A TALE OF TWO BOUNDARIES: BEGINNING AND END OF THE PALEOCENE

The boundaries of Epochs of the Cenozoic are based on marine fossils, thus closely linked to evolution of life. The Paleocene Epoch is bracketed by dramatic events, starting with the asteroid impact that caused a mass extinction (Cretaceous/Paleogene boundary [K/Pg]), and ending with extreme global warming, the Paleocene/Eocene Thermal Maximum (PETM), due to massive emission of isotopically negative carbon compounds into the ocean-atmosphere. Studies of protistan assemblages (calcareous nannoplankton, foraminifera) are of prime importance in documenting events at both boundaries and their effects on marine life. At the K/Pg, pelagic calcifiers (calcareous nannoplankton and planktic foraminifera) suffered severe extinction (>95% of species), contrasting with relatively minor effects at the PETM (latitudinal migration, temporary changes in assemblage composition, evolution of short-lived species). In sharp contrast, extinction of deep-sea benthic foraminifera did not happen at the K/Pg but at the PETM. The severe K/Pg extinction of pelagic calcifiers thus did not result in significant seafloor extinction, therefore does not imply long-term, extreme reduction in food supply to the seafloor. The extinction of zooplankton may not have reduced the biological pump greatly, because reduced pellet formation may have been compensated by reduced break-up of 'marine snow'. The PETM warming may have decreased food transfer to the deep sea by strongly increasing remineralization in the water column, causing deep-ocean starvation even at unchanged primary productivity. Ocean acidification may have been a major cause of the differential planktic-benthic patterns of extinction at the beginning and end of the Paleocene, because its effects depend upon the rate of acidification. The virtually instantaneous acidification caused by asteroid impact may have caused extinction of pelagic calcifiers, but was buffered before its effects could penetrate into the deep oceans, whereas the PETM acidification over timescales of millennia may played an important role in extinction of deep-sea calcifiers, with less severe effects on pelagic calcifiers. It remains a question whether future patterns of extinction due to anthropogenic ocean acidification at centennial timescales will resemble those of the K/Pg or the PETM.