Rock magnetic record of the Triassic-Jurassic transition in pelagic bedded chert of the Inuyama section, Japan

The end-Triassic mass extinction event is regarded as one of the five largest extinction events of the Phanerozoic. The emerging consensus points to volcanic activity at the Central Atlantic Magmatic Province (CAMP) as the ultimate cause of the extinction, yet the underlying mechanisms and the nature of global environmental changes that accompanied the biotic turnover remain elusive. We present a rock magnetic study of the extinction interval found within a continuous chert sequence that provides an uninterrupted record of pelagic sedimentation in the Panthalassa Ocean. The variations in the relative abundances and characteristics of authigenic magnetic phases indicate that the Triassic-Jurassic transition progressed in two stages. The initial stage, characterized by a disappearance of the previously ubiquitous magnetofossils, started a few tens of thousands of years to 100 k.y. prior to the formal Triassic-Jurassic boundary as identified by the diagnostic radiolarian species. The second stage, defined by significant changes in optical and magnetic properties of hematite pigment, lasted a few tens of thousands of years. The stepwise change in magnetic properties is suggestive of the protracted environmental deterioration, likely prompted by the early episodes of the CAMP volcanism, which was followed by a sudden ocean acidification event, perhaps triggered by a catastrophic release of gas hydrates.