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Volume 42 (2003)
- Issue 6 Pages 375-
- Issue 5 Pages 305-
- Issue 4 Pages 231-
- Issue 3 Pages 125-
- Issue 2 Pages 67-
- Issue 1 Pages 1-
Volume 42, Issue 2
Displaying 1-5 of 5 articles from this issue
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An Example from a Holocene Drowned Valley on the Southern Boso Peninsula, East JapanOsamu Fujiwara, Takanobu Kamataki, Toru Tamura2003 Volume 42 Issue 2 Pages 67-81
Published: April 01, 2003
Released on J-STAGE: August 21, 2009
JOURNAL FREE ACCESSTsunami deposits in an early Holocene drowned valley on the southern Boso Peninsula, Pacific coast of east Japan, comprise a stack of four units, Tna, Tnb, Tnc, and Tnd.
Unit Tna is composed of poorly sorted coarse to very coarse sand with high mud content, and erosionally it overlies the bay mud. This unit was deposited under the influence of landward paleocurrents in an upper flow regime, forming plane beds and antidunes.
Unit Tnb consists of a stack of several subunits and well reflects the tsunami waveform in detail. Each subunit, bounded by first-order erosion surfaces of hummocky cross-stratification, is attributed to a oscillating high-density current. The subunits are composed of lower inverse and upper normal graded intervals. Sorting becomes best in the middle horizon of the subunits. Mud content decreases upward in the inverse graded interval, and increases upward up to 20% in the normal graded interval. The inverse graded interval seems to be traction carpet and/or transport-lag. The normal graded interval suggests a long stagnant stage of current, which allows fallout of mud particles. Paleocurrents, reconstructed from the sedimentary structures, show that each subunit corresponds to the up-flow and/or return-flow.
The stack of subunits showing saw-toothed curve in grain-size parameters indicates the oscillation of high-energy currents with long period. This long period oscillating flow can be explained by a tsunami with a 10-minute order wave period. An extremely coarse-grained subunit intercalated in the middle horizon of unit Tnb can be explained as deposition from the largest wave of a tsunami wave train. Delayed arrival of the largest wave in a wave train is due to edge wave effect, indicating reflection on the continental shelf, and is a remarkable feature of tsunami that originate in deep ocean.
Unit Tnc also shows cyclic deposition of plant debris laminae and poorly sorted sandy silt layers, and is attributed to the relative small waves in the later stage of a tsunami.
Unit Tnd is composed of sandy silt layer including subsequent sinking of wood and plant debris after the tsunami.View full abstractDownload PDF (4378K) -
Study of Two Core Sediments Near Kitamiyamato Bank off Abashiri, HokkaidoNoriko Kawamura, Ken Ikehara, Hirokuni Oda, Masayuki Torii2003 Volume 42 Issue 2 Pages 83-97
Published: April 01, 2003
Released on J-STAGE: August 21, 2009
JOURNAL FREE ACCESSMarine sediment formed at a high sedimentation rate is distributed in the Okhotsk Sea, off Abashiri, Hokkaido. In order to make a detailed study of environmental changes in the area, we applied rock magnetic methods to two gravity cores covering 2, 700yrs (sediment rate about 150cm/yrs), GH00-1001 (water depth 461m) and GH00-1006 (water depth 1, 348m), recovered from the northern and southern basins near Kitamiyamato Bank. The dominant magnetic mineral in both cores is magnetite. A variety of rock magnetic parameters show three types of downcore variation, which are spiky anomalies suggesting the presence of volcanic ash and basaltic granules. Stepping down of the parameters indicates chemical change in a sedimentary environment, and the rather broad peaks suggest changes in the origin of detrital grains. Based on magnetic parameters, we also magnetically illustrated that increase in grain-size below -300cm (GH00-1001) and -150cm (GH00-1006) is a result of the dissolution of the fine part of magnetic grains in a reducing environment. Magnetic parameters are sensitive enough to pinpoint an episodic input of detrital material of different grain size and mineralogy at 275cm and 420cm (GH00-1001), which corresponds to the Kofun cold age (AD246-732).View full abstractDownload PDF (2119K) -
Akihisa Kitamura, Tomoki Kase, Shu-ichi Ohashi, Mayumi Hiramoto, Yosit ...2003 Volume 42 Issue 2 Pages 99-104
Published: April 01, 2003
Released on J-STAGE: August 21, 2009
JOURNAL FREE ACCESSThe closed and lightless conditions of submarine limestone caves prevent benthos having the potential to mix sediments from settling on the cave floor. Thus the sedimentary record may provide an excellent record of climatic and oceanic variability in a coral reef. In this study, we obtained a 43cm-long sediment core from a submarine cave (about 10m long, 20m deep) of Ie Island off Okinawa Island, and estimated deposition rates from the AMS 14C ages of molluscs. The sediment consists of massive calcareous silt and yields coccolithic, benthic, and planktonic foraminifer. The result of 14C dating indicates that the sediment core covers the past 1, 500yrs and the deposition rate ranges between 2cm/100yrs and 4cm/100yrs.View full abstractDownload PDF (804K) -
Akihiro Koizumi2003 Volume 42 Issue 2 Pages 105-111
Published: April 01, 2003
Released on J-STAGE: August 21, 2009
JOURNAL FREE ACCESSSkeletal material of a large-sized canid, Canis (Xenocyon) falconeri, was excavated from the mud sediments of the upper part of the Kasumi Formation (Plio-Pleistocene). The age of the second Horinouchi ash bed above the fossil bed is estimated as 1.6Ma. I estimate the age of this fossil at about 1.8Ma.
Although the skeletal elements were greatly disarticulated, it was possible to identify them as belonging to a single individual. The molar size of this canid is comparable with the extant northern races of Canis lupus. The carnassials have moderately hypercarnivorous characters in that the hypocone of the upper first molar is smaller than the protocone and is placed distally, and the entoconid of the lower first molar is very small but distinct. This Canid's upper fourth premolar is brachydont and the protocone relatively small. The lower third molar is present.
This is the first record of a fossil canid from the Japanese Plio-Pleistocene and is worth examining in Japanese Plio-Pleistocene vertebrate fauna.View full abstractDownload PDF (1500K) -
Estimation by Soil Color Reflectance Indices (L*, a*, and b*)Fucheng Qi, Kunihiko Endo, Xiangmin Zheng, Liming Zhou2003 Volume 42 Issue 2 Pages 113-119
Published: April 01, 2003
Released on J-STAGE: August 21, 2009
JOURNAL FREE ACCESSBased on a multi-proxy record of the loess-paleosol sequences, we studied the millennial climate change and environmental variation in Shengshan Island in the East China Sea for the last 50, 000 years. The proxy color reflectance indices (L*, a*, and b*) of the loess-paleosol sequence in Shengshan show a series of millennial oscillations. The relationship between the standard curve of δ18O in GRIP and sequential changes of L*, a*, and b* indices in the loess-paleosol section of Shengshan during last 50, 000 years showed that L*, a*, and b* are sensitive indicators for climatic change in southeastern China. Patterns of magnetic susceptibility change were very similar to those of L*, a*, and b*. Soil color reflectance indices are evidently very available as a proxy record for identification of loess-paleosol transition, like magnetic susceptibility. Based on soil color proxy, this study identified horizons of Heinrich events (H1 to H5) and interstadial peaks (IS1, 2, 4, 8, and 12) in the loess-paleosol sequence at Shengshan. The Shengshan loess section is a typical loess-paleosol sequence recording climatic oscillations for the last 50, 000 years in the East China Sea. Due to its special location, the section provided new evidence that climatic change indicated by dust sediments in the Loess Plateau is comparable to change in the Korean Peninsula and the Japanese Islands.View full abstractDownload PDF (902K)
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