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