Sediment distribution and depositional processes along the fluvial to marine transition zone of the Mekong River delta,Vietnam     

Marcello Gugliotta  Yoshiki Saito  Van Lap Nguyen  Thi Kim Oanh Ta  Toru Tamura 《Sedimentology》2019,66(1):146-164

The area of coastal rivers with a combination of fluvial, tidal and wave processes is defined as the fluvial to marine transition zone and can extend up to several hundreds of kilometres upstream of the river mouth. The aim of this study is to improve the understanding of sediment distribution and depositional processes along the fluvial to marine transition zone using a comprehensive dataset of channel bed sediment samples collected from the Mekong River delta. Six sediment types were identified and were interpreted to reflect the combined action of fluvial and marine processes. Based on sediment‐type associations, the Mekong fluvial to marine transition zone could be subdivided into an upstream tract and a downstream tract; the boundary between these two tracts is identified 80 to 100 km upstream of the river mouth. The upstream tract is characterized by gravelly sand and sand and occasional heterolithic rhythmites, suggesting bed‐load supply and deposition mainly controlled by fluvial processes with subordinate tidal influence. The downstream tract is characterized by heterolithic rhythmites with subordinate sand and mud, suggesting suspended‐load supply and deposition mainly controlled by tidal processes with subordinate fluvial influence. Sediment distributions during wet and dry seasons suggest significant seasonal changes in sediment dynamic and depositional processes along the fluvial to marine transition zone. The upstream tract shows strong fluvial depositional processes with subordinate tidal influence during the wet season and no deposition with weak fluvial and tidal processes during the dry season. The downstream tract shows strong coexisting fluvial and tidal depositional processes during the wet season and strong tidal depositional processes with negligible fluvial influence during the dry season. Turbidity maxima are present along the downstream tract of the fluvial to marine transition zone during both wet and dry seasons and are driven by a combination of fluvial, tidal and wave processes.  相似文献   

The missing half of the subduction factory: shipboard results from the Izu rear arc,IODP Expedition 350     

Cathy J. Busby  Yoshihiko Tamura  Peter Blum  Gilles Guèrin  Graham D. M. Andrews  Abigail K. Barker 《International Geology Review》2017,59(13):1677-1708

IODP Expedition 350 was the first to be drilled in the rear part of the Izu-Bonin, although several sites had been drilled in the arc axis to fore-arc region; the scientific objective was to understand the evolution of the Izu rear arc, by drilling a deep-water volcaniclastic section with a long temporal record (Site U1437). The Izu rear arc is dominated by a series of basaltic to dacitic seamount chains up to ~100-km long roughly perpendicular to the arc front. Dredge samples from these are geochemically distinct from arc front rocks, and drilling was undertaken to understand this arc asymmetry. Site U1437 lies in an ~20-km-wide basin between two rear arc seamount chains, ~90-km west of the arc front, and was drilled to 1804 m below the sea floor (mbsf) with excellent recovery. We expected to drill a volcaniclastic apron, but the section is much more mud-rich than expected (~60%), and the remaining fraction of the section is much finer-grained than predicted from its position within the Izu arc, composed half of ashes/tuffs, and half of lapilli tuffs of fine grain size (clasts <3 cm). Volcanic blocks (>6.4 cm) are only sparsely scattered through the lowermost 25% of the section, and only one igneous unit was encountered, a rhyolite peperite intrusion at ~1390 mbsf. The lowest biostratigaphic datum is at 867 mbsf (~6.5 Ma), the lowest palaeomagnetic datum is at ~1300 mbsf (~9 Ma), and the rhyolite peperite at ~1390 mbsf has yielded a U–Pb zircon concordia intercept age of (13.6 + 1.6/?1.7) Ma. Both arc front and rear arc sources contributed to the fine-grained (distal) tephras of the upper 1320 m, but the coarse-grained (proximal) volcaniclastics in the lowest 25% of the section are geochemically similar to the arc front, suggesting arc asymmetry is not recorded in rocks older than ~13 Ma.  相似文献   

Circumstellar matter in the symbiotic binary CH cyg during its current outburst     

A. Skopal  M. F. Bode  M. Bryce  J. Meaburn  S. Tamura 《Astrophysics and Space Science》1995,224(1-2):559-560

We present optical high-resolution spectroscopy and UBV photometry of the symbiotic binary CH Cyg during the current outburst which began in 1992 February.  相似文献   

Local structure of (Ca, Sr)2 (Mg, Co, Zn) Si2O7 melilite solid-solution with modulated structure     

Takuo Tamura  Akira Yoshiasa  Kazuaki Iishi  Setsuo Takeno  Hironobu Maeda  Shuichi Emura  Kichiro Koto 《Physics and Chemistry of Minerals》1996,23(2):81-88

The local structure around Co, Zn and Sr atoms in incommensurately modulated, melilite-type X₂T¹ T ₂ ² O₇ (X=Ca and Sr, T¹=Mg, Co and Zn, T²=Si) solid-solutions has been investigated by EXAFS analyses. The modulated structure was confirmed in Ca_2-xSr_xCoSi₂O₇ solid-solutions with X=0.0 to 0.6 and for both Ca₂Mg_1-YCo_YSi₂O₇ and Ca₂Mg_1-YZn_YSi₂O₇ solid-solutions over the whole compositional range at room temperature. The actual bond-distances determined by the EXAFS method for the T¹ site (Co-, Zn-O) in the modulated structure are longer than the mean bond-distances obtained from the X-ray diffraction method. This is attributable to the libration of the T¹ tetrahedra. In the Ca_1-XSr_XCoSi₂O₇ solid-solution both the Sr-O and Co-O distances by the EXAFS method for the X-site increase from Ca end-member to Sr end-member. These increases are respectively 0.8% and 0.6%. This means the local expansions of the tetrahedral sheets and of the XO polyhedra are well matched. In the modulated Ca₂Co_1-YMg_YSi₂O₇ and Ca₂Zn_1-YMg_YSi₂O₇ solid-solutions, the actual Co-O and Zn-O distances for the T¹-sites are nearly constant in the whole compositional range. The compositional variations of the local structure around the cations in the solid-solution are different for the X and T¹ sites. It is concluded that the local geometric restriction for the size of substituted cation in X site is larger than that in T¹ site. The dimension of the tetrahedral sheet puts restriction on the size of the cations situated at the interlayer X sites. In other words, the different behavior of the local geometric restriction between the X and T¹ sites is an important feature of the melilite structure and is also related to the modulated structure.  相似文献   

Medieval coastal inundation revealed by a sand layer on the Ita lowland adjacent to the Suruga Trough,central Japan     

Yuki Sawai  Koichiro Tanigawa  Toru Tamura  Yuichi Namegaya 《Natural Hazards》2016,80(1):505-519

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High-resolution imaging polarimetry of HL Tau and magnetic field structure     

P. W. Lucas  Misato Fukagawa  Motohide Tamura  A. F. Beckford  Yoichi Itoh  Koji Murakawa  Hiroshi Suto  Saeko S. Hayashi  Yumiko Oasa  Takahiro Naoi  Yoshiyuki Doi  Noboru Ebizuka  Norio Kaifu 《Monthly notices of the Royal Astronomical Society》2004,352(4):1347-1364

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Laboratory production of monophase pyrrhotite grains using solid-solid reaction and their characteristic infrared spectra     

Y. Kimura  K. Tamura  H. Chihara  C. Kaito 《Icarus》2005,177(1):280-285

A new method of producing pyrrhotite grains, which are most commonly found in cometary material and interplanetary dust particles, was developed. Pyrrhotite grains in the monophase having a 7C structure were predominately produced using a solid-solid reaction between iron and sulfur grains at room temperature. The characteristic infrared peaks were observed at 602, 563, and 397 cm⁻¹ (16.6, 17.8, and 25.2 μm).  相似文献   

Inaugural Address 25:1          下载免费PDF全文

Tetsuji Muto  Yoshihiko Tamura 《Island Arc》2016,25(1):3-3

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The discovery of an M4+T8.5 binary system     

Ben Burningham  D. J. Pinfield  S. K. Leggett  C. G. Tinney  M. C. Liu †  D. Homeier  A. A. West  A. Day-Jones  N. Huelamo  T. J. Dupuy  Z. Zhang  D. N. Murray  N. Lodieu  D. Barrado y Navascués  S. Folkes  M. C. Galvez-Ortiz  H. R. A. Jones  P. W. Lucas  M. Morales Calderon  M. Tamura 《Monthly notices of the Royal Astronomical Society》2009,395(3):1237-1248

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Spectrophotometric scan of comet 1973 XII Kohoutek     

Hisashi S. Ishii  Tetsuo Yamamoto  Shin'ichi Tamura 《Earth, Moon, and Planets》1981,25(4):437-450

Photoelectric spectrophotometric scans of Comet 1973 XII Kohoutek were made on November 25.85 and 28.85 UT, 1973. The vibrational temperature of C₂, the total numbers of CN and of C₂, and the production rates for CN and C₂ are derived from the observed fluxes. The temperature of C₂ was 4900 K on 25.85 November and 4300 K on 28.85 November. Mean abundance ratio of C₂ to CN was about 2.7. From the variation of production rates with heliocentric distance, it is shown that there was an outburst on 25.85 November in both CN and C₂ productions. For C₂, the production rates are derived by using two coma models, i.e. the Haser's parent-daughter model and the model relevant to a proposition of Yamamoto (1981b) that C₂ is formed via two-step photodissociation of its parent molecules. By comparing the production rate derived from the two models, it can be supported that C₂ is formed via two-step photodissociation rather than one-step photodissociation. In consequence, it is shown that the variation of the production rate with heliocentric distance is largely modified compared with that derived from the Haser's model.  相似文献