Dino-Killing Chicxulub Asteroid Inhibited Photosynthesis, Cooled Earth for Up To Four Years
A team of researchers from the National Center for Atmospheric Research, the University of Colorado Boulder and NASA has used a world-class computer model to paint a rich picture of how Earth’s conditions might have looked during the K-Pg extinction.
Paleontologists estimate that more than three-quarters of all species on Earth, including all non-avian dinosaurs, disappeared about 65 million years ago, at the Cretaceous-Paleogene boundary, an event known as the K-Pg extinction.
Evidence shows that the mass extinction occurred at the same time that a 10-km-diameter asteroid, dubbed Chicxulub, hit our planet in what is now the Yucatán Peninsula. The collision would have triggered earthquakes, tsunamis, and even volcanic eruptions.
Researchers also calculate that the force of the impact would have launched vaporized rock high above Earth’s surface, where it would have condensed into small particles known as spherules.
As the spherules fell back to Earth, they would have been heated by friction to temperatures high enough to spark global fires and broil Earth’s surface. A thin layer of spherules can be found worldwide in the geologic record.
“The extinction of many of the large animals on land could have been caused by the immediate aftermath of the impact, but animals that lived in the oceans or those that could burrow underground or slip underwater temporarily could have survived,” said lead author Dr. Charles Bardeen, of the National Center for Atmospheric Research (NCAR).
“Our study picks up the story after the initial effects — after the earthquakes and the tsunamis and the broiling. We wanted to look at the long-term consequences of the amount of soot we think was created and what those consequences might have meant for the animals that were left.”
Dr. Bardeen and co-authors used the NCAR-based Community Earth System Model (CESM) to simulate the effect of the soot on global climate going forward.
The team used the most recent estimates of the amount of fine soot found in the layer of rock left after the Chicxulub impact (15,000 million tons), as well as larger and smaller amounts, to quantify the climate’s sensitivity to more or less extensive fires.
In the simulations, soot heated by the Sun was lofted higher and higher into the atmosphere, eventually forming a global barrier that blocked the vast majority of sunlight from reaching Earth’s surface.
“At first it would have been about as dark as a moonlit night,” said co-author Owen ‘Brian’ Toon, a professor in the Department of Atmospheric and Oceanic Sciences and a research associate in the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder.
While the skies would have gradually brightened, photosynthesis would have been impossible for up to two years, according to the simulations.
Because many of the plants on land would have already been incinerated in the fires, the darkness would likely have had its greatest impact on phytoplankton, which underpin the ocean food chain.
The loss of these tiny organisms would have had a ripple effect through the ocean, eventually devastating many species of marine life.
The researchers also found that photosynthesis would have been temporarily blocked even at much lower levels of soot.
For example, in a simulation using only 5,000 million tons of soot — about a third of the best estimate from measurements — photosynthesis would still have been impossible for an entire year.
In the simulations, the loss of sunlight caused a steep decline in average temperatures at Earth’s surface, with a drop of 50 degrees Fahrenheit (28 degrees Celsius) over land and 20 degrees Fahrenheit (11 degrees Celsius) over the oceans. These freezing temperatures persisted at middle latitudes for 3-4 years.
While Earth’s surface cooled in the study scenarios, the atmosphere higher up in the stratosphere actually became much warmer as the soot absorbed light from the Sun.
The warmer temperatures caused ozone destruction and allowed for large quantities of water vapor to be stored in the upper atmosphere. The water vapor then chemically reacted in the stratosphere to produce hydrogen compounds that led to further ozone destruction. The resulting ozone loss would have allowed damaging doses of UV light to reach Earth’s surface after the soot cleared.
“The large reservoir of water in the upper atmosphere formed in the simulations also caused the layer of sunlight-blocking soot to be removed abruptly after lingering for years,” the authors said.
“As the soot began to settle out of the stratosphere, the air began to cool. This cooling, in turn, caused water vapor to condense into ice particles, which washed even more soot out of the atmosphere.”
“As a result of this feedback loop — cooling causing precipitation that caused more cooling — the thinning soot layer disappeared in just a few months.”
The research appears this week in the Proceedings of the National Academy of Sciences.
Charles G. Bardeen et al. On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections. PNAS, published online August 21, 2017; doi: 10.1073/pnas.1708980114