INTRODUCTION

The Paleocene/Eocene Thermal Maximum (PETM) was an interval of rapid global warming ~55 m.y. ago at the Paleocene/Eocene (P/E) boundary. The event is associated with an array of faunal and floral changes: the largest deep sea foraminiferal extinction event in the last 90 m.y., rapid planktonic foraminiferal diversification, appearance of several land mammal groups, transient turnover in nannoplankton assemblages, and a bloom of the shallow-water dinoflagellate genus Apectodinium (e.g., Maas et al., 1995; Kelly et al., 1998; Thomas, 1998; Crouch et al., 2001; Bralower, 2002).

Associated with these biotic changes is a negative carbon isotope excursion (CIE) of –2 to –3 in marine and terrestrial reservoirs (e.g., Kennett and Stott, 1991; Koch et al., 1992; Bains et al., 1999). The magnitude, global nature, and abrupt onset (<10⁴ yr) of the CIE require rapid transfer of a massive amount of isotopically light carbon to the ocean-atmosphere system (e.g., Dickens et al., 1995). Several sources for this light carbon have been proposed including volcanic outgassing (e.g., Eldholm and Thomas, 1993), rapid burning of terrestrial organic matter (Kurtz et al., 2003), impact of a comet (Kent et al., 2003), and massive dissociation of methane hydrates along continental margins (e.g., Dickens et al., 1995, 1997; Katz et al., 1999; Svenson et al., 2004). Because the reservoir of methane hydrate in continental margin sediments is extremely large and their carbon isotopic composition is highly depleted (¹³C value of approximately –60) (e.g., Kvenvolden, 1993), hydrate dissociation most readily explains the magnitude and rate of onset of the CIE (e.g., Dickens et al., 1997).

One of the main effects of introduction of a large amount of CH₄ or CO₂ into the ocean-atmosphere system is a rapid shoaling of the lysocline, the depth range of increased CaCO₃ solubility, and the calcite compensation depth (CCD), the depth below which all CaCO₃ is dissolved (e.g., Dickens et al., 1997). Ample evidence exists for shoaling of both surfaces in widespread locations (e.g., Lu and Keller, 1993; Schmitz et al., 1996; Thomas and Shackleton, 1996; Bralower et al., 1997; Thomas et al., 1999; Erbacher, Mosher, Malone, et al., 2004; Zachos, Kroon, Blum, et al., 2004; Zachos et al., 2005). The source of carbon, and especially whether it is introduced in the deep ocean or atmosphere, will greatly affect lysocline and CCD behavior (e.g., Dickens et al., 1997; Dickens, 2000). For example, if CH₄ reached the atmosphere prior to oxidation or if CO₂ was not completely dissolved in the deep ocean (e.g., Dickens et al., 1997; Dickens, 2000; Thomas et al., 2002), less shoaling would occur than if oxidation of CH₄ or dissolution of CO₂ occurred entirely in the deep ocean. The extent of shoaling would vary between basins largely depending on the source of the CH₄ or CO₂ and deepwater circulation patterns. Thus, the behavior of the lysocline and CCD in different parts of the ocean can be used to constrain the source of carbon during the PETM and its distribution through the atmosphere and oceans (Zachos et al., 2005).

The timing and scale of carbon sequestration can also be constrained from the lysocline and CCD. As CO₂ is removed from the system, primarily through weathering of silicates and organic carbon burial, the concentration of bicarbonate begins to rise, and the lysocline and CCD deepen, resulting in a transient supersaturation of bicarbonate in the deep sea. In theory, the lysocline may overshoot its preexcursion level during this recovery period (Dickens et al., 1997).

Global warming is another anticipated outcome of input of a massive amount of CH₄ or CO₂ to the ocean-atmosphere system. Stable isotope and trace element data suggest that sea-surface temperatures in the PETM increased by ~8°C at high latitudes and ~5°C at low latitudes and in deep waters (e.g., Kennett and Stott, 1991; Bralower et al., 1995; Zachos et al., 2003). However, several PETM stable isotope records have been obscured by recrystallization due to burial, which prohibits paleotemperature estimates (Bralower et al., 1997; Norris, Kroon, Klaus, et al., 1998). In addition, lysocline and CCD fluctuations could result in partial dissolution of, and overgrowth on, foraminiferal tests, altering isotope signatures. Rapid lysocline and CCD shoaling would result in large-scale carbonate dissolution, increasing the concentration of bicarbonate in pore waters. As the lysocline and CCD recover, supersaturation of bicarbonate at the seafloor would enhance precipitation of secondary calcite. Constraining the changes in the lysocline and CCD would help interpretation of stable isotope and trace metal ratios of altered PETM foraminifers.

The extent and rate of lysocline and CCD shoaling during the PETM can be approximated in closely spaced sites along a depth transect in middle to lower bathyal water depths using visual observations of foraminiferal preservation and a suite of other preservational proxies. One of the goals of Ocean Drilling Program (ODP) Leg 198 to Shatsky Rise, a large igneous province in the western Pacific Ocean (Fig. F1), was to obtain such a depth transect of PETM sites (Bralower, Premoli Silva, Malone, et al., 2002), one that would also complement a similar depth transect planned for the Atlantic (Leg 208) (Zachos, Kroon, Blum, et al., 2004). The PETM was recovered at relatively shallow burial depths at four sites on the Southern High of Shatsky Rise (Sites 1209–1212) (Tables T1, T2, T3, T4), ranging from 2387 to 2907 m water depth. In these sections, the PETM interval corresponds to an 8- to 23-cm-thick layer of clayey nannofossil ooze with a sharp base and a gradational upper contact (Fig. F2). An extremely thin (1 mm) dark brown clay seam lies at the base of this contact in several locations. At the deepest site drilled on Shatsky Rise (Site 1208 on the Central High; 3346 m water depth), the PETM interval is highly condensed (<3 cm), lies in a dark claystone with few nannofossils and almost no foraminifers, and was clearly close to the CCD before, during, and after the event. At all sites, the CIE was identified using bulk carbon isotope measurements. Plate reconstructions and benthic foraminiferal assemblages suggest that Sites 1209–1212 were located at ~20°N and ~2400–2900 m water depth during the PETM, similar to their present water depths (Bralower et al., 2002).

In this investigation, potential carbonate preservational proxies were evaluated in sections from the Shatsky Rise depth transect and used to interpret the response of the tropical Pacific lysocline during the PETM.