London: A new study has determined that the extinctions that happened 65 million years were more severe in northern oceans than the southern ones.
The extraterrestrial body that slammed into Earth 65 million years ago is best known for killing off the dinosaurs.
But it also snuffed out more than 90 percent of the tiny plankton species that made up the base of the food web in the oceans.
By sifting through geological records of ancient sediments from around the globe, palaeoceanographers have culled clues about how the impact caused so much havoc.
According to a report in Nature News, the researchers said 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 percent 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 percent 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 percent of its nannoplankton species.
"There`s an incredibly strong correlation between extinction rate and latitude,"
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," said Bralower.
Their slow recovery in the north would have impeded the resurgence of the whole northern oceanic food web.
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.