Abstracts of Annual Meeting of the Geochemical Society of Japan

Many researchers pointed out that decadal change in deep-sea circulation in the Japan Sea are strongly related with the global climate change. However, little is known about changes in fluxes and elemental composition of sinking particles in the sea. In this study, we assessed temporal changes in elemental composition of sinking particle in the Japan Sea by comparing results of a sediment trap experiment in middle 1980s with those in end of 1990s. The result indicated that (1) the contribution of Asian dust in sinking particles decreased significantly, (2) horizontal transport of re-suspended particles from the slope regions was weakened, and (3) there was no significant increase in vertical transport of anthropogenic heavy metals to the interior of the Japan Sea.

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A model of the earth's mantle evolution is presented based on a series of numerical models of mantle convection with moving plates and mantle magmatism. In the early earth just after the magma ocean disappeared, the strong internal heating due to abundant heat producing elements frequently induced mantle overturn. The mantle overturn, in turn, induced vigorous mantle magmatism on one hand, and vigorously stirred the mantle on the other hand. As a result, the mantle remained chemically rather homogeneous. The mantle overturn also induced chaotic and intermittent plate tectonics. As the internal heating rate declined, however, mantle overturn stopped and stable superplumes began to develop in the deep mantle as a hot pile of subducted oceanic crusts. The stable superplumes made plate tectonics more continuous and regular.

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One of the Earth's unique features among the known terrestrial planets is the presence of a chemically evolved massive crust – the continental crust. The timing of its emergence and subsequent evolution are the subject of a long-standing debate, with models ranging from early formation with little subsequent growth to progressive growth with a near-steady rate or with pulses at a high rate. Temporal variations of radiogenic isotope compositions in granitic (sensu lato) crust are of major importance because they can potentially constrain models of continental growth and the amount of sedimentary recycling through geological time. Yet the significant diversity and heterogeneity of granitic crust have prevented us from arriving at a reliable secular trend. Here we employ U-Pb and Hf isotope systematics of detrital zircons from four large rivers to examine Hf isotope secular trends in granitic crusts. We found that Hf isotope compositions of granitic crusts deviate from the mantle evolution line from about 3.2 to 2.0 Gyr ago, and the deviation declines between 2.0 and 1.3 Gyr ago and again increases afterwards. The findings, combined with the lack of zircons older than 3.2 Gyr in river sands, provide evidence for the emergence of large-scale continents at 3.2 Gyr ago with further rapid growth at around 1.3 Gyr, which resulted in the major growth of the sedimentary mass between 3.2 and 1.3 Gyr ago and the predominance of its cannibalistic recycling later.

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As the economic activity of developing countries increases, various environmental problems have been elicited. Air pollution effect much larger area. Since lead is in airborne pollutants and has distinctive isotope ratios, lead isotopes ratio is useful tracer for their provenance. We measured lead isotopes in settling particles in the western subarctic Pacific which considered to be contaminated by little anthropogenic lead. We estimated that the amount of the anthropogenic lead and its provenance.

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