RESUMED OCEANIC UPWELLING IN THE EARLY TRIASSIC FROM ISOTOPIC ANALYSIS OF A SPATHIAN ATOLL

The Early Triassic represents a protracted interval of recovery for life on Earth following the end-Permian mass extinction, the greatest loss of marine species of the Phanerozoic. This recovery was interrupted by a second hyperthermal episode in the Smithian substage, which may have delayed recovery further. The Jesmond section, which is part of the exotic Cache Creek Terrane of British Columbia, Canada, represents an early Spathian-age atoll structure, and records conditions in the Panthalassic Ocean following the Smithian thermal maximum, the hottest interval of the Early Triassic. Isotopic analysis of organic carbon and nitrogen from the Jesmond carbonates provides insight into open ocean conditions in the Early Triassic.

Previous studies of Panthalassic and outer-Tethyan sections, including the Sverdrup Basin of the Canadian High Arctic and the Chaohu Lake area of China’s Anhui Province, show evidence for ocean stratification and nitrogen limitation throughout the Early Triassic, with nitrogen fixation acting as the primary source of bioavailable nitrogen. Nitrogen isotope data from the Jesmond section show distinct enrichments in ¹⁵N, with values over 7‰, as is characteristic of productive coastal upwelling zones today. This suggests that western-boundary upwelling in the Panthalassic Ocean resumed as early as the Spathian, outboard of the arcuate terranes to the west of subtropical Pangaea. Organic carbon isotope data reflects primary production dominated by layered microbial mats, which likely contained a substantial proportion of sulfur oxidizing bacteria.