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doi:10.2204/iodp.proc.320321.208.2013

Introduction

Profound changes in global climate and carbon cycling occurred during the Paleocene and Eocene epochs, including at the beginning (~65 Ma) and at the end (~34 Ma). These changes are evidenced in marine sediment by major variations in the stable oxygen and carbon isotope composition (δ¹⁸O and δ¹³C) of carbonate (e.g, Shackleton, 1986; Zachos et al., 2001, 2008). Some Paleogene carbon cycle perturbations also appear to manifest as fluctuations in the lysocline and carbonate compensation depth (CCD) (e.g., Coxall et al., 2005; Bohaty et al., 2009; Leon-Rodriguez and Dickens, 2010), depth horizons in the ocean that relate to ocean chemistry and the accumulation of carbonate on the seafloor (Boudreau et al., 2010).

Long-term, low temporal resolution records of stable isotopes and carbonate accumulation have been available for years (e.g., Van Andel, 1975; Shackleton, 1986; Zachos et al., 2001; Rea and Lyle, 2005). Higher resolution stable isotope and carbonate records also have been generated across shorter time intervals of the Paleogene at many locations. Over the last few years, however, it has become clear that the available records are inadequate to fully appreciate Paleogene climate and carbon cycling because the early Paleogene seems to have been far more dynamic than once envisioned. As a case in point, vigorous current debate concerns the magnitude of the carbon isotope perturbation and seafloor carbonate dissolution across the Paleocene Eocene Thermal Maximum (PETM) around ~56 Ma (Zeebe et al., 2009; Cui et al., 2011; Dickens, 2011; Sluijs and Dickens, 2012), perhaps the best-studied event of the Paleogene. The now-abundant records spanning the PETM in marine sediment cores strongly suggest that the δ¹³C of carbonate depends on the location and phase of carbonate being examined (McInerney and Wing, 2011; Sluijs and Dickens, 2012) and that seafloor carbonate dissolution varied significantly between ocean basins (Zeebe and Zachos, 2007; Leon-Rodriguez and Dickens, 2010). Similar issues regarding the magnitude of the δ¹³C excursion and carbonate dissolution appear to mark the Middle Eocene Climatic Optimum (MECO) (Bohaty et al., 2009).

Coring during Expedition 320/321 recovered middle and upper Eocene sediment sequences at Sites U1331–U1333 that accumulated on the seafloor near the Equator (see the “Expedition 320/321 summary” chapter [Pälike et al., 2010]). From preliminary results, it appears that a series of carbonate accumulation events (CAEs) occurred in the equatorial Pacific during the middle and late Eocene (Pälike et al., 2010, 2012). These events were not obvious in previous CCD reconstructions (Van Andel, 1975; Rea and Lyle, 2005). This may reflect the past locations and depths of various sites drilled so far in the region; in particular, Ocean Drilling Program (ODP) Sites 1215 and 1219–1221 were all further north and deeper in the middle Eocene. In any case, links between the newly discovered CAEs and global changes in climate and carbon cycling are not clear.

In this report, we present records of δ¹³C and δ¹⁸O for samples of bulk carbonate that accumulated during the Eocene at Sites U1331–U1333. Although it is tempting to link portions of these records with those published at other locations (e.g., Shackleton, 1986; Coxall et al., 2005; Bohaty et al., 2009), additional analyses and work will be required to do this correctly. It also remains unclear how the isotope records relate to the CAEs from a mechanistic perspective.

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