The exact causes of the long- and short-term Paleogene warm episodes remain enigmatic. Several pieces of geochemical evidence, including changes in the mean ocean ¹³C and alkalinity, point toward greenhouse forcing (Shackleton, 1986; Kennett and Stott, 1991; Zachos et al., 1993; Thomas and Shackleton, 1996). Samples recovered during Leg 198 will help constrain the nature and causes of these warm episodes.

In terms of the rate and degree of warming, the LPTM is unprecedented in Earth history (Fig. F6). The deep-sea and high-latitude oceans warmed by 4°C and 8°C, respectively. The carbon isotopic composition of the ocean decreased by 3‰–4‰ coeval with the warming event, suggesting a massive perturbation to the global carbon cycle (Fig. F7) (Kennett and Stott, 1991; Bains et al., 1999). The large magnitude and rate (~–3‰–4‰/5 k.y.) of the carbon isotope excursion (CIE) is consistent with the sudden injection of a large volume of methane from clathrates stored in continental slope sediments (Dickens et al., 1995, 1997). Much of this methane would have quickly converted to CO₂, stripping O₂ from deep waters, contributing to the major extinction event of benthic foraminifers (Thomas, 1990), and lowering alkalinity. The result should be a sharp rise in the level of the lysocline and CCD (Dickens, 2000). Both CO₂ and CH₄ would also have immediately contributed to greenhouse warming.

The Leg 198 depth transect will help us determine (1) the magnitude of the tropical Pacific sea-surface and deep-water temperatures increase during the LPTM; (2) whether or not the Pacific lysocline and CCD shoaled during the CIE, whether or not bottom-water oxygenation decreased, and how these changes fit with geochemical models of clathrate release; (3) the response of planktonic and benthic populations to the LPTM in the subtropical Pacific; and (4) whether or not there is a change in the distribution of bottom-water carbon isotopes prior to and/or during the LPTM signaling possible circulation changes.

Several investigators have suggested that early Cenozoic global warming would have altered deep- ocean circulation patterns by reducing the density of surface waters in high latitudes (Kennett and Shackleton, 1976; Wright and Miller, 1993; Zachos et al., 1993). This, in turn, would permit increased downwelling of highly saline but warmer waters in subtropical oceans. Such reversals or switches in circulation probably occurred suddenly rather than gradually. In fact, it has been suggested that a sudden change in intermediate-water circulation patterns may have occurred just prior to the LPTM, possibly triggering the dissociation of clathrates (Bralower et al., 1997a). There may have been additional, abrupt warming intervals in the late Paleocene and early Eocene (Thomas and Zachos, 1999; Thomas et al., 2000). These "hyperthermals" were characterized by changes in the assemblage composition of benthic foraminifers corresponding to negative shifts in planktonic and benthic foraminiferal ¹⁸O and ¹³C values. The ultimate cause of the hyperthermals may be similar to the LPTM, driven by the release of greenhouse gas.

Leg 198 samples will be used to assess regional and global circulation changes during the Paleogene. Major changes in the sources of waters bathing Shatsky Rise might be reflected in the spatial and vertical distribution of carbon isotope ratios in bottom waters as well as in benthic foraminiferal assemblage patterns. Several studies have shown that throughout the late Paleocene and early Eocene, the most negative deep-ocean carbon isotope values were consistently recorded by benthic foraminifers from Shatsky Rise (Miller et al., 1987b; Pak and Miller, 1992; Corfield et al., 1992). Such a pattern is similar to that in the modern ocean, implying older, nutrient-enriched waters in the Pacific, and younger, nutrient-depleted waters in the high latitudes. Although Site 577 is discontinuous across the Paleocene/Eocene boundary, isotope data from Site 865 on Allison Guyot in the equatorial Pacific suggest a possible reduction, if not reversal, in the ¹³C gradient between the shallow Pacific and the rest of the ocean (Bralower et al., 1995). If this was true, it would be consistent with increased production of intermediate waters in low latitudes. In summary, Leg 198 samples will help address whether there is evidence of warmer, more saline deep waters at times during the Paleogene and how export production in the Pacific changed from the Paleocene to the Eocene.

The Eocene–Oligocene represents the final transition from a "greenhouse" to an "icehouse" world. Although this transition occurred over a period of 18 m.y., stable isotopic records reveal that much of the cooling occurred over relatively brief intervals in the late early Eocene (~50–51 Ma) and earliest Oligocene (~33 Ma) (Fig. F6) (e.g., Kennett, 1977; Miller et al., 1987a; Stott et al., 1990; Miller et al., 1991; Zachos et al., 1996). Furthermore, small, ephemeral ice sheets were probably present on Antarctica sometime after the first event (Browning et al., 1996). The first large permanent ice sheets became established much later, most likely during the early Oligocene event (Zachos et al., 1992a). Current reconstructions of ocean temperature and chemistry for the Eocene and Oligocene, however, are based primarily on pelagic sediments collected in the Atlantic and Indian Oceans (Miller et al., 1987a; Zachos et al., 1992b, 1996). Very few sections suitable for such work have been recovered from the Pacific (Miller and Thomas, 1985; Miller and Fairbanks, 1985). As a consequence, we still lack a robust understanding of how global ocean chemistry and circulation evolved in response to high-latitude cooling and glaciation.

Leg 198 sections across the Eocene–Oligocene transition will provide a vertical depth transect of ocean chemistry and temperature changes during this important climatic transition. These sections will allow us to determine whether the basin-to-basin deep carbon isotope gradient changed during the Eocene–Oligocene transition in response to high-latitude cooling and glaciation, and how the lysocline/CCD in the Pacific responded to the rapid high-latitude cooling/glaciations.