About 35 million years ago, an asteroid hit the ocean off the east coast of North America. The impact formed a 25-mile-diameter crater that is now buried beneath Chesapeake Bay, a Virginia and Maryland estuary. From this impact, the surrounding area experienced fires, earthquakes, falling molten glass droplets, an air blast and a devastating tsunami.
While the resulting "Chesapeake Bay impact crater" is now fully buried, it was discovered in the early 1990s through scientific drilling. It is now ranked as the largest known impact crater in the United States and the 15th largest on earth.
When the asteroid hit, it also produced a blow-ejecta layer, which includes tectites (natural glass formed by debris under meteorite impact) and shocked zircon crystals ejected from the impact area. Researchers refer to this layer as the "North American litter field," which covers a region of about 4 million square miles, about 10 times the size of Texas. Some ejecta landed on land, while the rest was immediately cooled by contact with seawater and then sank to the seabed.
A research team, including Arizona State University School of Earth and Space Exploration researcher and lead author Marc Biren, along with co-lessors Jo-Anne Wartho, Matthijs Van Soest and Kip Hodges, received drill samples from the Ocean Drilling Project website 1073 and dated them with the "uranium-thorium-helium technology" for the first time.
Their research was recently published in the international journal Meteoritics & Planetary Science .
"Determining the exact and exact ages of power events is crucial to our understanding of the history of the earth," Biren said. "In recent years, for example, the scientific community has recognized the importance of impact events on Earth's geological and biological history, including the 65 million-year-old dinosaur extinction event associated with the great chicxulub impact crater."
teams studied zircon crystals in particular because they retain evidence of shock metamorphism, which is caused by shock pressure and high temperatures associated with impact events. The dated crystals were small, about the thickness of a human hair.
"The key to our investigation was zircon – or to be more precise: zirconium silicate – crystals that we found in the oceanic sediments in a borehole, located nearly 400 kilometers (250 miles) northeast of the impact site, in the Atlantic," says co-author Wartho, who started the study when she was lab manager at Mass Spectrometry Lab at ASU.
For this study, Biren worked with co-authors Wartho (now working at GEOMAR Helmholtz Center for Ocean Research Kiel), Van Soest and Hodges to prepare samples for analysis and to date zircon crystals using the uranium-thorium-helium dating method. Biren then identified and processed shocked zircon fragments for imaging and chemical analysis with an electron microbe.
"This research adds a tool for investigators that date soil impact," Biren said. "Our results show the viability of the uranium-thorium-helium dating method for use in similar cases, where shocked materials were discarded from the crater and then allowed to cool rapidly, especially in cases where the sample size is small."
Transformations to Granular Zircon Revealed: Meteor Crater, Arizona
M. B. Biren et al, (U-Th) / He zircon dating of Chesapeake Bay distal impact ejecta from ODP site 1073, Meteoritics & Planetary Science (2019). DOI: 10.1111 / maps.13316
Scientists study the largest impact crater in the United States, buried for 35 million years (2019, August 13)
retrieved August 13, 2019
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