Two massive asteroids hit Earth around 35.65 million years ago, but did not lead to any lasting changes in the Earth's climate, according to a new study by UCL researchers.
The rocks, both several miles wide, hit Earth about 25,000 years apart, leaving the 60-mile (100km) Popigai crater in Siberia, Russia, and the 25-55 mile (40-85km) 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 a lasting shift in climate in the 150,000 years that followed the impacts.
The researchers inferred the past climate by looking at isotopes (atom types) in the fossils of tiny, shelled organisms that lived in the sea or on the seafloor at the time. The pattern of isotopes reflects how warm the waters were when the organisms were alive.
Co-author Professor Bridget Wade (UCL Earth Sciences) said: "What is remarkable about our results is that there was no real change following the impacts. We expected the isotopes to shift in one direction or another, indicating warmer or cooler waters, but this did not happen. These large asteroid impacts occurred and, over the long term, our planet seemed to carry on as usual.
"However, our study would not have picked up shorter-term changes over tens or hundreds of years, as the samples were every 11,000 years. Over a human time scale, these asteroid impacts would be a disaster. They would create a massive shockwave and tsunami, there would be widespread fires, and large amounts of dust would be sent into the air, blocking out sunlight.
"Modelling studies of the larger Chicxulub impact, which killed off the dinosaurs, also suggest a shift in climate on a much smaller time scale of less than 25 years.
"So we still need to know what is coming and fund missions to prevent future collisions."
The research team, including Professor Wade and MSc Geosciences student Natalie Cheng, analysed isotopes in over 1,500 fossils of single-celled organisms called foraminifera, both those that lived close to the surface of the ocean (planktonic foraminifera) and on the seafloor (benthic foraminifera).
These fossils ranged from 35.5 to 35.9 million years old and were found embedded within three metres of a rock core taken from underneath the Gulf of Mexico by the scientific Deep Sea Drilling Project.
The two major asteroids that hit during that time have been estimated to be 3-5 miles (5-8km) and 2-3 miles (3-5km) wide. The larger of the two, which created the Popigai crater, was about as wide as Everest is tall.
In addition to these two impacts, existing evidence suggests three smaller asteroids also hit Earth during this time – the late Eocene epoch – pointing to a disturbance in our solar system's asteroid belt.
Previous investigations into the climate of the time had been inconclusive, the researchers noted, with some linking the asteroid impacts with accelerated cooling and others with episodes of warmer temperatures.
However, these studies were conducted at lower resolution, looking at samples at greater intervals than 11,000 years, and their analysis was more limited – for instance, only looking at species of benthic foraminifera that lived on the seafloor.
By using fossils that lived at different ocean depths, the new study provides a more complete picture of how the oceans responded to the impact events.
The researchers looked at carbon and oxygen isotopes in multiple species of planktonic and benthic foraminifera.
They found shifts in isotopes about 100,000 years prior to the two asteroid impacts, suggesting a warming of about 2 degrees C in the surface ocean and a 1 degree C cooling in deep water. But no shifts were found around the time of the impacts or afterwards.
Within the rock, the researchers also found evidence of the two major impacts in the form of thousands of tiny droplets of glass, or silica. These form after silica-containing rocks get vaporised by an asteroid. The silica end up in the atmosphere, but solidify into droplets as they cool.
Co-author and MSc Geosciences graduate Natalie Cheng said: "Given that the Chicxulub impact likely led to a major extinction event, we were curious to investigate whether what appeared as a series of sizeable asteroid impacts during the Eocene also caused long-lasting climate changes. We were surprised to discover that there were no significant climate responses to these impacts.
"It was fascinating to read Earth's climate history from the chemistry preserved in microfossils. It was especially interesting to work with our selection of foraminifera species and discover beautiful specimens of microspherules along the way."
The study received funding from the UK's Natural Environment Research Council (NERC).
