BIOTIC EXTINCTIONS, DISSOLUTION, AND OCEAN ACIDIFICATION PRIOR TO THE CRETACEOUS/PALEOGENE (K/PG) BOUNDARY IN THE TROPICAL PACIFIC

The several million years preceding the K/Pg boundary has been the focus of many studies. Changes in ocean circulation and sea level, extinctions, and major volcanic events have all been documented for this interval. Important research questions these changes raise include the climate dynamics during the warm, but not hot, time after the decay of the Late Cretaceous greenhouse interval and the stability of ecosystems prior to the mass extinctions at the end-Cretaceous. Biotic perturbations and changes in ocean circulation during the Maastrichtian stage of the latest Cretaceous raise questions about whether the biosphere was preconditioned for the end-Cretaceous mass extinction of calcareous plankton. A brief acme of inoceramid clams at ~71 Ma at Shatsky Rise in the tropical North Pacific was followed by their extinction during the “mid-Maastrichtian event” at 70.1 Ma associated with an abrupt warming of deep waters. This was followed by an interval of intense dissolution beginning ~67.8 Ma at ODP Site 1209 (2387 m water depth). The transition into the dissolution interval was initially gradual, then rapid, and is defined by a low planktic/benthic (P/B) ratio, highly fragmented planktic foraminifera, an absence of larger taxa, abundance of smaller taxa, and low planktic foraminiferal and nannofossil species richness. This dissolution event is followed by a gradual recovery in carbonate preservation beginning ~700 kyr prior to the K/Pg boundary.

Was the dissolution event caused by a change in deep water circulation, migration of the site out of the high productivity tropical belt, or ocean acidification associated with Deccan Traps volcanism? A second dissolution event is documented ~200 kyr before the boundary as evidenced by a marked increase in planktic fragmentation, but without a change in P/B. Our data show that changing deep water masses, coupled with reduced productivity and associated decrease in pelagic carbonate flux was responsible for the first dissolution interval (67.8-66.4 Ma), while Deccan Traps volcanism may have caused surface ocean acidification ~200-kyr prior to the K/Pg mass extinction event.