2:15 PM - 2:30 PM
[MIS28-03] Repeated organic-rich deposition in Early Triassic pelagic deep Panthalassa and its relationship with global seafloor anoxia
Keywords:Black claystone, chert gap, siliceous claystone, total organic carbon, stable carbon isotope, conodont
Deep-sea sedimentary rocks deposited in pelagic Panthalassa during the Early Triassic are characterised by the presence of black organic-rich claystone and the general absence of radiolarian chert. This was considered to be a result of one of the most extensive episodes of oceanic oxygen depletion in the Phanerozoic. However, while oceanic anoxia repeatedly occurred throughout the Early Triassic according to studies on peri-continental sections, black claystone deposition in pelagic Panthalassa was thought to be restricted to earliest Triassic times.
In this study, the lithostratigraphy, conodont biostratigraphy and chemical composition are investigated for the Lower Triassic sedimentary sequence deposited in pelagic deep-water areas of Panthalassa with some new data for the upper part (upper Olenekian). Sedimentation rates and major element compositions of upper Olenekian to lower Anisian strata support published findings that the absence of chert is a consequence of increased clastic input while there is no apparent evidence of radiolarian extinction due to oceanic anoxia. Correlation of several sections from the Jurassic accretionary complex of Japan based on conodont biozonation reveals that the deposition of extensive black claystone occurred in the Induan, early Spathian (early late Olenekian) and across the Olenekian–Anisian boundary. Though a complete assessment is pending, sedimentary and geochemical evidence of anoxia in the water column and bottom waters have been obtained from the black claystone lithofacies. Conodont biozonation and stable carbon isotope curves allowed the correlation of the lithostratigraphy of pelagic deep Panthalassa and uranium (U) isotope records from shallow-marine carbonate, the latter of which is used as a proxy of the areal extent of anoxic seafloor on the global scale. Results showed that organic-rich deposition in pelagic Panthalassa was generally coeval with episodes of expanded anoxic seafloor, although the two phenomena are decoupled in Smithian (early Olenekian) times. This implies 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. Black claystone lithofacies straddling the Olenekian–Anisian boundary is particularly important, because it represents the first finding of a laterally extensive deposit associated with oceanic anoxia during this time interval.
In this study, the lithostratigraphy, conodont biostratigraphy and chemical composition are investigated for the Lower Triassic sedimentary sequence deposited in pelagic deep-water areas of Panthalassa with some new data for the upper part (upper Olenekian). Sedimentation rates and major element compositions of upper Olenekian to lower Anisian strata support published findings that the absence of chert is a consequence of increased clastic input while there is no apparent evidence of radiolarian extinction due to oceanic anoxia. Correlation of several sections from the Jurassic accretionary complex of Japan based on conodont biozonation reveals that the deposition of extensive black claystone occurred in the Induan, early Spathian (early late Olenekian) and across the Olenekian–Anisian boundary. Though a complete assessment is pending, sedimentary and geochemical evidence of anoxia in the water column and bottom waters have been obtained from the black claystone lithofacies. Conodont biozonation and stable carbon isotope curves allowed the correlation of the lithostratigraphy of pelagic deep Panthalassa and uranium (U) isotope records from shallow-marine carbonate, the latter of which is used as a proxy of the areal extent of anoxic seafloor on the global scale. Results showed that organic-rich deposition in pelagic Panthalassa was generally coeval with episodes of expanded anoxic seafloor, although the two phenomena are decoupled in Smithian (early Olenekian) times. This implies 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. Black claystone lithofacies straddling the Olenekian–Anisian boundary is particularly important, because it represents the first finding of a laterally extensive deposit associated with oceanic anoxia during this time interval.