The researchers report in Nature Geoscience1 today that the most severe extinctions of nannoplankton happened in the northern oceans and that the ecosystems there took 300,000 years to recover, much longer than in the south. Given that pattern, the researchers speculate that the direction of the impact caused long-lasting darkness in the Northern Hemisphere and metal-poisoning in the northern oceans.
Nannoplankton with calcium-based shells were the primary photosynthetic producers in the oceans until 65 million years ago, at the boundary between the Cretaceous and Palaeogene periods. But 93% of those species went extinct - along with ammonites, large marine reptiles such as the plesiosaurs, and all the dinosaurs. The extinctions have been linked to the Chicxulub impact crater, which is buried beneath the Yucatán peninsula in Mexico.
To trace the geographical distribution of the extinctions, Timothy Bralower from Pennsylvania State University in University Park and his colleagues examined published records that analysed fossil nannoplankton at 17 sites spread across the globe. They found that up to 98% of species went extinct in the northern oceans, whereas rates in the southern ocean were lower; the most southerly site at the time lost 73% of its nannoplankton species. "There's an incredibly strong correlation between extinction rate and latitude," says Bralower.
The Southern and Indian oceans fared better in other ways as well. Species diversity was less affected, and the normal species assemblages returned almost immediately. But it took up to 300,000 years after the impact for species diversity to recover in the northern oceans, according to the researchers.
Phytoplankton probably influenced the restoration of the entire marine ecosystem, says Bralower. Their slow recovery in the north would have impeded the resurgence of the whole northern oceanic food web.
Other researchers have previously detected latitudinal differences in the catastrophe. North American land plants were particularly hard hit compared with species on southern continents. Geological evidence suggests that the body that hit Earth was travelling from the southeast to the northwest. That kind of collision, along with the planet's rotation, would have thrown up more debris over the Northern Hemisphere and blocked out the sun for an extended period.
"Suppression of photosynthesis and darkness would explain the extinctions and the diversity drop," Bralower says.
But the delayed recovery in the north was a mystery. "You'd expect that once it got light again, they would start flourishing again. Why would it take up to 300,000 years to come to normal diversity in the Northern Hemisphere?" comments Bralower.
The team tested several hypotheses, such as ocean cooling and acidification, but decided that metal poisoning is the only mechanism that could have caused that delay. They surmise that the impact spewed heavy metals into the sky and that these landed mostly in the northern oceans. High concentrations of metals can be toxic to living organisms and inhibit reproduction. And the suppression of photosynthesis in the north would have prevented plankton from taking up the toxic metals and clearing them out of the surface ocean.
"Until this study, you could imagine no environmental mechanism to inhibit the recovery of plankton after light conditions were restored. So what we've done is put numbers on how long it took for critical parts of the food chain to recover," Bralower says. "It was staggeringly long."
Although several geoscientists are impressed by the latitudinal differences reported in the new paper, they are sceptical about the metal hypothesis.
"The reality is, the ocean is one big ocean," says Steven D'Hondt, an oceanographer at the University of Rhode Island in Narragansett. He says the ocean would mix in a thousand years and spread any heavy metals around. To explain the different rates of recovery, he says, he would "poke around in evolutionary ecology to find possible explanations".
James Zachos, a palaeoceanographer from the University of California, Santa Cruz, agrees. "I suspect these findings might be telling us more about the ecosystem recovery process from severe extinction, rather than about the extinction process," he says.
Previous studies of marine molluscs have shown that widespread lineages survived the extinction better than ones with fewer species and more restricted ranges, according to David Jablonski, a palaeontologist at the University of Chicago in Illinois. The mollusc data2 suggest that extinction intensity was fairly uniform across the globe, says Jablonski. But he adds: "If you squint your eyes a little at my data, high southern-latitude mollusc extinction is a little milder than elsewhere."
According to Bralower, species that lived in the high southern latitudes were adapted to low light and high metal concentrations and this allowed them to survive the immediate effects of the impact. "Adaptation is a critical factor in survivorship and the effect of the impact is built on top of that," he says.
Even if nannoplankton in the south suffered less than those in the north, it was "still a hell of a hit", says Jablonski. "No matter what, it was a bad time to be phytoplankton at the end of the Cretaceous period anywhere in the world."
1. Jiang, S., Bralower, T. J., Patzkowsky, M. E., Kump, L. R. & Schueth, J. D. Nature Geosci. online publication doi:10.1038/ngeo775 (2010).
2. Raup, D. M. & Jablonski, D., Science 260, 971-973 (1993).