Trace-element behavior and redox facies in core shales of Upper Pennsylvanian Kansas-type cyclothems

Abstract

The black shale submember of core shales of Pennsylvanian Kansas-type cyclothems is highly enriched in redox-sensitive trace elements (TEs) relative to the overlying gray shale submember as well as to many other modern and ancient organic-rich deposits. Controls on trace-element behavior in the black shale can be inferred from trace-element enrichment factors (EFs, relative to gray shale) and the relationship of TEs to total organic carbon (TOC). Most TEs conformed to one of two patterns: (1) Mo, U, V, Zn, and Pb exhibited moderate EFs and strong covariation with TOC in samples having <10 wt.% TOC but large EFs and weak covariation with TOC in samples having >10 wt.% TOC; and (2) Cu, Ni, Cr, and Co exhibited low EFs and moderate to strong covariation with TOC in all samples. These patterns are inferred to represent different responses to benthic redox 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 H 2S, 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 H 2S and resident mainly in the organic carbon or detrital fractions of the sediment. These inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples: euxinic conditions were considered to have existed if at least two of four “redox-indicator” trace elements (Mo, U, V, Zn) showed euxinic levels of enrichment, and nonsulfidic anoxic conditions were inferred otherwise. The validity of the procedure is indicated by (1) agreement among all four “redox indicators” for a large majority of samples (69% of 185); (2) among samples yielding a mixed redox signal, a systematic sequence of TE enrichment (V→Zn→Mo→U); and (3) for TEs of “weak euxinic affinity”, reduced variance among samples representing each redox facies. Sequential enrichment may be a response to differential redox thresholds for accumulation (e.g., V) or postdepositional remobilization of trace elements (e.g., U) and may provide a basis for finer assessment of redox conditions in low-oxygen paleoenvironments than permitted by simple classification schemes. The multiproxy procedure for redox-facies analysis developed in this study is likely to be more reliable than widely used single-proxy indicators based on trace elements (e.g., Mo, authigenic U, or V/(V+Ni)) or C–S–Fe systematics (e.g., S/TOC or DOP).