Massive asteroid impacts have not changed Earth’s climate in the long term

Two massive asteroids hit Earth around 35.65 million years ago but did not cause lasting changes to Earth’s climate, according to a new study by UCL researchers.

The rocks, both several miles wide, struck Earth about 25,000 years apart, leaving behind the 60-mile (100 km) Popigai Crater in Siberia, Russia, and the 25-55 mile (40-85 km) large crater in the Chesapeake Bay in the United States – the fourth and fifth largest known asteroid craters on Earth.

The new study, published in the journal Communications Earth & Environment, found no evidence of permanent climate change in the 150,000 years following the impacts.

Researchers inferred past climates by studying isotopes (types of atoms) in the fossils of tiny, armored organisms that lived in the sea or on the seafloor at the time. The isotope pattern reflects how warm the water was when the organisms were alive.

Co-author Professor Bridget Wade (UCL Earth Sciences) said: “The remarkable thing about our results is that there were no real changes after the impacts.” We expected the isotopes to shift in one direction or the other, which is due to warmer or cooler water, but that didn’t happen. These big asteroid impacts happened and in the long run our planet seemed to be fine as it was.

“However, our study would not have captured short-term changes over tens or hundreds of years because sampling occurred every 11,000 years. On a human time scale, these asteroid impacts would be catastrophic. They would create a massive shockwave and tsunami, widespread fires would occur, and large amounts of dust would be thrown into the air and block sunlight.

“Modeling studies of the larger Chicxulub impact that killed the dinosaurs also suggest climate change over a much shorter period of time, less than 25 years.”

“So we still need to know what’s coming and fund missions to prevent future collisions.”

The research team, including Professor Wade and MSc Earth Sciences student Natalie Cheng, analyzed isotopes in over 1,500 fossils of single-celled organisms called foraminifera, both those that lived near the sea surface (planktonic foraminifera) and on the seafloor (benthic foraminifera).

These fossils were between 35.5 and 35.9 million years old and were found within three meters of a rock core beneath the Gulf of Mexico as part of the scientific Deep Sea Drilling Project.

The two large asteroids that struck during this time were estimated to be 3–5 miles (5–8 km) in size and 2–3 miles (3–5 km) wide. The larger of the two, which created Popigai Crater, was about as wide as Everest is tall.

In addition to these two impacts, existing evidence suggests that three smaller asteroids also struck Earth during this period – the Late Eocene – suggesting a disturbance in our solar system’s asteroid belt.

Previous studies of the climate at that time had not produced conclusive results, the researchers noted. Some linked the asteroid impacts to accelerated cooling, others to episodes of warmer temperatures.

However, these studies were conducted at a lower resolution, examining samples at intervals greater than 11,000 years, and their analysis was more limited – for example, only species of benthic foraminifera that lived on the seafloor were examined.

By using fossils that lived at different ocean depths, the new study provides a more comprehensive picture of how the oceans responded to the impact events.

The researchers examined carbon and oxygen isotopes in several species of planktonic and benthic foraminifera.

They found isotope shifts about 100,000 years before the two asteroid impacts, suggesting a warming of the sea surface by about 2 degrees Celsius and a cooling of deep water by 1 degree Celsius. However, no displacements were detected either at the time of the impacts or afterwards.

The researchers also found evidence of the two large impacts in the rock in the form of thousands of tiny glass droplets or silica. These are created after rock containing siliceous acid is vaporized by an asteroid. The silica enters the atmosphere but solidifies into droplets as it cools.

Co-author and MSc Earth Sciences graduate Natalie Cheng said: “Given that the Chicxulub impact likely resulted in a major extinction event, we were curious to investigate whether what looked like a series of significant asteroid impacts during the Eocene also caused long-lasting climate changes.” . We were surprised to find that there was no significant climate response to these impacts.

“It was fascinating to read about Earth’s climate history through the chemistry preserved in microfossils. It was particularly interesting to work with our selection of foraminiferal species and discover beautiful examples of microspheres.”

Microscope image of silica droplets or microspheres found in rock. Photo credit: Natalie Cheng / Bridget Wade

The study was funded by the UK Natural Environment Research Council (NERC).

There are no paleoclimatic anomalies associated with the late Eocene extraterrestrial impacts, Communication Earth & Environment (free access)

Astrobiology