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doi:10.2204/iodp.proc.320321.209.2013

Introduction

The eastern equatorial Pacific (EEP) delineates a region between 15°N and 15°S latitude, and between 150°W and the coasts of Central and South America (Fig. F1). Large zonal and meridional variations in sea surface properties (e.g., temperature, salinity, and primary productivity) characterize this area. The variance has a complex origin reflecting multiple influences; it also changes significantly on interannual timescales with El Niño Southern Oscillation (e.g., Trenberth and Caron, 2000; Pennington et al., 2006). In short, the EEP is one of the most dynamic regions of the world’s oceans.

There is considerable interest in reconstructing past sea surface properties within this region, especially during warm intervals of the late Miocene and early Pliocene (e.g., Lawrence et al., 2006; Brierley et al., 2009). However, a basic issue confronts such studies; given the extreme spatial and temporal complexities of surface water across the modern tropical Pacific, compelling arguments for regionally meaningful changes in the past require lengthy records at multiple locations that can be correlated at high temporal resolution.

Numerous sites have been drilled and cored in the EEP over the past 40 y. These include eight locations targeted by the Integrated Ocean Drilling Program (IODP) during Expeditions 320 and 321 in 2009 (see the “Expedition 320/321 summary” chapter [Pälike et al., 2010]; Fig. F1). Before the community can begin to link records between these sites and others, composite depth sections are needed. Deep-sea coring at a specific drill hole proceeds through the collection of discrete sediment cores, each retrieved after an increment of drilling. On the R/V JOIDES Resolution, these cores, in most cases, should be 9.5 m in length. However, the coring process is imperfect. First, some recovered cores are less than the drilling increment, presumably because sediment has been lost. Second, the coring process typically leaves gaps (and occasionally overlaps) between successive cores. Third, there are sometimes sediment intervals disturbed by drilling. Because of these issues, coring at a single drill hole recovers a discontinuous stratigraphic record, whereas coring multiple proximal holes at a single location can lead to a continuous stratigraphic record, or “composite depth section” (e.g., Hagelberg et al., 1995).

Composite depth sections are made from spliced portions of cores from different holes. Correlative horizons are found in cores from two or more holes. By choosing such horizons that occur toward the base of a core in one hole and toward the top of a deeper core in another hole, tie points can be set so that stratigraphic records cross gaps and eliminate overlaps between successive cores in a single hole. Over the last few years, preliminary splicing has been done during the drilling expedition to evaluate stratigraphic completeness and to enhance sampling strategy. Invariably, however, some of the tie points are incorrect. Perhaps as important, intervals of core that are not part of the splice are not correlated to the composite depth section. At a drill site with three or more holes, these “off-splice” sediment intervals may comprise 60%–70% of the recovered sediment, but often need to be compressed or expanded, so that they can be aligned to features seen in the composite section. (e.g., Pälike et al., 2005; Westerhold and Röhl, 2006)

Revised composite depth sections have been presented for four of the sites drilled during Expedition 320/321: IODP Sites U1331, U1332, U1333, and U1334 (Westerhold et al., 2012). Here, we offer revised composite depth sections for IODP Sites U1336, U1337, and U1338. Age models for these sites and revised composite depth sections for other sites in the EEP await additional work.

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