Innovations and Biogeochemical Revolutions - The Geological Carbon Cycle

Main Points:

  1. The Geological Carbon Cycle
  2. The Vediobionts
  3. The Agronomic Revolution

I. The Geological Carbon Cycle

Let me pose two sets of observations for you:

First, astrophysicists have long held that our sun should have an evolution though time similar to those inferred from the study of other stars of similar size in the universe.

Our sun should have begun rather small and dim and grown in diameter through time. The amount of sunlight reaching the Earth should thus have increased by from 15% to 30% since the earth formed some 4.5 billion years ago.

If nothing else was different than today, this would mean the surface of the earth world have changed in temperature tremendously, and no liquid water could have been present on the Earth prior to 2 billion years ago.

However, we see instead by looking at the geological record, that there has been liquid water on the earth since it its crust solidified, and in general the Earth's surface seems to have remained within a surprisingly narrow range.

Obviously something has changed and changed in a way to make the Earth continuously habitable. How could this happen.

Second, CO2 as measured in our atmosphere shows a remarkable seasonal cycle.

This is the so called Keeling Curve from the Mauna Loa Observatory shows this spectacularly as well as dramatic upward trend in CO2 caused by fossil fuel burning.

Where does this cycle come from?

It comes from the seasonal cycle of photosynthesis and the asymmetry in land mass area between the northern and southern hemispheres.

Obviously plants exert a great effect on CO2 levels.

So what maintains the CO2 levels over long periods of time?

First, lets look at some numbers.

Reservoirs Sub-Reservoir Amount (10^15 g C)
Atmosphere   720
  Land 827
  Oceans 2
Oceans (dissolved)   38,000
  Organic Matter 15,000,000
  Carbonate Rocks 20,000,000

These numbers tell us a lot about the nature of the system.

Two kinds of biogeochemical cycles maintain the Earth's atmospheric levels of CO2: fast and slow.

  1. The fast cycle operates on time scales of hundreds to thousands of years.
  2. The second operates on hundred of thousands to millions of years.

Both are essential.

First, the fast cycle - this is the one familiar to most geochemists.

The critical chemical reactions are:

Photosynthesis and Respiration:

CO2 + H2O + e = CH2O + O2


CO2 + H2O = H2CO3 = H+ + HCO3-

Calcium Carbonate dissolution and precipitation:

Ca2+ + 2HCO3-= CaCO3 + H2O + CO2

Carbonate equilibrium in seawater:

H2CO3 = H+ + CHO3- = H+ + CO32-

Photosynthesis and respiration are the clear controllers of the seasonal cycle of CO2.

Note also that any carbon not immediately respired results in the accumulation of O2 in the atmosphere. We have O2 in the atmosphere because of the C buried as organic matter in sediments and rocks.

A negative feed back loop keeps O2 levels from getting to high:

If O2 levels get to high, land biomass will burn and photosynthesis will go down,
and O2 will go down.
Also the more carbon is buried, the more nutrients are buried, putting another break on the system.

CO2 in the atmosphere is in equilibrium with the ocean. the ocean has a vast amount of carbon in it in the form of carbonate (CO32-), and bicarbonate (HCO3-).

The equations above result in a chemical equilibrium between the oceans and atmosphere such that if perturbed by, say adding more CO2 to the atmosphere, ocean chemistry responds by shifting to absorb most of the CO2 arriving at a new equilibrium. This response to a perturbation of equalibrium is called LeChatlier's Principle.

Over hundreds to thousands of years, adding more CO2 to the atmosphere is just sucked up by the ocean, lowering the pH and thus producing more bicarbonate to neutralize it from carbonate thus driving the equilibrium equation back towards the acid side. Lowering atmospheric CO2 has the opposite effect, and results in the precipitation of CaCO3. This effect was spectacularly observed in the water pool in the lung of the Biosphere 2 Center in Fall, 1995.
Because the ratio of ocean C to atmospheric C is about 50 to 1, doubling or tripling atmospheric CO2 does little to the oceans or the net atmospheric CO2 on the long run. The only reason we are having an effect on the atmosphere is because the RATE of the input exceed that of the removal by the oceans! Over thousands of year our contribution to the atmosphere via fossil fuel burning would be nil.
Note also that if we look just at the fast cycle, the precipitation of CaCO3 is a source of CO2!

OK, but why settle on say 250 ppm instead of other amounts. Well, this must be a function of the amount of carbonate in the oceans.

That is controlled by the long term cycle of carbon.

Here the critical relationships are termed the UREY reactions, which are:

Calcium and magnesium silicate weathering and metamorphism:

CaSiO3 + CO2 = CaCO3 + SiO2
MgSiO3 + CO2 = MgCO3 + SiO2

With some intermediates this is for Ca silicates:

CaSiO3 + 3H2O + 2CO2 = Ca2+ + 2HCO3-+ H++ Si(OH)4 = CaCO3 + SiO2 + H2O + CO2

Note that here, it takes 2 CO2 molecules to weather one CaSiO3 molecules and when CaCO3 is precipitated one molecule of CO2 is released.

So there is a net loss of one molecule of CO2 for every molecule of CaSiO3 weathered and the precipitation of carbonates is a net sink for CO2 not a source!

Thus, the burial of organic carbon and carbonate carbon are the controllers of O2 in the atmosphere and the carbonate pool in the oceans, respectively. The latter controls the CO2 in the atmosphere.

Because of plate tectonics nearly all of this buried carbon is returned via subduction and metamorphism over about 200 million years.

In total about 0.2 x 1015 g of C is buried each year and just about that is returned by outgassing.

We can look at all of this in a plate tectonic context.

In the above diagram, Corg is organic carbon, primarily the breakdown products of carbohydrates produced by photosynthesis.


So, its chemical weathering that controls the flow of Ca2+ and HCO3- to the oceans where it ends up being buried as carbonate.

But as the joke goes, what then controls the rate of chemical weathering? Another set of feedbacks operate here. The most obvious one is temperature.

If CO2 in the atmosphere goes up, temperature goes up,
But if temperature goes up, chemical reaction rates go up
If chemical reaction rates goes up, chemical weathering of Ca and Mg minerals goes up
CaCO3 precipitation goes up
and so the CO2 goes down.
and the temperature goes down
If CO2 goes down,
temperature goes down,
weathering rates go down,
CaCO3 precipitation goes down
and CO2 accumulates in the atmosphere
and the temperature goes up.

This negative feedback loop looks like it might regulate CO2 just like a thermostat.

But there are other feedbacks involved:

Plants add CO2 to soils via respiration and their ultimate decay and respiration by bacteria.
They also have organic acids of their own which results also in added chemical weathering.
They also hold the water in the soil longer
Thus plants are said to fertilize chemical weathering
But plants use CO2 as their source of carbon, so more CO2 makes plants grow faster,
which makes weathering go faster too.
This is an another negative feedback
But, faster plant growth is limited by nutrient availability,
but that is a positive function of weathering.
which could compensate for the grater rate of plant growth
This is a positive feedback for the plants.
But a positive feed back on temperature is that: higher CO2 leads to warmer temperatures,
which leads to more evaporation,
which puts more of the greenhouse gas, water vapor in the atmosphere,
which makes it warmer.

So the picture is complicated. Then how do we reconcile the Faint Young Sun Paradox?

Well you would need about 1000 x present CO2 levels to compensate. Or some other greenhouse gasses, such as methane (CH4). This is possible and has been proposed as the solution. Methane may have especially important in the early atmosphere because of the lack of lots of O2.

The most important lesson of all this, is that, the composition of the Earth's atmosphere is constantly maintained by life.

II - Bacterial Communities

While macroscopic organisms (i.e. multicellular Eukaryota) play critical parts in regulating some of the rates of carbon and other material cycling, as we shall see, most of the actual physiological work is done by bacteria.

By at least 2 billion years ago, most of the basic physiological processes outlined on the previous lecture had evolved.

A characteristic of precambrian sediments is that they were often colonized by microbial mats. Today obvious microbial mats are seen where grazing and burrowing by animals is restricted by the hostility of the environment. This allow the mats to develop much as they did in the Precambrian.

An isolated lagoon of hypersaline water (salina) in sand dunes in the city of Puerto Penasco, Sonora, Mexico, at the north end of the Gulf of California, gives us a nice example of a microbial mat communities, as well as some extremeomophile bacteria.

This lagoon is so saline that crystals of gypsum (calcium sulfate - CaSO4) and halite (salt - NaCl) are forming in the water. Just below the transparent crystals is a slimy multicolored mat. This mat is gelatinous in texture and cuts, quite easily. Here, I have cut out cubes of the mat so that we can see some details of the mat community.

The uppermost layer (1) is a layer of cycanobacteria, which has carotene pigments to protect it against ultraviolet light. Below that (2) is a layer of purple photosynthetic sulfur-oxidizing bacteria. Below that are green sulfur-oxidizing bacteria, methane-oxidizing bacteria, and a transition into sulfur-reducing bacteria (3). Sulfur-reducing bacteria dominate layer (4) and persist probably about a meter down into the sediments, where they give way to methane-producing Archaea.

So, photosynthesis dominates the uppermost layers (1, 2) along with a mix of heterotrophic sulfur oxidizers (2) using H2S produced by the sulfur-reducing (chemoautotrophic) bacteria from layers (3) and (4). The sulfur reducing bacteria are using sulfate (SO42-) from seawater, diffusing down from above, as an electron acceptor and releasing H2S as waste. Virtually all of the iron in the sediment (which is soluble because this is a highly reducing environment) combines with some of the H2S to produce various iron sulfides, which give the sediment a black color. Eventually these iron sulfides would convert of iron pyrite. The rest of the H2S gets used up by the sulfur oxidizers (2) as an electron donor, except for some which gets into the atmosphere giving the locality a wonderful "rotten egg" smell that a lot of the inhabitants assume is pollution. Deeper in the sediment (below 4) the sulfate is all used up by the sulfur-reducing bacteria, and CO2 produced by anaerobic heterotrophy of organic matter gets used as an electron accepter by methanogenic bacteria (Archaea) with the production of methane (CH4) most of which is consume by methane-oxidizing bacteria in (3), but some leaks out into the atmosphere where it acts as a greenhouse gas.

This microbial community is much more complicated than I have described here and almost certain has hundreds of different microbial species in it, with many intertwined biogeochemical loops.

A critical point is that the community is self-organizing and of great complexity, yet is represents one of the oldest types of communities on Earth.

Other examples of complex microbial communities include soils, the water column in lakes and oceans, and our guts.

III. Vediobionts

Eukaryotic cells appeared about 1.9 billion years ago (middle Proterozoic), but for much of the time since then until about 670 million years ago, we see little evidence of macroscopic animals. However, at about 670 million years ago we see clear evidence of complex multicellular organisms that look, at least like animals.

(from Miller Museum Online Exhibit)

EdiacariaThe most common forms occur in what is called the Ediacaran assemblage, first identified in Australia, but present world-wide.

Ediacarans are rather large forms that all seem to be sac or quilted in structure. Although superficially similar to several animal phyla, it is unclear if any belong to extant groups. Adolph Sielacher places them all in their own phylum, he calls the Vendizoa.

These creatures are found in very shallow water environments and it looks like they depended on a large surface area, perhaps to get as much O2 as possible.

The appearance of large size suggests developmental processes and origin the homeobox system - key to metazoan life.

Allows for ordered developmental sequence and body part homology. This allows far greater levels of complexity and the development of hierarchical levels of organization of sub-units into larger units.

At the same time we see bioturbation which implies the origin of a coelom. A sac separating an outer body wall from organs inside an animal.

The basic structure of a coelom.

This allows larger organisms that can have a hydrostatic skeleton and the ability to bend an twist and push even thought hey are large. Major modification of the sediments result. And oxidative processes can now go on at depth. Hence the efficiency of use of carbon fixed by photosynthesis increases.

IV. The Agronomic Revolution

The Ediacaran assemblage colonized the bacterial mats - the dominant ecosystem type for over 2 billion years. Very few of the Ediacarian organisms could burrow into the mud. This all changed about 540 million years ago at the beginning of the Cambrian period. Apparently, all major phyla of animals appear at this time and many enter the mud for protection and for food. This not only aerated the mud, but also wiped out the mat communities, except in places like salinas where they still survive because of the exclusion of nearly all macroscopic animals. This so called "agronomic revolution" (Sielacher, 1997), changed marine ecosystems fundamentally.


(from Seilacher, 1998)

An excellent example of radically the agronomic revolution changed the world is visible in another lagoon near Puerto Penasco, Mexico. This one also sits in sand dunes but has an open channel to the Gulf of California. This lagoon, the Estero Morua, is how the salina we described previously must have looked like before it became landlocked.

Estero Morua

The Estero Morua, Sonora, Mexico.

The same kind of sandy substrate that hosted the microbial mats in the salina, here have no mats at all. There is no smell of H2S. Instead, the sands are home to millions of deep burrowing arthropods.Shrimp dance

At left is an example part of the sand flat on which students from the 1997 class of Biosphere 2's Summer course on "Field Methods in Earth System Science" are performing the "shrimp dance". The students rock their legs from side to side and sink into the sand, usually nearly up to their hips. Then they extract themselves, or attempt to, before the tide comes up.Shrimp in hand

Hundreds of small "shrimp" come to the surface. These belong to the decapod crustacean genus Callianassa which live in elaborate and deep burrows.

Below is a Callianassa (Callianassa truncata) from Italy.


The species from this lagoon is Callianassa californiensis, which looks virtually the same. Only males have claws so unequal in size. The tiny Callianassa makes burrows that can extend for more that a meter below the surface.callianassa burrows

At right is an example of a burrow system filled with epoxy and then excavated. This example is also from Italy, but the estero examples are very similar. By constructing and living in these burrows and eating the organic matter in the sediment the ghost shrimp massively ventilate the sediments, which would otherwise lack oxygen. The same process eliminates any possibility of the formation of microbial mats. Now the bacteria must live as coatings on grains and in the fluid between grains.

The Estero Morua is home to several oyster farms. In previous years the students enjoyed several dozen oysters fresh from the estero, some trying them for the very first time. They were DELICIOUS! But, remember, oysters are detritus feeders and since they are what they eat, they are always an assault on your immune systems. Thus, there is always a slight risk of, well, a reaction as a new bacterial community tries to set itself up in your gut.

Eating oysters at estero morua

VII. The Cambrian Explosion

At 540 million years ago there is a massive increase in the kinds of multicellular animal present, especially those with coeloms.

This is the Cambrian explosion. Famous example if the Burgess shale. It looks like all major animal phyla, perhaps all of them, were around by the middle Cambrian. By the end of the Cambrian the Phanerozoic pattern of marine organisms was pretty much established.

We will look at this in the next lecture.

Updated March 3, 2005
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