THE CARBON CYCLE
I. Global Carbon pools
Most Carbon is stored in sedimentary rocks in the crust as limestone (CaCO 3)
or dolomite (CaMg(CO3)2), which are in large part created and deposited by
living organisms (corals, clams, algae). Additional carbon pools in the crust
are present in coal, oil, and natural gas deposits.
Oceans are the second largest pool, where Carbon is stored as Carbonic
Acid, which forms when Carbon Dioxide combines with water (CO2 + H2O =
H2CO3). This molecule is soluble in water, creating a very weak acid, and is
responsible for the fact that rainwater has always been slightly acidic.
The atmosphere currently has the smallest global carbon pool, where carbon
is stored as Carbon Dioxide (CO2), Carbon Monoxide (CO), or Methane
(natural gas; CH4).
The biosphere is also an important pool, where carbon is stored as organic
compounds in organisms (forest trees are particularly important), the soils,
and as peat deposits (very common in cold environments).
Cycling times in these pools varies:
Pool
Cycling time
Biological pool
less than 1000 years
Crust
100 million years
II. Biological uses of Carbon.
Carbon makes up the backbone of ALL organic molecules.
III. The Carbon cycle.
What's more interesting is that carbon exists on earth in organic (sugars,
proteins, fats) and inorganic (carbon dioxide, methane) forms, and that lifeforms are able to move carbon between these forms.
Creation of organic from inorganic carbon:
Is done by photosynthesizing organisms (plants, algae, seaweeds) through
process of photosynthesis. In this, organism absorbs sunlight (roughly 1/2 of
total spectrum which reaches the surface), and converts this light energy to
chemical energy (by creating ATP from ADP). This energy is then used to
make sugar by combining inorganic carbon dioxide with inorganic water:
12H2O + 6CO2 + energy = C6H12O6 + 6O2 + 6H2O
Once captured as a simple organic compound, this carbon can then be used
to create all other organic molecules.
Creation of inorganic carbon from organic carbon:
This process occurs through respiration or combustion, in which organic
molecules are combined with oxygen to create carbon dioxide and water:
C6H12O6 + 6O2 = 6CO2 + 6H20 + energy
When this reaction occurs in our bodies, it is called respiration.
When this reaction occurs abiotically (through burning of organic matter) it is
called combustion.
Long-term carbon storage from the atmosphere and biosphere to the crust
happens in water, where carbonic acid dissolves. If a Calcium ion comes in
contact with this compound, it will adhere to it, creating Calcium Carbonate, or
limestone (CaCO3). As this compound is insoluble, it precipitates out of the
water, moving the carbon into the crustal pool.
IV. Human alterations to the Carbon Cycle
Since the discovery of fire, humans have been able to generate energy for our
use through the burning of organic matter. Until the mid-1700's most of the
material burned was recently alive (wood, peat, grass, cow dung). At this
time, we learned how to harness the energy released in fire through engines
of various types. This discovery powered the industrial revolution, and our
current life style.
However, there is not enough living biomass on the planet to meet these
energy needs. So, we began using carbon fuels (oil, coal, natural gas) which
are stored in the crust. In this way, we have been taking from the crustal pool
and giving to the atmospheric pool.
This can be seen in the yearly trend in global atmospheric carbon dioxide
levels. During each year, atmospheric carbon dioxide levels peak in Northern
Hemisphere winter, and are lowest in Northern Hemisphere
summer. However, in each year, the yearly maxima and minima are higher
than the year before it.
Why is this important? As we found out earlier, carbon dioxide makes the
atmosphere opaque to infrared light energy. As such, more carbon dioxide in
the atmosphere will allow for more heat to be trapped. This will likely cause
the global air temperature to increase.
One main factor which will dictate how bad this will become is the world's
oceans. If the extra carbon can be removed by the oceans through
production of more limestone, then atmospheric carbon dioxide levels may not
stay permanently high. In fact, this may already be occurring, as not all of the
carbon emitted by humans can be accounted for by the measured increase in
atmospheric carbon dioxide levels.