OCEAN ACIDIFICATION DURING THE LATE MAASTRICHTIAN, SHATSKY RISE, NW PACIFIC

The K/Pg boundary has been the focus of many latest Cretaceous studies with much attention on the several million years preceding the boundary where changes in ocean circulation and sea level, biotic extinctions, and major volcanic events took place. This all suggests that an impact may not have been the sole cause of the end-Cretaceous extinctions. Most intriguing is a dissolution event at Shatksy Rise that started ~1.6 myr before the K/Pg boundary where planktic foraminiferal preservation is extremely poor.

This dissolution event is characterized by chalky, highly fragmented planktic forams, increased dissolution of larger taxa, abundance of tiny planktic forams, greatly reduced P/B ratio, a sharp decrease in planktic and nannofossil species richness, and a significant decrease in benthic foram accumulation rates The onset of dissolution is clearly seen in the core where there is a distinct color change at Site 1210B. The dissolution event is preceded by a transitional interval beginning ~68 Ma that while containing well-preserved benthic forams is composed of planktic assemblages with highly variable preservation with increasing fragmentation towards the boundary as well as nannofossil species gradually declining in diversity.

Population counts (>63 μm) through the dissolution interval reveal assemblages of juvenile biserial, trochospiral, and planispiral taxa; there are no ‘blooms’ of triserial Guembelitria in the uppermost Maastrichtian at Site 1209. Recovery in calcareous plankton diversity and P/B ratio begins ~200 kyr before the boundary at 66.2 Ma. There is a gradual change in benthic d¹³C to more positive values at 66.7 Ma, but no corresponding change in Nd isotopes; therefore, dissolution probably was not attributed to a change in water mass/circulation.

The cause of the dissolution is unknown. Was it limited to Shatsky Rise, or was it more widespread, perhaps the result of a global acidification event coincident with the onset of Deccan Traps volcanism? The absence of other records of latest Maastrichtian dissolution adds to the mystery of the Shatsky Rise record. However, the recovery of planktic foram and calcareous nannofossil diversity prior to the K/Pg boundary demonstrates that ocean acidification was not integral to the mass extinction of calcareous plankton that marks the end-Cretaceous.