OCEANIC PRODUCTIVITY AFTER THE CRETACEOUS/PALEOGENE IMPACT: THE VIEW FROM THE DEEP

More than four decades have passed since Walter Alvarez contributed to bring mass extinctions to the attention of broad audiences, inspiring multidisciplinary research on a wide variety of topics, ranging from the Cretaceous/Paleogene (K/Pg) and other impact events to astronomy, climate modeling and the centuries-long debate whether apparent extinctions are real or due to imperfection of the fossil record. Despite intensive, wide-ranging studies, many questions remain on ecosystems during the aftermath of extinctions, and on species extinction estimates in the absence of knowledge of present biotic diversity. We summarize knowledge on an integral part of these questions, i.e., open ocean productivity / export productivity after the K/Pg mass extinction, with its feedback on the global carbon cycle.

We compile benthic foraminiferal data (extinction rates, diversity, assemblage composition and morphogroups) across the K/Pg boundary at geographically and bathymetrically diverse sites using records at differing time resolution, in order to analyze extinction patterns in Earth's largest habitat, the deep seafloor, arriving at two main conclusions. First, we find no significant links between the (low) rate of extinction of benthic foraminiferal species or their temporary decrease in diversity and the distance from Chicxulub crater, water depth, and paleolatitude: benthic foraminiferal records show strong post-impact variability in (geographic and bathymetric) space and time, supporting the existence of post-extinction heterogeneous oceans with local-to-regional plankton blooms. Second, we present evidence that this apparent variability might at least in part be due to variable incompleteness of the geological record at high time resolution, i.e., millennial resolution or less. Even incomplete records will document extinction, which is irreversible, but detailed patterns of events (e.g., short-term severe cooling superimposed on long-term warming, short-term acidification superimposed on long-term oversaturation), thus understanding of pathways of environmental change and causation can be understood from high-resolution records only. We need more of such records to enable us to evaluate the 'tempo and mode' of past extinctions and compare these to present and future extinction patterns.