Classic crime mysteries work because almost every character ends up being a murder suspect. The demise of the non-avian dinosaurs looks a lot like that. The Chicxulub effect and its aftereffects have created a large pool of potentially deadly suspects. from? A giant fireball and a massive tsunami? Wild fluctuations in climate? Global forest fires? Black skies hinder photosynthesis? Everything mentioned above?
Modeling of these effects, combined with data on the pattern of extinction, has led to differing opinions about what proved decisive regarding the extinction of many species. In the latest look at the end-Cretaceous extinction, a team of scientists based largely in Brussels revisited sediments laid down in the wake of the impact, and found that much of the debris came from fine dust. When this dust is introduced into climate models, global temperatures drop by up to 25 degrees Celsius, and photosynthesis stops for about two years.
From dust to dust
There was a lot going on in the atmosphere in the years after the impact. Debris resulting from the impact would have returned to Earth’s atmosphere, burning in the process into fine, sulfur-rich rock particles. The heat from this process could have ignited massive forest fires, adding a lot of soot to the mix. It all originated with debris from the collision that remained within the atmosphere.
This has led to the idea of ”effect winter”, where less sunlight reaches the Earth, causing a significant drop in temperature and possibly halting photosynthesis. The three main components of atmospheric dust — soot, rocky debris and sulfur-rich particles — have been blamed, but modeling has raised questions about whether any of them are present at sufficient levels to cause an impact winter.
To get a better idea of what’s going on, the researchers behind this new paper revisited sediments at a site called Tanis in North Dakota, where tsunami debris washed ashore in the immediate aftermath of the impact. Also found: Shocked quartz crystals that would have arrived at the site directly from the impact within hours. Above all those layers of impact debris is an iridium-rich layer of dust composed largely of silicate material that was ejected from the impact site and gradually settled over the next few years.
The researchers scanned this layer using a laser diffraction imager, which allowed them to estimate the size of the dust particles that formed these deposits. The average particle size turned out to be much smaller than what most research assumed would result from the collision. This smaller size will have an impact on how long the dust remains in the atmosphere, as well as how it interacts with sunlight.
To understand how this affects events following the Chicxulub impact, the researchers linked it to a global climate model.
The greatest joy
The researchers allowed the model to reach a steady state for end-Cretaceous continental formation and atmospheric composition. Depending on the season, this has produced global temperatures between 15° and 19°C. Then they injected a lot of stuff into that atmosphere, based on estimates generated elsewhere: 1018 grams of silicate dust, 1016 grams of soot, and 1017 Grams of sulfur dioxide. They also ran simulations of each of the impact debris individually for comparison.
The effects were dramatic. The climate model’s most extreme cases saw temperatures drop by up to 25 degrees Celsius. Conditions remained “extreme” for at least five years, and temperatures remained below pre-impact conditions for about 20 years.
Each type of material has a distinct half-life in the atmosphere and reacts in different ways to light, both incoming sunlight and infrared radiation traveling from Earth to space. As a result, the effect of a mixture of them is different from the effect of any single type of particle in the atmosphere. The fine dust grains used in this work remained in the atmosphere for more than twice as long as the coarser dust explored in previous models.
In addition to the dramatic change in climate, researchers estimate that it takes less than two weeks after the impact of photosynthesis to stop globally. It stays away from the world for at least 1.7 years when a partial return occurs during the Southern Hemisphere summer. (This is consistent with extinctions being more severe in the Northern Hemisphere.) Photosynthesis remained suppressed for up to four years after the impact.
The findings help explain why scientific crimes are so difficult to solve. It only took a small change in the size of particles blasted aloft at Chicxulub to dramatically alter climate dynamics in the following years. Hopefully, over time, data from other sites will help us sort out the complex events that came after the impact.
Natural Earth Sciences, 2023. DOI: 10.1038/s41561-023-01290-4 (About digital IDs).
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