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doi:10.2204/iodp.proc.320321.219.2016

Introduction

A key goal of Integrated Ocean Drilling Program (IODP) Expedition 320/321 (Pacific Equatorial Age Transect [PEAT]) was to recover a series of sediment sections from the Pacific paleoequator region preserving critical intervals of Cenozoic paleoceanographic and paleoclimatic conditions and development (see the “Expedition 320/321 summary” chapter [Pälike et al., 2010]). The age transect, or flow-line concept, of Pälike et al. (2010) made use of Pacific plate motion and placed sites close to the Pacific paleoequator for carefully selected time intervals that would ensure near-continuous recovery of optimally preserved early Eocene through recent sediment sections. Eight sites were drilled (Sites U1331–U1338) with present locations along an oblique north–south transect from 12°4′N, 142°10′W (old end-member Site U1331; 5116 m water depth), to 2°30′N, 117°58′W (young end-member Site U1338; 4200 m water depth). Establishing an age model for each site was a crucial component of the expedition objectives.

Sediment sections recovered from three holes at Site U1338 represent the interval from the late early Miocene at ~18 Ma to the recent. These three holes were spliced onboard using physical property data into a single continuous section with coherent depth progression from the water/sediment interface to the sediment–basalt transition, permitting determination of linear sedimentation rates from biomagnetostratigraphic data (see Fig. F14 in the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b]).

IODP employs three different core depth concepts for recovered sediments. The baseline depth is in meters below seafloor (mbsf), or core depth below seafloor (CSF-A, in meters). When splicing a single continuous section from multiple holes at a single drill site, a second set of depths are assigned to the cores: composite depth below seafloor (CCSF-A, in meters). Empirically, we have learned that CCSF-A depths lie deeper than CSF-A depths and that the spliced section typically has grown in length by about 10%. In the case of Site U1338, this value was 11%, calculated by linear regression of CSF-A versus CCSF-A for all Site U1338 cores (see Fig. F44 in the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b]). This growth factor distorts sedimentation and mass accumulation rates. To compensate for this distortion, CCSF-A depths for Site U1338 were divided by a growth factor of 1.11, resulting in a third set of depths, the corrected composite depths (CCSF-B, in meters) (see Table T23 in the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b]). Subsequently, CCSF-A depth data from Site U1338 were revised by Wilkens et al. (2013), who also provide a discussion of IODP depth scale terminology.

Here, we combine revised diatom and partly revised calcareous nannofossil biostratigraphic data with shipboard magnetostratigraphic data to produce a biomagnetostratigraphic age model for Site U1338 using the Wilkens et al. (2013) revised depth data.

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