What percentage of the Earth's crust is made up of organic material?

Question

I would like to know what percentage of the Earth's crust is made up of organic material (dead plants & animals, aka soil)?

Also, if you happen to know... what is the rate at which organic material on the Earth expands or grows deeper?

Answer 1

Interesting question. There are several types of organic matter in the Earth's crust: soil/sediment organics and fossil organics (coal, oil) at deeper levels. I suppose the total percentage of organic material in the crust should be small, but here's an attempt to put some numbers behind that:

A high average organic content for soil is 5%, and the soil layer is just a couple meters deep. Let's assume that the ocean sediment layer is similar in organic content and somewhat deeper than soil layers. Therefore, let's assume the depth of the soil/sediment layer is averages 10 meters. This yields an average 50 cm depth of organic matter in the soil/sediment layer.

Coal is the primary component of fossil organics. An area rich in coal would have multiple seams, with each seam being tens of meters thick, so let's assume a total depth of 100 meters. Perhaps 1% of Earth's surface contains such rich coal deposits, so spread over the entire Earth's surface, this would yield an average coal depth of 1 meter. *

The depth of the Earth's crust varies from about 5km on the ocean floor to about 40km over land, and there's roughly 3x more ocean area than land, so let's say the global average depth of crust is 15km.

Using the assumptions above, all of which purposefully lean towards the upside, a first order approximation of the organic content of the Earth's crust would be: (.5 m soil + 1.0 m fossil organics) / (15,000m) = 0.0001

So, as an upper limit, perhaps about 0.01% of the Earth's crust would be organic material.

....

Another answer here looks at the question from a geological perspective, and includes limestone as organic because it is derived from organic material, which is true. But from a chemical perspective, limestone (CaCO3) is usually considered to be inorganic, because it doesn't contain a carbon-hydrogen (C-H) bond. And from a biology perspective, even coal would be considered inorganic, because it isn't biologically active. Unfortunately, the distinction between organic and inorganic is, in some cases, a matter of definition....

http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html

.... and so the answer to your question would also depend on which definition of "organic" is most relevant to your problem.

Answer 2

Wiki explains it better than I can - Today's oil formed from the preserved remains of prehistoric zooplankton and algae, which had settled to a sea or lake bottom in large quantities under anoxic conditions (the remains of prehistoric terrestrial plants, on the other hand, tended to form coal). Over geological time the organic matter mixed with mud, and was buried under heavy layers of sediment resulting in high levels of heat and pressure (diagenesis). This process caused the organic matter to change, first into a waxy material known as kerogen, which is found in various oil shales around the world, and then with more heat into liquid and gaseous hydrocarbons via a process known as catagenesis.

Answer 3

Limestone is actually the largest portion of the earth's crust that is made up of organic material. My calculations show it is about 0.3% by volume alone. Other organic material will probably not add another 0.1% by volume.

Soil is a much smaller percentage of organic material on the planet, although that reservoir of carbon is very important in its role in global climate and the short-term global carbon cycle. Soil ranges from 0-99% organic material. Not all of the earth's land is covered by soil. Soil that does have high organic content is often very thin.

There are much larger stores of organic material in the earth's crust. The largest one is limestone. Other organic material that is found in the crust is petroleum and natural gas (about 3 Trillion barrels remain). Many shales contain anywhere from 2-20% organic material. Coal is another significant store of organic material in the crust that is nearly 100% organic material.

Since:
The American Geological Institute defines organic rock as:
"A sedimentary rock consisting primarily of organisms (plant or animal) such as of material that originally formed part of the skeleton or tissues of an animal. Cf: biogenic rock.
from: Bates, R. and J. Jackson, editors, 1984, Dictionary of Geological Terms, page 359


Just the carbon stored in limestone is about 48,000,000 Gigatons.

http://www.carleton.edu/departments/geol/DaveSTELLA/Carbon/long_term_carbon.htm
Using the molar weight of calcium carbonate to convert to the amount of "organic rock" in the crust, the quantity is
400,000,000 Gigatons. A gigaton is ONE BILLION METRIC TONS.

Let's assume an average density for limestone is about 2.71. This is a value typically used when analyzing well logs of deeply buried limestone deposits. That calculation gives us a volume of about 150,000,000 cubic kilometers of limestone, or about 35,400,000 cubic miles.

Now to calculate the volume of the earth's crust, we have the area of the earth of 510,065,600 km2. If we assume an average thickness of crust of 100 km, then the volume of the earth's crust is 510,065,600,000 cubic km. Divide the amount of limestone (which is by far the largest quantity of earth's organic material on the planet) and you get only 0.29 or about 0.3 % of the crust is organic material in the form of limestone.

Since estimates are that we have 3 Trillion barrels of oil left in the earth, that converts to 477 billion cubic meters, or 477 cubic kilometers. That adds another very small percentage.

Estimates of all the coal remaining on earth are estimated at 984 billion tons. That's the proven reserves, and there may be that much more that hasn't been found. There is 1.3 tons of coal in a cubic meter. That gives a volume of coal in earth's crust as 1,500 cubic kilometers, again a very small percentage of the crust.

It is hard to say there is any pattern for the "rate at which organic material ...expands or grows deeper." Organic material such as coal, oil, even natural gas tends to be altered with depth and above certain temperatures changes entirely to inorganic carbon in the form of graphite.

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Edit: my estimate of the thickness of the earth's crust is probably far to high, as the USGS crustal model shows it may be more like 20-30 km on average. That only means my final estimate may approach 0.9% for limestone alone, and the total could be close to 1%.

Answer 4

I cannot give you a definitive number. But I can tell you its very very small.