The Eocene/Oligocene (E/O) boundary represents an important point in the transition from the greenhouse world of the Cretaceous and early Paleogene into the late Paleogene-Neogene "icehouse." Attempts to estimate global ice volumes from deep-sea benthic ¹⁸O records have prompted very different conclusions as to the timing of the onset of the accumulation of continental-scale ice sheets. These range from the Early Cretaceous (Matthews and Poore, 1980) to the middle Miocene (Shackleton and Kennett, 1975). However, recent improvements in the stratigraphic resolution of the ¹⁸O record have led to suggestions that either the late middle Eocene (~43 Ma) or the earliest Oligocene (~34 Ma) are better estimates of the timing of the initiation of greenhouse to icehouse transition (Shackleton, 1986; Miller et al., 1987, 1991; Zachos et al., 1992). Supporting evidence for this interpretation comes from oceanic records of ice-rafted debris, weathered clay mineral compositions, microfossil assemblages, and sequence stratigraphic analyses (Kennett and Barker, 1990; Browning et al., 1996). Yet, the rarity of complete deep-sea sections across these intervals has limited our understanding of the dynamics of this important step to the modern icehouse world.

The Eocene–Oligocene transition is marked by a large rapid increase in the benthic foraminiferal calcite ¹⁸O record in earliest Oligocene time (Oi-1; Fig. F5). This excursion was first ascribed to a 5°C temperature drop associated with the onset of thermohaline circulation, but more recently Oi-1 has been associated with the onset of continental ice accumulation on Antarctica. Such confusion reflects the long-standing difficulty of separating the effects of temperature and ice on benthic ¹⁸O. Recent application of an independent paleothermometry technique based on Mg/Ca in benthic foraminifers shows no significant change corresponding to "Oi-1" (Lear et al., 2000). This result suggests that all of the ¹⁸O increase associated with Oi-1 can be ascribed to ice growth with no concomitant decrease in polar temperatures. Apparently, according to these data, the trigger for continental glaciation lay in the hydrological cycle rather than the carbon cycle. Specifically, it has been proposed that the opening of the Australian-Antarctic seaway in earliest Oligocene time might have enhanced the supply of moisture as snow to the Antarctica interior (Lear et al., 2000). Support for this suggestion exists from both ocean drilling and numerical modeling experiments (Bartek et al., 1992; Hine et al., 1999). On the other hand, paleoproductivity studies suggest that the contemporaneous increase in seawater ¹³C was driven by increased rates of organic carbon burial in marine sediments and that this factor may be implicated in global cooling and ice sheet growth (Diester-Hass and Zahn, 2001). Either way, long-standing global compilations indicate a pronounced deepening of the CCD associated with the Eocene–Oligocene transition (van Andel et al., 1975) Unfortunately, our most complete records of the E/O boundary come from only two mid-latitude sites (Deep Sea Drilling Project [DSDP] Site 522 and ODP Site 744). Leg 199 offers an excellent opportunity to generate low-latitude records of the Eocene–Oligocene transition and thereby fully evaluate the competing roles played by global cooling and ice growth in the transition from the Cretaceous greenhouse into the Neogene "icehouse."

Widespread evidence now exists to support the occurrence of at least two large closely spaced extraterrestrial impact events on Earth during early late Eocene time. In particular, two large craters (order ~100 km diameter; Chesapeake Bay, North America and Popigai, Northern Siberia) have been proposed to explain impact-ejecta strewn fields that are documented in deep-sea sediments from around the world (e.g., Koeberl et al., 1996; Bottomley et al., 1997). Proxy records for fine-grained extraterrestrial dust (³He measurements) in correlative marine carbonate strata have been interpreted as evidence for a comet shower triggered by an impulsive perturbation of the Oort Cloud (Farley et al., 1998). Intriguingly, unlike the more famous and pronounced precursor extraterrestrial impact event at Cretaceous/Paleogene boundary time, biostratigraphic studies indicate that the late Eocene impact horizons do not correspond to major extinctions among marine organisms. Only four or five radiolarian species seem to disappear from the record accompanied by modest compositional changes in planktonic foraminifers and organic walled dinoflagellate cysts (Sanfilippo et al., 1985; Keller, 1986; Brinkhuis and Biffi, 1993). On the other hand, although little evidence exists in the literature for climate change across the Cretaceous–Paleogene transition, recent work has suggested that the late Eocene impact event was associated with a short-term (~100 ka), albeit modest (maximum 2°C), cooling event at high latitude (Vonhof et al., 2000). Leg 199 presents an ideal opportunity to study the climatic and biotic effects of impacts that were too small to precipitate global mass extinctions but were apparently large enough to have engendered regional changes in climate.

In many ways the Oligocene (~33.5–23.5 Ma) represents something of the "neglected middle child" of Cenozoic paleoceanography—caught between the early Paleogene greenhouse and the well-developed Neogene "icehouse." This situation is at least partly attributable to the perception that Oligocene marks a prolonged interval of relative stasis in paleoclimate and biotic turnover, reflected in deep-sea micropaleontological communities by conservative body plans confusing taxonomies and low biostratigraphic resolution. However, in some respects the Oligocene represents the most interesting piece of the Cenozoic paleoceanographic puzzle because it offers an opportunity to unravel the processes that lie behind the transition from a world free of large-scale continental icecaps and rapid eustatic sea level oscillations to one dominated by these climatic changes. To a large extent, recent progress in our understanding of Oligocene paleoclimates has been driven by seismic stratigraphy and scientific drilling in continental margin sequences (e.g. Browning et al., 1996; Miller et al., 1996; Pekar et al., 2000). In contrast, modern benthic stable isotope compilations (Fig. F5) show that our paleoclimate records for the deep oceans through the Oligocene rely heavily on old DSDP sites, largely from the Atlantic Ocean (Miller et al., 1987, 1988, 1990, 1991, 1993). Leg 199 offers an excellent opportunity to generate low-latitude deep-sea records throughout the Oligocene and thereby test models developed from continental margin sequences for the pattern and timing of changes in global temperature and continental ice volume.

Nearly 30 yr ago, DSDP rotary drilling and coring of the central Pacific equatorial mound of sediments (e.g., DSDP Legs 5, 8, 9, and 16) (Fig. F1) established the general pattern of equatorial sediment accumulation and plate migration through the Neogene and late Paleogene (e.g., van Andel et al., 1975; Berger and Winterer, 1974; Leinen, 1979). However, the rotary coring technology available to these very early legs could not provide undisturbed sections or complete recovery and was utterly defeated by middle Eocene chert layers encountered in some of the more deeply buried sections. Since these early efforts, one leg (DSDP Leg 85; Mayer et al., 1985) has revisited the region and obtained hydraulic piston core samples of Neogene sediments. However, the Paleogene sections of the Leg 85 holes were rotary cored because they were deeply buried beneath the Neogene equatorial sediment mound. Much of the Paleogene section was not recovered. For these reasons, even the broad outlines of equatorial sediment accumulation in Paleogene sediments and the link of sedimentation to early Paleogene climate remain poorly defined. The P/E boundary interval in the central tropical Pacific Ocean has not been sampled, primarily because the DSDP sites were mostly placed on crust younger than the boundary. DSDP Site 163 (Fig. F1), on early Campanian–age crust, is the only drill site in the central tropical Pacific that has drilled the interval. Unfortunately, there was poor recovery over the lower Eocene caused by closely spaced cherts. The middle Eocene has been sampled at more sites. North of the Clipperton Fracture Zone, where Leg 199 sites are situated, drilling at DSDP Sites 161 and 162 recovered continuous sediment sequences, albeit with core gaps and disturbed by rotary coring, to middle Eocene basalt crust. Sediments from Sites 40 and 41, between the Clarion and Molokai Fracture Zones, were upper to middle Eocene radiolarian oozes beneath a 10 m thick layer of red clay.

Leg 199 drilling was designed to accomplish the following scientific objectives:

1. To define sedimentation, paleoproductivity, circulation, and wind patterns in the Eocene equatorial Pacific;
2. To study the Paleocene–Eocene and Eocene–Oligocene transitions in the equatorial Pacific, as well as other boundaries mentioned above; and
3. To obtain complete, continuous Oligocene and lower Micene paleoceanographic records to study the effects of glaciation in Antarctica upon equatorial Pacific circulation.

1. To collect continuous sequences of Paleogene biogenic sediments and thereby improve Paleogene biostratigraphy, tie this stratigraphy to paleomagnetic chronostratigraphy and generate high-resolution paleoceanographic records from the Paleogene central tropical Pacific Ocean;
2. To place new constraints on the late Paleocene and early Eocene equatorial position using paleomagnetic and micropaleontologic indicators;
3. To link seismic stratigraphy from the site survey to sediment chronostratigraphy to extend the Neogene equatorial Pacific seismic stratigraphy (Mayer et al., 1985, 1986; Bloomer et al., 1995) back in time;
4. To document the transition between Asian and American dust sources to understand the primary structure of the Paleogene wind field;
5. To provide new constraints on the early Paleogene mass balance of carbonate and opal burial and to track the Eocene movement of the CCD in detail;
6. To generate primary geochemical information needed to understand the widespread formation of Eocene cherts; and
7. To collect basal hydrothermal sediment sections for study of hydrothermal activity in the early Paleogene.