The end of the last ice age was a busy time in North America. As a whole, the world seemed to be in the process of exiting the long cold snap nearly 15,000 years ago, but there was a sudden return to icy conditions that lasted nearly 2,000 years. At about the same time, the first humans made their presence felt in North America, and many of the larger species of fauna went extinct. Over the past few years, a series of papers have built the case that everything other than the arrival of humans could be tied together nicely by the impact of a series of carbon-rich meteors or comets that burst in the air over North America, setting off massive fires locally, and blocking the sun globally. Today, PNAS is releasing a paper that calls all of that into question for a very simple reason: they tried to reproduce some of the results, and failed.

The events in question take place during a period called the Younger Dryas, named for a plant that did well during the cold. As the graphs below show, temperatures had been rising steadily from the depths of the ice age until 12,900 years ago, at which point they quickly reverted to their previous, frigid levels. It took well over 1,000 years before they'd return to the sort of warm that we've enjoyed for the last 10,000 years or so. But climate wasn't the only thing that was changing; North America saw major species losses, with a lot of its megafauna going extinct within a relatively short period of time.

The plunge in temperatures during the Younger Dryas correlate with a rise in fresh water reaching the oceans.

Traditionally, that has been ascribed to the combination of the arrival of the Clovis culture's hunters and climate change. The climate change itself was a bit harder to pin down, but a number of people have pointed a scientific finger at the draining of massive lakes, such as Lake Agassiz, that had built up behind glacial dams. During the first warming, these dams broke, releasing massive amounts of fresh water into the North Atlantic, and altering the ocean's circulation in a way that returned the planet to its icy state.

The new papers, generally published by a relatively small set of researchers, challenged those interpretations. They focused on the presence of dark organic sediments, called "black mats," that are associated with many of the first Clovis sites. The collaborators have argued that these dark sediments literally represent burnt material, with massive fires set off by the impact of extraterrestrial objects. Reports linked the black mats to magnetic and metallic grains, chemicals specific to soot, and, perhaps most tellingly, nanodiamonds, which can only form under a limited number of high-temperature, high-pressure conditions, such as those created by an impact.

The new paper challenges only one aspect of that data: the presence of high levels microscopic magnetic spheres in the same sediment layers as the other signs of a potential impact. These items do form at a constant rate from the impact of micrometeors, but may appear at higher numbers during a period of more intense bombardment. The authors tracked the levels of these magnetic microspheres at a series of seven Clovis-era sites, including two used by the authors to argue in favor of an impact. In one case, they obtained samples only a few centimeters away from the previous ones.

What they found, however, is nothing like the results that appeared in the previous papers. "Although concentrations of magnetic grains vary by more than two orders of magnitude among all study sites, no individual site shows clear evidence of uniquely enhanced levels of magnetic grains in YDB samples," the authors state, noting that the majority of their sites actually show a decrease at this time. They term this "a discrepancy between the two studies that is particularly troublesome." They suggest that local conditions, such as the presence of flowing water, might dominate a site's properties and cause extensive variability.

Obviously, that's just one piece of evidence out of a more extensive list that has been used to argue in favor of an impact. But the authors note that a separate study has raised questions about how extensive the evidence for massive fires is. In addition, the radically different results may suggest that there's a problem with the method of assigning samples to specific dates, which might call into question some of the more compelling findings, like the presence of nanodiamonds.

In any case, it's an interesting scientific controversy that doesn't appear to be going away anytime soon, as those who favor the impact idea will undoubtedly have more to add. It doesn't really say much about our understanding of climate change, as either or both of the events—an impact and the draining of Lake Agassiz—seem more than sufficient to have global consequences.

But the controversy also says important things about science in general. Various critics of science argue that contrary views don't get published, or complain that historical science is immune to the sort of reproducibility that takes place in laboratory-based science. The back-and-forth here shows that neither of those appears to be especially true.

PNAS, 2009. DOI: 10.1073/pnas.0907857106