Chromium geochemistry of the ca. 1.85 Ga Flin Flon paleosol
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Bibliographic record
Abstract
Abstract Fractionation of stable Cr isotopes has been measured in Archaean paleosols and marine sedimentary rocks and interpreted to record the terrestrial oxidation of Cr( III ) to Cr( VI ), providing possible indirect evidence for the emergence of oxygenic photosynthesis. However, these fractionations occur amidst evidence from other geochemical proxies for a pervasively anoxic atmosphere. This study examined the Cr geochemistry of the ca. 1.85 Ga Flin Flon paleosol, which developed under an atmosphere unambiguously oxidising enough to quantitatively convert Fe( II ) to Fe( III ) during pedogenesis. The paleosol shows an extreme range in Cr isotope composition of 2.76 ‰ δ 53/52 Cr. The protolith greenstone (δ 53/52 Cr: −0.23 ‰), the deepest weathering horizon (δ 53/52 Cr: −0.15 to −0.23 ‰) and a residual corestone in the upper paleosol (δ 53/52 Cr: −0.01 ‰) all exhibit Cr isotopic compositions comparable to unaltered igneous rocks. The most significant isotopic fractionation is preserved in the areas influenced by oxidative subaerial weathering (i.e. increase in Fe( III )/Fe( II )) and the greatest loss of mobile elements. The uppermost paleosol horizon is both Cr and Mn depleted and offset to significantly 53 Cr‐enriched compositions (δ 53/52 Cr values between +1.50 and +2.38 ‰), which is not easily modelled with the oxidation of Cr( III ) and loss of isotopically heavy Cr( VI ). Instead, the currently preferred model for these data invokes the open‐system removal of isotopically light aqueous Cr( III ) during either pedogenesis or subsequent hydrothermal/metamorphic alteration. The 53 Cr enrichment would then represent the preferential dissolution or complexation of isotopically light aqueous Cr( III ) species (enhanced by lower pH conditions and possibly the presence of complexing ligands) and/or the residual signature from preferential adsorption of isotopically heavy Cr( III ). Both scenarios would contradict the widely held assumption that only redox reactions of Cr can generate large magnitude isotopic fractionations and, if substantiated, non‐redox isotope effects would complicate the conclusive fingerprinting of ancient atmospheric O 2 from Cr isotope data alone.
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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) | 0.017 | 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 it