Is the Lomagundi Event present on the Rae craton? A case study from the Murmac Bay Group
Bibliographic record
Abstract
Shortly after the initial rise of atmospheric oxygen in the Paleoproterozoic Era, a major perturbation occurred in the global carbon cycle, which is manifested as a long-lived positive carbon isotope excursion recorded in ∼2.22–2.06 Ga carbonate rocks, known as the Lomagundi Event. Beyond its significance for evolving seawater composition, this geochemical event can be used as an indirect age marker in Paleoproterozoic sedimentary successions. Documenting further occurrences of this event in other Paleoproterozoic carbonate rocks confirms that the event was global and reflects ambient seawater composition. This event, however, has only been documented in two successions in Canada, despite the ubiquity of Paleoproterozoic-aged rocks on the Canadian Shield. Our study focuses on metacarbonate rocks from the Paleoproterozoic-aged Murmac Bay Group on the southwestern margin of the Rae craton in northern Saskatchewan. Measured δ13C values (up to 7.8‰) fall within the range of the Lomagundi Event, but most values are relatively low, suggesting CO2 loss (decarbonation) altered the δ13C values. Stable carbon isotope data coupled with major element geochemical data allowed us to account for the degree to which the values have changed due to metamorphic CO2 loss. We also compare δ13C values from micro-drilled (dolomite and calcite phases) with whole-rock δ13C values from the same samples to characterize the δ13C composition of pre-metamorphic carbonate minerals. Both measured and corrected δ13C values place the Murmac Bay Group metacarbonates within the range of values that characterize the Lomagundi Event and indicate, for the first time, the presence of the Lomagundi Event on the Rae craton.
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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 | 0.002 | 0.002 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 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".