MOLYBDENUM ISOTOPE RECORD OF THE HUSHPUCKNEY SHALE (MISSOURIAN STAGE, EASTERN KANSAS): IMPLICATIONS FOR REGIONAL AND GLOBAL ANOXIA

The molybdenum isotope system is emerging as an indicator of global redox conditions. Molybdenum isotope variations preserved in euxinic sediments may reflect changes in the relative proportions of euxinic, suboxic and oxic sedimentation in the global oceans through time. A shift to lighter d^97/95Mo values may reflect expanded anoxia while higher d^97/95Mo values may indicate a relative increase of oxic or suboxic marine deposition.

We find that anoxic (non-sulfidic) and euxinic sediments of the core black shales of the Hushpuckney Shale have d^97/95Mo values ranging from 0.20‰ to 0.76‰. These values are significantly lighter (by about ~0.8‰) than d^97/95Mo values of Holocene Black Sea sediments. If euxinic conditions led to quantitative removal of Mo from the water, the light isotopic compositions may suggest that the global area of euxinic sedimentation was up to an order of magnitude larger than today, similar to the mid-Proterozoic. However, the pre-glacial climate conditions during the Pennsylvanian and the overall ocean-atmosphere oxygenation of the Phanerozoic make this scenario unlikely.

Alternatively, considerable regional changes in the riverine Mo flux into the Appalachian basin in response to climate changes and lack of extensive water exchange between the epicontinental Appalachian basin and the global ocean could explain the observed d^97/95Mo values. The lightest Mo isotope values (0.20-0.55‰) are observed in the so-called ‘illitic zone' of the core shale, which coincides with the interglacial highstand of the Swope cyclothem. An increase in precipitation and run-off into the Appalachian basin would have led to a strengthening of the pycnocline with low-salinity surface waters forcing a stable stratification. With reduced open-ocean water exchange and strong euxinic conditions, the d^97/95Mo values would have approached the d^97/95Mo values of the continental Mo source (reported values for granites, basalts, and molybdenites fall between -0.01±0.27‰) of the overlying water masses. Anoxic (non-sulfidic) sediments of the transgressive and early regressive intervals of the Hushpuckney Shale may represent less salinity stratification and less restriction of circulation. In these intervals d^97/95Mo values are heavier (~0.7‰). Further work on local water column redox conditions will aid in interpretation of observed isotopic variations.