The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 118 , Issue 3
Showing 1-4 articles out of 4 articles from the selected issue
Articles
  • Kyoko S. Kataoka, Yoshitaka Nagahashi
    2012 Volume 118 Issue 3 Pages 139-156
    Published: March 15, 2012
    Released: August 03, 2012
    JOURNALS FREE ACCESS
    Few of the Japanese Pliocene to early Pleistocene tephra beds have been correlated to their source volcanoes, mainly because of the rapid degradation of proximal ejecta after eruptions. The present paper discusses the source areas of non-marine, distal tephra beds intercalated in the Plio-Pleistocene Second Setouchi Supergroup, central Japan, by focusing on the facies and thickness variations of both pyroclastic fall deposits and fluvially reworked deposits. Many of the examined tephra beds have thick (1—10 m) reworked/resedimented portions, indicating the transportation and deposition of tephra material by fluvial activity—a significant process during tephra bed formation. The tephra beds are divided into the following types based on the ratio of the thickness of the primary fall part to that of the reworked part: A-type, tephra consisting mainly of reworked part; B-type, reworked part dominant; C-type, fall part dominant; and D-type, tephra consisting mainly of fall part. The presence of pumice pebbles and cobbles in an A-type bed suggests the tephra was derived mainly from the resedimentation of ignimbrite emplaced by the nearest source volcano. The C- and D-type beds indicate eruptive activity at distant volcanic areas, with the consequence that the alluvial area received ashfall only. The present results indicate that 15 of the examined tephra beds were derived from large ignimbrites, and 5 from southern Kyushu or Tohoku (mainly distal ashfall). Thus, the sedimentology of distal resedimented tephra beds provides important clues (although indirect) to the source area, eruptive style, history, magnitude, and paleogeographical background of ancient tephras and eruptions.
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  • Akihiro Kano
    2012 Volume 118 Issue 3 Pages 157-171
    Published: March 15, 2012
    Released: August 03, 2012
    JOURNALS FREE ACCESS
    Stalagmites are important archives of the terrestrial paleoclimate during the late Pleistocene and Holocene, and have the advantage of precise dating by U-series methods and lamina counting. In addition, fast-track analysis of milligram-sized carbonate subsamples allows for very high time resolution. Oxygen isotopic age-profiles of stalagmites from caves in south China have provided standard paleoclimatic records that have revealed temporal changes in the intensity of the summer Asian Monsoon. However, quantitative evaluations of temperature and rainfall from stalagmite records are difficult because the stable isotope values and trace element contents in the carbonate fraction are controlled by complex interactions among processes in the atmosphere, in the soil layer, and in the cave. The stalagmite oxygen isotope ratio varies with rainfall intensity, rainfall seasonality, temperature, and evaporation of water, while the carbon isotope ratio reflects the drip rate of water onto the stalagmite, vegetation type, and soil moisture. Another important effect is prior calcite precipitation (i.e., the degree of calcite precipitation from the water before dripping onto a stalagmite), as this factor has a strong influence on the carbon isotope ratio and trace element contents. Despite the complexity of the controlling processes, stalagmite age-profiles from different regions commonly show synchronous changes in isotopic and chemical proxies, especially for certain climatic events such as the B?lling warming. These findings support the existence of a climatic teleconnection in the Northern Hemisphere. Importantly, the age-profiles of isotopic and chemical proxies contain paleoclimatic information. Novel approaches in the field of stalagmite paleoclimatology will enable separation of the effects of temperature and rainfall amount on oxygen isotope values.
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  • Takeshi Yoshida, Osamu Kazaoka, Mio Takeuchi, Takashi Kusuda, Kunio Fu ...
    2012 Volume 118 Issue 3 Pages 172-183
    Published: March 15, 2012
    Released: August 03, 2012
    JOURNALS FREE ACCESS
    In the Kujukuri-hama area of Chiba, Japan, Holocene series unconformably overlie the Pleistocene Kazusa Group, which was deposited mainly in a deep-sea environment and contains natural gas (methane) dissolved in water. These conditions allow gas to emerge from the ground as it moves upward through faults and sandy layers.
    In May and August 2007, water in tidal pools at central and southern Kujukuri-hama became cloudy. A field survey revealed that methane gas was seeping into the pools where the cloudiness occurred and that bluish gray sand, the color of which indicates reducing conditions, occurs within the area of the gas seep. Yellow groundwater was found immediately beneath the area of bluish gray sand, and the groundwater became cloudy as it emerged into the tide pools.
    Analyses of the pool water revealed a state of sulfate reduction (anoxic diagenesis) and that the cloudiness resulted from the presence of elemental sulfur. Therefore, the water in the tidal pools became yellow or cloudy in the presence of polysulfide ions or due to irregular reflections of sunlight induced by colloidal sulfur.
    The 16S rRNA genes of anaerobic methanotrophic archaea belonging to ANME-1 were detected from yellow groundwater near the surface of a sandy beach, raising the possibility that anaerobic oxidation was involved in the reduction of the sulfate ions. Based on these findings, it is proposed that ANME-1 are active within methane gas seeping from the Kazusa Group to the surface, resulting in reducing conditions.
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Note
  • Toshiyuki Yoshikawa
    2012 Volume 118 Issue 3 Pages 184-189
    Published: March 15, 2012
    Released: August 03, 2012
    JOURNALS FREE ACCESS
    Today's portable computing devices are suitable for geological field surveys because digital field data are easily stored and shared. However, most specialized systems for this purpose are expensive and require specific software and/or hardware. Widespread use of such systems would require readily available hardware and software. As a case study, a portable computer and software for general use were tested in a geological field survey, yielding the following results. The advantages of using a digital device were text input using a clipboard extension utility, sketching over a digital photograph, GPS assistance, and the portability of reference data. Disadvantages include the lower quality of route maps, difficulty in sketching by hand, limited battery life, and the need to protect the device from water and dust. Despite these limitations, the use of portable computing devices is already advantageous in some cases and has the potential for greater convenience during field work.
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