Tracing The Provenance Of Luochuan And Lingtai Loess-paleosol Based On Zircon U-Pb Ages

The change of dust potential provenances in the Chinese Loess Plateau not only contains the information of global cooling,the uplift of the Tibetan Plateau and the drought in the interior of Asia since the Cenozoic,but also closely relates with the paleoatmospheric circulation changes.Loess-paleosol,as the most important sedimentary sequence accumulated on the Chinese Loess Plateau since Quaternary with the most complete,the thickest and the best continuity,together with ice cores from polar glaciers and marine oxygen-isotope serves as the three pillars for the study of the global climate change.Zircon,an accessory mineral with high closure temperature,stable chemical properties and strong anti-interference ability,is common in loess.And thanks to the rapid development of mineral micro-in-situ analysis technology,zircon U-Pb dating based on LA-ICP-MS method can be successfully applied to loess tracing provenance.In this study,we use zircon U-Pb ages for provenance tracing of 10layers(S0,L1,L2,S2,L15,L18,L22,L24,L29,L33)of loess-paleosol sequence of Luochuan and Lingtai profile,respectively.The dataset in multi-dimensional space is reduced and simplified to project in low-dimensional space using the multi-dimensional scaling —MDS,and The similarity of zircon U-Pb age distribution between the study area and the potential source area was shown using line connection points in order to explore the changes of loess-paleosol provenance in the Chinese Loess Plateau since Quaternary.The results show that :(1)The upper sections(S0,L1,L2,S2,L15)in Luochuan and Lingtai sections have similar zircon U-Pb age distribution,which is inferred to have the same material source area,while the lower sections(L18,L22,L24,L29,L33)have obvious different age distribution with the upper sections.But the lower sections(L18,L22,L24,L29,L33)in Luochuan and Lingtai sections have similar age distribution,which is inferred to have similar material source area;(2)Compared with the zircon age combination of potential provenances around the Loess Plateau,it is found that the sediments in the upper sections of the Luochuan and Lingtai profile mainly come from the north to the northeast of the Tibetan Plateau in the northwest of the Chinese Loess Plateau,while the sediments in the under sections mainly come from the eastern Mu Us Desert in the northern the Chinese Loess Plateau,which is composed of the North China Craton,the central Asian orogen and the in-situ sediments;(3)The provenance difference occurred during the deposition period from L18 to L15,approximately at 1.1～1.32 Ma.During this period,an earth orbit-scale event occurredMid-Pleistocene Transition that began around 1.2 Ma.Since the MPT,the ice volume in the Northern Hemisphere has increased significantly and accompanied by the global cooling,as well as the periodic increase in the uplift height and width of the Tibetan Plateau,so that the intensity of the East Asian winter monsoon has increased significantly.Combined with provenance changes,it is likely that the direction of the East Asian winter monsoon changes from north to northwest since the MPT event.As the main driving force of the formation process of loess material,the change of the intensity and direction of the East Asian monsoon is the main reason for the provenance difference.Therefore,it implies that the dust source center of the Chinese Loess Plateau has changed from north to northwest.Before the MPT,the material of the Chinese Loess Plateau mainly came from the eastern Mu Us Desert formed by the sedimentary centers around North China Craton,the central Asian orogen,the recycling of underlying sedimentary and the in-situ sediments materials.Since the MPT,the detrital materials from the north to northeast of the Tibetan Plateau have played a leading role in the contribution of eolian materials to the Chinese Loess Plateau.