Two more trochoid species are added to the gastropod fauna in North Knoll of Iheya Ridge, Okinawa Trough, based on new collection of Shinkai 2000 : Iheyaspira n. gen. et n. sp. characterized by skeneid-like shell and 12 lateral teeth of radula, and Margarites ryukyuensis n. sp.

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We present ²¹⁰Pb/²²⁶Ra and ⁸⁷Sr/⁸⁶Sr data of barite separates from mineralized hydrothermal sulfate-sulfide chimneys collected from two young and active back-arc basins (i.e., Okinawa Trough and southern Mariana Trough), to examine the age of chimney formation and the relationship between the fluid chemistry and the field occurrence. Precipitation ages of purified barites in the mineralized chimneys are 24.8 (±1.3)-73.9 (±13.6) years BP₂₀₀₅ for chimney #220 from the Hakurei site in the Izena Cauldron (middle Okinawa Trough), 14.1 (±0.6)-53.4 (±2.8) years BP₂₀₀₅ for chimney #562-R13 from the Yaeyama graben (southern Okinawa Trough), and 29.8 (±1.4)-39.2 (±2.9) years BP₂₀₀₅ for chimney #903-R7 from the Archaean site in the southern Mariana Trough. Therefore, the time required for hydrothermal fluid venting to form these chimneys is of the order of decades. ⁸⁷Sr/⁸⁶Sr values for barites from chimney #220 are ∼ 0.7094; these values are higher than those of the JADE site (0.7091) at the slope of the Izena Cauldron and ambient seawater (0.70918). Given the relatively high Ag and Sb contents in chimney #220 bulk samples, we suggest that interactions have occurred between the hydrothermal fluid and the sediment cover. In contrast, barites from the southern Mariana Trough have relatively low ⁸⁷Sr/⁸⁶Sr values (∼ 0.7046), implying that magmatic fluids play a significant role in mineralization in this sediment-starved setting.

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Mineralogically investigated were massive sulfide samples of chimneys and mounds collected during the "SHINKAI 6500" dives (Dive 562 and Dive 567) from two basalt-hosted hydrothermal ore deposits, being at a short distance of about 1 km, occurring in the eastern end of the Yaeyama Central Graben, the Southern Okinawa Trough. In both sample series (#562- and #567- series), ore minerals are dominated by low-iron sphalerite with small amounts of chalcopyrite and galena. Sphalerite in the #567-series samples shows somewhat wide compositional variation, ranging from 0.4 to 3.8 mole%FeS, with the highest mode of about 1 to 2 mole%FeS. Significant amounts of pyrite and secondary marcasite are present in the #562-series samples with subordinate bornite, while in the #567-series samples, they are lacking, but instead tetrahedrite-tennantite series with atomic ratios of Sb/(Sb+As) being 0.2 to 0.8 is abundant and contains Ag up to 0.7 wt% with mostly less than 0.5 wt%. Neither pyrrhotite nor isocubanite is encountered in the two sample series. Microthermometries of two-phase (liquid + gas) fluid inclusions trapped in calcite (#567-series) have revealed the following results: homogenization temperatures (not corrected for pressure) of 242 to 317 ℃ and salinities of 5.1 to 7.6 wt.%NaCl equiv., being a little more saline than the seawater. The observed mineral assemblages and sphalerite compositions obtained for the Yaeyama Central Graben deposits lead us to indicate that the basalt-hosted mineralizations might have occurred under rather higher sulfidation state. From the mineral associations observed, it is also indicated that both hydrothermal deposits are characterized by a metal association of essentially Zn-Pb-Cu. The mineral assemblages and ore textures observed are similar to those of the Kuroko deposits rather than the Besshi-type deposits, as well as their metal association (Cu-Pb-Zn).

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Within neutrally buoyant hydrothermal plumes derived from the Yonaguni Knoll IV hydrothermal field (24°51´ N, 122°42´ E, D = 1,370-1,385 m) in the southwestern Okinawa Trough back-arc basin, we obtained profiles of dissolved manganese (Mn), helium isotopes (³He/⁴He), methane (CH₄) and its stable carbon isotope ratio (δ¹³C_PDB) in young to old plume waters. We first mapped the spatial distribution of hydrothermal plumes by towing a fixed array of optical sensors (Miniature Autonomous Plume Recorders: MAPRs) above the field. We then made water column observations and samplings using a CTD (Conductivity, Temperature, and Depth sensors)-Carousel package with a transmissometer and Niskin-X bottles at five locations, with distances between 0.6 and 6.1 km from the hydrothermally active center so far discovered. Vertical profiles of light transmission and chemical tracers indicated triple-layered plumes, the centers of which were at depths of 700-800 m, ∼1,050 m and ∼1,200 m. The CH₄ concentrations and δ¹³C values for the 1,200-m plume ranged between 1,026 and 10 nmol/kg, and between -22.4 and +40.4‰ (the highest δ¹³C value yet reported for oceanic CH₄), respectively, indicating active microbial CH₄ oxidation accompanied by the δ¹³C increase for residual CH₄. The δ¹³C(CH₄) was shown to be useful for tracing such a “microbial plume” whose CH₄ concentration has already fallen to almost the background level. By applying the Rayleigh distillation equation for a closed system, we estimated the kinetic isotope fractionation factor of 1.012 for the CH₄ oxidation process occurring within the hydrothermal plumes deeper than 1,000 m.

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本論文では, 東アジア域内自由化, 特にカボタージュ権の緩和による社会厚生の変化および空港のサービスレベルの変化について検討を加えた.まずクールノー型量的競争を仮定した航空旅客輸送市場モデルを開発し, 均衡条件の導出を行った.次にモデルの再現性を確認し高い再現性を確認した.最後にカボタージュ権緩和をタグエンド・カボタージュ実施に限定し, 日本および香港を含む中国との間で行われた場合を想定した.このとき, 日本側ゲートウェイ空港を関西空港, 中国側を上海空港とした.結果として, 日本および中国国内市場に互いのナショナル.キャリアの参入が生じ, 社会厚生が向上することがわかつた.

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2004年までに日本の熱水噴出域および冷水湧出域から記録された軟体動物を総括し,リストを作成した。記録された74種(多板綱2種,二枚貝綱30種,腹足綱42種)のそれぞれについて,ホロタイプの所蔵先と登録番号,模式産地,地理的分布域・生息深度,生息環境を列挙した。他の環境・地域との比較により,日本の熱水噴出域および冷水湧出域に生息する軟体動物相には下記のような特徴があるといえる。(1)日本の周辺はシロウリガイ属の多様性が世界で最も高い海域である。(2)大型になる二枚貝類では,シンカイヒバリガイ属がその次に多様性が高い。(3)多くの種は熱水噴出域または冷水湧出域のどちらかに固有であるが,シロウリガイ属とシンカイヒバリガイ属には両者の環境に生息する種がある。(4)深海の軟体動物相を特徴づける原鰓亜綱はきわめて乏しく,異靭帯亜綱は全く記録されていない。(5)熱水噴出域または冷水湧出域に固有の笠型貝類の種が多い。(6)ネオンファルス科に分類されているカイコウケシッブシタダミを除き,ネオンファルス上科の種は分布していない。(7)新生腹足類ではハイカブリニナ属が多く,ハイカブリニナ科の大型種,アルビンガイ属(Alviniconcha)やヨモツヘグイニナ属(Ifremeria)は日本には分布していない。(8)タマガイ科の捕食者は化石記録では過去にシロウリガイ類を捕食していたことが知られているが,現生の群集では確認されていない。(9)新腹足目に属する肉食者の一部は,周囲の環境から一時的に化学合成群集に侵入する種である。(10)沖縄トラフから記載された多板綱2種は,熱水噴出域に生息する多板類の世界で唯一の記録である。(11)溝腹綱,尾腔綱,単板綱,掘足綱,頭足綱は海外の産地では報告例があるが,国内の産地からは発見されていない。

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Chemical and isotopic compositions of seeping gas from the coastal area of Yagaji Island were measured. The gases composed mainly of methane. The δ¹³C and δD of methane suggest that the origin of the methane is thermal decomposition of organic matter. The δ¹³C of methane and CH₄/(C₂H₆+C₃H₈) ratio in the gases suggest long distance migration from the sources. The ³He/⁴He ratios in the Yagaji gases suggest thermal decomposition of organic matter occurs in deeply-buried sediments. The Yagaji gases would be generated from marine organic matter in the Motobu group around 4 km below the ground.

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Chemical and isotopic compositions of hydrothermal fluids from Hatoma Knoll in the southern Okinawa Trough were investigated. The hydrothermal fluids were derived from a single pure hydrothermal fluid source, but they underwent phase separation beneath the seafloor prior to venting. Only vapor-like fluids vent at the Hatoma system, and the most active area is around the center of the crater, based on the location of the maximum temperature and the lowest Cl^– concentrations. Compared with other hydrothermal systems in the world, at Hatoma the pH and alkalinity, as well as the B, NH₄⁺, K, Li, CO₂, and CH₄ concentrations, were higher, and the Fe and Al concentrations were lower, suggesting that the characteristics of the Hatoma hydrothermal fluids are comparable to those of the other Okinawa Trough hydrothermal fluids. Helium isotope ratios were lower than those of sediment-starved hydrothermal systems, suggesting that ⁴He derived from the sediment is supplied to the hydrothermal fluids in Hatoma Knoll. The carbon isotope ratios of CO₂ in the hydrothermal fluids indicate an influence of organic carbon decomposition. The carbon isotope ratios of CH₄ in the hydrothermal fluids imply that most of the CH₄ originated from microbial methane produced in a recharge zone of the hydrothermal system. Although sediment influences are a typical feature of Okinawan Trough hydrothermal fluids, the Hatoma hydrothermal system has the lowest carbon isotope ratios of CH₄ among them, which suggests that Hatoma is the most highly influenced by the sediments in the recharge zone. Thus, the degree of the sediment influences has a variable in each hydrothermal field in the Okinawa Trough.

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In this paper I provide an overview of previous studies of crustal fluids around the Ryukyu Islands and a summary of the interpreted origin and implications of the methane present in those fluids. The chemical compositions of crustal fluids around the Ryukyu Islands fall into two groups: one group with methane contents as high as 80%, and another with methane contents of about 60%. Methane in crustal fluids can originate either from basement rocks or from organic-rich layers overlying basement rocks. If the basement rocks of the Ryukyu Islands are the source rock of the methane in crustal fluids, there might be huge accumulations of natural gas around the Ryukyu Islands, requiring information about the areal extent and depth of such accumulations to quantify possible gas resources. On the other hand, organic-rich layers overlying basement can be also the origin of the methane in hydrothermal fluids in the southern Okinawa Trough. Thus, understanding the distribution of crustal fluids on land can lead to a better understanding of the distribution of crustal fluids below the nearby deep sea floor. Increasing our knowledge of the origin and distribution of crustal fluids will improve our understanding of mass flux in the Earth's crust.

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