Provenance Of Clay Minerals In The Sediments From The Amami Sankaku Basin Since Miocene

Amami Sankaku Basin(ASB), located in northwestern Philippine Sea, is a major sink area of Asian dust. Therefore, Asian dust signal obtained from the sediment in the ASB is very important to trace the paleoclimatic and envrionmental evolution of the Asian continent. High quality sediment section, recovered from ASB during International Ocean Discovery Program(IODP) Expedition 351, provides a unique opportunity to study the paleoclimate of this area. In this paper, the clay minerals in hole U1438 A and U1438 B retrieved during Expedition 351, were systematically analyzed. The clay mineral assemblages, content, mineralogical and micromorphology characteristics were investigated with X-ray diffraction and scanning electron microscopy. The analysis results show that the clay minerals in the Amami Sankaku Basin are mainly composed of smectite, illite, chlorite and kaolinite.The smectite(average 55%) and illite(average 35%) are the major component, and chlorite(average 7%) and kaolinite(average 3%) are the minor component in the clay minerals. The chemical index of illite in most samples is less than 0.5, and with an average of 0.3. The illite crystallinity values vary between 0.18 and 0.43°Δ2θ, and with an average of 0.3°Δ2θ. The illite crystallinity values in most samples are less than 0.4°Δ2θ. Scanning electron microscopy(SEM) images show that the morphology of smectite is irregular, with flocculent, flower-shaped, curled flake, and illite and chlorite are irregular flake, with clear edges, round corners and abrasion marks.Based on comparison of clay mineral assemblages, crystallographic and microscopic morphological features between ASB and the potential sources area(e.g.Asian continent, Luzon islands, western Philippine Basin, Shikoku Basin), the provenance of the four clay minerals is analyzed. The high content and typical authigenic morphology(flower-shaped, flocculent) for semctite indicate that smectite was mainly derived from weathering and alteration of volcanic materials around ASB.The crystallinity and chemical index of illite indicates it was formed in a cold and dry environment dominated by physical weathering. The crystallinity of illite is similar with that of Chinese loess and paleosol. In addition, the morphology of illite is irregular flake with round corner and abrasion mark. All above evidences indicate that illite originated from Asian continent. The variation of chlorite and kaolinite since Miocene is similar with illite, and the three minerals show a high positive correlation.Therefore, we argue that chlorite and kaolinite are also derived from Asian continent.The variation of the content of the four clay minerals can be divided into three stages since Miocene, Stage 1(early Miocene, 23-16Ma), the clay mineral is dominantly composed of smectite(90%), with minor illte(8%). and chlorite and kaolinite are too low to be identified. Stage 2(middle to late Miocene, 16-5.3Ma), the content of smectite decreased to 61%, whereas illite(30%), chlorite and kaolinite(10%)increased. Stage 3(Pliocene to present, 5.3Ma-present), the content of illite(46%)increased significantly, and the content of chlorite and kaolinite also increased(up to13%), whereas the content of smectite(41%) decreased significantly.The significant variation of clay minerals in Amami Sankaku Basin since Miocene was resulted from the tectonic activities and climate change. In the early Miocene(23-16Ma), the variation of clay mineral is mainly influenced by the tectonic activities. Frequent volcanism related to the tectonic activities produce large amounts of vocanic materials. The alteration of the volcanic material resulted in the relatively high conent of semctite. Since middle Miocene(16Ma to present), global cooling forced the aridity of Asia continent, and intensification of atmosphere circulation,caused the increase of illite, chlorite and kaolinite. The variation of illite contentrevealed the strengthening of East Asian winter mosoon for three times(approximately 15 Ma, 8Ma and 3Ma) since Miocene. In late Pleistocene, high ratios of(illite+chlorite)/ smectite and(illite+chlorite)/ kaolinite during glacial period indicate that much more Asian dust was input into the Amami Sankaku Basin, which are responded to the aridity of Asian continent and strengthened east Asian Monsoon during glacial period.