In the past 30 years, many researchers have investigated the relationship between extraterrestrial impacts and mass extinction events of the Phanerozoic. Nevertheless, such a relationship has only been confirmed for the Cretaceous/Paleogene (K/Pg) boundary at 65.5 Ma. Here, we review existing geological evidence for impact events during the Phanerozoic, recent advances in studies of impact frequency, and the distribution of crater size on the Moon. We conclude that the magnitude of the K/Pg boundary impact was exceptionally large among Phanerozoic events, and may have been a once in 500-1000 Myr event. The mechanism of extinction at the K/Pg boundary, associated with extraterrestrial impact, remains poorly understood, in part because the size and formation process of the impact crater (Chicxulub Crater) are poorly constrained. The recovery of drillcore from within the crater is required to clarify the mechanism of the K/Pg mass extinction.

View full abstract

Isotopic ages (K-Ar hornblende: 17.5±0.6 Ma; K-Ar biotite: 17.3±0.4 Ma; Rb-Sr biotite: 17.3±0.1 Ma) were determined for a ferrogabbro of the Pankenushi gabbroic complex, Hidaka magmatic belt, central Hokkaido. These ages are consistent with a previously reported U-Pb zircon age (18.5 Ma), indicating the Pankenushi complex was emplaced in the Early Miocene. Closure temperatures were estimated for the K-Ar hornblende and biotite ages. A very high cooling rate (10²–10³ °C/myr) suggests that the intrusion of mafic magma with an N-MORB signature was shortly followed by rapid uplift and denudation associated with the eastward tilting of the crustal sequence of the Hidaka magmatic belt. The tectonic setting of the magmatism may have been the opening of the Kuril backarc basin or collision between the Kuril arc and the NE Japan arc.

View full abstract

The Paleocene-Eocene thermal maximum (PETM) represents one of the most prominent and abrupt global warming events in Earth's history. Across the Paleocene-Eocene boundary (PEB; ca. 55 Ma), extreme global warming by 4–8 °C coincided with rapid ocean acidification, mass extinction in the deep ocean, and a distinct negative carbon isotope excursion (CIE) in both the marine and terrestrial environments. These lines of evidence strongly suggest a massive and rapid injection of ¹³C-depleted carbon into the atmosphere and ocean, thereby inducing global warming. However, the origin and magnitude of the massive carbon injection across the PEB remain unresolved.
We reconstructed the perturbation of the global carbon cycle across the PEB using an atmosphere-ocean box model, thereby obtaining a precise picture of the PETM event. It is likely that the PETM was triggered by the volcanism that produced the North Atlantic Igneous Province (NAIP). The model results indicate that the CIE of −3‰ during the PETM was caused by inputs of 2,200 Gt-C of thermogenic methane (triggered by the NAIP volcanism) and 1,300–3,300 Gt-C of biogenic methane following the dissociation of sub-seafloor gas hydrates. The destabilization of gas hydrate was probably due to a reduction in hydrostatic pressure in the sub-seafloor or the warming of bottom water due to a significant uplift of the seafloor related to eruption of the NAIP.
Although the CIE can be readily reconstructed, the calculated temperature rise is 2.6 °C at most under the generally accepted range for equilibrium climate sensitivity (rise of 1.5–4.5 °C per doubling of the atmospheric CO₂ concentration); hence, we cannot reproduce the global warming of 4–8 °C inferred from the geological record during the PETM. Our results indicate that climate sensitivity at this time may have been much higher than previously considered or that unknown feedback and forcings (other than atmospheric CO₂) were responsible for a major component of the PETM warming.

View full abstract

This paper attempts to construct the sequence stratigraphic framework for the lower to middle portion of thick turbiditic basin-filling successions of the Niigata-Shin'etsu basin, Northern Fossa Magna, central Japan, located in a tectonically active arc-arc junction zone between the NE and SW Japan arcs. Analyses of outcrop sections, well logs and 2D seismic sections in the Nishikubiki-Takada Plain area reveal that the targeted stratigraphic interval, including the upper Neogene Nambayama, Nodani, Kawazume and Nadachi Formations, can be divided into nine third-order depositional sequences: Sequence Nkb-W, -V, -U, -T, -S, -Q, -P, -K and -KZ in a stratigraphic ascending order. In accordance with previously proposed turbidite sequence models, each depositional sequence shows characteristic stacking patterns of turbidite facies successions, which can be used for dividing the sequences into systems tracts. The lower to middle part of the sequence, consisting of relatively sandy turbidites with stacks of upward-fining and overlying upward-coarsening successions, may correspond to FSST (falling-stage systems tract) and LST (lowstand systems tract). The overlying upward-fining part corresponds to TST (transgressive systems tract), and the obscure upward-coarsening part at the top of the sequence, corresponds to HST (highstand systems tract). The TST to HST intervals of some sequences contain characteristic foraminiferal assemblage zones; e.g., Globorotalia rikuchuensis zone, Blue zone, and No. 3 Globorotalia inflata bed, all of which indicate the inflow of an open-sea ocean current into the Sea of Japan during a relative sea-level rise and highstand. This characteristic pattern of turbidite sequence succession seems to vary in response to the submarine-fan type; e.g., sandy radial fan and channel levee system. The present results represent fundamental information for basin-wide correlations and the reconstruction of tectono-sedimentary processes of the Fossa Magna region.

View full abstract

Based on a single specimen from Serpukhovian (Late Mississippian/late Early Carboniferous) limestone, Niigata Prefecture, central Japan, we describe a new species of cyclid crustacean, Cyclus tazawai, as the first record of the order from East Asia. Comparable Early Carboniferous species with C. tazawai are C. torosus Woodward, 1870, C. simulans Reed, 1908 and C. jonesianus Woodward, 1870, all of which were reported from Ireland. Previously, the Early Carboniferous cyclids have been regarded to be endemic in Euramerica. The new species may have had a parasitic relationship with cartilaginous fishes.

View full abstract

[in Japanese]

View full abstract