Data report: X-ray fluorescence studies of Site U1457 sediments, Laxmi Basin, Arabian Sea

Abstract

Bulk sediment chemistry was measured at 2 cm resolution along cores from International Ocean Discovery Program (IODP) Site U1457 using the X-ray fluorescence (XRF) core scanner at the IODP Gulf Coast Repository. The Pleistocene splice section assembled from Holes U1457A and U1457B was scanned in its entirety, and nearly continuous sediment bulk chemistry profiles were constructed to a depth of 125 m core composite depth below seafloor (CCSF). Some sections of Hole U1457C were also scanned: (1) an upper Miocene hemipelagic section and (2) a 30 m lower Paleocene section directly overlying basalt. In the Pleistocene spliced sections, 2 cm spacing represents a sampling resolution of 150–300 y, whereas in the upper Miocene section this spacing represents about 500 y between samples. We report data and acquisition conditions for major and many minor elements. We find large variability in CaCO3 content in the Pleistocene section, from around 14 to 89 wt%. We used discrete shipboard CaCO3 measurements to calibrate the XRF Ca data. CaCO3 has cyclic variability and correlates with light sediment colors. Variation in aluminosilicate elements is largely caused by changes in dilution by CaCO3. The lower part of the spliced section, presumably representing distal Indus Fan deposits, has a distinctive but more uniform composition than the upper part. Introduction Variability in chemical composition of deep-sea sediments provides important data needed to understand changes in primary productivity, the carbon cycle, and sources of aluminosilicates. However, chemical analyses are typically too slow and expensive for high-resolution study of chemical composition covering long intervals of time. They also consume the sediments, so only limited numbers of analyses can be performed. Nondestructive X-ray fluorescence (XRF) core scanning provides the potential to produce high-resolution chemical profiles that can be calibrated with a small number of discrete chemical analyses. XRF is an X-ray optical technique that can measure most major and some minor elements. It is an economical way to extract bulk chemical data over longer sediment/rock profiles where thousands of analyses might be needed to resolve lithostratigraphic changes in the section. The method can be used to gather chemical data at a vertical resolution similar to that of physical property data collection by track systems on the JOIDES Resolution (e.g., Westerhold and Röhl, 2009; Lyle et al., 2012). These chemical measurements can augment physical property measurements to study cyclostratigraphy or rapid changes in sedimentation. If calibrated, the XRF data can be used to understand the long-term evolution of biogeochemical cycles and to identify the provenance of aluminosilicate sediments (Lyle and Baldauf, 2015). Data acquisition methods Data in this report were acquired at the IODP Gulf Coast Repository in College Station, Texas (http://iodp.tamu.edu/labs/xrf) using a third-generation Avaatech XRF core 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 M. Lyle et al. Data report: X-ray fluorescence studies of Site U1457 sediments halves for elements between Al and U in the periodic table. The Xray tube and detector apparatus is mounted on a moving track so that multiple spots at different positions along a split core section can be analyzed during each scanning run, and multiple scans with different settings can be automatically programmed (Richter et al., 2006). The operator controls 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 the Site U1457 (Figure F1) XRF scans, sample spacing was set at 2 cm intervals downcore in each core section, and separate scans at two voltages were used. 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 for cores collected using the advanced piston corer [APC]). The first scan was performed at 10 kV, 800 μA, 15 s live time, and no filter for the elements Al, Si, S, Cl, K, Ca, Ti, Mn, and Fe. A second scan was performed at 30 kV, 1000 μA, 20 s live time, and Pd-thick filter for Ni, Cu, Zn, Br, Rb, Sr, Y, Zr, Nb, Mo, Pb, and Bi. Of these elements, Br, Rb, Sr, and Zr had sufficiently large signals to justify further data reduction. Each core section was removed from refrigeration at least 2 h before scanning and was covered about 15 min before being placed on the scanner with 4 μm thick Ultralene plastic film (SPEX Centriprep, Inc.). The 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 core sections. Plastic film placed over cool core sections can cause water condensation onto the film, which severely reduces light-element XRF peak areas by absorbing the emitted low-energy X-rays (Lyle et al., 2012). All core sections contained within the continuous spliced section of Site U1457 were analyzed. Where the sediment splice changed from one hole to the other, both sections at the tie point were scanned in their entirety. All of the sections within the revised Site U1457 splice listed in Lyle and Saraswat (2019) were scanned. All data gathered, including the overlaps, are included in Table T1 (Hole U1457A) and Table T2 (Hole U1457B). Table T3 contains the Hole U1457C data, and Table T4 contains the Site U1457 splice data, which represents the data along a continuous sediment section for the upper 125 m CCSF. All tables contain both the raw peak areas and the normalized median-scaled (NMS)-reduced data, described below. Because there is more than one method of data reduction for XRF scan data (Weltje and Tjallingii, 2008; Lyle et al., 2012; Weltje et al., 2015), we report both raw peak area data and processed data in Tables T1, T2, and T3. The raw data thus will allow reprocessing in the future, if warranted. Data reduction methods for later calibration—normalized median-scaled (NMS) data Data reduction was achieved through a simple two-step method, as explained in Lyle et al. (2012): (1) The data were first median scaled by setting the median peak area of each element to a model percent derived from average graywacke (Wedepohl, 1995). The range in the peak area data was then converted into a raw percentage by dividing the peak area of each individual sample by the median peak area and then multiplying the result by the model median elemental abundance. (2) Because the resulting raw data rarely summed to 100% of the major elemental oxides, they were then normalized so that the major rock-forming elemental oxides summed to 100%. The normalization was achieved by dividing the raw sum of the major oxides into 100, and then multiplying each raw oxide by this factor. Minor elements were normalized by the same factor. The normalization step is used to eliminate variability caused by differences in porosity or cracks or by XRF source intensity variation (Lyle et al., 2012). The NMS method of data reduction has a few similarities and several differences to that of Weltje and Tjallingii (2008) and the further elucidation of the method in Weltje et al. (2015). Weltje and Tjallingii (2008) normalize each elemental peak area first by dividing by the sum of the total raw peak areas. They then log transform these ratios to reduce the range between major and minor XRFemitters, like our median-scaling step. Finally, they solve a matrix of XRF element/element ratios for composition. Weltje et al. (2015) investigated the use of various matrix solutions to get final composiFigure F1. Shaded bathymetric map of the Arabian Sea showing the location of Site U1457 (see the Site U1457 chapter [Pandey et al., 2016]). Site U1457 is located near the south coast of India, in the Laxmi Basin of the Arabian Sea. Yellow circles = sites drilled during Expedition 355, red stars = earlier scientific drilling sites, pink line = approximate extent of Indus Fan after Kolla and Coumes (1987), yellow dashed lines with question marks = the speculated locations of the continent/ocean boundary depending on whether Laxmi Basin is floored by oceanic or continental crust, gray lines with numbers = magnetic anomalies from Royer et al. (2002). Ba y o f B en ga l Arabian Sea axmi idge