Devonian ( <i>c.</i> 388–375 Ma) Horn River Group of Mackenzie Platform (NW Canada) is an open-shelf succession recording oceanic anoxic events
Bibliographic record
Abstract
At least four horizons of enhanced anoxia (anoxic horizons; AHs) are recognized in the uppermost Eifelian–Middle Frasnian mudrock-dominated strata of the Mackenzie Valley and Peel area of NW Canada. Aluminium-normalized Mo and U logs in two cored sections reveal AH-I at the Eifelian–Givetian boundary, AH-II in basal Frasnian, and AH-III and AH-IV bundled in the Middle Frasnian interval. These four horizons are characterized by attenuated siliciclastic components. Spectral gamma-ray K + Th and U are the best tools to trace these horizons in wells and outcrops. AHs are biostratigraphically correlated with ‘black-shale events’ in several basins of the world. Depositional environment is depicted as a stratified basin where the water-column chemocline defined co-sedimentation of anoxic mudrocks in topographic lows and oxic grey shales and carbonate banks on seafloor elevations. Based on inductively coupled plasma elemental data from 1687 samples, siliciclastic-lean basinal mudrock units that host AHs are strongly enriched in Mo (median EFMo ∼ 97–172 EFMo/EFU ≈ (3–3.5) × SW, where EFMo and EFU are respectively Al-normalized Mo and U in enrichment factor notation and SW is average present-day seawater value) compared with siliciclastic-rich units (median EFMo ∼ 17–37) and show strong EFU/EFMo covariation ( r ≈ 0.8 in Canol Formation and Bluefish Member). Supported by a lack of geological evidence for an oceanographic barrier, this enrichment indicates unrestricted water exchange with Panthalassa. At the same time, development of oligotrophy is indicated by a lack of P enrichment and weak to non-existent enrichment in Zn and Cu. These features are reconciled through a model by earlier workers that involves a global shift to a warm greenhouse mode with slowed oceanic convection, expanded oxygen minimum zones and a failure of nutrient resupply from the upwelling. The onset of mass degassing in continental large igneous provinces represents a potential trigger for this mid-Devonian shift. Devonian black-shale events in this scenario represent genuine oceanic anoxic events marking hothouse episodes in their nascent form. Supplementary material: Details of methods, analytical protocols and data scatterplots, stratigraphic cross-sections showing traceability of anoxic horizons, and inductively coupled plasma elemental and Rock-Eval 6 data used in this study are available at: https://doi.org/10.6084/m9.figshare.c.4212428
Stored with the screening record, where it is evidence for the labels above.
How this classification was reachedexpand
The three-model screen
all 5,600 screened works →All three models called this out of scope.
Devonian stratigraphy and geochemistry of anoxic horizons in NW Canada; the object is geological history.
This geological study investigates Devonian anoxic events in northwestern Canada, not research practice.
Devonian stratigraphy and anoxic events of the Horn River Group; earth science domain.
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 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.007 | 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".