Boring is beautiful when you’re studying a calamity, especially one as spectacular as the mass extinction that wiped out the dinosaurs. That’s because exciting sediments, full of variations and gaps, make it hard to disentangle the extinction signal from the noise of natural variability.
So you could say that James Witts, of the University of Leeds in the UK, lucked-out with an especially boring batch of sediments in Seymour Island on the Antarctic Peninsula (the part on the map that points up to South America). His study, recently published in the journal Nature Communications, catches the extinction of marine life in one of the most detailed records ever published for the end-Cretaceous. As Witts describes it:
The sedimentology is consistently, remarkably boring. More than 1,000 meters of sandy silt and silty sand!
It took about 4 million years to deposit that sand in a sea bed over the crucial time before, during, and after the mass extinction. In all that time fossils accumulated steadily – mollusks, sharks, corals, crustaceans, marine reptiles – until suddenly they all stopped. The sediments continued steadily accumulating, but all the Cretaceous fossils disappeared within a few meters of each other. Rare fossils disappeared sooner, common fossils disappeared later, but there’s a fossil-free gap right below the layer that marks the end of the Cretaceous.
It doesn’t look like the environmental setting over the extinction itself changed significantly, so we can discount any rapid changes in water depth having an effect on the pattern of extinction we see from the fossil record.
Then there is a layer of dead fish.
Antarctic marine life in the late Cretaceous, including the paperclip-shaped ammonite “Diplomoceras.” Illustration: James McKay
The remarkable thing about these fish is that they are mostly whole and un-scavenged, either because the usual scavengers were extinct, or because of low oxygen conditions triggered by microbe blooms in an ocean whose food web was unbalanced by the extinction:
The anoxia story was a surprise to us. In such a shallow setting it appears unusual. I imagine a scenario like parts of the Gulf of Mexico today, with input of material from rivers driving changes in ocean oxygen on a rapid (maybe seasonal?) scale.
A final fossil find is very unlikely to be the last individual of that species that ever lived on Earth, so it’s likely that a species lived on for some time after its last fossil was preserved. Witts used statistics on how frequently fossils of a given species show up in the sediments, to predict the likely gap between fossils if the species was still alive. He found that, in all likelihood, all the last Cretaceous species continued to live until within about 30,000 to 70,000 years of the end of the Cretaceous, and plausibly even less than that. This shows that the mass extinction was geologically “fast,” rather than a drawn-out attrition of species over hundreds of thousands of years.
Ultimately, one of the problems with studies on the end-Cretaceous extinction is that we are pushing the limits of the resolution of many fossil records, as well as the proxies for environmental change.
Recent rock dating has put the Chicxulub asteroid impact at the same time as massive eruptions in India, and shows that the main eruptions were alsogeologically rapid. So even though Witts’ study used a combination of magnetic field reversals, strontium isotope values, and fossil population changes to tie down the dates for his sediments, each of those has an uncertainty too big for a precise match to either the impact or the eruptions.
Also, the iridium measurements at Seymour Island used to mark the time of the asteroid impact date back to 1994, and come from a different location than Witts’ study, which may add further date uncertainty.
But dates aside, Witts’ catalogue of over 6,000 fossils shows no hint of increasing environmental stress before the extinction, despite an episode of climate warming that lasted over a million years, before cooling at the end of the Cretaceous. This is in contrast to studies elsewhere, which found signs of adeteriorating environment for a few hundred thousand years before the extinction; in Antarctica, life seems to have been boringly oblivious until the end.
In the ongoing debate over the ultimate killer of Cretaceous life, these findings support a geologically rapid cause (anything from tens of millennia to instant), rather than slowly building environmental stress:
Posted by howardlee on Wednesday, 15 June, 2016
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