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doi:10.2204/iodp.proc.320321.201.2012

Introduction

Integrated Ocean Drilling Program (IODP) Expedition 320/321 (Pacific Equatorial Age Transect [PEAT]) cored eight sites (U1331–U1338) during March–July in 2009 (Fig. F1), recovering an age transect at the Pacific paleoequatorial region from the time of maximum Cenozoic warmth in the Eocene, through initial major glaciations in the Oligocene, to the present (see the “Expedition 320/321 summary” chapter [Expedition 320/321 Scientists, 2010a]; Lyle et al., 2009). The overall aim was to obtain continuous and well-preserved calcareous sediment sections for specific time slices. Of major importance for the objectives of the PEAT expeditions is the assembly of an integrated bio-, chemo-, and magnetostratigraphy at the Equator, hereafter referred to as the Cenozoic megasplice.

In addition to those from the PEAT drill sites, key records for the breakthrough in reconstructing the equatorial climate system of the late Eocene and Oligocene epochs were recovered during Ocean Drilling Program (ODP) Leg 199 (Lyle, Wilson, Janecek, et al., 2002), Site 1218 in particular (Fig. F1). Data from Site 1218 allowed astronomical calibration of the entire Oligocene (Wade and Pälike, 2004; Coxall et al., 2005; Pälike et al., 2006), but the lack of carbonate in the uppermost Eocene at this site made detailed time control much less robust. Although the paleomagnetic record for these time intervals is of high quality (e.g., Lanci et al., 2004, 2005), global stratigraphic correlation is hindered by the lower mass accumulation rate, the absence of a detailed isotope stratigraphy, and low-resolution biostratigraphic control. In order to facilitate the development of an integrated magneto- and biostratigraphic framework with a stable isotope stratigraphy (necessary to enable global correlation), recovery of Eocene carbonate sediment with a high-quality magnetostratigraphy was targeted and successfully retrieved during Expedition 320.

During Expedition 320/321, at least three holes at each site were cored and used to construct composite sections (see the “Methods” chapter [Expedition 320/321 Scientists, 2010b]) in order to assure the recovery of a complete stratigraphic section needed for the assembly of the Cenozoic megasplice. As shown at ODP Leg 199 Sites 1218 and 1219 (Pälike et al., 2005), extensive postcruise work is required to reevaluate the shipboard composite depth stratigraphy and to provide a high-resolution revised meters composite depth (rmcd) scale. In addition, squeezing and stretching of cored intervals is necessary to compensate for depth distortion within individual cores (Hagelberg et al., 1992). To locate hiatuses and condensed intervals, it is essential to do a site-to-site correlation using physical property data (Shackleton et al., 1995, 1999; Shackleton and Crowhurst, 1997; Pälike et al., 2005; Westerhold and Röhl, 2006; Westerhold et al., 2007, 2008). Subsequently, the correlation allows integration of any kind of data from one site to another. A prerequisite for correlation is that decimeter-scale features in the sedimentary record can be correlated between holes and, if possible, between sites.

Both Leg 199 and Expedition 320/321 magnetic susceptibility and gamma ray attenuation (GRA) bulk density data can be correlated over large distances (>1000 km) across the Pacific seafloor (Pälike et al., 2005, 2009). Physical property data, a proxy for calcium carbonate oscillations, at Sites U1331 and U1332 show a remarkable match with those from ODP Site 1220, a site with an excellent magnetostratigraphy. Similarly, Sites U1333 and U1334 can be correlated to Site 1218. Together, these sites provide a coherent and integrated record for the equatorial Pacific and enable study of sedimentation patterns and mass accumulation rates at orbital resolution (Pälike et al., 2009). Here, we present revised composite depth scales and revised spliced composite records for Expedition 320 Sites U1331, U1332, U1333, and U1334 and Leg 199 Sites 1218, 1219, and 1220 (Fig. F1). We also correlate and integrate Leg 199 and Expedition 320 physical property and stratigraphic data (including shipboard and revised biostratigraphic data) to define a complete high-resolution time series for the middle–late Eocene, the entire Oligocene, and the early Miocene.

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