Extraterrestrial Impact Preceded Ancient Global Warming Event
By Mary L. Martially
Rensselaer Polytechnic Institute
October 13, 2016

A comet strike may have triggered the Paleocene-Eocene Thermal Maximum 
(PETM), 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. 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 tomorrow in 
the journal Science.

"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 
spike in global temperatures of about 5 to 8 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 temperature.

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 the PETM.

"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 period. 

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 microtektites.


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