Five Billion Years of Solitude (19 page)

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 involving 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.

The rise of vascular land plants caused so much carbon sequestration during the late Devonian and early Carboniferous that atmospheric CO
₂
levels plummeted. The diminished greenhouse effect dropped global temperatures by only a few degrees, but that seemingly slight change was enough to tip the world into a long-term ice age. Ice caps formed and grew at the poles as cooler summers failed to melt the accumulated snows of previous winters. The bright white spreading glaciers reflected more sunlight into space than the darker lands and seas, driving temperatures lower still. On average, every few tens of thousands of years or so, glaciers advanced from the poles into lower latitudes, locking water in their icy clutches to reduce global sea levels and turn climates more arid. Each time, terrestrial species in polar and temperate latitudes were forced down to the tropics ahead of advancing walls of ice four kilometers (two and a half miles) high. Each time, falling sea levels exposed the life-packed continental shelves to open air, disrupting marine ecosystems. Inevitably the glacial advance would wane, the walls of ice would retreat to the poles, and marine and terrestrial life would once again thrive in an interglacial period.

For a hundred million years, throughout the Carboniferous and most of the following geological period, the Permian, Earth’s ice caps endured, occasionally sending glaciers down from the poles. Those ice caps finally melted away around 260 million years ago, when increased volcanic activity and decreased oceanic absorption of carbon rapidly pumped atmospheric CO
₂
back to mid-Devonian levels. Abundant polar ice would not return to our planet until around 35 million years ago. Those polar ice sheets expanded just over two and a half million years ago, when the outpourings of undersea volcanoes formed the
Isthmus of Panama and sutured together North and South America, creating new oceanic and atmospheric circulation patterns that further lowered global temperatures. This occurred at the dawn of the Quaternary Period, a span of time that, at its tail end, would give rise to anatomically modern humans. Since the Quaternary’s beginning, and even today, with Antarctica and Greenland still locked in ice, the Earth has technically been in an ice age. That this is a rather remarkable state of affairs has only very recently come to be appreciated. Polar ice caps, despite their presence for the entirety of human history, are surprisingly infrequent occurrences in the history of Earth. As far as geologists can discern, over the course of its 4.5-billion-year existence, ice caps have graced our planet’s poles for only a sum total of about 600 million years—about an eighth of the Earth’s life thus far.

In our present ice age, glacial walls of ice repeatedly pulsed from the Arctic to cover the sites of modern-day Toronto, New York City, and Chicago, as well as much of northern Pennsylvania. They carved out Hudson Bay and the Great Lakes, and at their edges spat out glacial moraines—chunks of broken land that became places such as Long Island and Cape Cod. The glaciers last retreated some 12,000 years ago, at the beginning of an interglacial epoch we call the Holocene. The rise of agriculture, cities, commerce, industry, science, and technology that we recognize as human civilization and chronicle as human history has all occurred within the abnormally mild and stable Holocene interglacial, the climatic equivalent of a twelve-thousand-year summer.

The signs of glacial advance and retreat can be tracked in sedimentary rocks and isotopic analysis of seawater, but some of the most high-fidelity evidence of climate oscillations comes from within the glaciers themselves, in bubbles of trapped, ancient air. Found in ice cores extracted from today’s melting glaciers, each bubble is a snapshot of the atmosphere on a day in the distant past, when a minuscule puff of air was trapped in fresh-fallen snow that became part of the ice. The oldest bubbles are of impressive vintage—detailed analysis revealed they formed some 800,000 years ago. In aggregate, the gas in the
bubbles tracked the changing composition of Earth’s atmosphere for the majority of the past million years, long before modern humans ambled onto the planetary scene. The bubbles display a clear pattern that connects greenhouse-gas levels to glaciation: when glaciers were advancing, out of every million molecules of air trapped in their ice approximately 200 were CO
₂
. When glaciers were retreating, the amount of CO
₂
in ice-locked air rose to about 300 parts per million (ppm).