Mangled microfossils may mark impact sites

From Washington, D.C., at the American Geophysical Union meeting

Scientists studying rocks extracted from sediments deep beneath eastern Virginia say they may have found a new way to identify the locations of ancient, hidden craters blasted by extraterrestrial objects. Just look for broken or twisted microfossils.

TINY DINOS. Deformed dinoflagellate cyst (above) experienced the intense shock waves generated by an impact from space. An intact cyst of the same species is shown below.

The high-pressure seismic vibrations generated by the impacts of meteorites are strong enough to fracture quartz crystals and melt minerals in nearby rocks (see this week’s article; “Presto, Change-o!“).

Those same shock waves can have similar effects on microfossils, say Lucy E. Edwards and Jean M. Self-Trail of the U.S. Geological Survey in Reston, Va. They’ve examined many rock samples drilled from several sites around Norfolk, which lies on the edge of a 35-million-year-old crater now buried beneath the mouth of the Chesapeake Bay.

Scientists know that when the object that created this crater crash-landed east of the ancient Appalachians, sea level was higher and the impact site was located in about 300 meters of water. The seafloor sediments in place at the time contained tiny living marine organisms, called dinoflagellates, which spend part of their life cycle in a cyst buried in ocean-floor ooze. Those cysts, which are made of a waxlike material, typically have delicate protrusions.

In rocks that formed from ooze present during the impact, the dinoflagellate cysts are twisted and fragmented. Often, the cysts’ protrusions are melted and fused to other particles in the sediments. These features are testaments to the heat and intense pressures generated by the extraterrestrial impact, says Edwards.

Other rocks beneath the impact site contain the tiny mineral skeletons of Discoaster multiradiatus, a species of marine plankton that lived only between 55 million and 56 million years ago. When intact, the skeletons–true to the creature’s name–are small disks with a sunburst of ridges on each side. But many Discoaster fossils from the Chesapeake sediments have been damaged, says Self-Trail. In some cases, the edges of the disk have been sheared off, leaving the skeletons with a pentagonal shape. That sort of damage hasn’t been seen in Discoaster fossils unearthed in other areas, she notes.

Damaged microfossils have been observed at other impact sites, but Edwards says that scientists had attributed the microscopic carnage to poor preservation, not the impacts. She and Self-Trail suggest that rocks bearing damaged microfossils now might serve as hallmarks to the gargantuan shock waves generated by an extraterrestrial impact.

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