The sudden cooling period that began about 12,800 years ago—the Younger Dryas Cold Event—has long puzzled scientists because it bucked a global warming trend. Findings from a recent study provide new support to a much-debated 2007 hypothesis, which posits that fragments from a disintegrating comet collided with Earth and caused massive wildfires that paradoxically triggered a significant temperature drop. The 2007 hypothesis, which focused on evidence from Clovis sites in North America, was proposed by Richard B. Firestone, at the University of California, Berkeley, with colleagues.
This new, larger study, is reported in two parts by lead author Wendy S. Wolbach, professor of chemistry at DePaul University, Chicago, Illinois, with thirty other scientists, several of whom participated in the 2007 study, including Wolbach and Firestone. This multidisciplinary collaboration examined twenty-six sites throughout the Northern Hemisphere. In layers from the Younger Dryas, the researchers discovered anomalously high concentrations of unusual materials, including high-temperature iron-rich spherules; silica-rich glassy spherules; meltglass; and nanodiamonds, iridium, platinum, and osmium—all suggestive of cosmic impacts.
Additional investigation of ice core sequences from Greenland, Antarctica, and Russia showed that the start of the Younger Dryas coincided with peaks in aerosols associated with wildfires, such as nitrate, ammonium, oxalate, and formate. These peaks, along with sea salt and continental dust levels in ice layers, and carbon dioxide records from ice cores, suggest that the Younger Dryas began with raging wildfires that consumed some nine percent of the Earth’s land biomass.
Taken together, these multiple lines of geological evidence suggest the Younger Dryas was sparked by an Earth collision with multiple fragments of a disintegrating comet at least one hundred kilometers in diameter. Such comet fragments, the researchers hypothesize, sparked fires of a massive scale, with sun-blocking smoke causing the onset of an “impact winter” that “flipped [the Northern hemisphere] back into almost ice-age conditions,” said one of the researchers, Allen West, at the Comet Research Group in Prescott, Arizona. Nearly 13,000 years after the fact, these geological data, though still disputed by some, are the closest we can get to a smoking gun. (Journal of Geology)