Redox‐State and Bioproductivity of the Glaciogenic Early Cryogenian Rapitan Iron Formation (Cranswick River, Canada): Constraints From Combined Cadmium—Chromium Isotopes
Bibliographic record
Abstract
Abstract The Rapitan Group (Northwest Territories, Canada) includes banded iron formation (BIF), an unusual sediment type that is associated with late Neoproterozoic glaciations, in this case, the early Cryogenian Sturtian glaciation. New non‐traditional stable isotope data from jasper and hematite iron formation (IF) from the Cranswick River area contribute new insights regarding water chemistry changes in the respective depositional basin during glacier advance and retreat. Cr isotope signatures are characterized by strongly positively fractionated δ53Cr values (+0.68–1.47‰), attesting to strongly oxidized surface waters during the entire depositional episode. Relatively weakly fractionated δ114Cd isotope signatures (−0.02 to +0.28‰), and the absence of organic matter in the samples, indicate a low bioproductivity during BIF deposition. Detailed fluctuations over an intermittent glacier advance and retreat cycle recorded in the studied section, however, reveal an ephemeral increase in primary production (excursion to slightly elevated δ114Cd values) just after deposition of a diamictite layer, which agrees with corresponding elevated δ53Cr values. This evidence attests to the sensitivity of the Cd‐Cr double isotope tracer applied to BIFs to reconstruct changes in the surface waters of depositional basins and track climate changes that accompanied the glacial episodes. When compared to data from the younger (Marinoan/Ediacaran) Urucum BIF in Brazil, the Rapitan section differs by lack of negative Ce anomalies in its rare earth element patterns, implying that water column oxidation was not as strong during the early Cryogenian as compared to the Marinoan/Ediacaran glacial episodes.
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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 | 0.001 | 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) | 0.001 | 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".