ORGANIC-RICH DEPOSITION IN EARLY TRIASSIC PELAGIC DEEP PANTHALASSA AND THEIR RELATIONSHIP WITH GLOBAL-SCALE OCEANIC OXYGEN DEPLETION

The Early Triassic is known for one of the most extensive oceanic oxygen depletion in the Phanerozoic. High-resolution studies on oceanic regions near land masses show that there were multiple phases of oceanic anoxia during the Early Triassic. Oceanic anoxia is also recorded in the absence of chert and presence of organic-rich (carbonaceous) black claystone within sediments deposited on the deep-sea floor of remotely pelagic areas of Panthalassa, which became the basis for the proposal of the Early Triassic “superanoxia”. However, the black claystone was thought to be restricted to the basal part of the Lower Triassic, seemingly contradicting with evidence of repeated anoxia from other oceanic regions.

We investigated the lithostratigraphy, conodont biostratigraphy and chemical composition of multiple Lower Triassic sections in Japan, deposited in pelagic deep-water areas of Panthalassa. Combined with data published in previous studies, our results indicate that the absence of chert records increased clastic input rather than radiolarian extinction due to oceanic anoxia. Deposition of extensive black claystone in pelagic deep Panthalassa occurred in the Induan, across the Smithian–Spathian (early–late Olenekian) boundary and across the Olenekian–Anisian boundary. The three episodes of black claystone deposition are associated with sedimentary and geochemical evidence of anoxia in the water column and, to a lesser degree, the bottom waters. Organic-rich deposition in pelagic deep Panthalassa generally correlates with uranium-isotopic evidence of expanded anoxic seafloor, although they are decoupled in Smithian (early Olenekian) times. The black claystone straddling Olenekian to –Anisian boundary represents the first finding of a laterally extensive deposit associated with oceanic anoxia during this time interval. We infer that the deposition of black organic-rich claystone in pelagic deep Panthalassa reflects global trends in the spatial extent of bottom water anoxia during the Early Triassic, probably controlled mainly by nutrient input to the oceans.