Stable Carbon and Oxygen Isotope Evidence of Cretaceous Sea-Level Fluctuations Recorded in Septarian Concretions from Pueblo, Colorado, U.S.A.
Bibliographic record
Abstract
Abstract Limestone concretions in the Blue Hill Member of the Carlile Formation (middle Middle Turonian) exposed in the Lake Pueblo State Park, Colorado, U.S.A., range in size from 20-cm-diameter, approximately spherical forms to flattened spheroids >2 m in width and >1 m in height. The earliest matrix and septarian calcite cement, with δ13C and δ18O values ranging mostly from -18‰ to -8‰ (PDB) and -3‰ to -8‰ (PDB), respectively, precipitated from diluted seawater during shallow burial where sulfate reduction largely controlled pore-water bicarbonate composition. In contrast, the latest septarian cements exhibit very depleted δ18O values (∼-16‰) with δ13C values approaching values of -1‰ to -3‰, reflecting a strong meteoric-water component to pore fluids. Overall, δ13C and δ18O values are inversely covariant. Concretion matrix calcite and septarian cements precipitated from mixtures of modified marine and meteoric fluids. Pore-water reversals resulted from sea-level fluctuations leading up to and associated with the formation of a sequence boundary, or possibly the development of parasequences near the top of the Blue Hill Member. Relative sea-level falls caused pore-water freshening as the coastal mixing zone migrated seaward. Similarly, modified marine-derived pore waters replaced fresher pore fluids accompanying subsequent marine flooding. Carbonate cements precipitated during this active hydrologic history recorded the isotopic characteristics of their parent fluids. Negative δ18O values of the meteoric end-member calcite compositions suggest derivation from high-altitude precipitation in the Sevier highlands to the west. Septarian cements in concretions, despite their enclosure in relatively impermeable host sediment, can act as sensitive indicators of relative sea-level fluctuations.
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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 | 0.001 | 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) | 0.001 | 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".