When Argentina Got Dusted

A massive asteroid smacked coastal Argentina 3.3 million years ago, perhaps cooling climate and driving some of the region's mammals to extinction, researchers report in tomorrow's Science. Although the impact looks real, some experts say that its connection to climate change or extinctions is doubtful.

Peter Schultz of Brown University in Providence, Rhode Island, got his first clue to the impact 5 years ago on a visit to Argentina, when an interpreter mentioned odd green glass she had picked up as a child. Schultz, a cratering specialist, eventually explored sea cliffs of windblown dust deposits called loess near the coastal town of Miramar, working with geologist Marcelo Zarate of the Regional Center of Scientific and Technical Investigations in Mendoza, Argentina. The cliffs expose a layer of glassy, bubble-filled slabs up to 2 meters across; called escorias locally and first reported in 1865, these rocks had been attributed to everything from lightning strikes to ancient human-tended fires.

But after close study, Schultz, Zarate, and their colleagues conclude that an impact had fused loess into glassy slabs and flung them across at least 50 kilometers of Argentina's central coast. The glass has streaky flow patterns typical of rapidly cooled impact glass, mineral breakdown products that require temperatures even hotter than those of lightning and volcanoes, and a chemical composition resembling that of the local loess. Schultz presumes that a body a kilometer or so in diameter hit just offshore, producing a now-buried crater.

Radiometric dating of the glass showed that the object struck 3.3 million years ago and within about 100,000 years of an abrupt, temporary 2oC cooling of ocean bottom waters recorded in Atlantic and Pacific sediments. What's more, a major, sudden extinction at about this time wiped out 36 genera of mammals, mostly kinds known only from that region. Schultz and colleagues suggest that the impact either blasted the local fauna into extinction or induced global climate change, which triggered extinctions in southern South America. "Right now we only regard this as a coincidence," says Schultz--but an intriguing one, he says.

Other researchers say the apparent correlation could be meaningless. "Any particular moment would show quite a few extinctions" simply because the extinction rate in that geologic interval is high, says paleontologist David Webb of the University of Florida, Gainesville. Something big hit Argentina 3 million years ago, researchers agree, but to find out if it had any lasting effects on animals may take another decade of work.

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