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LEG 208 SYNTHESIS (continued)


The soft, weakly magnetized carbonate sediments recovered on 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, a good 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 at the Pliocene/Pleistocene boundary is identified at all sites except Site 1262.

Sites 1265 and 1266 combined to produce a good upper Miocene through Oligocene sequence. This includes the excellent resolution at Site 1265 of chron C6Cn 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 Oligocene/Miocene 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 coring or by XBC coring 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 help constrain the placement of the P/E boundary within the chron. Unfortunately, the top of Chron C24r is not well defined in the pass-through inclination data at most sites. Data from the lower Eocene is characterized by a large amount of scatter, as well as a frequent bias in the data toward negative values. At Sites 1262 and 1266, we believe the boundary is resolved in at least one hole, but we do not place a high degree of confidence in either of these boundary assignments. It is hoped that discrete sample analysis will provide cleaner data and allow for the precise constraint of this boundary. The base of Chron C24r is clearly defined at several sites, although high-frequency discrete sampling should help to place the boundary more precisely than is 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 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. Unfortunately, the base of the chron falls in a core break in both holes.

Cycle Stratigraphy and Orbital Rhythms

Marked cyclic variations are observed in MS and color reflectance data throughout the Maastrichtian to Holocene section at all the sites. These variations are expressed as more or less strong lithologic cycles that have frequencies at the decimeter to meter scale. This cyclic variability was used to correlate between parallel holes drilled at all sites and to define a composite section for each site. About 300 major cycles (peaks) in MS correlate very well between all Leg 208 sites (Fig. F35). They were calibrated using biostratigraphic datums and used to construct a refined initial-cycle-tuned age model for the complete Leg 208 ~74-m.y.-long 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 will be the objective of intense postcruise studies. Nevertheless, initial time-series analysis on selected time series representative for distinct intervals (late Maastrichtian, early Eocene, early to early late Miocene) were already done during the cruise. Continuous wavelet analysis allows rapid localization of repetitive patterns (e.g., Torrence and Compo, 1998). The 100-k.y. cyclicity is very strong in the Miocene, whereas a 40-k.y. periodicity is less pronounced (Fig. F39). Oscillations in the composition of the pelagic sediments at frequencies to that of the Earth's precessional (~20 k.y.) cycle of insolation occur through most of the time intervals drilled during Leg 208. Also the modulation by the shorter (~100 k.y.) eccentricity cycle is nicely documented. Figures F40 and F41 show examples for the Eocene and Maastrichtian. The bedding cycle patterns in Leg 208 sites were sufficiently distinct that they could easily be correlated between sites, allowing for the development of a detailed cycle stratigraphy for nearly the entire Cenozoic.

Depositional History of Walvis Ridge and Cenozoic Paleoceanography

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 each of these advantages were clearly evident, particularly for establishing the long-term sediment accumulation history of the ridge. With the lithostratigraphic and biomagnetostratigraphic data, we were able to construct relatively coarse-scale sediment accumulation curves for each site (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 which derives ~1.8–2.0 km of subsidence over the last 65 m.y. Although the absolute ages are coarse, the relative correlations are precise as a result of the application of cycle stratigraphic tie points to the Leg 208 sites, a first for a group of sites arrayed in a depth transect. Using the MS records, a total of 286 common points of correlation were identified over the Cenozoic and Maastrichtian. These points serve as the primary tie points for site-to-site correlation.

In general, carbonate accumulation rates are highest in the Maastrichtian, Paleocene and early Eocene, and Pleistocene, and lowest in the middle and late Eocene and early–middle Miocene (Fig. F45). These time-dependant changes become much more pronounced with depth, 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 rise 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.

Critical Events

A major achievement of Leg 208 was the recovery of continuous undisturbed cores spanning over a half-dozen "critical" intervals or events. All the events were recovered in at least two sites, and at least three of the early Cenozoic events were recovered in five sites. The Leg 208 cores allow each 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 events such as extinctions or abundance acmes indicative of unusual environmental stress. The most prominent, the P/E and K/P boundaries, are characterized by relatively rapid and extreme change. The other events, although less extreme, show characteristics that indicate brief extremes in climate and/or ocean carbon chemistry. This includes the mid-Paleocene biotic event at 58.2 Ma, 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 P/E boundarylike clay layers, but of a smaller scale, were identified in the upper Paleocene and lower Eocene of all sites. The most distinct of these smaller events includes what is affectionately referred to as the Eocene layer of mysterious origin, or "ELMO," event at 53 Ma. 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., 1999).

The Cretaceous/Paleogene Boundary

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 APC cored. 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 and, to a higher degree, orangish Maastrichtian chalks were recovered below the boundary.

The boundary succession is marked by an irregular boundary between the light reddish brown and brown clay-bearing nannofossil ooze with foraminifers of uppermost Maastrichtian age (nannofossil Zone CC26). Color cyclicity is recorded in both color lightness and chromaticity. Within the darker lithology, bioturbation is preserved. Two prominent ash layers were identified just below the K/P boundary at Site 1267. At the boundary, this lithology is abruptly overlain by a 2- to 3-cm-thick dark red to reddish brown clay- and Fe oxide–bearing nonbioturbated foraminifer nannofossil ooze/chalk. Microtectites are present within this reddish transition zone directly at the boundary at Site 1262. The sediment grades upward into a moderately bioturbated brown nannofossil- and foraminifer-bearing clay (foraminiferal zones P and P1a). The K/P boundary is marked by decreased carbonate deposition/preservation and increased clay, iron oxide, and volcanic ash accumulation, both of which produce distinctive increases in MS and lightness (Fig. F46).

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 foraminifer zone and Micula prinsii nannofossil zone (CC26). The brown nannofossil- and foraminifer-bearing clay contains high abundance of W. hornerstownensis, Chilobuembelina midwayensis, and Chilobuembelina 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 subzones P to 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 foraminifer 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 food 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 samples 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 boundaries at Site 1262 is the presence of greenish unaltered ovoid spherules that are concentrated in the first few centimeter of the basal Paleocene. No spherules have been found higher up in the sediments.

The K/P boundary is similar to records from Blake Nose (western North Atlantic Ocean) and Shatsky Rise (Pacific Ocean). The Site 1262 record shows a microtectite layer (ejecta fallout) capped by a reddish to brown iron oxide layer. This layer is overlain by a dark brown to red clay that contains planktonic foraminifers diagnostic for Zone P. At Walvis Ridge, in contrast to the other records, the entire early Danian is recovered in brown nannofossil and foraminifer-bearing clay, whereas at Blake Nose and Shatsky Rise the increasing carbonate content results in white foraminifer-nannofossil ooze. The ultrafine micrite at Blake Nose and Shatsky Rise is assumed to be related to the collapse of the marine biosphere that resulted in a drop in the CCD. Although Zone P is normally unrecovered or poorly preserved in most sites, the substantial thickness of the uppermost Maastrichtian M. 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.

Mid-Paleocene Biotic Event

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. The event has also been identified at ODP Leg 198 Sites 1209–1212 (Bralower, T.J., Premoli Silva, I., Malone, M.J., et al., 2002). 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).

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 low-diversity largely dissolved 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.

Paleocene–Eocene Thermal Maximum

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. We successfully drilled the PETM interval in multiple holes at five drill sites (e.g., Sites 1262, 1263, 1265, 1266, 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 never 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 foraminifer fragmentation, and unusually small pre-extinction benthic foraminifer species. 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 ~10 to ~20 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., 1999), 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 (more than 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. F48A). 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 foraminifer assemblages occur near the base of the clay layer (Fig. F48B) 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 foraminifer 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. F48C) 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 foraminifer compositions shift from earliest Eocene low-diversity diminutive assemblages to early Eocene moderate-diversity assemblages (Fig. F48D, F48E), and nannofossil assemblages show a marked relative decrease in Fasciculithus spp. and increase in Zygrhablithus bijugatus (Fig. F48C). This relative increase in Z. bijugatus typically coincides with the BEE in other regions (e.g., Shatsky Rise and central Pacific), but occurs much 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 magnetic susceptibility 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), which predict a transient lysocline overdeepening to result from the oceanic mass-balancing of increased bicarbonate, carbonate, and Ca++ concentrations (via enhanced chemical weathering and runoff) as concurrent carbonate dissolution decreased CO2-derived H+ concentrations and increased Ca++ concentrations (Broecker and Peng, 1982; Walker and Kasting, 1992).

Chron C24N "ELMO" Event

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 early Eocene (Figure F49). This layer, tentatively called the "ELMO" event, was a faithful indicator point for estimating the depth to the underlying P/E boundary in the parallel holes of each site. The ELMO event shows similar color characteristics as the P/E boundary layer as well as a drop in the MS values and calcium carbonate content of the sediment and an increase in natural gamma radiation values but not as intense. The ELMO event is characterized by a split drop in the 1-cm point MS records of all sites, which are marked by events A and B in Figure F49. At the shallowest sites (1263 to 1266), events A and B are most distinctly developed, whereas in the deepest sites (1267 and 1262) they are merged together. At the latter sites, an additional drop in MS values is observed slightly above the ELMO event. At the middepth Site 1266, a thin white-colored layer is found immediately above the red-colored layer.

This layer, which appears to be present in the MS records of sites drilled on Shatsky Rise, is associated with benthic foraminifer assemblages similar in character to those of the PETM. This implies a transient shift of paleoenvironmental conditions toward those documented for the PETM.

Eocene/Oligocene Boundary and Early Oligocene Glacial Maximum

Sediment recording the response of tropical South Atlantic sediment to global cooling and CCD deepening at the Eocene–Oligocene transition was recovered across a broad depth range on the northeastern flank of Walvis Ridge (Sites 1262, 1263, 1265, 1266, and 1267 in a total of nine holes). Reworking, downslope transport, and dissolution of microfossils prevent a detailed stratigraphy from being established across the E/O boundary. Nonetheless, the available nannofossil and planktonic foraminiferal data, combined with magnetostratigraphy, suggests that the boundary interval is relatively complete at the shallowest Site 1263 but highly condensed or unconformable at the deeper Sites 1262 and 1267. At the middepth Sites 1265 and 1266, the upper Eocene and lower Oligocene sediments show extensive reworking and dissolution (Fig. F50). Despite the extensive reworking, systematic pronounced lithostratigraphic changes are observed at all sites across this transition. That they are clearly correlatable on a coarse scale suggests these changes reflect on regional and global events.

The deep Sites 1262 and 1267 record abrupt lithologic changes over a ~0.5-m interval in the uppermost Eocene and lowermost Oligocene, from brown clay below to light brown to gray nannofossil ooze or foraminifer-bearing nannofossil ooze above. This transition is associated with a distinct decrease in MS and a corresponding increase in color reflectance lightness (L*); gamma ray attenuation (GRA) bulk density exhibits no uniform trend across these two sites. Carbonate content increases from <20 wt% to >80 wt% at the deeper sites. In comparison to the condensed deep sites, the E/O transition interval is greatly expanded at the two shallow sites. Here, decreases in MS and GRA bulk density and an increase in L* occur gradually over a 2- to 5-m petrographic shift from light brown clay-bearing nannofossil ooze to very pale brown nannofossil ooze.

The E/O boundary is associated with a marked increase in sedimentation rates and a general improvement in microfossil preservation at all sites. Whereas late Eocene nannofossils in most sections show a high degree of etching, early Oligocene assemblages show less dissolution and slightly more overgrowth. At the deepest site in the transect, planktonic foraminifers are largely absent because of intense carbonate dissolution; the Site 1262 transition interval is defined by nannofossil zones. Comparatively, the succession of nannofossil events that characterize the Eocene–Oligocene transition, including the distinctive boundary between Subzones CP16c and CP16b (NP22/NP21), first occurrence of Ericsonia formosa, have been recognized at the shallowest Site 1263. Planktonic foraminifers generally show better preservation, but are much more fragmented across the Eocene–Oligocene transition in this site as well. At all sites, the lithologic transition appears to be accompanied by a significant increase in carbonate accumulation rates as well (Fig. F50).

Increased carbonate content and improved microfossil preservation across the boundary interval indicate that the lysocline and CCD deepened substantially and rapidly along this transect during the Eocene–Oligocene transition. In the latest Eocene, the CCD on Walvis Ridge was between the paleodepths of Sites 1266 and 1267; based on the highly condensed upper Eocene sequence of Site 1267, we infer that it was likely nearer the paleodepth of Site 1266. After the Eocene shoaling, the CCD returned to a depth well below Site 1262. The "Oi-1" is estimated mainly from color reflectance L* (Fig. F50) and magnetostratigraphy, which places C13N at this level. At all sites, the carbonate values peak for a short interval that corresponds to this normal. These events suggest that this CCD migration may be related to the first widely accepted sustained glaciation of Antarctica. This shift is also observed in other ocean basins (e.g., Zachos et al., 1996) including the Pacific where the timing is tightly constrained by cores recovered by Legs 198 and 199 (Bralower, T.J., Premoli Silva, I., Malone, M.J., et al., 2002; Lyle, Wilson, Janecek, et al., 2002). By tightly constraining the timing and magnitude on a global scale, it should be possible to determine what role, if any, climate change played in driving this unique transition in ocean chemistry.

Early Oligocene Braarudosphaera Blooms

ODP Leg 208 recovered several intervals from the early Oligocene that contain nannofloras highly enriched in calcareous debris derived from Braarudosphaera dinocysts. 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 has 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 atop the Rio Grande Rise, early Oligocene Braarudosphaera deposits have been 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 have been recovered from Sites 362, 363, 522, 526 (Bolli et al., 1978; Hsü 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 glaring 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, 1970).

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., in press). The coherent structure of these stable isotope stratigraphies argue against large-scale redepositional mechanisms. This interpretation is corroborated by the presence of Braarudosphaera layers draped atop topographic highs along Walvis Ridge as those at Site 526. Hence, open-ocean Braarudosphaera layers recovered by 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.

Early Miocene High Abundance of Bolivinids

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 can not 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, 574, and 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 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 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, and 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, we identified the HAB event at Sites 1264 and 1265, where its lower boundary is coeval (at shipboard sample resolution) with the lowermost occurrence of S. belemnos 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 detailed comparison of the timing and intensity of the HAB event over a depth range of >1000 m will be possible.

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