EARLY MISSISSIPPIAN OCEAN ANOXIA DRIVING CLIMATE COOLING AND INCREASED GLACIATION: TESTING THE HYPOTHESIS USING URANIUM ISOTOPES IN MARINE CARBONATES

The Early Mississippian (Tournaisian) positive δ¹³C excursion (or TICE) is one of the largest recorded in the Phanerozoic with magnitudes of change of up to +7‰. It is associated with a positive δ¹⁸O shift signaling global cooling and an increase in Southern Hemisphere continental glaciation. We are testing the hypothesis that increased ocean anoxia, organic matter burial, and pCO₂ reduction was the cause of the positive δ¹³C shift, global cooling, and increased glaciation. We are testing this hypothesis using uranium isotopes (δ²³⁸U) of Lower Mississippian marine limestones from the Pahranagat Range of southern Nevada as a global seawater redox proxy.

The Early Mississippian δ²³⁸U curve is characterized by a gradual ~0.4‰ negative shift that coincides with the onset of the positive TICE excursion in the Upper S. crenulata-S. isosticha conodont Zone. This is followed by a gradual ~0.25‰ positive shift that is complete before the TICE ends (before Lower G. typicus Zone). The lack of correlation among δ²³⁸U values and water-depth dependent facies changes, terrestrial element proxies (Al, Th), redox-sensitive metals (U, V, Mo, Re) and diagenetic proxies (Ca/Mg, Mn/Sr) suggests that the measured δ²³⁸U curve represents a global seawater redox signal. The coincidence in timing between the onset of the negative δ²³⁸U and positive δ¹³C shifts supports the hypothesis that a large increase of organic matter burial and subsequent positive δ¹³C shift, pCO₂ drawdown, and cooling was enhanced by expanded ocean anoxia. The decoupling of δ²³⁸U from δ¹³C after the initial shifts suggests the effects of the difference between C versus U ocean reservoir size or spatial differences in C and U ocean sinks.