Tetsuji Onoue

Deep-sea record of impact apparently unrelated to mass extinction in the Late Triassic

The 34-million-year (My) interval of the Late Triassic is marked by the formation of several large impact structures on Earth. Late Triassic impact events have been considered a factor in biotic extinction events in the Late Triassic (e.g., end-Triassic extinction event), but this scenario remains controversial because of a lack of stratigraphic records of ejecta deposits. Here, we report evidence for an impact event (platinum group elements anomaly with nickel-rich magnetite and microspherules) from the middle Norian (Upper Triassic) deep-sea sediment in Japan. This includes anomalously high abundances of iridium, up to 41.5 parts per billion (ppb), in the ejecta deposit, which suggests that the iridium-enriched ejecta layers of the Late Triassic may be found on a global scale. The ejecta deposit is constrained by microfossils that suggest correlation with the 215.5-Mya, 100-km-wide Manicouagan impact crater in Canada. Our analysis of radiolarians shows no evidence of a mass extinction event across the impact event horizon, and no contemporaneous faunal turnover is seen in other marine planktons. However, such an event has been reported among marine faunas and terrestrial tetrapods and floras in North America. We, therefore, suggest that the Manicouagan impact triggered the extinction of terrestrial and marine organisms near the impact site but not within the pelagic marine realm.

Composition and accretion rate of fossil micrometeorites recovered in Middle Triassic deep-sea deposits

Micrometeorites, which are submillimeter-sized extraterrestrial particles that survive atmospheric entry, originate from dust-producing objects such as comets and asteroids. Although ancient micrometeorites found in sedimentary rocks are of key interest as a historical record of meteoroid populations in the solar system, they are rare and prone to severe chemical weathering. Here we report the recovery of well-preserved micrometeorites, older than 240 Ma, in radiolarian chert from Japan. The collection of micrometeorites comprised 258 cosmic spherules, which are particles that totally melted during atmospheric entry, and 2 coarse-grained unmelted micrometeorites. These micrometeorites are much older than any previous micrometeorite collection in the sedimentary record. Using this collection, we calculated the accretion rate of iron-type cosmic spherules to the Earth during the Anisian Stage of the Middle Triassic. The estimated accretion rate for Anisian iron-type spherules smaller than 125 μm is 25 ± 8 t yr −1 . Analysis of the accretion rate for cosmic spherules also reveals high accretion rates of small spherules (∼8–36 μm) for a 0.74 m.y. period in the late Anisian. However, the possible link between an enhancement in the accretion rate of small cosmic spherules in the late Anisian and variations in the flux of extraterrestrial matter to the Earth requires further scrutiny.

Late Triassic bivalves from Sambosan accretionary complex, southwest Japan, and their biogeographic implications

ABSTRACT This paper records eight species of Late Triassic Tethyan bivalves from the oceanic limestone and associated basaltic volcaniclastic rocks which belong to the Sambosan accretionary complex in southwest Japan. The bivalve-bearing succession consists of basaltic volcaniclastic rocks (ca. 60 m thick) conformably succeeded by dark gray bedded limestone (ca. 10 m thick), which is, in turn, overlain by massive limestone (ca. 50 m thick). The succession is dated as Carnian to Norian by means of conodonts and bivalves. Upper Triassic bivalves, including Gruenewaldia decussata, G. woehrmanni and Costatoria goldfussi, are recorded from the examined succession. Their occurrence implies a Tethyan affinity for the examined Sambosan oceanic rocks. No bivalve species characteristic of a high latitudinal zone occurs in this succession.

Osmium isotope evidence for a large Late Triassic impact event

Anomalously high platinum group element concentrations have previously been reported for Upper Triassic deep-sea sediments, which are interpreted to be derived from an extraterrestrial impact event. Here we report the osmium (Os) isotope fingerprint of an extraterrestrial impact from Upper Triassic chert successions in Japan. Os isotope data exhibit a marked negative excursion from an initial Os isotope ratio (187Os/188Osi) of ∼0.477 to unradiogenic values of ∼0.126 in a platinum group element-enriched claystone layer, indicating the input of meteorite-derived Os into the sediments. The timing of the Os isotope excursion coincides with both elevated Os concentrations and low Re/Os ratios. The magnitude of this negative Os isotope excursion is comparable to those found at Cretaceous–Paleogene boundary sites. These geochemical lines of evidence demonstrate that a large impactor (3.3–7.8 km in diameter) produced a global decrease in seawater 187Os/188Os ratios in the Late Triassic.

Groundwater helium anomaly reflects strain change during the 2016 Kumamoto earthquake in Southwest Japan

Geochemical monitoring of groundwater and soil gas emission pointed out precursor and/or coseismic anomalies of noble gases associated with earthquakes, but there was lack of plausible physico-chemical basis. A laboratory experiment of rock fracturing and noble gas emission was conducted, but there is no quantitative connection between the laboratory results and observation in field. We report here deep groundwater helium anomalies related to the 2016 Kumamoto earthquake, which is an inland crustal earthquake with a strike-slip fault and a shallow hypocenter (10 km depth) close to highly populated areas in Southwest Japan. The observed helium isotope changes, soon after the earthquake, are quantitatively coupled with volumetric strain changes estimated from a fault model, which can be explained by experimental studies of helium degassing during compressional loading of rock samples. Groundwater helium is considered as an effective strain gauge. This suggests the first quantitative linkage between geochemical and seismological observations and may open the possibility to develop a new monitoring system to detect a possible strain change prior to a hazardous earthquake in regions where conventional borehole strain meter is not available.

Bolide impact triggered the Late Triassic extinction event in equatorial Panthalassa

Extinctions within major pelagic groups (e.g., radiolarians and conodonts) occurred in a stepwise fashion during the last 15 Myr of the Triassic. Although a marked decline in the diversity of pelagic faunas began at the end of the middle Norian, the cause of the middle Norian extinction is uncertain. Here we show a possible link between the end-middle Norian radiolarian extinction and a bolide impact. Two palaeoenvironmental events occurred during the initial phase of the radiolarian extinction interval: (1) a post-impact shutdown of primary and biogenic silica production within a time span of 104–105 yr, and (2) a sustained reduction in the sinking flux of radiolarian silica for ~0.3 Myr after the impact. The catastrophic collapse of the pelagic ecosystem at this time was probably the dominant factor responsible for the end-middle Norian conodont extinction.

Aulotortus friedli from the Upper Triassic gravitational flow deposits of the Kumagawa River (Kyushu, Southwest Japan)

Abstract. The benthic foraminifer Aulotortus friedli is reported from the Upper Triassic carbonates (capped seamount) of the Sambosan Accretionary Complex in Kyushu, Southwest Japan. Specimens were collected from shallow-water limestone clasts in a debris flow sequence near the Ose locality along the Kumagawa River. The limestone clasts mainly represent shoal facies where the benthic foraminifer is extremely abundant. Because of the remarkable preservation of the Japanese specimens, they allow further description of the involutinid A. friedli. In addition, a detailed sedimentary and environmental analysis of the gravitational deposits where A. friedli occurs is given. Using microfacies analysis together with the biostratigraphic range of A. friedli and associated foraminifers, a new interpretation is proposed of a seamount collapse process that occurred during the Norian-Rhaetian and preceded the major collapse event of the Sambosan seamount, which has been placed in the Middle to Late Jurassic.

Upper Triassic (Carnian) reef biota from the Sambosan Accretionary Complex, Kyushu, Japan

Calcified sponges, algae, and reef problematica are abundant yet poorly known from the Triassic of Japan. They are abundant in shallow-water carbonate, redeposited blocks of the Sambosan Accretionary Complex, Konosé Group, and southern Kyushu. Based on study of thin-sections from reef limestone exposed along the Kuma River, some important organisms and reef microfacies are described, which seem typical of Upper Triassic reef complexes. The most abundant reef organisms are hypercalcified sponges, including sphinctozoans, inozoans and chaetetids, followed by cyanophycean algae (including “Tubiphytes”-like organisms), and solenoporacean red algae. Loose sponge spicules in one thin-section also indicate the occurrence of rare hexactinellid sponges. Chambered demosponges described from the Konosé carbonate rocks include Solenolmia manon manon (Münster), Colospongia sp., Jablonskyia andrusovi (Jablonský), several unidentified chambered sponges as well as the inozoid Permocorynella sp. 1 and Permocorynella sp. 2. Also present are chaetetid sponges and solenoporacean red algae belonging to Parachaetetes cassianus (Flügel) and Parachaetetes? sp. or Solenopora? sp. Especially abundant in thin-sections are cyanophyceans and “Tubiphytes”-like organisms. Among the organisms is Cladogirvanella Ott and Hedstroemia sp. The composition of the biota and presence of typical problematic organisms increases our knowledge of shallow-water Upper Triassic carbonate rocks in a remote setting in western Panthalassa. The composition of the biota indicates a mostly Carnian age. Most comparable organisms are known from both the northeastern and southern Tethys.

Mid-oceanic shallow-water carbonates of the Panthalassa domain: new microfacies data from the Sambosan Accretionary Complex, Shikoku Island, Japan

During the Late Triassic, carbonate platforms expanded on continental shelves and island arcs. They have been studied in detail in the Tethys realm but coeval mid-oceanic shallow-water environments of the Panthalassa domain have received less attention. To fill this gap, we investigated the Upper Triassic limestone of the Sambosan Accretionary Complex (SAC), southwest Japan. A comprehensive sampling of the Upper Triassic limestone has been performed in 16 localities at Shikoku Island. Eight microfacies, ranging from pelagic to lagoonal settings, were identified, including two microfacies that are described in the SAC for the first time. Quantitative microfacies analysis assesses the composition of the SAC limestone and its biostratigraphy is refined. Finally, a speculative depositional model of the SAC carbonate platforms is proposed based on modern and ancient analogues. Instead of a true atoll-type platform with well-defined facies belts as suggested by previous studies, this new model suggests that the typical Sambosan platform was more probably a carbonate bank with submerged margins and a mosaic of microfacies in the platform interior. Special attention is given to the factors that have probably controlled the carbonate sedimentation at the top of the seamount(s).

Integrated Upper Triassic Conodont and Radiolarian Biostratigraphies of the Panthalassa Ocean

Abstract. The Late Triassic conodont biostratigraphy of two pelagic chert sections (sections N and Q) in the Inuyama area, central Japan, was investigated to calibrate the Triassic radiolarian zonation proposed by Sugiyama in 1997 with the conodont zones and the standard Triassic timescale. Based on the stratigraphic distributions of marker species, six conodont zones were defined: the Paragondolella? tadpole interval Zone, the Quadralella tuvalica interval Zone, the Epigondolella quadrata interval Zone, the E. triangularis interval Zone, the Mockina postera interval Zone, and the M. bidentata Zone. These conodont zones are comparable to the standard Carnian and Norian conodont zones of North America and the Tethys. The Carnian-Norian boundary in the sections studied is tentatively placed between the last occurrence of a Carnian species (Q. tuvalica) and the first occurrences of Norian species (E. quadrata and E. spatulata). The intercalibrated conodont—radiolarian biostratigraphy from the sections we studied accurately calibrates the radiolarian zones in Japan with standard chronostratigraphic stages and substages.