Provenance Of Loess In Central Great Plain,U.S.,and Their Paleoclamitic Implications

The key role of dust on whole earth system has aroused more and more attention.Loess is one of the most important deposition of dust,study of it have involved in many ascepts.The most mature and successful research on loess is Asian,especially the Chinese Loess Plateau(CLP)until now.Research of loess on North American(NA)has a 70 year history,however,there is still some controversy.The most important area of Quaternary dust deposition in North America is in the Mississippi system and central Great Plains.It was agreed that loess in Mississippi system is controlled by glaciate,and highly related to Laurentia Ice Sheet.However,previous work has documented some of the highest loess accumulation rates known worldwide in Great Plain.To interpret the paleoclimatic information recorded in thick,some times rapidly accumulate loess in the central Great Plains,it is essential to understand the nature of the loess sources,and whether they are glacially influenced streams,unglaciated landscapes,or a mixture of the two.In this paper,we used Nd-Sr isotopic system,major and trace element,mineral composition,magnetic susceptibility and content of CaCO3 to address these issues from a new perspective,extending previous studies that focused entirely on the predominant sources of Late Pleistocene loess.We found that:(1)The major elements of loess in American central Great Plains are quite different from Chinese Loess Plateau(CLP)and Upper continental crust(UCC).The primary feature is that content of Fe and Mg in Great Plains is much lower than Chinese loess and UCC.And content of Na and K is similar,all lower than UCC.The distribution pattern of trace element in Great Plains is similar to Chinese loess,but Zr and Sr is higher than Chinese loess.The distribution pattern of rare earth element(REE)is quite similar,and all exhibition negative anomaly of Eu.Exceptly,most light rare earth element(LREE)is higher than UCC,but most heavy rare earth element(HREE)is lower than UCC.(2)The loess in American central Great Plains suffered weathering strongly.However,it is remain weak compared with Chinese loess(3)Middle Pleistocene to Holocene loess units with widely varying accumulation rates are all closely linked to rocks that crop out in unglaciated landscapes of the western Great Plains,which are the primary sources of the loess.However,there was a small but significant contribution from a secondary source,likely the Platte River system,which carried sediment from glaciated areas of the Rocky Mountains.Or Precambrian Canadian shield,which can be by produced by LIS's grinding and carried by ice sheet and wind.(4)The contribution from this secondary source probably including glacially eroded sediment was greater in Marine Isotope Stage(MIS)3 and early MIS 2,but decreased somewhat about 22-18 ka.(5)The highest mass accumulation rates(MARs)in this section occurredafter this shift,best explained by environmental changes driving greater dust emission from nonglacial dryland sources,as the Laurentide Ice Sheet retreated from its maximum extent.But interesting in all,the high MARs is controlled by climate directly.