A DISSOLUTION EVENT IN THE NORTH PACIFIC (SHATSKY RISE) PRIOR TO THE K/PG BOUNDARY

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.

We document an intriguing dissolution event at Shatsky Rise that began ~67.8 Ma at Site 1209 (2387 m). The event is marked by poor planktic foraminiferal preservation and sharply reduced calcareous plankton diversity. The shift into the dissolution interval is gradual. Within the interval, the planktic/benthic (P/B) ratio is low, planktic foraminifera are highly fragmented, larger taxa are mostly absent, small taxa are relatively abundant, planktic foraminifera and nannofossil species richness are low, and the accumulation rate in benthic foraminifera is low. The event is followed by abrupt recovery ~300 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?

Based on benthic oxygen and carbon isotopes and Nd analyses of fish debris, we propose the dissolution was caused by a change in water mass properties. Cooler, more corrosive waters affected Site 1211 (2907 m) as early as 68.3 Ma and reached Site 1209 at ~67.8 Ma. The most intense dissolution occurred from ~66.8 to 66.3 Ma with foraminiferal preservation and P/B ratio punctuated by a number of intense ~100-kyr cycles of dissolution that occurred during times of negative shifts in benthic δ¹³C. At ~66.3 Ma there is an abrupt +0.7‰ shift in benthic δ¹³C and a -0.7‰ shift in δ¹⁸O values coinciding with a sharp recovery in the P/B ratio. The interval from 66.3 to 66.1 Ma records less fragmentation and the recovery of planktic foraminiferal and nannofossil diversity. The 100-kyr prior to the K/Pg boundary is marked by positive shifts of 0.6‰ and 0.4‰ in benthic δ¹³C and δ¹⁸O values, respectively, suggesting another change in deep/intermediate waters prior to the end-Cretaceous mass extinctions.