研究業績

The late Norian “chaotic carbon episode” in the Late Triassic has been identified in North America, Italy, and China, reflecting a global carbon isotope perturbation. Although it has been suggested that catastrophic events, such as the emplacement of large igneous provinces, were the cause of this perturbation, direct evidence for volcanic forcing is lacking. In order to investigate the cause of the global “chaotic carbon episode” in the late Norian, we obtained osmium and organic carbon isotope data (187Os/188Os and δ13Corg) and major, trace, and highly siderophile element data and documented changes in radiolarian assemblages in a middle–upper Norian bedded chert succession in the Sakahogi section, Japan, deposited in a low-latitude zone of the Panthalassa Ocean. The δ13Corg data exhibit a significant carbon isotope excursion (CIE) during the late Norian (Sevatian 1) Mockina bidentata conodont zone. High-resolution 187Os/188Os data exhibit two negative shifts during the CIE, which are interpreted as related to volcanic activity. The biostratigraphic age and Os isotopic mass balance considerations for the ocean suggest that the onset of Angayucham volcanism was the source of unradiogenic Os that caused the decrease in seawater 187Os/188Os ratios during the Sevatian. A remarkable diversification of radiolarians with a large Mesosaturnalis bloom occurred during the low-187Os/188Os interval. This suggests that the Sevatian Mesosaturnalis species are likely indicators of the onset of Angayucham volcanism.