Oxygen, a waste gas expelled by plants, is the critical component of air that our lungs crave.  Our breath is designed solely to extract oxygen from the atmosphere.  Breathing is an act that remains virtually forgotten until it is threatened...then it becomes an instant crisis.
      Oxygen gas occupies less than  21% of the volume of the atmosphere.  Most of the air that we breathe is gaseous nitrogen, a substance we cannot use.  It is that one-fifth of the air we breathe that powers our oxidative metabolism.  The intense reactivity of oxygen causes a release of energy when it combines with other molecules....a property many organisms take advantage of.

        It may be surprising to learn that oxygen occupies a larger volume percentage of solid rock than air.  The fact that oxygen is very chemically reactive causes it to combine with virtually all substances it comes into contact with. The Earth's crust consists essentially of compounds formed from such reactions.  These reactions are naturally favored; they will happen spontaneously under normal Earth conditions.  Although favored, it can take a long time for such a reaction to occur.  The interactions of water and atmosphere with geological and biological materials occur over an extensive range of environments.  Each environment influences the types and rates of chemical reactions differently and oxygen chemistry is highly variable as a result.  The complex chemistry of Earth is a diverse manifestation of many reactions occurring simultaneously.  Oxygen's natural reactivity would cause its complete depletion from the atmosphere if it were not for the persistent replenishment of it as a waste gas from photosynthesis.

        The oxygen gas necessary to sustain life is created by life itself.  Photosynthetic organisms (plants) are responsible for the development and continued input of oxygen into the atmosphere.  Once expelled as a waste gas from the photosynthesizers, oxygen becomes available to other organisms that breathe oxygen as fuel.  Over the past hundreds of millions of years, according to the fossil record, lifeforms developed sophisticated systems that utilize the power available in atmospheric oxygen. Such an evolutionary development required that a fairly steady concentration of atmospheric oxygen be maintained.  Drastic changes in oxygen concentration over time would prevent the steady development of lifeforms depicted in the fossil record.  The maintenance of a steady concentration over geologic time is a great mystery, considering the highly variable components involved in the cycling of atmospheric oxygen .

test your knowledge: try the oxysphere quiz for this section

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prelude, imagemap, introduction, atmos evolution, earth systems, reactivity, organic matters, phosphorus, oceanic processes,
photochemistry, fire, weathering, burial and tectonics, anthropogenic influence, glossary, references, quiz

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OXYSPHERE
EVOLUTION of the ATMOSPHERE

        The early Earth did not contain oxygen gas when it formed.  The oxygen had already combined with other elements.  Other volatile substances present during the formation of the solar system became the Earth's first atmosphere, modified by volcanic activity and held near Earth's surface by gravity.  Evidence suggests that this early atmosphere was likely lost to space: certain inert gases found in the rest of the solar system are now absent in Earth's atmosphere. It has been hypothesized that a collision of the early Earth with another large planet-like body may have caused the early atmosphere to escape Earth's gravity.  Methane (CH₄), carbon dioxide (CO₂), ammonia (NH₃), and water vapor (H₂O) then became primary atmospheric gases due to volcanic outgassing.

       The Sun's ultraviolet rays caused molecules of CH₄, NH₃, and H₂O to photodissociate (break apart by light energy).  The broken pieces of the molecules recombined to form a variety of other compounds such as CO₂ and N₂.  These gases supplemented volcanic emissions and accumulated in the atmosphere.  Water vapor condensed as it accumulated; a hydrologic cycle developed and strengthened, and oceans filled the low areas.  CO₂ gas, being quite soluble in water, was removed from the atmosphere in large amounts.  Carbonic acid has been a principal component of chemical weathering ever since.  The precipitation of limestones (CaCO₃) from seawater became a process of long-term storage for large amounts of atmospheric CO_2.
 
 

         Oxygen increased in the atmosphere through the development of photosynthetic organisms in the oceans. Marine plants produced oxygen as a hazardous waste gas. This gas was poisonous to the early lifeforms struggling to survive in an unforgiving world. The corrosiveness of oxygen required that organisms develop protective membranes to limit oxygen's contact with delicate tissues.

Oxygen eventually accumulated in the atmosphere.  Dioxygen molecules (O₂) permeated the air, naturally circulating into the upper atmosphere.  Energetic UV rays from the sun in the upper atmosphere split oxygen molecules apart, allowing the ozone molecule (O₃) to be created from the broken parts. Ozone molecules also split apart when hit by solar UV radiation; it is this absorption of solar rays that helps protect life from Sun's damaging energy.  All of the radiation absorbed through the splitting of oxygen and ozone molecules results in a radiation shield in the stratosphere.  With ozone in place as a radiation shield, life was then able to populate the land.

        Atmospheric oxygen colors our world by oxidizing minerals and scattering our sun’s rays.  Oxidized compounds, like the minerals in weathered rocks, tend to be bright colors such as yellow, orange, red, purple, etc.  Unoxidized compounds are often dark colors such as brown, green, black, and grey. The sky is blue because sunlight is scattered by particles in the atmosphere, many of which are oxygen molecules.