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RESULTS (continued)

Oligocene–Miocene Boundary Interval

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 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 deep-water 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 Mi1 event with erosion surfaces and inferred sea level falls provide support for the hypotheses that both involve 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 almost 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) that appear 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 Oligocene–Miocene and Eocene–Oligocene transitions) sedimentary sequences recovered at Sites 1218 and 1219 as seen in 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 within 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, which is supported by available bio- and magnetostratigraphic datum points that occur at both sites. MST data from Site 1218 and Site 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 calcium carbonate present in the two sites, probably due to a different paleodepth with respect to the CCD.

The magnetic reversal records from both sites are good and allow the construction of a detailed, yet preliminary, timescale. The application of this common timescale to data from both sites allows the calculation of linear sedimentation rates. 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 rate at both Sites 1218 and 1219 corresponds to the presence of a calcium carbonate-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 the evaluation of the quality of previous age calibrations of datum events. For example, the last occurrence of the calcareous nannofossil R. umbilicus (14 to an exaggerated jump in sedimentation rates at both sites (Fig. F25). Interpolated between the base of magnetochron C12n and the top of magnetochron 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 more than 1° latitude and 7° longitude, 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).

Eocene–Oligocene Transition

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 et al., 1999). Leg 199 recovered E/O boundary sections from five Northern Hemisphere sites (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 opal-rich and carbonate-poor to carbonate-rich and opal-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 (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 in 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 within calcareous nannofossil Subzones CP16a and CP16b (NP21). The age of the P18/P16 boundary, and thus the E/O boundary, is presently estimated to 33.70 Ma on the seafloor magnetic anomaly 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 paleomagnetic reversal stratigraphies for all of the Leg 199 sites drilled on 56-Ma crust (Sites 1217, 1219, 1220, 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 the 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 37.7-Ma isochron, calculated through linear interpolation between paleomagnetic and/or biostratigraphic indications (Fig. F28; Table T2).

Discoaster barbadiensis and D. 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 within Zone NP21 (CP16c), above the extinction of the last Eocene discoasters. By assuming a linear sedimentation rate within Zone NP21 in this composite section, an age estimate of 33.3 Ma was obtained for the initial change (midpoint of transition) in lithology, and an estimate of 32.9 Ma for the midpoint of the second, final step. 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 in middle of Oi-1 (33.–33.1 Ma) (Zachos et al., 2001a) (Fig. F5) on the common timescale used (Cande and Kent, 1995). 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 k.y. and then another rapid increase in CaCO3 over 10–20 k.y (see "Paleogene CCD").

The above 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 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 site. 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.

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