Marine osmium isotope record during the Carnian "pluvial episode" (Late Triassic) in the pelagic Panthalassa Ocean

The Carnian Pluvial Episode (CPE) was a global environmental change and biotic crisis that occurred during theCarnian (Late Triassic). The climate during the CPE was characterized by a short-lived period of extreme rainfall,and an extinction of marine taxa is known to have occurred during the latest Julian (i.e. early Carnian). Althoughthese events are considered to have been caused by the Wrangellia Flood Basalt (FB) volcanism, existing studieshave found little direct evidence to support this. We investigated the temporal relationship between the eruptionof Wrangellia FB and CPE using high-resolution microfossil biostratigraphy and paleo-seawater Os isotope dataof an Upper Triassic bedded chert succession from an accretionary complex in Japan, which accumulated in apelagic deep-sea environment in an equatorial region of the Panthalassa Ocean. Our biostratigraphic analysis,based on conodonts and radiolarians, and osmium isotope data show: (i) a continuous decline of initial Os isotoperatios (187Os/188Osi) in the early Julian; (ii) low 187Os/188Osi ratios during the late Julian; and (iii) an abruptincrease in 187Os/188Osi ratios at the end of the Julian. The decrease in 187Os/188Osi ratios throughout the Juliansuggests an increased input of unradiogenic Os from the eruption of the Wrangellia FB into the ocean. Moreover,redox-sensitive elements, such as V and U, increased abruptly at the end of the Julian, which is the first evidenceof reducing conditions during the CPE within the pelagic deep-sea Panthalassa Ocean. Marine anoxic event in thelate Julian has been recognized from widespread deposition of black shales and organic-rich marls in intermediateto shallow water Tethyan sections. Thus, oxygen-depleted conditions occurred at the Tethyan shallowcontinental margin, as well as in the pelagic deep-sea Panthalassa Ocean, at the end of Wrangellia FB volcanism.