July 22 , 2016

by Marcia Bjornerud

At least five times in the past five hundred million years, the normally meticulous scalpel of natural selection, which excises this moth or spares that finch on account of the tiniest differences in wing color or beak shape, has become the evolutionary equivalent of a machete. Whole taxonomic groups of organisms—not merely individuals or species but genera, families, and orders—have been cut down in swift, indiscriminate strokes. After each of these mass extinctions, life on Earth eventually recovered but was irrevocably changed, with the creatures that survived, as much by happenstance as hard-earned fitness, becoming the unlikely founders of brave new biospheres. The blue whales, polar bears, and Siberian tigers that today symbolize the threat of extinction in the Anthropocene, the geologic age wrought by humans, owe their very existence to the demise of the most charismatic of all megafauna, the dinosaurs, at the close of the Cretaceous period.

Of the great mass extinctions, the end-Cretaceous event ranks third or fourth in severity by most measures, but it certainly looms largest in the popular imagination. The dinosaurs were not only dramatic in life but also operatic in death. In 1980, Luis Alvarez, a Nobel Prize-winning physicist, with his son Walter, a Berkeley geologist, and two of the elder Alvarez’s colleagues at Lawrence Berkeley National Laboratory, proposed what has become the preëminent theory of the dinosaurs’ annihilation. They proposed that an asteroid some six miles wide struck Earth at the same geologic instant that the dinosaurs and many other organisms went extinct. There was a searing pulse of heat, after which pulverized rock was ejected into the stratosphere, creating a dusty shroud that put the entire planet in shadow. Photosynthesis ceased; the herbivores died, then the carnivores. The idea, published just a month after the ash-spewing eruption of Mt. St. Helens, resonated with the existential anxieties of the late Cold War and seized the public’s attention.

Yet among geoscientists, there has been a lingering sense that the story has not been told in full. Initially, the extraterrestrial-impact theory seemed intellectually distasteful. It was too tidy, violating a deep-seated taboo against catastrophic explanations for geologic phenomena. Worse, the senior Alvarez antagonized paleontologists who had spent years puzzling over the Cretaceous fossil record by publicly dismissing them as “stamp collectors.” In the next ten years, as the search for the impact site finally led to the discovery of a buried crater of precisely the right age off the coast of Mexico’s Yucatán peninsula, most paleontologists swallowed their indignation and came to embrace the asteroid hypothesis. But still they questioned whether the fallout scenario could account for all the particulars. The extinction had, in fact, been somewhat selective: while most dinosaurs, pterosaurs, and large marine reptiles died off, a small group of theropods (the predecessors of birds), as well as crocodilians, snakes, amphibians, and mammals, survived. In the oceans, the coiled ammonites that had ruled the Mesozoic seas were virtually wiped out, while bony fish suffered relatively few losses. Certain groups of clams and corals were decimated, as were the most important primary producers of the time, tiny planktonic foraminifera—one-celled organisms that create minute and wondrously elaborate shells of calcite. But foraminifera that lived in deeper water and nearer the equator fared better than those from the ocean surface and the high-latitude regions. Surely the mute fossils had something important to say about the horrors they had seen.

By the time the Yucatán crater was identified, in 1990, the Berlin Wall had fallen. As the threat of nuclear holocaust began to fade from the collective consciousness, it was replaced by a growing awareness that environmental malefaction might be humanity’s downfall. Acid rain was shown to be devastating forests in New England and Scandinavia, the legacy of sulfurous emissions from decades of coal burning. The pattern of marine extinction at the end of the Cretaceous suddenly looked very much like what one would expect in an ocean that had become soured, the creatures’ shells broken down by sulfuric acid. And the rocks in the Yucatán crater had plenty of sulfur in them: they included thick layers of a mineral called anhydrite, or calcium sulfate, which would have been vaporized in the impact, hurled into the atmosphere, and then precipitated as burning acid rain. The 1991 eruption of Mt. Pinatubo, in the Philippines—ten times more powerful than that of Mt. St. Helens—provided further insight. The eruption injected enough sulfate particles into the stratosphere to counteract, for two years, the inexorable climb in global temperatures related to rising greenhouse-gas concentrations. The immense volumes of brimstone blasted from the hundred-and-fifty-mile-wide Yucatán crater could have caused far more severe cooling—devastating to organisms accustomed to the warm Cretaceous world—before falling out of the atmosphere as the rain from hell. It seemed, then, that sulfur, not dust, must be the real culprit in the end-Cretaceous extinction.

But for the past twenty years, many paleontologists, with their renowned philatelic attention to detail, have remained unsatisfied with this explanation, too. Caustic acid rain should have been especially harmful to freshwater ecosystems, yet species in these environments, including frogs and other amphibians sensitive to changes in water chemistry, had survival rates of close to ninety per cent—far higher than those that lived on dry land, where only twelve per cent endured the cataclysm. The failure of any of the proposed kill mechanisms to account for the details of the fossil record has led some paleontologists to propose that the asteroid was not a lone assassin but struck a global ecosystem already weakened by other injuries. The most frequently cited accomplice is volcanic activity, in particular the eruptions that produced the Deccan Traps, a mile-thick stack of basalt flows in present-day India. For tens of thousands of years leading up to the extinction, the oozing lavas released enormous quantities of carbon dioxide. In recent reconstructions of the Cretaceous finale, the murderous asteroid has been forced to share the stage with unglamorous greenhouse gases.

Now, a paper just published in Scientific Reports has named another possible conspirator: crude oil. According to Kunio Kaiho and his colleagues at Tohoku University, in Sendai, Japan, the sudden ignition of underground oil at the Yucatán impact site could have jetted into the upper atmosphere a mass of fine black carbon, also known as soot. Human-made black carbon, the bane of Beijing, remains in the lower atmosphere for only a matter of days before falling back to the surface, where it warms the planet by absorbing heat. But black carbon injected into the stratosphere would have the opposite effect, acting as a long-lived sunshade that could abruptly cool Earth and inhibit photosynthesis over a period of years. Kaiho’s team suggests that the asteroid may have sent up as much as three billion tons of soot, hundreds of times more than the world’s industries release each year. Petroleum—the ectoplasm of ancient organisms, our shameful Anthropocene addiction—may have come back to haunt the dinosaurs, too.

To test the new theory, Kaiho’s team compared samples of sooty material found in late Cretaceous rocks from both close to the impact (in Haiti) and far away (Spain). They found that the samples were chemically similar to each other and bore the molecular fingerprint of crude-oil combustion—a particular hydrocarbon called superbenzene, or coronene, for its six-ring structure. They point out that the rocks at the Yucatán impact site are known to bear oil: indeed, drill cores taken in the nineteen-fifties by the Mexican oil company Pemex were key to the discovery of the crater. They then calculated how much black carbon could have been produced by flash-heating, and ran high-resolution computer simulations to explore the specific effects at different locations of both soot and sulfate in the stratosphere, month by month.

The modelling shows that the cooling effects of sulfate aerosols alone would have been smaller than previously thought, but that the combination of sulfur and soot can explain some of the subtler aspects of the end-Cretaceous fossil record. In particular, the simulations reveal more dramatic cooling in polar than in equatorial regions, consistent with the pattern of marine extinctions. The models also suggest that severe drought would have transformed once lush regions within fifteen degrees of the equator, already devastated by limited photosynthesis, into Saharas. In these areas, large herbivores dependent on fresh greens, and the carnivores that ate them, would have quickly starved. Freshwater organisms that could live off plant detritus, in rivers still running by rain from higher latitudes, would have been among the few survivors. Hence the lucky frogs.

The Kaiho paper is certain to feed the end-Cretaceous controversy. But the ferocity of the debate suggests that more than science is at stake. Catastrophes of the past are an irresistible backdrop onto which we project our own fears of apocalypse, the doomsday scenarios that trouble our sleep. They allow us to peer timidly into the abyss and then draw our feet back again, glancing nervously about at this Anthropocene world. If we’re fortunate, we’ll just be naturally selected.