Redox condition and nitrogen cycle in the Permian deep mid-ocean: A possible contrast between Panthalassa and Tethys

To constrain the redox conditions and related nitrogen cycles during the Middle Permian (Guadalupian) to latest Late Permian (Lopingian) deep mid-Panthalassa, we determined the abundances of major, trace, and rare earth elements along with the carbon and nitrogen isotope ratios in shales interbedded with deep-sea cherts that are well-exposed at the Gujo-Hachiman section in the Mino-Tanba belt, SW Japan. The positive Ce anomalies together with low Mo and U enrichment factors (MoEF < 1.0; 0.7 < UEF < 2.5) during the Guadalupian and the most of the Lopingian indicate that the deep mid-Panthalassa was under oxic condition. On the other hand, the slightly higher MoEF (2.0 < MoEF < 9.2) and UEF (0.9 < UEF < 2.5) values in some of the middle-late Lopingian shales suggest deposition under intermittent suboxic condition. These new findings indicate that the redox condition in the deep mid-Panthalassa never reached an anoxic condition during the Guadalupian and Lopingian until the Permian-Triassic boundary (P-TB) interval. In view of the redox state from the Guadalupian to latest Lopingian, our newly obtained δ15NTN values (−0.12‰ to +2.57‰) indicate nitrate-rich conditions in the mid-Panthalassa along with large isotopic fractionation during nitrate assimilation. However, unlike the oxic and nitrate-rich deep-Panthalassa, we speculate that oxygen-depleted (i.e., anoxic/euxinic) and bioavailable nitrogen-poor conditions developed in the deep Tethys immediately before the Guadalupian-Lopingian boundary (G-LB). These environmental stresses were potentially driven by a global cooling episode (Kamura event) together with the unique paleogeography, i.e., no contact with polar ice caps in the Tethyan Ocean. Upwelling of the anoxic watermass accumulated in the deep Tethys during the global cooling episode likely triggered oceanic anoxia in the shallow-marine regions around the G-LB, which eventually resulted in the G-LB mass extinction.