Controls on uranium isotope fractionation in the late Paleoproterozoic ocean
Bibliographic record
Abstract
Uranium isotope data from Proterozoic carbonates (δ 238 U carb ) and black shales (δ 238 U auth ) are enigmatic. Average Proterozoic δ 238 U carb (approximating Proterozoic seawater, δ 238 U sw ) is similar to modern river/seawater δ 238 U, and Proterozoic black shales do not always record highly fractionated δ 238 U auth compared to contemporaneous δ 238 U carb . However, very light δ 238 U carb and heavy δ 238 U auth for the widely anoxic Proterozoic oceans was expected because large isotope fractionations accompany U reduction in anoxic environments. To address this enigma, we report black shale δ 238 U auth through a well-characterised multi-core transect in the late Paleoproterozoic Animikie Basin, North America. There is a wide range of δ 238 U auth , from –0.52‰ to 0.53‰, that generally correlates with organic carbon enrichments. Heavy δ 238 U auth in organic-rich shallow shelf sediments within and near a euxinic wedge are attributed to enhanced productivity with vigorous sediment microbial activity, neutrally charged aqueous U species that slow reaction kinetics, and redoxcline fluctuations. In less organic-rich sediments of the anoxic-ferruginous deep shelf environment, characterized by lower productivity and plentiful reduced iron availability, light δ 238 U auth may reflect rapid U reduction or adsorption to solid Fe/Mn species. Hence, for the widely anoxic Proterozoic oceans, we propose that large isotopic fractionations (0.4 to 1.2‰) were associated with highly productive areas on anoxic continental margins, and muted isotopic fractionations (–0.1 to 0.4‰) occurred in anoxic deep-ocean environments. Low-productivity Proterozoic oceans yielded sediments with δ 238 U carb and δ 238 U auth close to modern river/seawater values, whereas higher-productivity basins (e.g., Animikie Basin) resulted in lower δ 238 U carb and more variable δ 238 U auth .
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| 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) | 0.000 | 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 teacher head, 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".