Impact Event and Radiolarian Faunal Turnover across the Middle–upper Norian Transition in the Upper Triassic of Japan

Abstract:

It has long been recognized that marine biotic turnover events took place at around the Norian/Rhaetian boundary in the Late Triassic. Although the causes of these Norian to Rhaetian biotic turnover events are still the subject of debate, extraterrestrial impacts have been proposed to account for the biotic turnover. A Norian impact event has been inferred from a platinum group element (PGE) concentration anomaly, Os isotope negative excursion, and the presence of Ni-rich magnetite and microspherules, in claystone layer within middle to upper Norian bedded chert succession in the Sakahogi section, Mino Belt, central Japan. Previous osmium isotope studies have revealed that the anomalously high PGE abundances resulted from a large chondritic impactor (3.3–7.8 km in diameter). To evaluate the biotic and environmental effects of such an impact event, we report on new micropaleontological data from the Sakahogi section across the ejecta layer.

Detailed high-resolution sampling and biostratigraphic data showed that the ejecta layer occurs at the base of the radiolarian Trialatus robustus–Lysemelas olbia zone and is correlated with the middle to late Norian boundary. The biostratigraphc analysis revealed that no mass extinction of radiolarians occurred at the impact event horizon. Only one species became extinct at the ejecta horizon and the extinction rate of radiolarians is estimated to be about 5% at the horizon. However, major turnovers in radiolarians occur above the ejecta horizon, in the Trialatus robustus–Lysemelas olbia zone; this turnover is associated with deposition of a spicular chert, suggesting temporal changes in marine ecosystems after the impact event.