Unmixing Grain Size From Sediments And Its Environmental Implications In The Arid Region Of Northwestern China

Aeolian sand as the main component of the desert is affected by the wind transport in the process of the provenances to the sedimentary region.And the change of the intensity and variation of the wind transport as the results of land-atmosphere interaction,the aeolian sand is sensitive to the land-atmosphere interaction.Therefore,we can define the provenances of desert material and its transport process,and the desert evolution clearly through the study in grain size composition,transport process and sedimentary environment of aeolian sand.Grain size distributions in sediments as a powerful proxy is widely used for global or regional environmental change research,however,the traditional parameters of grain size can not indicate clear environment information,resulting in the grain size has multiplicity of solution.With the development of the statistics,through the mathematical methods,the mixed grain size can be unmixing end members and obtain clear environment information of end members.In further studies,select appropriate unmixing grain analysis models to separate grain size components of aeolian sediments in arid desert region and to clarify their environmental information are needed.In this study,252 surface aeolian sand samples in Alashan desert and 107 surface lake sediment samples of the five lakes in the hinterland of the Badain Jaran Desert without runoff recharge were selected for examining the spatial distribution of grain size.Moreover,deferent unmixing grain size analysis modals were used to calculate end members from all grain size measurements,and the environmental significance of the end members was determined.Finally,on the basis of previous studies,deferent unmixing grain size analysis models were applied to the lacustrine sediments of huahai section,and the end members of different models unmixing grain size analysis were compared and analyzed.The results go as follows :(1)A three-end-member model explains more than 90 % of the grain size distributions of aeolian sand of Tengger desert(WT1-WT3)and Ulan Buh desert(WL1-WL3)calculated by Weibull function.The results of the composition and the spatial distributions of end members in Tengger desert indicated that the content of WT1 is affected by the lake sediments transported in the prevailing wind direction;WT2 content is affected by the coarse grain size distributed between the sand dunes,the content of WT3 is affected by the coarse grain stoped by sand dunes and its spatial distributions can indicates the direction of the provenances;The results of the composition and the spatial distributions of end members in Ulan Buh desert indicated WL1 content is affected by the fine particles in prevailing wind direction,WL2 content is affected by the coarse particles in Yellow River carried by the wind,and WL3 content is affected by the coarse particles stoped by dunes in prevailing wind direction.(2)For lakes in the hinterland of the Badain Jaran Desert with no runoff recharge,sediments in the lake are transported by the wind alone.Median and mean grain sizes in the lake sediment decrease from the nearshore to the offshore,deep-water zone,and are affected by wave sorting.However,the lowest median and mean grain sizes are not found in the middle of the lake,which differs from lakes supplied by surface runoff.A two-end-member model explains more than 90 % of the grain size distributions of lake sediments.EM1 is interpreted as wind transported sediment,and EM2 is interpreted as sediment perturbed by lake waves after wind transport.Changes in the modal grain sizes of EM2 are related to lake area.Changes in the content of EM2(%)can reveal approximate lake level fluctuations at low temporal resolution.(3)In this study,lognormal and NMF was selected and a two-end-member of two diferent umixing grain size analysis modals explains more than 90 % of the grain size distributions of the lacustrine sediments of huahai section.However,the coefficient of NMF(95.6 %)is higher than the LOG(94.2 %),indicating that the unmixing results of NMF is more close to the original data than the LOG.Through the comparison of the end members from two unmixing grain size analysis and the modern surface sediment grain size samples,it showed that NMF and LOG also can separate the end members with geography significance,but the end members of NMF more conform to the original sediment grain size than LOG.And the results showed that change in content of EM1 consistent with the detrital mineral with the depth of the section,it means the increase of content of EM1 also indicates the increase of surface runoff into the lake.The change of EM2 content is consistent with change of the TOC content indicating the change of effective humidity in the huahai area,so that the change of EM2 content can reconstruct the evolution of lake area in the arid area.These results show that(1)the Weibull function can be applied to the surface aeolian sand of the desert,and separate the end members with geographical significance.(2)For lakes in the hinterland of deserts which are not recharged by river runoff,grain size can be used as a proxy for past climatic change and environmental evolution at low resolutions.The water level of a lake is negatively related to the median grain size of sediments in the lake center,but positively correlated with EM2 contents.(3)NMF is more suitable than lognormal for the lacustrine sediments in arid region of desert.These understandings can not only provide proxies with clear environmental significance for the internal relationship between regional and global environmental changes,but also provide new evidence for the hot issue of the process and mechanism of the interaction between the earth's surface layers and the spheres.