Tracking the redox history and nitrogen cycle in the pelagic Panthalassic deep ocean in the Middle Triassic to Early Jurassic: Insights from redox-sensitive elements and nitrogen isotopes

•We made a detailed geological map at Katsuyama using high resolution aerial photos.•Distinct high Ce and Mn contents across the Tr–J boundary indicate an oxic condition.•Enrichments of Mo and U in Anisian and Toarcian black shales support deep-sea euxinia.•δ15NTN in Anisian black shales may attribute to a prominent ammonium assimilation.•δ15NTN values in Toarcian black shales indicate an enhance nitrogen fixation.

In order to clarify the redox history of the central Panthalassic Ocean in the Middle Triassic (Anisian) to Early Jurassic (Toarcian), we determined the abundances of major, trace and rare earth elements, and organic carbon and total nitrogen isotopes from well-exposed shales interbedded with deep-sea cherts from Inuyama, southwest Japan. A distinct positive Ce anomaly accompanying high Mn and weak Mo enrichments was detected in the transitional sequence across the Tr–J boundary, which indicates deposition under a more oxic condition than at any other period in the study section. The oxic period lasted for 130–170 kyr, and coincided with a faunal turnover at the end of the Triassic. On the other hand, strong enrichments of U, V, Mo, TOC and TN in Anisian and Toarcian black shales suggest deep ocean anoxia in both intervals. Highly concentrated Mo contents in some black shales indicate euxinia in the pelagic Panthalassic deep ocean. δ15NTN values are low, down to − 2.0 ‰, in such black shales enriched in redox-sensitive elements, compared with other shales. This low δ15NTN values during recurrent oceanic euxinic events in the Anisian may indicate strong assimilation of ammonium, but the possibility of enhanced nitrogen fixation cannot be ruled out especially for the Toarcian anoxic event. This work provides the first documentation of the long-term redox history in the pelagic Panthalassic deep ocean, from the Middle Triassic to the Early Jurassic, based on redox-sensitive elements in shales, and it reveals probable links between oceanic redox changes and biological activity.

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