric system experiencing two steady states, one with lower (0.02%) atmospheric oxygen content, and the other with higher (21% or more) oxygen content . The Great Oxygenation Event can then be understood as a switch between the lower to the upper stable steady states . </P> <P> Another factor is the presence of hydrogen gas . The appearance of cyanobacteria might explain the decline of hydrogen gas and why Earth's air is so oxygen - rich . </P>

Name the biological process that removed oxygen from the atmosphere