Exploring the depositional, diagenetic and carbon-oxygen isotope record of an evolving Ordovician carbonate system, Tarim Basin, China
Bibliographic record
Abstract
Bulk-rock based carbon-oxygen chemostratigraphy should be combined with a detailed understanding of depositional facies (mineralogy, porosity), its 2D-chronostratigraphic architecture, and diagenesis. The Ordovician of the western Tarim Basin recorded a peculiar litho-biostratigraphic succession. The Darriwilian Yijianfang Formation formed part of a carbonate ramp dominated by filter feeders. Toward its top, there is condensation succeeded by a multi-Myr hiatus. The hiatus correlative succession is a black-shale (Darriwilian to early Sandbian Saergan Formation) preserved in slope-to-basin settings. A marine red-bed interval (Sandbian Tumuxiuke Formation) diachronously succeeded toward a more basin-wide record. Finally, the late Sandbian to Katian Lianglitag Formation re-established a shallow-water carbonate factory (ramp-to-platform), but this time being highly productive and hosting a diversifying assemblage of benthic primary producers. By exploring diagenesis associated with a first component-specific data-set of δ13C - δ18O values and by integrating and filtering respective literature bulk-rock data, a synoptic chemo-chronostratigraphic sequence is presented. It displays segments lasting from tens of Myrs to several 100 kyrs. There is a long-term trend of increasing δ13C values culminating in the early Katian at 3.2 ‰ followed by a steady decrease. This tipping point is associated with a precursory baseline shift that interferes with the short-term Guttenberg carbon-isotope excursion presumably associated with a positive shift of δ18O values. The baseline shift was driven by regional effects of photosynthesis and a boosting (dasycladacean-related) production of aragonite along the Sandbian-Katian boundary interval. There is a medium-term (Darriwilian to earliest Sandbian) negative δ13C excursion coinciding with both a basal positive δ18O excursion and the demise of the Darriwilian carbonate ramp (Suecicus-Event, new term). It might represent the effects of volcanism/SO2-outgassing during the switch from a passive to an active continental arc. This event masquerades the elsewhere recorded middle Darriwilian carbon-isotope excursion. Caution is needed to consider the Tarim realm for global Ordovician chemostratigraphy.
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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 | 0.000 | 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".