The Carbon Cycle

This one is for those of you who were daydreaming through biology class at the end of the school year (you know who you are).  The carbon cycle is one of several ecological cycles we're currently messing up, along with the phosphorus cycle and nitrogen cycle (more on that one in a later post).  Just in case you've forgotten what the carbon cycle is, here's a diagram summarizing it:


Image from http://www.marietta.edu/~biol/biomes/ecosystems.htm
Confusing, right?  Let's break it down by following one single carbon dioxide molecule as it goes through the cycle on land, starting in the atmosphere.
  1. CO2 diffuses into a leaf via stomata and into a mesophyll cell inside the leaf.  There it diffuses into the chloroplast inside the cell.
  2. Once in the chloroplast, the carbon atom is combined with two others to make sugar via the Calvin cycle.  That sugar is then either used by the plant for energy (via cellular respiration in mitochondria), make other molecules (amino acids, fatty acids, nucleic acids, more complex sugars), or stored.
  3. Herbivores and omnivores eat the plants and then omnivores and carnivores eat the herbivores.  At each stage, sugars, fatty acids, and proteins are burned for energy via cellular respiration, which turns the carbon atoms in the sugars, fatty acids, and proteins back into CO2 via glycolysis and the Krebs cycle.  That CO2 from cellular respiration is what we breath back out into the atmosphere.
  4. Anything that is not eaten eventually dies, where decomposers in the soil then eat the organic molecules and burn them for energy via cellular respiration.
  5. Once back in the atmosphere, the cycle starts all over again.
Note that all of the CO2 we breath out comes originally from plants which took the CO2 out of the air in the first place.  All breathing does is close the loop started by photosynthesis.  This part of the cycle is the rapid carbon cycle and is the reason global carbon dioxide levels decrease during the Northern Hemisphere summer (more land area = more plants to absorb CO2) and increases during the Northern Hemisphere winter.

There are two large sinks by which CO2 is removed from the cycle.  Both operate on geologic time scales across millions of years.  The first is limestone.  Carbon dioxide dissolves into water to form carbonic acid.

CO2 + H2O → H2CO3

This is why increasing CO2 levels in the atmosphere lowers the pH in the oceans.  More CO2 in the atmosphere = more CO2 dissolving into water (Henry's Law for those who remember it) = more carbonic acid.  Carbonic acid then loses a hydrogen to form bicarbonate.

H2CO3 → H+ + HCO3-

Bicarbonate then joins with Ca2+ to form calcium carbonate, also known as limestone.

Ca + 2HCO3- → CaCO3 + CO2 + H2O

Limestone formation was a major sink for carbon dioxide during the Ordovician period, which is characterized by limestone beds interspersed with sandstone and shale.  CO2 levels dropped from nearly 6,000 ppmv down to around 4,000 ppmv during the Ordovician (Royer 2006), mostly due to limestone.  Limestone also forms when other rock types weather.  Most types of rocks–basalts, fieldspars, quartz, etc–are silicate rocks.  During the weathering process, carbon dioxide and rain form carbonic acid, which then combines with silicate:

2CO2 + 3H2O + CaSiO3 → Ca2+ + 2HCO3- + H4SiO4

The calcium and bicarbonate then flow into the groundwater and from there out into the ocean, where they combine to form limestone.  Weathering has played a dominant role in determining the level of carbon dioxide in the atmosphere over the Tertiary Period, with the uplift of the Himalayas and resultant increase in weathering decreasing carbon dioxide levels from ~1,000 ppmv down to ~300 ppmv over that time.

The other major sink?  Fossil fuel formation.  Coal formation buried terrestrial plants deep in the Earth.  Oil formation did the same for marine plants.  That buried the carbon within those plants, keeping it (and the carbon dioxide that would form from it) out of circulation.

Note also that there is only one natural mechanism that counterbalances the limestone sink: volcanic eruptions.  The unnatural mechanism that we humans are engaged in now?  Digging up fossil fuels, burning it, and thereby releasing that long-buried carbon dioxide back into the atmosphere.

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