Scientists have provided the first physical evidence that a comet or other celestial body struck Earth during a notable warming period 56 million years ago. At the time, massive amounts of carbon entered land and sea, and global temperatures rose by more than five degrees Celsius.
Distinctive silicate glass spherules found in rock dating to this warm period, called the Paleocene-Eocene Thermal Maximum or PETM, may be debris ejected during a meteorite impact, say the scientists. Their report is published in the 14 October issue of the journal Science.
Electron backscatter images of the glassy spherules from the U.S. East Coast show signs of alteration by a massive impact. | M.F. Schaller et al., Science (2016)
What effect — if any — such an impact may have had on this period of global environmental change 55 to 56 million years ago remains unclear. However, scientists say it is important to study the PETM because it is perhaps the best past event by which to understand the potential impacts of global climate warming seen today.
“It is extremely useful to study all geological instances where a rise or release of carbon dioxide coincides with a warming,” said study lead author Morgan Schaller, an assistant professor in the department of earth and environmental sciences at Rensselaer Polytechnic Institute. “It helps us understand spatial patterns of climate change and the timescales over which these consequences are relevant.”
In recent years, efforts to study the PETM have become a major point of interest for scientists. Even so, the causes of this warming event remain a mystery.
One of the potential causes of the massive injection of carbon into the atmosphere during the PETM is an extraterrestrial impact on Earth. By studying sections of marine shelf on the Atlantic Coastal Plain associated with the period between the Paleocene and Eocene epochs, Schaller and colleagues uncovered some of the first direct evidence for such an impact event.
The spherules, known as microtektites, are droplets of molten rock that were melted and thrown out of the impact crater by the energy of the projectile, or condensed from rock that was vaporized upon impact. They have distinctive features, like micro-craters on their surfaces, that are best interpreted as melted terrestrial debris ejected during a meteorite impact.
Just how the impact event would have looked, and how the debris would have fallen out of it, is not entirely certain.
“It’s really too early to say where the impact hit or how big it may have been,” Schaller said. “Based on what we do know, we can say that the impact was small and close to the sites we studied, or large and far away.”
Earlier research efforts in the same area of the Atlantic may not have detected these spherules for a variety of reasons, Schaller says, including the difficulty associated with detecting these translucent to dark-colored objects on a dark background like a black lab tray.
In fact, Schaller and co-author Megan Fung, a graduate student at Rensselaer Polytechnic Institute, discovered them completely by accident.
“We were not looking for this material,” Schaller said. “As a summer project, I had asked Megan to search through samples for microfossils so we could use them for geochemistry … She did not have great luck, so I dumped the material on a tray for a final look before giving up on it. I then spotted the spherules. Megan and I then went hunting for more of them at a bunch of different sites.”
The spherules they captured were recovered from four sites extending from the southern New Jersey coast to the coast of Florida.
“The real icing on the cake,” Schaller continued, “was the identification of shocked quartz inclusions in our spherules … a sign of an impact event.”
The results of Schaller, Fung and his team will prompt further investigations into the possible influence of an impact event on the global environmental change that characterized this notable warming period in Earth’s ancient history.