A Sequence-Stratigraphic Perspective of the Late Ordovician Mass Extinction
Bibliographic record
Abstract
Mass extinctions throughout the history of life are a remarkable source of information on the relationship between past changes to the Earth System and global biodiversity. To understand the dynamics of Earth System changes and their impact on biodiversity, investigations of the pattern and drivers of faunal turnover heavily rely on fossil occurrences from measured stratigraphic columns. Traditionally, a cluster of last occurrences in a stratigraphic column is interpreted as evidence of a pulse of extinction; however, application of the principles of sequence stratigraphy to the fossil record have demonstrated that the structure of the stratigraphic record is a primary control on the distribution of occurrences in the fossil record. Significantly, this stratigraphic control on fossil occurrences results in the predictable clustering of last occurrences at major sequence-stratigraphic surfaces. These stratigraphically generated clusters of last occurrences mask the underlying pattern of faunal turnover in the fossil record, and while this control on fossil occurrences has been documented, there are no tested methods that can distinguish stratigraphically generated clusters of last occurrences when studying mass extinctions.The Late Ordovician mass extinction during the Hirnantian Stage (445.2–443.1 Ma) offers a promising case study of the stratigraphic control on fossil occurrences and its impact on our understanding of the pattern and drivers of faunal turnover. In the fossil record, the Late Ordovician mass extinction commonly is expressed as two clusters of last occurrences of species. These clusters of last occurrences coincide with major sequence stratigraphic surfaces that represent large changes in glacio-eustatic sea level. While the two clusters of last occurrences attributed to the Late Ordovician mass extinction are often interpreted at face value as two pulses of extinction, their association with major stratigraphic surface merits a thorough reassessment of the Upper Ordovician fossil record from a sequence-stratigraphic perspective to evaluate this face-value reading of the fossil record. In this dissertation, I develop a framework that can be used to determine the underlying pattern, timing, and therefore drivers, of mass extinctions in the fossil record with the ultimate goal of applying this method to an empirical stratigraphic record to understand the Late Ordovician mass extinction. In Chapter 1, I introduce the reader to the field of stratigraphic paleobiology and its importance for studying the fossil record of mass extinction events. In Chapter 2, I use forward modelling of plausible Late Ordovician mass extinction scenarios in a hypothetical stratigraphic record to develop a methodology that can be used to deconvolve the biological and geological signatures of the fossil record and determine the underlying pattern of a mass extinction based on field-collected data. Critically, this method requires a well-resolved sequence stratigraphic framework to contextualize patterns of faunal turnover in the fossil record. In Chapters 3 and 4, I undertake the development of such a stratigraphic correlation framework for the exceptional Upper Ordovician stratigraphic record on Anticosti Island (Québec, Canada), which preserves one of the thickest Ordovician–Silurian boundary sections in the world. In Chapter 3, I describe a new, biostratigraphically relevant species of brachiopod, Hirnantia notiskuani, that constrains the age of the Ellis Bay Formation to the Hirnantian Stage. Using this biostratigraphic constraint, in Chapter 4, I combine sedimentological, geochemical, and stratigraphic data to develop a high-resolution sequence stratigraphic framework for Upper Ordovician strata on Anticosti Island that will enable the methodology developed in Chapter 2 to the fossil record. In Chapter 5, I conclude with remarks on each of the three chapters and outline a vision for future work in studying the Late Ordovician mass extinction on Anticosti Island.
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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 | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.004 | 0.003 |
| Science and technology studies | 0.001 | 0.003 |
| Scholarly communication | 0.002 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.004 | 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".