SEQUENCE STRATIGRAPHIC CONTROL ON THE MOLYBDENUM ISOTOPE RECORD OF THE HUSHPUCKNEY SHALE: IMPLICATIONS FOR REGIONAL PALEOCEANOGRAPHIC CONDITIONS

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 in oxic or suboxic marine deposition.

Contrary to modern environments where marginal, anoxic settings capture the heavy molybdenum isotope value of the open ocean, black shales of the Hushpuckney Shale deposited under the most oxygen-depleted conditions during sea level highstand and maximum pycnocline strength conditions record the lightest values. This indicates that unique paleoceanographic conditions in the Late Paleozoic Midcontinent Sea played a mayor role in governing the molybdenum isotope record. In particular, lateral advection of oxygen depleted ocean waters through the Permian Basin Seaway over hundreds of kilometers led to constant molybdenum scavenging, driving the molybdenum isotopes to lighter values in the epicontinental sea. During regression, when the pycnocline strengths diminished and oxygen depletion was not as severe, molybdenum isotopes have heavier values. Hence, the results demonstrate that interpretations of the molybdenum isotope record of ancient epicontinental sea deposits need to take local paleoceanographic conditions into consideration, as local effects might obscure the global signal.