OVERVIEW OF SCIENTIFIC OBJECTIVESLeg 198 is designed to obtain a depth transect of the Cretaceous through the Paleogene Pacific Ocean to advance our understanding of the behavior of Earth's climate during "greenhouse" intervals. The Cretaceous and Paleogene were characterized by some of the most equable climates of the Phanerozoic. In addition, these intervals contain some of the most abrupt and transient climatic changes in the geologic record. Examples include the Paleocene-Eocene thermal maximum (e.g., Kennett and Stott, 1991; Zachos et al., 1993), the middle Maastrichtian deep-water event (e.g., MacLeod and Huber, 1996), and the Cenomanian/Turonian boundary oceanic anoxic event (OAE) (e.g., Schlanger et al., 1987). These intervals were characterized by major changes in oceanic circulation, which had a pronounced effect on geochemical cycling and a major impact on marine biotas.
One of the largest obstacles facing our understanding of the climate of the Cretaceous and Paleogene is that many good stratigraphic sections have been buried at depths where diagenetic alteration has obscured stable isotope and other climate proxies. In other sequences, spot coring, coring gaps, and drilling disturbance hinder detailed paleoceanographic studies. The result is that site coverage is uneven and almost nonexistent in some areas. This is especially the case for the Pacific Ocean. The aerial extent and importance of the Pacific in global circulation, however, makes it a critical target for drilling of warm climatic intervals.
One of the most promising locations in the Pacific for recovering Cretaceous and Paleogene sediments at relatively shallow burial depths is Shatsky Rise, a medium-sized large igneous province (LIP) in the west-central Pacific (Fig. 1). Shatsky Rise has been the target of three Deep Sea Drilling Project (DSDP) expeditions, Legs 6, 32, and 86, and Ocean Drilling Program (ODP) Leg 132. The highest quality record was obtained during Leg 86 at Site 577, which was limited to the Paleogene and uppermost Maastrichtian. Some sites in the older legs were spot-cored, and chert lowered recovery in others, especially in the Cretaceous. Yet even with an extremely patchy record, sea-surface temperature (SST) estimates obtained from isotopic analyses of Shatsky Rise sediments have provided key points in our understanding of Cretaceous and Paleogene climates. The Leg 198 depth transect will provide key data from the tropical Pacific as well as a more representative open-ocean signal than exists with the current data.
The majority of sites used in current paleoceanographic investigations were situated at relatively shallow paleodepths, mostly less than 2000 m. The selected Shatsky sites provide a depth transect spanning 2500 m of the water column, providing us with the opportunity to sample true intermediate and deep waters. Even though the paleodepth estimates for the Leg 198 sites are somewhat uncertain, their broad depth range ensures that the deepest sites will lie below previously drilled sites. Existing recovery from Shatsky demonstrates that pristine, isotope-grade material will be recovered for the entire interval of interest. This will allow the following specific objectives to be addressed through depth-transect drilling.
Tectonic and Stratigraphic Evolution of Shatsky Rise | Table of Contents