BIOL 1108K Study Guide for Exam 3 Chapter 25-33

Terms in this set (166)

Human beings, through their use of technology, have also become important players in the carbon cycle with significant consequences for Earth's surface environments.

Each year, photosynthetic organisms in the oceans fix carbon dioxide into organic carbon molecules, removing an estimated 92 gigatons of carbon from the atmosphere.

Land plants also fix carbon dioxide, removing another 120 gigatons of carbon from the air.

On the other hand, heterotrophic organisms convert organic molecules back to carbon dioxide, largely through respiration.

The amount of carbon dioxide returned to the atmosphere by respiring and fermenting organisms very nearly, but not quite, balances its removal by photosynthetic organisms.

The small difference between the amount of carbon fixed by photosynthesis and the amount returned by heterotrophs to the environment as carbon dioxide...

...can be accounted for, in part, by organic matter that gets buried in sediments after organisms die.

The sedimentary carbon, which includes oil and coal, is eventually returned to the atmosphere as carbon dioxide, in part by their natural oxidation by microorganisms...

...and in part by volcanoes, which recycle buried and subsequently subducted organic carbon back to the atmosphere as carbon dioxide.


Another way that carbon moves through the biosphere involves weathering and carbonate deposition.

Carbon dioxide reacts with water to produce carbonic acid, which reacts with rocks, effectively transferring carbon from atmospheric carbon dioxide to bicarbonate and carbonate ions dissolved in water.

These ions react with calcium ions in the oceans or in lakes to produce calcium carbonate that accumulates on the sea or lake floor as limestone.

Collectively, all of these biological and geological processes help balance the carbon cycle.

The left and right summary columns do not sum to zero, in large part because of uncertainties in our estimates of carbon transfer among different reservoirs.

However, the overall picture changes when we also consider the effect of human activities.

By burning fossil fuels, humans oxidize sedimentary organic carbon on a grand scale, introducing about 8 gigatons of carbon to the atmosphere each year,...

...a number much larger than the amount introduced by volcanoes or other natural processes.

Clear cutting and burning forests also result in the oxidation of organic carbon to carbon dioxide. On the other hand, the restoration of forests increases the transfer of atmospheric carbon dioxide to biomass.

Because human activities do not significantly affect the processes that remove carbon dioxide from the atmosphere, the amount of carbon dioxide in the atmosphere measurably increases over time.
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