Cyclic Sedimentation in the Mississippian Pride Shale: Quantitative Paleoenvironmental Analysis of Tidal Rhythmites Using X-Ray Fluorescence Scanning and Advanced Spectral Methods

Tidal rhythmites offer a unique opportunity to better constrain the orbital dynamics of the Earth-Sun-Moon system (e.g., lunar recession rate, changes in Earth's precessional constant), and provide high-resolution paleoclimate archives with exceptional time control. The Mississippian Pride Shale of West Virginia is an unusually thick tidal rhythmite deposit (~60 m), representing hundreds to thousands of years of nearly continuous sedimentation. Previous analyses of the Pride Shale, using visual observations of layer thicknesses, identified a hierarchy of tidal cycles (semidiurnal to the 18.6-year nodal cycle) and an annual monsoonal climate cycle. In this study, we develop a new methodology for the investigation of ancient tidal rhythmites, using high-resolution X-ray fluorescence (XRF) scanning and advanced spectral methods. Importantly, the XRF-scanning technique provides the opportunity to deconvolve multiple paleoenvironmental signals (detrital flux, biogenic flux, redox state) preserved within these deposits, and thus allows a more complete analysis of their mechanism of formation. Assessment of the individual paleoenvironmental proxy records using advanced spectral methods (time-frequency analysis, coherency analysis, etc.) permits evaluation of their linkage to tidal and climatic forcing agents. This coupled XRF-scanning and spectral approach provides an opportunity to extract important paleoclimate information from tidal rhythmites. For this study, samples of the Pride Shale have been collected using a variety of approaches, including the extraction of continuous sections from outcrop using a concrete saw. Analyses of the Pride Shale samples suggest robust detrital (e.g., titanium), biogenic (e.g., silicon/titanium), and redox proxies (e.g., total sulfur), with variable sensitivity to the individual forcing mechanisms.