Abstracts of Annual Meeting of the Geochemical Society of Japan

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Cretaceous to Paleogene granitic rocks in South Korea occur, from NW to SE, in the Ogcheon Belt and the Gyeongsang Basin, which are widely exposed over an area of more than 90000km2. The granitic rocks provide an important opportunity to study temporal and spatial changes in magmatism at the eastern margin of the Eurasian continent, which have also played major roles in growth of the continent. U-Pb zircon age determinations using ultra-violet (UV)-laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were performed on total twenty-six samples at Earthquake Research Institute, University of Tokyo. The obtained ages range from 97 Ma to 48 Ma rather continuously, although the magmatic activity appears to have clustered in four stages, 97 to 80 Ma, 75-66 Ma, 61-59 Ma and 48-47 Ma, respectively, with gaps of ~5-10 m.y. between the stages. Furthermore, the overall age distribution shows systematical southeastward younging. These zircon age data are compared with the contemporaneous granitic rocks in Japan to investigate the temporal and spatial variations. The age data compiled from literature and a few preliminary zircon ages we have newly analyzed for SW Japan show that the granitic rocks in South Korea exhibit a clear younging trend toward the contemporary trench, in contrast with that younging toward the continent in SW Japan. The petrological and geodynamical constraints suggest that the granitic magmatism in Korea has been mostly generated from the partial melting of a basaltic lower crust, accompanied by a magma mixing process with mantle-derived basaltic magma which might have played an important role as a heat source. Therefore, during Cretaceous to Paleogene magmatism in Korea, the lower crust and the upper mantle must have been melted, and moreover the melting region migrated several hundred km over 50 m.y. toward the trench. Based on a large-scale numerical model of mantle convection, it is inferred that such melting regime and its migration toward a trench can occur associated with lithospheric delamination which migrates trench ward due to the large-scale mantle flow induced by subduction. On the other hand, the contemporaneous magmatic activity in SW Japan, which shows an opposite polarity younging toward the continent, cannot be attributed to the same mechanism as in South Korea. Formation of the Cretaceous granitic batholiths in SW Japan had occurred associated with the paired metamorphism, Sanbagawa (high-P/T type) and Ryoke (low-P/T type) metamorphism. Thermal effect of Cretaceous ridge subduction in this region is proposed to explain the origin of both granitic magmatism and paired metamorphism, which could have propagated toward the continental side as the ridge subducted.

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Settling particles were collected by time-series sediment trap moored at station KNOT (44N 155E, water depth 770m) from October 25th 1999 to June 20th 2001, and from June 25th 2002 to May 11th 2006. Particulate nitrogen contents and its isotope ratios (d15N) in the collected samples were measured. The results of d15N showed the general variations that are lower during the spring bloom periods and higher during winter periods. In order to analyze quantitatively the factors controlling such variations, we used an ecosystem model including nitrogen isotopes. We will present the analyses obtained with the model.

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Seven coral reef communities are defined on Shiraho fringing reef, Ishigaki Island, Japan. Then, metabolisms (net photosynthesis and calcification rates) of ten sites including six communities, which are occupied most area on the reef flat and slope, were measured by in situ incubations. In the results, net photosynthesis on the reef flat was totally positive, but reef flat acts as a source for CO₂, because calcification/photosynthesis ratio is 2.5 (> 1.67). Moreover, net photosynthesis on the reef slope was negative. It is expected that almost all excess organic productions are effused to the outer reef and are consumed by the community on the reef slope. Therefore, it is, considered that total net organic production on the whole reef system is almost zero and the whole reef system also acts as a source for CO₂.

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Biogenic volatile organic compounds emitted from the ocean are known to play an important role in the marine atmospheric chemistry. In this study, we investigated diurnal variations of isoprene, DMS and methyl iodide in the atmosphere, based on the high frequency VOC monitoring at Hateruma Island(24.1ºN, 123.8ºE). Isoprene in the marine atmosphere was found to be greatly reduced at mid-day. Such a diurnal variation of isoprene was completely different from those observed in the terrestrial forests. DMS and methyl iodide did not show so significant diurnal change as isoprene.

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