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doi:10.2204/iodp.proc.320321.203.2012

Data acquisition methods

Data in this data report were acquired at the IODP Gulf Coast Repository in College Station, Texas (USA) (odases.tamu.edu/research-facilities/xrf-request/), using a third generation Avaatech XRF scanner with a Canberra X-PIPS SDD, model SXD 15C-150-500 150 eV resolution X-ray detector. The XRF scanner is configured to analyze split sediment core halves for elements between Al and U in the periodic table. The X-ray tube and detector apparatus is mounted on a moving track so that multiple spots at different depths can be analyzed on a split core during the scanning run and multiple scans with different settings can be automatically programmed (Richter et al., 2006). Many parameters are controlled by the operator. For example, there are controls for X-ray tube current, voltage, measurement time (live time), X-ray filters used, and area of X-ray illumination. The downcore position step is precise to 0.1 mm.

For Site U1338 XRF scans, sample spacing along each core section was set at 2.5 cm intervals and separate scans at two voltages were used. One scan was performed at 10 kV for the elements Al, Si, S, Cl, K, Ca, Ti, Mn, and Fe, and a repeat scan was performed at 50 kV for Ba. The voltage used for elements measured is determined by the energy needed to excite the appropriate characteristic X-rays. The X-ray illumination area was set at 1.0 cm in the downcore direction and 1.2 cm in the cross-core direction, and the scan was run down the center of the split core half (6.8 cm total diameter). Both scans were done with an X-ray tube current of 2 mA. Settings used for Site U1338 10 kV XRF scans are 2 mA tube current, no filter, and a detector live time of 20 s; for the 50 kV scan the settings are 2 mA current, Cu filter, and a detector live time of 10 s.

After consultations with colleagues in Bremen, Germany, we now use lower power to preserve tube life and reduce possibility of peak overlap problems. The raw X-ray peak areas are proportional to the power applied. Figure F3A is a comparison of data from XRF scans done at the Gulf Coast Repository on Section 321-U1337A-12H-2 using two different X-ray tube currents at 10 kV. For all elements the peak area measured is proportional to the tube current multiplied by the count time. The slope for each elemental data set is near the slope of 0.375 expected from the power-time ratios between the two runs. Figure F3B is a similar comparison between two different Avaatech scanners with different detectors: the Bremen MARUM XRF3 scanner (then equipped with a Canberra SXP 5C-200-1500 V2 200 eV resolution detector) versus the Gulf Coast Repository scanner (Canberra SXD 15C-150-500 150 eV resolution detector) on the Eocene/Oligocene boundary section of Hole U1333C (Sections 320-U1333C-14H-4 and 14H-5). The raw data in this comparison are also linearly proportional to power, although different sensitivities of the SXP versus SXD X-ray detectors add an additional linear factor to the count differences. In each example, there is more variability within the light element (Al) measurement where counts are low and air absorption of the low-energy X-rays is more significant. Nevertheless, the different scanners detect a similar chemical signal.

Prior to scanning, each core section was removed from refrigeration at least 2 h before scanning and was covered ~15 min before the scanning with 4 µm thick Ultralene plastic film (SPEX Centriprep, Inc.). Ultralene film protects the detector face from becoming sediment covered and contaminated during the scan. It is important to wait until the core sections warm to room temperature before putting the film on them. Plastic film placed over cool core sections can lead to water condensation on the film and severely reduce light element XRF peak areas by absorbing the emitted low-energy X-rays. We observed in one test a 25%–50% reduction in Ca peak area comparing a cold-run core section to the same section after it was allowed to warm and the Ultralene was replaced. The difference in measured peak area between the warm and cold core was not constant in this particular test but increased downcore on the cold core as the condensation continued to form. Also see Tjallingii et al. (2007) for a discussion of water on light element XRF intensities.

Only core sections along the continuous spliced section of Site U1338 were analyzed, not every core section recovered at the site. We XRF-scanned every archive core half in the Site U1338 splice table (see Table T24 in the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b]). If the splice transferred from one hole to the next in the middle of a section, we ran both entire sections. Therefore, most jump points in the splice have significant overlap. In a few cases where the splice was being revised (J. Dickens, pers. comm., 2011) we also scanned additional sections to help determine the best splice revision. All data gathered, including the overlaps, are included in Table T1. Table T2 has only the data following the published Site U1338 splice (see Table T24 in the “Site U1338” chapter [Expedition 320/321 Scientists, 2010b]), with minor revisions where the meters composite depth (mcd) depths did not match at the tabulated splice point and a revision between 40 and 45 mcd (core composite depth below seafloor [CCSF], method A [overlapping]; see the “Methods” chapter [Expedition 320/321 Scientists, 2010a]) where a mismatch was found by Dickens. Table T2 contains both the raw and normalized median-scaled (NMS) reduced data, described below.

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