Life returned to crater of Cretaceous asteroid in the blink of an eye

Attachment 10657Usually, new studies of the dino-killing mass extinction at the end of the Cretaceous provide another view into just how bloody awful it was. But if you’re a glass-half-full kind of person, it’s interesting to think about how quickly life recovered—not on timescales relevant to an individual organism, necessarily, but in terms of species and ecosystems.A research cruise recently drilled a rock core into the Chicxulub Crater where an asteroid fell 66 million years ago. Coring the deeper rock helped show test models of the impossibly jello-like behavior of the bedrock during the impact, but there are also sedimentary rocks on top that were formed some time after the collision.

Researchers who have looked elsewhere have noticed that life recovered more slowly in the Gulf of Mexico and North Atlantic than in other ocean basins, taking about 300,000 years. One hypothesis to explain this is that concentrations of toxic metals were high near the impact crater. If that’s true, recovery should be slowest at ground zero. But that’s not what a huge team led by the University of Texas at Austin’s Christopher Lowery found when they examined rocks that might record the first few years after the asteroid hit.
Layer cake

They found two rock layers that, between them, cover the first 200,000 years or so after impact. Immediately above the jumbled chaos of crater rock, there is a 76-centimeter-thick brown limestone, above which you find a thicker white limestone.

The question of how much time is recorded in that brown layer of limestone is key. The first constraint is that the fossilized life just above in the white limestone shows that the brown layer cannot have taken more than 30,000 years to form. But the researchers also measured helium-3 concentrations in the core, which actually provides an indication of how quickly the sediment piled up before it became rock. Those measurements point to the brown layer spanning less than 1,000 years. That’s the shortest period of time the technique can discern, so this leaves open the possibility that it’s much less than 1,000 years.

That brings us to the researchers’ final step. After the impact, a ton of kicked-up mud would have settled to the seafloor. The brown limestone shows some very fine layering shaped by energetic currents you wouldn’t normally find—like the sloshing after an asteroid smacks into the ocean. That leads to the conclusion that this brown limestone is just that settling mud, with normal sediment accumulation only resuming much later with the white limestone. If that’s true, physics tells us the brown layer would have settled out in just six years or so.
In the mud

That makes the fossils in these rocks particularly interesting. While the lower part of the brown limestone contains just older fossils that were kicked up with the rest of the mud, the upper part of this layer contains tiny worm burrows that shows things were living there while the mud was still settling. Tiny fossilized foraminifera species known to have survived the mass extinction also start showing up in the upper part of this layer. That would mean life returned to the crater within a matter of years.

In the white layer of limestone above, which probably begins recording history 30,000 years after the impact, the diversity and number of tiny fossils increases considerably. And measurements of barium, titanium, and iron show that the mass of growing life was much higher, as well. This evidence paints a picture of a healthy, productive ecosystem that had basically returned to "normal" function (minus the species that disappeared, of course). That's much faster than the 300,000 years it took elsewhere in the Gulf of Mexico or Atlantic.

If life returned faster at ground zero than it did farther from the crater, then there wasn’t some sort of toxic 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 relate to a complex set of ecosystem factors, like differing conditions and species competition.

And despite the incredible violence of the impact—enough to vaporize some bedrock, jumble the rest, and probably induce new hydrothermal vents—some of the luckier species seem to have called the crater itself home much sooner than we might have guessed.