INTRODUCTION

The Paleogene represents a climatically dynamic period in Earth history. Stable isotope and other temperature 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., 2001, 1994). 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 shifts in carbon isotope patterns and 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., 1995, 1992; 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). This event, like the PETM, has caused large-scale perturbations in ocean chemistry and paleoecology (Barrera and Huber, 1993, 1991; 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 as key variables. Theoretical models have invoked 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., 1995, 1992). Similarly, the Oligocene glaciation has been attributed to both the initiation of the Antarctic Circumpolar Current 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., 1997, 1995; Paytan et al., 1998; Pearson and Palmer, 2000; Schmitz et al., 1997; Stott et al., 1990; Zachos et al., 1993).

Progress in characterizing Paleogene oceanography and climate history, particularly the transient events, has been hampered 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 (Bralower et al., 1997, 1995; 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. The ODP Leg 208 proposal, which resulted from the PPG discussions, described as its major goal the acquisition of sediment archives necessary to characterize such short-term changes in ocean chemistry and circulation that theoretically should have accompanied these climatic extremes, as well as their effects on the oceanic biota. Acquisition of more complete records of these events at a higher temporal resolution is required for both formulating and testing hypotheses on the origin of these and similar events.

Walvis Ridge, located in the eastern South Atlantic Ocean (Fig. F1), is one of the few known locations where it is possible to recover Paleogene sediments, including the PETM and EOGM, over a broad range of depths, including depths exceeding 4.5 km. 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 between depths of 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 the poor recovery (~50%–75%) and coring disturbances, 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 (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 and Weissert, 1985, and papers within; Shackleton, 1987). Nearly complete PETM intervals were recovered at the shallowest and deepest Sites 525 and 527. Stable isotope analysis of foraminifers recovered from these sites has helped to 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 (poor recovery).

During the winter of 2000, a seismic survey of southeastern Walvis Ridge was undertaken by the Meteor (Cruise M49/1; Spieß et al., 2003). The survey extended coverage of the Leg 74 sites to the north and northeast, where more continuous and slightly thicker sediment sequences were discovered (Fig. F2). The higher-fidelity multichannel seismic 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. The Leg 208 sites cover a paleodepth range of 2.2 km, sufficient to constrain large, as well as subtle, changes in the chemistry of deep and intermediate waters. Advances in coring technology and drilling strategies (i.e., multiple-hole composite sections) allowed for 100% recovery of sequences that were only partially recovered during Leg 74. In addition, recent advances in data acquisition and cyclostratigraphy enabled high-precision correlation and dating of these sediments.

Walvis Ridge is a northeast-southwest–trending aseismic ridge that divides the eastern South Atlantic Ocean into two basins, the Angola Basin to the north and the Cape Basin to the south (Figs. F1, F3). The ridge consists of a series of interconnected crustal blocks that slope gradually toward the northwest and more steeply toward the southeast. Magnetic and gravity anomalies indicate that Walvis Ridge was formed by hotspot volcanism near the spreading ridge as the Atlantic basin gradually widened (Rabinowitz and Simpson, 1984) and is assumed to have subsequently followed a simple thermal subsidence of ~1.1 km since the Maastrichtian (Moore, Rabinowitz, et al., 1984).

Pelagic sediments drape most of the ridge and generally increase in thickness toward the continental margin (Moore, Rabinowitz, et al., 1984). In the vicinity of the primary Leg 208 target area on the southeastern portion of the ridge, sediment thickness varies from ~300 m on the deep (>4.5 km) seafloor adjacent to the ridge to ~600 m near the summit (~2.5 km), a pattern that is clearly expressed by seismic sequences in the multichannel seismic profiles (Fig. F4). The sediments are primarily calcareous oozes and chalks that range in age from Campanian to Holocene. The Neogene sequences consist primarily of nannofossil and foraminiferal nannofossil oozes with relatively high carbonate contents, often in excess of 90 wt%, although lower through middle Miocene sections are characterized by low values (0%–20%). Turbidites and slumps are present in some intervals. The underlying Paleogene sediments are dominated by nannofossil and foraminifer-bearing nannofossil oozes to chalks. Carbonate contents are also high, generally in excess of 80 wt% through most of the Paleocene, Eocene, and Oligocene with the exception of several short carbonate-poor intervals at the deeper sites (e.g., Paleocene and upper Eocene of Site 527) that represent episodes of CCD shoaling. A few thin chert layers are present below the upper Paleocene of the shallowest Sites 525 and 528 and in the lower Eocene of Site 529. Slump deposits are present in the upper Paleocene at Site 529. In most sections, calcareous microfossil preservation varies from good to excellent. The natural remnant magnetism of the Leg 74 Walvis Ridge sediments appears to be strong and stable. As a result, the quality of the magnetic polarity records is excellent, particularly in the Upper Cretaceous and lower Paleocene intervals (Chave, 1984).

Sediment accumulation rates on Walvis Ridge have varied considerably with time. On average, the highest rates (~8–13 m/m.y.) occur in the Pleistocene, Paleocene, and Maastrichtian intervals. Much lower sedimentation rates (1–5 m/m.y.) as well as unconformities are common at most sites in the Neogene. For example, most of the Pleistocene is absent at the shallowest sites (Sites 525 and 526), whereas the lower and middle Miocene is absent at several of the deeper sites. The Neogene unconformities appear to be predominantly erosional in nature, although dissolution clearly affected deposition on deeper portions of the ridge (i.e., lower–middle Miocene at Site 527). Unconformities are also present in the upper Eocene and Oligocene. In contrast, at middepths (Site 529) most of the Paleogene is present but the middle–upper Miocene is either condensed or absent. In the deepest section (Site 527), the middle–upper Eocene is relatively condensed and the Oligocene and Miocene are highly condensed and/or absent. Nevertheless, it appears that for most of the early Paleogene, deposition was more or less continuous over much of the ridge. This continuity in sediment accumulation is reflected to some extent by similar patterns in the low-resolution carbon isotope stratigraphies for four of the Leg 74 sites (Shackleton and Hall, 1984; Shackleton, 1987).

Cyclical variations in sedimentation are evident in various lithologic and physical property indexes, particularly in the more expanded Maastrichtian and Paleocene sequences. Spectral analysis of sediment color banding reveals the presence of a strong precessional beat in upper Maastrichtian and lower Paleocene sediments at Sites 525 and 528 (Herbert and D'Hondt, 1990). Similar cycles, although more subtle, are present in the upper Paleocene and lower Eocene as well.

During Leg 208, sediments from Walvis Ridge were recovered that are suitable for addressing a number of important questions concerning the regional and global response to critical climatic events in the Paleogene. In addition, the overlying sediments at several sites will be used to resolve various aspects of the regional response to Neogene climate transitions. Here we discuss the critical climatic events of interest and other associated scientific objectives.

Early Eocene Climatic Optimum

The EECO represents the most recent episode of sustained global warmth. For nearly 2 m.y. of the early Eocene, global climate was warm and ice free. Paleontologic and isotopic proxies indicate that the high-latitude seas and bottom waters of the mid- and upper ocean were as much as 8°C warmer than at present (Fig. F5) (Miller et al., 1987; Shackleton and Boersma, 1981; Stott et al., 1990; Zachos et al., 1994). The biogeographic ranges of subtropical to temperate terrestrial fauna and flora extended well into polar latitudes (Axelrod, 1984; Estes and Hutchison, 1980; Wolfe, 1980), and polar ice sheets were small or nonexistent.

Several critical issues concerning the EECO need to be addressed. The first issue concerns the nature of climate variability during this period. At present, not a single marine record details paleoceanographic or climatic variability of this interval on orbital timescales. As a result, the approximate duration of the EECO and climate stability during this period are unknown. The second issue concerns the origin of the EECO. Empirical and theoretical geochemical studies suggest that greenhouse gas levels were significantly higher, possibly six times that of preindustrial levels, at the peak of the EECO (Berner, 1991). These estimates are supported to some extent by climate modeling in which the observed EECO meridional thermal gradients could only be attained in simulations with greenhouse gas levels six to eight times those of preindustrial levels (Sloan and Rea, 1996) and are further supported by a boron isotope record that indicates unusually low pH for the Eocene surface ocean, consistent with high pCO₂ (Fig. F6) (Pearson and Palmer, 2000). The third issue concerns the underlying mechanism for driving changes in greenhouse gas levels. Why did CO₂ levels increase in the early Eocene? Was the rate of mantle outgassing higher as suggested by geochemical models (Berner et al., 1983; Schrag, 2002)? If so, was the dissolved carbon content of the ocean higher than that of the present (Walker et al., 1981)? Constraints on the CCD and lysocline depths and their depth changes on orbital timescales might provide insight into these and related questions.

Paleocene/Eocene Thermal Maximum

In terms of rate and degree of warming, the PETM is unprecedented in Earth's history. Isotope records suggest that at 55 Ma the deep ocean and high-latitude surface waters warmed by 4° and 8°C, respectively, in a period of <40 k.y. (Fig. F7). This period of extreme warmth, which lasted <150 k.y., triggered profound changes in global precipitation and continental weathering patterns (e.g., Gibson et al., 1993; Kaiho et al., 1996; Robert and Kennett, 1994). The PETM also affected biota on a global scale, triggering rapid turnover of marine plankton (Kelly et al., 1996; Bralower, 2002), the largest mass extinction of deep-sea benthic foraminifers of the last 90 m.y. (Thomas and Shackleton, 1996; Thomas, 1990), and a rapid radiation of land mammals (Clyde and Gingerich, 1998; Koch et al., 1992; Rea et al., 1990).

Several forcing mechanisms have been proposed for the PETM, including massive outgassing associated with rifted margin volcanism (Eldholm and Thomas, 1993) and sudden dissociation of methane gas hydrates stored on continental shelves and slopes (Dickens et al., 1995, 1997). Both hypotheses were inspired, in part, by marine and terrestrial carbon isotope records that show an abrupt 3–4 decrease in the ocean/atmosphere inorganic carbon reservoirs in <2 k.y. (Röhl et al., 2000; Bralower et al., 1997, 1995; Kennett and Stott, 1991; Thomas and Shackleton, 1996; Koch et al., 1995, 1992). Such a large and rapid carbon isotope excursion requires input of a large volume of isotopically depleted carbon from an external reservoir into the ocean/atmosphere system. In terms of fluxes and isotopic mass balances, the hydrate dissociation model provides the most plausible solution. If estimates are correct, dissociation of only a fraction of the total reservoir of methane hydrate stored on continental margins is sufficient to generate the observed isotopic excursion. In principle, with warming of deep waters, shelf and slope hydrates became unstable, triggering a catastrophic release of CH₄ and greenhouse warming.

Massive methane dissociation should have had profound effects on ocean chemistry. Dickens et al. (1997) used a box model to simulate the effects of releasing ~1.1 x 10³ Gt of methane (immediately oxidized to carbon dioxide) directly into the atmosphere on ocean carbonate chemistry (Fig. F8). The amount of carbon added to the system was determined from mass balance calculations assuming a ¹³C of –60 for bacterially produced methane. This exercise found several notable effects, including a dramatic increase in weathering rates, a reduction in ocean pH, and a shoaling of the CCD and lysocline all within several thousand years (Fig. F8). The effects on the ocean are greatest in areas proximal to deepwater formation where the excess CO₂ enters the deep ocean via convective processes. The CCD is eventually restored, although not to its original position, as the system appears to initially overcompensate before returning to a steady state. This predicted pattern is consistent with the PETM low-carbonate or clay layers in many deep-sea PETM sites (Fig. F9).

The methane hydrate dissociation model can be tested with deep-sea drilling. A series of paleodepth transects in each of the major ocean basins would allow for characterization of the lysocline/CCD changes during this event. The initial shoaling should be expressed as a dissolution interval or hiatus in deeper sites coincident with the benthic foraminiferal extinction and carbon isotope excursion. The model-predicted overshoot or overcompensation should be preserved as well. Existing PETM sequences show a CCD reduction in carbonate content to varying degrees coincident with the excursion. However, without vertically offset sites, it is not yet possible to quantify the extent to which the reduction in carbonate content reflects changes in carbonate dissolution or production. With a series of drill sites placed along vertical depth transects, it should be possible to separate effects of preservation from production. A Pacific Ocean depth transect was recently completed during ODP Leg 198 (Bralower, Premoli Silva, Malone, et al., 2002), and Walvis Ridge was targeted to provide a complementary Atlantic Ocean depth transect. Walvis Ridge is well suited for a depth transect, as rates of sedimentation are fairly high and continuous through the upper Paleocene–lower Eocene over most of the ridge. Leg 208 drilling on the ridge recovered the sequences needed to constrain how the deep- to intermediate-ocean carbonate chemistry changed during this important event.

An interesting new development is the emergence of evidence of additional biotic events and carbon isotope excursions. Faunal and isotope data from Sites 690 and 865 (Thomas and Zachos, 2000; Thomas et al., 2000) point toward other times with conditions potentially similar to, but not as extreme as, those of the PETM during the late Paleocene and early Eocene. If these lesser events have a similar origin as the PETM, we would expect lesser but similar scale responses in ocean carbonate chemistry. Drilling at Blake Nose showed similar repetitions of the climatological/sedimentological conditions of the Paleocene/Eocene (P/E) boundary. A long gamma ray log and X-ray fluorescence (XRF) Fe data (Röhl et al., 2003) from Site 1051 showed a series of major peaks that represent a reduction in the carbonate content and/or increased terrigenous flux augmenting the clay content. These records revealed cyclicity of clay accumulation on a timescale of ~2 m.y., which most likely reflects climate cycles, although the precise nature of the climatic changes is unknown. The gamma ray and XRF Fe data records showed peaks during the carbon isotope event but also near the top of Chron C24r (Shipboard Scientific Party, 1998; Röhl et al., 2003). Similar cycles persist throughout the Eocene, indicating that this warm greenhouse period was not stable (e.g., Wade and Kroon, 2002). The long-term periodicity in these records may reflect a long-period orbital cycle, although a continuous record, spanning at least 8 m.y. of the upper Paleocene and lower Eocene, is required to evaluate the possibility of Milankovitch-driven climate.

Middle Eocene to Early Oligocene Glaciation(s)

The primary transition from "greenhouse" to "icehouse" conditions occurred during the middle Eocene to early Oligocene. Although this interval encompasses 18 m.y., stable isotopic records reveal a steplike pattern, with much of the change occurring over a few relatively brief steps in the middle Eocene (~52 Ma) and earliest Oligocene (Fig. F10) (EOGM; ~33.4 Ma) (e.g., Miller et al., 1987; Kennett and Stott, 1991; Zachos et al., 1996). The first event marks the onset of ephemeral glacial activity (Browning et al., 1996; Hambrey et al., 1991), whereas the second and larger of the two steps, the EOGM, represents the first appearance of permanent ice sheets on Antarctica (Barrera and Huber, 1991; Zachos et al., 1992). Furthermore, the highest-resolution deep-sea isotope records of this transition indicate that the final transition may have been modulated by orbital forcing (Diester-Haass and Zahn, 1996).

The climatic transition during the EOGM should have had a dramatic effect on oceanic/atmospheric circulation patterns and continental weathering rates and, hence, ocean chemistry. For example, reconstructions of bottom water isotope patterns hint at a brief pulse of a proto-North Atlantic Deep Water (NADW) coincident with the EOGM (Miller et al., 1991; Zachos et al., 1996). Also, carbonate sediment accumulation patterns suggest a significant deepening of the CCD at the Eocene/Oligocene (E/O) boundary (Peterson and Backman, 1990; van Andel, 1975; Lyle, Wilson, Janecek, et al., 2002). The latter is consistent with a sudden lowering of sea level and/or increased chemical weathering of the continents, both of which would increase the flux of dissolved ions to the deep sea. However, the CCD on a regional or local scale is also sensitive to changes in the rates and patterns of carbonate production. A depth transect is required to separate the relative contributions of each process and to more robustly constrain rates of change.

Oceanic Recovery from the Cretaceous/Paleogene Mass Extinction

The Cretaceous/Paleogene (K/P) impact and mass extinction triggered a number of changes in oceanic properties, some long lasting. These changes include (1) a drastic decrease in the export production as indicated by decreased carbon isotopic gradients (Fig. F11) (Hsü et al., 1982; D'Hondt et al., 1998; Stott and Kennett, 1989; Zachos et al., 1989) and reduced barium accumulation rates, (2) a sharp decrease in deep-sea carbonate accumulation (D'Hondt and Keller, 1991; Zachos and Arthur, 1986), and (3) at least in the South Atlantic Ocean, enhanced 100-k.y. oscillations in deep-sea sedimentation (D'Hondt et al., 1996). Each of these changes was likely caused by the profound changes in pelagic ecosystems at the time of the K/P mass extinction, changes that were sustained for a half-million years or longer. The overall recovery is still poorly constrained. For example, although we know that deep-sea carbonate accumulation did not recover for >2 m.y. after the mass extinction (Zachos and Arthur, 1986), a long-term record of deep-sea carbonate accumulation has only been generated for a single site (ODP Site 1001) where carbonate accumulation rates did not recover until 4 m.y. into the Paleogene (D'Hondt et al., 1998). Poor core recovery has precluded successful reconstruction in previously drilled South Atlantic Ocean sites. The improved recovery through modern ODP drilling techniques, however, made it possible to acquire a complete section cored with the APC in two holes at Site 1262.

The K/P mass extinction effectively changed the state of the global ecosystem, and documentation of the timing of these recoveries and their relationships to other paleoceanographic properties will provide a critical test of the coupling between oceans, climate, and biota. There is little reason to believe that any physical consequences of the K/P impact could have lingered for more than a few thousand years. Consequently, the Maastrichtian through Paleocene record of deep-sea sediments provides an ideal opportunity for testing the sensitivity of the global environment to biological disaster. For example, the long-term decrease in deep-sea carbonate accumulation might have changed oceanic alkalinity and atmospheric CO₂ concentrations. If so, the interval of decreased carbonate accumulation should be marked by changes in various paleoceanographic proxies, such as increased foraminiferal preservation and migration of the lysocline or CCD. Similarly, the 100-k.y. oscillations in lower Paleocene deep-sea sediments may have resulted from a decreased ability of postextinction biota to buffer seasonal and Milankovitch-scale climate change (Herbert et al., 1995; Herbert and D'Hondt, 1990; D'Hondt et al., 1996). If so, these oscillations should correspond to similar oscillations in various paleoceanographic proxies, such as carbon isotopic differences between planktonic and benthic foraminifers.

Paleogene Deepwater Circulation and Chemical Gradients

Changes in either the meridional thermal gradient or precipitation patterns can dramatically alter the mode of ocean circulation, thermohaline or otherwise (Broecker, 1997). Warming of polar regions coupled with increased precipitation, for example, would tend to lower the density of high-latitude surface waters and thereby inhibit sinking. This, in turn, might be balanced by increased convection elsewhere, possibly in subtropical regions where high rates of evaporation raise seawater salinity and density. Given the extreme thermal gradients and precipitation patterns, it is likely that deep convection during the PETM, and possibly during the EECO, was affected at high latitudes. If true, there should be evidence for this in deepwater chemical gradients as inferred from stable carbon isotopes and the distribution of carbonates on the seafloor.

A shift in the source of waters bathing Walvis Ridge should be reflected in carbon isotope, carbonate dissolution, and benthic assemblage patterns, as well as in patterns of other isotopes with short residence times (i.e., Nd). Through much of the late Paleocene and early Eocene, the most negative deep-ocean carbon isotope values were consistently recorded by benthic foraminifers from the Pacific Ocean (Kennett and Stott, 1990; Corfield, 1994; Corfield and Norris, 1996; Corfield and Cartlidge, 1992; Pak and Miller, 1992). Although this gradient is of the same sign as the modern one, it is much smaller, indicating that Pacific Ocean deep waters were only slightly older than Atlantic Ocean deep waters. As such, one would predict a similar CCD in the two basins. Low-resolution records of carbonate accumulation on Walvis Ridge for the Cenozoic suggest large-scale changes in the CCD at the P/E and E/O boundaries, although the timing and magnitude of these "events" are not well defined (Moore, Rabinowitz, et al., 1984).

Neogene Paleoceanography

Although the main focus for ODP Leg 208 was the recovery of Paleogene sediments, the younger part of the Walvis Ridge record is of considerable interest to paleoceanographers. The overlying Neogene sediments recovered during Leg 74 served to resolve several important paleoceanographic issues. For example, Shackleton et al. (1984) used oxygen isotopes to reveal long-term temperature trends of surface and deep waters for the Neogene in the Walvis Ridge area, and Moore, Rabinowitz, et al. (1984) used microfossil preservation to reconstruct carbonate saturation changes.

Today the lysocline depth is present at the boundary between NADW and Antarctic Bottom Water (AABW) between 4.0 and 4.8 km on the western side of the Mid-Atlantic Ridge. On the eastern side of the Mid-Atlantic Ridge, Walvis Ridge divides the South Atlantic Ocean into the northern Angola and southern Cape Basins. This basinal topography blocks AABW from directly entering the Angola Basin; instead, AABW enters through the Romanche Fracture Zone, where it mixes with NADW and forms a less defined NADW–AABW transition at ~5 km (Berger et al., 2002; Volbers and Henrich, 2002). Thus, the lysocline is deeper in the eastern Atlantic Ocean, although Schmiedl et al. (1997) argue that AABW is present at depths of ~4200 m to the north of Walvis Ridge.

Microfossils are markedly better preserved in the upper Miocene (Moore, Rabinowitz, et al., 1984) at Walvis Ridge, suggesting the onset or intensification of NADW production and its increased influence within the Angola Basin. Another step of increased carbonate preservation occurs near the beginning of the Pliocene. Detailed reconstruction from DSDP Leg 74 sites, however, was hampered by poorly constrained age models and low recovery.

As part of ODP Leg 208, we planned to employ cyclostratigraphy to improve the accuracy of dating by tuning the sedimentary cycles to the astronomical cycles of Milankovitch as far back as possible. This technique has been used to improve the dating of the sedimentary record during several paleoceanographic legs. The astronomical timescale has been established back to the late Oligocene by using cyclostratigraphy of sediments from drill holes at Ceara Rise (Lourens et al., in press; H. Pälike, pers. comm., 2003). The Walvis Ridge sediments show distinct cycles in certain intervals detected by visual means (Borella, 1984). However, sedimentary cycles were previously documented primarily in the Cretaceous and Paleogene parts of the geological record.

Oxygen isotope records of carbonate microfossils will identify the long-term trends in the climate system and particularly the high-resolution structure of the mid-Miocene Climatic Optimum (MCO). High-resolution stable isotope records in an astronomically tuned time frame are expected to document small-scale ice expansions (so-called Mi events; e.g., Miller et al., 1991) and associated sea level changes during the Miocene. At present, the amplitude of the ¹⁸O profiles recording the intensity of these Mi events is poorly known.

The Leg 208 depth transect is ideal for studying the vertical structure of the ocean throughout the Neogene. Carbon isotope records should establish deepwater characteristics of the waters bathing Walvis Ridge within the Angola Basin. Carbonate preservation profiles and benthic foraminiferal assemblages will be used to document the evolution of Neogene deep water (e.g., presence and relative abundance of Cibicidoides wuellerstorfi). Identification of the timing of the intensification of NADW influence in the South Atlantic Ocean within the Miocene will be a primary focus of our study. Numerous microfossil datums will be calibrated employing the astronomically tuned Neogene timescale.

The eastern South Atlantic Ocean is characterized by intense upwelling in the Benguelan system. Although the Leg 208 sites are located outside of the modern upwelling realm, stronger trade wind activity during glacials could have extended these upwelling filaments to Walvis Ridge (Little et al., 1997). Leg 208 sediments may, therefore, hold clues on upwelling intensity during the Pliocene–Pleistocene period through planktonic foraminiferal assemblages that are sensitive to productivity changes. Moreover, benthic foraminiferal assemblages will provide information on seasonal variability in export productivity (e.g., relative abundance of Epistominella exigua and Alabaminella weddellensis).

Cenozoic Cyclostratigraphy

One of the main objectives for Leg 208 drilling is to establish a cyclostratigraphy throughout the entire Cenozoic in order to construct an astronomical timescale for each of the sites. Establishment of such an astronomical timescale is critical for virtually all the objectives of Leg 208 drilling, especially for the upper Maastrichtian through lower Eocene, which currently lacks an orbitally tuned timescale. This lack of high-quality, high-resolution, and multicore sequences along a depth transect has slowed progress in reconstructing Paleogene paleoceanography and climate history. An orbitally tuned timescale is also essential for resolving rates of paleoceanographic change associated with the abrupt, short-lived events.

To meet these scientific objectives, the primary drilling focus was to recover sediments representing each of the target intervals over a depth range of 2 km. Moreover, it was essential that these intervals be stratigraphically continuous and have sufficiently high sedimentation rates to resolve orbital cycles to at least the 41-k.y. periodicity. Reconstruction of the nature of the oceanic response to each of the target events further required that we recover relatively unlithified sequences, from which well-preserved microfossils could be extracted for geochemical analysis. The preservation of these calcareous microfossils does not need to be perfect but sufficient to resolve relative differences in isotopic and elemental ratios of benthic and planktonic fauna as influenced by changes in the oceanic thermal and chemical structure.

Site Survey and Coring Strategy

During Leg 74 drilling in the Walvis Ridge area, the PETM and other critical intervals were not recovered at all sites, in part because of poor recovery and in part because of local unconformities. These unconformities, which appear erosional in nature, were most common in the Eocene and Oligocene sediments, particularly at shallow to middepth sites. These sites (DSDP Sites 525, 528, and 529) were located near gaps (channels) in the ridge that might explain the discontinuous nature of sedimentation. Based on seismic surveys, areas to the east of the existing sites appeared to provide more continuous sequences, which have a higher potential of recovering critical intervals. Thus, Meteor Cruise M49/1 (see report by Spieß et al., 2003) surveyed an area along the northeastern flank of Walvis Ridge that included the Leg 74 sites (Fig. F1). The main survey grid focused on a region to the north and east of the Leg 74 sites, away from a large channel that dissects the ridge to the southwest (Fig. F2). A seismic grid was established with several lines crossing existing sites in order to establish ages of key regional reflectors and sedimentary packages. Because of extensive upper Cenozoic downslope sediment transport throughout the region, the survey also targeted several isolated bathymetric highs where sediment transport might be minimal. The resultant M49/1 multichannel seismic profiles were then used with existing DSDP site data to locate stratigraphically continuous sequences of lower Cenozoic sediment at relatively shallow burial depths.

A depth transect with 5 primary and 11 alternate sites was designed using high-resolution Meteor Cruise seismic data (Fig. F2; Table T1). Based on the seismic profiles, the unconformities encountered during Leg 74 appeared to be highly localized, and more expanded, possibly continuous, sequences were identified. The potential to recover the P/E boundary and other key target intervals was maximized by selecting sites that had relatively thick Paleogene sequences and thin Neogene cover. In some instances, this approach required placing sites in channels where the Neogene was thinner but lower Paleogene horizons and bedding appeared uniform. A large number of alternate sites were identified for flexibility to move to a new site in case a local unconformity should be encountered at the P/E boundary.

The primary proposed sites for Leg 208, WALV-8A (Site 1264), -8E (Site 1263), -9B (Site 1265), -10F (Site 1266), -11B (Site 1267), and -12A (Site 1262), spanned a water depth range from 2507 to 4760 m. The stratigraphic targets for the two shallowest proposed sites, WALV-8E and -9B (water depths of 2717 and 3059 m), included the PETM and EOGM in upper Paleocene to lower Oligocene chalks and oozes. We avoided areas near DSDP Site 525, where the upper Eocene and lower Oligocene are absent, and areas near Site 529, where several slumps were identified in Eocene sections. Proposed Site WALV-10F (Site 1266), located to the south of Site 528 at an intermediate depth of 3811 m, appeared to contain a similar Paleogene sequence but with much thinner Neogene cover. The deepest proposed site, WALV-12A (Site 1262), was located well north of DSDP Site 527 at 4770 m. Proposed Site WALV-11B was located near Site 527 but at a slightly shallower water depth of 4365 m.

Upper Paleocene sediments were drilled at five sites (Sites 1262 and 1264–1267), the upper Maastrichtian and lower Paleocene at two sites (Sites 1262 and 1267), and the Neogene at all six sites, including Site 1264 from which the lower Oligocene through Pleistocene was recovered. At least two APC and/or XCB drilled holes were taken at each site to ensure recovery of a complete sequence, and cores were overlapped between holes to facilitate compilation of a composite section. During drilling, composite sections were assembled using core log data (magnetic susceptibility [MS], natural gamma radiation [NGR], and color reflectance) for correlation. In several cases, a third or fourth hole was cored to ensure recovery of at least two complete copies of the critical intervals.