OVERVIEW OF SCIENTIFIC OBJECTIVES

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.

• Fill the Pacific gap in current paleoenvironmental data sets to complete global reconstructions and modeling of the physical and chemical parameters of warm oceans.
• Reconcile the nature of climatic forcing during the Cretaceous and Paleogene by monitoring changes in the properties of surface and deep waters. This will help to constrain the character and stability of intermediate and deep-water circulation, vertical thermal gradients, and basin fractionation during ancient intervals of extreme warmth.
• Understand how apparently cool tropical sea-surface temperatures and low meridional thermal gradients were maintained during "greenhouse" climate intervals.
• Shed light on the origin of transient climatic events such as the Eocene/Oligocene boundary, the Paleocene-Eocene Thermal Maximum (PETM), late Paleocene and early Eocene hyperthermals, and the mid-Maastrichtian events. The depth transect will also help address questions concerning the nature of chemical (i.e., carbonate compensation depth [CCD], nutrients, and oxygenation) and physical oceanographic changes (temperature gradients) during these events.
• Understand water column stratification during mid-Cretaceous OAEs as well as obtain complete records of organic-rich sediments suitable for detailed paleontological and geochemical investigations. These data will allow us to more fully determine the response of marine biotas to abrupt environmental changes and to constrain changes in carbon and nutrient cycling during the OAEs.