| 其他摘要 | The Australian strewn field, covering at least one-tenth of the Earth's surface, is the largest and youngest of the four known tektite strewn fields. Microtektites belonging to the Australasian strewn field have been found in more 40 deep-sea cores throughout much of the Indian Ocean, the western equatorial Pacific Ocean, and the Sulu, Celebes and Philippine Seas. The distribution of the Australian microtektites and impact debris shows a radial and somewhat concentric pattern toward Indochina, which is believed by most workers to be source area of the Australian microtektites. The South China Sea is the largest marginal sea in the western Pacific, located on the triple junction of Pacific Ocean, Eurasian and Indian Ocean plates. As a marginal sea, the South China Sea has higher sedimentation rates, and provides an ideal basis for high-resolution paleoceanographic changes. The late-Pleistocene strata keep a great deal of records about paleoceanography and paleoclimate in the northern margin of the South China Sea. The South China locates adjacent enough to the source region (12° N, 106° E), usually to be believed as ideal zone for integrated investigate of Australian microtektites. ODP Site 1144 is located 20° 3.18'N, 117° 25.14' E, recovered a l.lMa continuous sequence of high sedimentation rate hemipelagic sediments, the part of which contain a large number of microtektites. The core was sampled continuously through the microtektite layer every 1-2 cm. Many microtektites were collected from sediment samples at depths of 386.17-386.27mcd. More than 969 microtektites and 1543 fragments scatter across a 10cm interval were recovered from ODP Shell44.At the peak concentration centered at 386.23m, 194 microtektites and 239 fragments were counted. On the base on of collecting microtekties, we do some work about sampling planktonic foraminifer, which is G.sacculifer or Grubber. Based on geographic distribution in size, shape, petrography and composition of Australasian microtektites from ODP Site 1144, and C and O isotope of foraminifer, some new idea and conclusion have been obtained in this paper: In reporting on some of the physical and geochemical properties of Australasian microtektites, Previous works were developed and these microtektites have distinguished two groups, normal and bottle-green microtektites. Based on physical and physical and geochemical properties of miecrotektites sampled from ODP Sitell44, we suggest the microtektites should be falling into the normal group. The evidences of surface characters shown some solution processes in the duration of microtektites deposition, but we still do not completely understand the process of aerodynamics now. Like as the origins and formed processes of lightning fulgurites and impact glass, some properties in physical shape, size, main and trace element of microtektites from ODP Sitell44 imply the origin of microtektites seems to be only one possible explanation: partial melting. Form our data of major element, trace element, REE, we conclude that original materials of microtektites must the terrestrial rock, especially uppercrustal sedimentary rocks. The microtektites of ODP Sitell44 reveal signatures of two or more probably, three different precursory rocks. One of them is young and unleached clastic sediments, the other components are likely to be clays and/or sandstones. The result is a combination of only two-parent material, which may involve other rock. For example, dolomitite may be one of them. This conclusion shows that the complex nature of parent rocks of Australasian microtektites Based on the data derived from ODP Sitel 144 and other cores in the South China Sea and Australian strewn field, the present result confirms microtektite layer, which is located on high-resolution sediments, occurs below the Brunhes/Matuyama boundary. The Australian microtektites of the low sedimentation rate cores occur at or above the B/M boundary. The post deposition processes may affect both the mirotektites and B/M magnetic polarity records: bioturbation and post depositional magnetization (PDRM) acquisition. Maximum amplitude of bioturbation is thought to be about 8cm. Sedimentation rate strongly influences the depth of PDRM acquisition. PDRM acquisition in low sedimentation rate cores is very high, whereas 16cm PDRM depth is appropriate in high sedimentation rate sediments. Bioturbation and PDRM depth affect the stratigraphical position of microtektites and B/M boundary. Above reason may be attributed to error of the position of B/M, which probably leads to the complexity of microtektite deposition age. Geographic variations in the concentration of Australasian microtektites in 44 cores from the Indian Ocean, western equatorial Pacific Ocean, and the Philippine, Celebes, and Sulu Seas were used to predict the location of the Australasian tektite source crater and to estimate its size. Our research suggests some an uncertainty of previous predicted crater and size. Previous authors predicted that the source crater of the Australian strewn field is located in central Indochina. The close proximity of ODP Site 1144 to the predicted source crater as compared to previously examined cores in the Australian strewn field, implied that the microtektite concentration would be high. The high concentration and percentage of fragments at Site 1144, which is located in the northern part of the South China Sea, suggest that the source crater is Therefore, it is clear from the above discussion and previous works that an important area would be an option as more preciously study Australian microtektite event in South China, especially the South China Sea. We suggest new additional site data should be continued to obtain in the South China Sea so that farther integrated investigate are developed. Based on the consequences in groups, origins and climate effects of impact events of Australian microtektites and δ C & δO isotope of planktonic foraminifer from ODP Site 1144 in the South China Sea, these agreements have some beneficial factors or combination of factors to develop investigating forward for microtektites of the South China Sea including the whole Australian strewn field. In addition, it also has been provided a new approach to investigate microtektites in the future. Acknowledgements. This study is a contribution to the ODP program. At the same time, I am grateful for Dr. Zhao Quanhong, Dr. Jian Zhimin, Dr. Cheng Xinrong providing some samples. Especially I am indebted to and appreciate Academician Ouyang Zhiyuan, Dr. Li Chunlai, Dr. Liu Jianzhong, Dr. Wang Shijie, and Dr. Zhou Yongliao in three postgraduate duration. I appreciate also the helpfulness and generosity of Dr. Xu Tao, Dr. Zuo Wei, Dr. Liu Jianjun and other members of the Cosmochemistry Unit, National astronomical observatory, CAS. Additional generosity support was provided from the Education Office of Guiyang institute of Geochemistry, CAS. |
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