Chemostratigraphy of the uppermost Cambrian at the Ordovician GSSP
Bibliographic record
Abstract
Chemostratigraphy is an important tool for correlating layered sedimentary rock successions. Preserved/near primary carbon isotope signatures in marine carbonates can provide high-resolution profiles for sedimentary sequences supplementing the need for distinguishing fossils from different depositional environments and those lacking fossil materials. The Global Boundary Stratotype Section and Point (GSSP) of the Cambrian‒Ordovician boundary is located at Green Point in the Green Point Formation of the Cow Head Group in western Newfoundland, Canada. To reconstruct a continuous and high-resolution chemostratigraphy from the Cambrian‒Ordovician boundary to the Furongian Series Stage 10, we included the δ13C results of the Green Point Formation covering the Ordovician GSSP interval (Azmy et al., 2014). The Green Point Formation through the base of Ordovician GSSP consists of alternating dark gray to black shale and thin ribbon limestone rhythmites, with few fossils. The samples are micritic limestone, dolomitic limestone, and dolostone. They were determined to be in primary to near-primary condition based on multiple screening tests. Cathodoluminescence screening reveals dull to bright luminescence of the samples indicative of good preservation for many of them. The δ13Ccarb and δ18O values of the Green Point carbonates range from -6.44‰ to +0.33‰ (VPDB) and from -8.63‰ to -5.67‰ (VPDB), respectively, with poor correlation. Mn/Sr ratios range from 0.63 to 9.82, with no correlation to δ13Ccarb, but with ratios supporting the near primary nature of the δ13C values. Carbon isotope compositions of the Green Point Formation below the Ordovician GSSP fluctuate but remaine essentially invariantly negative. The δ13C values reveal a negative excursion at and below the Cambrian‒Ordovician boundary, which may correlate with the Top of Cambrian Carbon Isotope Excursion (TOCE) and its significant negative excursion. A nadir of -6.44 ‰ at the base of the Eoconodontus conodont zone marks the proposed GSSP for the base of the Furongian Series Stage 10. The lower excursion may be correlated with the Hellnmaria-Red Tops Boundary (HERB) carbon isotope excursion found in sequences in the United States of America, Australia, and north China. Without an adequate record of conodonts, high-resolution chemostratigraphic trends of carbon isotope compositions facilitate the correlation of intercontinental and intracontinental sequences.
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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 | 0.005 | 0.003 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.005 | 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".