New evidence for a long Rhaetian from a Panthalassan succession (Wrangell Mountains, Alaska) and regional differences in carbon cycle perturbations at the Triassic-Jurassic transition

The end-Triassic mass extinction is one of the big fiveextinction events in Phanerozoic Earth history. It is linked with the emplacement of the Central Atlantic Magmatic Province and a host of interconnected environmental and climatic responses that caused profound deterioration of terrestrial and marine biospheres. Current understanding, however, is hampered by (i) a geographically limited set of localities and data; (ii) incomplete stratigraphic records caused by low relative sea-level in European sections during the Late Triassic and earliest Jurassic; and (iii) major discrepancies in the estimated duration of the latest Triassic Rhaetian that limit spatiotemporal evaluation of climatic and biotic responses locally and globally. Here, we investigate the Late Triassic-Early Jurassic time interval from a stratigraphically well-preserved sedimentary succession deposited in tropical oceanic Panthalassa. We present diverse new data from the lower McCarthy Formation exposed at Grotto Creek (Wrangell Mountains, southern Alaska), including ammonoid, bivalve, hydrozoan, and conodont biostratigraphy; organic carbon isotope (d13Corg) stratigraphy; and CA-ID TIMS zircon U-Pb dates. These data are consistent with a Norian-Rhaetian Boundary (NRB) of similar to 209 Ma, providing new evidence to support a long duration of the Rhaetian. They also constrain the Triassic-Jurassic boundary (TJB) to a similar to 6 m interval in the section. Our TJB delta C-13(org) record from Grotto Creek, in conjunction with previous data, demonstrates consistent features that not only appear correlative on a global scale but also shows local heterogeneities compared to some Tethyan records. Notably, smaller excursions within a large negative carbon isotope excursion [NCIE] known from Tethyan localities are absent in Panthalassan records. This new comparative isotopic record becomes useful for (i) distinguishing regional overprinting of the global signal; (ii) raising questions about the ubiquity of smaller-scale NCIEs across the TJB; and (iii) highlighting the largely unresolved regional vs. global scale of some presumed carbon cycle perturbations. These paleontological and geochemical data establish the Grotto Creek section as an important Upper Triassic to Lower Jurassic succession due to its paleogeographic position and complete marine record. Our record represents the best documentation of the NRB and TJB intervals from Wrangellia, and likely the entire North American Cordillera. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study provides new evidence supporting a long duration of the Rhaetian and constraints the Triassic-Jurassic boundary to a 6-meter interval in the section based on diverse biostratigraphic and isotopic data from the Late Triassic-Early Jurassic sedimentary succession in southern Alaska. The record from Grotto Creek, in conjunction with previous data, shows consistent features on a global scale but also reveals local heterogeneities compared to some Tethyan records, raising questions about regional overprinting of the global signal and highlighting unresolved regional vs. global scale of some presumed carbon cycle perturbations.