INTRODUCTION

The Eocene, a 21 m.y. epoch of the Cenozoic, is well known for stable, warm climate conditions, but there is surprisingly little detailed information about paleoceanographic conditions that were a major part of the climate machine. Modeling studies and proxy data suggest that a significant contribution to warm conditions was provided by atmospheric CO₂ (Pearson and Palmer, 2000; Demicco et al., 2003; Shellito et al., 2003; Royer et al., 2004). Others have challenged this assertion, however (Veizer et al., 2000). One of the constraints on changes in atmospheric CO₂ is ocean pH—low ocean pH supports high atmospheric CO₂ (Royer et al., 2004), or, vice versa, maintenance of high atmospheric CO₂ drives ocean pH down (Caldeira and Wickett, 2003). Low ocean pH lowers the activity of CO₃^2– in seawater, causing higher net dissolution of CaCO₃ and smaller magnitudes of CaCO₃ burial in the deep ocean. The carbonate compensation depth (CCD) is thus an indicator of ocean pH because the depth-dependent increase in CaCO₃ dissolution favors preferential loss of carbonate from deeper ocean regions. Calcium content of the oceans probably changed during the Cenozoic (Hardie, 1996; Horita et al., 2002) and could also affect the CCD, but changes in the weathering cycles needed to change Ca are typically much slower than changes in the C system. The residence time of Ca relative to the carbon cycle (~10⁶ yr residence time for Ca vs. 10⁵ yr for C) (Broecker, 1971) is a rough indicator of the relative response times. Significant changes in ocean pH and atmospheric CO₂, if driven by changes in biogeochemical cycles, can happen much more rapidly (on the order of 10³ yr), as is being shown by the modern anthropogenic CO₂ transient (Caldeira and Wickett, 2003) and the Paleocene/Eocene boundary event (Dickens et al., 1995, 1997; Thomas et al., 2002).

Ocean Drilling Program (ODP) Leg 199 is the first drilling transect across a major cog in the Eocene ocean-atmosphere climate system, the equatorial Pacific (Lyle, Wilson, Janecek, et al., 2002). The eastern equatorial Pacific, the Eocene paleoposition of the Leg 199 transect, has always been a major locus for insolation heating of the oceans and, therefore, a major factor in ocean heat balance (Huber and Caballero, 2003). In addition, the Pacific was volumetrically larger in the Eocene, composing roughly two-thirds of all the tropical oceans rather than the modern one-half.

In the modern oceans, the tropics are a major area where CaCO₃ is deposited. The tropics should also have been important in the Eocene as a locus of pelagic carbonate production, if not burial. However, previous drilling has shown that the CCD was shallow in the Eocene and radically deepened during the Eocene–Oligocene transition (van Andel, 1975; Berger et al., 1981; Delaney and Boyle, 1988; Peterson et al., 1992; Coxall et al., 2005) associated with the first major Antarctic glaciation, Oi-1 (Miller et al., 1991; Zachos et al., 2001). Deep Sea Drilling Project (DSDP) drilling also showed that there is a high temporal variability to CaCO₃ deposition in the Eocene tropics (van Andel, Heath, et al., 1973). The timing of this variability has not yet been well documented, nor are the processes well understood.

In this paper, we collected detailed data for two Leg 199 drill sites (Sites 1218 and 1219) and developed detailed biogenic sedimentation time series (see also Olivarez Lyle and Lyle, this volume). We explore how carbonate burial varied during a 12-m.y. interval in the middle and late Eocene from ~46 Ma to the Eocene/Oligocene boundary (33.6 Ma), a period in which the CCD was extremely shallow and CaCO₃ burial in the deep ocean was low. We found long-period carbonate cycles in the equatorial Pacific sediments and link these cycles to initial Antarctic glaciations. We combined information from three drill sites (Sites 1218, 1219, and 1220) to study the change in the CCD and productivity for a critical 4-m.y. subinterval (42–38 Ma) for which we have overlapping records and found that the distribution of carbonate cannot be explained by depth-dependent dissolution in a one-dimensional ocean. Instead, we found a shallower CCD at the equator than at a paleoposition at 2°S, a situation reversed from modern conditions. The deeper off-equator CCD, which was probably caused by elevated carbonate production away from the equator, fits with Leg 199 observations of early Eocene CaCO₃ deposition (Rea and Lyle, 2005) north of the equator. Similarly, relatively high CaCO₃ deposition in the Antarctic region over the Eocene (Nelson and Cooke, 2001) suggests carbonate burial in the Pacific was organized in a different manner than in the Holocene.

We also found that the buildup of the largest of these carbonate accumulation events (CAEs), beginning at 41.5 Ma, was caused by increased production of carbonate. The appearance of CaCO₃ in the sediments is linked to polar glaciation. The end of CAE-3, the largest event, was caused by enhanced dissolution, with important implications for the carbon cycle.