Local Controls on Carbon Cycling in the Ordovician Midcontinent Region of North America, with Implications for Carbon Isotope Secular Curves
Bibliographic record
Abstract
Abstract The carbon isotope record from ancient epicontinental seas may contain much more of a local-scale carbon cycling signal than is generally appreciated. A unique opportunity exists to examine this issue in the case of the Late Ordovician Mohawkian Sea of eastern Laurentia, where the Millbrig K-bentonite stratigraphic framework has been used to delineate a time slice at 454 Ma, extending over ~ 1,500,000 km2 of the eastern United States. Across the time slice, carbonate and organic carbon δ13C vary by 4.5‰ and 7.5‰ respectively, a spatial variation that is as large as temporal (secular) changes in epeiric-sea δ13C that have been reported in the past. These new data are considered in the context of geographic variations in lithological, biological, and other geochemical sediment characteristics. Collectively, these sediment properties distinguish regions of the Mohawkian Sea which likely differed in terms of the nature and relative importance of carbon cycling processes. Water-column depth and structure, and barriers to free exchange of water across the Mohawkian Sea, may have been overarching factors in the development of these regions, raising the possibility that changes in circulation patterns, such as those caused by sea-level change, played a role in driving secular carbon isotope excursions by changing the rate of exchange of dissolved inorganic carbon between water masses. If the observed effects of local carbon cycling on the distribution of Mohawkian Sea δ13C were commonplace in ancient epicontinental marine environments, it would imply that local-scale carbon cycling may have left a nontrivial imprint on epeiric-sea records of secular variations in δ13C, in addition to the imprint left by changes in the global carbon cycle. This may have contributed to the broad scatter in δ13C values observed in the Paleozoic portion of the global carbon isotope secular curve.
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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 | 0.001 | 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) | 0.002 | 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".