Did you know that fossil fuels – coal, oil, natural gas: the much-reviled hydrocarbons of today – are remains of long-dead organisms, essentially graveyards from hundreds of million years ago that were absorbed into the Earth’s bedrock? That coal is extracted from what’s left of ancient tropical forests and oil and natural gas from the remains of ancient marine microorganisms?
In these climate-challenged times, these should be well-known facts. But I wasn’t aware of them for many years. I’d thought of fossil fuels the same way I thought of minerals: as resources to be extracted from the earth, more abundant in some parts of the world than others. The qualifier fossil, which so unequivocally points to ancient life and distinguishes the fuels from the minerals, had completely escaped my attention. What kinds of fossils had left hydrocarbon molecules in such large quantities that are now fueling our lives and warming the world? How and when were they formed? I’d never bothered to consider these questions. Just goes on to show how shallow our understanding of everyday concepts can be! (Ask me about the phases of the moon or how a refrigerator works and I might struggle in the same way.)
The upside is that ignorance can turn into a source of wonder. I see now that every time we power our cars with gasoline, or consume electricity that comes from a coal or natural gas plant, or use the myriad products of petroleum (from asphalt to plastic bags to synthetic fibers), we are linked (however indirectly) to lifeforms from hundreds of millions of years ago. Fossil fuels should never have been burned in such large quantities, but they are a fascinating illustration of how the Earth’s deep history – stuff that we think belongs only to natural history museums and geology textbooks – is tangibly a part of our daily lives.
The Plants That Became Coal
Modern coal deposits date back to the tropical forests of the Carboniferous (literally coal-bearing) geologic period, which began about 323 million years ago. According to Wikipedia, coal is “formed when dead plant matter decays into peat which is converted into coal by the heat and pressure of deep burial over millions of years.”
Rocks from that period reveal the haunting imprints of plants that grew in these tropical forests. Take this fossil exhibit from one of my favorite spots in Amherst — the Beneski Museum of Natural History. It shows the 310-million-year-old trace of an ancient fern in a slice of Ohio rock. While the ferns I see every day in Massachusetts are short and carpet forest floors, back in the Carboniferous, they were tree ferns that grew nearly 65 feet tall (there are still tree ferns today in the tropics: the first image below shows the silhouette of one I recently saw in Costa Rica).
The environmental journalist Janet Marinelli describes other classes of plants that dominated the Carboniferous in this article . In addition to ferns, she highlights lycopods, ancestors of today’s club mosses. I have learned to recognize club mosses (second image below) on my daily walks. They are only a few inches tall — I incorrectly thought of them as ‘baby conifers’ or ‘hemlock nurseries’ — but a few hundred million years ago, they grew to an astonishing 130 feet. Marinelli also mentions calamites, ancestors of modern horsetails that I often spot along trails (third image below). They are short, but in the swamp forests of the Carboniferous, they rose to 50 feet.
Ancient ferns, club mosses, horsetails, and the earliest ancestors of the conifers: these, then, were the kinds of plants that got buried under dirt and rock for hundreds of millions of years. Heat and pressure slowly converted them to coal.
Here are some artistic renditions of what a tropical coal forest might have looked like. The visual feel of such a forest and the species of animals that inhabited it would have been quite different. Flowering plants and trees that dominate landscapes around the world today had not evolved at the time – they evolved much more “recently”, in the last 100 million years. This is why you won’t find oak, banyan, acacia, or any wildflower fossils in remnants of coal forests. The earth was also much warmer and more humid during the Carboniferous and the continents were differently aligned – North America, South America, and Africa were lashed together into a single supercontinent called Pangea. Places such as Pennsylvania and Virginia, where modern coal deposits are found, were at that time closer to the equator.
(If all this talk of geologic periods and tectonic activity is too much, I totally understand! There was a time when my eyes would glaze over too — the esoteric names and jargon-filled descriptions seemed so inaccessible. It wasn’t until I began to connect them with what we now take for granted that the Earth’s deep history started to resonate powerfully. Take something as vital as oxygen — the planet didn’t have much oxygen at all until the photosynthetic activity of cyanobacteria, starting two billion years ago, made it abundant on Earth, in the process creating the ozone layer that now protects us from harmful radiation. Or consider how plants and fungi established a symbiotic partnership on land 400 million years ago, turning barren continental rocks into the fauna-filled forests, savannas, and agricultural landscapes of today. Or consider how a large asteroid crashed into the Earth 66 million years ago, ending the reign of the dinosaurs, but not all of them – for we still have some of their descendants, the 9000-odd species of birds on Earth today.)
The Microorganisms that Became Oil and Natural Gas
The story of oil and natural gas is more complicated – at least for someone like me with only a faint idea of geology and microbiology. The current theory is that oil comes from the remains of tiny organisms that once thrived in warm, shallow seas in the Cretaceous and Jurassic geologic periods. This is roughly 200 to 66 million years ago, during the age of the dinosaurs. The sources of oil and natural gas are therefore younger than the coal forests of the Carboniferous.
I recently learned that plankton is the name given to a diverse class of marine micro-organisms that turned into oil and natural gas. A common feature of plankton is that they drift in the water and are unable to propel themselves against currents of water or wind. They include both microscopic plants (phytoplankton), which serve the same indispensable purpose as land plants, fixing the sun’s energy and thereby enabling other lifeforms; and microscopic animals (zooplankton) which feed on phytoplankton. The zooplankton in turn are a major source of food for larger organisms – whales, for example, feed on a type of zooplankton called krill.
Unlike land plants, which I can see and appreciate easily, these microscopic lifeforms are harder to relate to. (I can’t remember the last time I peered into a microscope – was it back in college or high school?) A 2010 New York Times article by William Broad discusses a type of single-celled phytoplankton called diatoms and claims they are “the source of the vast majority of the world’s oil”. There’s a lovely image in that article (below) that shows the varied geometry of diatoms – elongated, triangular, circular, star-shaped – and their iridescent colors.
If the marine micro-organisms theory is correct, then it implies that any oil-rich terrestrial region – North Dakota, Texas, the Middle East – must have been underwater once. (Such is the nature of continental movements that even North Dakota which seems so far inland and landlocked now was once covered by a shallow sea!) Just as important, the seas that covered the region must have had plenty of sunlight and nutrients that enabled an abundance of marine micro-organisms. When the shallow seas withdrew to expose land, the sediments – accumulated remains of dead micro-organisms collected over millions of years – became a source of oil and natural gas. The graphic above (from here) conveys it quite well.
The Middle East, which used to be under the Tethys Ocean, is an excellent example. The Tethys no longer exists but it was a predecessor to today’s Mediterranean Sea, the Indian Ocean, and the Black and Caspian Seas. When the continents slowly changed their alignments, the Tethys receded, exposing the sediment-rich Middle East. Thus, a fortuitous combination of tectonic activity and ancient marine life made the Middle East rich in oil and geopolitically important in the 20th century. Another example, I suppose, of the Earth’s deep history manifesting in the modern world!
§
In closing, I should mention two books I leafed my way through while researching this article. They provide further context on the topics I’ve discussed here. The first is Echoes of Life: What Fossil Molecules Reveal about Earth History and the second is Vanished Ocean: How Tethys Reshaped the World. I also highly recommend the Deep Time exhibit at the Smithsonian National Museum of Natural History — it provides an engaging and awe-inspiring overview of Earth’s four-billion-year-old history.