Geology Of Georgia

An Afterthought on Notes on Part One of an Occasional Series:
The Appalachian Plateau

The story of carbon is the story of the planet. I realized that I had been discussing the Carboniferous rocks of northwest Georgia without ever mentioning the carbon.

The Earth was very hot four billion years ago and the atmosphere was unbreathable, all methane, carbon dioxide, and hydrogen sulfide. Until 2 billion years ago, free oxygen was rare as an atmospheric gas. Pure iron exposed to the air back then would not have rusted. The oxygen we breathe in the air today accumulated in the atmosphere as a waste gas, a byproduct of carbon dioxide/water based photosynthesis. There is little doubt that organisms produced oxygen before 2 billion years ago, but this oxygen was unable to accumulate as a gas because the iron dissolved in seawater combined with the oxygen to form rust (iron oxide). The chemically inactive iron oxide sank and accumulated on the sea floor, forming the vast Precambrian iron formations; much of the iron mined today is derived from these formations. Only until all the iron was removed from the sea water did the oxygen begin to accumulate in the atmosphere, up to its present concentration of about 21 percent.

With oxygen available in the atmosphere, life began to flourish. But life was very different during the Carboniferous Period. Primitive sharks flourished in the pre-Tethys ocean that surrounded the northern and southern, pre-Pangaea land masses. The climate was probably something like that of today - a south polar ice cap was beginning to form, and there were tropical forests of club mosses around the equator. Huge dragonflies with 30-inch wingspans swarmed on land, and pelycosaurs, early, mammal-like reptiles, flourished during the Carboniferous. The best-known pelycosaur is Dimetrodon, the one with the great big sail on its back. Nobody knows what Dimetrodon did with its sail. It may have been a solar panel to help the animal warm up to a temperature where it could use its muscles, and/or perhaps it was a radiator to cool down in the shade, when things got too hot. Or it could have been a sexual advertisement, a bony equivalent of a peacock's fan.

Most significantly, though, the Carboniferous was the time of vast tropical swamps of giant club moss trees. Nature hadn't yet learned to break down cellulose. When a giant club moss tree fell, it lay on the ground and got buried by the next tree that fell. The trees wouldn't decay, because the bacteria which now break down cellulose hadn't yet evolved (there was no need to, because there hadn't been a supply of cellulose to feed the bacteria up until this point). In the course of millions of years of trees falling on trees during the Carboniferous, almost all the carbon got taken from the atmosphere, leaving carbon dioxide as only 0.03% of the modern atmosphere. The carbon was buried underground in the form of the great Carboniferous coalfields, where it stayed until yesterday, geologically speaking.

Today, a log that falls is digested by funguses and microbes, and all the carbon goes back into the atmosphere. There can never be another Carboniferous because you can't ask nature to unlearn how to biodegrade cellulose.

Mammals came along when the world cooled off, and now he have a very clever mammal that's taking all the carbon stored as underground coal and putting it back up into the atmosphere. Mammals are taking the iron buried as the Precambrian iron oxide deposits and manufacturing automobiles, which put more carbon back up into the atmosphere. Once we burn up all the coal and natural gas, we'll have an antique atmosphere - a hot, nasty atmosphere that no one's seen for three hundred million years.

According to Jonathan Franzen (The Corrections), the moral of this story is don't recycle plastic - send it to a landfill. Get that carbon back underground. Bury it. Put the genie back in the lithic bottle.