The Study Of Asian Eolian Dust Record In Chinese Loess And Pacific Sediments

Eolian dust is an important part in the Earth System Science.Dust transmission and deposition processes have a crucial influence on rainfall,climate and marine ecosystems.Asian arid interior is the second largest dust emitted center all over the world.The eolian dust emitted from Asian interior formed the thick Chinese loess plateau in a short distance.The dust can reach the North Pacific,the Greenland ice core and even the Alps in the European after a long distance.Dust releasing mechanism would be resolved by delineating dust sources and tracking the history of the evolution of dust sources at different time-scales.It is also an important reference for modern sandstorm forecast and management.Both the Chinese Loess Plateau(CLP)and north Pacific oceans are the key deposition regions of Asia dust.Paleoclimate proxies were interpreted mostly in case of the loess source stability,and thus desiphering the provenance of Asian dust is becoming particularly critical.However,great controversies still exist in loess source research in recent years.The study of potential source areas,especially the Gobi Desert,was insufficient;the specific source of Asian dust is still not clear and its evolution could not come to agreement,either.The eolian dust sources archived in Pacific sediments are not very clear at both tectonic and orbital time scales.In addition,the study about the loess deposits and Pacific sediments lack the direct contact with each other.This article firstly conducts supplemental geochemical characteristics of potential source areas,attempting to further delineate Asian dust Endmember features.Then,the provenance evolution history of Chinese loess since 2.7 Ma at tectonic and orbital sclaes was studied,using the Nd and Sr isotopes of a restricted grain size silicate fraction of Xifeng loess.We studied source evolutions of north Pacific sediments since late Oligocene at tectonic scale and also the evolutions during the Plio-Pleistocene transition and last five glacial-interglacial cycles at orbital time scales.Finally,we comprehensively discussed the discrepancies of source evolutions recorded in loess and Pacific sediments.In order to further understand the geochemical characteristics of potential source areas,the trace elements concentrations of both the<75 ?m and<5 ?m silicate fractions from almost all potential deserts were studied.We selected the source indicators with no obvious grain size effect,small changes inside one single source region but variable changes among the different source regions.In addition,the study mainly emphasizes the detrital zircon U-Pb age distribution and Nd-Sr isotopic compositions from the Gobi Desert samples.Combined with previous studies,the results show that the characteristics of detrital zircon age assemblages and Nd-Sr isotope of Chinese loess are similar to that of North Tibetan Plateau(NTP).But they could not be entirely explained only by the materials from NTP.We propose that Gobi desert is another major source.Asian dust has two-endmember features,namely,the NTP and the Gobi Altay mountains(GAMs).The Nd and Sr isotope of silicate fractions with limited grain size(28-45 ?m)was measured to study the source evolution of Chinese loess since 2.7 Ma at both tectonic and orbital time scales.This kind of restricted fraction could ger rid of the influence of grain size and wind sorting on Nd and Sr isotopes.The results indicated no obvious provenance change during the glacial-interglacial cycles,but the source was gradually evolved from the NTP to the GAMs since 2.7 Ma at tectonic scales.Both tectonic activities and Palaeoclimate change would have controlled the source shift.With respect to tectonic uplift of NTP,the later Uplift of the GAMs(5±3 Ma)can produce large amounts of debris to Alxa arid land and then was transported to the Loess Plateau by the gradually intensified East Asian Winter Monsoon(EAWM).The Published Sr isotopes of Northwest Pacific deposits supported our speculation.North Pacific sediment is another important agent to study the evolution of Asian dust.Previous studies have found that the Pacific sediments are mainly derived from eolian dust in Asian interior.We systemically measured the Sr-Nd isotopes,trace elements concentrations from sixteen surface samples of sixteen northwest Pacific sites and other three time-series sites,namely sites 1208 and 1209 as well as the core LL44-GPC3 in north central Pacific Ocean.Clay minerals were also analysed from 15 samples of site 1208.The data showed that the Northwest Pacific sediments are the mixture of Taklimakan dust and volcanic ash.Interestingly,we detected the signal of Gobi dust for the first time at site 1208.In order to track the released mechanism of Gobi dust,Nd and Sr isotopic compositions of high-resolution sediments from site 1208 during 2.85-2.6 Ma interval and 0-500 kyr samples from site 1209 were systemically measured.A sudden release of Gobi dust was detected between 2.72 Ma and 2.70 Ma when the large-scale Arctic ice sheet began to emerge at that time,with duration of only 20 thousand years.Globally cooling,large-scale growth of the Arctic ice sheet and the strengthening Siberia high pressure and winter monsoon might be the main mechanism of Gobi dust release and transportation.Taking into account that the Gobi dust was absent in other northwest and north central Pacific sites,we deduce that the Taklimakan dust was predominantly transported by westerly wind,whereas Gobi dust was mainly marigated to the northwest Pacific oceans by the EAWM under specific climatic conditions.The new eolian flux record was recovered from site 1208 in northwest Pacific ocean.The eolian dust flux gradually increased since 13.5 Ma,synchronous with the gradually global cooling after MMCO,indicating that the decreasing temperature may have played a dominant role in promoting the aridity of the Asian interior.Tectonic activities processes might supply amounts of debris,which is also an important factor for the dust release.Great discrepensies existed for the dust source evolutions between the Pacific sediments and the Loess records.Pacific sediments were mainly derived from the Taklimakan dust by westerly wind with long-distance transportation and thus recorded the evolutions of materials from NTP,while the Chinese loess was eolian dust deposition transported mainly by the near-surface winter monsoon,reflecting the binary mixed signals of NTP and GAMs endmembers.We tentatively suggest that different atmospheric circulations and sources probably result in the differences of their source shift record.