Unit 1: Many Planets, One Earth // Section 6: Atmospheric Oxygen
A stable climate is only one key requirement for the complex life forms that populate Earth today. Multi-cellular organisms also need a ready supply of oxygen for respiration. Today oxygen makes up about 20 percent of Earth's atmosphere, but for the first two billion years after Earth formed, its atmosphere was anoxic (oxygen-free). About 2.3 billion years ago, oxygen increased from a trace gas to perhaps one percent of Earth's atmosphere. Another jump took place about 600 million years ago, paving the way for multi-cellular life forms to expand during the Cambrian Explosion.
Oxygen is a highly reactive gas that combines readily with other elements like hydrogen, carbon, and iron. Many metals react directly with oxygen in the air to form metal oxides. For example, rust is an oxide that forms when iron reacts with oxygen in the presence of water. This process is called oxidation, a term for reactions in which a substance loses electrons and become more positively charged. In this case, iron loses electrons to oxygen (Fig. 12). What little free oxygen was produced in Earth's atmosphere during the Archean eon would have quickly reacted with other gases or with minerals in surface rock formations, leaving none available for respiration.
Figure 12. Oxidation of iron to form rust
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Geologists trace the rise of atmospheric oxygen by looking for oxidation products in ancient rock formations. We know that very little oxygen was present during the Archean eon because sulfide minerals like pyrite (fool's gold), which normally oxidize and are destroyed in today's surface environment, are found in river deposits dating from that time. Other Archean rocks contain banded iron formations (BIFs)—the sedimentary beds described in section 5 that record periods when waters contained high concentrations of iron. These formations tell us that ancient oceans were rich in iron, creating a large sink that consumed any available free oxygen.
Scientists agree that atmospheric oxygen levels increased about 2.3 billion years ago to a level that may have constituted about 1 percent of the atmosphere. One indicator is the presence of rock deposits called red beds, which started to form about 2.2 billion years ago and are familiar to travelers who have visited canyons in Arizona or Utah. These strata of reddish sedimentary rock, which formed from soils rich in iron oxides, are basically the opposite of BIFs: they indicate that enough oxygen had accumulated in the atmosphere to oxidize iron present in soil. If the atmosphere had still been anoxic, iron in these soils would have remained in solution and would have been washed away by rainfall and river flows. Other evidence comes from changes in sulfur isotope ratios in rocks, which indicate that about 2.4 billion years ago sulfur chemistry changed in ways consistent with increasing atmospheric oxygen.
Why did oxygen levels rise? Cyanobacteria, the first organisms capable of producing oxygen through photosynthesis, emerged well before the first step up in atmospheric oxygen concentrations, perhaps as early as 2.7 billion years ago. Their oxygen output helped to fill up the chemical sinks, such as iron in soils, that removed oxygen from the air. But plant photosynthesis alone would not have provided enough oxygen to account for this increase, because heterotrophs (organisms that are not able to make their own food) respire oxygen and use it to metabolize organic material. If all new plant growth is consumed by animals that feed on living plants and decomposers that break down dead plant material, carbon and oxygen cycle in what is essentially a closed loop and net atmospheric oxygen levels remain unchanged (Fig. 13).
Figure 13. Cycling of carbon and oxygen
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However, material can leak out of this loop and alter carbon-oxygen balances. If organic matter produced by photosynthesis is buried in sediments before it decomposes (for example, dead trees may fall into a lake and sink into the lake bottom), it is no longer available for respiration. The oxygen that decomposers would have consumed as they broke it down goes unused, increasing atmospheric oxygen concentrations. Many researchers theorize that this process caused the initial rise in atmospheric oxygen.
Some scientists suspect that atmospheric oxygen increased again about 600 million years ago to levels closer to the composition of our modern atmosphere. The main evidence is simply that many different groups of organisms suddenly became much larger at this time. Biologists argue that it is difficult for large, multicellular animals to exist if oxygen levels are extremely low, as such animals cannot survive without a fairly high amount of oxygen. However, scientists are still not sure what caused a jump in oxygen at this time.
One clue may be the strange association of jumps in atmospheric oxygen with snowball glaciations. Indeed, the jumps in atmospheric oxygen at 2.3 billion years ago and 600 million years ago do seem to be associated with Snowball Earth episodes (Fig. 14). However, scientists are still unsure exactly what the connection might be between the extreme ice ages and changes in the oxygen content of the atmosphere.
Figure 14. Atmospheric oxygen levels over geological time
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Source: © Snowball Earth.org.
Why have atmospheric oxygen levels stayed relatively stable since this second jump? As discussed above, the carbon-oxygen cycle is a closed system that keeps levels of both elements fairly constant. The system contains a powerful negative feedback mechanism, based on the fact that most animals need oxygen for respiration. If atmospheric oxygen levels rose substantially today, marine zooplankton would eat and respire organic matter produced by algae in the ocean at an increased rate, so a lower fraction of organic matter would be buried, canceling the effect. Falling oxygen levels would reduce feeding and respiration by zooplankton, so more of the organic matter produced by algae would end up in sediments and oxygen would rise again. Fluctuations in either direction thus generate changes that push oxygen levels back toward a steady state.
Forest fires also help to keep oxygen levels steady through a negative feedback. Combustion is a rapid oxidation reaction, so increasing the amount of available oxygen will promote a bigger reaction. Rising atmospheric oxygen levels would make forest fires more common, but these fires would consume large amounts of oxygen, driving concentrations back downward.