Cryptic oxygen in the Precambrian : redox proxies and isotopic perspectives
Bibliographic record
Abstract
The emergence of oxygenic photosynthesis and the oxygenation of surface environments represent key transitions in Earth’s history. Long considered to have begun exclusively during the Great Oxidation Event (GOE, ~2.4 Ga), significant oxygen production is now thought to have started earlier, in the form of localized and transient processes. Geochemical and isotopic proxies, together with geochronological constraints, indeed suggest the existence of oxygen oases prior to the GOE, although their global impact remains uncertain. This thesis investigates the evolution of continental weathering, nutrient fluxes, and redox conditions from the Paleoarchean to the Paleoproterozoic through three complementary case studies. The Moodies Group (~3.2 Ga, South Africa) preserves well-constrained primary geochemical signals largely controlled by sedimentary provenance, but shows limited evidence for strong oxidation. The Red Lake carbonate platform (~2.9 Ga, Canada) documents an early oxygen oasis, with molybdenum isotopic compositions indicating a restricted marine reservoir dominated by hydrothermal inputs. Finally, the Lephala Formation (~2.3 Ga, Botswana) records the formation of manganese nodules followed by later oxidative alteration, highlighting the differential preservation of isotopic systems. Together, these results demonstrate that early oxygenation occurred within localized redox niches, emphasizing the need for integrated multiproxy approaches to constrain their evolution and environmental impact.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| 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".