Abstract:
The mid Cretaceous (Barremian-Turonian) and early Paleogene were characterized by some of the most equable climates of the Phanerozoic and are among the best known ancient "greenhouse" climate intervals. In addition, these intervals contain some of the most abrupt and transient climatic changes in the geologic record, including the latest Paleocene thermal maximum, the mid-Maastrichtian deep-water event, and the early Aptian Oceanic Anoxic Event. These transitions involved dramatic changes in oceanic circulation, geochemical cycling, and marine biotas. Ocean Drilling Program Leg 198 on Shatsky Rise is designed to address the causes and consequences of Cretaceous and Paleogene global warmth. The objectives are to address the long-term climatic transition into and out of "greenhouse" climate as well as abrupt climatic events. Shatsky Rise, a medium-sized large igneous province in the west-central Pacific, contains sediments of Cretaceous and Paleogene age at relatively shallow burial depths. As a result, sediments of both ages can be reached readily through drilling and fossil materials are sufficiently well preserved for stable isotope and trace element analyses and for faunal and floral assemblage studies. The Leg 198 sites comprise a depth transect that has been designed to characterize changes in the nature of surface and deep waters through time, including vertical gradients of temperature, oxygenation, and corrosiveness. Combined with the results of previous and future legs, Leg 198 drilling will help determine (1) the frequency, amplitude, and forcing of climate change in warm intervals; (2) latitudinal thermal gradients in discreet mid-Cretaceous to Paleogene time slices; and (3) changes in the sources of deep water and vertical ocean structure through time. Shatsky Rise has been the target of three Deep Sea Drilling Project legs, but most sites were spot-cored or plagued by low recovery, especially in the Cretaceous. Previous drilling was centered on the southern part of Shatsky Rise. Leg 198 includes sites on the central and northern parts of the rise where the stratigraphy is less well known. Chert provides a significant recovery problem in the Cretaceous, thus, we have attempted to position sites at locations with poorly developed reflectors. In addition, the drilling strategy includes use of extended core barrel coring combined with the motor driven core barrel to increase recovery in cherty intervals. In intervals of poor core recovery, Formation MicroScanner and gamma-ray logs will help determine the stratigraphic extent of diagnostic sediments, such as black shale.