Chicxulub Impact Occurred during Northern Spring or Summer: Study

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About 66 million years ago, a 10-km-wide asteroid crashed into Earth near the site of the small town of Chicxulub in what is now Mexico. The impact unleashed an incredible amount of climate-changing gases into the atmosphere, triggering a chain of events that led to the extinction of non-avian dinosaurs and 75% of life on the planet. According to an histological and histo-isotopic analysis of a unique impact-triggered assemblage of fossil fish from North Dakota, the United States, the Chicxulub impact occurred during boreal spring/summer, shortly after the spawning season for fish and most continental species.

The Chicxulub impact 66 million years ago generated a tsunami-like wave in an inland sea that killed and buried fish, mammals, insects and a Triceratops, the first victims of a cataclysm that led to Earth’s last mass extinction. Image credit: Robert DePalma.

“Time of year plays an important role in many biological functions such as reproduction, feeding strategies, host-parasite interactions, seasonal dormancy, and breeding patterns,” said Dr. Robert DePalma, a researcher with Charles E. Schmidt College of Science at Florida Atlantic University and the University of Manchester.

“Hence, it is no surprise that the time of year for a global-scale hazard can play a big role in how harshly it impacts life.”

“The seasonal timing of the Chicxulub impact has therefore been a critical question for the story of the end-Cretaceous extinction. Until now, the answer to that question has remained unclear.”

Dr. DePalma and colleagues examined the Tanis locality in southwestern North Dakota to understand the inner workings of the extinction event.

“This unique site in North Dakota had yielded a wealth of new and exciting information,” said Dr. Anton Oleinik, a researcher at Florida Atlantic University.

“Field data collected at the site, after hard work that went into analyzing it, provided us with new incredibly detailed insight of not only what happened at the Cretaceous-Paleogene boundary, but also exactly when it happened.”

The unique structure and pattern of the growth lines in fossil fish bones from the Tanis site showed that all of the examined fish died during the spring-summer growth phase.

The isotopic analysis of the growth lines provided independent confirmation of this, showing a yearly oscillation that also terminated during the spring-summer growth.

The researchers further supported their findings by overlaying multiple additional lines of evidence.

Examination of juvenile fossil fish was supported in part by cutting-edge synchrotron-rapid-scanning X-ray fluorescence (SRS-XRF), providing a novel way of seasonally dating the deposit.

Comparing the sizes of the youngest fish to modern growth rates enabled the scientists to predict how long after hatching the fish were buried.

Comparing this to known modern spawning seasons enabled them to deduce what seasonal range was represented by the deposit at Tanis — spring to summer, just as indicated by the bones.

“The beauty of any great discovery such as this is that it is a chance to give back to the scientific community, and to the world,” Dr. DePalma said.

“It not only answers important questions, but also sparks new minds to reach forward and achieve.”

The study was published in the journal Scientific Reports.

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R.A. DePalma et al. 2021. Seasonal calibration of the end-cretaceous Chicxulub impact event. Sci Rep 11, 23704; doi: 10.1038/s41598-021-03232-9

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