Ocean Drilling Program, United States

During Ocean Drilling Program (ODP) Leg 208, we will drill a depth transect of five sites between 2500 and 4800 m water depth targeting early Cenozoic sediments on the northeastern flank of the Walvis Ridge. Previous drilling in this region (Deep Sea Drilling Project [DSDP] Leg 74) recovered pelagic oozes and chalk spanning both the Paleocene/Eocene and Eocene/Oligocene boundaries. Our drilling objective is to recover fully intact sequences spanning both of these "critical" intervals as well as the Cretaceous/Tertiary (K/T) boundary at the deep end-member sites. New multichannel seismic data (Meteor cruise M49/1) along with existing information from DSDP Leg 74 sites were used to identify site locations where continuous sequences of early Cenozoic sediment should be present. Double/triple advanced hydraulic piston coring and double extended core barrel coring, high-resolution physical property measurements, and construction of composite sections will be employed at each site to ensure 100% recovery of the sedimentary section. These composite sections will provide a detailed history of paleoceanographic variations associated with several prominent episodes of early Cenozoic climate change including the early Eocene Climate Optimum (EECO), the Paleocene-Eocene Thermal Maximum (PETM, also known as LPTM), and the Early Oligocene Glacial Maximum (EOGM). In particular, the transect will enable characterization of depth-dependent changes in deepwater chemistry and circulation associated with these extreme climatic states. This will facilitate testing of the leading hypothesis for the cause of the PETM and carbon isotope excursion, the abrupt dissociation of as much as 2000 gigatons of marine methane hydrate. Numerical modeling demonstrates that the injection of such a large mass of carbon to the ocean/atmosphere should have triggered an abrupt (<10 k.y.) shoaling of the carbonate compensation depth and lysocline, followed by a gradual recovery. Sediment cores recovered during this leg will be used to constrain both the rate and scale of recovery, as well changes in other aspects of deep-sea chemistry. The Leg 208 transect will complement a transect drilled on the southern Shatsky Rise during ODP Leg 198.