The Two Exchange Systems that Alter the CO2 Concentration of the Atmosphere

How is it possible for organisms to alter the carbon dioxide concentration of the atmosphere? It is through two exchange systems that organisms can alter the carbon dioxide concentration of the atmosphere. First, is the land-based or terrestrial exchange system by which carbon is exchanged between land plants and both soil and the atmosphere; Second, the marine system which exchanges carbon within the oceans and between the oceans and the atmosphere, The basic cycle begins when photosynthesizing plants use carbon dioxide (CO2) found in the atmosphere or dissolved in water.

Using sunlight plants (autotrophs, meaning self-feeders) take up CO2 from the air and combine it with hydrogen in water to produce molecules of simple organic compounds (glucose) and oxygen. This can be represented by the following biochemical reaction: sunlight 6 CO2 + 6 H20 C6 H12 06 + 6 02 (equation 1) glucose Some of this carbon is incorporated into plant tissue as carbohydrates, fats, and protein; the rest is returned to the atmosphere or water by respiration, a slow form of burning which is the reverse of equation 1.

Some of the carbon is not re-released into the atmosphere and is therefore fixed into the living tissues. The remainder of carbon is thus passed on to herbivores that eat the plants and thereby use, rearrange, and degrade the carbon compounds. Ultimately, all the carbon compounds are broken down by decomposition and thereafter into fossilized carbon remains, and so are removed from the atmosphere-hydrosphere (the total body of water including oceans, rivers, and lakes) system. In addition to the terrestrial exchange system by which carbon is exchanged between land plants and both soil and the atmosphere, vast quantities of carbon dioxide from volcanic emissions have become trapped in sedimentary rocks, mainly by biological precipitation from seawater.

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Looking at the record of sedimentary rocks, geologists note, that carbonate did not become abundant until nearly 2 x 103 MA ago, although calcareous stromatolites as old as 3 x 103 MA are known. Carbonate sediments became increasingly abundant in more recent geological history. There seems to be a strong link between occurrence and the availability of marine organisms, such as blue-green bacteria, capable of removing CaCo3 from seawater and it in limestone. In short, the atmosphere has evolved through the fixation of carbonate in limestone and carbon in fossil fuels, both reducing the levels of CO2 in the atmosphere, and the latter producing atmospheric oxygen. What is the geological evidence that organisms may be responsible for the present-day, oxygen-rich composition of the atmosphere?

While photosynthetic life reduced the carbon dioxide content of the atmosphere, it also started to produce oxygen. For a long time, the oxygen produced did not build up in the atmosphere, since it was taken up by rocks. To this day, the majority of oxygen produced over time is locked up in the ancient “banded rock” and “red bed” formations. It was not until probably only 1 billion years ago, that the reservoirs of oxidizable rock became saturated and the free oxygen began to accumulate in the atmosphere. This had two important consequences. First, it set the stage for the advent of aerobic (oxygen-based) respiration. Second, as ultraviolet light split oxygen molecules, ozone was formed, resulting in the ozone layer that now serves as a shield against UV light. Through this protection photosynthetic organisms evolved and started to have a major impact on the oxygen content of the atmosphere. These organisms fed off atmospheric carbon dioxide and converted much of it into marine sediments consisting of the innumerable shells of sea creatures. Geologic evidence for the origin of early life comes from the study of rocks and fossils. Fossils are the remains of organisms that became trapped and preserved in ancient sedimentary rock. Microfossils found in rocks dating back more than 3.5 billion years provide the only direct evidence of early life. Fossils are not uniformly abundant throughout the Earth’s long rock record; most are found in rocks that were deposited only during the last 600 million years. This more recent period is called the Phanerozoic and the abundant fossil record attests to the great diversity of life forms following the earlier Precambrian period which lasted over the first 4 billion years of Earth’s history. Early stromatolites are layered hemispherical masses of limestones deposited by the descendants of the blue-green algae micro-organisms. The chemical traces of the activities of early life are seen in certain complex carbon substances preserved in rocks and the more spectacular stromatolites. The most famous find was in 1954 and is called the “Gunflint Chert Find”. These stromatolitic rocks are off the northern shores of Lake Superior in Minnesota and Ontario. Precambrian microfossils living in ancient oceans were trapped initially in soft gelatinous silica which, over many years, hardened, preserving the soft organisms in the rock. These organisms have similarities with prokaryotic cells living today in anaerobic environments, such as hydrothermal undersea vents. They are tiny and have to be magnified 1000 or 2000 times to be viewed. While the Gunflint Chert finds are 2 billion years old, even older fossils have been found and it now appears that life stretches back perhaps 4 billion years.