Planetary Sciences News

“Glass Pearls” in Clamshells Point to Ancient Meteor Impact

Research suggests that the spherical structures, smaller than grains of sand, may be microtektites, but additional investigations are needed to verify their identity.

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Mike Meyer recently returned to investigating 83 mysterious objects from his past.

Collected while sifting through fossilized clams in a Florida quarry in 2006, the identities of these microscopic objects were unknown for more than a decade. Many of the glassy spheres, about 200 micrometers in diameter, are translucent, and others have frosty surfaces. Some have bumps or cracks on their surfaces, likely caused by the abrasive action of sand grains, said Meyer, an Earth systems scientist at Harrisburg University in Pennsylvania.

More than a decade later, Meyer and coauthors of a study published in Meteoritics and Planetary Science analyzed the physical characteristics and elemental compositions of the mysterious microspherules. The team concluded that the curious objects “are likely microtektites or a closely related type of material.”

Microtektites are tiny bodies of natural glass formed from terrestrial debris ejected during meteorite impacts.

Meyer estimates the objects to be 1 to 2 million years old on the basis of the current consensus on the age of the shell beds where they were found. However, the beads could have been transported from elsewhere before they were enclosed in the shells, introducing the possibility that they are older.

An Unexpected Find Prompts Waiting for Additional Resources

Meyer serendipitously discovered the silica-rich “glass pearls” when he was a University of South Florida undergraduate student participating in a summer fieldwork project led by Roger Portell, director of the invertebrate paleontology collection at the Florida Museum of Natural History and a coauthor on the new study.

Meyer and other students searched for benthic foraminifera enclosed in fossilized clams in a Sarasota, Fla., quarry. There, a 4.5-meter-tall section of the Plio-Pleistocene Upper Tamiami Formation was exposed. (The quarry is now sealed up and part of a development, according to Meyer.) Meyer and the other students collected eight sediment samples and used five different sizes of meshes to sieve the samples.

The microspherules were found only with a 98-micrometer sieve, the study notes. A paintbrush coated with tragacanth gum was used to remove the tiny structures from the samples and mount them on micropaleontology slides.

Two microtektites mounted on a brown micropaleontology slide
Experts think many more microtektites have yet to be found in Florida. Credit: Mike Meyer

Right after happening upon the glass spheres, Meyer emailed researchers to ask if any of them knew what he might have. Some experts told him the microspherules were “weird looking.” They suggested different techniques he might use to further study them, but those required expensive equipment and detailed analyses outside of the scope of the resources available to Meyer as a college student.

Meyer postponed his quest to identify the pearly unknowns.

A Reignited Search

After Meyer earned his doctorate and began working at Harrisburg University, he resumed looking for answers.

Coal ash and fly ash contamination were effectively ruled out as the source of the microspherules. Coal ash particles are usually between 0.1 and 20 micrometers in diameter and tend to have irregular shapes, the researchers wrote. “Fly ash is also more heterogeneous in composition,” with higher concentrations of aluminum and iron than those of the tiny spheres.

Other types of contamination were also noted as unlikely sources, given that the specimens are found only in certain layers of raw sediment and within articulated shells.

The microspherules’ spectroscopy data were compared to those of volcanic rocks, microtektites, and micrometeorites. “A volcanic origin is unlikely,” not only because there aren’t any known volcanoes nearby but because the microspherules have high sodium concentrations.

The microspherules’ high sodium concentration also indicates the objects are not micrometeorites. The sodium suggests “significant evaporation has not occurred, ruling out micrometeorites, since they experience substantial heating and evaporation during atmospheric entry,” according to the study.

Meyer and his coauthors ultimately suggest the microspherules are microtektites from a previously unknown impact event. As for the high sodium concentrations? They could be explained by a meteor’s impact into carbonate-rich sediments, a smaller impact, or one that occurred close to where the objects were deposited.

The study is “fascinating,” but more work is needed to constrain the ages of the microspherules, study the specific cause of their high sodium concentration, and investigate the impact that may have created them, said Scott Harris, a planetary geologist at Fernbank Science Center in Atlanta who wasn’t involved with the study.

Additional specimens are needed for these analyses, and Meyer has asked Florida fossil clubs to share similar microspherules as they are found.

—Rachel Crowell (@writesRCrowell), Freelance Science Journalist

Citation: Crowell, R. (2019), “Glass pearls” in clamshells point to ancient meteor impact, Eos, 100, https://doi.org/10.1029/2019EO133383. Published on 24 September 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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