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doi:10.2204/iodp.proc.320321.216.2014

XRF scanning of Site U1335–U1337 spliced sections

Sites U1335–U1337 exhibit the characteristic variations in sedimentary calcium carbonate content that result in the common seismic stratigraphy found throughout the equatorial Pacific east of Hawaii (Mayer et al., 1986). It has been difficult in the past to understand and determine what forcing mechanisms caused these carbonate cycles, which in turn caused these common seismic horizons, because they were poorly resolved by low-resolution shipboard sampling and analysis. Through the use of XRF scanning and discrete geochemical sampling to calibrate the XRF scan data, we can potentially determine calcium carbonate content, biogenic opal, biogenic barium (bio-Ba), and clay content in fine enough detail to better understand why sedimentary carbonate has varied in the equatorial Pacific.

XRF scanning can be used to study the biogeochemically active elements Ca, Si, and Ba in order to understand changes in productivity (Lyle et al., 2012). These data can then be compared to changes in preservation to further understand changes in the overall carbon cycle. XRF scan data can also be used to measure aluminosilicate elements including Al, K, and Ti to better understand the dust input to the region (Lyle et al., 2012). Measuring redox-sensitive elements such as Fe and Mn gives information about the sedimentary redox environment, as well as a measure of distal hydrothermal plume deposition near the basement (Lyle et al., 2012).

XRF uses the characteristic fluorescence of elements exposed to high-energy X-ray illumination to estimate a sample’s chemical composition. High-energy X-ray photons eject inner-shell electrons from atoms being illuminated by the X-rays (Jansen et al., 1998), and the vacancies are filled by outer shell electrons. The excess energy is released as characteristic X-ray fluorescence energies for each element. The intensity of the fluorescence can be used to determine the abundance of different elements within the illuminated sample. With a scanning XRF, the detector is moved by a stepping system along a sediment core so that multiple measurements can be taken automatically (Richter et al., 2006).