END-CRETACEOUS MARINE MASS EXTINCTION NOT CAUSED BY PRODUCTIVITY COLLAPSE

An asteroid impact at the end of the Cretaceous caused mass extinction, but extinction mechanisms are not well understood. The collapse of sea-surface to sea-floor carbon isotope gradients has been interpreted as reflecting a global collapse of primary productivity (‘Strangelove Ocean’) or export productivity (‘Living Ocean’), which then caused mass extinction higher in the marine food chain. Evidence from organic biomarkers, however, indicates a rapid recovery of primary productivity, and non-calcifying phytoplankton did not suffer extreme extinction. In addition, phytoplankton-dependent benthic foraminifera on the deep-sea floor did not suffer significant extinction, suggesting that export productivity persisted at a level sufficient to support their populations. We compare benthic foraminiferal faunal records with benthic and bulk stable carbon isotope records from ODP and DSDP sites in the Pacific, Southeast Atlantic and Southern Oceans. Benthic foraminiferal accumulation rates and species relative abundances indicate that directly after the impact organic matter export to the benthic faunas declined but persisted in the SE Atlantic and Southern Oceans, and temporarily increased in the Pacific, although the carbon isotope gradient in these same samples collapsed. We argue that the carbon isotope records were severely affected by changes in relative strength of the biological and solubility pumps in the oceans, and by the mass extinction of the surface-dwelling carriers of the isotope signal (nannoplankton and planktic foraminifers). For example, in the earliest Paleocene the contribution to the bulk carbon isotope signal of isotopically light calcareous dinocysts increased, and planktic foraminifera were represented by very small, non-symbiont bearing specimens. We conclude that end-Cretaceous decrease in export productivity was moderate, regional, and insufficient to explain marine mass extinction. A transient episode of surface ocean acidification may have been the main cause of extinction of calcifying plankton and ammonites, and recovery of productivity in terms of biomass (though not in terms of biodiversity) may have been as fast in the oceans as on land.