| The eolian dust from Asia continent is not only a major component of the deep sea sediments in the northwestern Pacific, but also an important proxyof palaeoclimate of Asian continent.Eolian dust in the deep-sea sediment of northwestern Pacific record the aridity history of dust source area(Asian continent) and the atmospheric circulation of East Asia, and plays a significant role in the evolution of the global climate and environment. Amami Sankaku Basin(ASB) is located in the northwestern Philippines Sea, with a relatively flat topography and lack of large-scale ocean currents. The sedimentary section is continuous since Miocene. Furthermore,ASB is influenced by the westerlies and the east Asian monsoon, therefore, ASB is an ideal place to research the eolian dust record and its palaeoclimate implications. The grain size composition in deep-sea sediments is an important indicator to tracing source and palaeoclimate.According to the characteristics of grain size frequency distribution, the component and provenance of eolian dust can be identified, and the grain size of eolian dust can be used to indicate the strength of wind.In this paper, the grain size composition of detrital sediments in hole U1438 A and hole U1438 B was analyzed. Both holes were collected from the ASBin the northwestern Philippine Sea during the International Ocean Discovery Program( IODP) Expedition 351. Different independent grain size components were separated by using Weibull function fitting, and the content of each grain-size copmonent is calculated, and the source of each grain-size component was analyzed. In addition, the environmentally sensitive size population in the detrital sedimentwas extracted by using the principle componenet analysis(PCA)and the grain size-standard deviation methods, and compared with the paleoclimate proxies. Based on above study, the input history of Asian dust to the ASB and the response of the different environmentally sensitive size populations to the Asian inland dry/wet condition and the atmospheric circulation model in tectonic and orbital time scale.The grain size analysis of detrital sediment in hole U1438 A and hole U1438 B showed that the grain size distribution display a four-peak pattern. In hole U1438 A,the mean grain size of the detrital sediment over the past 350 ka is approximately 13.1μm, ranging from 0.04 to 160 μm. The average contents of sand, silt and clay are approximately 1.5%, 74.7% and 23.8% respectively.Four independent grain-size components were separated by using Weibull distribution function:(1)the ultra-fine component varies from 0.04 to 0.9 μm, with a size mode at approximately 0.25 μm and occupy about 0.96%, which may be genetically related to marine authigenetic clay;(2) the fine-grained fraction ranges from 0.2 to 32 μm, with a size mode at approximately 3.5 μm,and with an average content of 29.2%. This fraction is mainly derived from Asian dust;(3) the coarse-grained fractions varies from 0.3 to 90 μm, with a size mode at about 10 μm, and with an average content of 54.4%; and(4) the ultra-coarse-grained fractions varies from 3 to 160 μm, with a size mode at about 40 μm, and witn an average content of 15.5%. Both the coarse and ultra-coarse components represent volcanic materials which are mainly derived from the ridges and islands around ASB. In hole U1438 B,the mean grain size of the detrital sediment over the past 25 Ma is approximately 14.8 μm, ranging from 0.04 to 160 μm. The contents of sand, silt and clay are approximately 3.3%, 72% and 24.7% respectively. Four independent grain size components in hole U1438 B were separated by using Weibull distribution function and the result is similar to hole U1438 A.(1) The ultra-fine component varis from 0.04 to 0.9 μm, with a size mode at about 0.25 μm, and with an average content of 1.1%, which was derived from the authigenic clay;(2) the fine-grained fraction ranges from 0.2 to 32 μm, with a size mode at about3.5 μm,and with an average content of 29.5%, which was mainly derived from Asian dust;(3) the coarse-grained fractions varies from 0.3 to 90 μm, with a size mode at about 10.3 μm, and an average content of 60.3%; and(4)the ultra-coarse-grained fractions varies from 3 to 160 μm, with a size mode at about 32.5 μm, and with an average of 9.1%. Both the coarse and ultra-coarse components represent volcanic materials which weremainly derived from the ridges and islands around ASB.Based on the variation of 1.8-6μm/14-22μm ratio and 0.9-3μm/>10μm ratio, and the content of fine-grained fraction in hole U1438 B, the input history of Asian dust to ASB can be divided into six stages:(1) 25.8~23Ma,the fine-grained dust was high, and the coarse-grained volcanic material was low.(2) 23-17.3Ma, fine-grained dust increased gradually, and coarse-grained volcanic materials decreased.(3)17.3-13.8Ma, fine-grained dust increased significantly, and volcanic material decreased.(4)13.8~9Ma, fine-grained dust decrease and volcanic material increase.(5) 9-2.5Ma,fine-grained dust increased significantly and volcanic materials decreased.(6) 2.5Ma to present, dust and volcanic material fluctuated significantly. On tectonic time scale, the variation of Asian dust respond to the enhanced aridity in the Asian continent and the strengthened atmospheric circulation forced by the uplift of Tibetan Plateau and the increase of global ice volume. On orbital time scale, the variation of the ratio of 1.3-2.2μm/28-40 μm and 0.9-3 μm/>10 μm in hole U1438 A show higher value during glacial period than that during interglacial, which is also identical with the variation of the mass accumulation of eolian dust in the North Pacific and Chinese Loess Plateau, and grain size in Chinese Loess Plateau. We argue that the increase of eolian fraction was driven by the enhanced aridity of Asian continent and strengthened East Asia Winter Monsoon(EAWM)/westerly during glacial period.Therefore, the ratio of 0.9-3 μm/>10 μm can be used as a proxy of the increased aridity and enhanced atmospheric circulation of Asian continent.These results suggest that the grain size composition of the detrial sediment in the ASB can be used to reconstruct the history of Asian aridity and atmospheric circulation since Miocene. |
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