Geographic controls on nannoplankton extinction across the Cretaceous/Palaeogene boundary

Calcareous nannoplankton, a large group of marine autotrophs that produce carbonate skeletons, were decimated to less than 10% of species during the Cretaceous/Palaeogene boundary mass extinction, 65 million years ago. Although the mass extinction followed an impact event, the exact cause of the nannoplankton mortality is not well understood. Here we assess the timing and spatial variability of nannoplankton extinction by analysing nannofossil counts in Cretaceous/Palaeogene boundary sections from all of the main ocean basins. We find that extinction rates were higher in the Northern Hemisphere oceans, and diversity remained low for 310,000 years. In contrast, Southern Hemisphere oceans showed lower extinction rates, and a nearly immediate recovery of normal nannoplankton populations. We propose that the oblique, northward impact concentrated ejected particulates into the Northern Hemisphere, blocking sunlight and suppressing photosynthesis. Increased rates of extinction and a prolonged recovery would then be associated with the greatest concentration of particulates. We speculate that metal poisoning from fallout of the particulates may have exacerbated and extended the nannoplankton crisis in the Northern Hemisphere, and thereby slowed the recovery of the Northern Hemisphere marine food web.