Browsing by Author "Tokuyama, A."

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    Solute generation and CO2 consumption during silicate weathering under subtropical, humid climate, northern Okinawa Island, Japan
    (Elsevier, 2007) Vuai, S. A. H.; Tokuyama, A.
    The chemical characteristics of freshwaters draining the silicate rocks in the northern part of Okinawa Island were studied to understand solute generation processes, and to determine rates of chemical weathering and CO2 consumption. It was observed that the water chemistry is highly influenced by marine aerosols, contributing more than 60% of total solute. Significant positive correlations observed for chloride versus dissolved silica and chloride versus bicarbonate suggest a strong influence of evapotranspiration on the seasonality of solute concentration. It was also found that chemical weathering has been highly advanced in which the dominant kaolinite minerals are being gibbsitized. Carbonic acid was found to be the major chemical weathering agent, releasing greater than 80% of weathering-derived dissolved cations and silica while the remaining portion was attributed to weathering by sulfuric acid generated via oxidation of pyrite contained in the rocks. The flux of basic cations, weathering-derived silica and CO2 consumption were relatively high due to favourable climatic condition, topography and high rate of mechanical erosion. Silicate weathering rates for basic cations were estimated to be 6.7–9.7 ton km− 2 y− 1. Carbon dioxide consumed by silicate weathering was 334–471 kmol km− 2 y− 1 which was slightly higher than that consumed by carbonate weathering. In general, divalent cations (Mg and Ca) and bicarbonate alkalinity derived from carbonate dissolution were higher than those from silicate weathering. As a consequence, the evolution of chemical species in the freshwaters of northern area of Okinawa Island to a large extent could be explained by mixing of two components, characterized by waters with Na+ and Cl− as predominant species and waters enriched with Ca2+ and HCO3−.
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    Trend of trace metals in precipitation around Okinawa Island, Japan
    (Elsevier, 2011) Vuai, S. A. H.; Tokuyama, A.
    Okinawa Island is located between mainland Japan to the northeast, Taiwan to the southwest and China to the west. In recent years, precipitation with relatively low pH has been observed in the island during winter seasons. This study was conducted to evaluate the potential sources and factors affecting distribution of trace metals in the atmosphere around the northern area of Okinawa Island. The results showed that the trace metal contamination was low comparable to rural areas of less industrialized countries. The annual average depositions were 6.0, 5.2, 4.4, 2.7 and 20.4 kg km− 2 y− 1 for Al, Fe, Mn, Cu, and Zn, respectively. The metals in the precipitation were mainly derived from the dissolution of wind-blown soil particles. The distribution of trace metals across the island showed the influence of road traffic as a source and cause of atmospheric pollution. Aluminium and Fe were found to be high closer to the main road and decreased further away from the road, while Zn and Cu were strongly correlated with average mean pH of precipitation. The variation of concentrations in the island was attributed to the distance from the main road, average rain pH at a particular sampling location and wind speed and direction. A strong correlation between lithological elements (Al and Fe) and anthropogenic elements (Zn and Cu) indicates that they were carried by the same particles and experienced similar scavenging mechanism during dissolution process.