Extraterrestrial Impact Event Recorded in the Late Triassic Deep-Sea Deposits from Japan

Abstract:

The Late Triassic is marked by the formation of several large impact structures on the Earth, including the 90-km-diameter Manicouagan crater in Canada (Spray and Kelly, 1998). Anomalously high platinum group element (PGE) concentrations have been reported from the Upper Triassic deep-sea deposits in Japan, which have been interpreted to be derived from an extraterrestrial impact event forming the Manicouagan crater (Onoue et al., 2012; Sato et al., 2013). However, previous studies have not clarified the composition of the projectile. Here we report the PGE element ratios and osmium (Os) isotopic compositions of the Upper Triassic ejecta layers in Japan in order to understand the projectile component.

Evidence for the Late Triassic impact event has been discovered in the deep-sea claystone layers at four bedded chert sections in the Japanese accretionary complex. The claystone layers contain microspherules, Ni-rich magnetite grains and high abundance of PGEs (Onoue et al., 2012). Biostratigraphic analysis of radiolarian fossils from the studied sections revealed that the claystone layers occur embedded in the upper middle Norian (Sugiyama, 1997; Onoue et al., 2012).

Identification of the projectile is attempted by comparing the isotope and elemental ratios of the ejecta deposit with similar data obtained from meteorites. The Ru/Ir and Pt/Ir ratios of all the claystone samples from the study sites are plotted along the mixing line between chondrites and upper continental crust. Although chondrites cannot be distinguished from iron meteorites by using PGE/Ir ratios, the claystone layers have Cr/Ir ratios between 10⁴–10⁵, indicating that the claystone layers are clearly contaminated by chondritic material. The Os isotope data show an abrupt and marked negative excursion from an initial Os isotope ratio of ~0.477 to unradiogenic values of ~0.126 in a claystone layer within a middle Norian bedded chert, indicating the input of chondrite-derived Os into seawater. The size of the projectile can be inferred from the sedimentary Os concentrations and isotope ratios, by making reasonable estimates of the range in the amount of Os released from the projectile into seawater. Assuming 22–100% release of chondrite-derived Os into seawater, the impactor is calculated to be 3.3–7.8 km in diameter.