DEVELOPING A PORTABLE XRF METHOD FOR COASTAL MARINE SEDIMENT CORE SCANNING: AN EXAMPLE FROM THE MONTEREY BAY CONTINENTAL SHELF

Portable X-ray fluorescence (p-XRF) has revolutionized geochemical analysis in the earth sciences. We describe the adaptation of a p-XRF instrument to the routine analysis of split marine and lake sediment cores using a custom-designed track system. Inspired by the larger and more expensive automated sediment core XRF systems in use around the world, our approach combines the greater affordability of the p-XRF with a highly transportable core track design. Discussion of the p-XRF track system will include: sediment core spatial resolution (e.g., minimum step-size limits), analytical reproducibility, and quantitative calibration using loose-powder Certified Reference Materials (CRMs). Successful data acquisition from several marine and lake depocenters using our modified p-XRF system will be discussed, with a special emphasis placed on p-XRF data from a newly-acquired continental shelf sediment core from offshore Pt. Año Nuevo near Monterey Bay, California, that extends continuously from ~AD 800 to 2014. Compositional analysis of the p-XRF data using several multivariate statistical techniques (including cluster analysis, discriminant function analysis, and principal component analysis) indicates three distinct geochemical units, which are largely defined by relative changes in the proportions of inferred lithogenic (Si, Ti, K, and Zr), biogenic (Br ± S), and porewater sediment components (Cl ± S). Comparison of these p-XRF compositional trends against X-ray computed-tomography (CT) scan results shows good correspondence with geophysical stratigraphic patterns, while a sequential Student’s t-test regime shift analysis of Br concentrations suggests two significant increases in export primary productivity along the central California continental shelf at ~AD 1400 and ~AD 1700.