Sidebar – the Earth’s crust is not quite fully oxidized

We aerobes are thankful that we have an oxidizing atmosphere, at least, that it contains copious oxygen as its oxidant. Still, we need reduced compounds in our own metabolism and, for the last 170 years or so, in fossil fuels to power our industrial society with all the risks appertaining to that use.

We owe the existence of reduced compounds proximately to plants who use photosynthetic electron transport to reduce CO2 and water to sugars, oxygen, and some new water. Many plants also reduce nitrate acquired from soil as a nitrogen source. They and cyanobacteria at the start of it all reduced carbon and made the oxygen in the atmosphere.

In the larger, geological scheme, Earth inherited a party reduced composition in its crust and mantle, as well as a very reduced iron-nickel mass in its core. This can be seen in the elemental composition of he crust estimated by a variety of means. It’s clear that most of the crust is metal oxides but there is a slight deficit in the oxidation state of the crust as a whole. From published sources on the abundances of the elements I calculated the oxygen demand:

Calculations by the author

The Earth was fortunate in being small enough so as not to acquire a thick and stifling hydrogen layer yet big enough to retain its water – poor Mars – and far enough from the Sun to avoid a runaway greenhouse effect as Venus experienced.

The atmosphere began in a reduced state with methane. That methane was oxidized away. The ocean is quite oxic now, though it began in a reduced state. Until oxygenic photosynthesis liberated copious oxygen, the seas were green with dissolved ferrous iron. That iron is now almost all tied up in the several hundred meters thicknesses of Red Beds as ferric and mixed ferrous/ferric oxides. Iron is the most limiting nutrient for marine life in large areas, including the South Pacific. The Atlantic gets more iron from iron-bearing sands blowing off deserts such as the Sahara. The Southern Hemisphere has much less desert area and lacks much of the input.