TRACE ELEMENT BEHAVIOR AND REDOX FACIES IN CORE SHALES OF UPPER PENNSYLVANIAN KANSAS-TYPE CYCLOTHEMS

The black shale submembers of core shales of Pennsylvanian “Kansas-type” cyclothems are highly enriched in redox-sensitive trace elements (TEs) relative to overlying gray shale submembers as well as to many other modern and ancient organic-rich deposits. Controls on trace-element behavior in core shales can be elucidated from the relationships of their Al-normalized concentrations to TOC and other TEs. In low-TOC samples (<~10 wt%) most TEs exhibit strong positive covariation with TOC, but in high-TOC samples (>~10 wt%) two distinctly different patterns of TE enrichment are observed: (1) Mo, U, V, Zn, and Pb are highly enriched but weakly covariant with TOC; and (2) Cu, Ni, Cr, and Co are weakly enriched but strongly covariant with TOC. These patterns are inferred to represent different responses to sulfidic anoxic conditions. The first pattern is characteristic of TEs of “strong euxinic affinity,” i.e., those taken up in solid solution by Fe-sulfide or involved in other reactions catalyzed by free H₂S, and resident mainly in authigenic phases, whereas the second pattern is characteristic of TEs of “weak euxinic affinity,” i.e., those not strongly influenced by the presence of free H₂S and resident mainly in the organic C or detrital fractions of the sediment. The redox facies of black shale samples were assessed based on four “redox indicator” TEs of strong euxinic affinity (i.e., Mo, U, V, Zn). The majority of samples (69% of 185) exhibited agreement among all four redox indicators, a high level of internal consistency validating the procedure. Samples yielding a mixed redox signal showed a systematic sequence of TE enrichment: first V, then Zn and Mo, and finally U. This sequence may reflect (1) initiation of V enrichment under non-sulfidic anoxic conditions through semi-reduction from V(V) to V(IV); (2) complete reduction of V to V(III) and precipitation of authigenic phases containing V, U, Mo, and Zn under euxinic conditions; and (3) preferential loss of authigenic U from euxinic facies samples through post-depositional remobilization under conditions of rising or fluctuating benthic O₂ levels. The multiproxy procedure for redox facies analysis developed in this study may be more reliable than commonly used single-proxy trace-element methods (e.g., Mo, authigenic U, or V/(V+Ni)) or C-S-Fe systematics (e.g., S/TOC or DOP).