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Upwelling, Productivity, and Equatorial Sedimentation

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 the 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 to this narrowly focused Neogene arrangement (Moore et al., in press). Sedimentation rates appear significantly slower in the Eocene, and the distribution of sediments from seismic reflection profiles appears different than a pattern resulting from a single linear source of biogenic sediments along the equator. 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.

Pacific Plate Movement and the Cessation of Biogenic Sedimentation

The central equatorial Pacific is unique in the world's oceans because the path of plate motion carries this 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 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) and 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 paleomagnetic 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 ultimate scientific goal for Leg 199.

Early Paleogene Warm Climates

The early Paleogene (~60–45 Ma) witnessed the warmest global climates recorded on Earth in the entire Cenozoic. It has long been appreciated that abyssal ocean-water temperatures were significantly warmer than today during the early Paleogene (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 for warmer surface temperatures in high-latitude oceans (and the interiors of continents) than today is widespread (Zachos et al., 1993). Diverse sets of paleontological, sedimentary, and geochemical proxy records indicate that, at this 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. Finally, 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).

ODP Leg 199, the "Paleogene Equatorial Transect," was designed to drill a lower Paleogene transect across the world's most long-lived wind-driven current structure that 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, and 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.

General Circulation Model Reconstructions

Model reconstructions of 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. Thus, 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. 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 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. Model results such as these can then be used as a set of hypotheses to be tested by drilling, recovery of sediments, and the proper paleoceanographic analyses.

Paleoceanographic Time Slices

We have known for many years that the early Paleogene, particularly one interval in the early Eocene (~53–50 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). 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 through 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 from the perspective of 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.

Paleocene/Eocene Boundary

It is now well accepted that the Paleocene/Eocene (P/E) boundary involved 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 (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 analyses (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 the Earth 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 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, 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-like geochemical signals in the Mesozoic (e.g., Opdyke et al., 1999; Hesselbo et al., 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.

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