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doi:10.2204/iodp.proc.320321.205.2013

Introduction

A primary objective of the Integrated Ocean Drilling Program (IODP) Pacific Equatorial Age Transect (PEAT) project is to study the evolution of Earth systems in the equatorial Pacific from 50 to 0 Ma (see the “Expedition 320/321 summary” chapter [Pälike et al., 2010]). This objective requires the production of continuous sediment records sampled frequently enough to resolve the effects of orbital forcing. It has long been known that the burial of CaCO₃ in the equatorial Pacific has varied on all timescales driven by climate-related changes in ocean chemistry and/or productivity (Hays et al., 1969; van Andel and Moore, 1974; Mayer et al., 1986; Lyle, 2003). X-ray fluorescence (XRF) scanning, because it is a fast, low-cost measurement, can potentially measure frequently enough to achieve sample spacings of <5 k.y. and resolve an orbitally forced signal on 10 m.y. time frames. We report here the construction of a CaCO₃ profile for the 414 m thick sediment column at Site U1338 based on calibrated XRF measurements.

Site U1338 (2°30.469′N, 117°58.178′W; 4200 m water depth) (Fig. F1) is on 18 Ma ocean crust buried by 414 m of pelagic sediment (see the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b]). The sediment drapes over the topography so that the ~200 m abyssal hill relief on the basement is still visible at the seafloor despite the 400 m of sediment cover (Tominaga et al., 2011).

Lyle et al. (2012) presents a method to condition XRF data for later calibration that works well for carbonate sediment from the equatorial Pacific. The methodology (1) scales the range of individual element XRF peak areas to better match the range of sediment elemental concentrations by scaling the median peak area to the median of measured bulk sediment composition, followed by (2) normalization of the sediment composition to 100% assuming a set of model sediment components.

The scaling process is needed because characteristic X-ray production by different elements depends not only on an element’s bulk concentration in sediment but also on its efficiency at absorbing and fluorescing X-rays. The normalization process is needed because XRF is a volume measurement, not a mass measurement. Each XRF measurement illuminates a certain volume of sediment, but the mass of sediment illuminated in different samples may not be the same because of porosity. The illumination volume is also dependent on composition, which affects penetration efficiency and absorption characteristics of different X-ray energies (Tjallingii et al., 2007). The composition effect will most strongly affect estimated composition if the surface region has a different composition than layers below. Because of the volume measurement problem, raw elemental peak areas are affected by both porosity and composition (Fig. F2). These volume effects create biases between individual measurements as well as systematic offsets over a large sediment column. Normalizing total component percentages to 100% eliminates much of this effect.

In this paper, we show how the XRF data from Site U1338 were calibrated and how errors were estimated, and we tabulate both the calibration data set and the XRF-estimated CaCO₃. The XRF data set has a time resolution between samples of ~2 k.y. in the uppermost Miocene to Holocene sediments and ≤1 k.y. in the sediment section below. The time resolution changes because of slower sedimentation in the upper section of Site U1338 (see the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b])

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