Tiny phytoplankton in the oceans of the Northern Hemisphere had far larger extinction rates during the mass extinction event 65 million years ago than those living in the Southern Hemisphere, according to a paper published online this week in Nature Geoscience. The recovery of the Northern Hemisphere phytoplankton also occurred significantly later than recovery in the southern oceans.
Populations of phytoplankton smaller than 20 micrometres were decimated during the Cretaceous/Tertiary mass extinction, which is linked to an impact event. Timothy Bralower and colleagues propose that the clouds of debris from the impact ― which would have blocked the sunlight that these phytoplankton needed to grow, and poisoned them as the metal-laden dust fell to the ocean surface ― were concentrated in the Northern Hemisphere, leading to the higher extinction rates. The team also suggests that the recovery of marine diversity in the north may have been hindered by the phytoplankton's slower start in this region.