Katsuhiko Suzuki

Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event

The Early Cretaceous Ontong Java Plateau was emplaced at almost the same time as marine biotic changes that culminated in oceanic anoxic event 1 (OAE1a). A causative link between these events has been suggested, but direct evidence has been lacking until now. New Os isotope measurements across the Lower Aptian “Selli Level” black shale deposited during OAE1a in central Italy reveal two negative excursions in marine 187 Os/ 188 Os ratios within a period of 2 Ma starting above the Barremian-Aptian boundary and ending just above the Selli Level horizon, suggesting an order-of-magnitude increase in the global fl ux of unradiogenic Os. The results are consistent with early and major phases of eruption of the Ontong Java Plateau. The latter phase is estimated to have been as short as ~1 Ma and may have induced widespread oceanic stratifi cation that triggered OAE1a.

Geochemical characteristics and origin of the HIMU reservoir: A possible mantle plume source in the lower mantle

Combined Pb-Sr-Nd-Hf-Os isotopes, together with major and trace element compositions, were determined from clinopyroxene and olivine phenocrysts, along with whole rocks, for ocean island basalts with high μ (μ = 238U/204Pb) (HIMU) and enriched mantle isotopic characteristics from Cook-Austral Islands. Clinopyroxene and olivine separates record reliable isotopic information of the sources because of minimized in situ radiogenic ingrowth and their lower susceptibility to crustal contamination. Coherent isotopic systematics in multi-isotope spaces defined by the HIMU samples are best explained by recent mixing of melts derived from the HIMU reservoir and the local shallow mantle. The isotopic compositions of the HIMU reservoir are constrained to be low ɛNd (≤+4), low ɛHf (≤+3), and moderately radiogenic 187Os/188Os (0.14–0.15) in association with radiogenic Pb isotopes (206Pb/204Pb ≥ 21.5). Since ancient oceanic crust would have had exceptionally radiogenic 187Os/188Os, moderately high 187Os/188Os precludes recycled oceanic crust as the only contributor to the HIMU reservoir. Instead, mantle metasomatized with partial melts from subducted oceanic crust is a likely candidate for the HIMU reservoir. Moreover, partial melting of oceanic crust in equilibrium with Mg perovskite would fractionate U/Pb, Sm/Nd, and Lu/Hf, which are in accordance with the time-integrated U/Pb, Sm/Nd, and Lu/Hf deduced from Pb, Nd, and Hf isotopic compositions of the HIMU reservoir, respectively, with a formation age of 2–3 Ga. We thus propose that the HIMU reservoir was formed by hybridization of a subducted oceanic crust-derived melt with the ambient mantle and then stored for several billion years in the lower mantle.

Petrological and Nd-Sr-Os isotopic constraints on the origin of high-Mg adakitic rocks from the North China Craton: Tectonic implications

The Mesozoic high-Mg dioritic rocks in the North China Craton have been suggested to be part of adakitic rocks. The origin of the high-Mg diorites has been attributed to equilibration of partial melts from delaminated mafic crust (eclogite) with mantle peridotite. Here we present petrological and Os isotopic data against the delamination model, and propose a process of magma mixing between siliceous crustal melts and basaltic magma from metasomatized mantle in a post-kinematic setting for their origin. The magma mixing process is supported by (1) euhedral overgrowths of high-Ca plagioclase and high-Mg pyroxene over low-Ca plagioclase and low-Mg pyroxene, respectively, and (2) highly radiogenic Os isotopic compositions, and negatively correlated Nd and Sr isotopic ratios. Our proposed model is probably applicable to the general mode of origin and tectonic settings of high-Mg adakitic magmas.

Marine osmium isotope record across the Triassic-Jurassic boundary from a Pacific pelagic site

The Triassic-Jurassic (T-J) boundary ca. 200 Ma represents one of the major mass extinction events of the Phanerozoic; however, the cause of this event remains controversial because of a paucity of geological evidence. In this study we present an isotopic record of osmium extracted from a bedded chert succession across the T-J boundary in the Kurusu section of Japan, deposited within a Paleo-Pacific (Panthalassa) deep basin. The data show a gradual decrease in seawater 187 Os/ 188 Os values during the Rhaetian, followed by a sharp increase in the latest Rhaetian, and a subsequent stable phase across the T-J boundary. The decreasing trend of 187 Os/ 188 Os values during the Rhaetian indicates a gradual increase in the relative supply rate of unradiogenic Os from the mantle associated with emplacement of the Central Atlantic Magmatic Province. The subsequent shift toward radiogenic values reflects an increased supply of radiogenic Os due to enhanced continental weathering. This interval marks more negative isotopic values of organic carbon, the onset of radiolarian faunal turnover, and conodont extinctions, indicating that the rapid increase in continental weathering rate was closely linked to the perturbation of the carbon cycle and the T-J biotic crisis.

Visualization of geoscience data on Google Earth: Development of a data converter system for seismic tomographic models

We have developed a visualization system for multidisciplinary geoscience data, which visualizes seismic tomographic models, geochemical datasets of rocks, and geomagnetic field models by exploiting Google Earth technologies. As Google Earth supports the ad hoc language, Keyhole Markup Language (KML), we have developed software packages to convert datasets of different fields of geosciences into KML files; we call these software packages KML generators. The software consists of two components: the engines of the KML generator and the user interface (UI). We provide both desktop and web UI applications for the KML generators. The web applications are now available (http://www.jamstec.go.jp/pacific21/google_earth). The KML generators provide a graphical UI and a flexible visualization scheme that enable both expert and nonexpert users to handle various geoscience data. In this paper, we describe a visualization schema of seismic tomography on Google Earth. The KML generator for the tomography enables us to display vertical and horizontal cross sections of the model on Google Earth in three-dimensions (3D), which can be useful for understanding the structure of the Earths interior. In our visualization system, with multiple KML files produced from the KML generators, various geoscience data can be visualized with the same 3D graphics. This contribution can promote cross-disciplinary studies and provide new insights into the Earths dynamics.

Mechanoradical H2 generation during simulated faulting: Implications for an earthquake‐driven subsurface biosphere

[1]xa0Molecular hydrogen, H2, is the key component to link the inorganic lithosphere with the subsurface biosphere. Geochemical and microbiological characterizations of natural hydrothermal fields strongly suggested that H2 is an important energy source in subsurface microbial ecosystems because of its metabolic versatility. One of the possible sources of H2 has been considered as earthquakes: mechanoradical reactions on fault surfaces generate H2 during earthquake faulting. However it is unclear whether faulting can generate abundant H2 to sustain subsurface chemolithoautotrophic microorganisms, such as methanogens. Here we present the result of high velocity friction experiments aimed to estimate the amount of H2 generated during earthquakes. Our results show that H2 generation increases with frictional work (i.e., earthquake magnitude) and that a H2 concentration of more than 1.1 mol/kg of fluid can be achieved in a fault zone after earthquakes of even small magnitudes. The estimated earthquake-derived H2 concentration is sufficiently high to sustain a H2-based subsurface lithoautotrophic microbial ecosystem. Furthermore, earthquakes have initiated on the Earth at least since tectonic plate movement began ∼3.8 Ga, implying the possible existence of ancient earthquake-driven ecosystems. Seismic H2 based subsurface ecosystems might exist not only over the Earth but also other planets.

Tritium in Japanese precipitation following the March 2011 Fukushima Daiichi Nuclear Plant accident

Tritium concentrations in Japanese precipitation samples collected after the March 2011 accident at the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) were measured. Values exceeding the pre-accident background were detected at three out of seven localities (Tsukuba, Kashiwa and Hongo) southwest of the FNPP1 at distances varying between 170 and 220 km from the source. The highest tritium content was found in the first rainfall in Tsukuba after the accident; however concentrations were 500 times less than the regulatory limit for tritium in drinking water. Tritium concentrations decreased steadily and rapidly with time, becoming indistinguishable from the pre-accident values within five weeks. The atmospheric tritium activities in the vicinity of the FNPP1 during the earliest stage of the accident was estimated to be 1.5×10(3) Bq/m(3), which is potentially capable of producing rainwater exceeding the regulatory limit, but only in the immediate vicinity of the source.

Prokaryotic Abundance and Community Composition in a Freshwater Iron-Rich Microbial Mat at Circumneutral pH

The abundance, diversity and composition of bacterial and archaeal communities in a freshwater iron-rich microbial mat were investigated using culture-dependent and culture-independent methods. The sampling site is a mixing zone where ferrous-iron-rich fluids encounter oxygen-rich environments. Quantitative PCR analysis shows that Bacteria dominated the mat community (>99% of the total cell numbers). Phylotypes related to iron-oxidizers in Gallionellaceae, methano/methylotrophs in Methylophilaceae and Methylococcaceae, sulfide-oxidizers in Sulfuricurvum and an uncultured clone group, called Terrestrial group I or the 1068 group, in the Epsilonproteobacteria were detected in the clone library from the original sample and/or the enrichment cultures. This result suggests that these members may play a role in Fe, S and C cycling in the mixing zone. Although Archaea were minor constituents numerically, phylogenetic analysis indicates that unique and diverse yet-uncultivated Archaea are present in the iron-rich mat. The phylotypes of these yet-uncultivated Archaea belong to environmental clone groups that have been recovered from other mixing zones in terrestrial and marine environments, and some of our phylotypes have significantly low similarity (80% or lower) with the archaeal clones reported previously. Our results provide further insights into the bacterial and archaeal communities in a microaerobic iron-rich freshwater environment in mixing zones.