MAASTRICHTIAN OCEAN-CLIMATE EVOLUTION: A VIEW FROM THE TROPICAL PACIFIC

Well-preserved, Maastrichtian sections recovered from four sites (1209-1212) during Ocean Drilling Program (ODP) Leg 198 to Shatsky Rise in the northwest Pacific provide insight into widespread oceanographic, biotic, and climatic changes that occurred during latest Cretaceous time. Low-resolution isotopic records from these sites have revealed temporal changes in the vertical structure of the water column and the character of water masses that bathed the rise. These data indicated that during the earliest Maastrichtian, Shatsky Rise lay within a relatively warm, well-oxygenated intermediate water mass that originated in the North Pacific. By late Maastrichtian time, however, this warmer water mass had been displaced by a cooler water mass that originated in the Southern Ocean. The present study uses high-resolution isotopic records derived from benthic foraminifera to examine in detail the transition between early and late Maastrichtian circulation modes. The mid-Maastrichtian transition is manifest in the sedimentary record as a prominent 1 to 2-m-thick horizon over shallow portions of Shatsky Rise, which contains a series of layers containing large shell fragments of Inoceramus, a wide-ranging bivalve. High-resolution, isotopic records through this interval suggest that the inoceramids flourished during a brief time when a warm, saline intermediate water mass, possibly from the Western Tethys, moved over the rise. The last occurrence of inoceramids at Shatsky Rise coincides with the withdrawal of this warmer water mass and, on a broader scale, with the global extinction of the inoceramid bivalves and other significant ecological and climatic changes. Results implicate changes in the sources and distribution patterns of intermediate water masses as a major driver for latest Cretaceous global change.