THE RECORD OF ENVIRONMENTAL CHANGE IN NORTHEASTERN PANTHALASSA DIRECTLY PRECEDING THE END-TRIASSIC MASS EXTINCTION EVENT

A compilation of environmental information across the end-Triassic mass extinction (ETE) from Nevada to British Columbia reveals potentially stressed environmental/dysoxic conditions preceding the ETE in northeastern Panthalassa. The Ferguson Hill locality, central Nevada, records the initial negative carbon isotope excursion (INIE) of 1.7‰ found worldwide along with the highest occurrence of Choristoceras crickmayi,and presence of elevated Hg indicating the onset of CAMP volcanism. A precursor negative carbon isotope excursion (PCIE) of 1.6‰ is documented 7m below the extinction horizon. Macrofaunal and petrographic analyses show a lack of bioclasts within the precursor and INIE intervals, which suggests the cause of the carbon isotope shift may be connected to deleterious environmental effects. An identical PCIE documented in Tethys and the Germanic basin, which predates the ETE by ~100,000 k.y., suggests that observed shifts in the carbon cycle occurred worldwide. At Ferguson Hill, an elevated amount of sulphide pseudomorphs (goethite framboids after pyrite) was identified within the PCIE interval followed by episodic re-occurrence up-section and then by steady increase starting 2m below the extinction horizon. The presence of sulphide pseudomorphs suggests episodic dysoxic conditions within the sediment and potentially within the water column preceding the ETE. The upper part of the section starting 3m below the extinction horizon is characterized by the appearance of chemosymbiotic bivalves and low oxygen tolerant taxa illustrating a gradual change to tolerance of dysoxic conditions by benthic fauna.

We also investigated three sites at Williston Lake, British Columbia, which document occurrence of multiple phosphorite deposits in the interval preceding the ETE. The genesis of modern phosphorites in similar environments requires fluctuating redox conditions, from oxic to euxinic, linked to organic matter delivery in upwelling zones. Thus, phosphorite deposits may provide direct biosedimentary evidence that episodic shelf euxinia existed at the Williston Lake site leading up to the ETE. Both sites record stressed conditions before the ETE, which possibly were precursors for reduced environmental stability during initial CAMP volcanism in northeastern Panthalassa.