Geological Society of America

T185 Greenhouse to Icehouse Transition: Global Events of the Devonian, Carboniferous, and Early Permian (Posters)

Tuesday, 6 November 2018
9:00 AM - 6:30 PM
Halls J-K

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

Paper number: T185-7
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Keyi Cheng Presenting Author
University of New Mexico, Albuquerque, NM
Maya Elrick
University of New Mexico Dept Earth & Planetary Sciences, Albuquerque, NM
Stephen J. Romaniello
Arizona State University, Tempe, AZ
Geoffrey Gilleaudeau
George Mason University, Fairfax, VA
Ganqing Jiang
University of Nevada Las Vegas, Las Vegas, NV

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-319511

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