MOLYBDENUM AND URANIUM ISOTOPE CONSTRAINTS ON REDOX AND ENVIRONMENTAL CONDITIONS IN THE LATE PENNSYLVANIAN MIDCONTINENT SEA

Late Pennsylvanian cyclothems of the Midcontinent region of North America contain core black shales that represent deepwater sedimentation under anoxic conditions (Heckel, 1977; Algeo et al., 2004). The reducing character of these core shales resulted in strong authigenic enrichment of molybdenum (Mo) and uranium (U) (Algeo and Tribovillard, 2009). Mo and U isotopic profiles exhibit correlative, opposite excursions (negative for d⁹⁸Mo, positive for d²³⁸U) within each core shale (Herrmann et al., 2012, and in prep.). Spatial variation in Mo- and U-isotopes across the Midcontinent Shelf provides insight regarding controls on these isotopic systems. For molybdenum, proximal shelf areas (SW Iowa) show smaller ranges of d⁹⁸Mo (~0.5‰ vs. ~1.0‰) and higher minimum values (+0.7‰ vs. +0.5‰) than distal-shelf areas (S Kansas). For uranium, proximal shelf areas show smaller ranges of δ²³⁸U (~0.5‰ vs. ~1.0‰) and lower maximum values (0.40‰ vs. 0.65‰) than distal-shelf areas. Simple seawater redox controls ought to produce positive covariation of d⁹⁸Mo with d²³⁸U (Poulsen et al., 2006; Weyer et al., 2008), so the pattern of negative covariation observed in these core shales documents more complex controls. Herrmann et al. (2012, and in prep.) concluded that the U isotope system responded primarily to a global-ocean redox control overprinted by minor local influences, whereas the Mo isotope system was probably influenced strongly by negative fractionation associated with uptake of Mo on Mn-Fe-oxyhydroxides. In this study, we apply a box model to investigate lateral variation in Mo- and U-isotopic variation across the Midcontinent Shelf to test these hypotheses. Our results suggest that the mechanisms inferred by Herrmann et al. (2012, and in prep.) are viable and can fully account for the observed patterns of Mo- and U-isotopic variation.