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        Oceans cover over two-thirds of Earth's surface.  Free oxygen gas first became introduced in the oceans several billion years ago as a waste product of photosynthesis. The accumulation of dissolved oxygen in the water was toxic to the early organisms and their survival depended upon their ability to develop protective tissues.
        Marine photosynthesizers at the surface of the Earth's oceans were the first forms of life to successfully populate Earth.  They continue to be the greatest source of gaseous oxygen today.
        Photosynthetic activity occurs in the uppermost regions of the water column where sunlight is sufficiently energetic to power it.  This upper sunlit water is called the photic zone.  It varies in depth depending upon the angle of the Sun's rays and the clarity of the water.
Summer Sunshine
        During summer days, the incident angles of solar radiation are highest, causing the deepest and most energetic penetration of sunlight into the water column.  Photosynthetic activity is greatest during this time and much oxygen waste gas accumulates in the water.
Thermocline prevents mixing
        Surface waters expand when they heat up in the summer sunshine. This increases their buoyancy which *may  result in the development of a warm and stable layer at the surface. This warm water rests on top of colder water and may extend only as deep as wave action can mix warm surface water into the ocean.  This warm layer acts like a lid, preventing the mixing of surface waters with colder, deeper water.  Excess oxygen gas from photosynthesis builds up in this mixed layer and is subsequently released into the atmosphere.
Heating releases gases
       The amount of dissolved gas that water can hold depends upon its temperature. Seawater can hold fewer dissolved gas molecules when it heats up. Oxygen (and other gases) come out of solution and enter the atmosphere as the water is heated.
        The three major dissolved gases in the surface waters of the ocean are oxygen, nitrogen, and argon.  The ratio that they exist dissolved in the ocean is not  the same as is the ratio existing in the atmosphere.  The ratio of O2:N2:Ar in the surface waters is  approximately 20:36:1 [5b].  The atmospheric ratio of 21:78:1 shows that more oxygen gas dissolves in the sea than does nitrogen gas.   Nitrogen gas is not as soluble as oxygen and argon, so when water releases its dissolved atmospheric gases, the air becomes relatively enriched in oxygen.
        When the surface waters heat up, a slight enrichment of atmospheric oxygen occurs in the overlying air.

Winter characteristics
        During winter periods, colder water absorbs more dissolved gases.  As this happens, a slight depletion of oxygen from the overlying atmosphere occurs at the sea surface as oxygen gas is absorbed.  The angle of sunlight during winter is lower as well; photosynthesis is greatly diminished without intense sunshine penetrating the water.  With the diminished creation of oxygen gas by photosynthesizers, there is more room for the absorption of oxygen from the atmosphere.
         Low sunlight angles allow cold atmospheric temperatures to predominate, especially at higher latitudes.  Wintertime surface waters will be prone to sink due to an increased density from the colder temperatures.  As dense surface waters sink, they transport absorbed gases along with them.
        The descending surface waters are replaced by slightly less dense water from below in a mixing process called convection.  This causes a mixing of surface and deeper waters and a subsequent redistribution of oxygen from the atmosphere to the deep ocean.  Rising deeper water is often enriched in nutrients which is then also redistributed during winter convection.  Colder regions of the ocean support larger animal populations due to the mixing of dissolved gases from above and nutrients from below [3a].

        The change in the levels of atmospheric oxygen over an ocean is different than it is over land because oxygen gas is soluble in seawater.  The solubility creates smaller fluctuations in the levels of atmospheric oxygen over an ocean compared to the direct exchanges that land plants have with the atmosphere. Additionally, due to the time necessary for convection and diffusion processes to permit the mixing of oceanic and atmospheric gases, a slight delay exists in maximum and minimum concentrations over an ocean compared to the direct exchanges of land plants.
       The surface of the Earth's Southern Hemisphere is dominated by ocean.  Earth's continents are concentrated in the Northern Hemisphere.  This difference is represented by the seasonal atmospheric gas levels between the two hemispheres.
    The following figure illustrates the effects an ocean has on the levels of two atmospheric gases: oxygen and carbon dioxide.  Carbon dioxide is many times more soluble in water than oxygen and this is reflected in a smaller change in the amplitude of its atmospheric concentration.   Carbon dioxide is 34 times more soluble in the ocean than oxygen.  Its annual fluctuation in the Southern Hemisphere exhibits a much smaller amplitude than does its northern counterpart.

Comparing differences in atmospheric oxygen and carbon dioxide levels between hemispheres [5c].

        What is initially striking about the figure is the near anti-correlation between gas curves from opposite hemispheres.  The seasonal fluctuation of photosynthetic gases causes a maximum oxygen release during the Northern Hemisphere's late summer (August - September) while at the same time, oxygen is at a minimum in the Southern Hemisphere's late winter.

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