Causes and Effects of the Oxygenation of Earth's Atmosphere
by Peter Crisci
Earth's atmosphere varied greatly in composition before finally developing into its current form. Over the course of eons, the amount of atmospheric oxygen slowly increased, largely due to the activity of photosynthetic organisms, though other factors such as volcanism also played an important role. Evidence for all this can be found in the different elements, minerals, and formations that developed as side effects of changing atmospheric composition. As a result of oxygenation, Earth experienced radical change overall, and forced to adapt, life expanded dramatically in diversity and complexity. This paper will explore in further detail various causes and effects of increasing oxygen levels in the atmosphere.
History of the Atmosphere
Earth's first atmosphere likely consisted primarily of hydrogen and helium. It dissipated relatively quickly, however, because the planet does not have enough mass to permanently retain an atmosphere of such light elements and did not yet have a magnetic field to shield it from the erosive effects of the solar wind. Later, Earth developed an atmosphere largely composed of heavier gases released through volcanic activity, such as ammonia, carbon dioxide, hydrogen sulfide, methane, and sulfur dioxide. At this point, there still would not have been any significant amount of atmospheric oxygen. 
Aside from there being no major source of oxygen at that time, the conditions in the early atmosphere would not have allowed it to persist. The ongoing formation of sulfates and iron ores consumed most of the oxygen that did find its way into the atmosphere.  Additionally, more abundant gases like methane and hydrogen sulfide may have reacted with and more or less eliminated any oxygen remaining. 
Oxygenation finally occurred, for the most part, in two major phases.  The first, the Great Oxygenation Event, began around 2.45 to 2.2 billion years ago. As a result, oxygen levels increased in the shallow ocean and atmosphere to about 15 percent by 1.8 billion years ago.  The second phase took place in stages from around 800 to 542 million years ago and raised oxygen levels in both the atmosphere and deep ocean.  Overall, gradually rising oxygen levels marked the Proterozoic, though atmospheric oxygen probably did not reach modern levels, 21 percent, until about 400 million years ago. 
Sources of Oxygen
No way is known of forming an atmosphere with a high concentration of oxygen without some form of life to generate it.  The oxygen in Earth's atmosphere largely resulted from cyanobacteria, which release it through photosynthesis. The organisms emerged 3.5 billion years ago and became well-established during the Proterozoic.  From then, continuing for up to several hundred million years, dissolved iron in the ocean reacted with any oxygen they released. When this finally exhausted the iron, oxygen levels in the ocean gradually rose to saturation, at which point the gas was free to diffuse into the atmosphere. 
However, cyanobacteria cannot account for all atmospheric oxygen. For instance, basins that were once believed to have been habitats for the organisms are now thought to have been unsuitable for them, too hot due to hydrothermal vents. Also, whether or not structures originally believed to be fossils of ancient cyanobacteria are really fossils at all was called into question in 2002. 
Another source of oxygen in the atmosphere is the oceans themselves. Ultraviolet radiation from the sun breaks water molecules down into their constituent elements, hydrogen and oxygen, which then enter the atmosphere. However, this process accounts for only around one to two percent of atmospheric oxygen. 
The arrival of life on land also relates to increasing oxygen levels in the atmosphere. While there is little direct evidence of the earliest terrestrial life, it is thought that the expansion of life onto land began at least one billion years ago, initially with protists, mosses, and other primitive organisms that, like cyanobacteria, release oxygen as a byproduct of their metabolism. Moreover, this colonization would have increased weathering on land so as to promote the sequestration of carbon, helping to make way for the development of an oxygen-rich atmosphere. 
Finally, eventual changes in volcanic activity also allowed oxygen levels to rise. As Earth's interior cooled, volcanic activity declined and, as a result, so did the amount of volcanic gases in the atmosphere that would have reacted with and consumed oxygen. Also, as oxidized rocks subducted and became incorporated into the planet's interior, the oxygen those rocks contained eventually entered the atmosphere through volcanoes. 
Evidence for Oxygenation Events
Because iron and oxygen react so readily with one another, deposits of oxidized iron can form a record of oxygen levels in the atmosphere. One example is red beds, rock formations rich in hematite, which is composed from oxidized iron.  Another such record is the banded iron formations, rock on the seafloor in which iron-rich layers alternate with iron-poor ones, recording the cyanobacteria's release of oxygen. Finally, iron oxide found in ancient soils suggests the presence of an atmosphere containing significant oxygen. 
The presence of certain minerals and elements can suggest a great deal about oxygen levels. Finding minerals like pyrite and uraninite, which only form in oxygen-poor environments, shows that oxygen levels must have been lower in the past.  Similarly, the disappearance after 2.41 billion years ago of certain sulfur isotopes, which can only form in the absence of oxygen, implies that significant amounts of oxygen appeared at that time. 
Chromium can also form a record of the oxygenation of the atmosphere. When it weathers from rocks on land in the presence of the oxygen, it is eventually carried to the oceans in the form of oxidized hexavalent chromium. There it reacts with iron and settles to the seafloor to become part of the banded iron formations.  Earlier than 2.7 billion years ago, oxidized hexavalent chromium is not found there, so there were not likely large amounts of oxygen in the atmosphere at that time.  Later, through the increasing presence of that form of chromium, the banded iron formations reveal fluctuations in atmospheric oxygen levels during the early Paleozoic, before the Great Oxygenation Event, which correspond to smaller changes in the amount of oxygen in the atmosphere. 
Consequences of Oxygenation
Early on, oxygen would have posed a threat to life. Had higher levels existed sooner, life may not have developed at all because the compounds that resulted in the first organisms cannot form under such conditions; oxygen interferes, for example, with the chemical reactions that form amino acids. Later, oxygen threatened the existence of various species. Because it can react destructively with organic compounds, many forms of primitive bacteria, among other organisms, cannot survive in the presence of oxygen. 
In the long run, however, it obviously proved a great benefit to life. Although many species became extinct as a result of increasing oxygen levels, the transition was slow enough that life in general could adapt.  Some of the bacteria that survived the transition, for instance, evolved into the first eukaryotes. 
Oxygen drove life to develop the means to process and eventually to take advantage of it, such as for producing larger amounts of energy more efficiently, which supported, among other things, the evolution of more complex bodies.  Furthermore, the development of photosynthesis as a way to make use of oxygen, initially in the cyanobacteria, allowed for the emergence of plants. Increasing quantities of oxygen in the atmosphere was a major cause of the Cambrian Explosion and the tremendous variety of life that exists today. 
Finally, oxygen brought about great changes for Earth itself, in addition to those already alluded to, like new rock and mineral formations. The oxygenation of the atmosphere allowed for the development of the ozone layer, an essential shield against ultraviolet radiation that enabled life to move to shallow water and to land.  Another possible effect of oxygenation was the cooling of the planet. Oxygen in the atmosphere may have consumed methane, a powerful greenhouse gas, and drove down the temperature of the planet by reducing the greenhouse effect. Higher oxygen levels may have triggered the Huronian Glaciation, one of the earliest and most severe ice ages, in addition to other periods of cooling. 
The atmospheric changes that took place over the history of the earth dramatically show that an oxygen-rich atmosphere not only is an element of the modern environment but also was a major cause of its emergence. However, while the most profound consequence of oxygenation was its effect on the environment and consequently on life, driving it to take on a tremendous number of new forms, there are other ramifications to consider. Also significant is how the atmosphere's development exemplifies the drastic effects small changes can have. In this case, for instance, the metabolism of bacteria, over enormous periods of time, was enough to remake the entire planet. This makes it difficult not to appreciate that change is always at work, even if it is progressing imperceptibly.
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19 Frei 250
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28 LeVine; Lane; Knauth 731
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