Molecular Oxygen (O₂)

Surprisingly few attempts to quantify the geochemical cycling of oxygen exist in the literature. In part this may be because globally it is not a limiting element or nutrient. In part this may also be because the slow turnover times of the pools and the corresponding difficulties of experimentally measuring these changes. Below is a figure using the numbers for oxygen pools and fluxes from Keeling, Najjar, Bender and Tans (1993) Global Biogeochemical cycles 7; 37-67.

The figure above depicts pools and fluxes of oxygen. The values in parenthesis < > are estimates of oxygen pool sizes. The dotted arrows are fluxes interconnecting pools and are either chemical or biological in origin. The rates of the fluxes are given in curly brackets {}.

• The largest oxygen pool is found within sedimentary rocks and contain >96% of earths oxygen. There is little real change of this pool, as it for the most part is inaccessible to chemical reactions.  The small change is due to weathering and volcanoes that consume a small amount of O₂.
• The atmosphere contains the second largest pool with about 3.7% of earth's total oxygen in the form of O₂. 
• Other pools that are significant for oxygen cycling are the oxygen pools associated with biological material, such as that found in the deep ocean and as long lived terrestrial biota.
• One other (non-equilibrium) pool is that derived from fossil fuel reserves that when extracted and burnt will require O₂ for combustion.

Oxygen Fluxes in Action

The largest flux involved with oxygen is the exchange of 140 x 10¹⁵ moles of O₂ between the atmosphere and surface waters of the ocean. This is essentially only a mixing phenomenon of surface waters and the atmosphere. Colder waters contain more O₂ and ocean currents also distribute the oxygen throughout the ocean to deeper waters.

The most spectacular chemistry is the (bio)geochemical cycling of oxygen.  This is the result of biology driving a critical water <--> O₂ cycle on earth.  This process is the result of two opposing reactions.  One reaction, the photosynthetic splitting of water that liberates O₂.  This reaction is common to all plants, algae and cyanobacteria.  The second reaction opposing this are a variety of O₂ consumption reactions that are found in biology. The principle reaction is aerobic respiration, but there are others.  The term gross primary production (GPP) is a measure of total photosynthesis and is presented here in units corresponding to the O₂ generated by photosynthesis. Yet with all photosynthetic organisms there is also an intrinsic component due to respiration. This value is shown above as the net respiration rate and will reduce the GPP values to a quantity termed net primary production (NPP). The remaining biomass derived from NPP is then partitioned as long lived biota. Yet eventually too this is consumed by respiration via decomposition of wood, cellulose. This aerobic decomposition consumes O₂ and returns to water via biochemical reactions.  The total fluxes are 9.2×10¹⁵ mols for terrestrial plants and 4.3 ×10¹⁵ mols for aquatic plants. These fluxes with the respiration results in the complete turnover of 3.7×10¹⁹ mols of atmospheric O₂ in the atmosphere in about ~3 million years. See the work of Keeling et.al., (1993) for further numerical discussions.