Preservation of carbon isotopes in kerogen from thermally altered Mesoproterozoic lacustrine microbialites
Bibliographic record
Abstract
Stable carbon isotope geochemistry is a well-established and reliable tool for studying metabolisms of microbial communities in the Precambrian record; however, the isotopic effects of high-temperature alteration from igneous intrusions (i.e., contact metamorphism) have not been thoroughly explored. The Mesoproterozoic (∼1.4 Ga) Middlebrun Bay Member of the Rossport Formation, Sibley Group, in Ontario, Canada, is composed of carbonaceous stromatolites and microbial laminites preserved in an evaporitic, lacustrine chert–carbonate deposit and is cross-cut by an intrusive mafic sill at the studied locality. Sedimentary organic matter (kerogen) was investigated along two vertical stratigraphic transects to determine the spatial variability of its geochemical preservation. Thermal alteration of the preserved kerogen (as measured by Raman spectroscopy) increased toward the mafic sill, but the alteration was greater for kerogen preserved in carbonate mineralogies compared to that preserved in quartz (chert). Bulk δ13Corg values fluctuate throughout each vertical section, with a total average of −28.2‰ ± 0.8‰; however, values are unexpectedly lower for samples near the mafic sill, approaching −30‰, inconsistent with previously reported patterns. These measurements indicate that thermal alteration of sedimentary rocks does not universally result in 13C enrichment and increased δ13Corg values and suggests that ancient kerogen may be preferentially shielded from postdepositional heating effects due to micrometre-scale differences in mineralogy.
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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 | 0.000 | 0.000 |
| 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".