BACKGROUND AND OBJECTIVES

The complex system of equatorial currents is one of the most persistent and clear traces of wind-driven circulation in the oceans. The Leg 199 drilling program (Fig. F1; Table T1) was designed to study the evolution of this system in the Pacific Ocean as Earth went from maximum Cenozoic warmth to initial Antarctic glaciations. In the Neogene, the northern position of the Inter-tropical Convergence Zone (ITCZ) relative to the equator has given rise to tradewind-induced divergence at the equator and has caused a narrow band of equatorial upwelling (Fig. F2) as well as an equatorially asymmetric zonal current system. High productivity associated with equatorial upwelling results in a high rain of biogenic debris to the seafloor within 1.5°-2° of the geographic equator, with peak values restricted to an even narrower zone. In the Pacific Ocean, this biogenic rain has built, over Neogene time, a mound of almost pure calcareous and siliceous sediments stretching along the equatorial region and reaching a thickness of >500 m (Fig. F3). The little that we knew about Paleogene sequences in the central tropical Pacific Ocean prior to Leg 199 suggested that pre-Oligocene latitudinal patterns of sedimentation were significantly different than this narrowly focused Neogene arrangement (Moore et al., 2002). Sedimentation rates in the Eocene appear to be significantly slower than Neogene rates, and the distribution of sediments from seismic reflection profiles appears different than a pattern resulting from a narrow, linear, well-focused source of biogenic sediments along the equator. Ocean Drilling Program (ODP) Leg 199 was designed to collect sediments along a latitudinal transect in order to better understand Eocene and Oligocene sedimentation patterns and thereby reconstruct the dynamics of Eocene biogeochemical cycles and paleoceanography.

The central equatorial Pacific is unique in the world's oceans because the path of plate motion carries the linear trace of equatorial upwelling and productivity northward with time (van Andel, 1974). There are two clear implications of this northward plate motion: (1) the thickest part of the Neogene equatorial mound of biogenic sediment is displaced several degrees to the north of the equator and (2) sediments deposited a few tens of millions of years ago have moved completely out of the region of high sediment flux. In principle, this set of circumstances presents an excellent opportunity to recover relatively undisturbed sediments through climatologically and paleoceanographically important time intervals that are notorious for extensive chert formation (e.g., the middle Eocene) thus grossly undersampled in the deep oceans.

Despite the high quality of plate tectonic models, the path of movement of the Pacific plate is still not perfectly understood, and it is not yet possible for us to backtrack drill sites to their paleopositions in the early Eocene with confidence. Therefore, paleomagnetic studies aimed at locating our drill sites with respect to the Eocene equator were an important component of the Leg 199 shipboard science. An equally important aim of the shipboard paleomagnetic program was to cross-calibrate geomagnetic reversal stratigraphy with biostratigraphy. In fact, the likelihood of recovering sediments with compositional changes linked to orbitally forced insolation variability made the production of astronomically calibrated timescales an important scientific goal for Leg 199.

The early Paleogene, particularly the early Eocene (~55-49 Ma), witnessed the warmest global climates recorded on Earth in the entire Cenozoic, and it has long been appreciated that abyssal ocean-water temperatures were significantly warmer than today (e.g., Shackleton and Kennett, 1975). This observation has led to widespread speculation that these warm temperatures originated through low-latitude water mass formation (e.g., Brass et al., 1982). Yet, this explanation has always posed formidable problems for dynamicists, and recent numerical experiments support the alternative, more conservative, view that the warm, abyssal water masses of the Paleocene and Eocene are more likely linked to deepwater formation in high-latitude regions having warmer sea-surface temperatures than today (Bice and Marotzke, 2001). Support is widespread for the view that surface temperatures were warmer in high-latitude oceans (and the interiors of continents) during the early Paleogene (Zachos et al., 1993). Diverse sets of paleontological, sedimentary, and geochemical proxy records indicate that, at that time, subtropical to temperate fauna and flora extended to subpolar regions of both hemispheres where continental polar ice sheets were conspicuously absent (Shackleton and Kennett, 1975). The surface-temperature regime of the early Paleogene tropics is an ongoing subject of investigation (Bralower et al., 1995; Andreasson and Schmitz, 1998; Pearson et al., 2001), but the very warm temperatures (~12°C) estimated for high latitudes and deep waters mean that there can be little doubt that latitudinal temperature gradients during the early Paleogene were substantially smaller than today (Crowley and Zachos, 2000). This observation raises an intriguing paleoclimate problem because, if warmer high-latitude climates depend on enhanced wind-driven ocean currents or wind-carried heat and moisture to transport heat to the poles, it is difficult to explain how this transport was maintained under the weaker pole-to-equator thermal gradients. Instead, weaker latitudinal temperature gradients should give rise to weaker winds and diminished wind-driven transport. This apparent paradox is a persistent problem in numerical general circulation model reconstructions of warm paleoclimates (Barron and Washington, 1984; Manabe and Bryan, 1985; Sloan and Huber, 2001).

New data from the tropical oceans are necessary to define the climatic and oceanographic processes associated with early Paleogene warmth. Measurement of tropical sea-surface temperature gradients, for example, is an important way to distinguish between greenhouse-induced warming of the poles and warming by either atmospheric or oceanic heat transport (e.g., Crowley, 1991; Bralower et al., 1995; Wilson and Opdyke, 1996; Wilson and Norris, 2001; Pearson et al., 2001). Data on winds and currents are needed to partition heat transport between the atmosphere and oceans. The patterns of tropical wind and ocean circulation are key elements of global circulation, and existing records indicate that these patterns may have been markedly different from today during the early Paleogene (Hovan and Rea, 1992; Janecek and Rea, 1983; Rea et al., 1990).

Leg 199 was designed to drill a lower Paleogene transect across the world's most long-lived wind-driven current structure, which features the confluence of the northern and southern hemispheric winds and has a pattern, strength, and biogenic productivity linked to global climate patterns. The lower Eocene transect was aimed at providing basic records of sediment composition, mass accumulation, plankton communities, sea-surface and abyssal temperatures, and paleoproductivity across the paleoequator. These sorts of data are needed in order to assess the stability of the water column, the magnitude of heat transfer out of the tropics, surface ocean circulation, the location and strength of the trade wind belts, and the location of the ITCZ. The location of the ITCZ and the transition from the trade winds to the westerlies can be determined by the changes in the composition and rates of deposition of wind-blown dust, whereas mass accumulation rates (MARs) of biogenic debris can be used to assess the position and the strength of upwelling zones. Stable carbon isotope data will be used to assess nutrient flows in the water column and to constrain the global carbon cycle.

Model reconstructions of the Eocene climate have been hampered by a severe lack of data to constrain marine boundary conditions and, until recently, the need to specify either the atmospheric or oceanic part of the heat transport equation. In addition, lack of adequate control on bathymetry and topography in the Eocene, particularly in ocean gateway regions, severely weakens the capacity to model climate accurately from first principles (Deconto et al., 2000). Finally, computational constraints generally force the number of model years to be short (a few thousand years or less) so long-term transitions cannot be easily modeled. Most paleoclimate modeling is, therefore, performed on time slices for which reasonable initial conditions are established. In order to adequately understand Paleogene ocean dynamics and climate, there must be a strong interaction between paleoceanographers who gather new observations and climate modelers who make use of them to develop more realistic models (Sloan and Huber, 2001; Huber and Sloan, 2000).

One recent development in paleoclimate modeling is the application of coupled, ocean-atmosphere, global circulation models (e.g., Huber, this volume, and references therein). These types of models produce physically consistent worlds and can be used to study ocean-atmosphere interactions rather than to specify them. An example of the results of such modeling can be seen in Figure F4 with the estimated 56-Ma paleopositions of the Leg 199 drill sites superimposed. The figure shows annual upwelling velocity and indicates that the eastern Pacific should have been a relatively productive region, with respect to much of the world ocean, in the early Eocene.

We have known for many years that the early Paleogene, particularly the early Eocene (~55-49 Ma), represents the most extreme long-lived interval of global warming witnessed on Earth since the well-documented mid-Cretaceous greenhouse (e.g., Shackleton and Kennett, 1975; Wilson and Norris, 2001). Yet little is known about the number of constituent hyperthermals, the range of temperatures, or their effects on biotic evolution (Thomas and Zachos, 1999). Similarly, although we know that the Eocene greenhouse period was followed by a long shift toward lower temperatures and ice sheet growth into the late Eocene-Oligocene, the detailed history of these events and consequences for oceanic and atmospheric circulation, carbon cycling, and biotic evolution are only vaguely understood. The late Eocene is also interesting with respect to the response of global climate and biodiversity to the history of large impact extraterrestrial bodies on Earth. Thus, the Paleogene can be thought of as containing numerous critical time slices that provide an excellent opportunity to improve our understanding of important paleoclimatic problems involving the dynamics of greenhouse gas release, warm climate stability, biotic turnover associated with extreme climate transitions, extraterrestrial impacts, and initiation of major continental ice sheets.

It is now well accepted that a substantial (~5°-7°C) warming in the Southern Ocean and subtropics, a 35%-50% extinction of deep-sea benthic foraminifers, and rapid perturbation to the global geochemical carbon cycle took place across the Paleocene/Eocene (P/E) boundary (e.g., Kennett and Stott, 1991; Zachos et al., 1993; Koch et al., 1992). A growing body of evidence attributes these events to the massive release and oxidation of methane from the marine gas hydrate reservoir (e.g., Dickens et al., 1997; Katz et al., 1999). High-resolution stable-isotope records (Bains et al., 1999) and orbitally tuned chronologies from sites in the Atlantic and off Antarctica (Norris and Röhl, 1999) suggest that carbon release occurred extremely rapidly (within one precession cycle). Thus, the P/E boundary event may represent the best example in the geologic record of the response of Earth's ocean-atmosphere climate system to greenhouse warming on a timescale approaching that of the ongoing global anthropogenic experiment. Recently, elevated barium accumulation rates have been reported across the P/E boundary in various deep-sea sites. These data have been interpreted in terms of enhanced deposition of organic matter in deep-sea sediments that may have acted as a negative feedback on atmospheric CO₂ levels and global temperatures to return Earth to average late Paleocene conditions (Bains et al., 2000). Alternatively, this signal may reflect increased ocean barite saturation levels possibly driven by the injection of barium into the global ocean from the marine gas hydrate reservoir (Dickens et al., 2001).

Whereas recently developed data sets lend considerable support to the methane hydrate hypothesis, considerable uncertainty remains about the mechanism and location of carbon release, the response of the calcite compensation depth (CCD), and biotic overturn. Furthermore, the recent discovery of P/E-boundary-like geochemical signals in the Mesozoic (e.g., Opdyke et al., 1999; Hesselbo et al., 2000; Weissert, 2000) raises the distinct possibility that further "hyperthermals" might have existed in Paleogene time (Thomas and Zachos, 1999). Leg 199 presents a major opportunity to help improve our understanding of the climatic chain of events during the Paleogene. Results from the leg should prove particularly useful given the volumetric significance of the Pacific Ocean to geochemical mass-balance simulations (e.g., Dickens et al., 1997) and the current paucity of P/E boundary records from the basin.

The Eocene-Oligocene transition represents an important point in the shift 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 leading 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 near-contemporaneous increase in seawater ¹³C was driven by increased rates of C_org burial in marine sediments, and this factor may be implicated in global cooling and ice sheet growth (Diester-Hass and Zahn, 2001). Either way, long-standing global lithologic 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 Eocene/Oligocene (E/O) boundary come from two midlatitude 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/Tertiary (K/T) 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 K/T boundary, recent work has suggested that the late Eocene impact event was associated with a short-term (~100 k.y.), 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 (~33.7-23.8 Ma) represents something of a "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 the 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 in order to 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 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 early 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 Miocene paleoceanographic records to study the effects of glaciation in Antarctica upon equatorial Pacific circulation.

In addition, the following important but more general objectives shaped the Leg 199 drilling plan:

1. To collect continuous sequences of Paleogene biogenic sediments, thereby improving Paleogene biostratigraphy, tie this to magnetostratigraphy, 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 in order 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 American and Asian dust sources in order 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 (SiO₂) 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.