RESULTS

The sediments drilled during the Paleogene equatorial transect fall into the following five broad litho-chronostratigraphic units (Fig. F6): (1) a surficial clay unit, sometimes containing a basal radiolarian ooze with a basal age ranging from the middle Miocene in the south of the transect (Site 1218) to the early-middle Eocene in the north (e.g., Sites 1215 and 1216); (2) a nannofossil ooze/chalk unit whose base is at the E/O boundary and whose top is of early Miocene age in the south (Sites 1218 and 1219) and Oligocene age in the central part of the transect (Sites 1217, 1220, 1221, and 1222); Oligocene-Miocene carbonates are nonexistent in the north (Sites 1215 and 1216); (3) a middle-upper Eocene radiolarian ooze and radiolarian clay that is present at all sites except those in the north (Site 1215 and 1216); (4) a lower middle-lower Eocene unit composed of cherts, clays, and radiolarian ooze that is present in varying thicknesses at all Leg 199 sites along the 56-Ma transect except one in the extreme north (Site 1215); and (5) a lower Eocene-upper Paleocene nannofossil ooze or chalk resting upon basalt basement that is recovered at all sites (except Sites 1216 and 1222 where cherts overlie basement and the relevant stratigraphic interval was not drilled).

Surficial Clay and Radiolarian Ooze

The upper clay unit is composed of wind-blown dust, clays and radiolarians eroded and reworked from older sediment outcrops, authigenic minerals, and (in sites marginal to the modern equatorial region) freshly deposited radiolarians.

The clay mineral composition of eolian dust transported to Leg 199 sites may provide a way to track the latitudinal position of the ITCZ (Fig. F7). Enhanced precipitation in the ITCZ helps to "wash out" dust particles from the atmosphere, forming an effective barrier to interhemispheric dust transport. In other words, dust generated in the Northern Hemisphere is deposited only in latitudes north of the ITCZ. Today, Asian dust (rich in quartz and illite clays) dominates the composition of dust deposited in the Pacific north of the ITCZ. In contrast, just south of the ITCZ, a clay mineralogy dominated by smectite is transported via the trade winds from Central and South American source regions. During Leg 199, we used light absorption spectroscopy (LAS) (see Vanden Berg and Jarrard, this volume) to produce shipboard estimates of clay mineralogy. In Figure F7, plots of LAS-determined mineralogical change downcore are shown vs. depth for Sites 1215-1219. Using paleoposition information, one can then identify the change in ITCZ through time by the change from a smectite-rich to illite-rich clay suite (Fig. F8).

Radiolarian ooze or clayey radiolarian ooze are important sediment lithologies in the basal part of the surficial clay unit in Leg 199 sites drilled near the modern equator. These oozes are found at Sites 1219-1221 (Fig. F6). In each case, the site occupied an off-equatorial position at the time that the radiolarian ooze was being deposited in the range of 3°-6° north of the paleoequator. Thus, in the Neogene and latest Oligocene, radiolarian ooze is deposited on the fringes of the equatorial upwelling zone. The zone of radiolarian ooze deposition is bound on the south by the appearance of foraminifers and disappears to the north as primary productivity decreases.

Oligocene-Lower Miocene Nannofossil Ooze/Chalk

The second broad litho-chronostratigraphic unit along the Leg 199 transect is the nannofossil ooze that first appears at the base of the Oligocene. The clay or radiolarian ooze to carbonate transition is an outstanding marker for the E/O boundary because it is very abrupt (see "Eocene-Oligocene Transition"). The unit has cyclic variations of carbonate content related to orbitally driven changes in insolation. This cyclic variability will allow us to construct an orbitally tuned Oligocene age model of the sediments and to calibrate ages for magnetostratigraphy and biostratigraphy (see "Stratigraphic Intercalibrations").

The nannofossil ooze/chalk unit is thickest at sites near the Oligocene equatorial region and thins to the north (Fig. F6). Nevertheless, traces of the lower Oligocene carbonate can be found as far north as Site 1217 (16°52´N). At the beginning of the Oligocene, Site 1217 was located ~9° north of the equator at a water depth of 4400 m.

The age of the uppermost nannofossil ooze/chalks depends on the paleoposition and water depth of each drill site. In the south (Sites 1218 and 1219), the uppermost nannofossil ooze unit has an age near the late Miocene. Traversing north, the uppermost nannofossil ooze at Site 1220 (10°11´N) is late Oligocene in age, whereas at Sites 1217 (16°52´N), 1221 (12°02´N), and 1222 (13°49´N), the uppermost nannofossil oozes are early Oligocene in age.

Late-Middle Eocene Radiolarian Ooze

The third broad litho-chronostratigraphic unit along the Leg 199 transect consists of radiolarian ooze, clayey radiolarian ooze, and radiolarian clays (Fig. F6). Clay content tends to be higher at the top and bottom of the unit. The radiolarian oozes are characteristic of the Eocene section and are the most enigmatic facies drilled during Leg 199, from a modern perspective, because of the lack of a true modern analog.

In the southern sites of the Leg 199 transect (Site 1219) and in shallower drill sites (Site 1218 and DSDP Site 162), carbonate can be found at low concentrations in the upper Eocene and at higher levels in the middle Eocene, especially in Chron C18 (~39-41 Ma). For the rest of the transect, calcium carbonate (CaCO₃) is absent from this unit even in sites located at the contemporaneous paleoequator. The latitudinal width of the late-middle Eocene zone of radiolarian deposition varies through time. The middle Eocene, between ~40 and 45 Ma, represents the greatest latitudinal expansion—radiolarian oozes were found from the equator to a paleolatitude between 10° and 11°N (between DSDP Site 40 and Site 1216). In contrast, radiolarian deposition is confined to within a few degrees of the equator in the uppermost Eocene. Along the 56-Ma transect, radiolarian oozes are deposited only to the south of Site 1221, located at a paleolatitude of ~3°N. The upper Eocene-lower Oligocene sequence is condensed at Site 1221, and Sites 1222 and 162 both have a hiatus just below the Oligocene.

One of the striking features of a unit with such high biogenic silica content is the relative absence of diatoms. Only one major interval in the Eocene radiolarian ooze contains a relatively high diatom content (>10% diatoms in smear slides; Fig. F9). High numbers of diatoms appear in Chron C18 (radiolarian Zone RP15; 39-39.6 Ma) and are associated with the appearance of carbonates in the same sediment interval at Sites 1218 and 1219.

Lower Middle-Lower Eocene Cherts, Clay, and Radiolarian Ooze

The fourth broad litho-chronostratigraphic unit along the Leg 199 transect consists of cherts, clays, and radiolarian oozes (Fig. F6). This unit proved predictably troublesome to recover. Leg 199 drilling results indicate that cherts are present at the boundary between the lower and middle Eocene throughout the transect with the exception of Site 1215 in the extreme north. A younger cherty chalk interval is present at Site 1218 near the middle/upper Eocene boundary, but this interval was continuously cored and recovered with the extended core barrel (XCB) (Fig. F10).

The lower-middle Eocene chert interval is relatively thin at Site 1219 in the south (~8 m thick; 7°48´N) but thickens to the north. At Site 1220 (10°11´N), the unit, including recovered cores of early Eocene radiolarian ooze, spanned 40 m. Poorly recovered cherty intervals span 30-40 m at Sites 1217 (16°52´N), 1221 (12°02´N), and 1222 (13°49´N). The unit is chronologically extensive. It typically first appears in radiolarian Zone RP11-12 (~45-49 Ma) and continues into the lower chalk unit around the top of radiolarian Zone RP7 (~53 Ma). The relationship between these cherts and the lower nannofossil chalk is not yet clear. At Site 1219, a transition from zeolitic clay to nannofossil chalk was recovered, suggesting that the base of the unit may be marked by a gradational transition from clay to CaCO₃. However, the chert recovered was associated with most of the basal nannofossil chalk recovered on the leg; this lower unit is not free from chertification.

Lower Eocene-Upper Paleocene Nannofossil Ooze or Chalk

The basal litho-chronostratigraphic unit along the Leg 199 transect consists of nannofossil ooze or chalk of early Eocene-upper Paleocene age overlying basement basalt. Despite high numbers in the rest of the Eocene, radiolarians are rare to absent in the basal carbonates. This unit is present at all sites except perhaps Sites 1216 (where the relevant stratigraphic interval was not drilled) and 1222, (where only chert was recovered in the one hole drilled to basement). In three of the seven sites targeted to recover the P/E boundary (Sites 1215, 1220, and 1221), this objective was fulfilled (see "P/E Boundary"). Where present, the basal carbonate unit is most lithified in the south, where overburden is greatest (~250 m burial depth; Sites 1218 and 1219). In contrast, at Site 1215, where the burial depth is significantly shallower (~25 m), we recovered a nannofossil ooze. At three of the sites containing basal chalks (Sites 1217, 1218, and 1220), the sediments are partially to extensively dolomitized. The origin of this dolomite is unclear, but dolomitization appears to be related to proximity to basement rather than location along the latitudinal transect.

One of the most striking features of the Paleogene equatorial Pacific sediments is the rapid appearance and disappearance of CaCO₃ through the stratigraphic record. Leg 199 drilling has significantly increased stratigraphic control for many of the Paleogene and early Neogene carbonate-noncarbonate transitions.

Van Andel et al. (1975) compiled DSDP data from all the drill sites in the region and used subsidence curves of ocean crust to develop the Cenozoic history of CaCO₃ deposition. This compilation shows that the Eocene was marked by a shallow CCD or level in the water column where the pelagic rain rate of biogenic carbonate is equal to the rate of depth-dependent dissolution. Specifically, the Eocene CCD in the equatorial Pacific is estimated to have been ~3400 m shoaling to ~3200 m beyond 4° from the equator to either the north or south. Furthermore, one of the most prominent changes in the CCD through time captured in this early compilation was a pronounced deepening (by ~1600 m in the equatorial region) associated with the approximate transition from the Eocene to the Oligocene. Shipboard results from Leg 199 support this early interpretation of events. In fact, our shipboard data demonstrate that the correlation between the Eocene-Oligocene transition and the CCD deepening is strikingly precise and consistent across wide tracts of the tropical Pacific. These findings, raise the intriguing possibility that this pronounced deepening of the CCD is in some way related to the first widely accepted sustained glaciation in Antarctica (Oi-1; Fig. F5). The nature of the relationship between these two significant paleoceanographic signals will be an important component of shore-based Leg 199 research. In addition, our shipboard data also demonstrate that the transition from silica-rich/carbonate-poor Eocene to silica-poor/carbonate-rich Oligocene deep-sea sediments is remarkably rapid. The transition from carbonate-free to carbonate-bearing sediments is sharp (typically occurring over a 10- to 20-cm interval [Fig. F11]), implying that it took only a few tens of thousands of years to introduce the changes in ocean chemistry that depressed the CCD to >1 km below its Eocene depth.

Leg 199 sediments define other major CCD changes in the Eocene that will be better studied after the cruise. The Paleocene-lower Eocene basal chalks define a relatively shallow CCD in the beginning of the interval drilled during Leg 199 (Fig. F12). The transition from chalk to clay and radiolarian ooze occurs when near-equatorial sites passed through a water depth of only ~3200-3300 m (Sites 1219-1221), whereas the off-equator sites record a shift from carbonate to clay when they passed below 3400-3600 meters below sea level (mbsl) (Sites 1215 and 1217). In contrast, the Oligocene and Neogene CCD is ~4500-4600 m (Fig. F12). The difference in early Eocene CCD between equatorial and northern sites is greater than the errors associated with the estimation and is the reverse of the Neogene trend. Typically, in the Neogene, the CCD is deeper beneath the equatorial region because of higher carbonate production in the equatorial region relative to periequatorial regions, but this situation seems to have been reversed in the early Eocene.

Although carbonate disappears from the equatorial Pacific in the lower Eocene, CaCO₃ is not absent from the Eocene record. A prominent CaCO₃ event appears in Chron C18r at ~40-41 Ma (defined by Site 1219 in the south) that can also be found at DSDP Site 162 (van Andel et. al, 1973) (Fig. F12). Smaller carbonate events occur in Chron C20n (~43 Ma) and near the base of C17n (~38 Ma) at Site 1219. The younger event can also be found at Site 1218. In contrast, none of these carbonate events can be found further north, nearer the middle Eocene paleoequator (Site 1220). In fact, there is no CaCO₃ in the middle and late Eocene section at Sites 1220, 1221, or 1222.

The transition from chalks to radiolarite at ~40 Ma in Site 1218 appears as abrupt as the transition from radiolarian clay to nannofossil oozes at the E/O boundary. The rapid transitions of sediment type in either direction suggest that there is a strong climate switch at work. Sediments with poor carbonate preservation have also been identified during each of the Oligocene intervals that correlate to warm intervals between the "Oi" glacial advances of Miller et al. (1991). Thus, the Leg 199 shipboard scientists have developed a working hypothesis that carbonate deposition in the tropical Pacific is in some way associated with continental glaciations, whereas tropical Pacific radiolarian oozes are associated with warm global climates. If this hypothesis is substantiated, one implication would be that the transition from cold to warm climates can be as abrupt as the transition from warm to cold climates.

Drill sites move with their respective plates and must be backtracked in space as well as depth. Van Andel et al. (1975) recognized the paramount importance of backtracking tropical Pacific drill sites with respect to the motion of the Pacific plate because the equatorial upwelling system leaves a strong imprint on the sediment column when the site passes underneath the equator. This generalization holds for the Oligocene and Neogene. We can recognize the passage of Site 1219 under the equator in the Oligocene by the sedimentary changes, but we find confusing sedimentary signals in the earlier record.

We expected that sedimentation patterns might be different in the Eocene; therefore, we made it a primary leg objective to obtain independent evidence for the position of the paleoequator through the Paleogene. This objective is especially important because the backtrack paths we have used to find paleopositions of Leg 199 sites assume a fixed Hawaiian hotspot, and there is good evidence that the Hawaiian hotspot moved with respect to the Pacific plate prior to 42 Ma (Tarduno, Duncan, Scholl, et al., 2002). In addition, small errors in the plate tectonic model, when propogated over long periods of time, may lead to relatively large errors in a drill site position over long intervals of time.

Shipboard whole-core paleomagnetic analysis has been sufficient to make a preliminary definition of the position of the paleoequator and the change in paleolatitude of Leg 199 drill sites in the Oligocene and in the late middle Miocene (Figs. F13, F14). These data will be refined postcruise with discrete sample analyses. They suggest that the fixed hotspot backtrack of paleopositions is sufficiently accurate to estimate paleopositions back to the middle Eocene. Paleomagnetic analysis of older sediments should yield further evidence for equatorial positions in the early Eocene.

Another primary objective of Leg 199 science is to assess the level of productivity over the Paleogene by using biogenic MARs in concert with other geochemical and micropaleontological data. With this goal in mind we designed a shipboard program to monitor the composition of pore waters in the sediments and measure the bulk chemical composition of recovered sediments.

Organic Carbon Diagenesis and Interstitial Waters

In the context of productivity and biogenic sediment flux considerations, it is notable that none of the Leg 199 sediments contained significant organic carbon (C_org). As measured on board the ship, C_org levels in Leg 199 sediments were consistently low (~0.1-0.2 wt%), essentially the detection limit for the analysis at sea. Chemical gradients in Leg 199 interstitial waters primarily reflect the relatively limited organic matter diagenesis as well as dissolution of biogenic silica and varying amounts of diffusive influence of reactions in the underlying basalt (Fig. F15). Sulfate concentrations are high (>25 mM) throughout the transect, indicating little oxidation of labile organic matter (Fig. F15). These high sulfate levels mean that barite in the sediments will be well preserved and as a result will potentially be useful for paleoproductivity studies. This is especially important because it appears that any organic matter that may have been delivered to these sites has long since been degraded, if in fact it ever was deposited at all. Ammonium, another by-product of organic matter degradation, is only present in extremely low levels at all sites.

Dissolved silica concentrations generally increase with depth at all sites (Fig. F15). These high interstitial water silica values are consistent with the presence and dissolution of biogenic silica throughout the sediment. Strontium concentrations are generally constant at about seawater value (87 mM) for several of the sites containing little carbonate (Sites 1215-1217, 1220, and 1221), but strontium increases with depth at Sites 1218 and 1219 (Fig. F15) reflecting the presence and dissolution of biogenic carbonate.

Calcium, magnesium, and potassium concentrations at most Leg 199 sites show little evidence for exchange with basalt and subsequent diffusion (Fig. F15). However, the increase in calcium concentration and decrease with depth in magnesium, potassium, and lithium concentrations at Site 1219 are significantly greater than that seen at other Leg 199 sites. These patterns are consistent with alteration of basement rocks and with the recovery of highly altered basalt at Site 1219, unlike the other sites in the Leg 199 transect. The higher levels of lithium at Site 1215 relative to other Leg 199 sites could be linked to the volcanic ash layers recovered at that site.

Bulk-Sediment Analyses

The Leg 199 shipboard geochemical program differed from typical ODP protocol by incorporating relatively detailed downcore profiles of C_org composition. The resulting profiles primarily reflect the shifts in lithology between sediments dominated by silica to those dominated by carbonate (Fig. F16). When these data are combined with sedimentation rates (Fig. F17) and bulk density data, it is possible to examine burial fluxes through the Paleogene. MAR calculations are a means to distinguish rates of elemental deposition even under conditions of significant dilution by other sedimentary phases. This is best shown by comparing Si weight percent profiles at Sites 1218 and 1219 to the Si MAR profiles (Fig. F16). Low Si contents between 20 and 30 Ma are primarily caused by dilution of Si by CaCO₃. The MAR profile indicates that Si fluxes decreased much less dramatically than percentage data seems to indicate.

Changes in Si MARs should reflect biogenic SiO₂ production because the detrital Si contribution by aluminosilicates is relatively low and constant. Al analyses reflect the detrital aluminosilicate contribution (Fig. F18) and are relatively low with respect to the high Si MAR in the Eocene and Oligocene. A rough indicator of baseline aluminosilicate contribution to Si MARs is given by the Si MAR in the period younger than 15 Ma when clays were the principal sediment accumulating at all Leg 199 drill sites. By comparison to this baseline (15 Ma to present value), it is clear that biogenic Si deposition has decreased since a peak in the middle Eocene between ~38 and 45 Ma.

Virtually all calcium MAR results from the deposition of CaCO₃ (Fig. F16). High Ca contents and high Ca MARs occur in the high-CaCO₃ intervals of the Oligocene and lower Miocene. Smaller events can be seen in the middle Eocene, particularly ~40 Ma. The lower Eocene Ca MAR is approximately one-half to two-thirds of the high rates of deposition in the Oligocene. Phosphorus burial has a primary source term driven by export productivity (Fig. F18). The phosphorus MAR peaks in the early Oligocene and also in the period ~40-45 Ma in the middle Eocene, which suggests that this part of the middle Eocene had elevated paleoproductivity relative to the late Eocene.

Ratios to an element assumed to be constantly delivered (or nearly so) is another quick way to assess changes in deposition. Figure F19 illustrates this approach by showing ratios of Si, Ba, and Al to Ti. We assume that titanium is bound to aluminosilicates and has relatively constant deposition. The plots of Si/Ti and Ba/Ti suggest that there was relatively high burial of Ba and Si during the Eocene. The relatively constant Al/Ti is an indication that clay minerals are a primary contributor of the Ti flux.

Latitudinal MAR Transects

Observing MARs in time slices along latitudinal transects is another way to assess how sedimentation in the Eocene differs from that in the Oligocene and Neogene. We have chosen three time slices for shipboard comparison: (1) an early Eocene time slice (50-55 Ma), when all parts of the 56-Ma transect were above the CCD; (2) a middle Eocene time slice (38-45 Ma), during the period of highest radiolarian ooze deposition; and (3) an Oligocene time slice (25-34 Ma), when high CaCO₃ sedimentation had been established.

The Si MAR is highest in the middle Eocene (Fig. F20), but the latitudinal gradient of the Si MAR does not peak in the equatorial region as it does in the Neogene. Instead, the Si MAR increases southward. A high Si MAR in the middle Eocene may be due, in part, to the fact that Eocene radiolarians are heavily silicified (Moore, 1969). An individual middle Eocene radiolarian test has an average weight about four or five times that of an average Pleistocene radiolarian test and is much less susceptible to dissolution at the seafloor. The high Si MAR may represent low dissolution prior to burial as well as relatively high Si flux to the sediments.

In the early Eocene, we observe low Si MARs at all latitudes—sufficiently low that a large contribution of Si to sediments must be from clay minerals. Only the Oligocene has a MAR pattern that resembles that of the Neogene with the highest MAR peaking in the equatorial region. The flux of Si at the Oligocene equator is roughly equivalent to the modern MAR at 110°W (Lyle, 1992). Lyle (1992) expressed the modern flux as SiO₂ rather than biogenic Si. The SiO₂ fluxes in the Holocene equatorial region are equivalent, however, to ~50 mg/cm²/k.y. biogenic Si.

Ca MARs in the early Eocene time slice (Fig. F21) are actually equivalent to or higher than modern fluxes of Ca from CaCO₃ along a 110°W transect, except in the equatorial region (Lyle, 1992). In the early Eocene, however, there is no well-developed equatorial maximum in CaCO₃ deposition. In fact, CaCO₃ burial in the equatorial region may be only half as much as burial in the subtropical flank of the transect. In the middle Eocene, essentially no CaCO₃ is deposited anywhere except the southernmost part of the transect, which reflects the shallow middle Eocene CCD and the lack of CaCO₃ deposition in the equatorial region. Only the Oligocene latitudinal transect resembles a Neogene equatorial profile. CaCO₃ deposition at the equator is as high as Holocene deposition.

It is clear from our shipboard examination that sedimentation patterns of the Eocene are significantly different from modern equatorial sedimentation. Postcruise analyses utilizing other paleoproductivity indicators will be needed to understand the development of primary and export productivity through the Eocene. Nevertheless, there are few indicators that productivity was high in the Eocene central tropical Pacific Ocean.

One important problem to be approached by postcruise studies is whether C_org levels are low in Eocene sediments because of a long period of exposure to oxidants diffusing into the sediment from seawater or whether C_org levels were never high in the first place. Shipboard paleomagnetic studies provide circumstantial evidence for the riddle of low C_org. Preservation of the paleomagnetic record in sediments strongly depends on diagenesis. For example, Fe³⁺-Fe²⁺ reduction dissolves magnetite, leading to a decrease of both magnetic susceptibility and magnetic intensity of the sediments. Along the Leg 138 transect, the Neogene equivalent of the Leg 199 transect, sites at the equator or under regions of relatively high C_org deposition have magnetostratigraphic records limited to the upper sediments (Mayer, Pisias, Janecek, et al., 1992), whereas sites under regions of low primary productivity have more extended magnetostratigraphic records. The long magnetostratigraphic records obtained on the Leg 199 transect, along with the absence of a systematic decrease in magnetic intensity and susceptibility downcore, suggest that there never were high levels of C_org deposition in the Eocene.

A major success of Leg 199 is the recovery of continuous sedimentary records with uninterrupted sets of distinct Cenozoic geomagnetic polarity chrons from the paleoequatorial Pacific Ocean (Fig. F22). The sedimentation rates of the recovered and complete composite sections provide acceptable resolution for meaningful magnetobiochronologic calibrations for the early middle Eocene-early Miocene time interval. Biogenic silica is ubiquitously present and will permit, for the first time, the establishment of a precise Cenozoic biochronology of radiolarians, diatoms, and silicoflagellates from a tropical Pacific Ocean setting.

Biogenic carbonate is variably preserved and exhibits moderate to complete dissolution. The upper Paleocene-lower Eocene and Oligocene-lower Miocene intervals have the best preservation, which offers huge potential for establishing a tropical Pacific Ocean biochronology of calcareous nannofossils and planktonic foraminifers tied to magnetostratigraphy. Nannofossils can also be calibrated through much of the Eocene. As most existing age estimates for these groups have been derived from the Atlantic Ocean region, the establishment of an accurate biochronology based on Leg 199 sediments will permit assessments of the degree of interbasin synchrony among the calcareous plankton.

It follows that the sediments recovered during Leg 199 from the paleoequatorial Pacific Ocean will be an exceptionally valuable reference material for years to come for paleoceanography as well as studies of the evolution, biochronology, and intergroup and interbasin correlations among siliceous and calcareous micro- and nannofossils.

One of the enduring legacies of Leg 199 will undoubtedly be the revision of tropical radiolarian biostratigraphy. The opportunity to tie radiolarian biostratigraphic events directly to unambiguous magnetochronology will provide some of the first direct ties between the tropical siliceous microfossil record and the absolute timescale of the Paleogene. Furthermore, the repeated recovery of lower Miocene, Oligocene, and middle and upper Eocene radiolarian-rich sediments at four sites makes it possible to check the reliability of radiolarian events.

Most radiolarian bioevents have been calibrated indirectly to the absolute timescale by correlation to calcareous nannofossil biostratigraphy. During Leg 199, radiolarian bioevents were accompanied by high-resolution magnetostratigraphy, which allows the ages of biozone boundaries to be determined directly from the Cande and Kent (1995), Hilgen (1991a, 1991b), and Shackleton et al. (1995) timescales. Figure F23 shows a comparison between the newly determined zonal boundary ages, calibrated using the reversal stratigraphy from Sites 1218 to 1220, with those estimated by Sanfilippo and Nigrini (1998; SN98). The age estimates of SN98 are based on an unpublished catalog and chart constructed from a reexamination of all Paleogene low- and middle-latitude DSDP/ODP sites from Leg 1 through 135 in which there is a recognizable radiolarian fauna. The published information was reevaluated using current, uniform species concepts and integrated, where possible, with published nannofossil and paleomagnetic data. Sanfilippo and Nigrini (1998) cautioned that their chronology of Paleogene radiolarian zonal boundary events is at best a good approximation.

However, it is apparent that most radiolarian events are synchronous (or nearly so) among the Leg 199 sites. The Leg 199 age estimates tend to be older than the published estimates for 11 of 17 events, whereas the others agree to ~100 k.y. or less of previously published ages. Discrepancies in published vs. Leg 199 ages may reflect, for example, uncertainties in the age estimates of SN98 ("at best a good approximation"), paleobiogeographic diachroneity of the events, and difficulties in consistently recognizing evolutionary transitional events. The current radiolarian tropical biostratigraphic zonation includes numerous evolutionary transitions that tend to be subject to interpretation more than distinct first or last occurrence datums.

Postcruise refinements of the biostratigraphy of each site will improve the overall estimate of the ages for both zone marker events and recognition of new bioevents. A cycle stratigraphy based on physical properties measurements in each core will help refine the age estimates of biostratigraphic datums in magnetochrons. The ability to compare the timing of the same events at different Leg 199 sites can identify biostratigraphic datums that may be locally unreliable owing to reworking or latitudinal differences in the timing of evolutionary first or last appearances of species. The abundant, well preserved radiolarian assemblages will permit the recognition of other, easily identified datum levels with a consequent increase in the resolution of siliceous biostratigraphy.

In compilations of benthic foraminifer stable-isotope data, one of the most striking features of the Cenozoic record is also one of the least studied—the late Oligocene (Zachos et al., 2001a, 2001b) (Fig. F5). Much of the Cenozoic record younger than ~51 Ma is characterized by a series of increases in benthic foraminifer ¹⁸O that record the gradual global refrigeration and polar ice sheet growth that led up to the current icehouse planetary climate. The late Oligocene is a prominent exception to this descent into a glaciated world. Compiled oxygen isotope data from benthic foraminifers display an ~1.5 shift to more negative values that results in isotopic ratios in the uppermost Oligocene that are the same as, or even more negative than, the upper Eocene prior to the first large-scale glacial advance on Antarctica (Zachos et al., 2001a). This decrease appears to represent the largest event of its kind in the pre-Pleistocene record of the Cenozoic. At least two competing end-member hypotheses exist to explain this isotope shift: either Antarctica was rapidly deglaciated to a large extent or global deepwater temperatures warmed by 5°-7°C. To date, geochemical data are too sparse for the late Oligocene to either test the deglaciation hypothesis or evaluate the rate and timing of the change in ocean chemistry associated with this climate transition. In fact, because our current records for this time interval are derived from several different sites that lack adequate stratigraphic overlap, even the magnitude and rate of ¹⁸O decrease across this time interval are poorly constrained.

The late Oligocene deglacial/warm climate state persisted for ~1.5-2 m.y. and was terminated at the Oligocene/Miocene (O/M) boundary by an episode of polar ice buildup and/or global cooling—the Mi-1 event (Fig. F24). An astronomically tuned record in the equatorial Atlantic has been used to suggest that the Mi-1 event represents a glaciation triggered by changes in orbital insolation (e.g., Zachos et al., 2001b). The coincidence of both the ¹⁸O decrease in the late Oligocene and the Mi-1 event with erosion surfaces and inferred sea level falls provides support for the hypotheses that both involved significant changes in polar ice sheets. However, direct confirmation from coordinated shifts in both benthic and planktonic stable-isotope records or from temperature proxies such as Mg/Ca are mostly lacking.

During Leg 199, we recovered remarkably complete sequences through the upper Oligocene and lower Miocene at Sites 1218 and 1219. These sites display unambiguous magnetostratigraphy, a distinct record of cyclic alternations in sediment physical properties that offers potential for development of an astronomically tuned timescale, and a series of biostratigraphic events in calcareous nannofossils, planktonic foraminifers, and radiolarians that afford direct correlation to previously drilled sites that lack magnetostratigraphy. Sedimentation rates through the late Oligocene and O/M boundary average ~1 cm/k.y. and will permit the development of a standard Pacific reference stable-isotope stratigraphy and Mg/Ca data set that can be correlated at orbital resolution to records throughout the tropical and subtropical oceans. The excellent timescales for Site 1218 and 1219 and potential for further refinements will help to evaluate the rates of change in ice volume and deep-ocean temperatures.

In the context of paleoceanographic research, perhaps the single most important scientific rationale that lies behind scientific ocean drilling is the argument that deep-sea sediments provide the most stratigraphically complete and globally representative proxy records of paleoclimate change. One of the outstanding highlights of Leg 199, in general, is our recovery of stratigraphically complete sequences (the sections drilled are virtually free of hiatuses at the biostratigraphic zone and magnetochron level) representative of substantial tracts of the central tropical Pacific Ocean. Undoubtedly the most elegant demonstration of this important leg highlight is the detailed stratigraphic correlation of Oligocene (including the Eocene-Oligocene and Oligocene-Miocene transitions) sedimentary sequences recovered at Sites 1218 and 1219 as seen in multisensor track (MST) data. These data show consistent cycles between these two sites on a submeter scale, allowing a detailed correlation between the two sites. Consistent cycles persisted over at least 22 m.y., from the early Miocene to the middle Eocene. During coring of Site 1219, it was possible to predict a priori both bio- and magnetostratigraphic datum points from those obtained at Site 1218. This important shipboard finding suggests that it is possible to identify signals that act on a larger scale in the equatorial Pacific, and a close comparison allows the identification and estimation of core gaps and short (subzonal scale) hiatuses.

The quality of the MST data obtained allowed the shipboard construction of not only spliced records from both sites but also an intersite correlation that is supported by available bio- and magnetostratigraphic datum points, which occur at both sites. MST data from Sites 1218 and 1219, illustrated in Figure F25, were adjusted to a common timescale that was obtained mostly from magnetic reversals. The two sites show very similar records down to a small scale for gamma ray attenuation (GRA) bulk density, magnetic susceptibility, and color reflectance data. Yet, the different paleodepths of both sites also lead to differences and offsets (particularly in the Eocene part of the interval) that are mostly attributed to different amounts of CaCO₃ present in the two sites, probably because of a different paleodepth with respect to the CCD.

The magnetic reversal records from Sites 1218 and 1219 are good and allow the construction of a detailed preliminary timescale. The application of this common timescale to data from both sites allows the calculation of average linear sedimentation rates (LSRs). These are also illustrated in Figure F25, averaged over 400-k.y. segments. Sedimentation rates from both sites track each other well, and Site 1219 shows a consistently slower sedimentation rate throughout the younger two-thirds of the Oligocene. Between ~40 and 42 Ma, an increase in sedimentation rates at both Sites 1218 and 1219 corresponds to the presence of a CaCO₃-rich section during a time when the CCD must have fluctuated strongly.

On the shipboard timescale, the MST data show quasi-cyclic patterns where the dominant frequency appears to change throughout time partly as a function of lithology. These cycles are consistent with an orbital forcing that has been observed in the Miocene, Oligocene, and Eocene (Shackleton et al., 1999; Shackleton et al., 2000; Pälike et al., 2001). The lithologic cycles observed will allow the postcruise generation of a detailed astronomical age calibration of bio- and magnetostratigraphic datums throughout the Oligocene and early Miocene. In detail, the shipboard data already allow us to evaluate the quality of previous age calibrations of datum events. For example, the last occurrence of the calcareous nannofossil Reticulofenestra umbilicus (14 (close to 32 Ma) leads to an exaggerated jump in sedimentation rates at both sites (Fig. F25). Interpolated between the base of Chron C12n and the top of Chron C13n, the stratigraphic position of this biostratigraphic datum suggests an older age that would be compatible with that given by Shackleton et al. (1999). This is just one example of how further postcruise studies will allow the refinement of many datum events throughout the early Cenozoic.

The remarkable fidelity of the correlation between these two sites, separated by >1° latitude and 7° longitude (~800 km), suggests that drilling results from these two sites are representative of large-scale paleoceanographic forcing functions in the late Paleogene eastern equatorial Pacific Ocean. We anticipate that the continuously cored sediments from Site 1218 with supplementary control from correlative sediments in Site 1219 will provide a paleoceanographic reference section for the late Paleogene tropical Pacific Ocean. In particular, these two sites offer an excellent opportunity to generate high-resolution geochemical records in deep-sea foraminiferal calcite with excellent age control throughout the entire Oligocene from a single deep Pacific Ocean site and thereby test models for the pattern and timing of changes in global temperature and continental ice volume developed from Atlantic Ocean DSDP sites and recent ODP transects on continental margin sequences (e.g., Miller et al., 1998, 1991, 1987) and Antarctic drilling (e.g., Wilson et al., 1998).

A major highlight of Leg 199 is the recovery of multiple E/O boundary sections from the central tropical Pacific Ocean (Fig. F11). Elsewhere, in the deep oceans, this important paleoceanographic boundary is often marked by condensed sequences containing poorly preserved microfossils or a hiatus. For these reasons, reliable geochemical records across the E/O boundary are rare and limited to mid- to high-latitude sites from the Southern Hemisphere (e.g., Zachos et al., 1996; Diester-Haas and Zahn, 2001; Gersonde, Hedell, Blum, et al., 1999). Leg 199 recovered E/O boundary sections from five Northern Hemisphere sites (1217, 1218, 1219, 1220, and 1221). Taken together, these sites provide a valuable opportunity to study the chain of events across the E/O boundary within the framework of a depth and latitudinal transect (Figs. F1, F11). Throughout this transect, the transition from the Eocene to the Oligocene is instantly recognizable by a sharp upsection shift from SiO₂-rich and carbonate-poor to carbonate-rich and SiO₂-poor sediments (Fig. F11).

The pronounced lithologic transition associated with the E/O boundary is sharper in all of the Leg 199 sites drilled on older (~56 Ma) ocean crust than the single site (1218) drilled on 42-Ma crust. Furthermore, among the sites situated on 56-Ma crust, the thickness of the carbonate-rich lowermost Oligocene sediments and their carbonate content generally decreases with increases in both latitude and water depth (Fig. F11). These observations indicate that the CCD deepened substantially and rapidly during the Eocene-Oligocene transition (see "Paleogene CCD").

In detail, the Eocene-Oligocene transition at Site 1218 is marked by a distinct two-step upsection shift from dark radiolarian-rich clay to pale nannofossil chalk (Fig. F26). This two-step shift from carbonate-poor to carbonate-rich sediments is also evident as a two-step increase in GRA bulk density and decrease in magnetic susceptibility values in MST data (Fig. F26). At the other deeper and/or higher latitude sites, the lithologic and physical properties transition across the E/O boundary is more of a single, sharp step (Fig. F26).

One stratigraphic complication that we faced during Leg 199 is that the E/O boundary is formally defined by the extinction of the planktonic foraminifer genus Hantkenina (Zone P18/P16 boundary, Premoli-Silva et al., 1988), but planktonic foraminifers are absent in sediments of this age in all sites. The extinction of the planktonic foraminifer genus Hantkenina occurs toward the younger end of Chron C13r and in calcareous nannofossil Subzones CP16a and CP16b (NP21). The age of the P18/P16 boundary, thus, the E/O boundary, is presently estimated to 33.70 Ma on the seafloor magnetic anomaly-based timescale (Cande and Kent, 1995). This age estimate for the E/O boundary will likely be further refined as soon as an astronomically tuned timescale becomes confidently established across the Eocene-Oligocene transition interval.

For the above reasons, the exact placement of the E/O boundary at Leg 199 sites will remain unresolved, perhaps until the problem can be addressed through shore-based high-resolution stable-isotope stratigraphy. Nevertheless, the availability of high-quality magnetic reversal stratigraphies for all of the Leg 199 sites drilled on 56-Ma crust (Sites 1217, 1219, 1220, and 1221) combined with high-resolution nannofossil biostratigraphy made it possible to establish good shipboard approximations for the position of the boundary (Fig. F27). Specifically, age control across the Eocene-Oligocene transition in Leg 199 sites is provided by magnetostratigraphy at four of the five sites where the E/O boundary interval was recovered by the ODP advanced piston corer (APC) (Sites 1217, 1219, 1220, and 1221). Calcareous nannofossil and radiolarian biostratigraphy provides the age control at Site 1218 and aids the identification of the geomagnetic polarity zones in the four remaining sites. In order to compare the timing of the lithologic change from dark-colored Eocene radiolarite to light-colored Oligocene nannofossil chalk among the five Leg 199 sites with E/O boundary intervals, we have aligned all sites along a 33.7-Ma isochron, calculated through linear interpolation between paleomagnetic and/or biostratigraphic indications (Fig. F28; Table T2).

Discoaster barbadiensis and Discoaster saipanensis are constrained to have disappeared over narrow (~20-30 cm) intervals in Site 1218 sediments just below the major change in lithology (Fig. F27). These findings reveal that the entire two-step change in lithology occurred in Zone NP21 (CP16c) above the extinction of the last Eocene discoasters. Based on LSRs, the estimated position of the E/O boundary is at 243.3 meters composite depth (mcd), whereas the midpoint of the initial change in carbonate composition occurs at ~242 mcd (Fig. F27). The midpoint of the second, final step in carbonate composition occurs at ~240 mcd. Thus, our records from Site 1218 imply that the change in CCD occurred in the earliest Oligocene in two steps, as a rapid increase in CaCO₃ over 10-20 k.y. followed by a pause of ~100-200 k.y. and then another rapid increase in CaCO₃ over 10-20 k.y (see "Paleogene CCD"). Furthermore, the boundary condition change of the ocean-climate system that caused the first step of this drastic deepening of the CCD and accompanying change in sedimentation in the tropical Pacific Ocean thus occurred near the onset of Oi-1 (33.-33.1 Ma) (Zachos et al., 2001a) (Fig. F5) on the common timescale used (Cande and Kent, 1995).

The above findings raise the intriguing possibility that this pronounced deepening of the CCD associated with the Eocene-Oligocene transition is in some way related to the first widely accepted sustained glaciation in Antarctica (Oi-1; Fig. F5). The nature of the relationship between these two important paleoceanographic signals will be an important component of shore-based Leg 199 research. Calcareous benthic foraminifer assemblages indicate lowermost bathyal and upper abyssal paleodepths at Site 1218. The calculated age-depth curve for Site 1218 indicates a paleodepth of 3700 boundary at this site (Fig. F12). Examination of test walls under transmitted light indicates that benthic foraminifers are well preserved and suitable for benthic foraminifer stable-isotope stratigraphy. Average sedimentation rates in the lower Oligocene are relatively high for a deep-ocean Pacific setting (~1-2 cm/k.y.). Together with the sections from the sites drilled on older (~56 Ma) crust, the section from Site 1218 offers exciting prospects for shore-based investigation of the first Pacific Ocean depth and latitudinal transect across this paleoceanographically important interval. For example, application of combined stable isotope and Mg/Ca records in benthic foraminiferal calcite will allow us to separate the temperature and ice volume components of "Oi-1" (e.g., Lear et al., 2000). This information, together with new constraints on the phase relationships between these signals and the deepening CCD will help to evaluate the role of the hydrological cycle vs. the carbon cycle in triggering the onset of the first persistent large-scale ice sheets during the Cenozoic.

The difference between Eocene equatorial Pacific sediments and those of the Oligocene and Neogene is striking. Everywhere along the Leg 199 transect, upper lower Eocene to uppermost Eocene sediments consist primarily of one microfossil group, the radiolarians, mixed with varying amounts of clay. In contrast, Oligocene sediments are dominantly nannofossil rich and also contain a diverse mixture of foraminifers, radiolarians, and diatoms. One of the primary postcruise tasks of Leg 199 is to decipher why radiolarian ooze is so common in the Eocene.

The task is made more difficult because analog Neogene and Holocene radiolarian oozes are rare and occupy a specific zone in the equatorial sedimentation regime that is not the same as the equivalent deposit in the Eocene. For example, the "type" Eocene radiolarian ooze (e.g., the middle Eocene ooze of Site 1220 at ~120 meters below seafloor [mbsf]; more specifically, Core 199-1220B-9H; roughly Chron C20n or 43 Ma) consists of 80%-90% radiolarians, <2% diatoms, and 10%-15% clay. The rest of the sediment is made up of opaque minerals. The sedimentation rate for this interval is 8.3 m/m.y. The total thickness of middle-upper Eocene radiolarian ooze at Site 1220 is 80 m, and it was deposited over 13 m.y. During that time, Site 1220 traversed from ~1°S to 1.5°N, based upon a fixed hotspot model, to determine paleopositions. A comparison between these deposits and a good example of a lower Miocene radiolarian ooze cored in the same site shows that the Miocene sediments are also composed dominantly of radiolarians (40%-80%), with traces of diatoms (<2%), and moderate amounts of clay (~20%). However, the remainder of the Miocene sediment is made up of nannofossils. The sedimentation rate is only 2.3 m/m.y., the total thickness of the radiolarian ooze is 18 m, and the sediment was deposited when Site 1220 was between the latitudes of 4° and 6°N. This deposit was formed just beneath the early Miocene CCD but also at the edge of the equatorial productivity zone, which seems typical for modern radiolarian oozes.

Neogene material deposited beneath the equatorial region, even if it was deposited below the CCD, contains large numbers of diatoms and would not be equivalent to the Eocene radiolarian oozes. Site 849 (Leg 138), located at the modern equator, is roughly at the equivalent longitude and latitude as occupied by Site 1220 in the early Eocene. The sediments of Site 849 contain from two to ten times as many diatoms as radiolarians throughout the entire sediment section, based upon smear slides (Mayer, Pisias, Janecek, et al., 1992). In other words, an Eocene radiolarian ooze cannot simply be created by shoaling the CCD in a modern equatorial environment. Diatom production in the Eocene must have been significantly lower than Neogene diatom production. The lack of diatoms is another indication, along with the low levels of C_org deposition (see "Fluxes and Paleoproductivity") that productivity levels in the Eocene were significantly below Neogene levels.

Diatoms are occasionally found in the Eocene radiolarian oozes in relatively large abundances (Fig. F29). We checked whether the presence of diatoms is related to when Leg 199 drill sites crossed the paleoequatorial region. Green arrows on Figure F29 mark stratigraphic intervals where diatom numbers increase to >5% in smear slide counts. In the Eocene section, they occur primarily around one interval marked by radiolarian Zone RP15 (~39-41.5 Ma based on the preliminary Leg 199 age recalibration) rather than clustering near where we expect equator crossings. The RP15 interval is also the time of maximum radiolarian ooze deposition at Site 1217, then at a paleolatitude of ~8°N. In contrast, maximum sedimentation rates of radiolarian ooze occurred roughly coincident with paleoequator crossings. The presence of diatoms seems to be related to an ecological event that is also associated with carbonate deposition at Sites 1218 and 1219, not with passage through the equatorial zone.

One important observation is that the radiolarian deposition zone in the uppermost Eocene is restricted to Sites 1218, 1219, and 1220 in the Leg 199 transect. These sites lie in a paleolatitudinal range from 1° to 2°N. All other sites exhibit a hiatus during this interval or have only clay deposition. The uppermost Eocene is marked by a hiatus of ~2 m.y. at Site 1221 (12°02´N) that expands to 5-6 m.y. at Site 1222 (13°49´N) and at DSDP Site 162 (14°52´N), based upon a recalibration of the DSDP biostratigraphy. Sediments deposited in the late Eocene at Site 1217 (16°52´N) are zeolitic clays. Assuming that radiolarian deposition marks some moderate level of productivity, the latest Eocene was a truly impoverished interval for regions beyond several degrees of latitude from the equator. In contrast, in middle Eocene time, significant radiolarian production occurred over a broad latitudinal range. Siliceous middle Eocene sediments are formed along the entire transect save only for the northernmost sites (1215 and 1216), suggesting the zone of radiolarian production was broader in middle Eocene time (reaching ~10°N latitude) than it was in the late Eocene (when it was only a few degrees wide). Therefore, not only was there a significant change in the type of sediments deposited in the equatorial Pacific during the later Paleogene but also a distinct narrowing of the focus of siliceous sedimentation between the middle Eocene and late Eocene.

A principal objective of Leg 199 was to recover shallowly buried sections across the P/E boundary that could be used for paleoclimate and evolutionary studies. It has been known for several years (e.g., Thomas, 1990, 1998; Kaiho et al., 1996; Aubry et al., 1996, 1998; Kelly et al., 1996, 1998; Sanfilippo and Blome, 2001) that the negative carbon isotope anomaly and transient global warming associated with the P/E boundary are accompanied by carbonate-poor sediments overlying more calcareous sediments. A mass extinction of benthic foraminifers occurs at the same time as the carbon-isotope anomaly, which suggests that the boundary records major changes in deep-ocean chemistry and habitats. Carbonate preservation tends to be poorest at the initiation of the carbon isotope anomaly and then improves over an interval several tens of centimeters to several meters thick that records several hundred thousand years of sedimentation. Two drill sites (690 and 1051) have been reported with an interval of laminated sediment just above the base of the carbon isotope anomaly. Laminations have been interpreted to record low-oxygen environments that prevailed during the initial phases of the P/E boundary and may be partially responsible for the benthic foraminifer extinction (Kaiho et al., 1996; Thomas and Shackleton, 1996). Dickens et al. (1995, 2001) have suggested that outgassing of submarine gas hydrate deposits could produce both low-oxygen conditions and carbonate dissolution by oxidation of methane in the water column. However, other than the changes in carbonate content and laminated sediments, most P/E boundary sections display a relatively simple internal stratigraphy.

Leg 199 drilled the first P/E boundary sections (Sites 1215, 1220, and 1221) ever to be sampled in the central tropical Pacific. Furthermore, these sites record a more complex stratigraphy for the P/E boundary than has been recognized previously (Figs. F30, F31). Sediments from Sites 1220 and 1221 change from calcareous ooze containing Paleocene microbiotas to a distinctive layered sedimentary unit in the vicinity of the benthic foraminifer extinction. The first beds at or above the benthic foraminifer extinction horizon consist of brown sediments 8-12 cm thick. The brown bed contains layers of volcanic ash at Site 1220 but is massive at Site 1221. The top of the brown bed is burrowed and overlain by ~2 cm of rose-pink-colored sediment that displays laminations and a few discrete burrows. A layer of black, manganiferous sediments overlies the rose-pink bed and grades upward into brownish, partly laminated and burrowed calcareous clays. The brown, pink, and black beds all have low carbonate contents typical of the core of the P/E boundary at other localities. However, the interbedding of sediments with different color and sedimentary structures has not been observed in P/E boundary sections from elsewhere in the world. The presence of similar sequences of color bands at Sites 1220 and 1221, situated >200 km from one another, suggests that the stratigraphy of these P/E boundaries has at least regional significance and may record changes in deep-ocean chemistry and sedimentation that reflect different phases of the development of the boundary interval on a global scale.

Biostratigraphy of the P/E Boundary Interval

Although the P/E boundary is marked by a major extinction of benthic foraminifers (~53% species extinction), surface-ocean microbiotas display mostly temporary changes in taxonomic diversity. Unfortunately, most P/E boundary sections are highly condensed or have severely dissolved calcareous faunas, so the sequence of biotic events has not been worked out in detail. A few tropical-subtropical sites record the presence of three short-lived species of planktonic foraminifers ("excursion fauna") (Kelly et al., 1998) that are believed to have evolved shortly after the beginning of the global climate change associated with the P/E boundary and then became extinct ~200 k.y. later. Calcareous nannofossils show elevated rates of both extinction and speciation in the ~500 k.y. after the onset of Eocene time, but the sequence of evolutionary turnovers is still much debated.

Results for Leg 199 drill sites suggest that, contrary to previous interpretations, the various biotic events associated with P/E boundary time (e.g., the extinction of the nannofossil species Fasciculithus tympaniformis and many benthic foraminifer taxa together with the evolution of the planktonic foraminifer excursion fauna) are not synchronous in the P/E boundary interval. At Sites 1220 and 1221, Paleocene benthic foraminifers become extinct at the start of the deposition of the sequence of multicolored beds (Fig. F32). The reduction in carbonate content (from ~80 to ~12 wt%) associated with the benthic extinction and reduced preservation of foraminifers makes it difficult to define the precise level of the extinction. However, a sharp drop in carbonate content is observed at the same level as the carbon isotope excursion in other deep-sea sites. Therefore, it is reasonable to conclude that the base of the multicolored beds at Leg 199 sites will ultimately prove to contain the initiation of the carbon-isotope anomaly. Data from Site 1220 make it quite clear that the excursion fauna (composed of the planktonic foraminifers, Acarinina africana, Acarinina sibaiyaensis, and Morzovella allisonensis) predates the extinction of Paleocene benthic foraminifers. Hence, Leg 199 results strongly suggest that the excursion fauna evolved before the P/E boundary. These species also range above the top of the multicolored beds and may survive termination of the carbon isotope anomaly. Counts of the abundance of the calcareous nannofossils at Sites 1220 and 1221 show that Fasciculithus is replaced by Rhomboaster well above the multicolored beds believed to represent the start of the Paleocene-Eocene thermal maximum. We have also documented the appearance of Thoracosphaera cysts in the boundary interval. Thoracosphaerid blooms are frequently associated with sediments immediately above the K/T mass extinction horizon and have been interpreted to represent an opportunistic "disaster" flora (Brinkhuis and Biffi, 1993).

Geochemical Profiles of the P/E Boundary Interval

The shipboard inorganic geochemistry protocol for Leg 199 incorporated a program of bulk-sediment analysis, including a detailed analysis of the P/E boundary interval at Sites 1220 and 1221 (Fig. F33). Results for the two sites are very similar although changes in elemental concentrations are somewhat sharper at Site 1220 than Site 1221. Both sites have a significant enrichment in Mn concentration associated with the black layer, which probably contains Mn oxides. The dark, carbonate-poor portion of the P/E boundary at Sites 1220 and 1221 is associated with high levels of Si, Al, Ti, Fe, and Mg. This is consistent with a higher proportion of silicate minerals such as clays compared to the more carbonate-rich portions of the sediment column and in keeping with the interpretation that the boundary lithology represents an interval of pronounced shoaling of the CCD.

At Sites 1220 and 1221, the P/E boundary is associated with a double peak in levels of Ba and P across the interval of major color change. Superimposed on the center of these broad increases in Ba and P is an interval of very low Ba and P, which corresponds to the black Mn-rich layer and the red layer. Minima in Ba and P also correspond to a minimum in the Sr profile. The Ba and P profiles may be explained in several ways. One possibility is that the double peak seen in the profiles for these elements reflects real changes in the rate of Ba and P delivery to seafloor during P/E boundary time. Alternatively, it is possible that the rates of Ba and P delivery to seafloor were high throughout P/E boundary time, and the double peak is a dilution artifact arising from a shorter interval of rapidly accumulating black and red sediments, possibly as a result of nearby hydrothermal activity. A third possibility is that the double peak in Ba and P arises from diagenetic remobilization of primary Ba and P signals. Regardless, the levels of Ba and P recorded across the P/E boundary at Sites 1220 and 1221 are high in comparison to those measured in other Leg 199 sediments. This observation, together with the congruency of behavior between the two elements, raises the intriguing possibility that the shipboard data record an increase in surface-ocean productivity across this important paleoceanographic interval (Bains et al., 2000) rather than an increase in barite saturation arising from the injection of Ba into the global ocean from the marine gas hydrate reservoir (Dickens et al., 2001).