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INTRODUCTION (continued)

Scientific Objectives

Leg 208 scientists recovered sediments from Walvis Ridge 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.

Early Eocene Climatic Optimum

The EECO represents the most recent episode of sustained global warmth. For nearly 2 m.y. of the early Eocene, the global climate was warm and ice free. Paleontologic and isotopic proxies indicate that the high-latitude seas and bottom waters were as much as 8°C warmer than at present (Fig. F5) (Miller et al., 1987; Shackleton and Boersma, 1981; Zachos et al., 1994; Stott et al., 1990). 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 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; Pearson and Palmer, 2000). 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 the present (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 this 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 planktonic organisms in the ocean (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 the most 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 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 and 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 could become unstable, triggering a catastrophic release of CH4 and immediate greenhouse warming through positive feedback.

A massive methane dissociation event should have 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 were greatest in areas proximal to deepwater formation where the excess CO2 entered 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 clathrate 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 foraminifer extinction and carbon isotope excursion. The model-predicted overshoot should be preserved as well. Existing PETM sequences show a 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 microfossil preservation or the depth range over which the carbonate changes are present. 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 by 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-ocean carbonate chemistry changed during the 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 shows similar repetition of the climatological/ sedimentological conditions of the Paleocene/Eocene (P/E) boundary. A long-term record of the gamma ray log (X-ray fluorescence [XRF] Fe data) (Röhl et al., 2003) of Site 1051 shows a series of major peaks that represent a reduction in the carbonate content and/or increased terrigenous flux augmenting the clay content. Cyclicity of clay accumulation on the timescale of ~2 m.y. most likely reflects climate cycles, although the precise nature of the climatic changes is unknown. One of the gamma ray (or XRF Fe data) maxima coincides with the carbon isotope event and peaks again near the top of Chron C24r (Shipboard Scientific Party, 1998, Röhl et al., 2003). Similar cycles persist throughout the Eocene, indicating 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 (~2 m.y.). 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 "ice-house" 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 in just a few relatively brief steps in the earliest–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 an increasing chemical weathering of 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 and sustained for a half-million years or longer by the slow ecological recovery on evolutionary timescales. The final recovery from several of these changes is 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), the final recovery of deep-sea carbonate accumulation has only been identified at a single Caribbean 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 identification of this final recovery in previously drilled South Atlantic Ocean sites. However, the improved recovery through modern ODP drilling techniques made it possible to acquire a complete APC-cored section in two holes at Site 1262.

The K/P mass extinction effectively changed the state of the global ecosystem, and documenting 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 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 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 Basin. This basinal topography blocks the 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.

Microfossil preservation improved markedly in the late 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. However, detailed reconstruction from DSDP Leg 74 sites were hampered by poorly constrained age models and low recovery.

As part of ODP Leg 208, we intend 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 in sediments of drill holes at Ceara Rise (Lourens et al., in press). 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. High-resolution natural gamma radiation and magnetic susceptibility variability examinations on board the JOIDES Resolution during Leg 208, combined with multiple drill holes at every site, have revealed cycles throughout the Neogene.

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 depth transect recovered during ODP Leg 208 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 deepwater (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 ODP 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). ODP 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 ODP 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 ODP 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.

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