Surface sediment portable XRF geochemistry of the American Samoa EEZ abyssal plain collected during cruise 2026-604-FA

Surface sediment samples were collected from the Samoan Basin, an abyssal region in the Pacific Ocean on expedition 2026-604-FA. This region is within the U.S. Exclusive Economic Zone (EEZ) offshore American Samoa. These samples were analyzed by portable X-ray fluorometry (XRF) to measure geochemical concentrations of major, minor, and some trace elements to determine if there was any surficial geochemical heterogeneity between box core stations. Samples were collected from the upper ~2 centimeters of each box core and placed in a clean 9-cm-diameter petri dish, which was then oven dried at 40°C for two days. Each sample was ground to a homogenous fine powder in a ceramic mortar and pestle and loaded into a loose-powder sample cup equipped with a 4-micron-thick polypropylene X-ray transparent thin film with a polyfill backing to minimize porosity. Each sample was analyzed in triplicate on a Bruker Tracer 5i portable XRF in lab bench mode, where the loose-powder sample cup was placed directly on the XRF measurement window within a shielded chamber. Samples were run in batches of five or more, with both an instrument monitor (pure SiO2) and manufacturer standard (Bruker mudrock tile) run in triplicate both before and after the samples. Two Certified Reference Materials (CRMs) were analyzed as part of each analytical batch (marine sediment standards MAG-1 and MESS-4) to assess instrument accuracy and reproducibility. Following data collection, the Bruker XRF concentrations were calibrated using a set of 33 rock and sediment CRMs in loose-powder sample cup form, as well as a set of six custom standards for calibrating bromine (Br) concentrations.