The Indication Of Aeolian Quartz In The Amami Sankaku Basin To The Tectonic-scale Paleoclimate Changes Of East Asia Since Miocene

The northwestern Pacific is located in the downwind direction of the westerly and the East Asian winter monsoon,and it is an important‘sink’area of Asian dust.The aeolian component in the deep sea sediment record tectonic-scale paleoclimate change of East Asia.The Amami Sankaku Basin is an abyssal oceanic basin and located in the northwestern Pacific.International Ocean Discovery Program（IODP）Expedition 351 retrieved a long and continuous advanced piston core（Hole U1438）in the basin,which provide a good opportunity to address research on the paleoclimate change of East Asia since Miocene.Quartz is an important component of sediment,and has relatively stable physical and chemical characteristic,so it is a reliable proxy of sediment source and paleoclimate change.Quartz is ubiquitous in hole U1438.In order to identify the source of quartz in the hole,the regional characteristics and source of quartz in the Philippine Sea are studied.The author select some typical surface and core-top sediments in sites near the Luzon Island,on the Benham Rise and deep water basin in the western Philippine Sea,and near the Kyushu Palau Ridge and in the Parece Vela Basin in the eastern Philippines Sea.The quartz in the sediments was seperated with a sequential physical and chemichal leaching procedure,and then the grain size,morphology,δ¹⁸O values and crystallinity index of quartz were analysized.The results show that the quartz in the Philippine Sea is composed by three endmembers:the endmember 1（EM1,average mode at 2.9μm）,endmember 2（EM2,average modeat 8.2μm）and endmember 3（EM3,average modeat 21.7μm）.EM1 quartz has typical aeolian quartz morphology,with higherδ¹⁸O values and lower crystallinity index,the author argue that EM1 quartz is mainly derived from the Taklamakan desert.The EM2 quartz is angular with corrosion marks on the surface,and EM3 is also angular but with little corrosion.The EM2 and EM3 quartz has lowerδ¹⁸O values and higher crystallinity index.The author argue that the EM2 and EM3 quartz are from volcanic islands.So the EM1 quartz in the Philippine Sea can be used as proxy of paleoclimate in the East Asia.The grain size,morphology,δ¹⁸O values and crystallinity index of the quartz in hole U1438 were analysed.The grain size distribution of quartz are bimodal,trimodal and four model characteristics.The Weibull distribution function is used to separate different endmember:the grain size of EM1 quartz,vary from 0.04μm to 21.18μm,with a mode at³.11μm,the grain size of EM2 quartz range from 0.04μm to 65.34μm,with a mode at⁹.15μm.The EM1 and EM2 quartz are subangular/subrounded,with collision pits and upturned plate on the surface,which is the typical characteristics of aeolian quartz.EM1 and EM2 quartz have higherδ¹⁸O values and lower crystallinity index.In the plot ofδ¹⁸O values against crystallinity index of quartz,EM1 quartz is on the mixing line of volcanic island and Taklimakan Desert,while EM2 quartz is on the mixing line of volcanic island and Mongolia Gobi.Therefore,the author argue that EM1 quartz is transported from Taklimakan Desert by prevailing westerly,and EM2quartz is transported from Mongolia Gobi by winter monsoon.The grain size distribution of EM3 and EM4 quartz are wider with mode at²7.76μm and>50μm,respectively.The EM3 and EM4 quartz are angular,and with smooth surface or dissolution pits in some samples.The lowδ¹⁸O values and high crystallinity index for EM3 and EM4 quartz are similar with those in northwestern Pacific islands,which indicate they are derived from the volcanic islands.The vertical variation of aeolian quartz content,flux,and EM1 and EM2 quartz flux are similar and can be divided into four stages:（1）23-15 Myr,they are very low and keep stable,（2）15-7.6 Myr,they gradually increase at approximately 15 Myr,and have a peak at 10-11 Myr,（3）7.6-3.6 Myr,significant increase occurred during this period,and peak at 5.3 Myr,（4）3.6 Myr to present,significantly increase occurred at2.5 Myr and 1.2 Myr.The variation of quartz flux indicate an enhanced aridificaiton in Taklimakan Desert and Mongolia Gobi since 15 Myr,especially at Pliocene and Quaternary,which is similar with the dust flux variation of North Pacific,and coupling with the global cooling trend recorded in the deep sea oxygen isotopes,but inconsistent with uplift of Tibetan Plateau.Therefore,the author argue that the aridification of East Asia continent is mainly controlled by global cooling.The variation of mode grain size of EM1 and EM2 quartz of hole U1438 indicate the intensity variation of the westerly and winter monsoon,respectively.The westerly and winter monsoon strengthened since Miocene and can be divided into four stages:23-15 Myr,15-7.6 Myr,7.6-3.6 Myr and 3.6 Myr to present.There isn’t significant variation in the intensity of westerly and winter monsoon before 7.6 Myr,indicating a weak influence of global cooling and Tibetan Plateau on the atmospheric circulation.At 7.6 Myr,the increased EM1 and EM2 quartz modal grain size indicate the strengthening of westerly wind and winter monsoon.After 3.6 Myr,the stable of EM1quartz modal grain size indicates that the intensity of westerly wind is stable,while the rapid increase of EM2 quartz modal grain size indicates that the intensity of winter monsoon increase.The author argue that global cooling and Tibetan Plateau may be two potential mechanisms that drive the strengthening of westerly wind and winter monsoon.