For scientists interested in what the world looked and felt like millions of years ago, coal mines are as good as it gets. While coal may be a major culprit in global warming, there is no place like a coal mine for studying climate change in the past and its likely effects on our own world. Mining companies know this, and for whatever reason, be it good citizenship or simply good public relations, they frequently lend paleontologists a hand.
Consider, for example, Cerrejón, an immense set of open pit coal mines in northern Colombia near the Caribbean coast. The pits are huge, circular, moonscape scars in the earth with shaley slopes that dump runoff water into green crater lakes where no plant dares grow and no bird dares swim. Once in a while, dynamite collapses part of the surrounding wall, and enormous cranes collect the coal while methane fires belch from fissures in the cliffs high above.
But there’s something else. The shale slopes at Cerrejón have preserved the fossil record of an entire tropical ecosystem as it existed 58 million years ago. By looking at the fossils, paleontologists can tell what the ancient climate at Cerrejón was like (hotter and wetter than it is today) and what the foliage was like (very lush and similar to today’s Amazon jungle). The animals were huge. Cerrejón had river turtles with shells the size of kitchen tables that could seat six, and at the top of the food chain was Titanoboa cerrejonensis, a 45-foot, 2,500-pound serpent. Titanoboa was a true river monster—the largest snake ever known to have existed, and about five times the size of the Amazon anaconda, the biggest snake alive today.
What the coal shows is that Cerrejón produced these giant creatures at a time when mean ambient temperatures in the tropics were in the high 80s, about six degrees warmer than today and about as warm as temperatures can be without risking a massive die-off. Yet Titanoboa and the forest where it lived apparently thrived.
And what coal mining gives you is access to such a place. Cerrejón is the only complete ancient tropical ecosystem that is available for study. We can assume that there are plenty of fossils in the tropics, but they are buried in the jungles somewhere in the middle of nowhere. University grants can’t pay for big excavations on spec, but coal companies will, because coal is well worth the investment.
And since paleontologists and coal companies have known for at least a century that the mines are treasure chests of ancient secrets, many companies have staff scientists who look for unusual formations and alert researches when they find something good. When I traveled to Cerrejón a couple of years ago, the multinational company that runs it, Carbones del Cerrejón Ltd. did as much as possible to assist visiting paleontologists, providing everything from hard hats to radio-equipped truck drivers who knew when the dynamite was ready to go off and where to seek shelter when it happened.
Coal deposits are compressed peat swamps that began as dry-land tropical forests. Temperatures grow colder in the early phase of a natural climate cycle, and polar ice sheets expand and sea level drops. When the cycle begins to change, the ice has not yet reached its maximum extent, but rains increase in lower latitudes, changing the dry forest into swamp. The ice warms but does not melt immediately. When it does, it happens quickly and dramatically, causing sharp rises in sea level. Salt water floods the swamp, the forest dies, mud, silt and sand tamp down everything, and coal forms. Then the cycle begins again.
The good stuff—spores, seeds, pollen, cell walls and bones—is not only in the coal itself, but in the layers of sedimentary rock on the floor and the roof of each coal deposit. Think about the layers as a sandwich: The coal is the filling and the sedimentary layers are the bread. The bottom slice shows us what was in the forest when the cycle began; the top slice has everything that was there when the forest died. Each cycle, known as a “cyclothem,” lasts about 100,000 to 400,000 years, depending on where it is.
There is nothing on Earth any better than a coal mine for analyzing ancient ecosystems. The information contained in the coal is “so dense” given the compressed volume of vegetation, notes Illinois State Geological Survey geologist Scott Elrick, “that it goes with without saying that you’re going to have a huge fossil presence.”
Elrick is part of an informal network of scientists who monitor coal mining throughout the United States and the world for opportunities to study new strata as they are exposed. In 2006, Elrick and a colleague discovered four square miles of fossil forest in the ceiling of Peabody Energy’s Vermilion Grove tunnel mine in eastern Illinois, preserved elegantly 306 million years ago when a series of earthquakes allowed the sea to bury the entire forest in a single catastrophic event.
Exploring coal mines can be an urgent undertaking, for the fossils, once revealed, do not linger. Once Peabody was done mining a section of coal seam at Vermilion Grove, the company put wire fencing against the ceiling to keep loose pieces of the shale roof from falling to the tunnel floor. Researchers needed to bring flashlights, take pictures, collect samples, and leave before the formation crumbled away.
The good news is that coal science is a renewable resource. Once the old formation is gone, an entirely new fossil exposure is uncovered. At Cerrejón, tropical rains every spring and summer cut huge erosion channels in the shale and mudstone, sending fossils and bones to a watery grave in the crater far below. But in November, researchers can find a whole new set of skeletal remains drying in the sun.
The fossil plants and animals tell scientists a lot about what the prehistoric world was like during each glacial cycle. But researchers are taking the analysis a step further. Global warming and cooling occurs in part as a result of periodic changes in the Earth’s orientation during orbit. But the warming trends are accompanied by dramatic rises in atmospheric carbon dioxide. The key question—for our time—is whether CO2 is the driver.
Coal is “organic climate,” said paleoclimatologist Isabel Montanez of the University of California-Davis. “It tells us when it was wet and when it was dry.” And coal can be a tool in reconstructing atmospheric CO2 at particular geological moments. “It’s an environmental dipstick,” Montanez said, “that allows me to tie sea levels to the CO2 in the atmosphere to continental climate.”
Montanez stays in touch with Elrick and other network members to find where to explore next. For the past two years she has been analyzing fossil plant stomata—the pores through which plants absorb the CO2 they need for photosynthesis. Her samples come from coal mines in many parts of the world; much of the recent work has been done in the Donets Basin, in the Ukraine. The “stuff on the bottom” of a coal seam is the tail end of a glacial period, she said. Carbon dioxide is low and the plants have “lots more stomata” to get what they need. “Above the coal, there are fewer and fewer stomata,” she added.
That’s when CO2 is at its highest levels—and when when the floods come. Then everything in the swamps dies or gets washed out to sea where plankton suck up all the loose carbon, and atmospheric CO2 plunges once again.
So what does this mean for us today? Quite a lot, it turns out. The Earth is in an “interglacial” period right now. Carbon dioxide is rising and the planet is warming. Ice melts, the polar permafrost and tundra thaw out, microbes on land have their own feast of dead vegetation and begin to emit methane—a powerful greenhouse gas—in large enough quantities to increase warming.
The coal tells us that, by itself, there is nothing unusual about climate change, “and when we tell people about it, they think that because it’s a natural process, they don’t have to worry about CO2 emissions,” Montanez said. “But what we’re doing is not natural.” The natural cycle takes several hundred thousand years. Plants and animals have time to evolve. With the atmospheric changes occurring these days, however, this cycle could happen in centuries. Many plants and critters will not have enough time to adapt.
The coal also tells us that as atmospheric CO2 rises, the warming effect accelerates. Add the auto emissions and the exhaust from coal-fired power plants for an extra jolt of CO2, and it’s like spiking the punch. “It will make a huge difference, there’s no doubt about that,” Montanez said. “The question is when.”