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doi:10.2204/iodp.proc.320321.203.2012

Introduction

A primary objective of the Integrated Ocean Drilling Program (IODP) Pacific Equatorial Age Transect (PEAT) project is to produce continuous records that track the effects of climate change in the equatorial Pacific with enough detail to resolve orbitally forced climate cycles. A significant part of climate change is recorded by variability in the chemical composition of sediments, but this information typically is hard to extract at a reasonable cost.

X-ray fluorescence (XRF) scanning is potentially an economical way to extract the chemical data—it is an X-ray optical technique that can measure most major elements and some minor ones in ~20–30 s per measurement. This method can be used to gather chemical data at vertical spacing similar to that at which physical properties data are gathered (e.g., Westerhold and Röhl, 2009). These chemical measurements can augment physical properties measurements to study cyclostratigraphy, and if calibrated, XRF scan data can be used to understand the long-term evolution of biogeochemical cycles.

In this data report, we present the results of XRF scanning on the spliced sedimentary section of PEAT Site U1338 and describe a basic technique to normalize and calibrate the data for further geochemical study. Both the raw and normalized data along the splice are presented in tables. Data at this sampling resolution for the first time allows the study of geochemical cycles for long periods in the Miocene and of how biogeochemical changes are associated with long-term changes in global climate.

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

Site U1338 has the characteristic variations in sedimentary calcium carbonate content that result in the common seismic stratigraphy that is found throughout the equatorial Pacific region east of Hawaii (Mayer et al., 1985, 1986; see the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b]; Tominaga et al., 2011). It has been a long-standing scientific problem to understand what forcing mechanisms and associated variations in global geochemical cycles caused the carbonate cycles that in turn caused the common seismic horizons across the equatorial Pacific. XRF scanning potentially can determine calcite contents with sufficient detail to better understand both the links between physical properties and sediment carbonate contents and why sedimentary carbonate varied throughout the eastern equatorial Pacific.

XRF studies of biogeochemically active elements Ca, Si, and Ba also can be used to understand changes in productivity and can be compared to changes in preservation to better understand changes in the carbon cycle. XRF scanning also can measure aluminosilicate elements (Al, K, and Ti) to understand dust deposition in the equatorial Pacific, whereas measurements of the redox-sensitive elements Fe and Mn can be used to study changes in the sedimentary redox environment and hydrothermal activity. Dymond (1981) shows how chemical data can be used to discern sediment processes in the eastern equatorial Pacific.

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