Jerry Lei

Topic

Paleoenvironmental interpretations of the Late Triassic marine realm across the Canadian Cordillera: Slow burn of the end-Triassic mass extinction

School of Earth and Ocean Sciences

Date & location

• Wednesday, March 6, 2024
• 2:30 P.M.
• Clearihue Building, Room B019

Examining Committee

Supervisory Committee

• Dr. Jon Husson, School of Earth and Ocean Sciences, University of Victoria (Supervisor)
• Dr. Blake Dyer, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Vera Poseplova, School of Earth and Ocean Sciences, UVic (Member)
• Dr. Martyn Golding, Geological Survey of Canada, Natural Resources Canada (Outside Member)
• Dr. Terry Lacourse, Department of Biology, UVic (Outside Member)

External Examiner

• Dr. Matthew Saltzman, School of Earth Sciences, Ohio State University

Chair of Oral Examination

• Dr. Min Tsao, Department of Mathematics and Statistics, UVic

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 marks 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.

Conodont recovery and Re–Os isochron ages on Mount Sinwa attribute an instance of coral reef collapse to paleoenvironmental disturbance around the NRB. Measurements of ⁸⁷Sr/⁸⁶Sr ratios suggest a trend of gradual increase across the late Norian, but this Panthalassan dataset notably does not replicate the sudden drop previously observed in Tethyan values, indicating the NRB 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.

Morphometric analyses of two conodont species that are especially prolific in the Norian and Rhaetian strata of Panthalassa, Mockina carinata and Mockina englandi, demonstrate a gradual reduction of platform width across the NRB. These intraspecific trends are likely a more conservative parallel to concurrent intergeneric 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 experienced 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 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 excursions observed 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 that the ecological distress which 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 tests the hypothesis of a protracted ETME which 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.