by Lee Billings
Mike Arthur was a sedimentary geologist. Viewing walls of rock with alternating bands of limestone, sandstone, shale, and coal was for him like reading stories, ones written in stone. He was also a geochemist. With the help of a hammer, a sample bag, and a bit of laboratory wizardry, he could discern the subtle chemical signals in rock layers that revealed ancient, long-vanished environments—the flora and fauna, the weather and geography, and how each former world developed, flourished, and finally passed away, largely forgotten but for those lithic memories.
Paleoclimates and past global climate changes were his specialty, as seen through his research emphasis, the formation of black shales. Black shales are compactions of clay, mud, and silt formed in deep water and made the color of jet by their heavy loads of organic carbon. Organic carbon—the stuff from which plants and animals are made—is normally quickly eaten and recycled in a water column. But when organic detritus drifts to the stagnant bottom of a deep body of water, the absence of sunlight and oxygen can stave off the creatures that would otherwise churn through and consume the remains. Undisturbed, layers of carbon-laden silt and mud accumulate, compress, and sink deeper beneath the Earth’s surface, where a slow geothermal simmer cooks them into black shale. Given sufficient heat, pressure, and time, a fraction of the carbon in organic-rich black shale transforms into petroleum, and further cooking will crack the oil into methane and a handful of other volatile organic compounds collectively and colloquially known as natural gas. To Arthur, instances of worldwide black shale deposition were signposts of past pulses of global warming: as temperatures climbed and sea levels rose, the deepening, tepid oceans would have lost much of their ability to mix oxygen-rich surface water to the bottom. Anoxia would set in, and rich deep-sea ecosystems would dissolve into sulfurous, bacteria-infused black mud.
Arthur’s research into black shales initially took him around the country and the world, but by the early 1990s he had decided to settle in Pennsylvania. There, he realized, a good bit of the crucial evidence he needed to study black shales—and the Earth’s fluctuating climate over the past 500 million years or so—could essentially be found right in his backyard, in the Allegheny Plateau. The Allegheny Plateau boasts some of the world’s largest black shale deposits. In their finest details, the shales and their surrounding rocks told of the comings and goings of mountain ranges, glaciers, and vast inland seas in Pennsylvania’s deep past.
Pennsylvania’s rocks are also intimately linked to our planet’s climatic present and its future. The inexorably rising temperatures—temperatures that were sending glaciers and polar ice into retreat, strengthening storms, shifting animal migration patterns, and making Arthur reconsider his greenhouse seedlings—in a way had come from the very ground beneath his feet. The additional warmth came chiefly from rising levels of atmospheric carbon dioxide, CO2, a gas prodigiously produced by the combustion of fossil fuels. Carbon dioxide is transparent to visible light but absorbs a good fraction of infrared light—that is, light we perceive as radiant thermal heat. Sunlight readily passes through the gas on its way to shine on Earth’s surface, but when the warmed surface re-radiates that light skyward in the infrared, it becomes trapped by the absorptive blanket of CO2. This is the basis of the well-known “greenhouse effect,” and CO2’s greenhouse effect is believed to be the primary architect of Earth’s climate now and for the last half billion years. Humans had been gradually raising the atmospheric levels of CO2 and other greenhouse gases for thousands of years, mostly through agriculture, but the rate of increase had greatly accelerated in the industrialized boom times of the past century. Much of that sudden surge had its roots in the rocks of the Allegheny Plateau, which runs through Pennsylvania and into portions of surrounding states.
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The largest known anthracite coal deposit on Earth was discovered in northeastern Pennsylvania in the latter half of the eighteenth century, supposedly when a hunter building a campfire accidentally set a nearby outcropping of crystalline black rock ablaze. By the mid-1800s, Pennsylvania anthracite had supplanted wood as the preferred method for heating homes in the United States, and coal mining had become a major industry throughout the Allegheny. At about the same time, Pennsylvania gave birth to the global petroleum industry, when drillers of salt wells found their work hampered by thick, viscous upwellings of black “rock oil.” The first petroleum refinery was built in Pittsburgh in 1853, and the first oil well in the United States was drilled near Titusville, Pennsylvania, in 1859. Petroleum found its killer app in Henry Ford’s Model T, which first rolled off a Michigan assembly line in 1908. The U.S. natural gas industry was actually birthed just north of the Pennsylvania state line, with a well drilled in Fredonia, New York, but the black shale deposit from which it came proved to have its bulk in Pennsylvania territory.
Riding on the surge of ancient carbon, Pennsylvania’s economy boomed. Oil wells and mine shafts soon suffused the Allegheny rock, and refineries, pipelines, and railroads sprouted like weeds across the state. Like most booms, this one was short-lived. Output from the state’s oil fields had already begun to decline by the dawn of the twentieth century, and was progressively overshadowed by immense newly discovered fields in Texas, Venezuela, Saudi Arabia, the Gulf of Mexico, and elsewhere. By the 1950s, Pennsylvania’s Allegheny rocks still contained abundant coal and gas, but in a world increasingly addicted to oil, market forces dictated that those less-profitable fuels simply be left in the ground.
Pennsylvania’s energy fortunes sharply rebounded in the first decade of the new millennium. As oil production from conventional, easily accessible reservoirs peaked, energy companies devised new methods to wring more oil and gas from harder-to-reach, “unconventional” source rocks. The most successful new method was hydraulic fracturing, or fracking, which squeezed previously inaccessible natural gas from deeply buried shales. When a gas-bearing shale lies beneath miles of rock, as it does throughout the Allegheny, the resulting pressure can lock gas within the formation. Pumping millions of gallons of high-pressure, chemical-laced water down a borehole, however, splinters the shale rock, and granules of sand or ceramic added to the slurry prop open the fractures. The locked-in gas, now liberated, streams through the cracks and back up the borehole, to be collected, compressed, and sold.
Fracking, combined with technology for drilling wells not only down but also laterally across layers of rock, offered a way to tap the biggest black shale formation in the Allegheny: the Marcellus. It was named for a small town in upstate New York where it jutted from the ground in sheer, flaky cliffs of carbon, and its expanse stretched westward from New York’s Finger Lakes to the eastern half of Ohio, and south down to Maryland and West Virginia. But the Marcellus’s concentrated carbonic heart could be found a mile or more beneath most of Pennsylvania, conveniently abutting major, energy-hungry metropolitan areas across the northeast United States. Comparing production rates of Marcellus fracking operations with the deposit’s extent, thickness, depth of burial, and shale porosity, one of Mike Arthur’s Penn State colleagues, the geologist Terry Engelder, estimated the formation might hold nearly 500 trillion cubic feet of recoverable gas. That would be enough to make the Marcellus the second-largest known gas field on Earth, enough to supply the entirety of the United State’s energy needs for two decades.
As word spread of Engelder’s Marcellus evaluations, energy companies great and small swooped in, buying up leases by the truckload in rural communities. A new boom began. Some farmers with huge tracts of land overlying productive parts of the Marcellus became millionaires overnight. Restaurants, motels, and other businesses sprang up to meet the needs of an inrushing flood of new workers. But the boom had a dark side, too. Long stretches of backwoods roadway buckled beneath roughshod convoys of heavy trucks, and sylvan forest glades disappeared beneath parking-lot-size concrete drill pads and miles of snaking pipeline. Natural gas linked to nearby fracking operations found its way into well water, and concerns grew about the possibility of
fracking’s proprietary chemical cocktails contaminating regional lakes, rivers, and aquifers. Public opposition soared, particularly in major cities served by the vulnerable watersheds. Penn State, keenly aware of its long and lucrative association with the oil and gas industry, attempted to walk the line between opposition and support. It formed the Marcellus Center for Outreach and Research in 2010 to engage with and educate all the region’s stakeholders on the pros and cons of further developing the shale. The university chose Mike Arthur to serve as codirector of its new center.
On that unseasonably warm October day in 2011, a few hours after he had arrived from his farm, Arthur and I sat in his fifth-floor office talking about the Marcellus. He pulled up an animated time-lapse map on his desktop computer to show me the year-by-year progression of Marcellus shale drilling in Pennsylvania. The deposit’s expanse was indicated by the color yellow, which filled most of the state, with a dot for each new well. Sixty dots sprinkled the yellow state map for 2007, the year of Engelder’s initial estimate. In 2008, the number of new wells jumped to 229. Six hundred eighty-five wells were drilled in 2009, followed by another 1,395 in 2010. Nineteen hundred and twenty more had come online in 2011. On Arthur’s computer screen, yellow, pockmarked Pennsylvania looked like a slice of Swiss cheese.
I asked Arthur to give me the gist of how all that energy, all that carbon, had found its way a mile and a half below Pennsylvania. He gestured to the map, to the south central portion of the state, where an arc of gray, wrinkled land crested above the surrounding yellow. There were no drill dots on those gray folds, because there was little shale beneath them. They were the Allegheny Mountains, a northern offshoot of the vast Appalachian Range. Geologists believe they peaked approximately 290 million years ago, in a mountain-building event called the Allegheny orogeny, one tiny event in the motions of Earth’s tectonic plates that gradually thrust Europe, Asia, and Africa all against what is now North America to form the supercontinent of Pangaea. The Alleghenies had likely once been at least as tall as the Rockies or the Alps, or even the Himalayas, before being worn into gentle, rolling ranges by hundreds of millions of years of wind and rain. Beneath the Allegheny surface folds, Arthur said, there were layers of debris from a succession of more ancient, eroded ranges, each linked with its own pulse of mountain building and tectonic collision. One of those pulses, associated with the Acadian orogeny nearly 400 million years ago, in the midst of a span of time we know as the Devonian Period, was what had set the stage for the Marcellus.
The world was warm during much of the middle Devonian, too warm for polar ice caps. Some of the water that would otherwise have been locked up as ice was instead thinly spread over the North American interior as a shallow inland sea. Most of what is now Pennsylvania was flat, and underwater. Meandering continental drift had yet to transport it to its present northerly locale—at the time, it was in tropical latitudes. Phytoplankton, fish, and squid-like nautiloids thrived amid coral reefs and sponges in the warm, clear seawater. In death, their calcareous bodies, skeletons, and shells came to rest in thick layers of white lime mud on the seafloor dozens of feet below. The remains gradually hardened into rock layers of calcium carbonate—limestone. Eastward was an ocean, though not the Atlantic. It was the Paleo-Tethys, and it was disappearing, squeezing shut between landmasses on geological collision courses. Island arcs appeared on the eastern horizon, harbingers of the Acadian orogeny, foot soldiers on the front line of a tectonic advance. Over tens of millions of years, the island arcs approached and collided with the continent, slowly lifting mountains on the land like folds rising in a rug pushed across a slippery tile floor. Ranges took root in what would later become New York, New Jersey, Massachusetts, Delaware, New Hampshire, Maryland, and south central Pennsylvania. Pressed down by the weight of the surrounding mountains, the crust—the planar lime-mud floor of the sea—subsided and sank perhaps 200 meters (700 feet), centimeters per millennium, carrying the seafloor ecosystem down to destruction, far below the penetrative power of life-giving sunlight. Algae, phytoplankton, and the rare fish were all that was left behind in the open surface waters. In the dark depths of that sunken inland sea, the Marcellus shale was born.
“Picture this sea, surrounded by mountains at least a mile high, largely cut off from the world ocean,” Arthur said. “The mountains made their own weather, and then slowly weathered away. It’s called an orographic effect. They lifted up air masses and formed storms that rained out over the peaks. Erosion carried huge volumes of sediment and nutrients into the water. Iron, copper, zinc, phosphorus, molybdenum. The nutrient influx really ramped up the productivity of the algae and the phytoplankton, which bloomed, died, and decomposed on the seafloor. The decomposition used up a lot of oxygen, more than could be replaced by turnover and circulation in the deep water. That was great news for anaerobic, sulfate-reducing bacteria already living on the bottom. Oxygen is toxic to them; they are some of the planet’s most ancient organisms, from before our atmosphere had abundant oxygen. Anyway, they release hydrogen sulfide, which is toxic to most everything else. So those bacteria really knocked out whatever benthic ecosystem was left. After that, whatever organic matter settled to the bottom had practically nothing to decompose and recycle its carbon. The environment shaped the bugs, and the bugs in turn shaped the environment. That coevolution was what made the Marcellus.”
Over the course of about two million years, a fine particulate rain—countless trillions of little deaths—continually drifted down to the anoxic bottom, forming layer after layer of pristine organic carbon. At last, the underlying crust accommodated the weight of the mountains, found equilibrium, and stopped subsiding. Sediments continued to course in from the eroding ranges, piling on over the thick black mud, burying an entire sea’s worth of carbon. Eventually they piled so high the seafloor was raised once again into sunlight, and oxygen-rich, clear-water ecosystems returned—but only for a short while. Almost entirely filled with accumulating sediments, and now fully cut off from the global ocean, the vast basin’s last vestige of sea gradually evaporated. Millions more years passed, and the mountains wore down to stubs, burying what would become the Marcellus even deeper beneath their scattered strata.
Removed from its ancient context, the creation of the Marcellus struck me as eerily familiar. A new source of energy and nutrients flows into an isolated population. The population balloons and blindly grows, occasionally crashing when it surpasses the carrying capacity of its environment. The modern drill rigs shattering stone to harvest carbon from boom-and-bust waves of ancient death suddenly seemed like echoes, portents of history repeating itself on the largest of scales.
And yet, as grand as the changes were that created the Marcellus—the collisions of continents, the rise and fall of mountains, the burial of an entire sea—they paled in comparison to an even greater global transformation that began at approximately the same time, Arthur explained. The Marcellus was the last major black shale that contained no significant debris from land plants, he said. When the mountains rose around that nameless sea, they were likely bald, and the rivers that washed down from their steep slopes flowed in roaring braids through a landscape devoid of vegetation other than scattered mosses, lichens, and fungi. At that point some 390 million years ago, a point seemingly so far removed from the present day, the planet was already well over four billion years old. And in all of that time not even a single green leaf had graced the entire terrestrial world.
“This was a time of transition, when vascular plants were just beginning to colonize the land,” Arthur told me. “They start cropping up in black shales just above the Marcellus, and as the shales get younger you start seeing more and more evidence of land plants. You get into the late Devonian rocks, and you can see fossilized land plants that seem to first be colonizing around riverbanks and shorelines. They had yet to fully invade other life zones farther from the water. It’s kinda cool.”
Two evolutionary innovations spurred the colonization of land, each involv
ing the harvest and transport of water. Land plants “vascularized,” developing roots to draw water and nutrients from the earth, and they also began building their bodies from lignin, a durable carbon-rich macromolecule strong enough to bear water’s heavy weight. The resulting vascular, lignin-rich plants propagated across the continents. They doubled the planet’s photosynthetic productivity and dramatically altered the planet’s carbon cycle. Once again, life and its environment were shaping each other in a powerful, world-changing feedback loop.
In death, the durable lignin in the leaves, stems, trunks, and roots of the new land plants resisted easy decay. When submerged by floods and sedimentation, all that vegetal carbon became locked away for hundreds of millions of years. Over time the plant remains turned to peat, then lignite, and finally coal as their depth and duration of burial increased. The process peaked in a 60-million-year geological period that followed the Devonian, when so much lignin-locked carbon was buried and converted to coal that it formed massive deposits around the globe, including the high-grade anthracite and great coal measures of Pennsylvania and the surrounding Appalachian states. Geologists appropriately call this time the Carboniferous Period.
Back in the late Devonian, oxygen that would have otherwise bonded with carbon decomposing in the open air instead built up in the atmosphere, probably reaching concentrations nearly double that of the present day. This rise in atmospheric oxygen coincided with the first insects and amphibians leaving their aquatic environments to fly, crawl, and walk the Earth. In Pennsylvania and elsewhere, their fossilized remains are often found in late-Devonian “red beds,” deposits of iron-rich sedimentary rock that rusted when they were saturated with atmospheric oxygen. The high oxygen levels and new abundant fuel from land plants also increased the frequency and severity of wildfires, which may have prompted the evolutionary shift from fragile spores to hardier seeds that could endure high-temperature, low-moisture conditions during and after a conflagration. The emergence of seeds allowed plants to propagate from the moist coasts and lowlands into drier highland environments. For the first time in Earth’s history, mountains and continental interiors were blanketed in green.
