Organic geochemical study on the relationship between primary productivity of palaeocean and evolution of marine invertebrates

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Outcrops containing the Permian/Triassic (P/T) boundary exposed in the Tanba and Chichibu terranes, Southwest Japan, have intercalated organic rich black shales. The initial organic carbon concentration was evaluated to be about 3% (average), considering their maturity levels. Kerogens do not contain any debris of higher plant origins, but are mostly composed of structureless amorphous ones. Biomarkers show that black shales comprise organic matter derived mainly from plankton with the additional organic matter related tothe bacterial activity. Palaeocean at the time of black shale formation was comparatively active. The stratified ocean with well developed oxygen minimum layr rich in nutrients was intermittently destroyed due to the tectonically and climatically induced active ocean current systems in the P/T transition time, resulting in upwelling for the remarkably high primary production. Expanding oxygen minimum layr in the Late Permian and its abrupt destruction can be one of crucial causes for the mass extinction of marine invertebrates and their evolution.