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 employed by 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.
Leg 208 scientists cored a total of six sites between 2.5 and 4.8 km on Walvis Ridge. Upper Maastrichtian through Pleistocene sediments were recovered at Sites 1262 (4759 m) and 1267 (4354 m), upper Paleocene through Pleistocene sediments at Sites 1266 (3797 m), 1265 (3059 m), and 1263 (2717 m), and lower Oligocene through Pleistocene sediments at Site 1264 (2507 m) (Fig. F33).
Overall, the Pleistocene through upper Miocene section is well represented at all sites and 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 1263 and 1265 and condensed at Site 1264. At least part of the lower Oligocene has been identified at all sites. 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.
In sharp contrast, the primary drilling target, the lower Eocene through upper Paleocene, is well represented at all sites where drilled, 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. 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 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 at 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 interrupted by turbidites. At Site 1264, most of the upper Miocene is expanded but the middle and lower Miocene is complete and 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 foraminifer 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 their absence 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 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 1263 and 1265 and condensed at Site 1264, and 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 rare or have a discontinuous distribution at all sites, but the marker species S. distentus and S. ciperoensis provide biostratigraphic control. Rich "Braarudosphaera layers" occur in Zone NP23 (CP18) at Sites 1264 and 1265. Helicosphaera 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, least at Site 1264. In situ components are mainly long-lived 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 marking 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 the 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, 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 sub-optimal environmental conditions. Biostratigraphic subdivisions of lower Eocene sediments was hindered by the absence of as Planorotalites palmerae and general scarcity of Morozovella formosa, which likely reflects unfavorable ecological conditions.
Because of condensed sections, unconformities and reworking the upper Eocene–lower Oligocene benthic foraminiferal faunal turnover marked by the uppermost occurrence of Nuttallides truempyi could not be documented in detail. 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 clayey layer just above the boundary as defined by the uppermost occurrence of the benthic foraminifer S. beccariiformis in the benthic extinction event (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/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 at the other sites.
There is no evidence for unconformities in the Paleocene, but there is some dissolution, reworking, and downslope transport in 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 NP3/NP4 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 foraminifer assemblages across the K/P boundary at both sites do not exhibit any evidence of significant extinction.