SEA-LEVEL AND PLATE TECTONIC EFFECTS

     The most obvious effect of plate tectonics is the changing position of the continents through time.  As you can see in the world map of the Carboniferous (below), things were a bit different.   Plattsburgh was close to the equator, while places like India were very far to the south of their present position.  While different latitudinal position will, of course, dictate different climatic conditions, other results of continental motion can be even more important.

     The presence of a large continental mass at either pole helps to cause in inequitable climate over the surface of the earth in which there are extreme differences between the temperature at the equator and that at the poles.  Why?   Because water retains heat.  It has a very high heat capacity and tends to gain or lose heat slowly.  Think about how long it takes Lake Champlain to warm up in the Spring, and how long it takes it to cool in the winter.  Open, polar oceans will moderate the climate, whereas land masses with very little heat retention tend to contribute to extremes in temperate.

     Mountain building due to continental collision can also have large effects on climate.  In the map above, the mountain belts connecting North America, South America, Africa and Europe are thought to have affected atmospheric circulation and produced arid conditions in North America (western side of the supercontinent, as opposed to extremely moist conditions to the southeast.  These orographic effects are common today in places like Asia, where the moist environment of India ends at the Tibetan Plateau where moisture is blocked, resulting in the arid conditions in Mongolia to the northeast.

     The rate  of sea-floor spreading also affects the climate by determining the presence or absence of epicontinental seas.  During times of fast sea-floor spreading, large parts of the continents are covered by shallow seas which tend to moderate the climate.  When the rate sea-floor spreading is slow, the seas drain off of the continents, and the moderating effect is lost, result in more climatic extremes.

CO₂ Cycle

     The ultimate source of atmospheric carbon dioxide is volcanoes.  Emission of CO₂ into the atmosphere leads to the generation of naturally occurring carbonic acid through a reaction of carbon dioxide with water in the atmosphere.

CO₂ + H₂O -> H₂CO₃.

Carbonic acid is a natural weathering agent which breaks down minerals in rocks.  The reaction with carbonates (limestone, dolomite, marble) leads to rapid weathering and the formation of bicarbonate ion (HCO₃^-) which is transported into the oceans.

H₂CO₃ + CaCO₃ -> Ca⁺² + HCO₃^-

Weathering of silicate minerals also uses up carbonic acid and produces silicic acid (H₄SiO₄) and carbonate ion (CO₃^-2) which are also transported into the oceans. 

     In the oceans animals and plants use the carbonate, silica and calcium to produce shells which then end up being deposited on the ocean floor, producing sedimentary deposits.

     We can see that abundant weathering will remove carbon dioxide from the atmosphere, which over the long term would have the effect of lowering temperature. 

      Mountain building and oceanic regression (retreat of the sea from the continents) will generally increase the amount of rock available for weathering.  Weathering rate will increase as temperature rises (most chemical reactions take place more quickly under warm temperatures), and as moisture increases (both precipitation and groundwater carry carbonic acid).

PLANT LIFE AND ATMOSPHERIC CHEMISTRY

     It is also important to note that the evolution of plants has also affected the carbon dioxide balance of the atmosphere.  As plants photosynthesize they remove carbon dioxide from the atmosphere and generate oxygen (see figure below).  When plant material decomposes it is oxidized, which uses up oxygen and gives off carbon dioxide.  But when plant debris is stored within

the earth in the form of coal (bogs are anoxic and acidic, hence no bacteria to oxidize the plant debris) and other organics (petrochemicals for one), carbon dioxide is removed from the cycle and stored within the earth.  Excess burial of organics in sedimentary rocks (especially those deposited in large anoxic ocean basins) will result in the atmosphere becoming more oxygen rich, while excess erosion and oxidation of organic-rich sedimentary rocks will result in the atmosphere becoming more carbon dioxide rich.   Note in the figure below that as plant life evolved and expanded across the continents through time, the level of CO₂ in the atmosphere decreased.  This model is based on the increasing amount of carbon-rich sediments found in progressively younger rock.

It is interesting to note that erosion and oxidation of organic-rich sedimentary rocks, and the burning of fossil fuels are to large extent the same process with the main difference being the rate at which CO₂ is produced.

RETURN TO EXTINCTION NOTES DIRECTORY