Calcite and aragonite produced by plankton sinks toward the seafloor throughout the oceans. At some depth, depending on mineralogy, this carbonate begins to dissolve because of unfavorable physiochemical conditions. The carbonate compensation depth (CCD) is the depth where carbonate dissolution exceeds carbonate supply so that no carbonate accumulates on the seafloor (Murray and Renard, 1891; Bramlette, 1961). The water column above the CCD where the preservation of carbonate decreases noticeably (often ~2 km in depth) is the lysocline (Berger, 1971). Early experiments evaluating the lysocline involved measuring weight loss in suspended calcite spheres (Peterson, 1966). When samples of foraminiferal ooze are suspended in the lysocline, thin-walled planktonic tests preferentially dissolve and shell fragmentation increases (Berger, 1967).
The CCD and lysocline at a given location may change over geological time in response to variations in carbonate supply and properties of deep ocean waters (e.g., Berger, 1974, 1992; Farrell and Prell, 1989; Le and Shackleton, 1992; LaMontagne et al., 1996). Stable oxygen and carbon isotope records constructed using benthic foraminifers display a series of large-amplitude excursions during the Paleocene and Eocene at ~65–33.7 Ma, especially spanning the late Paleocene, the Initial Eocene Thermal Maximum (IETM), and the Early Eocene Climatic Optimum (EECO) (Fig. F1). These perturbations in 18O and 13C of deep-sea carbonate are generally interpreted as representing profound changes in early Cenozoic deepwater temperatures and global carbon cycling. As such, they might also correspond to times of significant depth variation in the CCD and lysocline. Enhanced deep-sea carbonate dissolution clearly coincided with a prominent –3 excursion in the 13C of benthic foraminifers at the IETM (Kennett and Stott, 1991; Schmitz et al., 1996; Thomas and Shackleton, 1996; Zachos et al., 2001), both phenomena signifying a rapid, massive input of carbon to the ocean and atmosphere (Dickens et al., 1997; Dickens, 2000). For most intervals of the Paleocene and Eocene, however, established records of deep-sea carbonate dissolution (van Andel, 1975; Lyle, Wilson, Janacek, et al., 2002) are not sufficiently well constrained for correlation with isotopic perturbations and, by inference, episodes of global oceanographic change.
During Leg 198, a series of locations was cored on Shatsky Rise in the North Pacific (Fig. F2) to evaluate the record of Cretaceous and Paleogene oceanographic change (Shipboard Scientific Party, 2002). During the Paleocene and Eocene, the crest of Shatsky Rise was nominally at 1500 m water depth (Ito and Clift, 1998; Sager et al., 1999). The relatively thick and complete Paleogene sedimentary successions down the flanks of Shatsky Rise (Krasheninnikov, 1981; Sliter and Brown, 1993) should, therefore, hold a record of past fluctuations in carbonate dissolution. In this study, carbonate content, coarse size fraction, benthic foraminiferal abundance, and planktonic foraminiferal fragmentation were measured in samples of Paleocene and Eocene sediment from Holes 1209A and 1211A on the Southern High of Shatsky Rise (Fig. F2). Our primary aim was to document intervals of enhanced dissolution between 65 and 33.7 Ma. We also compare our records to the Paleogene CCD curve recently published for the equatorial Pacific Ocean (Lyle, Wilson, Janecek, et. al., 2002).