Oceanic water redox conditions of the region between Tethys and Panthalassa during the late early Triassic

Abstract

Fossil evidence of a complex food-chain in the marine ecosystem after the end-Permian mass extinction (EPME) event has been reported from the Osawa Formation, which consists of upper Olenekian (Spathian) shallow marine, mudstone-dominant strata in the South Kitakami Belt in Northeast Japan. This sedimentary sequence offers an important material that records the paleoenvironment of the oceanic region between the eastern Tethys and Panthalassa. This paper reports detailed records of the oceanic environment from two sections of the Osawa Formation focusing on the redox condition. Parallel, undisturbed laminae suggesting limited benthic activity are prominent in the middle to upper parts of the Osawa Formation, while disturbance of laminae by burrows occurs in the lower and uppermost parts, suggesting active benthic activity in an oxygen-rich depositional environment. The amount of pyrites (euhedral pyrite, pyrite framboids, and pyrite polyframboids) increases in the well-laminated mudstones. A relatively broad size distribution of pyrite framboids indicates that these mudstones were deposited under dysoxic conditions and suggest that the water column was not sulfidic. Redox-sensitive elements such as Mn, V, and U show values close to the average values of the upper continental crust throughout the Osawa Formation. Mo is moderately enriched in the well-laminated and pyrite-abundant mudstone horizons although the degree of enrichment is lower than known examples of modern sulfidic basins. These data suggest that the condition of the bottom water in this oceanic region during the Spathian varied between oxic and dysoxic (weakly oxygenated) and was never sulfidic. Such an oceanic environment characterized by mildly reducing bottom water and oxic water column appears to have been the settings for the recovery and establishment of the Early Triassic marine ecosystem characterized by diverse nektonic motile animals and limited diversity of benthic, in-motile animals.