Maximum (PETM) is a rapid warming of the Earth caused by an accumulation
of atmospheric carbon dioxide 56 million years ago, which offers analogs to global warming today. A comet strike may have triggered the PETM, suggested a new study.
Sorting through samples of sediment from the
time period, researchers at Rensselaer Polytechnic Institute discovered
evidence of the strike in the form of microtektites - tiny dark glassy
spheres typically formed by extraterrestrial impacts. The research will
be published in the journal Science
‘A comet strike may have triggered the Paleocene-Eocene Thermal Maximum (PETM), suggested a new study.’
"This tells us that there was an extraterrestrial impact at the time
this sediment was deposited - a space rock hit the planet," said Morgan
Schaller, an assistant professor of earth and environmental sciences at
Rensselaer, and corresponding author of the paper. "The coincidence of
an impact with a major climate change is nothing short of remarkable."
Schaller is joined in the research by Rensselaer professor Miriam Katz
and graduate student Megan Fung, James Wright of Rutgers University, and
Dennis Kent of Columbia University.
Schaller was searching for fossilized remains of Foraminifera, a
tiny organism that produces a shell, when he first noticed a
microtektite in the sediment he was examining. Although it is common for
researchers to search for fossilized remains in PETM sediments,
microtektites have not been previously detected.
Schaller and his team
theorize this is because microtektites are typically dark in color, and
do not stand out on the black sorting tray researchers use to search for
light-colored fossilized remains. Once Schaller noticed the first
microtektite, the researchers switched to a white sorting tray, and
began to find more.
At peak abundance, the research team found as many as three
microtektites per gram of sediment examined. Microtektites are typically
spherical, or tear-drop shaped, and are formed by an impact powerful
enough to melt and vaporize the target area, casting molten ejecta into
the atmosphere. Some microtektites from the samples contained "shocked
quartz," definitive evidence of their impact origin, and exhibited
microcraters or were sintered together, evidence of the speed at which
they were traveling as they solidified and hit the ground.
Atmospheric carbon dioxide increased rapidly during the PETM, and an
accompanying spike in global temperatures of about five to eight degrees
Celsius lasted for about 150,000 years. Although this much is known, the
source of the carbon dioxide had not been determined, and little is
known about the exact sequence of events - such as how rapidly carbon
dioxide entered the atmosphere, how quickly and at what rate
temperatures began to rise, and how long it took to reach a global high
One clue can be found in a sudden shift in the ratio of carbon
isotopes (atoms containing a number of neutrons unequal to the protons
in their nucleus) in certain fossils from the time period. In
particular, Foraminifera, or "forams," produce a shell whose chemistry
is representative of atmospheric and ocean carbon isotopes. The research
team initially set out to examine the ratio of carbon isotopes in
Foraminifera fossils over time, to more closely pinpoint events during
"In sediment records, when you look at the ratio of carbon-12 to
carbon-13 in a particular species, you see that it's stable and then it
abruptly shifts, wiggles back and forth and slowly returns to pre-event
values over hundreds of thousands of years," Schaller said. "This
evidence defines the event, and tells us that the atmosphere changed, in
particular adding carbon from a source depleted in carbon-13. A comet
impact on its own may have contributed carbon to the atmosphere, but is
too small to explain the whole event and more likely acts as a trigger
for additional carbon releases from other sources."
As a source of fossils, the team used sediment cores - cylinders
of sediment extracted vertically from sediment deposits with a hollow
bit - known to correspond to the time period of the PETM. Sediments near
the top are more recent, those further down are older, and signature
layers indicating known events are used to calibrate the timescale
represented in the sample. The team chose cores from three sites -
Wilson Lake and Millville in New Jersey, and Blake Nose, an underwater
site east of Florida - known for a rich sedimentary record of the time
As Schaller tells it, the discovery of microtektites was "completely
by accident." Ordinarily, the team passes samples through sieves of
various sizes, to isolate samples most likely to contain forams. The
tektites, which are smaller than most forams, would have been largely
removed in this process.
"We were having lousy luck looking for forams, and I was frustrated.
I went to the lab and dumped a sample on the sorting tray without
sieving it, and there it was," Schaller said. "It was a stunning moment.
I knew what I was looking at was not normal."
Once the team made the discovery, they obtained a sample from a
fourth site - Medford - where the unit is naturally exposed at the
surface, to rule out the possibility that the samples had been
contaminated by the drilling process. The Medford samples also contained