Abstracts of Annual Meeting of the Geochemical Society of Japan

Vertical distributions of ^239+240Pu activity and ²⁴⁰Pu/²³⁹Pu atom ratio in settling particles and the particulate ^239+240Pu fluxes in the water column in the western Northwest Pacific were investigated. Time-series settling particle samples were collected using sediment traps to examine the seasonal variation of Pu flux and ²⁴⁰Pu/²³⁹Pu atom ratio. The high (> 0.18) ²⁴⁰Pu/²³⁹Pu atom ratios in settling particles indicate that plutonium from the Pacific Proving Grounds (PPG) source in the central Pacific is transported toward the western Northwest Pacific. Evidence indicates that Pu scavenging onto the settling particles is strongly dependent upon the bulk mass flux. The results suggest that advective lateral transport of dissolved Pu from the open ocean to the ocean margin and removal of Pu into the margin sediments by particle scavenging is a common phenomenon in the Pacific Ocean.

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Nitrous oxide (N2O) is a greenhouse gas and is also involved in the stratospheric ozone depletion. The ocean is recognized as one of the important sources of atmospheric N2O, contributing to the total sources by more than 17%. Nitrous oxide is produced as a by-product of nitrification or an intermediate of denitrification. However the estimated oceanic N2O flux and relative contributions of the two processes still have large uncertainty. Recently, isotopomer ratios (nitrogen and oxygen isotope ratio including intramolecular 15N-site preference) have been found to be a useful tool to investigate production/consumption processes because they reflect isotopic fractionation that is characteristic to each process. In this study, we analyzed the production/consumption processes of N2O in the eastern North Pacific Ocean.

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In order to investigate the influence of watershed geology, a water sampling survey was conducted on Ishigaki and Iriomote Islands, southwest of Japan, whose basement rocks are carbonate and siliceous sandstone, respectively. Samples from Miyara River in Ishigaki showed generally high total alkalinities and DIC concentrations reflecting the effect of carbonate erosion and those concentrations drastically increased downriver. In contrast, samples from Urauchi River in Iriomote showed lower DIC concentrations, reflecting slow chemical erosion of silicate rocks and less groundwater contribution. The stable carbon isotope (d¹³C) values of high-DIC samples suggest that dissolution of carbonate rocks are enhanced by continental and/or other acidic matters.

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Geochemical data including 389 XRF analyses, 203 ICP-MS trace element analyses, 188 Sr-Nd, 120 Pb, and 83 Hf isotope compositions were examined to reveal the origin of the late Cenozoic magmatic activity (~15 mys) in the SW Japan arc. This activity includes OIB-type alkali basalts, IAB-type sub-alkali basalts, HMA, and adakites (ADK). Activity in the SW Japan volcanic arc commenced at ~15 Ma when the back arc basin (BAB) of the Japan Sea opened. Initiation of subduction of the young Shikoku Basin Philippine Sea plate (PHS) occurred at the same time. At ~15 Ma, OIB activity occurred in the rear arc due to upwelling of the asthenosphere by BAB opening, whereas IAB and Setouchi HMA were produced in the forearc due to dehydration and melting of the young slab. ADK and IAB activity between these two areas began at about 8-9 Ma This ADK-IAB zone migrated to the rear arc by 2 Ma, and has remained active to the present. At the same time, OIB activity ceased in the area where ADK-IAB activity began. OIB activity does not require slab inputs and thus originates from asthenospheric mantle. In contrast, IAB, HMA, and ADK obviously require slab fluids and melts. The temporal rear arc-ward migration of the slab-related magmas indicates progressive penetration of the PHS into the anomalously hot mantle asthenosphere that was emplaced during BAB opening. Interaction of BAB opening tectonics and initiation of young PHS slab subduction combined to form the complex volcanism present in the SW Japan arc.

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