DENVER—Some 56 million years ago, carbon surged into Earth's atmosphere, raising temperatures by 5°C to 8°C and causing huge wildlife migrations—a scenario that might mirror the world's future, thanks to global warming. But what triggered this so-called Paleocene-Eocene thermal maximum (PETM) has remained a mystery.
Now, in new work presented on 27 September at the annual meeting of the Geological Society of America and published this week in Science, a group of scientists bolsters its claim that a small comet impact kicked off the PETM, stirring up the carbon just 10 million years after a similar event decimated the dinosaurs. The group announced the discovery of glassy, dark beads, set in eight sediment cores tied to the PETM's start—spheres that are often associated with extraterrestrial strikes.
The critical evidence was hardly the result of a targeted campaign, according to Morgan Schaller, a geochemist at the Rensselaer Polytechnic Institute in Troy, New York, who presented the team's work. The spheres were hiding in plain sight—in sediments off the coast of New Jersey.
For a summer project, Schaller and Megan Fung, his graduate student and co-author, combed through the cores, looking for the fossils of microscopic organisms called foraminifera, often used as a dating tool. But instead of “forams,” they discovered a series of dark, glassy spheres. The spheres looked like microtektites, the debris created and tossed aside when comets or asteroids strike Earth at high speeds. This was a surprise to the team: These sediments had been studied many times before. The spheres may have blended against the background of the black trays that are commonly used to hunt for light-colored forams, as visible as a full moon in the night.
The team is convinced the glassy spherules weren’t erupted from a volcano—another way they could have been made. Their water content is less than 0.03%, much lower than volcanic spheres, and they contain inclusions of the fused quartz glass that is characteristic of a hot impact. Their chemistry is different from microtektites from other known impacts. But the spheres will still face a high bar before being accepted as the real thing by other geologists.
Separate work by Fung clinches the case for an impact, the team noted at the geology meeting. Three of the cores she examined had large spikes in charcoal immediately above (and, therefore, just after) the layers with the spheres. The charcoal, which contains signs of charred plants, points to widespread wildfires sparked by the impact, they said. PETM-associated sediments elsewhere in the world bear signs of similar charcoal events.
The story may appear to be all wrapped up, but the group’s interpretation is misguided, says Jerry Dickens, an oceanographer at Rice University in Houston, Texas, who attended the talks. “They have completely misinterpreted the data and missed the correct, and more cool, story.” Dickens does not doubt that the spheres originated in an impact, or that the charcoal stemmed from forest fires. But both the spheres and charcoal were likely present throughout the PETM-associated clays, not just in small layers at the start. As the PETM got going, and erosion rates sped up in the warming world, sediments rich in carbon and oxygen accrued at faster rates at the New Jersey sites. This abundance of oxygen and carbon would have fueled microbes to degrade the charcoal and spheres, eliminating evidence for them higher up in a way that they couldn't at the core's base. This vanished evidence, he said, results “in a strange thing where they imagine a boundary horizon where it looks very important, but it's not.”
Others at the session were more convinced. “It is a really amazing discovery,” says Birger Schmitz, a geologist at Lund University in Sweden who also attended the talks. “The data look sound.” He says the evidence points to a small impact event of an asteroid or comet, maybe a body a couple kilometers across. However, similar objects hit Earth without triggering a global disturbance, he notes. “I have no idea of how a small asteroid could have triggered all the things that happened during the PETM.” To spark such a large carbon influx, the strike must have hit an unusual carbon-filled place like an oil reservoir, he says.
News of the spherules has bounced around the community of PETM researchers for months, says Ellen Thomas, a geologist at Wesleyan University in Middletown, Connecticut. Thomas “absolutely” believes Schaller has found microtektites. But she is perplexed because she has since re-examined several different PETM cores from New Jersey and has not found any spherules; similarly, she has never seen them in global samples. If the team successfully dates the spherules to the start of the PETM, she will consider it real evidence of an impact. “If they have not dated them,” she says, “I think they may well be contamination.” The New Jersey cores were dug with rotary drills, and there's abundant contamination in the samples, along with many spherules dating to impacts from different eras.
If accepted, and that's a big if, the strike could join a list of events associated with the PETM's carbon injection. Many scientists believe the spike could have come from a chain reaction of events, starting with ocean volcanism cooking organic carbon out of rocks and into the atmosphere. Rising temperatures may have then released seafloor methane or thawed permafrost, driving up temperatures further.
The scientists are cautious about how a small impact might fit in that chain of climate events—not all extraterrestrial strikes are the same. The PETM strike may have been a world-changing event like the dinosaur killer just 10 million years earlier. Or, it could have been like the object that struck and excavated the Chesapeake Bay 35 million years ago: locally devastating, but globally survivable.