Impact relics discovered in shells

Two to three million years ago, a meteorite could have hit Florida

These tiny beads of rock-glass discovered a researcher inside shells. They testify that two to three million could have hit a Florida meteorite. © Kristen Grace / Florida Museum
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Unusual find: Inside the fossil shells researchers have discovered evidence for a previously unknown meteorite impact in Florida. The shells contained tiny beads of rock glass - typical testimonials of an impact. Their chemical composition also indicates an extraterrestrial origin. Thus, two to three million years ago, one or more meteorites could have landed on or near the Florida Peninsula.

While marks on the moon remain clearly visible over millions of years, this is different on Earth. The erosion by wind and water, but also the vegetation quickly make craters and other impact traces disappear. Often only subtle traces, such as tiny spheres of rock glass, so-called microtectites, reveal that once upon a time there was an impact.

Micrograph of one of the microtectites discovered in the mussels. © Mike Meyer / Meteoritics and Planetary Science

Mini globules in ancient shells

In a rather unusual place, Mike Meyer from Harrisburg University in Pennsylvania discovered such microtectites. While still a student, he had studied banks of fossil shells in collaboration with colleagues in Florida - actually looking for microfossils in their interior. But when he opened some of the shells and sifted through the accumulated sediment in them, he discovered tiny globules in them.

"They really stood out. Because grains of sand are typically rather irregular, potato-shaped, "says Meyer. "But I always found these tiny, perfect bullets." Meyer had found more than 80 of these small 200-micron rounds in the end. But at first nobody knew what it was. He kept it for more than ten years until he recently found and analyzed it.

Witnesses of an impact

The surprising result: The kolles were of extraterrestrial origin. Their physical characteristics and their enrichment with exotic metals suggest that they are microtectites - tiny evidence of a meteorite impact. The koles were formed when the tremendous energy of the impact melted and vaporized rocks and threw them into great heights. There, the drops of stone froze to the little balls before they fell back to earth. display

The microtectites now identified are the first ever found in Florida and the first discovered in fossil shells. "That really blew me away, " says Meyer. According to datings, the kegs were deposited about two to three million years ago. The researchers suspect that at that time one or more meteorites on or near the Florida Plateau must be taken.

Impact in the salt dome?

However, it is still puzzling why the microtectites contain unusually high levels of sodium. Normally, however, this rather volatile element evaporates, while a meteorite is destroyed by the earth's atmosphere. It is therefore unlikely that the sodium was brought from the impactor, Meyer and his colleagues. They rather suspect that it comes from the impact site.

"The high sodium content of microtectites is exciting because it suggests a relatively close impact location, " Meyer explains. "When the meteorite struck, he probably hit a rock salt deposit or the ocean." Both speak for a strike site in or near Florida. "But we still need more material and sites to confirm the source of these microtectites, " the researchers emphasize. They suspect that even more of these small tumblers of rock glass could hide in the shell bays of the region. (Meteoritics & Planetary Science, 2019; doi: 10.1111 / maps.13299)

Source: Florida Museum of Natural History

- Nadja Podbregar