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Yamazaki T.,Geological Survey of Japan | Ikehara M.,Kochi University
Paleoceanography | Year: 2012

In the Southern Ocean, magnetic mineral concentration increases in glacial periods. The variation pattern closely resembles eolian dust flux records from Antarctic ice cores, but the cause of the linkage remains unclear, as the dust flux is too small for the source of terrigenous materials in the Southern Ocean. We have conducted an environmental magnetic study of late Pleistocene sediments from the south Indian Ocean to investigate the origin of the magnetic concentration changes. Biogenic magnetites can be detected using the characteristics of almost no magnetostatic interactions and narrow coercivity distribution, reflecting occurrence of single-domain magnetites in a chain. We interpret that a non-interacting component on first-order reversal curve diagrams and low-coercivity components with small dispersion from isothermal remanent magnetization (IRM) component analyses represent biogenic magnetites, and that the interacting and middle-coercivity components represent terrigenous maghemites. The ratio of anhysteretic remanent magnetization susceptibility to saturation IRM reflects relative abundance of the biogenic and terrigenous components. It was revealed that biogenic magnetites are a dominant constituent of the magnetic minerals. In glacials, the abundance of both biogenic and terrigenous components increased with increased proportions of the latter. Increased ocean productivity in glacials is suggested from increased proportions of biogenic magnetites with elongated morphologies, indicative of less-oxic conditions, and increased sedimentation rates. These observations suggest that the increased magnetic concentration in glacials in the Southern Ocean may be explained by iron fertilization; the production of biogenic magnetites was enhanced associated with increased ocean productivity, which was fueled by increased eolian dust flux. © 2012 by the American Geophysical Union.

Magnetic properties are increasingly used for paleoclimatic and paleoceanographic studies. Utilizing recently developed proxies, an environmental magnetic study was conducted on the uppermost 12 m sediments of Integrated Ocean Drilling Program Site U1337 in the eastern equatorial Pacific Ocean. This interval is above the Fe-redox boundary, and covers the past ~800 k.y. The site is located near the present southern boundary of the Intertropical Convergence Zone (ITCZ), and is thus expected to be sensitive to variations of the ITCZ position. The ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (kARM/SIRM), first-order reversal curve diagrams, and IRM acquisition curves indicate that the magnetic mineral assemblage consists of a dominant biogenic component and a minor terrigenous component. Two groups, the biogenic soft (BS) and hard (BH), are identified for the biogenic component, and probably correspond to different magnetofossil morphology. The BH component, probably carried by elongated magnetofossils, increases in sediments of glacial periods, which are probably in less oxic conditions due to increased ocean productivity. This demonstrates that magnetofossil morphology, which can be discriminated by the rock magnetic technique, is a sensitive indicator of slight oxic-suboxic environmental fluctuations in sediments. Temporal variations of the terrigenous component, most likely transported as eolian dust, were estimated from the kARM/SIRM ratio and S ratio (ratio of a moderate field IRM to SIRM, representing relative contribution of lower- and higher-coercivity magnetic minerals); significant glacial-interglacial variations occurred at marine isotope stage (MIS) 10 and before, but not after. In addition, coeval upcore increases in sedimentation rates and the BH component were observed, suggesting increased productivity. These observations may indicate that the position of the ITCZ was more southward than today ca. 250 ka and before. © 2012 Geological Society of America.

Shinohara H.,Geological Survey of Japan
Earth, Planets and Space | Year: 2013

Volcanic gas compositions of Shinmoedake, Kirishima volcano, Japan were measured by Multi-GAS during the persistent degassing period with repeating Vulcanian eruptions from March 2011 to March 2012. In order to avoid risks due to eruptions, the measurements were performed with the Unmanned Aerial Vehicles (UAV) that fly through the plume with the Multi-GAS and by an automatic Multi-GAS monitoring station installed 5 km away from the summit. Based on the UAV measurements on May 18, 2011, most of the major volcanic gas components were quantified as CO2/SO2 = 8, SO2/H2S = 0.8, H2O/C02 = 70 and H2/SO2 = 0.03 (mol ratio), and the SO2/H2S ratio of the plume was quantified as 8 on March 15, 2011. The Multi-GAS monitoring station occasionally detected a dilute plume with an SO2/H2S ratio ranging from 0.8 to 3.3 from April 2011 to March 2012. The decrease of the SO 2/H2S ratio from March 15, 2011, to May 18, 2011, is interpreted as the result of a ten times increase of the degassing pressure. Based on the S02 fluxes and the gas compositions, the conduit magma convection is considered to be the gas supply mechanism at the Shinmoedake, and the degassing pressure changes are attributed to the change of depth of the convecting magma column top.

The mélanges of the Jurassic accretionary complexes of Japan have very complicated structures which were formed by the progressive disruption and mixing of Ocean Plate Stratigraphy (OPS) during various stages of accretionary processes along the ancient arc-trench system. The OPS is composed of Permian basalt-limestone-chert, Permian-Triassic boundary claystone, Early Triassic-Early Jurassic chert, Early Jurassic-earliest Cretaceous siliceous shale and Jurassic to early Cretaceous turbidite, in ascending order.The OPS was detached from the oceanic plate along a decollement, which developed within the P-T boundary claystone during the offscraping and underplating processes. Disruption and mixing first occurred along the decollement, then along successively developed out-of-sequence thrusts. Neighboring rocks along the faults are mixed to become various sized clasts and matrices of mélanges. Mappable slabs in the mélanges were survivors from tectonic disruption of the accreted OPS. On the surface of accretionary wedge, a part of the OPS was collapse by submarine sliding and mud diaprisim to form chaotic sediments on the top of the OPS. Thus, the full extent of accretionary complex was analyzed by age-stage development of mélange formation. In the mélanges, some of the large tectonic slabs in the mélanges kept the original order of the OPS even though they were deformed in various degrees. This suggests that the OPS was tectonically stacked during the offscraping process, and was gradually disrupted along the decollement and out-of-sequence thrusts during the accretionary processes that form the mélange structures. The mélanges structures mentioned above are shown in the geological maps at the scale of 1:50,000 recently published by the Geological Survey of Japan. These geological maps clearly show the mélange formation processes during ocean plate subduction along the Mesozoic Asian continental margin. © 2011 Elsevier B.V.

Shinohara H.,Geological Survey of Japan
Journal of Volcanology and Geothermal Research | Year: 2013

Global volatile fluxes from subaerial volcanoes at subduction zones were estimated based on a compilation of fluxes from various sources, including persistent degassing, hot and cold springs, soil degassing, and eruptions. Because worldwide comprehensive datasets are not available, especially for diffuse volatile discharges, volatile fluxes from Japan arcs were estimated based on detailed datasets, and the regional fluxes were extrapolated to the global flux with consideration of the regional characteristics of volcanic volatile compositions, which were estimated based on volcanic gas compositions of persistent degassing. The estimated global fluxes indicate that persistent degassing is the major source of volatiles, especially for S with a contribution of 80%. Diffuse discharges and persistent degassing are similarly important sources of H2O, CO2, and Cl, but the contribution of explosive eruptions is less than 15% for all the volatiles. The estimates of diffuse degassing fluxes include large errors due to limited data. However, the potential impact of these sources on the global flux indicates the importance of further studies to quantify these fluxes. The volatile budget of subduction zone volcanism was evaluated by comparing the estimated volatile fluxes, the volatile contents in the crust, and the primitive magma volatile contents. The contribution of volatiles remaining in the crust are not significant; however, consideration of lower crust foundering significantly alters the volatile budget estimate because the primitive magma supply rate should be significantly increased to account for the lower crust foundering. © 2013 Elsevier B.V.

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