地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
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121 巻 , 3 号
選択された号の論文の14件中1~14を表示しています
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  • 121 巻 (2012) 3 号 p. Cover03_1-Cover03_2
    公開日: 2012/07/09
    ジャーナル フリー
     南極前線帯付近の暴風圏(吠える40度,狂う50度,叫ぶ60度とも表現される)を超えてさらに南下すると,夏には比較的穏やかな南極の海が広がっている.寄港地を出港してからはまったく陸地を見ることがない南大洋での調査航海ではあるが,時々巨大な氷山に遭遇する.とくに,南大洋インド洋区ではリュツォ・ホルム湾やプリッツ湾の近くを航行する際に多くの氷山と出会った.時には,観測している間に数頭のクジラが船の周りに集まってくることもある.写真は,2頭のクジラが氷山を背景に悠然と回遊する様を撮影したものだ.南大洋では,豊富な栄養塩を利用した高い基礎生産を土台として,クジラを頂点とする豊かな生態系ピラミッドが保たれている.
    (写真・解説 池原 実 2008年2月14日撮影)
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総説
  • 田林 雄, 山室 真澄
    121 巻 (2012) 3 号 p. 411-420
    公開日: 2012/07/09
    ジャーナル フリー
     Nitrogen compound deposition as a result of human activities on the terrestrial surface has increased sharply during the last one hundred years, causing ecological problems. One problem is nitrogen saturation in forest ecosystems, which results in the forest ecosystem receiving more nitrogen than required. In recent years, nitrogen oxide emissions have been decreasing in Japan; however, the total deposition of nitrogen is not decreasing because of cross-boundary emissions from other countries. As depositions in forest ecosystems increase, the likelihood of nitrogen saturation increases. At the same time, certain species of trees, geological conditions, and climate will affect nitrogen saturation. In Japan, there is heavy precipitation in summer; therefore, decreases in nitrate concentrations of stream water cannot be observed. Stable isotope analysis is the key to elucidating nitrogen dynamics between the atmosphere and rivers. From the atmosphere to stream water, nitrogen compounds have many chemical and biological reactions. Because a stable isotope is an indicator of the progress of these reactions, frequent analyses of stable isotope compositions reveals how nitrogen moves from the atmosphere to stream water.
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論説
  • 越谷 賢, 丸井 敦尚
    121 巻 (2012) 3 号 p. 421-440
    公開日: 2012/07/09
    ジャーナル フリー
     Three-dimensional earth sciences information is being made available in digital formats because such information is useful and understandable to various users. In this study, we develop a three-dimensional hydrogeological model of Tsukuba City and the surrounding region, and develop a method for estimating the hydraulic conductivity of aquifers in combination with the three-dimensional hydrogeological model and other related hydrogeological information with the primary objective of proposing a method for evaluating groundwater resources. The three-dimensional hydrogeological model was developed using a geological map and drilling data with data interpolated using a geostatistical method. The model developed reproduced the data used well, and it was found to be accurate for practical use. Moreover, this model could be used for various types of visualization, from which information could be extracted and applied immediately to numerical simulations; this provided information that was useful and understandable for evaluating groundwater resources. The proposed method for estimating the hydraulic conductivity of aquifers was developed from hydraulic conductivities obtained from various drilling surveys, which were grouped together taking geology and stratigraphy into consideration based on a three-dimensional hydrogeological model, and using a statistical analysis. This method can be used to rapidly evaluate trends in hydraulic conductivity by integrating data distributed over a broad area. In addition, if a columnar section of an evaluated point is obtained, this method can be used to determine the actual hydraulic conductivity of aquifers. Therefore, these methods were found to be effective for evaluating groundwater resources.
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  • 若林 徹, 須貝 俊彦, 笹尾 英嗣, 大上 隆史
    121 巻 (2012) 3 号 p. 441-459
    公開日: 2012/07/09
    ジャーナル フリー
     To elucidate the background value and distribution of natural heavy metals in Holocene alluvial sediments, we examined the heavy metal contents (MnO, Fe2O3, Ni, Cu, and Zn) of five sediment cores drilled at various locations on the Nobi Plain, central Japan using an XRF analysis. The amounts of heavy metals were in proportion both to the amounts of clay fraction and Al2O3 contents of sediments. Ni, Cu, and Zn contents were higher in the clay mineral fraction than in the other fraction when applying the elutriation method. This suggests that Ni, Cu, and Zn are adsorbed onto the clay minerals. MnO was especially enriched in several horizons. This might be affected by its oxidized-reduced state. In addition, there is little difference in the heavy metal contents of terrestrial and marine sediments.
     Most of the heavy metal contents of samples studied were lower than those of the modern riverbed sediments from Imai et al., (2004). High concentrations of Cu were observed in subsurface sediment. These anomalous high concentrations of Cu are presumably due mainly to anthropogenic pollutions.
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討論
  • 石渡 明, 辻森 樹
    121 巻 (2012) 3 号 p. 460-470
    公開日: 2012/07/09
    ジャーナル フリー
     Maruyama et al. (2011) noted; “Some researchers insist that the Dabie-Su Lu metamorphic belt, which was formed by the North China-South China collision, and the Sangun metamorphic belt of Japan were generated along the same trench, based on their similar radiometric ages (Ishiwatari and Tsujimori, 2003; Ernst et al., 2007). This idea is completely wrong.” Omori and Isozaki (2011) also criticized these papers as “a synthesis that misleads to cause confusion” and “a strange paleogeographic reconstruction” using the same logic. These criticisms are based on the idea that the continental collision-type (A-type) and oceanic subduction-type (B-type; also called Cordilleran-type or Pacific-type) orogenic belts are different and exclusive of each other. However, it is evident that both A- and B-type processes took place simultaneously along each of the Appalachian-Caledonian and Hercynian belts (Maruyama et al., 1996), and the Himalayan continental collision and Indonesian oceanic subduction are currently occurring along the same plate boundary. We provide an objective discussion to counter those criticisms, and point out some discrepancies in the large-scale superficial nappe model of Omori and Isozaki (2011) for the geology of Korea. We believe that our “Yaeyama promontory” hypothesis has inspired constructive international discussions about the configuration of the Earliest Mesozoic collision belt in East Asia, and we think our sinuous configuration model still persists.
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小特集:南極寒冷圏の古環境学 —南大洋から地球環境変動を探る—
総説
  • 本山 功
    121 巻 (2012) 3 号 p. 478-492
    公開日: 2012/07/09
    ジャーナル フリー
     The Benguela Current is an eastern boundary current in the South Atlantic subtropical gyre, associated with strong coastal upwelling off Namibia, which plays a major role in heat transport from the Indian Ocean to the Atlantic Ocean through the Agulhas Current and in the global carbon cycle through high biologic productivity. Seven sedimentary cores (Sites 1081–1087) recovered during the Ocean Drilling Program Leg 175 cruise from continental slopes consist of upper Cenozoic continuous siliceous and calcareous hemipelagic sequences, which allowed high-resolution paleoceanographic analyses over the upwelling region from 20°S to 30°S. Onboard and post-cruise scientific research has revealed and discussed the history of the oceanic system and the relation to global climate changes as follows. (1) Calcareous sequences (Sites 1085 and 1087) in the southern part of the region record sedimentary inprints that can be correlated with the Miocene global carbonate crash events and latest Miocene to early Pliocene biogenic bloom with the first signal of wind-driven upwelling at 11.2 Ma. (2) Off Namibia diatom concentrations dramatically increased after 3.1 Ma and reached a maximum spanning from 2.6 to 2.0 Ma, which was called the Matuyama Diatom Maximum associated with a moderate increase in organic matter accumulation and lowering of sea-surface temperature. This elevated bioproductivity and cooling occurred in response to changes in water circulation caused by gateway closures and enhanced bipolar glaciation. During the last 2 million years, the decreasing trend of diatom deposition coincided with an overall increase of coastal upwelling intensity. (3) The Walvis Opal Paradox is another prominent feature observed in orbitally controlled climate cycles in the upwelling system during the Quaternary. It is characterized by a decrease of diatom/opal deposition, which coincided with increased upwelling during glacial periods and vice versa during interglacials. Its possible causes include waning of North Atlantic Deep Water production during glacials. Despite these great advances in the reconstruction of the evolution of the Benguela Current upwelling system, causal links to global climate and regional events in other oceans are less well understood. To evaluate the interplay between opal/organic carbon deposition in the upwelling system and a series of climatic, tectonic, oceanographic, and biologic events in the world ocean, a better understanding of sedimentary processes on shelves and slopes in terms of glacio-eustatic sea level changes, improvement of paleoproductivity reconstructions, and reevaluation of dissolution of siliceous microfossil shells is needed.
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  • 鈴木 紀毅
    121 巻 (2012) 3 号 p. 493-517
    公開日: 2012/07/09
    ジャーナル フリー
     Paleoceanographic and tectonic studies of the uppermost Cretaceous to the Quaternary around the Tasmania region based on ODP Leg 189 were reviewed from approximate 70 papers. The Tasmania Land Bridge blocked the seawater connection between the Pacific and Indian oceans by 35.4 Ma (timescale of CK 95), although a very shallow water path was already present from approximate 40 Ma. Prior to the opening of the Tasmanian Gateway, the Tasmanian region was strongly influenced by the 3rd order of the global eustaic change. After the Tasmanian Gateway substantially opened at 35.4 Ma, the bottom current strengthened between 33.5 and 30.2 Ma, and this gate opened fully after 30.2 Ma when nannofossil-ooze with planktonic foraminifers started to accumulate on the seafloor around the region.
     The paleoceanographic and paleoclimatic conditions underwent a major change in the latest Cretaceous (Maastrichtian at least), the earliest Paleocene to the latest Eocene, the latest Eocene to the early Oligocene, the late Oligocene to the early Miocene, the early to the late Miocene, and the Quaternary.
     This region was influenced by global climatic and paleoceanographic changes. A significant deepening of the Tasmanian Gateway occurred 1.7 my earlier than the Eocene-Oligocene boundary, and the surface water temperature rose during the significant opening of the gate. In the late Eocene, the climate in the summer of the Southern Hemisphere was relatively cool based on floral changes, although this cool condition did not affect to the marine faunal ecology.
     Bottom water around the Tasmania region was common in this region by 28 Ma, whereas the sea-surface condition was different before 22.8 Ma. The full development of the Antarctic Circumpolar Current that flow over the seafloor was estimated at 23.95 Ma. Global paleoceanographic events such as the middle Miocene Climatic Optimum, Monterey Excursion, the middle Miocene climatic transition, and carbonate crash (=late Miocene carbon shift) were detected in this region.
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  • 池原 実
    121 巻 (2012) 3 号 p. 518-535
    公開日: 2012/07/09
    ジャーナル フリー
     The Southern Ocean plays an important role in the global climate system both at present and in the geologic past. To resolve the causes and processes of atmospheric CO2 change, it is important to understand the mechanisms and processes of sub-systems in the Antarctic Cryosphere such as change of biological productivity, sea-surface temperature, surface water frontal system, sea ice distribution, and the Antarctic Ice Sheet during the glacial-interglacial climate cycle. A large number of float observations made recently suggest that mid-depth Southern Ocean temperatures rose 0.17°C between the 1950s and 1980s. The Southern Ocean is warming faster than the global oceans, and this is concentrated within the Antarctic Circumpolar Current (ACC). Warming is consistent with a poleward shift of the ACC, probably driven by long-term poleward shifts in the winds of the region, as represented by the southern annular mode. Changes to the extent of Antarctic sea ice are difficult to quantify for the pre-satellite observation era. However, a substantially larger set of proxy records based on whaling positions indicates that a larger southward shift of the summer sea ice edge occurred between the mid-1950s and early 1970s. In the glacial to interglacial cycle, ice-rafted debris (IRD) is an important proxy for reconstructing past iceberg discharges and sea ice expansions. However, it is necessary to specify the origin of IRD in the Southern Ocean, because IRD deposition on the pelagic seafloor is controlled not only by the dynamics of the Antarctic ice sheet but also by surface water conditions such as sea-surface temperature and oceanic front migrations. For example, several layers rich in volcanic tephra were deposited in the eastern Atlantic sector of the Southern Ocean. Deposition of the tephra-rich IRD layers was controlled by changes in sea-surface temperature and sea ice conditions in the Polar Frontal Zone of the South Atlantic, rather than Antarctic ice sheet dynamics. Thus, IRD deposition is a signal of the expansion of sea ice in the South Atlantic. According to IRD records, it seems that sea ice expansion events occurred suddenly in the Atlantic sector of the Southern Ocean during the last glacial period.
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  • 香月 興太
    121 巻 (2012) 3 号 p. 536-554
    公開日: 2012/07/09
    ジャーナル フリー
     Paleontological research in the Southern Ocean based on quantitative studies of diatoms has progressed rapidly during the last 20 years, particularly on the Last Glacial Maximum (LGM) and Holocene paleoenvironment. This review of diatom paleoceanography in the Southern Ocean starts with reconstructions of the sea-ice distribution and productivity during the LGM. Maximum winter sea-ice distribution at the LGM (concentration > 15%) extended in the Atlantic and Indian sector close to 47°S, and in the Pacific sector as far north as 57°S. On the other hand, summer sea-ice distribution at the LGM was close to the modern summer sea-ice distribution except for the Weddell Sea extending as far as 52°S. However, LGM summer sea-ice information is still rather limited. Primary productivity in the Southern Ocean is closely related to the seasonal sea-ice zone. Increasing primary production under the seasonal sea-ice zone during the last glacial is linked to higher iron input. Sea-ice plays an important role as an effective transporter of dust-borne iron, and enhances its bioavailability. Termination periods of the surface condition in the Southern Ocean are described in relation to mechanisms of sea-ice distribution changes and their causes. Sea-ice retreat during the deglacial period and sea-ice advance during Neoglaciation in the Southern Ocean basically depended on solar insolation changes. However, the timings of sea-ice distribution changes were differentiated in each region. The last section, including short-term climate oscillations and their mechanisms, are described. Abrupt variations of Holocene diatom changes as 200–300 year cycle fluctuations indicate rapid climate changes of the Southern Ocean, which were very closely correlated to the solar activity cycle.
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論説
  • 原田 尚美, 川上 創, 福田 美保, 宮下 航, 松本 剛
    121 巻 (2012) 3 号 p. 555-569
    公開日: 2012/07/09
    ジャーナル フリー
     Throughout geologic time, variations of atmospheric CO2 partial pressures (pCO2) have been associated with climate change. During glacial periods, increased productivity and an efficient biological pump in the North Pacific, equatorial Pacific, and Southern Oceans may have contributed to low atmospheric pCO2. Furthermore, changes in global ocean ventilation would have been considered to be among the most effective controllers of variations of atmospheric pCO2 glacial-interglacial time scale. However, there is still some controversy as to whether ventilation was effective everywhere during glacial periods, and whether intensification of marine productivity resulted in a decrease of atmospheric pCO2. Resolving this controversy requires more data from many regions regarding temporal changes in past export fluxes of biogenic materials, especially in the upwelling area, where active ocean ventilation and biological production are observed. The aim of this study is to identify changes in the 230Th-normalized export flux of biogenic components commonly used as proxies for paleoproductivity—namely total organic carbon (TOC), calcium carbonate (CaCO3), biogenic opal (SiOPAL), and opal and CaCO3 ratio (SiOPAL/CaCO3)—as recorded in a sediment core from 36°S off the central–south Chilean coast. The 230Th-normalized fluxes of biogenic components were low from 22,000 to 15,000 calendar years before the present (cal yr BP), indicating reduced primary productivity. From 13,000 to 10,000 cal yr BP, the 230Th-normalized flux of biogenic components increased, thereafter dropping between 8000 and 5000 cal yr BP, and again increasing slightly in the late Holocene (< 5000 cal yr BP). The changing 230Th-normalized fluxes of biogenic components are linked to upwelling activity, which was at its maximum during the late deglaciation during the past 22,000 yr. The high fluxes off the central–south Chilean coast might in part be not only due to enhanced coastal upwelling but also related to enhanced upwelling at higher latitudes of the polar frontal zone in the entire Southern Ocean, which would be related to the north-south migration of Southern Westerly Winds.
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世界露頭百選(No. 8)
  • 沈 樹忠
    121 巻 (2012) 3 号 p. 570-578
    公開日: 2012/07/09
    ジャーナル フリー
     中国南東部,浙江省の煤山採石場には一連の古生代ペルム紀/中生代三畳紀の見事な連続層が露出している。この地層断面はペルム紀末の大量絶滅のパタンと原因を解明するため世界で最も詳しく研究されてきた。多産する化石の層序に加えて,凝灰岩の正確な年代測定や炭素同位体比層序が明らかにされている。中でも,国際地質科学連合で承認された2つの地層境界の世界模式標準層序セクション・地点を含むDセクションは,重要である。最近,煤山ジオパークがオープンした。
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