Sixty-six million years ago, a giant rock from space crashed on Mexico’s Yucatan Peninsula, throwing up a cloud of dust big enough to envelop the planet in darkness for at least several months.
Scientists now generally accept that this was the event that killed the dinosaurs and a new study led by researchers at Scripps Institution of Oceanography, UC San Diego, establishes that the same event killed off some of the world’s fish – but not all of them.
The researchers found that the crash devastated marine life in the Atlantic Ocean for millions of years, but were surprised to also find that life marched on in the Pacific Ocean virtually unscathed.
“In the Pacific, the fish population doesn’t even hiccup but they get pummeled in the Atlantic,” said Richard Norris, a Scripps paleooceanographer and co-author of the study appearing Aug. 27 in the journal Nature Geoscience.
The study breaks new ground in countering the common assumption among scientists that the cataclysmic event, called the Cretaceous/Paleogene mass extinction, must have killed off life everywhere. It doesn’t, however, provide an answer to why marine life in the Pacific dealt with it so much better than their counterparts in the Atlantic.
It could be, said lead author Elizabeth Sibert, a graduate student in Norris’s lab at Scripps Oceanography, that the Atlantic was simply closer to the impact zone and therefore had to deal with more dust than the Pacific Ocean did. She and Norris suggested another possibility is that the period of darkness, which lasted only on a scale of months to a few years, was enough to throw off circulation in the Atlantic Ocean for several million years despite its brevity. That change in ocean circulation could have triggered a localized prolonged drop in fish abundance that did not occur in the Pacific.
The researchers analyzed fossil records left behind from the event. From a site in Gubbio, Italy containing sediments from that period in history associated with the ancient Atlantic Ocean, they were able to see even at fine time scales how marine life fared by observing the teeth and scales of the ancient fish embedded in the sediment. The team, which includes Pincelli Hull, a former student of Norris now at Yale University, performed the same analysis on fossil remains cored from deep-ocean sediment in the Pacific.
In both ocean basins, the record shows that the event wiped out many of the primary producers – small phytoplankton and zooplankton – at the base of the ocean food web and the top predators – such as mosasaurs and ammonites – at the other end of the web. Only in the Pacific Ocean, though, did they find that the smaller fishes in the middle survived, perhaps supported by phytoplankton that also survived the blast.
Sibert said that possible next steps toward explaining this ancient story of survival could include an examination of which specific fish groups survived and which didn’t. Also, she said, researchers may want to look at other major climate events from the past 100 million years to see what kinds of marine species were better able to withstand those events as well. Such detailed records are relatively hard to come by at present, she said.
“We have great records of phytoplankton but a one-celled organism doesn’t respond the same way fish do,” she said.
Norris said that one takeaway from the paper that makes the events of the mass extinction analogous to present-day climate change is that it shows how even brief events can have very long-lasting effects.
“This is an example of a tipping point being exceeded and we worry about tipping points happening in relation to climate change,” he said. “When those triggers get thrown, the earth can be tossed into a completely different state that can affect life for millions of years.”
NASA funded much of the research, which was supported by the International Ocean Discovery Program. The American Philosophical Society funded Sibert’s work at Gubbio, Italy.
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