About 66 million years ago, an asteroid the size of a city crashed into what is now the Yucatan Peninsula, ushering in a long period of darkness that extinguished the non-birds dinosaurs. Researchers have long debated exactly what aspect of this event, known as the Chicxulub Impact, caused the rapid climate change. Were they sulfur particles from evaporated sedimentary rocks? Soot from the ensuing global fires? Or dust from the Yucatan rock itself?
Now new research claims dust is the deadliest aspect of the impact. While soot and sulfur contributed to the global darkness and the shock winter that stopped photosynthesis for almost two years, fine granite dust dispersed by the impact remains in the atmosphere for up to 15 years. The asteroid impact set off an extinction spiral that killed 75% of all species on the planet.
“We found that the dust-induced disruption of photosynthetic activity was enormous, much larger than we had expected before this study,” lead researcher Jem Burke Sennellpostdoctoral fellow in planetary sciences at the Royal Observatory of Belgium, told Live Science.
The space rock that crashed into Earth at the end of Cretaceous period (145 million to 66 million years ago) left behind a crater 110 miles (180 kilometers) wide and 12 miles (20 kilometers) deep. The material that was in this gap quickly entered the atmosphere. In the first few hours after the impact, partially molten spherical particles of rock began to rain back to the surface hundreds of miles from the impact.
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But there were finer particles as well. In the new study, published today (October 30) in the journal Nature Geoscience, Sennell and his colleagues used data from a site in North Dakota called Tanis, where a 4.3-foot-thick (1.3-meter) rock preserves a snapshot of the debris rain after the impact. The researchers measured the sizes of the grains in this layer to determine what was ejected into the atmosphere by the collision. They then entered this information into a computer model of the global atmosphere.
The simulation suggests that within about a week, dust grains between about 0.8 and 8 micrometers in diameter circled the globe, essentially blanketing the atmosphere. These particles are smaller than the diameter of a typical human hair. Today, on Environmental Protection Agency lists particles less than 10 micrometers in diameter as “respirable particles” because they can easily enter the lungs.
The sudden blanketing of the atmosphere halted photosynthesis on Earth for about two weeks, the researchers report. It did not return for 620 days (about 1.7 years), and it took at least four years for plants to start photosynthesizing at the rate seen before the impact. (About half of the plant species disappeared after the impact of Chicxulub, researchers estimate (but plants do better than animals because their seeds can wait in dormancy for better conditions to sprout again.) The dust’s longevity proved disastrous for life: until sulfur particles began to fall out of the atmosphere within about 8.5 years, dust particles of this size can remain in the atmosphere for 15 years.
“The combined emissions of all these discharges lead to a drop in surface temperature of up to 15 degrees Celsius [27 degrees Fahrenheit]which are mostly driven by sulfur and dust,” Senel said.
The results are intriguing, he said Clay Tabora University of Connecticut paleoclimatologist who was not involved in the study, and information on the size of the dust from North Dakota will improve simulations of the post-impact climate.
But the study is unlikely to be the last word on whether soot particles, dust or sulfur contributed most to the late Cretaceous mass extinction. Different studies use different climate models, which can affect the results, and differences between the models used may explain disagreements among researchers about whether soot or dust had the biggest global impact, Tabor told Live Science.
“There are many important processes that can affect aerosol optical properties and atmospheric lifetime, but these processes can be difficult to accurately simulate, especially in the extreme case of the Chicxulub impact,” he said.