The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 116, Issue 2
Displaying 1-5 of 5 articles from this issue
Review
  • Takuya Sagawa
    2010 Volume 116 Issue 2 Pages 63-84
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    Planktonic foraminiferal Mg/Ca thermometry has been used in reconstructing past surface-ocean temperature from deep-sea sediments. Mg/Ca paleothermometry has the advantages of enabling reconstructions of past temperature with a precision of ±1° and reconstructions of salinity when combined with oxygen isotope measurements of foraminiferal shell. Previous paleoceanographic studies that employed this method have shed light on the important role of oceanic variability in global climate change at various time scales. However, the method has some limitations, including the fact that carbonate dissolution in the deep sea modifies the Mg/Ca value of foraminifera. The application of Mg/Ca thermometry to paleoceanographic studies, taking into account its basis and limitations, represents a powerful tool in terms of understanding the global climate system. This paper summarizes the principles, applications to paleoceanography, problems, and perspectives of Mg/Ca thermometry.
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Articles
  • Susumu Tanabe, Yoshiro Ishihara, Toshimichi Nakanishi
    2010 Volume 116 Issue 2 Pages 85-98
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    The latest Pleistocene to Holocene incised-valley fill (Alluvium) under the Tokyo and Nakagawa Lowlands has been divided into the underlying Nanagochi Formation and the overlying Yurakucho Formation since the 1960s. This division has been useful in the field of soil mechanics, because the Nanagochi Formation (deposited in a freshwater to brackish water setting) comprises stiff sediments with a high N value, whereas the Yurakucho Formation (deposited in a marine setting) comprises poorly consolidated sediments with a low N value. Based on the sedimentary facies identified in eight sediment cores recovered from these lowlands, in combination with radiocarbon age data, we divided the Alluvium into braided river, meandering river, estuary, spit, and delta systems, in ascending order. The fluvial and marine sedimentary systems yield high and low N values, respectively, and contain contrasting proportions of mud and water. Based on these results, we propose that the boundary between the Nanagochi and Yurakucho formations should be placed at the boundary between the meandering river and estuary systems. These systems are easily distinguished based on the presence of shells and bioturbation in the estuary
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  • Atsushi Noda, Seiichi Toshimitsu, Toshiyuki Kurihara, Hideki Iwano
    2010 Volume 116 Issue 2 Pages 99-113
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    The Izumi Group, which occurs from western Shikoku to the Izumi Mountains, Kii Peninsula, comprises sediments deposited in a strike-slip basin during the Late Cretaceous. Some differences exist in the lithostratigraphy and geological structure of the group where it occurs in western and eastern Shikoku; consequently, outcrops in central Shikoku may record evidence of the transition between the two areas. We conducted stratigraphical and geochronological surveys as part of compiling the “Niihama” geological map, and defined the following new formations: the Kussaki, Isoura, and Niihama formations. Fission-track dating of felsic tuff beds in the Niihama Formation yielded an age of 79.1±2.2 Ma. The radiolarian assemblages within mudstone of the Niihama Formation are DK assemblages, corresponding to the early-middle Campanian. The combined age data indicate a middle Campanian age for deposition of the Niihama Formation. The present results suggest that the Izumi Group was simultaneously deposited in western and central Shikoku. The lithology and geological structure in the study area are comparable with those in western and eastern Shikoku, indicating that the analyzed rocks represent strata deposited during a transitional stage of basin development and basin fill.
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Short Article
  • Tohru Yamanoi, Kiwako Saito, Takashi Matsubara, Kazuo Komori
    2010 Volume 116 Issue 2 Pages 114-117
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    Molluscan and plant fossils from ancient mangrove swamps are known from upper Lower to lower Middle Miocene sediments in Southwest Japan. Given that the mangrove-swamp-dwelling bivalve Geloina was recently found in the Tate Sandstone Member of the Kadonosawa Formation in the Ninohe district, northeastern Japan, it was expected that mangrove pollen would also be found at this site. To investigate this possibility, 26 samples were collected from the Tate Sandstone Member at two sites (Tate and Kitadate). The pollen composition at both sites is characteristic of the NP-2 zone of Neogene pollen zones (Yamanoi 1992a). Mangrove pollen (Kandelia sp.) fossils were first found in the uppermost part of the Tate Sandstone Member at the Kitadate site, representing the northernmost record of Miocene fossil mangrove in Japan. Based on present-day mangrove assemblages and distribution, the average wintertime temperature in the Ninohe area at the time represented by the pollen fossils is estimated to have been about 11°C higher than that of today.
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Report
  • Takayuki Uchino, Katsumi Ueno, Kiyoko Kuwahara
    2010 Volume 116 Issue 2 Pages 118-123
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    Radiolarian and fusuline fossils were discovered respectively from chert and limestone blocks in the Ashio Terrane of the Kambara Massif, Southwest Japan. Radiolarians in gray bedded chert indicate an age of late Middle Permian (Capitanian) to early Late Permian (Wuchiapingian), and those in black massive chert indicate a latest Permian (Changhsingian) age. This is the first report of Late Permian oceanic rocks in the Kambara Massif. Fusulines in a piece of limestone float indicate a latest Carboniferous (Kasimovian and late Gzhelian) age. This limestone is probably the same as that reported in a previous study, broadly inferred to be Late Carboniferous to Early Permian in age. Therefore, the age of the limestone is now well constrained. These fossil data are important in reconstructing the oceanic plate stratigraphy of the Ashio Terrane of the Kambara Massif.
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