Portable X‐ray fluorescence spectroscopy as a tool for cyclostratigraphy

Saker-Clark, Matthew; Kemp, David B. and Coe, Angela L. (2019). Portable X‐ray fluorescence spectroscopy as a tool for cyclostratigraphy. Geochemistry, Geophysics, Geosystems, 20(5) pp. 2531–2541.

DOI: https://doi.org/10.1029/2018gc007582

Abstract

Cyclostratigraphic studies are used to create relative and high‐resolution timescales for sedimentary successions based on identification of regular cycles in climate proxy data. This method typically requires the construction of long high‐resolution datasets. In this study, we have demonstrated the efficacy of portable X‐ray fluorescence spectroscopy (pXRF) as a non‐destructive method of generating compositional data for cyclostratigraphy. The rapidity (100 samples per day) and low cost of pXRF measurements provide advantages over relatively time‐consuming and costly elemental and stable isotopic measurements that are commonly used for cyclostratigraphy. The non‐destructive nature of pXRF also allows other geochemical analyses on the same samples. We present an optimized protocol for pXRF elemental concentration measurement in powdered rocks. The efficacy of this protocol for cyclostratigraphy is demonstrated through analysis of 360 Toarcian mudrock samples from North Yorkshire, UK, that were previously shown to exhibit astronomical forcing of [CaCO3], [S] and δ13Corg. Our study is the first to statistically compare the cyclostratigraphic results of pXRF analysis with more established combustion analysis. There are strong linear correlations of pXRF [Ca] with dry combustion elemental analyzer [CaCO3] (r2=0.7616), and pXRF [S] and [Fe] with dry combustion elemental analyzer [S] (r2=0.9632 and r2=0.9274 respectively). Spectral and cross‐spectral analysis demonstrates that cyclicity previously recognized in [S], significant above the 99.99% confidence level, is present above the 99.92% and 99.99% confidence levels in pXRF [S] and [Fe] data respectively. Cyclicity present in [CaCO3] data above the 99.96% confidence level is also present in pXRF [Ca] above the 98.12% confidence level.

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