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doi:10.2204/iodp.proc.320321.205.2013

Methods

XRF scan data were acquired at Texas A&M University (USA), and the CaCO₃ analyses were done at Stockholm University (Sweden).

XRF data acquisition

XRF scans were done using the Ocean Drilling and Sustainable Earth Science (ODASES) XRF scanning facility at the IODP Gulf Coast Repository (GCR) in College Station, Texas (USA) (odases.tamu.edu/research-facilities/xrf-request), using a third-generation Avaatech XRF scanner with a Canberra X-PIPS (passivated implanted planar silicon) silicon drift detector (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 are mounted on a moving track so that multiple spots at different depths can be analyzed on a split core in a single scanning run and multiple scans with different settings can be automatically programmed. Scan parameters are controlled by the operator, including X-ray tube current, tube voltage, measurement time (live time), X-ray filters used, and area of X-ray illumination. The downcore position step is also programmed and is precise to 0.1 mm. A basic description of first and second generation Avaatech scanners are given in Richter et al. (2006). The ODASES scanner is a third-generation Avaatech scanner.

For the Site U1338 XRF scans, each core section was removed from refrigeration at least 2 h before scanning and scraped to clean the split core surface. The surface was covered with 4 µm thick Ultralene plastic film (SPEX Centriprep, Inc.) to prevent contamination of the X-ray detector. Measurements were taken at 2.5 cm intervals, and separate scans at two voltages were done. 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. Both scans were done with an X-ray tube current of 2 mA. Settings used for the 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.

Further details about the XRF scanning procedure and reproducibility can be found in Lyle et al. (2012).

CaCO₃ analyses

Calcium carbonate concentrations were determined using a Model 5012 UIC coulometer. Samples were freeze-dried, ground to dry powder, and stored in a desiccator prior to analysis. In sediment samples with carbonate contents in excess of 25%, ~30 mg of the sediment powder was placed in a Teflon cup and weighed using a Sartorius MC5 microbalance, with a standard deviation of better than ±1 µg and a linearity of better than ±2 µg within 500 mg. The cup with its sediment powder was placed in the coulometer sample tube, and then 2 mL 2 M HCl was added, drenching the entire cup and the sediment powder in it. The operation of the coulometer followed procedures recommended by the instrument manufacturer. A few standards of pure calcium carbonate (Merck 2060, 99.95%–100.05% CaCO₃) were analyzed before and after the daily runs of sediment samples. Coulometer CaCO₃ values were recalculated using a calibration algorithm described by Mörth and Backman (2011), based on analyses of 460 pure calcium carbonate standards. They also demonstrated that in order to generate a precision of ±0.8%, at least 7 mg of 100% calcite is needed. Mörth and Backman (2011) show that a sample size of 35 mg will meet the ±0.8% criterion in all sediment samples ranging from 100% to 20% carbonate content. A sample containing only 10% carbonate will need a sample size of 70 mg in order to meet the ±0.8% criterion. The uncertainty increases in samples having <10% calcium carbonate.

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