The rate at which carbon is now accumulating in the atmosphere appears to be without precedent in the geological record. That makes it hard to find situations analogous to the current changing climate. Likely the closest analog occurred over 55 million years ago and has been termed the Paleocene/Eocene thermal maximum (PETM). During the PETM, there was a geologically rapid change in the amount of carbon in the atmosphere, which was followed by an equally sudden change in temperatures. The change upset ecosystems across the planet and led to a major extinction event in the oceans.

While these features are relatively easy to determine from the geological and fossil records, there's one major aspect of the PETM that has remained uncertain: where the carbon came from. While various plausible ideas have been floated, there was no definitive evidence backing up any of them.

A few years back, a couple of researchers from Rutgers University suggested that the carbon literally showed up from outer space, delivered by a comet. While the idea was met with skepticism at the time, the same team is back with more evidence to back their idea: debris that they claim is likely to have been from an impact.

Where’d all the carbon come from?

The story of the PETM is written in isotopes. The sudden change in temperature that occurred, for example, can be tracked through changes in the isotopes of oxygen present in the water cycle. And we can identify the source of this warming based on isotopes of carbon. Different processes, especially biological ones, have a slight bias toward heavier or lighter isotopes of carbon. As a result, the carbon in the atmosphere has a ratio of isotopes that roughly reflect its source. While this may change over time as different processes come to dominate the carbon flux, these changes tend to be gradual.

This situation is decidedly not true at the PETM. Here, the ratio between carbon isotopes changes suddenly in geological terms—most scientists would say within tens of thousands of years. For that change to happen, a large source of carbon with an isotope ratio different from the atmosphere must have been released.

There have been two leading contenders for where all this carbon came from. One idea holds that it came from the methane that's trapped in the sea bed in the form of clathrates, a sort of mixed water-methane ice that can form at high pressures and low temperatures. Were an external source of heat to raise the ocean temperatures sufficiently, the clathrates would be destabilized and would release their methane to the atmosphere. Of course, this idea raises questions about what set off the initial burst of warming.

The second idea solves the problem by proposing that the opening of the North Atlantic was at fault. As rifting started taking place east of Greenland, it might have burned off both clathrates and carbon-rich ocean sediments. These would have triggered greenhouse warming that could have destabilized clathrates elsewhere in the ocean.

Given the existing data, it's difficult to discern which of these ideas is correct or if something else entirely happened. James Wright and Morgan Schaller would argue in favor of "something else entirely."

Sudden impact?

Wright has been pushing the idea that the PETM could be explained by a comet since at least 2003. Comets are rich in organic materials, including methane ices, and Wright has estimated that a 10km wide comet would deliver enough of the right isotopes of carbon to at least partially explain the changes seen as the PETM started. In a 2013 PNAS paper, Wright and Schaller argued that they had found solid evidence for a cometary origin.

That evidence came in the form of two sediment cores obtained in New Jersey. In that paper, Wright and Schaller describe how the cores have a regular banded pattern that suggests they captured a periodic process. Most of the periodic climate processes we know take many thousands of years to occur, which would mean an impossibly slow rate of sediment deposition if these were the cause. The alternative shorter-term processes aren't typically as regular as the bands in the sediment. So Wright and Schaller argue that they're looking at annual bands, with the different colors representing seasonal changes.

Tracking the changes in isotopes across these bands, they find that the changes are nearly instantaneous. Carbon isotope shifts are accomplished in only 13 years, with most of the lag involving simply the diffusion of the carbon into the ocean and its incorporation into the sediments. This, the researchers argue, is evidence that the carbon was delivered all at once—hence the cometary source.

The rest of the research community, however, doesn't seem to have been convinced. Although their paper on the subject went through peer review, the authors had prearranged for a specific editor to handle the reviews. We talked to the editor, Columbia's Wally Broecker, at the time, and he said he had an interest in seeing it published, in part because he favored seeing radical and new ideas being considered.

Less than a year later, there were a series of responses published in PNAS that tore the initial work to pieces. An international team of scientists argued that the original claims were implausible. "Using basic carbon cycle and climate considerations, we show this is not feasible," they wrote. "In fact, Wright and Schaller’s isotope records indicate that the CIE onset took at least several millennia." A second response ascribed the apparent banding pattern to disturbances caused by drilling.

Try, try again

That history sets the stage for the new work, in which Wright and Schaller join a number of other researchers to look for overt signs of the impact their model needs. Their samples include the original cores from New Jersey, along with an ocean core taken off the coast of Florida. Searching through the cores, they find a collection of what they call "spherules," rounded clusters of silicate material a few hundred micrometers across. Their numbers peak at around the same time as the carbon isotope changes.

While similar forms can be produced by volcanic activity, the product of volcanoes is typically similar in composition; the chemistry of these spherules varies widely. The spherules also contain damage that suggests they impacted with other material at high velocities, and they have materials that can only form at temperatures above 1,750 degrees Celsius, which is hotter than volcanoes get. For all these reasons, plus a few others, the authors conclude that this material was created in an impact.

Will this be enough to convince everyone? Probably not. Sparse signs at three drilling sites isn't going to be viewed as a slam-dunk for the existence of a large impact. And it's somewhat odd that the ocean-based core, where less sediment was deposited over the same time period as the other two, still has similar numbers of spherules in a given volume.

The authors optimistically conclude, "These findings will motivate a search for impact ejecta at other sites to define the geographic footprint of the [Paleocene-Eocene] strewn field, which will ultimately constrain the currently unknown location of an impact crater." They're right about the first part, in that other people will undoubtedly start checking their sediment cores to see if they have similar evidence for an impact. But their first check will be about whether the evidence is there at all. And other researchers will undoubtedly consider whether the spherules could form through means that don't involve an impact.

Should the impact idea hold up, however, then it would be time to revisit its possible role in the PETM. After all, one of the responses to the PNAS publication suggested that the timing of carbon isotope changes aren't consistent with an impact.

While this story is still developing, it does provide an interesting window into the scientific process. Radical new ideas clearly can get a hearing, even if it's not an especially kind one. That lack of kindness in this case, however, was based on a simple scientific issue: the data presented in 2013 didn't make sense given what we already knew, and we had alternative explanations. On the plus side, that hostile reception caused these scientists to do exactly what they were supposed to do: go back and gather more evidence.

Science, 2016. DOI: 10.1126/science.aaf5466 (About DOIs).