Characterizing eukaryotic evolutionary environments across the Mesoproterozoic-Neoproterozoic boundary
Bibliographic record
Abstract
This thesis implements a combination of sedimentology, stratigraphy, geochemistry, and geochronology to characterize 1200 and 800 Ma eukaryotic evolutionary environments and elucidate the complex interplay between the Proterozoic biosphere, atmosphere, hydrosphere, and lithosphere. The late Mesoproterozoic Bylot Supergroup in the Borden Basin provides a rare window into late Mesoproterozoic biology and geochemistry but lacked reliable depositional ages beyond broad maximum and minimum constraints (1270–720 Ma). New Re-Os radiometric dates from the middle Bylot Supergroup constrain the first known appearance of Bangiomorpha pubescens, the earliest crown group and photosynthetic eukaryote to ca. 1045 Ma. We calibrated molecular clock analyses with this new fossil age in order to estimate the origin of the chloroplast via primary endosymbiosis at ca. 1250 Ma. This date provides a chronological framework for understanding the role of eukaryotic oxygenic photosynthesis in global biogeochemical cycles. Results from this study also indicate that deposition of the Bylot Supergroup coincided with peak collision of the Grenville orogeny and the amalgamation of Rodinia ca. 1090–1020 Ma. Furthermore, comparing the sedimentary records between various depocentres in the Borden Basin highlights asynchronous deposition across the basin and raises the possibility that the upper Bylot Supergroup is younger than previously considered.Radiogenic isotope (Sr, Os, and Nd) chemostratigraphic data from throughout the Bylot Supergroup elucidate complex, multi-phase tectonically-driven paleo-environmental changes in the epicratonic Borden Basin. These results demonstrate that tectonic events throughout the Uluksan Group's depositional history altered the configuration of the basin and drove episodic restriction and marine incursion. Variable marine and freshwater or brackish environments in epeiric seaways, like the Borden Basin, which were relatively widespread in the late Mesoproterozoic Era, may have exerted unique selective pressure on eukaryotic evolution. Marine limestone samples from the Bylot Supergroup provide a new age-calibrated record of the terminal Mesoproterozoic Sr isotopic composition of seawater. These data display a secular trend to more evolved compositions that likely reflects increased chemical weathering of active orogens during the amalgamation of Rodinia. Whereas some geochemical studies suggest atmospheric oxygen increased earlier, most metal proxies indicate late Proterozoic oxygenation commenced ca. 800 Ma along with eukaryotic diversification. A multi-proxy (iron isotopes, iron speciation, and redox-sensitive elements) investigation of transgressive shale intervals from the ca. 811 Ma upper Fifteenmile Group in Yukon reveals the redox structure and highlights the processes that governed the distribution of redox-sensitive metals within the basin. Relative sea-level change in a weakly restricted basin with a redox-stratified water column kick-started vigorous dissimilatory iron reduction and benthic particulate shuttling, which produced large variations in sedimentary redox proxies that do not directly reflect changes in the redox state of the global ocean at this time.
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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 | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".