Where oxygen exists on Earth and how much it moves

　I explained that climate change is "the history of the carbon cycle." In contrast, the evolution of life can be described as the "history of oxygen circulation."

【earth's oxygen】

　The major difference between oxygen and carbon is their abundance on Earth. Among the elements that make up the earth, oxygen (30%) is second only to iron (32%). Most oxygen exists in the Earth's crust and mantle as SiO₂ and MgO (Morgan, PNAS, 1980). By the way, most of the iron exists in the core, and also exists as iron oxide in the crust and mantle.

　The boiling point of silicate minerals (or minerals whose main composition is SiO₂) exceeds 2000 degrees, so even in planetary collisions, some of the constituent elements may not be broken apart. It has existed as a silicate mineral since the birth of the earth. The next most common type of rock is carbonate rock (CaCO₃), in which carbon dioxide from the atmosphere is fixed due to the action of the ocean. Calcium carbonate decomposes when heated, so it could not have existed in the early Earth. Next is seawater. Up to this point, the effects of living organisms will have little to do with it. (Some carbonate rocks are fixed by living organisms.)

　

Data taken from Garrels, et al., Controls of Atmospheric O2 and CO2: Past, Present, and Future, American Scientist, 64, 306-315 (1976).

【Life evolution and accumulation of biologically derived oxygen compounds: SO₄^2- and O₂】
　Sulfate ions (SO₄^2-), which are dissolved in seawater, are the second most abundant ions after oxygen contained in rocks and water. How did this come about? It is thought that SO₄^2- in seawater was extremely small until at least 3.5 billion years ago. The sulfur that comes from inside the earth is probably reductive H₂S. It is thought that photosynthetic bacteria in the early stages of life used H₂S to reduce carbon dioxide. At that time, SO₄^2- is emitted. (CaSO₄, which creates the striped pattern of stromatolites, has been found.) Even if sulfate ions leaked into seawater, they would have been quickly consumed by sulfate-reducing bacteria. (Up to this point, I have referred to Sakai, How is the oxidation state of sulfur determined in nature?, Chemistry and Education 46, (1998)) Sulfur-oxidizing bacteria are thought to have accumulated sulfate ions in seawater over hundreds of millions of years. Oxygenic photosynthetic bacteria are born and help accumulate sulfate ions in the ocean. Oxygen began to accumulate in seawater 2.5 billion years ago. Oxygen began to accumulate in the atmosphere about 2 billion years ago, and in the Phanerozoic era, land plants began to photosynthesize, reaching the current atmospheric oxygen level. The result is the amount of oxygen shown in the diagram above. Because very little oxygen dissolves in seawater, there is only about 1/100 of the oxygen in the ocean as there is in the atmosphere.

【Amount of oxygen moving around the biosphere】
　Currently, the amount of oxygen transferred by living organisms through respiration and photosynthesis is only 2.5×10¹⁵ mol per year (equal amounts for photosynthesis and respiration). In a year, only 1/15,200 of the total amount of oxygen in the atmosphere and oceans is used for biological activities. The amount of photosynthesis on land and in seawater is said to be about the same. If the amount of oxygen transferred by living organisms is simply divided into two by land and ocean, the amount transferred by living organisms in the ocean will be 1.25×10¹⁵ mol per year. Seawater originally contains only 2.7×10¹⁷ mol of oxygen. In deep seawater isolated from the atmosphere, the effects of oxygen consumption by living organisms can be seen over many years. In addition, the effects of oxygen generation may be seen locally even in the subsurface layer where light reaches. In ocean chemistry and biology, these signals are picked up and used to decipher the state of ocean ecology.