Abstract: The long standing issues that low values of magnetic susceptibility as well as uncertainty of magnetic mineral types of lake sediments in arid area of northwest China are widely existed,making it limited in using the environmental magnetism methods to estimate the lake evolution as well as to reconstruct their regional paleoenvironment and paleoclimate. Low values of magnetic susceptibility could enlarge the instrument error,while the uncertainty of magnetic mineral types of lake sediments may confuse researchers in studying the mechanism of enhanced magnetic susceptibility. To solve these problems,we extracted magnetic minerals with self-regulating magnetic mineral-extracting device from 5 samples chosen from different sedimentary units from the top 2.5 m of BST12B(42°00'42.9"N,87°09'53.8"E)drilling core which was collected in the depth of 9.2 m in the central area of Bosten Lake in winter of 2012 and had the total length of 51.52 m. We tested mineral compositions of all the 5 samples both before and after extracting using XRD(X-Ray Diffraction)method,which had been proved to be very efficient in identifying minerals quantificationally. Results show that all the lacustrine samples contained Magnetite and Pyrite,neither Pyrrhotite nor Goethite were detected in all the samples,which meant that the major magnetic minerals in Holocene lacustrine sediments of Bosten Lake were magnetite and bits of pyrite. Moreover, in comparison with the residual parts,the extracted parts enriched clay minerals,such as Chlorite and Mica, which were widely distributed in lacustrine sedimentary environment and had relatively high magnetic susceptibility value. While the concentration of the major minerals in our tested samples,Quartz,Feldspar and Calcite decreased dramatically. As a result,the values of magnetic susceptibility of these residual parts decreased synchronously, which indicated that the magnetism contribution of these clay minerals made to the lacustrine sediments in arid area should not be ignored. Combining the characteristics of magnetic minerals assemblage with other climate proxies that include particle size,color and concentration of Sulfur,we can infer that the Pyrite can be potentially used as a high lake water level indicator which has been used in the estimation of the Holocene lake evolution in Bosten Lake. The reconstructed result indicates that the lake water level of Bosten Lake was very low,or even just a seasonal waterlogged environment during the early Holocene,while the deep water environment sustained from middle to late Holocene except an episode of short-term shore-environment appeared which may result from an extremely drought event. As a summary,our work illustrates that the using of magnetic mineral extracting method is viable in exploring the lacustrine sediments environmental magnetism problems in arid areas, and is of significant meanings in studying lake evolution and environment change. The specific type of magnetic mineral,Pyrite could be potentially used in assessing lake level fluctuation.

Key words: arid area, Bosten Lake, Holocene, environmental magnetism, magnetic mineral