Bioturbating animals control the mobility of redox-sensitive trace elements in organic-rich mudstone

Poduska, Kristin M. and Harazim, Dario and Mcllroy, Duncan and Edwards, Nicolas P. and Wogelius, Roy A. and Manning, Phillip L. and Layne, Graham D. and Sokaras, Dimosthenis and Alonso-Mori, Roberto and Bergmann, Uwe (2015) Bioturbating animals control the mobility of redox-sensitive trace elements in organic-rich mudstone. Geology, 43 (11). pp. 1007-1010. ISSN 1943-2682

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Abstract

Bioturbating animals modify the original mineralogy, porosity, organic content, and fabric of mud, thus affecting the burial diagenetic pathways of potential hydrocarbon source, seal, and reservoir rocks. High-sensitivity, synchrotron rapid scanning X-ray fluorescence elemental mapping reveals that producers of phycosiphoniform burrows systematically partition redox-sensitive trace elements (i.e., Fe, V, Cr, Mn, Co, Ni, Cu, and As) in fine-grained siliciclastic rocks. Systematic differences in organic carbon content (total organic carbon >1.5 wt%) and quality (D13Corg~0.6‰) are measured between the burrow core and host sediment. The relative enrichment of redox-sensitive elements in the burrow core does not correlate with significant neo-formation of early diagenetic pyrite (via trace metal pyritization), but is best explained by physical concentration of clay- and silt-sized components. A measured loss (~-15%) of the large-ionic-radius elements Sr and Ba from both burrow halo and core is most likely associated with the release of Sr and Ba to pore waters during biological (in vivo) weathering of silt- to clay-sized lithic components and feldspar. This newly documented effect has significant potential to inform the interpretation of geochemical proxy and rock property data, particularly from shales, where elemental analyses are commonly employed to predict reservoir quality and support paleoenvironmental analysis.

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