Looking at rock cores drilled from the ocean floor, Bralower, associate professor of geology and of marine sciences, went back 55 million years to a time when evolution’s slow, steady rhythm suddenly sped up. It was a time when a host of mammals, from rodents to primates, materialized in North America. And a time when species after species of ancient plankton called “foraminifera” appeared in waters near the ocean surface, while up to half of their bottom-dwelling cousins vanished forever.
These changes took place in a geologic heartbeat called the “late Paleocene thermal maximum” (LPTM). The period, named for its temperature spikes of up to 14 degrees Farenheit in the deep oceans, spanned about 100,000 years—a moment so brief it rarely shows up in the fossil record.
“The period is marked by the biggest extinction event in the deep sea in the last hundred million years,” Bralower says. “Nothing else even comes close. But what’s really surprising is that at the same time there was rapid evolution.”
Usually, evolution is described as the culmination of eons and eons of persistent trial and error, not as a sudden burst of activity. And the few known evolutionary spurts are usually thought to follow major extinctions, not to coincide with them. The disappearance of the dinosaurs 65 million years ago, for example, is often perceived as having paved the way for large mammals.
But, in theory, climate change could drive evolution and extinction at the same time. In particular, during the LPTM, changing climate could have allowed land mammals to spread into new territories and disrupted the normal pattern of ocean currents, killing some foraminifera while fostering others. And after California researchers James Kennett and Lowell Stott found an unusual carbon composition in the sea creatures’ shell-like fossils, geologists identified a possible mechanism for the climate change: methane gas from beneath the ocean floor.
The gas, normally stored in tiny ice cages called “hydrates,” transforms into carbon dioxide if it escapes into the atmosphere. Researchers think a large, sudden release could create enough carbon dioxide to start a feedback loop which could substantially warm the climate. But the nagging question has always been: What would have triggered such a release?
In November, Bralower and his colleagues, including graduate student Debbie Thomas, published their answer in the journal Geology. At the start of it all, they said, were Caribbean volcanoes.
Actually,volcanoes—different ones—had topped the list of suspects once before. They had erupted in the North Atlantic Ocean sometime between 61 and 54 million years ago, and it was no small event: The lava that poured out created the North Atlantic igneous province, a submerged plateau the size of Texas. Geologists agree that these “gentle” eruptions contributed to a small temperature increase that had been under way worldwide for millions of years.
But some researchers also suggested that the eruptions, on their own, could have caused the much more significant warming that released the methane.
“The main problem with this idea is that the eruptions appear to have been going on for millions of years before the foraminifera disappeared,” Bralower says. “Why should the ocean waters be warming all that time, and one day the methane suddenly kicks in?”
Like other geologists who remained unconvinced by the explanation, Bralower began to look for a different event—one that had coincided with the extinction of the foraminifera. His search led him to the JOIDES Resolution (JOIDES is short for “Joint Oceanographic Institutions for Deep Earth Sampling”), a research ship funded by the National Science Foundation. For decades, the ship has sailed around the world drilling cores from the ocean floor.
Bralower signed on in late 1995 and set sail for the Caribbean. Near the coast of Colombia, the ship stopped to drill. Each day, as the cylindrical core came up in 10-meter sections, Bralower scanned the layers for evidence that he and the crew had succeeded in doing something very rare: capturing the LPTM in the rock record. Because the event was so brief, only six or seven useful oceanic cores have been located in decades of drilling.
But Bralower was lucky. After the sections were cut in half, he traced his way back to the depth where the LPTM should be. He saw a wispy layer, whose churned-up sediments spoke of ancient animals burrowing into the bottom, followed by undisturbed layers, suggesting an ocean floor bereft of life—strong evidence that he had found the moment when the bottom-dwelling foraminifera had vanished.
Still, Bralower didn’t know what had released the methane. And when the crew drilled at the second site, 250 miles from Haiti, the early sections were badly fragmented—which often means the LPTM will not be there. He was getting nervous.
Then, the crew pulled up a piece so stunning, Bralower could hardly believe it. Even before the core was cut he knew that the LPTM had been preserved. Through the protective lining that covered the core, Bralower saw an undisturbed layer like the one in the first core. Surrounding it was layer after layer of trademark red, green, and blue volcanic ash—the first evidence that Caribbean volcanoes erupted around the time the foraminifera went extinct.
The sheer quantity of ash suggested a large, explosive event. It was exactly the kind of trigger he had hoped to find.
Still, Bralower wondered why the first core had no hint of volcanic activity. About a year later, after picking through sample after sample from the first core, the answer finally came. He was looking at a piece from the boundary between the churned-up sediments and the undisturbed ones when something bright caught his eye. A closer look revealed that the glass-like grains which stood out against the dark grey rubble were feldspar, hornblende, and zircon—volcanic minerals. This meant there was a thin layer of volcanic ash right before the foraminifera disappeared from the first core. The eruption seemed to be linked to the extinction.
But he still needed final proof. Fortunately, Bralower had dated the cores by taking samples every one to two centimeters, rather than every meter, as most geologists do. He had adopted this more detailed—and more expensive—approach from Kennett and Stott, who had identified the unusual carbon composition in LPTM fossils and linked them to dramatic changes in ocean temperature. Bralower’s painstaking work paid off when he found the LPTM’s signature carbon composition in his own ash layers. He was convinced that Caribbean volcanoes had triggered the methane release.
“The strength of the argument is really the timing,” Bralower says. “There was a massive eruption of Caribbean volcanoes immediately before or right at the onset of the LPTM. The chance that it was a coincidence is minute.”
Bralower has identified two possible clusters of prehistoric volcanoes that could have been the source. He says he may never know which one was responsible, but he favors the arc closer to the Haiti site because that core had much thicker ash layers.
But a more important question is what these findings might tell us about evolution and the global climate, Bralower says. He thinks the explosive Caribbean eruptions spewed enough sulfurous particles into the upper atmosphere to deflect sunlight in that region. With less light reaching the tropics, the air and ocean surface temporarily cooled. The surface waters became denser and sank several thousand meters—far enough to reach the ocean floor in many places.
The sinking water, though cooler than normal surface temperatures, was warmer than usual for the bottom. And, because warm water holds less oxygen than cold water, the foraminifera on the ocean floor suffocated. The abrupt change in temperature also melted the ice holding the underground methane, which escaped into the atmosphere, causing global warming, which hastened the release of more methane and led to more warming.
“Once the cycle got going,” Bralower says, “it caused profound biological effects—extinctions on the sea floor, very rapid evolution of land mammals, and the equally impressive burst of evolution in the surface waters.”
All of this provides a worst-case scenario for global warming today, Bralower says. It suggests what could happen if we’re not careful. The other lesson we might learn from the LPTM, Bralower says, is that some of the most dramatic events can occur in a very short period of time. It’s a breakthrough he attributes to Kennett and Stott and their scrutiny of the fossil record.
“All these years, geologists have been skipping big sections of the rock record because the analyses are so costly,” Bralower says. “Who knows what we’ve been missing?”