and P1a).
The sediments recovered during Leg 208 have shed new light on the nature of short-term paleoceanographic events of the last 70 m.y., while also improving upon our understanding of the general long-term trends as established by earlier cruises. DSDP Leg 74, in particular, provided the essential groundwork for Leg 208 by defining the long-term depositional history of Walvis Ridge and surrounding basins and also by identifying the presence of critical intervals, including the P/E boundary (Moore, Rabinowitz, et al., 1984). This body of work, along with new multichannel seismic profiles (Spieß et al., 2003), was essential in developing the drilling strategy for Leg 208.
Despite its numerous seminal contributions, the critical deficiency of Leg 74 was a technological one, the inability to continuously core sedimentary sections with minimal disturbance. The consequences of this deficiency became increasingly evident as Leg 208 progressed. Because of the exceptional core recovery in multiple holes, we were able to resolve the complete spectrum of lithologic variability down to the centimeter scale, including the orbitally paced oscillations. As a consequence, Leg 208 documented the occurrence of a number of critical events of the Paleogene, several either previously undocumented or poorly constrained. These include the well-known events that served as the primary targets for this cruise, the P/E and K/P boundaries, but also several heretofore unrecognized events in the early Eocene and Paleocene. Finally, Leg 208, with the depth-transect approach, was also able to establish the character of both the long- and short-term changes in the vertical dimension, a constraint that is essential to understanding both the causes and consequences of change.
In the following section, we summarize the major findings of this leg, focusing primarily on those contributions that are novel or represent a significant improvement upon previous findings. We start with a brief summary of the chronostratigraphy, followed by an overview of the depositional history of Walvis Ridge and its relation to paleoceanographic change, and then focus on specific critical events that were documented at multiple holes.
During Leg 208, a total of six sites were cored between 2.5 and 4.8 km water depth on Walvis Ridge. Upper Maastrichtian through Pleistocene sediments were recovered at Sites 1262 (4759 m) and 1267 (4354 m); upper Paleocene through Pleistocene sediments were recovered at Sites 1266 (3797 m), 1265 (3059 m), and 1263 (2717 m); and lower Oligocene through Pleistocene sediments were recovered at Site 1264 (2507 m) (Fig. F33).
Overall, the Pleistocene through upper Miocene section is represented at all sites and is most expanded at Site 1264. The middle through lower Miocene is relatively condensed at Sites 1264 and 1265 and highly condensed and/or interrupted by unconformities at all other sites. The upper Oligocene is present at Sites 1264 and 1265 and partially present at Site 1263. At least part of the lower Oligocene has been identified at all sites, with the record most incomplete at Site 1262. The E/O boundary is not recovered at Site 1262 and 1267 and is present in intervals of intense reworking and dissolution at Sites 1265 and 1266. The upper and middle Eocene is highly condensed or unconformable at all sites except Site 1263. Microfossil assemblages in the middle Miocene through middle Eocene are affected to varying degrees by dissolution, reworking, and downslope transport, especially at Sites 1265–1267.
The lower Eocene through upper Paleocene is well represented at all sites, with generally good preservation of calcareous microflora and fauna except for dissolution in the clay layer just above the P/E boundary. The lower Paleocene and upper Maastrichtian are well represented at Sites 1267 and 1262. Preservation of microflora and fauna is generally good to moderate, with some reworking and dissolution in the lower upper Paleocene and dissolution in the Maastrichtian at Site 1267.
Pleistocene sediments were recovered at all sites, with the most expanded sections (~20 m) at Sites 1266 and 1267 and the most condensed sections at Sites 1263 and 1265. Calcareous nannofossils are abundant and have good preservation at all sites, except within slumps at Site 1262. A complete succession of nannofossil events is present at Sites 1264, 1266, and 1267. Pleistocene assemblages of planktonic foraminifers are a mixture of well-preserved subtropical to temperate planktonic foraminifers, with good preservation at Sites 1264 and 1265 and common reworking in the basal Pleistocene at Sites 1262 and 1265–1267. Benthic foraminifers are rare and well preserved at all sites and resemble faunas presently living in the Walvis Ridge region with depth-dependent assemblage composition.
The most expanded Pliocene sections were recovered at Sites 1264, 1266, and 1267. The lower Pliocene is missing at Site 1263, and the uppermost upper Pliocene is missing at Site 1265. Pliocene calcareous nannofloras consist of abundant nannofossils with good preservation. The midlatitude discoasterid markers Discoaster asymmetricus and Discoaster tamalis are common to abundant, and the occurrence of common Scyphosphaera spp. characterizes the lower Pliocene.
Planktonic foraminiferal biostratigraphic resolution through the upper Pliocene is limited by the scarcity of key tropical/subtropical age-diagnostic taxa (e.g., the absence of all menardellids and Globigerinoides fistulosus), probably because of the presence of cool temperate waters. Assemblages from Site 1264 are best preserved. Reworking is common at Sites 1262 and 1267, and there are numerous turbidites at Sites 1262 and 1266. Benthic foraminifers are generally rare but well preserved, resembling faunas presently living in the Walvis Ridge region. Downhole fluctuations in relative species abundance probably reflect variability in deepwater circulation and productivity.
At no site was a complete Miocene sequence recovered, but the upper Miocene is present at most sites. There are unconformities across the Miocene/Pliocene boundary at Sites 1263–1265, in the upper Miocene at Site 1266, and in the lower to middle Miocene at Site 1267. The Miocene is condensed at Sites 1263, 1262, and 1267 and at several sites is interrupted by turbidites. At Site 1264, most of the upper Miocene is expanded and the middle and lower Miocene are complete but relatively condensed. Calcareous nannofossil assemblages of uppermost and/or lowermost Miocene (including the Oligocene/Miocene [O/M] boundary) are rich and moderately well preserved at Sites 1264–1266. The upper Miocene markers Discoaster quinqueramus, Discoaster berggrenii, and Discoaster hamatus, the middle Miocene Helicosphaera ampliaperta and most of the helicoliths, and the lower Miocene Triquetrorhabdulus carinatus are absent at all sites. All Miocene assemblages are dominated by small- and medium-sized placoliths (Reticulofenestra spp. and Cyclicargolithus spp.). Miocene planktonic foraminiferal assemblages have generally good preservation at Site 1264 but have been affected by dissolution at all other sites. The subdivision of the upper–middle Miocene was hampered by the general absence of the Fohsella clade, probably because of temperate-water conditions, although the absence of this genus may also reflect evolution toward a more stenothermal ecology.
Middle–lower Miocene benthic foraminiferal assemblages are affected by downslope transport and reworking at all sites except Site 1264. The middle Miocene benthic foraminiferal turnover could be recognized at all sites but not documented in detail because of the unconformities and condensed sections. The lower Miocene bolivinid event (defined as the high abundance of bolivinids [HAB] event; ~18 Ma) was documented at Sites 1264 and 1265.
There are unconformities corresponding to part of the Oligocene at Sites 1262, 1263, 1266, and 1267. The upper Oligocene is present at Sites 1264 and 1265 and highly condensed at Site 1263. At least part of the lower Oligocene has been identified at all sites. Oligocene assemblages at all sites are affected by dissolution and/or reworking. Oligocene calcareous nannofossils are generally abundant and moderately preserved and show slight dissolution and low diversity. Reworked Eocene specimens are most common at Sites 1262 and 1265. Sphenoliths vary in abundance and are generally rare or have a discontinuous distribution at all sites, but the marker species Sphenolithus distentus and Sphenolithus ciperoensisprovide biostratigraphic control. Rich "Braarudosphaera layers" occur in Zone NP23 (CP18) at Sites 1264 and 1265. Helicosphaera spp. are always rare and absent in some intervals. The succession of lowermost Oligocene events could be identified at Sites 1263, 1265, 1266, and 1267, but the lowermost Oligocene biostratigraphic marker Isthmolithus recurvus, a "cool-water taxon," has a discontinuous distribution. Intense dissolution and extensive reworking of planktonic foraminifers made identification of the O/M boundary problematic at Sites 1262, 1263, 1266, and 1267. This interval contains well-preserved assemblages at Site 1265, but the record is interrupted by a slump. Lower Oligocene planktonic foraminiferal assemblages were recovered at all sites but were affected by intense dissolution.
Benthic foraminiferal assemblages are affected by dissolution, reworking of older material, and severe downslope transport at all sites, but least at Site 1264. In situ components are mainly long-lived and common uppermost Eocene through lower Miocene bathyal through abyssal species. Transported components reflect middle bathyal or greater depths.
Sections spanning the E/O boundary interval are incomplete or condensed. Sites 1263 and 1265 contain the most complete records, but even these are affected by dissolution and reworking. Calcareous nannofossil events delimiting this critical interval could be recognized at Sites 1263, 1265, and 1266. Preservation is moderate and specimens show dissolution, etching, and reworking. The uppermost Eocene, characterized by the uppermost occurrences of representatives of the rosette-shaped discoasters, contains reworked Paleocene and Eocene forms. Planktonic foraminiferal assemblages show severe dissolution and reworking even at Sites 1263 and 1265, and the scarcity of key marker taxa (e.g., Turborotalia cerroazulensis and Cribrohantkenina inflata) hindered evaluation of stratigraphic completeness. A winnowed well-sorted assemblage of thick-shelled Globigerinatheka spp. is present within the uppermost Eocene at Sites 1263, 1265, 1266, and 1267 and intercalated in lower Oligocene sediments at Site 1266.
The upper Eocene sections are condensed or interrupted by unconformities at all sites, and reworking is common. The upper part of the middle Eocene is incomplete at all sites except Site 1263. At Sites 1265 and 1266, the lower boundary of the middle Eocene is also marked by unconformities. Preservation of all calcareous microfossil groups improves in the lower Eocene at all sites but deteriorates because of dissolution just above the P/E boundary. Calcareous nannofossils are diverse but moderately preserved, and discoasters show strong overgrowth. Reworking is common in the middle and upper Eocene at Sites 1265 and 1266. The upper Eocene marker species Cribrocentrum reticulatum is absent at all sites. The absence of several key tropical-marker species of planktonic foraminifers reduced the level to which the Eocene could be subdivided. Specifically, upper through middle Eocene sections typically lack such marker species such as Orbulinoides beckmanni, Morozovella lehneri, and Hantkenina nuttalli. The scarcity of these biostratigraphically useful species is the result of strong carbonate dissolution and/or suboptimal environmental conditions. Biostratigraphic subdivision of lower Eocene sediments was hindered by the absence of as Planorotalites palmerae and the general scarcity of Morozovella formosa, which likely reflects unfavorable ecological conditions.
Condensed sections, unconformities, and reworking of the upper Eocene–lower Oligocene prevented detailed documentation of the benthic foraminiferal faunal turnover marked by the uppermost occurrence of Nuttallides truempyi. The presence of abundant Plectofrondicularia paucicostata indicates unusually intensive downslope transport in the upper Eocene at Sites 1265–1267. In contrast, lower Eocene benthic foraminiferal faunas are generally well preserved and show fluctuating species richness and relative abundance of abyssaminids and small smooth-walled bolivinids.
The P/E boundary was recovered at all sites except Site 1264 and is marked by a prominent clay layer, the lowermost few centimeters of which is barren or contains very few calcareous microfossils. Preservation of all microfossil groups deteriorates in the clay layer just above the boundary as defined by the uppermost occurrence of the benthic foraminifer S. beccariiformis in the BEE.
Calcareous nannofossil assemblages are abundant to sparse close to the boundary. Discoasters increase in abundance, and Rhomboaster spp. show their lowermost occurrence in the clay level. Specimens belonging to the Rhomboaster-Tribrachiatus plexus are poorly preserved because of recrystallization, which prevents the identification of the boundaries between Zones NP11, NP10, and NP9 at most sites. Fasciculithus and Zygrhablithus show a reversal in relative abundance just above the BEE. The clay layer contains few planktonic foraminifers. The Morozovella velascoensis clade is poorly represented in the Walvis Ridge region, and no "excursion" taxa were present in the P/E boundary interval. The overall scarcity of M. velascoensis made identification of the P5/P6a zonal boundary problematic.
The benthic foraminiferal extinction event was identified at all sites at the base of the clay layer. S. beccariiformis is present in the uppermost Paleocene sample just below the clay layer at all sites over the full depth transect. The lowermost few centimeters of the clay layer were barren at Sites 1262, 1267, and 1266 and contained a few small specimens at Sites 1265 and 1263. Postextinction faunas are dominated by minute N. truempyi, abyssaminids, clinapertinids, quadrimorphinids, small species of Bulimina, Aragonia aragonensis, and Tappanina selmensis. Abyssaminids and clinapertinids are more abundant at Sites 1262 and 1267, and buliminids, T. selmensis, and A. aragonensis are more abundant at the other sites.
There is no evidence for unconformities in the Paleocene, but there is evidence for dissolution, reworking, and downslope transport in some intervals of the lower upper Paleocene at Sites 1262 and 1267. Diverse assemblages of calcareous nannofossils with good to moderate preservation show characteristics of midlatitude assemblages. Reworked Cretaceous specimens are rare between the lower part of Zone NP8 and the K/P boundary at Sites 1262, 1266, and 1267. The "mid-Paleocene biotic event" is recognized by the lowermost occurrence of Heliolithus kleinpellii at the base of Zone NP6 (CP5) at Sites 1262 and 1267. The rarity of Ellipsolithus macellus in the lower part of its range prevents the recognition of the CP3/CP2 (NP4/NP3) zonal boundary.
Abundant large planktonic foraminifers are present through much of the Paleocene. Upper Paleocene assemblages are dominated by the genera Acarinina, Morozovella, Subbotina, and Globanomalina. The mid-Paleocene biotic event occurs in the lower part of the upper Paleocene (basal Subzone P4a) in a clayey interval at Sites 1262 and 1267 and is marked by a short-lived increase in the abundance of small igorinid taxa (Bralower, Premoli Silva, Malone, et al., 2002; Bralower et al., 2002). Lower Paleocene (Zone P2) assemblages are dominated by the praemuricate taxa.
Benthic foraminiferal assemblages are highly diverse in the upper Paleocene, but at the three deepest sites there are strong fluctuations in the relative abundance of Siphogenerinoides brevispinosa and Bulimina thanetensis. The lowermost occurrence of the latter may mark the benthic foraminiferal expression of the mid-Paleocene biotic event.
The Maastrichtian/Danian boundary was recovered at Sites 1262 and 1267 and appears to be complete at both sites, although the planktonic foraminiferal Zone P0 could not be identified. Danian calcareous nannofossil assemblages are abundant and moderately well preserved. Just above the K/P boundary, assemblages contain reworked Cretaceous specimens and are dominated by Thoracosphaera spp., Biantholithus sparsus, and Cyclagelosphaera reinhardtii. Calcareous nannofossils are abundant in the upper Maastrichtian. Preservation varies from good to moderate with strong dissolution and fragmentation in the uppermost Maastrichtian where the assemblages are mainly composed of solution-resistant species such as Micula straurophora, Micula murus, Watznaueria barnesae, Lithraphidites carniolensis, and Lithraphidites quadratus. Nephrolithus frequens, a cool-water marker species, is rare.
The Danian planktonic foraminiferal assemblages contain some reworked Cretaceous specimens and abundant but diminutive planktonic foraminifers (e.g., Woodringina hornerstownensis, Parasubbotina spp., Globoconusa daubjergensis) with highly variable preservation at Site 1267 and excellent preservation at Site 1262, particularly in the clay-rich sediments just above the K/P boundary. The Maastrichtian assemblages at both sites are moderately preserved and exhibit varying degrees of etching and dissolution. As observed elsewhere, benthic foraminiferal assemblages across the K/P boundary at both sites do not exhibit any evidence of significant extinction.
The soft, weakly magnetized carbonate sediments recovered during Leg 208 frequently produced erratic or seemingly biased inclination records, making magnetostratigraphic interpretations difficult or impossible over many intervals. Despite this, several polarity sequences were identifiable, including most of the major boundaries in the Pliocene–Pleistocene, an upper Miocene through Oligocene sequence at Sites 1265 and 1266, and an excellent Paleocene through Upper Cretaceous sequence at Sites 1262 and 1267.
Whereas the inclination record from the Pliocene–Pleistocene is not very clean at most sites, we were frequently able to identify major reversal boundaries. However, assignment to a particular boundary was often aided in this interval by biostratigraphic datums or cyclostratigraphy. Of particular note, Chron C2n, close to the Pliocene/Pleistocene boundary, is identified at all sites except Site 1262.
Sites 1265 and 1266 were combined to produce an interpretable upper Miocene through Oligocene sequence. In particular, this includes the excellent expression of Chron C6Cn at Site 1265 across the O/M boundary. This chron consists of three very distinctive short normal events which, combined with the biostratigraphic data and cyclostratigraphy, should allow for the refinement of the timescale across the O/M boundary.
The Eocene was generally not well resolved at any of the sites, but an excellent Paleocene to Upper Cretaceous polarity sequence was recovered at Sites 1262 and 1267 (Fig. F34). At these two sites, the Paleocene cores were recovered either by APC or XCB systems in well-lithified sediments, both of which served to recover relatively undisturbed sediments over this interval.
Identification of the upper and lower boundaries of Chron C24r will be important to constrain the position of the P/E boundary within this chron. Unfortunately, the top of Chron C24r is not well defined in the shipboard pass-through inclination data at most sites. Data from the lower Eocene are highly scattered and the inclination data are frequently biased toward negative values. It seems likely that the boundary is resolved in at least one hole of Sites 1262 and 1266, but discrete sample analysis should provide cleaner data to precisely constrain the top of Chron C24r. The base of Chron C24r is clearly defined at several sites, although high-resolution discrete samples should help to position or pinpoint the boundary more precisely than yet possible with the pass-through archive-half data.
The boundaries of Chron C29r have also been targeted for special attention to better constrain the reversal boundary ages around the K/P boundary. The inclination record in this interval at Site 1262 is characterized by high-frequency oscillations in both inclination and intensity. It is likely that the pass-through magnetometer maps these intensity oscillations into directional changes (Parker and Gee, 2002) and that discrete sample data will provide a clean record of the reversal boundaries. This problem is not seen at Site 1267; however, where the top of Chron C29r is relatively well defined, the base of the chron unfortunately falls in a core break in both holes.
Cyclic variations are observed in MS and color reflectance data throughout the Maastrichtian to Holocene section at all sites. These variations are expressed by lithologic changes at a decimeter to meter scale. The cyclic variability was used to correlate between parallel holes and to define a composite section for each site. About 300 characteristic features (peaks or troughs) in MS were identified and used to correlate between Leg 208 sites (Fig. F35). These tie points were dated using the age models of the individual sites. By adopting the average age of each tie point, a refined age model was constructed for the complete Leg 208 ~74-m.y. record (Table T2). The distinct record of cyclic alternations in sediment physical properties (Figs. F36, F37, F38) offers potential for refining the Neogene astronomical timescale and the development of an astronomically tuned timescale of the Paleogene as far back as the Late Cretaceous.
The detailed investigation of the sedimentary cycles and their relation to orbital forcing will be an important objective of postcruise studies. Nevertheless, preliminary continuous wavelet analysis (e.g., Torrence and Compo, 1998) on selected time intervals reveals a strong influence of the astronomical cycles of precession (~20 k.y.), obliquity (~40 k.y.), and eccentricity (~100 and ~400 k.y.) on the sedimentary cycle patterns of most of the Leg 208 sites. For instance, the 100-k.y. cyclicity is very strong in the lower Miocene, whereas the ~40- and ~20-k.y. periodicities are less well pronounced (Fig. F39). During the Eocene, both precession- and obliquity-related cycles as well as the 100-k.y. eccentricity cycle are recorded (Fig. F40). In the last example, a dominant influence of the precession cycle is shown during the Maastrichtian (Fig. F41).
A key aspect of the Leg 208 drilling strategy was the depth-transect approach, the basic objective of which is to constrain time-dependent changes in sediment properties as a function of depth. In principle, this strategy provides a number of advantages for paleoceanographic studies including the ability to (1) reconstruct changes in sediment production rates, (2) contour changes in the CCD and lysocline depths, (3) establish deepwater circulation patterns, and (4) splice stratigraphic gaps in high-resolution time series. For the Leg 208 transect, the benefits of these advantages were clearly evident, particularly for establishing the long-term sediment accumulation history of the ridge. Using the lithostratigraphic and biomagnetostratigraphic data, 1-m.y.-averaged sediment accumulation curves for each site were constructed (Figs. F42, F43, F44), distinguishing noncarbonate from carbonate accumulation. The accumulation curves were then combined with the subsidence curves to contour the depth-dependent changes in accumulation rates as a function of time (Fig. F45). The curves are based on a simple thermal subsidence model that derives ~1.8–2.0 km of subsidence over the last 65 m.y. Although the absolute age datings are preliminary, the relative correlations are precise as a result of the application of cyclostratigraphic tie points to the Leg 208 sites.
In general, carbonate accumulation rates were highest in the Maastrichtian, Paleocene, early Eocene, and Pleistocene and lowest in the middle and late Eocene and early–middle Miocene (Fig. F45). These time-dependant changes are much more pronounced at the deeper sites, indicating that changes in the level of the CCD and lysocline contributed to the observed patterns. The large-magnitude CCD events have been previously recognized and closely correspond in time to similar changes observed in other basins (Peterson and Backman, 1990; Lyle, Wilson, Janecek, et al., 2002).
The Leg 208 records show that from the K/P boundary to the early middle Eocene, the CCD appears to have maintained a position below the deepest Site 1262 (~3.5 km at 55 Ma), with one brief exception during the PETM. During the early middle Eocene, the CCD began to shoal and carbonate accumulation began to collapse, initially at Site 1262 at ~50 Ma then at Site 1267 at ~49 Ma. The CCD continued to ascend, eventually rising above Site 1266 to its shallowest level between 2.5 and 3.0 km at 44 Ma. During the late Eocene, the CCD deepened significantly in two steps, with the first at ~42 Ma and the second at ~36 Ma. The latter descent, which is global in extent, was relatively rapid and extreme. It appears that the CCD settled at depth well below Site 1262, but only briefly before gradually shoaling again within a few million years. From the mid-Oligocene to middle Miocene, the CCD fluctuated at a middepth level, roughly between 3.2 and 4.5 km, deepening briefly in the early Miocene. In the late middle Miocene, the CCD began a slow descent, eventually reaching a depth close to modern by the earliest Pliocene.
A major achievement of Leg 208 was the recovery of continuous undisturbed cores spanning over a half-dozen critical intervals or events. All events were recovered from at least two sites, and at least three of the early Cenozoic events were recovered at five sites. The Leg 208 cores allow each event to be observed in the context of orbitally paced oscillations in climate. These events are unique, as they stand out against the normal background variability of environmental change. They generally coincide with biotic perturbations such as extinctions or abundance acmes indicative of unusual environmental stress. The most prominent are the P/E and K/P boundaries, characterized by relatively rapid and extreme change. The other events, although less extreme, show characteristics that indicate brief extremes in climate and/or ocean carbonate chemistry. This includes the mid-Paleocene biotic event at 58.2 Ma (close to the lowermost occurrence of H. kleinpellii [Bralower et al., 2002]), the EOGM at 33.5 Ma, the early Oligocene Braarudosphaera layers at 28.5–30 Ma, and the early Miocene Bolivina acme at ~18 Ma. In addition, several previously unrecognized events, characterized by clay layers similar to the P/E boundary but of a smaller scale, were identified in the upper Paleocene and lower Eocene of all sites. The most distinct of these smaller events are two clay layers in the lower Eocene referred to as X and Y (see Fig. F37) events. The Y event occurs in Chron C24n at ~53 Ma, close to the uppermost occurrence of Discoaster multiradiatus. The assertion that these dissolution layers are linked to global events, and not regional, is based on their presence in other ocean basins, primarily the Pacific, where they are documented in cores recovered from Shatsky Rise (Bralower, Premoli Silva, Malone, et al., 2002). The documentation of such events is of importance, as their occurrence was predicted on the basis of anomalous excursions in benthic foraminiferal assemblages (Thomas et al., 2000).
A remarkably well-preserved complete K/P boundary was recovered at Walvis Ridge. The boundary was cored at two Sites 1262 and 1267. Double coring at these sites resulted in a total of four separate K/P records. In Hole 1267A, the boundary was biscuited because of XCB coring, whereas the record in Holes 1262B and 1262C was cored by the APC. The lithologic sequence in the K/P boundary interval is similar at both sites, as they differ in water depth by only 400 m today and had the same paleodepth during the K/P interval. At Site 1267, the boundary interval was more lithified, leading to a greater abundance of Maastrichtian chalks below the boundary.
Common to both Sites 1262 and 1267, the K/P boundary records an abrupt transition from highly cyclic Maastrichtian clay-bearing nannofossil ooze with foraminifers (nannofossil Zone CC26) to overlying Paleocene dark reddish brown clay-rich foraminifer-bearing nannofossil ooze and nannofossil clay. This boundary coincides with a distinctive increase in MS and a decrease in sediment lightness (Fig. F46), corresponding to an overall increase in the abundance of clays, oxides, and ash these earliest Paleocene sediments comprise. At Site 1262, microtectites are present at the boundary and overlying sediments grade upward into moderately bioturbated brown nannofossil- and foraminifer-bearing clay (foraminiferal Zones P and P1a).
Preliminary biostratigraphy at all of the K/P boundary sequences shows the well-established abrupt change in plankton assemblages across the boundary (Luterbacher and Premoli Silva, 1964; Thierstein, 1982; Monechi, 1985). The white nannofossil ooze below the boundary yields diverse assemblages of the uppermost Maastrichtian Abathomphalus mayaroensis planktonic foraminiferal zone and Chiloguembelina nannofossil Zone CC26. The brown nannofossil- and foraminifer-bearing clay contains a high abundance of W. hornerstownensis, Chiloguembelina midwayensis, and Chiloguembelina morsei as well as increasing abundance of Parvularugoglobigerina eugubina through the basal Paleocene (P). Based on this preliminary analysis, Zone P0 is not present at Walvis Ridge. Remarkably well preserved planktonic foraminifers of Zone P dominate the lowermost 20 cm of the Paleocene at Site 1262 (Fig. F46), whereas at Site 1267 the preservation within the similarly thick sequence is highly variable. At this site the dwarfed assemblages from within Zone P to Subzone P1b show fragmentation and some specimen overgrowth. Large reworked specimens of Maastrichtian foraminifers are present within Zone P at both sites, although to a lesser degree at Site 1267. The benthic foraminiferal fauna just above the boundary in Hole 1267A is distinctly different from samples above and below, since Bulimina kugleri becomes common and Praebulimina reussi becomes extinct. This opportunistic species is assumed to reflect high-nutrient conditions just above the boundary.
Nannofossils in the basal Danian sediments include survivor species such as Thoracosphaera spp. plus the first occurrence of B. sparsus, Markalius inversus, and C. reinhardtii. Both nannofossils and planktonic foraminifers show a complete succession typical for the Danian (Luterbacher and Premoli Silva, 1964; Monechi, 1985). The uppermost Maastrichtian at all sites shows signs of dissolution in both nannofossils and foraminifers. The planktonic foraminifers display etching and fragmentation. The minute thin-walled earliest Paleocene faunas, however, are remarkably well preserved at Site 1262, suggesting that a latest Maastrichtian lysocline shoaling resulted in sediments barren of foraminifers, just prior to the K/P boundary, followed by subsequent deepening in the earliest Paleocene. A significant feature of the K/P boundary at Site 1262 is the presence of greenish unaltered ovoid spherules that are concentrated in the first few centimeters of the basal Paleocene. No spherules were found higher in the core.
Although Zone P is normally unrecovered or poorly preserved at most sites, the substantial thickness of the uppermost Maastrichtian Micula prinsii Zone and the lowermost Danian P. eugubina Zone indicate the K/P boundary is paleontologically complete. Moreover, the cycle stratigraphy is very robust with distinct spectral peaks in the precession and eccentricity bands. Thus, the Walvis Ridge sections provide a well-preserved and relatively detailed record of this major extinction event and the subsequent biotic recovery.
A prominent 10- to 30-cm-thick dark brown clay-rich calcareous nannofossil ooze was found at Sites 1262 and 1267, and a 10-cm-thick brown nannofossil chalk was found at Site 1266. This layer shows a pronounced peak in MS that reflects an increase in clay content (Fig. F47). Preliminary micropaleontological investigations suggest that this interval represents a short-lived event of considerable evolutionary significance. This interval corresponds to the P4 Globanomalina pseudomenardii planktonic foraminiferal zone and coincides with the evolutionary first occurrence of the nannofossil H. kleinpellii, an important component of late Paleocene assemblages and a marker for the base of Zone CP5 (NP6) (early late Paleocene; ~58.2 Ma). The event was also identified at ODP Leg 198 Sites 1209–1212 (Bralower, Premoli Silva, Malone, et al., 2002).
Fundamental changes in faunal populations occur before, during, and after the deposition of the clay-rich ooze. Planktonic foraminifers in the clay-rich layer are characterized by a largely dissolved low-diversity assemblage dominated by representatives of the genus Igorina (mainly Igorina tadjikistanensis). This low-diversity assemblage suggests some kind of oceanic perturbation of unknown origin. Together with the documented severe dissolution in this interval, the observed lithologic changes are likely to represent a response to increased seafloor carbonate dissolution owing to a transient shoaling of the lysocline and CCD. Regardless of origin, it is now clear from the high-resolution stratigraphy of the Leg 208 sites that this is a global event. Shore-based isotopic investigations should shed light on the nature of this event.
The primary objective of Leg 208 was the recovery of a South Atlantic depth transect to reconstruct the tempo and mode of regional carbonate saturation response to the global carbon-cycle perturbation during the PETM. The PETM interval was successfully recovered in multiple holes at five sites (e.g., Sites 1262, 1263, 1265, 1266, and 1267) that covered a modern depth range of 2717 to 4755 m and an estimated paleodepth range of ~1500 to ~3500 m (Fig. F48). Shipboard physical property, lithologic, and biostratigraphic data indicate complete recovery of the Paleocene–Eocene transition interval at all sites except Site 1265, where drilling difficulties prevented recovery of the entire ooze–clay transition or the P/E transition interval had not been fully deposited. Major patterns within and between these sites are summarized and interpreted below in the depth domain; subsequent shore-based stable-isotope and cyclostratigraphic analyses will serve to test and refine these interpretations in the time domain.
In the uppermost Paleocene, nannofossil ooze predominates across the entire depth transect. Deeper sites have slightly lower carbonate content and markedly higher MS values, whereas intrasite variance in both parameters is minimal. Microfossil preservation varies between excellent and moderate both within and between sites. These patterns are consistent with a relatively stable latest Paleocene carbonate saturation profile of increasing undersaturation at greater paleodepth and a paleo-CCD well below the deepest site (~3500 m). Notably, at the deeper Sites 1267 and 1262, the uppermost centimeters of nannofossil ooze immediately underlying the PETM clay show slightly decreasing carbonate content, increasing MS, increasing planktonic foraminiferal fragmentation, and unusually small pre-extinction benthic foraminiferal specimens. These latest Paleocene changes likely represent some combination of syndepositional shoaling of the carbonate saturation profile and postdepositional carbonate dissolution "burndown."
At the onset of the PETM, carbonate content plummets to ~0 wt%, producing a pronounced lithologic shift from nannofossil ooze to a clay interval that roughly doubles in thickness from ~20 to ~50 cm down the depth transect. This general pattern of decreased carbonate content is present throughout marine PETM records (e.g., Bralower et al., 1997; Thomas and Shackleton, 1996; Thomas, 1998; Thomas et al., 2000) and is consistent with a massive methane flux to the ocean-atmosphere inorganic carbon reservoir that elevated pCO2 levels, decreased carbonate ion concentrations, and shoaled the carbonate saturation profile (Dickens et al., 1995, 1997). Commensurately increasing MS values show progressively more structure in the thicker clay layers of deeper sites, whereas the maximum values at each site coincide with the uppermost clay interval, where carbonate content begins to recover. These complex patterns represent some combination of time-transgressive shoaling of the paleo-CCD, differential burndown of previously deposited carbonate, and increased terrigenous input from enhanced chemical weathering and erosion. Most importantly, these new data clearly demonstrate that the South Atlantic paleo-CCD shoaled much more (>2000 m) than predicted by current models (~400 m) (Dickens et al., 1997), suggesting the release of a much larger volume of less isotopically negative methane or an incomplete understanding of carbon cycle dynamics.
Biostratigraphically, the onset of clay deposition coincides with the highest occurrence of the benthic foraminifer S. beccariiformis and other typical upper Paleocene taxa (Fig. F48). Calcareous microfossils are absent to extremely rare and generally poorly preserved in the lowermost clay, reflecting some combination of deleterious benthic conditions, decreased carbonate export production, and intensified carbonate dissolution. Lowermost Eocene benthic foraminiferal assemblages occur near the base of the clay layer (Fig. F48) and are extremely low in abundance and minute in size. A. aragonensis, T. selmensis, and Bulimina spp. predominate at shallower Sites 1263, 1265, and 1266, whereas abyssaminids and clinapertinids predominate at deeper Sites 1262 and 1267 and in the lowermost sample at the shallower sites. Nannofossils are common and show only slight dissolution. The last appearance of the planktonic foraminifer M. velascoensis roughly coincides with the onset of the PETM, and no related "excursion" taxa (e.g., Morozovella allisonensis, Acarinina sibaiyaensis, Acarinina africana) occur within the PETM—a biogeographic pattern that stands in stark contrast to those documented at lower and higher paleolatitudes (Kelly et al., 1998; Kelly, 2002). Planktonic foraminifers within the clay layer primarily consist of extremely rare and poorly preserved specimens of A. soldadoensis. Nannofossil assemblages within the PETM clay are markedly poorer in preservation, lower in abundance and richness, and predominated by discoasters.
The start of the PETM recovery interval may be defined as the onset of increasing carbonate content, which produced a gradational upsection lithologic sequence at each site of nannofossil-bearing clay, nannofossil clay, clay-bearing nannofossil ooze, and finally nannofossil ooze. Recovery intervals are thicker at shallower sites, likely reflecting higher overall mass accumulation rates coupled with the time-transgressive deepening of the carbonate saturation profile and commensurately earlier increases in carbonate mass accumulation rates. Magnetic susceptibility values are consistent with this scenario, with shallower sites showing more oscillations within the generally decreasing trends.
As carbonate content increased through the PETM recovery interval, planktonic foraminiferal preservation improved and faunal abundance and richness increased to include morozovellids, acarininids, subbotinids, and rare globanomalinids. The first occurrences of the nannofossils Rhomboaster cuspis and Rhomboaster calcitrapa, basal members of the Rhomboaster-Tribrachiatus lineage, are within the recovery interval (Fig. F48) and provide potentially isochronous biomarkers for intersite correlation. Above these nannofossil first occurrences, three prominent bioevents occur in varying stratigraphic order at all five sites: Benthic foraminiferal compositions shift from earliest Eocene low-diversity diminutive assemblages to early Eocene moderate-diversity assemblages (Fig. F48), and nannofossil assemblages show a marked relative decrease in Fasciculithus spp. and increase in Zygrhablithus bijugatus (Fig. F48). This relative increase in Z. bijugatus typically coincides with the BEE in other regions (e.g., Shatsky Rise and central Pacific), but occurs later at Walvis Ridge, and may be correlative with the Fasciculithus–Rhomboaster abundance reversal reported at equatorial Pacific Sites 1220 and 1221 (Lyle, Wilson, Janecek, et al., 2002). These intersite differences in bioevent ordination may represent incomplete sampling coverage or real paleoenvironmental differences between sites.
A subtle, but potentially important, final lithologic feature of the PETM recovery interval is the restabilization of carbonate content and MS values at slightly higher and lower values, respectively, than their pre-PETM values. This pattern is present at other PETM sites (e.g., Southern Ocean Site 690) and is consistent with numerical simulations by Dickens et al. (1997) that predict a transient lysocline overdeepening to result from the oceanic mass-balancing of increased bicarbonate, carbonate, and Ca2+ concentrations (by way of enhanced chemical weathering and runoff) as concurrent carbonate dissolution decreased CO2-derived H+ concentrations and increased Ca2+ concentrations (Broecker and Peng, 1982; Walker and Kasting, 1992).
At ~20 to 35 m above the P/E boundary, a red-colored 5- to 15-cm carbonate-depleted layer was found at all sites covering the lower Eocene (Fig. F49). This layer is tentatively placed in Chron C24n and called the Chron C24n event. This layer was a faithful indicator point for estimating the depth to the underlying P/E boundary in the parallel holes of each site. The Chron C24n event shows similar color characteristics to the P/E boundary layer as well as a drop in calcium carbonate content of the sediment and an increase in MS and NGR values, but these changes are not as intense. The Chron C24n event is characterized by a double peak in the 1-cm-sampled point MS records of all sites, which are marked by events a and b in Figure F49. At the shallowest sites (1263–1266), events a and b are most distinctly developed, whereas at the deepest sites (1267 and 1262) they are merged. At the latter sites, an additional peak in MS values is observed slightly above the main clay layer. At the middepth Site 1266, a thin white-colored layer is found immediately above the red-colored horizon. The uppermost occurrence of nannofossil D. multiradiatus is recorded close to the Chron C24n event.
This layer, which appears to be present in the MS records of sites drilled on Shatsky Rise, is associated with benthic foraminiferal assemblages similar in character to those of the PETM. This implies a transient shift of paleoenvironmental conditions toward those documented for the PETM.
Sediment recording the response of South Atlantic Ocean to global cooling and CCD deepening during the Eocene–Oligocene transition was recovered across a broad range of depths on the northeastern flank of Walvis Ridge during Leg 208, but the transition is not well preserved biostratigraphically. The shallower Sites 1263 and 1265 contain the most complete records, but even at these sites, calcareous nannofossil and benthic and planktonic foraminiferal assemblages are affected by dissolution, downslope transport, and reworking.
The E/O boundary (33.7 Ma) (Berggren et al., 1995) is by definition at the uppermost occurrence (top [T]) of the planktonic foraminiferal taxon Hantkenina spp. Partially dissolved specimens of this solution-susceptible species were present at the shallowest Sites 1263 and 1265, but as a result of strong reworking of Hantkenina spines, the exact position of the E/O boundary could not be precisely determined. The E/O boundary is bracketed by the T of Globigerinatheka spp. (34.3 Ma) and T. cerroazulensis lineage (33.8 Ma) and the T of Pseudohastigerina spp. (32.0 Ma). These are all last appearances and thus easily influenced by reworking.
Calcareous nannofossil preservation in the E/O boundary interval is moderate because of dissolution, etching, and common reworking. Nannofossil events that bracket the boundary are the uppermost occurrence of the rosette-shaped discoasters (T of Discoaster barbadiensis at 34.2 Ma; T of Discoaster saipanensis at 34.0 Ma) and that of Ericsonia formosa (32.9 Ma). The T of D. saipanensis and the T of E. formosa could be observed at all sites (Table T3), but these are uppermost occurrences and thus prone to reworking. The bottom of the increase in the abundance of Ericsonia obruta is at the E/O boundary (33.7 Ma) but was observed at Site 1263 only. The global increase in 
18O values in deep-sea benthic foraminifers (Oi-1; ~33.5 Ma) occurs within the interval between the T of D. saipanensis and the T of E. formosa (34.0–32.9 Ma) (e.g., Zachos et al., 2001).
At all sites, a distinct increase in sediment lightness (L*), probably indicating an increase in carbonate, occurs between the T of D. saipanensis and the T of E. formosa (i.e., between 34.0 and 32.9 Ma) (Fig. F50). This increase in L* is marked by a distinct decrease in MS, but there is no uniform trend in gamma ray attenuation (GRA) bulk density.
At the deep Sites 1262 and 1267, dissolution is severe, the section is biostratigraphically incomplete, and the lithologic change from brown clay below to light brown to gray nannofossil ooze or foraminifer-bearing nannofossil ooze above occurs abruptly over a ~0.5-m interval. At the shallower Sites 1263 and 1265, the increase in L* and the decrease in MS and GRA bulk density occur more gradually over a 2- to 5-m interval marked by a change in lithology from light brown clay-bearing nannofossil ooze below to very pale brown nannofossil ooze above. The increased carbonate content across the boundary interval at the two deeper sites indicates that the lysocline and CCD deepened substantially and rapidly at the Walvis Ridge transect sites. In the latest Eocene, the lysocline was between the paleodepths of Sites 1266 and 1267. During the E/O transition, the lysocline/CCD deepened abruptly to a depth below that of Site 1262. This significant downward shift in lysocline/CCD in the E/O boundary interval has also been observed in other ocean basins (e.g., Zachos et al., 1996; Lyle, Wilson, Janecek, et al., 2002) and possibly reflects an increase in mechanical and chemical weathering rates on continents and related changes in ocean chemistry associated with global cooling (EOGM). The peak in carbonate contents as inferred from L*, however, is transient, as values decline shortly thereafter. This peak in carbonate values corresponds with a magnetic "normal" that may represent Chron C13n.
Several lower Oligocene intervals that contain nannofloras highly enriched in Braarudosphaera debris were recovered during Leg 208. Although still poorly understood, the recurrence of Braarudosphaera layers is thought to reflect unusual paleoceanographic conditions. This inference is supported by the global occurrence of braarudosphaerids among survivor assemblages preserved immediately following the K/P mass extinction. The exotic character of Braarudosphaera assemblages is further enhanced by the scarcity of other contemporaneous nannofossil taxa.
Braarudosphaera layers were recovered from all sites except the two deepest, Sites 1262 (4759 m) and 1267 (4378 m). Cursory examination revealed that two separate braarudosphaerid-rich layers were recovered at Site 1263 (Fig. F51). The upper braarudosphaerid layer is preserved within Section 208-1263A-6H-2 (~50 mcd) and is assigned to nannofossil Zone NP23 and planktonic foraminiferal Zone P20, whereas the lower braarudosphaerid layer from Sample 208-1263A-9H-3, 27 cm (83.84 mcd), is assigned to nannofossil Zone NP21 and planktonic foraminiferal Zone P18. Only one prominent Braarudosphaera layer was recovered at Sites 1264 (Core 208-1264A-29H) and 1265 (Core 208-1265A-15H). Much like the upper braarudosphaerid layer from Site 1263, these layers are from sediments belonging to nannofossil Zone NP23 and may represent a single Braarudosphaera depositional event that blanketed Walvis Ridge. The exact thicknesses of these braarudosphaerid layers is presently unknown.
Previous drilling throughout the South Atlantic Ocean documented the presence of multiple Braarudosphaera-enriched layers in lower Oligocene sequences on both the Rio Grande Rise and Walvis Ridge. In the southwestern Atlantic Ocean on top of the Rio Grande Rise, early Oligocene Braarudosphaera deposits were recovered at DSDP Site 22 (Maxwell, Von Herzen, et al., 1970) and Site 516. One of the braarudosphaerid layers from DSDP Site 22, termed the "Maxwell Marker" for its distinctive acoustic properties, was described as an indurated chalk containing a nannoflora composed of 100% braarudosphaerid fragments (Maxwell, Von Herzen, et al., 1970).
In the southeastern Atlantic Ocean along Walvis Ridge, numerous Braarudosphaera layers from the early Oligocene were recovered from Sites 362, 363, 522, and 526 (Bolli, Ryan, et al., 1978; Hsü, LaBrecque, et al., 1984; Moore, Rabinowitz, et al., 1984). Braarudosphaera deposits around the Walvis Ridge region exhibit a strong spatial pattern, being most common and prominent closer to shore. Despite having been discontinuously cored, a lower Oligocene section drilled off the coast of Africa (Site 362) yielded at least 34 separate Braarudosphaera layers (Bukry, 1978).
Modern braarudosphaerids are most common in high-nutrient, low-salinity coastal waters and are extremely rare in today's open ocean. Moreover, the paleobiogeographic distribution of fossil braarudosphaerids, which extends from the Early Cretaceous to the Holocene, is strongly biased toward neritic coastal plain deposits (Perch-Nielsen, 1985). Thus, the temporal and spatial focusing of Braarudosphaera deposits in lower Oligocene sequences from the subtropical South Atlantic Ocean has been the source of much speculation among paleoceanographers.
Most hypotheses advanced to account for this biogeographical anomaly have invoked reduced sea-surface salinities (e.g., Bukry, 1978). It has been proposed that pulses of deglacial meltwater decreased sea-surface salinities throughout the subtropics, thereby fostering the Braarudosphaera blooms (Bukry, 1978). Other proposed mechanisms have speculated that upwelling of low-salinity nutrient-laden waters triggered Braarudosphaera blooms (Siesser, 1978; Melguen, 1978; Peleo-Alampay et al., 1999). Another suite of hypotheses invokes taphonomic redepositional processes such as submarine slumps and/or surface water current transport to explain the presence of "nearshore" braarudosphaerids in open-ocean, pelagic sediments (e.g., Maxwell, Von Herzen, et al., 1970; Maxwell et al., 1970).
A recent study of stable isotope data derived from assemblages of well-preserved depth-stratified foraminiferal species indicates that enhanced upwelling over glacial/interglacial timescales fueled the recurrence of massive Braarudosphaera blooms at Site 363 (Kelly et al., 2003). The coherent structure of these stable isotope stratigraphies argues against large-scale redepositional mechanisms. This interpretation is corroborated by the presence of Braarudosphaera layers draped atop topographic highs along Walvis Ridge like those at Site 526. Hence, open-ocean Braarudosphaera layers recovered during ODP Leg 208 at Sites 1263–1265 may reflect exceptionally large blooms that extended far offshore. These enigmatic deposits represent a wealth of untapped information about rhythmic changes to the ocean/climate system throughout the subtropical South Atlantic region.
Deep-sea benthic foraminifers show a gradual but profound faunal overturn in the middle Miocene, which started in the late early Miocene before the middle Miocene cooling (e.g., Thomas and Vincent, 1987). Before these gradual faunal changes, however, a highly unusual event occurred in benthic foraminiferal faunas in the western Atlantic and eastern Indian Oceans. Small, smooth species of the genus Bolivina reached extremely high relative abundances (>60%) at bathyal to abyssal open-ocean locations (Thomas, 1986; Smart, 1992; Smart and Murray, 1994; Smart and Ramsay, 1995). The bolivinids are very small and recognized only in studies of the small size fraction (>63 µm).
The event was called the HAB event (Smart and Ramsay, 1995) and cannot be explained by observations on recent benthic foraminifers: high relative abundances of bolivinids occur within oxygen minimum zones under zones of upwelling along continental margins and in the silled basins off California (e.g., Bernhard and Sen Gupta, 1999). It is not clear whether such bolivinid-rich assemblages form in response to the high organic flux, the lack of oxygen, or the combination of both. The HAB event is recognized throughout the eastern Atlantic Ocean, northwest Indian Ocean, and Mediterranean Sea (Smart and Ramsay, 1995) but not in the eastern equatorial Pacific (Sites 573–575) (Thomas, 1985) and eastern Indian Ocean (Site 758) (C.W. Smart, unpubl. data). Because of the spatial extent of the event, Smart and Ramsay (1995) speculated that the bolivinids outcompeted other species in locations bathed by low-oxygen waters derived from Tethyan sources that reached into the western Indian and eastern Atlantic Oceans. However, these explanations of the HAB event are unsatisfactory because there is no evidence in the sedimentary record for low-oxygen conditions or extremely high organic productivity during the event.
Pagani et al. (1999) identified a pronounced increase in the carbon isotopic composition of alkenones coeval with the HAB event at DSDP Site 608 in the North Atlantic Ocean. The carbon isotopic composition of alkenones is strongly controlled by nutrient concentrations in the modern ocean; therefore, these authors suggested that increased algal growth rates (responding to increased local availability of nutrients) and export productivity could have affected the benthic faunas. A linkage between the HAB event and primary producers in the surface waters is also suggested by the fact that the beginning of the HAB event is coeval with the lowermost occurrence followed by a strong increase in abundance of the nannofossil taxon Sphenolithus belemnos at DSDP Site 608 (Olafsson, 1991).
A scenario involving regional changes in surface water nutrients precludes a Tethyan role for the HAB event and supports the hypothesis that a nutrient-rich water mass (possibly similar to Antarctic Intermediate Water) was introduced into the ocean basins where the HAB event occurred or the hypothesis that the water-column stratification changed regionally. The timing of the end of the HAB event closely coincides with the termination of the early Miocene Climatic Optimum and possibly the rapid expansion of the East Antarctic ice sheet. Therefore, the oceanographic conditions responsible for the HAB event may have been due to changes in ocean heat transport that forced early to middle Miocene cooling.
Smart and Murray (1995) identified the HAB event at Walvis Ridge Site 529. During Leg 208, the HAB event was identified at Sites 1264 and 1265, where its lower boundary is coeval (at shipboard sample resolution) with the lowermost occurrence of S. belemnos, as it is at Site 608 in the northeastern Atlantic Ocean. The time interval in which the event occurred (range of S. belemnos) is present, although very thin, in the sediments at Site 1266 and may also yield these aberrant faunas. Preliminary shipboard cyclostratigraphy places the HAB event at Sites 1264 and 1265 in a very similar section of the record (Fig. F52), suggesting that a detailed comparison of the timing and intensity of the HAB event over a depth range of >1000 m will be possible.
