海洋理工学会誌
Online ISSN : 2188-3262
Print ISSN : 1341-2752
最新号
選択された号の論文の4件中1~4を表示しています
原著
  • 田崎 智晶, 市川 哲也, 赤司 有三, 加藤 敏朗, 宮向 智興, 鈴木 輝明, 高倍 昭洋
    2018 年 24 巻 1 号 p. 1-24
    発行日: 2018年
    公開日: 2018/08/30
    ジャーナル フリー

    Izonoura — located in the inner area of Matoya bay in Shima City, Mie Prefecture — prospered in the aquaculture of green laver (Monostroma nitidum). However, the production decreased rapidly after the 1980s, and the fishermen have demanded an investigation into its cause. We (Tasaki. et al., 2017) conducted continuous observations of several environmental elements and weather throughout the spring and summer in 2014, which revealed that the cause of the decrease in laver production amount was turbidity and clarified the generation mechanism of high turbidity. As a result, lowering of river discharge that was caused by the past dam construction in the basin is one of the reasons for the bottom sediment condition (Tasaki et al., 2017). In this study, we constructed SS-Diffusion models to reproduce the behavior of the SS observation numerically, and then examined some improvement cases to reduce turbidity in Izonoura. As a result, a decrease in the river flow quantity with past dam construction promoted SS habitation in Izonoura and confirmed that it was a factor in the quality of bottom aggravation. However, as a technique of environmental improvement, the simple recovery of the river flow quantity under the present situation promoted resuspension for the existing sedimentation and promoted environmental degradation. Therefore, having greatly restrained resuspension by the sediment removal of the old route and improving the bottom sediment quality in Izonoura, and then restoring river flow quantity, it was considered possible to restart Monostroma nitidum culture and autonomously improve the turbidity of Izonoura.

  • 奥村 真子, 三木 理, 上野 聡浩, 小杉 知佳, 吉村 航, 加藤 敏朗
    2018 年 24 巻 1 号 p. 25-33
    発行日: 2018年
    公開日: 2018/08/30
    ジャーナル フリー

    The present study is an attempt for assessing the relationship between iron deficiency and discoloration in Undaria pinnatifida sporophytes. First, thallus discoloration and the decrease in photosynthetic pigment contents were examined in Undaria pinnatifida by employing an indoor iron-deficiency experiment. When Undaria pinnatifida sporophytes were cultured under iron-deficient conditions, which the iron concentrations were between 2 and 5 μg/L, the growth decreased, the thalli became discolored, and the photosynthetic pigment contents decreased. Iron deficiency caused the sporophytes to become lighter (L*) and less red (a*). Therefore, it appears that iron deficiency induces the discoloration of Undaria pinnatifida sporophytes, similar to how nitrogen and phosphorus deficiencies perform. Next, we investigated how the mechanism involved in the addition of iron affects the discolored Undaria pinnatifida sporophytes due to iron deficiency by performing an indoor iron-addition experiment. Undaria pinnatifida sporophyte growth, thallus color, and photosynthetic pigment content were all restored by the addition of dissolved iron solution, which the concentrations were between 10 and 50 μg/L. Iron addition made the sporophytes recover their lightness (L*) and redness (a*). In addition, the color was restored more quickly as the amount of added iron was increased. The discoloration of iron-deficient Undaria pinnatifida sporophytes can thus be restored by iron addition.

寄稿
  • 前野 深, 安田 敦, 中野 俊, 吉本 充宏, 大湊 隆雄, 渡邉 篤志, 金子 隆之, 中田 節也, 武尾 実
    2018 年 24 巻 1 号 p. 35-44
    発行日: 2018年
    公開日: 2018/08/30
    ジャーナル フリー

    The island-forming eruption at Nishinoshima volcano, Ogasawara Islands, Japan, began its activity in November 2013 and once ended in late 2015 (1st stage). The eruption restarted in April 2017 and finally the activity ceased in August 2017 (2nd stage). In this paper we discuss the sequence of the eruption, and characteristics of magma, based on geological and petrological analyses. The eruptions were characterized by continuous Strombolian activity with lava effusion in both 1st and 2nd stages. One of the most intriguing characteristics of the products is a vast lava flow field, consisting of multiple lava lobes with clefts. The clefts are thought to be the products from lava inflation driven by an increase of internal pressure by successive injection of new lava into the lobes. During the landing survey in October 2016, we collected samples from lavas and fallout deposits. The petrological analyses were carried out together with other rock samples collected by different surveys at Nishinoshima. The 2013–2015 lava flows were andesite with <10 vol.% of phenocrysts. The whole-rock composition is 59.5–59.9 wt.% in SiO2. The petrological features of the 2013–2015 lava flows are similar to those of products from the past eruptions; however, the whole-rock compositions are clearly distinguished from the 1973–1974 products and the pre-1702 products, and lies on the narrow range between these two products. Magma temperature was estimated to be 1050°C using a pyroxene thermometry. Viscosity was estimated to be ~104–106 Pa · s based on petrological data. The variation of phenocryst compositions suggests that andesite magma derived from a crystal-rich shallower reservoir and a crystal-poor deeper reservoir. The depth of shallower magma reservoir was estimated to be 1.5–2 km equivalent to water saturation pressure, which is estimated based on melt inclusion analysis and thermodynamic calculations.

  • 武尾 実, 大湊 隆雄, 前野 深, 篠原 雅尚, 馬場 聖至, 渡邉 篤志, 市原 美恵, 西田 究, 金子 隆之, 安田 敦, 杉岡 裕 ...
    2018 年 24 巻 1 号 p. 45-56
    発行日: 2018年
    公開日: 2018/08/30
    ジャーナル フリー

    Nishinoshima is an andesitic stratovolcano located in Ogasawara Islands, Japan. In November 2013, island-forming eruption started. Before the eruption, Nishinoshima was a small island of the area of 0.29 km2 and elevation of 25 m but it had a huge edifice rising 3,000 m from the sea floor. By March 2016, area and elevation reached 2.7 km2 and 140 m, respectively. We conducted various types of geophysical observations at this “difficult-to-access island” (950 km from Tokyo taking 90 min by Jet plane, or 24 h by ship). In June 2016, we conducted airborne observations using unmanned helicopter, collecting 250 grams of scoria and detailed 4K images of lava flows. OBSs (Ocean Bottom Seismometers) were deployed around Nishinoshima in four periods. From February 2015 to May 2017, characteristic waveforms dominated at 4–8 Hz band were frequently observed. Comparisons with infrasonic records and video images revealed that the 4–8 Hz seismic signals were associated with eruptions at pyroclastic cone. The number of seismic signals of this type declined from July 2015, and disappeared in November 2015, suggesting that the eruptive activity started declining in July 2015 and ceased in the middle of November 2015. In October 2016, we landed and deployed a broadband seismometer and an infrasonic sensor in the old Nishinoshima, collecting a lot of new lava, deposits, and ash samples. We demonstrated a capacity of remote-island volcano monitoring system for one day test navigation circling around Nishinoshima. After one and a half year quiescence, a new eruptive phase started in April, 2017. Our on-land seismic sensor detected precursory signals as early as April 17. The seismometer also recorded characteristic waveforms during the very early stage of the new eruption phase before data transmission was terminated on April 21.

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