Paleoenvironmental interpretations of the Late Triassic marine realm across the Canadian Cordillera: Slow burn of the end-Triassic mass extinction
Date
2024
Authors
Lei, Jerry
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Abstract
Despite representing some of the most pivotal intervals in evolutionary history, the timing and tempo of mass extinction events have remained contentious. Many studies have contributed evidence suggesting that ecosystem disturbance associated with the end-Triassic mass extinction (ETME) began prior to the Triassic/Jurassic boundary (TJB), but the extent and duration of this leadup phase is not well established. This uncertainty is exacerbated by a comparative lack of studies investigating the ETME within the context of long-term Late Triassic trends, as well as by the dominance of Tethyan datasets in paleoenvironmental interpretations of the epoch. The research presented in this dissertation consists of a multi-faceted investigation of Panthalassan paleoenvironmental conditions spanning from the Norian/Rhaetian boundary (NRB) to across the TJB, as recorded in western Canadian marine strata.
An instance of coral reef collapse on Mount Sinwa, British Columbia, is associated with the paleoenvironmental disturbance around the NRB via conodont and Re–Os isochron age constraints. Ratios of 87Sr/86Sr are observed to gradually increase across the late Norian, as opposed to the sudden drop previously observed in Tethyan datasets, indicating the NRB disturbance was not triggered by mantle-derived volcanism on a global scale. A 3 – 4‰ negative excursion in δ13C values is captured in the latest Norian on Mount Sinwa, consistent with the global carbon cycling disruption proposed to occur around the NRB by prior studies.
The conodont species Mockina carinata and Mockina englandi are especially abundant in the Norian and Rhaetian strata of Panthalassa. Morphometric analyses on these two conodont species demonstrate a gradual reduction of platform width across the NRB. These intraspecific trends are likely a more conservative parallel to concurrent intergeneric morphology shifts observed in Tethyan conodonts, together potentially implying a global shift in conodont diet away from mineralized food sources during this time. This may suggest that the biomineralization pressure typically associated with the ETME began at a lesser severity around the NRB, and that conodont biodiversity underwent only limited recovery between the substantive turnover at the NRB and complete extinction of the class around the ETME. Specimens of both these species that have a mid-platform length to breadth ratio greater than 3:1 are observed exclusively in the Rhaetian, a clear sign of morphotype origination or subspeciation, with implications for improved biostratigraphic utility.
The compilation of δ13C values across stratigraphic sections from Williston Lake, Holberg Inlet, and Kyuquot Sound in the Canadian Cordillera develops a comprehensive Panthalassan record spanning from the Norian through into the Hettangian, with representation from a variety of depositional settings across a wide paleogeographic area. Three distinct negative excursions are observed, with one proximal to the NRB, one within the Rhaetian, and another across the TJB. The somewhat variable positions of these excursions suggest that the earliest “precursor” excursion associated with the Rhaetian leadup to the ETME may be indistinguishable from an excursion associated with the NRB. Some of the observed excursions are too large in magnitude to reflect shifts in global ocean water chemistry, necessitating a local-scale amplification mechanism, such as disturbance-triggered organic carbon respiration in a water column with restricted circulation. Nevertheless, this evidence for repeated carbon cycling instability indicates the ecological distress that initiated around the NRB persisted across the Rhaetian, escalating into the TJB.
Drawing from a combination of lithological, paleontological, and geochemical evidence from across the Canadian Cordillera, this dissertation supports the hypothesis of a protracted ETME that initiated as early as the NRB. With implications of elevated extinction pressure persisting for millions of years before the climax at the TJB, this research challenges preconceptions of the timescale in which mass extinction events ought to be envisioned.
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Keywords
conodont, carbon isotope, paleontology, geochemistry