354-3 High-resolution records of prolonged environmental perturbations during the end-Triassic mass extinction interval from the pelagic sediments in Panthalassa

Abstract:

The Late Triassic was marked by intense climate change and prolonged biotic turnover culminating in the end-Triassic mass extinction (ETE). Marine faunas experienced extinctions and marked size reduction across the Norian/Rhaetian boundary (NRB) prior to the Triassic/Jurassic boundary (TJB) (Lucas and Tanner, 2018). However, the cause of these environmental changes remains unclear due to the limited bio- and chemostratigraphic constraints during the Late Triassic and Early Jurassic. Here, we present integrated radiolarian and conodont biostratigraphy together with whole-rock geochemical data, including major and trace element concentrations, organic carbon isotope (δ¹³C_org), and osmium isotope (¹⁸⁷Os/¹⁸⁸Os) records from the pelagic chert succession in the Sakahogi section, Japan, spanning the late Norian to the Early Jurassic (Sinemurian).

Our biostratigraphic analyses identified radiolarian zones from the Skirt F Zone (TR8C) to Bipedis horiae Zone (JR0B) and conodont zonation Misikella posthernsteini Zone. These biozones indicate that the studied succession spans from the late Norian to the Sinemurian. Furthermore, the TJB is recognized by the first occurrence of the radiolarian Pantanellium tanuense.

Enrichment factors of redox-sensitive elements (V, Cr, Mn) indicate the development of suboxic conditions during the latest Rhaetian, followed by oxic conditions. Weathering proxies (Chemical Index of Alteration and Weathering Index of Parker) consistently indicate four features: intensification of chemical weathering during the late Norian, temporary weakening across the NRB, re-intensification during the early Rhaetian, and weakening followed by re-intensification during the late Rhaetian. The δ¹³C_org record shows three negative excursions in the late Rhaetian.

Measured ¹⁸⁷Os/¹⁸⁸Os ratios range from 0.5 to 0.8 and exhibit decreases during the latest Norian, the early Rhaetian, and the latest Rhaetian. These decreases coincide with enhanced chemical weathering, and the latest Rhaetian decrease also corresponds to three δ¹³C_org excursions and the inferred redox transition. Integrated biostratigraphic and geochemical records suggest that major environmental changes associated with the ETE developed progressively from the NRB to the TJB rather than representing only a short-term environmental perturbation at the TJB. The decreases in ¹⁸⁷Os/¹⁸⁸Os ratios may reflect unradiogenic Os inputs associated with Late Triassic large-scale magmatism episodes, including the emplacement of the Angayucham oceanic plateau and the Central Atlantic Magmatic Province. Further studies are required to clarify the relationship between volcanism and environmental perturbations.

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