Signs of comet collision found in 55-million-year-old rocks

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Glass blobs in rocks found along the US east coast point to a comet collision a few million years ago.
Marc Ward / Stocktrek Images / Getty Images

Glassy spheres discovered in sedimentary rock have tipped off geologists about a previously unknown prehistoric comet crash – one that may have triggered a period of intense global warming.

Morgan Schaller at the Rensselaer Polytechnic Institute in New York and colleagues found marble-like glassy spherules, known as microtektites, which they believe to be fragments of debris scattered into the air after an object collided with the Earth some 55 million years ago. 

They published their work in Science.

The distinct structures and unique appearances of spherules, as well as the way they’re positioned in sediment, can offer clues about historic impact events.

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Examples of a few of the spherules examined in the study.
M F Schaller et al, Science 2016

Schaller’s spherules were found in marine shelf sites on the Atlantic Coastal Plain, along the east coast of the US, dating back to the boundary between the Paleocene and Eocene epochs. 

This is known as the Paleocene-Eocene Thermal Maximum (PETM), one of the most dramatic climate events known to science. 

During this period, the global average temperature was 8 °C higher than it is today and the world largely devoid of ice. Massive amounts of carbon were injected into the atmosphere and oceans and many of the world’s organisms experienced drastic shifts in their evolution.

This intense warming is particularly relevant to us, because it marks the closest comparative event to the global warming evident today.

What may have kick-started the PETM is hotly debated, and theories stretch from volcanic degassing to the cycle of Earth’s orbit. Now, the possibility of a meteorite impact may be thrown into the mix.

To draw clues from the spherules, the researchers analysed the size, structure, layout and abundance of the particles they had uncovered, and compared the data to evidence of other impact sites.

Shape and colour of the fragments also offered clues about their origins.

“The spherules often have surface pits and in some cases microcraters,” the researchers write, “indicating relative velocities high enough to fracture the spherules on impact with one another, or other objects, after solidification.”

Not everyone’s convinced, though. 

Christian Koeberl, an impact specialist at the University of Vienna in Austria, said the spherules could have come from another time and been reworked into the PETM sediments.

The researchers did not directly use radiometric dating on the spherules themselves – just the surrounding sediment. 

But the next step, according to the research team, is to uncover spherules in more locations and start to figure out how far the debris spread. This will help them eventually narrow down a potential crater location to mark the comet’s impact.

Edit: this article originally stated the collision occurred 5.5 million years ago. This had been corrected.

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