.150 Ka Bp. The Salawusu River Valley Since The Particle Size Distribution Analysis, And Climate Change

Situated at the southeast of the Mu Us Desert, Ordos Plateau, the Milanggouwan stratigraphical section in the Salawusu River Valley keeps a record of 38 sedimentary cycles of alternate evolution of the aeolian dune layers with the fluvio-lacustrine facies and palaeosols since 150 ka BP. This paper, based on the analysis of 142 samples of grain-size and on the calculation and research results of the stratigraphical sequences in the section, has put forward the following views: 1. Grain-size accumulation percentage of the aeolian sands in the past 150 ka in Milanggouwan section is mainly characterized by the aeolian palaeo-mobile dune sands, which are composed of majority fine sands and minority very fine sands. There isn't almost any silt and clay. Those of the fluvial facies, especially the lacustrine facies and palaeosols, are mainly composed of very fine sands and also contain some quantities of silt and clay. 2. The general characteristics of the grain-size distribution of different sedimentary facies, especially the variations of the parameters Mz (mean particle diameter) and o (standard deviation) clearly show that the fluvio-lacustrine facies and palaeosols usually become finer in particles and worse in sorting than the aeolian sands. As regards Sk (bias angle) and Kg (kurtosis) values, the fluvio-lacustrine facies or palaeosols increases considerably compared with the aeolian sands. The former displays positive bias and the latter often approximately symmetrical distribution with only a minority negative bias. 3.At the vertical direction of the section, the grain-size distributional coarse and fine rhythms vary very pronouncedly with the alternation of sedimentary facies, which is clearly shown not only in the grain-size percentage in this section but also in the response from different grain-size parameters. Mz, a , Sk, Kg and SC/D (ratio of the sum of silt and clay to sand) of fluvio-lacustrine, palaeosols and aeolian sands constitute an interlocking, multi-fluctuation process curve with peak and valley values: the <& value of Mz, o and Sk values often increase accordingly from aeolian sands to its overlying fluvio-lacustrine facies or/and palaesols, the Kg value heightens, too, and the SC/D value also basically consists with its change steps. 4.The values of grain-size parameters lie on their eigenvalues. Therefore, the grain-size eigenvalues "bounce" with alternation of the different sedimentary facies in the section, whose changeregularity is coincident with that of grain-size parameters. Relatively speaking, however, taking the eigenvalue 50 as a boundary, "active eigenvalues" 75, 84 and 95 above the eigenvalue have wide bounce range and high sensitivity responding to the sedimentary facies ; in contrast with them, those "inert eigenvalues" 5, 16 and O25 below the above-cited eigenvalue, have relatively narrow bounce range and low sensitivity.This paper, based on the grain-size analysis and the comparative result of the Mz - o scatter gram of ancient aeolian sands in Milanggouwan section with the modern dunes in Mu Us Desert, holds that fossil eolian dunes are a proxy of the dune activation, and be regarded as "desert process" , while the overlying fluvio-lacustrine and palaeosols on the dunes are regarded as inter-desert process. Accordingly, the dune deposits of the section since 150 ka BP can be divided into 38 alternate evolvement processes of desert and inter-desert periods, which accord with the sedimentary cycles of the section. It is further suggested from now climate in the area that the 38 inter-succession processes of dune deposits and fluvio-lacustrine or palaeosols in past 150 ka actually result from the alternate evolvement of the ancient East Asia winter and summer monsoons in the Mu Us Desert during the long geologic times. The most direct embodiment is the fluctuant images of grain-size parameters brought about by the bounce of grain-size eigenvalues.In order to discuss dune sedimentary and the cause of its evolvement, in this paper we also contrast every oxygen isoto...