“The bigger they are, the harder they fall” might have applied to some individual behemoths during the hey-day of the dinosaurs, but it also held true for the rock from space that did them in.
As best we can make out, a 10 kilometer wide asteroid struck the Earth along the coast of the Yucatán Peninsula back then and produced a shockwave and fireball of unfathomable scale. As tsunamis swept across the Gulf of Mexico and wildfires raged, huge amounts of sulfur (from rock vaporized by the impact) and soot were lifted into the air, blocking sunlight from reaching the surface. With the fires followed by cold and greatly diminished photosynthesis (sunlight might have dropped by 80 percent), ecosystems collapsed.
To make things worse, when the skies cleared after a few years to a decade, the sulfur may have acidified the surface ocean. The long-lived greenhouse gases that came from the vaporized rock took over, producing sustained warming for millennia at least. Oh, and incredibly massive volcanic eruptions on the Indian subcontinent were already messing with Earth’s climate before the impact. It was a cruel pendulum of extremes.
Some of this story is clearly written in the geologic record, but other details come from a theoretical understanding of the consequences of an impact of this scale. Take that long winter, for example. We expect it must have happened because of the amount of material that would have gotten kicked up into the atmosphere, but it’s difficult for the rocks to record a change in climate that would have lasted a few decades at most. It gets even trickier when you remember that many of the plankton that can contain climate indicators were busy going extinct. A study led by Utrecht University’s Johan Vellekoop, however, has managed to extract some details from rocks along the Brazos River in Texas that tell us about that "impact winter".
Those rocks include sediments deposited in the aftermath of the impact. There’s a layer of tsunami-deposited sand beneath finer-grained sediments that settled slowly to the seafloor after that. It’s possible that the temperature contrast between the colder atmosphere and still-warm ocean fueled strong storms, and this may have stirred some of that sediment back up. That complicates the effort to figure out how long it took for those layers to be deposited.
Still, other research has estimated that these storms should have died down within less than a century, so the layers were interpreted to represent the first few decades immediately following the impact event.
Conveniently, there’s a climate indicator they could look for in those rocks. Although the calcium-carbonate-armored plankton usually used for this kind of thing are missing, the researchers could turn to an organism that leaves no preservable body parts. Microbes called Thaumarcheota may not build shells, but the composition of the lipids in their cell membranes depends on water temperature. And they’re chemosynthetic, deriving energy from ammonia rather than sunlight-driven photosynthesis. Those lipid molecules are pretty resilient and can be analyzed in marine sediments—even those that have turned to rock.
The analysis indicated local sea surface temperatures of 30-31°C prior to the impact. The data from the post-tsunami layers are a little variable (possibly due to the stirring up of the sediment by storms), but it records temperatures fully 2-7°C cooler than that. After that, temperatures rose to 1-2°C higher than they had been before the impact.
That matches the expected pattern of climatic upheaval and is the first direct evidence of a phase of colder temperatures, although there is also some evidence of cool-water plankton moving towards the equator. That helps increase our confidence that the terrible story we’re telling is the right one.
PNAS, 2014. DOI: 10.1073/pnas.1319253111 (About DOIs).
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