The Oxygen Budget

Atmospheric molecular oxygen is generated and consumed by a wide range of processes in the Earth system. Biological, chemical, and physical processes both on and beneath the Earth's surface all contribute to the budget for oxygen in the Earth's atmosphere. Figure 1 shows a simple representation

The atmosphere

1 100 000 Gt-O

CO

0.13 Gt-O/yr Net uptake by weathering of fossilzed carbon

Net emission due to burial of organic carbon

0.13 Gt-O/yr Net uptake by weathering of fossilzed carbon

Net emission due to burial of organic carbon

0.05 Gt-C/yr organic carbon burial

0.05 Gt-C/yr fossil carbon uplifting

0.05 Gt-C/yr organic carbon burial

Figure 1 A simple model of the oxygen cycle, and its relationship to the carbon cycle, that considers the three major reservoirs: the atmosphere, the biosphere, and the sediments. Redrawn from graph presented by http://atoc.colorado.edu/~fasullo/pjw_class/ images/oxygencycle.gif.

Figure 1 A simple model of the oxygen cycle, and its relationship to the carbon cycle, that considers the three major reservoirs: the atmosphere, the biosphere, and the sediments. Redrawn from graph presented by http://atoc.colorado.edu/~fasullo/pjw_class/ images/oxygencycle.gif.

of the budget for oxygen. This figure also shows how the oxygen budget is closely linked to the carbon cycle. While it is difficult to measure the annual fluxes shown in Figure 1, oxygen isotopes (17O,18O) do provide constraints leading to the approximate values shown.

The major mechanism by which molecular oxygen is produced on our planet is through photosynthesis. As seen in Figures 2 and 3, the net reaction of photosynthesis is to convert carbon dioxide (CO2) and water to molecular oxygen. This large annual flux is counteracted by the equally large annual flux from the effects of respiration in removing oxygen from the atmosphere.

Oxygen Budget
Figure 2 A representation of the basic exchange of atmospheric oxygen between biospheric systems.

Thus, as suggested by Figures 1 and 2, the current atmospheric equilibrium is maintained by this cycle between photosynthesis and respiration. As indicated in Figure 3, there are several slower removal processes in addition to this more rapid cycling.

The mean residence time of molecular oxygen in the atmosphere is roughly 4000 years. With the approximate balance between photolysis and respiration, this residence time is largely controlled by the long-term burial of reduced carbon in ocean sediments. This burial of organic matter results is initiated via the photosynthesis from organisms in the upper levels of the ocean, whose carbon then is deposited to deeper levels of the ocean. The actual rate of burial depends on the area of the ocean floor subject to anoxic conditions, which in turn varies inversely with the concentration of atmospheric oxygen. The balance between the burial of organic matter and its oxidation thus plays a significant role in the maintenance of the atmospheric molecular oxygen at 20.95% of the atmospheric density.

A large amount of oxygen has also been consumed by weathering of reduced crustal materials, such as those containing iron and sulfur, through the geologic history. However, the residence time of atmospheric oxygen at the current rate of exposure would be roughly 2 million years (My) due to removal mechanism.

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Responses

  • Yemane
    What is oxygen budget?
    2 years ago

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