第四紀研究
Online ISSN : 1881-8129
Print ISSN : 0418-2642
ISSN-L : 0418-2642
30 巻 , 4 号
選択された号の論文の6件中1~6を表示しています
  • 西田 史朗
    1991 年 30 巻 4 号 p. 239-250
    発行日: 1991/10/30
    公開日: 2009/08/21
    ジャーナル フリー
    Vertical variations of volcanic glass major element composition were examined on two representative volcanic ash layers of the Pleistocene Osaka Group, namely the Pink and Azuki Volcanic Ash Layers. Both layers are widely distributed in the Kinki district, well exposed in its surrounding hilly area, and are most reliable marker beds in their stratigraphy.
    The Pink Volcanic Ash Layer lies on the widespread Marine Clay 1 (Ma 1) horizon and is estimated at ca. 0.92 Ma in FT-age. The Azuki Volcanic Ash Layer is interbedded also in the transgressive Marine Clay 3 (Ma 3) and is measured at ca. 0.87 Ma in FT-age. At both sides of the Azuki we can recognize drastic climatic and vegitational changes in the fossil evidence.
    In this study major element composition of volcanic glass shards were analyzed with an energy-dispersive X-ray micro-spectrometry by grain-by-grain method. Analytical results were processed statistically and offered for further discussion.
    On the chemical stratigraphy, the Pink Volcanic Ash Layer shows a stable compositional succession in major elements through a whole layer, in spite of its clear lithological facies change, especially in grain size distribution and matrix clay color.
    The Azuki Volcanic Ash Layer shows two types of sedimentary facies. One is a direct fall and water-laid-type deposit, and the other is a water-transported-type deposit. Binocular observation shows that Azuki volcanic glasses are composed of two type of glass, colorless and colored. Color variation in the latter ranges from light brown to deep brown and sometimes opaque black. In this study black and opaque glass shards are rejected and restricted in transparent glass only.
    In Azuki Volcanic Ash glass, major element composition is stable in the colorless layer and changes drastically in the colored glass, about 10cm above the base. In addition, the colored Azuki Volcanic Ash Layer shows a gradual change in chemical composition within the layer, and chemical composition shows little difference in the two types of glass because of admixture and sorting during transportation.
  • 富樫 孝志, 松本 征夫
    1991 年 30 巻 4 号 p. 251-263
    発行日: 1991/10/30
    公開日: 2009/08/21
    ジャーナル フリー
    Sanukitic andesite, glassy andesite and obsidian are found from six sites, including Kozaki, Fujio-yama, Iwagami, Nagamasu, Minamikata and Amagoidai sites, in Yamaguchi prefecture; they range in age from palaeolithic to Jomon period. These volcanic rocks are carried from a distance. The authors undertook speculative research to determine the original localities of these rocks using three petrological methods: megascopic judgement, polarized microscopic judgement, and comparison of main chemical compositions by X-ray fluorescence analysis.
    With megascopic judgement, it was possible to identify the source of rocks only in some special cases. Examples are the greyish white obsidian from Kannon-zaki, Hime-shima island, Oita prefecture. This obsidian was found at six sites.
    Polarized microscopic judgement is an effective measure for the andesitic rocks.
    With its use, we identified sanukitic andesite from Hari-yama, Hiroshima prefecture, in five of the six sites (all except Amagoidai), another sanukitic andesite from Taku, Saga prefecture, in the Fujio-yama and Minamikata sites, and glassy andesite from Himeshima island, Oita prefecture, in all six sites.
    X-ray fluorescence analysis is a convenient method for identifying black obsidian. It enabled us to identify obsidian from Kumi, Oki-Dogo island, Shimane prefecture, in the Kozaki and Fujio-yama sites, another obsidian from Matsuura, Nagasaki prefecture, in the Iwagami site, and an obsidian from Koshi-dake, Saga prefecture, in all six sites.
  • 茅根 創
    1991 年 30 巻 4 号 p. 265-280
    発行日: 1991/10/30
    公開日: 2009/08/21
    ジャーナル フリー
    Five marine molluscan assemblages are identified in the present shallow sea bottom around the Futtu Spit on the west coast of the Boso Peninsula and in the Holocene marine sediments below the spit.
    (1) A tidal flat assemblage characterized by Crassostrea gigas, Batillaria multiformis and Cyclina sinensis is present in the sediments on Pleistocene bedrock. (2) An inner-bay mud-bottom assemblage composed of Raetellops pulchellus, Macoma tokyoensis and Dosinella penicilata is found in the mud sea bottom deeper than 12m around the spit and in the muddy sediments below the spit from 20m to 30m in depth, which filled the depression of the basement. (3) An inner-bay sand-bottom assemblage dominated by Musculus senhousia and Tapes philippinarum is distributed in the sand sea bottom from 0 to 12m in depth to the north of the spit. M. senhousia often forms mats and covers the sea bottom. (4) An inland-sea sand-bottom assemblage consists of Mactra chinensis, Phacosoma japonicum and other outer sea molluscans. It is present in the sand bottom with a depth shallower than 15m and in the sand sediments which form the spit. (5) An inland-sea sand-and-mud bottom assemblage composed of fragments of Kellia porculus and Tonna luteostoma is found only in the westernmost core below 10m.
    These molluscan assemblages change with change in the sedimentary facies. From 10, 000y. B. P. to 5, 000y. B. P., the sea level rose rapidly, and the Pleistocene bedrock was submerged. The tidal flat assemblage occurred on tidal flats overlain by mud, and the inner-bay mud-bottom assemblage was present in the mud sediments. From 5, 000y. B. P. to the present, the sea level has dropped slightly and the spit has prograded westward. The inland-sea sand-bottom assemblage was present in the sand sediments which form the spit. This assemblage is also found in the shallow water to the south of the spit. After the spit was formed, the inner-bay sand-bottom assemblage appeared in the shallow water to the north of the spit.
  • 沖津 進
    1991 年 30 巻 4 号 p. 281-290
    発行日: 1991/10/30
    公開日: 2009/08/21
    ジャーナル フリー
    The presence or absence of the Pinus pumila zone during the Last Glacial Age in Japan is discussed on the basis of the present distribution and growth of Pinus pumila, on the assumption that summer temperature declined and winter precipitation decreased. At present the height, coverage, mean annual stem elongation and production rate of Pinus pumila scrub correlate with each other. Expansion and reduction of Pinus pumila scrub in the Last Glacial Age can be inferred from those relationships.
    Mean annual stem elongation and production rate decreased with the reduction of summer temperature. Scrub height decreased with the decrease of snow depth around the stands. These facts suggest that the distribution of Pinus pumila scrub will be reduced when summer temperature declines and winter precipitation decreases. From this, it can be deduced that in the Last Glacial Age the distribution of Pinus pumila scrub is supposed to have strongly shrunk in the uppermost part of the mountains where the Pinus pumila zone is located at present; the Pinus pumila zone probably disappeared there. It may be possible that a Pinus pumila zone existed in area below the present-day lower limits of the periglacial landforms during the Last Glacial Age. However, the area is thought to have enjoyed a high summer temperature, which enabled the forests to be established there, as well as no serious deforestation agent. Thus the Pinus pumila zone was never established in this area as it was in the upper part of the high mountains.
    In conclusion, the vertical zonation of the upper part of the high mountains in the Last Glacial Age was Forest zone-Timberline ecotone-Real Alpine zone, in ascending order, while the present arrangement is Forest zone-Pinus pumila zone without no intermediate ecotone. Those two arrangements are quite different. The Pinus pumila zone finally appeared in the Post Glacial Age when summer temperature rose and winter precipitation increased.
  • 大村 明雄, 小林 由幸, 澤 祥
    1991 年 30 巻 4 号 p. 291-295
    発行日: 1991/10/30
    公開日: 2009/08/21
    ジャーナル フリー
  • 大河内 直彦, 平 朝彦
    1991 年 30 巻 4 号 p. 297-312
    発行日: 1991/10/30
    公開日: 2009/08/21
    ジャーナル フリー
    The environmental changes that occurred during deglaciation, revealed by deep sea sediments, are reviewed with special reference to deepwater circulation. New paleoceanographic tracers developed during the last decade show that global changes of deepwater properties occurred after the last glacial maximum. In the Atlantic, Cd/Ca and δ13C records show that the formation of North Atlantic Deep Water (NADW) decreased or stopped and that the Antarctic Ocean was the dominant source of deepwater in glacial time. The tracers also show that the deepwater was nutrient-rich and the intermediate water was nutrient-poor (bottom-heavy state) compared to the present condition. In the Pacific deepwater, paleochemistry has been less constrained due to paucity of data. Some δ13C records from the Pacific, however, were compiled, and they show that a “bottom-heavy state” may have occurred as in the Atlantic. More systematic investigations, including extensive coring programs, are required, especially in the southern ocean, in order to construct a global ocean model and to solve the mystery of the postglacial increase in atmospheric CO2 levels.
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