Riverine evidence for isotopic mass balance in the Earth's early sulfur cycle
Bibliographic record
Abstract
During a time of negligible atmospheric pO_2, Earth’s early sulfur cycle generated a spectacular geological signal seen as the anomalous fractionation of multiple sulfur isotopic ratios. The disappearance of this signal from the geologic record has been hypothesized to constrain the timing of atmospheric oxygenation, although interpretive challenges exist. Asymmetry in existing S isotopic data, for example, suggests that the Archaean crust was not mass balanced, with the implication that the loss of S isotope anomalies from the geologic record might lag the rise of atmospheric O_2. Here, we present new S isotopic analyses of modern surface and groundwaters that drain Archaean terrains in order to independently evaluate Archaean S cycle mass balance. Natural waters contain sulfur derived from the underlying bedrock and thus can be used to ascertain its S isotopic composition at scales larger than typical geological samples allow. Analyses of 52 water samples from Canada and South Africa suggest that the Archaean crust was mass balanced with an average multiple S isotopic composition equivalent to the bulk Earth. Overall, our work supports the hypothesis that the disappearance of multiple S isotope anomalies from the sedimentary record provides a robust proxy for the timing of the first rise in atmospheric O_2.
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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 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".