Geological Society of America

T221 Understanding the Neoproterozoic Earth-Life System

Tuesday, 24 September 2019
1:30 PM - 5:30 PM
Room 102AB, West Building

MULTIPLE NEGATIVE MOLYBDENUM ISOTOPE EXCURSIONS IN THE DOUSHANTUO FORMATION (SOUTH CHINA) FINGERPRINT COMPLEX REDOX-RELATED PROCESSES IN THE EDIACARAN NANHUA BASIN

Paper number: T221-10
Program: Technical Sessions
Time: 3:55 PM – 4:10 PM

Authors

Chadlin Ostrander Presenting Author
Arizona State University, Tempe, AZ
Ariel Anbar
Arizona State University, Tempe, AZ
Swapan K. Sahoo
University of Nevada, Las Vegas, Las Vegas, NV
Brian Kendall
University of Waterloo, Waterloo, ON
Ganqing Jiang
University of Nevada, Las Vegas, NV
Noah Planavsky
Yale University, New Haven, CT
Timothy Lyons
University of California, Riverside, Riverside, CA
Sune G. Nielsen
Woods Hole Oceanographic Institution, Woods Hole, MA
Jeremy D. Owens
Florida State University, Tallahassee, FL
Stephen J. Romaniello
Arizona State University, Tempe, AZ

Abstract

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 δ⁹⁸Mo_NIST+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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-341144

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