New Re–Os Geochronological Data from the Upper Doushantuo Formation: Age Constraint on the Shuram Excursion and Implication for the Ediacaran Fluctuated Continental Weathering
Bibliographic record
Abstract
High Resolution Image Download MS PowerPoint Slide 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, n = 13; 2σ), indicating a middle-late Ediacaran age for the upper Doushantuo Formation. This is supported by a negative δ 13 C carb 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 187 Os/ 188 Os and 87 Sr/ 86 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 187 Os/ 188 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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".