INOCERAMID EXTINCTION AND TWO DISSOLUTION EVENTS IN THE TROPICAL PACIFIC DURING THE MAASTRICHTIAN

Here we present the findings of a Late Cretaceous study, currently under review, in which biotic perturbations and changes in ocean circulation during the Maastrichtian stage raise questions about whether the biosphere was preconditioned for the end-Cretaceous mass extinction of calcareous plankton. We document three major events on Shatsky Rise in the tropical North Pacific. The first was a brief acme of inoceramid clams at ~71 Ma followed by their extinction during the “mid-Maastrichtian event” at 70.1 Ma, which we attribute to the abrupt warming of deep waters as indicated by benthic oxygen isotopes in this study. This was later followed by an interval of dissolution due to the incursion of a cooler, older water mass first observed at ~68.3 Ma at ODP Site 1211 (2907 m). Dissolution then intensified beginning ~67.6 Ma at ODP Site 1209 (2387 m). The late Maastrichtian dissolution interval was initially gradual, and is characterized by a low planktic/benthic (P/B) ratio, highly fragmented planktic foraminifera, mostly an absence of larger taxa, low abundances of smaller taxa, and low planktic foraminiferal and nannofossil species richness. This appears to be orbitally-driven as there was a step-wise decrease in P/B with each step taking place over a long eccentricity cycle superimposed with shorter 100 kyr decreases in P/B. Foraminiferal isotope data suggest that Shatsky Rise moved out of a zone of tropical divergence and into more oligotrophic waters of the North Pacific subtropical gyre. The reduced flux of carbonate contributed to the severity of the 1.2-myr late Maastrichtian dissolution interval. A change in water mass to warmer, younger and less corrosive deep waters brought about the rapid recovery in carbonate preservation and calcareous plankton diversity ~400 kyr prior to the K/Pg boundary. A second dissolution event occurred ~200 kyr before the boundary as evidenced by a renewed increase in planktic fragmentation, but without a decrease in P/B ratio. 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, while Deccan Traps volcanism may have caused surface ocean acidification ~200 kyr prior to the K/Pg mass extinction event.