High productivity in the earliest Triassic ocean: black shales, Southwest Japan

Abstract

Bedded cherts containing the Permian/Triassic (P/T) boundary exposed in the Tanba and Chichibu Belts, Southwest Japan, contain interbedded organic-rich black shales. The organic carbon concentration of black shales before thermal maturation 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 kerogen. Black shales comprise organic matter derived mainly from plankton with additional organic matter derived from bacterial activity, showing that the high primary production took place intermittently in the earliest Triassic. A stratified ocean with a well-developed oxygen-minimum layer rich in nutrients was intermittently disturbed due to tectonically and/or climatically induced active ocean current systems in the earliest Triassic, resulting in upwelling, causing remarkably high primary production, to form organic-rich black shales.