GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 221-9
Presentation Time: 3:55 PM


OSTRANDER, Chadlin M., School of Earth and Space Exploration,, Arizona State University, Tempe, AZ 85287-1404, SAHOO, Swapan K., Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154, KENDALL, Brian, Earth and Environmental Sciences, University of Waterloo, 200 University Ave W, Waterloo, ON N2L 3G1, Canada, JIANG, Ganqing, Department of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, PLANAVSKY, Noah J., Department of Geology and Geophysics, Yale University, New Haven, CT 06511, LYONS, Timothy W., Earth Sciences, University of California, Riverside, Riverside, CA 92521-0423, NIELSEN, Sune G., Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, OWENS, Jeremy D., Department of Earth, Ocean, and Atmospheric Sciences, Florida State University, Tallahassee, FL 32306, ROMANIELLO, Stephen J., School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287 and ANBAR, Ariel D., School of Molecular Sciences, Arizona State University, Tempe, AZ 85287-1404

The Ediacaran Doushantuo Formation (South China) offers one of the most complete and extensively studied records of end-Neoproterozoic biotic and environmental change. Local depositional conditions of these sedimentary rocks are not fully constrained, however, making it difficult to differentiate processes on the Ediacaran Earth that operated on a local scale from those that operated on a larger, perhaps global scale.

Here, we report multiple negative molybdenum (Mo) isotope excursions (to as low as δ98MoNIST+0.25= -2.24 ± 0.10‰; 2SD) in Doushantuo black shales from four separate sites (Rongxi, Taoying, Wuhe, and Yuanjia). We propose that these negative excursions can be explained by some combination of (a) enhanced local marine oxygenation and (b) changes in the degree of connectivity between the paleo basin and the open ocean.

Enhanced local marine oxygenation, by exposing more sediments in the paleo basin to H2S-poor conditions, could have hindered quantitative tetrathiomolybdate formation within these sediments. Local marine oxygenation could have also stimulated the operation of a local Mn oxide shuttle. Today, both of these processes are shown to promote retention of lighter-mass Mo isotopes in sediments. Alternatively, or in addition, the sedimentary basin where the Doushantuo Formation was originally deposited may not have maintained an uninterrupted connection with the open ocean during the entirety of the Ediacaran Period. The negative δ98Mo excursions occur coincident with sea level highstands that, by improving the connection between the sedimentary basin and the open ocean, could have also promoted local redox conditions unfavorable to quantitative tetrathiomolybdate formation and/or catalyzed the operation of an Mn oxide shuttle. When trying to infer temporal changes in ancient global ocean redox, it is important to consider the influence of variations in local redox conditions and sea level changes on stratigraphic trends in isotope compositions.