Driving through the coal country of northeastern Wyoming two weeks ago set me to thinking: where did all that coal come from? The quick answer: it came from swamps, a long, long time ago. We don’t give swamps much credit today, other than as a source of alligators and redneck reality TV, but without ancient swamps the world today would be a much, much different place.
Here’s a quick history lesson.
Most of the world’s carbon that we know today as coal and oil was laid down between 360 and 300 million years ago, between the Devonian and Permian Periods in a time called the Carboniferous Period, which was coined by two geologists, William Conybeare and William Phillips, studying England’s coal deposits way back in 1822. In North America, the Carboniferous is divided into the Mississippian and the (later) Pennsylvanian sub-periods.
It was a geologically active time as the Earth’s two massive continents, the southern Gondwana and the northern Laurasia, merged by the end of the Carboniferous into the mammoth Pangaea supercontinent (I’ve often wondered why the Earth didn’t wobble like a lopsided spinning top with so much landmass accumulated in one place!). Actually, Pangaea began its life as a massive O-shaped continent, which I didn’t know.
At the start of the Carboniferous, huge ice sheets at the southern pole locked up large amounts of water as ice, dropping sea levels significantly, which in turn led to big increases in tropical and swampy conditions across both continents. These conditions fostered the novel evolutionary development of bark-bearing trees, which included the fiber lignin, which when buried in the soil can linger relatively intact for thousands of years. The humid, swampy conditions also encouraged ferns and other seedless plant life to grow to huge proportions.
Shallow, warm oceans repeatedly flooded the continents, covering fallen trees and plants in water that apparently lacked bacteria, which is necessary for biological decomposition. With the flooding came sediments which covered and sealed the plant material. These layers, some of which were over 33 feet thick, eventually formed peat beds, which eventually became coal. The Carboniferous was also a time of active mountain-building as the supercontinent Pangaea came together. It was this process geological uplift and submersion that generated the intense heat and pressure required to transform the peat beds into coal and oil.
All this plant life resulted in the oxygen composition of the atmosphere rising to 35% (compared with 21% today), which is why the Carboniferous is sometimes referred to as the Age of Oxygen. Not surprisingly, the carbon dioxide content of the atmosphere lowered correspondingly, as plants ‘inhaled’ it in copious amounts. In fact, the CO2 level during the Carboniferous became the lowest in Earth’s history. This had two effects: (1) the vast amounts of CO2 that plants pulled from the air remain locked in their stalks after they fell into the swamps, which contributed to the carbon richness of the coal they were eventually transformed into; and (2) the sinking amount of CO2 in the atmosphere changed the planet’s climate – cooling it significantly.
In fact, climatologists credit the Carboniferous with creating the pattern of warming and cooling – Ice Ages separated by warm periods – that continued right up until the end of Holocene (when we messed it up by burning all that coal and oil).
Another important biological milestone was the development of the amniotic egg (such as a chicken egg today). A gradual drying of the climate on continents over the course of the Carboniferous encouraged reptiles to diversify and expand their territory, at the expense of some types of amphibians. This eventually resulted in adaptation of a hard-shelled egg. Scales too. In fact, it wouldn’t long (only another 100 million years) before ‘terrible lizards’ – dinosaurs – became a dominate species on the planet. Insects also grew well in the humid and high-oxygen conditions of the Carboniferous. One of the largest was an ancestor of the dragonfly, which had a wingspan of 60 to 75 centimeters. A giant millipede grew to be more than 1 ½ meters long, and had thirty pairs of legs.
Exciting stuff! With the advent of the Pangaea supercontinent, however, and the geological and biological changes that it inspired, the Carboniferous Period drew to a close.
Except, of course, it hasn’t. Every time you flip on a light or plug in an electronic gizmo, you step right back into the Carboniferous Period. As long as we continue to burn coal and oil, we’ll be revisiting the Age of Oxygen. As William Faulkner once said, the past is never really past, and of all the geological Periods on Earth, his quote best describes the Carboniferous. We can thank swamps for that.
By the way, coal use is on a serious decline in America, which frankly is good news. Here’s a great New York Times story that explains it:
And here’s a picture of a Carboniferous swamp: