New Re-Os Geochronological Data from the Upper Doushantuo Formation: Age Constraint on the Shuram Excursion and Implication for the Ediacaran Fluctuated Continental Weathering.

The largest negative carbon-isotope excursion in geological history has been reported by several studies of the upper Doushantuo Formation of South China, which has been correlated to the middle Ediacaran-Shuram excursion (SE). Due to a scarcity of radiometric age constraints on the excursion in South China, however, global correlations and comparisons of this event remain a debate. Here, we present Re-Os and carbon isotope data on organic-rich sediments obtained from a drill-core sample in the Chengkou area, the northeastern margin of the Yangtze Platform, and South China. The Re-Os geochronology yields a depositional age of 568 ± 15 Ma (Model 3, MSWD = 1.9, = 13; 2σ), indicating a middle-late Ediacaran age for the upper Doushantuo Formation. This is supported by a negative δC excursion, which can be reliably correlated to the SE sequences. This age is consistent with the Re-Os radioisotopic dates bracketing the Shuram peaks in Northwest Canada and Oman. A compilation of Os/Os and Sr/Sr isotope ratios as well as the contents of redox-sensitive elements (RSE) from organic-rich sediments deposited between 635 and 540 Ma shows that the radiogenic Os/Os ratios (>1.0) associated with enhanced oxidative weathering occurred at ca. 635, 580, and 560 Ma. As a result, accelerated influxes of nutrients stimulated primary productivity, promoting organic carbon burial and leading to ocean oxygenation. Additionally, elevated continental weathering could have delivered high fluxes of oxidants (e.g., sulfates) to oceans, resulting in transient ocean oxygenation. Corresponding to elevated radiogenic Os and Sr isotope ratios, the significant RSE enrichments at these three times indicate the presence of large marine RSE reservoirs and an oxygenated ocean. Therefore, the Re-Os age and initial Os isotope composition of organic-rich shale can be a sensitive tool for constraining the time interval of enhanced continental weathering and resulting pulses of ocean oxygenation during the Neoproterozoic era.