The Paleogene was a climatically dynamic period. Various climate proxies reveal a complex history of warming and cooling, characterized by periods of both gradual and rapid change (Miller et al., 1987; Miller and Katz, 1987; Stott et al., 1990; Zachos et al., 1994, 2001). Major events include a 1-m.y.-long global warming trend that began in the late Paleocene and climaxed in the early Eocene in a 1- to 2-m.y.-long climatic optimum (early Eocene Climatic Optimum [EECO]) and a 12-m.y.-long stepped cooling trend that began in the early middle Eocene and culminated in the earliest Oligocene with the appearance of continental-scale ice sheets (Hambrey et al., 1991; Zachos et al., 1992). One of the more prominent events is a transient but extreme greenhouse interval known as the Paleocene/Eocene Thermal Maximum (PETM) at ~55.0 Ma. Major changes in ocean chemistry, as inferred from carbon isotope anomalies, and changes in the distribution and preservation patterns of terrigenous and biogenic sediments on the seafloor (e.g., Bralower et al., 1995; Kennett and Stott, 1991; Robert and Kennett, 1997) characterize the PETM. In addition, distinct shifts in the distribution of key groups of fauna and flora occurred in the oceans and on land (e.g., Kelly et al., 1998; Koch et al., 1992, 1995; Thomas and Shackleton, 1996; Thomas, 1998; Wing, 1998). Another notable event is the earliest Oligocene Glacial Maximum (EOGM, or Oi-1), a brief but extreme glacial interval that occurred at ~33.4 Ma and marks the transition to permanent glacial conditions on Antarctica (e.g., Miller et al., 1987, 1991; Zachos et al., 1996; Coxall et al., 2005). This event, like the PETM, caused large-scale perturbations in ocean chemistry and paleoecology (Barrera and Huber, 1991, 1993; Salamy and Zachos, 1999; Thomas and Gooday, 1996; Thunell and Corliss, 1986). Multiple hypotheses exist to explain the large-scale, long-term changes in Paleogene climate, although none have yet gained universal acceptance. In general, among many factors, the role of ocean gateways (continental geography) and greenhouse gas levels are considered key variables. Theoretical models invoke either the absence of a circum-Antarctic current or higher greenhouse levels or some combination of both to account for the EECO (Barron, 1985; Bice et al., 2000; Sloan and Barron, 1992; Sloan and Rea, 1996; Sloan et al., 1992, 1995). Similarly, Oligocene glaciation has been attributed to both the initiation of the Antarctic Circumpolar Current (ACC) and a reduction in greenhouse gas levels (e.g., Kennett and Shackleton, 1976; Mikolajewicz et al., 1993; Oglesby, 1991; Raymo et al., 1990; Rind and Chandler, 1991; DeConto and Pollard, 2003). Some of the more abrupt transient excursions are more likely to have been forced by rapid changes in greenhouse gas levels because they occur over short timescales (e.g., 103–104 yr) and, most importantly, are accompanied by geochemical and isotopic anomalies suggestive of major perturbations in the carbon and sulfur cycles (Dickens et al., 1995, 1997; Paytan et al., 1998; Pearson and Palmer, 2000; Schmitz et al., 1997; Stott et al., 1990; Zachos et al., 1993).
Further progress in characterizing Paleogene oceanography and climate history, particularly the transient events and rapid shifts, was slowed by the lack of high-quality, high-resolution, multicored sequences. Most sites cored prior to Leg 198 suffer from poor recovery and drilling disturbance, and few were multicored or drilled as part of depth transects. The few exceptions are sites recovered during recent Ocean Drilling Program (ODP) legs including Sites 865, 999, 1001, 1051, and Bass River core hole in New Jersey (Bralower et al., 1995, 1997; Miller et al., 1998; Norris and Röhl, 1999; Röhl et al., 2000, 2001, 2003). High-resolution records produced from these sites have yielded a wealth of exciting, important evidence of climate change to be more fully explored with additional data. The ODP extreme climate advisory panel (Program Planning Group [PPG]) recognized the dearth of high-resolution records across climate transients, and the panel formulated new questions concerning extreme climates (Kroon et al., 2000) and potential drilling targets, among which was the Walvis Ridge area. Leg 208 was designed specifically to address this deficiency with a major goal of developing the high-fidelity records necessary to characterize short-term events, including the changes in ocean chemistry and circulation and biota that theoretically should have accompanied these climatic extremes. Walvis Ridge, located in the eastern South Atlantic Ocean (Fig. F1), was one of the few known locations where previous drilling recovered the PETM and EOGM over a broad depth range. The ridge was the target of drilling by Deep Sea Drilling Project (DSDP) Leg 74, which occupied Sites 525–529 on the northern flank of the ridge at water depths between 2.5 and 4.2 km (Moore, Rabinowitz, et al., 1984). Paleogene pelagic sediments characterized by moderate sedimentation rates (~6–15 m/m.y.) and good magnetic stratigraphy were recovered at each site. However, because of poor recovery (~50%–75%) and coring disturbance, especially with the rotary core barrel in unlithified sediments, only short segments of the sequences were recovered fully intact and none of the sequences were double cored. Technical problems combined with the lack of high-resolution shipboard core logs limited high-resolution cyclostratigraphic investigations to a few short segments of the Cretaceous/Paleogene (K/Pg) boundary interval (Herbert and D'Hondt, 1990). Nevertheless, subsequent shore-based studies of low-resolution samples collected from these cores were instrumental in adding to our understanding of long-term Maastrichtian and Paleogene paleoceanography of the South Atlantic Ocean (e.g., calcite compensation depth [CCD], carbon isotope stratigraphy, and deep-sea temperature/ice volume) (e.g., Moore, Rabinowitz, et al., 1984; Hsü, Labrecque, et al., 1984; Shackleton, 1987). Nearly complete PETM intervals were recovered at the shallowest and deepest Sites 525 and 527, respectively. Stable isotope analysis of foraminifers recovered from these sites helped constrain the magnitude of the deep Atlantic biogeochemical and environmental changes during this event (Thomas et al., 1999; Thomas and Shackleton, 1996). At the remaining sites, the PETM was not recovered because of core gaps.
During the winter of 2000, a seismic survey of southeastern Walvis Ridge was carried out by the Meteor (Cruise M49/1; Speiss et al., 2003) (Fig. F2). The survey extended coverage of the Leg 74 sites to the north and northeast, where more continuous and slightly thicker sediment sequences were discovered. The higher-fidelity multichannel seismic (MCS) data generated during the survey allowed identification of several areas where the PETM and other critical intervals could be recovered by advanced piston corer/extended core barrel (APC/XCB) drilling.
Utilizing data from Leg 74 and the new high-resolution MCS profiles of Meteor Cruise M49/1 (Spiess et al., 2003), Leg 208 successfully recovered fully intact, stratigraphically continuous sections of upper Cretaceous and Cenozoic strata at six sites on Walvis Ridge over a depth range of 2.2 km (Fig. F3), sufficient to constrain depth-dependent changes in the chemistry of deep and intermediate waters. Furthermore, offset drilling in multiple holes at each site allowed 100% recovery of sequences that were only partially recovered during Leg 74. Finally, recent advances in data acquisition and cyclostratigraphy enabled high-precision correlation and dating of these sediments and the assembly of composite sections.
The scientific objectives for Leg 208 are detailed in the Leg 208 Initial Reports volume (Zachos, Kroon, Blum, et al., 2004). The major objectives include the following: