A new depositional model for glaciogenic Neoproterozoic iron formation: insights from the chemostratigraphy and basin configuration of the Rapitan iron formation<sup>1</sup>Northwest Territories Geoscience Office Contribution 0052.
Bibliographic record
Abstract
Neoproterozoic iron formations record an unusual and apparently final recurrence of this sediment type after a hiatus of more than one billion years. Despite the unusual environmental conditions that led to their formation, specifically their association with glaciogenic deposits, Neoproterozoic iron formations have strongly influenced models for the Precambrian Earth’s surficial evolution and iron formation in general. A suite of high-quality trace element data for 42 samples from the Rapitan iron formation in northwestern Canada were used to reconstruct the configuration and redox evolution of the Rapitan Basin. Complete rare-earth element and yttrium (REE+Y) patterns demonstrate that the Rapitan Basin was hydrologically connected to the open ocean, but that local catchments of an evolved, possibly granitic composition supplied dissolved REE+Y, suggesting partial basin restriction. Molybdenum and U systematics are consistent and indicate a partly restricted, or “silled” basin. In contrast to modern analogues for such basins, such as the Cariaco Basin, the stratigraphic association with glaciogenic clastic rocks requires ice cover to be considered in basin reconstruction. Accordingly, the Rapitan iron formation was deposited through a complex interplay of processes: during intervals of ice cover, glacially sourced iron oxyhydroxides were bacterially reduced to dissolved ferrous iron, which was subsequently oxidized to ferric iron following ice withdrawal. During this time, extreme primary productivity in the shallow water column drove eutrophication at middle water depths and the production of a three-tiered stratified water column with ferruginous deep water, a thin euxinic wedge at middle depths, and oxic surface water. Regardless of the basinal redox conditions and depositional constraints, the absence of a positive Eu anomaly in the Rapitan iron formation suggests that the global ocean was fully oxygenated by the Neoproterozoic.
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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) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.002 | 0.001 |
| Open science | 0.002 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.006 | 0.001 |
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".