John Maisano/University of Texas
Usually, new studies of the mass extinction of dinosaurs at the end of the Cretaceous period paint a different picture of how terrible it was. But if you’re a glass-half-full person, it’s interesting to think about how quickly life recovered — not necessarily on timescales relevant to an individual organism, but in terms of species and ecosystems.
A research cruise recently drilled a rock core in Chicxulub Crater where an asteroid fell 66 million years ago. Drilling out the deeper rock helped to show test models of the rock’s impossible jello-like behavior during impact, but there are also sedimentary rocks on top that formed some time after the impact.
Researchers looking elsewhere have found that life in the Gulf of Mexico and the North Atlantic recovered more slowly than in other ocean basins, for some 300,000 years. One hypothesis to explain this is that the concentrations of toxic metals were high near the impact crater. If that’s true, the recovery should be slowest at Ground Zero. But that’s not what a huge team led by the University of Texas at Christopher Lowery in Austin discovered when they examined rocks that may be the first a few years after the impact of the asteroid.
Cake with layers
They found two rock layers that span the first 200,000 years after the impact. Directly above the tangled chaos of crater rock is a 76 centimeter thick brown limestone, above which you’ll find a thicker white limestone.
The central question is how much time is recorded in that brown layer of limestone. The first limitation is that the fossilized life just above the white limestone shows that the brown layer took no more than 30,000 years to form. But the researchers also measured helium-3 concentrations in the core, which actually gives an indication of how quickly the sediment accumulated before turning to rock. Those measurements indicate the brown layer spans less than 1,000 years. That’s the shortest time the technique can discern, so this leaves open the possibility that it… a lot less than 1000 years.
Which brings us to the researchers’ last step. After the impact, a ton of accumulated mud would have settled on the seabed. The brown limestone shows some very fine layers formed by energetic currents that you wouldn’t normally find, like the sloshing after an asteroid crashes into the ocean. That leads to the conclusion that this brown limestone is just that settling mud, with normal sediment accumulation not resuming until much later with the white limestone. If that’s true, physics tells us that the brown layer would have settled in about six years.
in the mud
That makes the fossils in these rocks particularly interesting. While the lower part of the brown limestone contains only older fossils that were kicked up with the rest of the mud, the upper part of this layer contains small worm holes that show that things lived while the mud was still settling. Small fossilized foraminifera known to have survived the mass extinction also appear in the upper part of this layer. That would mean that there would be life in the crater again within a few years.
In the white layer of limestone above, which probably begins to make history 30,000 years after the impact, the diversity and number of small fossils increases significantly. And measurements of barium, titanium and iron show that the mass of growing life was also much higher. This evidence paints a picture of a healthy, productive ecosystem that had essentially returned to “normal” function (minus the species that have disappeared, of course). That’s much faster than the 300,000 years it took elsewhere in the Gulf of Mexico or the Atlantic.
If life at Ground Zero returned faster than further from the crater, then there was no poisonous plume near the impact that poisoned the seas and slowed the recovery of life in the region. Instead, the researchers say, the variability from place to place must be related to a complex set of ecosystem factors, such as different conditions and species competition.
And despite the incredible violence of the impact — enough to vaporize some rock, scramble the rest, and likely spark new hydrothermal vents — some of the luckier species seem to have called the crater itself home much sooner than we might have. guessed.
Nature2018. DOI: 10.1038/s41586-018-0163-6 (About DOIs).