Geochemistry Of Rare Earth Elements And Its Control Factors In Watershed At Mount Huangshan Landscape

The distribution and fractionation mechanism of rare earth elements in theenvironment are important basis and prerequisite for some researches which utilize them as the geochemical tracer.Rare earth elements are often studied for some individual geological areas such as soil profile or river.The systematic study on its source,distribution and differentiation in mixed geological conditions are little reported and the mechanism of its migration in the complex environment are also need more data to elucidated.The Mount(Mt.)Huangshan landscape watershed is a typical mixed silicate area.The watershed drains originally from Yanshanian granite,and the middle and lower reaches flow the metamorphic sedimentary-volcanic rocks and micro-distributed carbonate rocks.The acidic and slightly acidic soil developed in acid granite are greatly distributed in the basin.In recent years,the acid rain happens frequently in this region.Scenic forest ecosystem has the risk of enhancing in soil and water loss and mineral nutrient loss.The complex geological conditions in Mt.Huangshan Landscape Watershed provide an ideal place to launch this research.In this study,the Watershed in North Mt.Huangshan(NMHW)is selected as sampling site.The samples in the riverine system(including dissolved state and particulate),groundwater,rainwater and soil in three weathering profiles in the Mt.Huangshan landscape were sampled and the rare earth elements were detected.This study is aimed at analyzing the cyclic process of rare earth elements from soil to riverine system in the drainage basin,especially the migration,and differentiation mechanism of them in the complexed conditions.The main conclusions are as follows:(1)Three weathering profiles from the source to downstream of the watershed were Selected.Based on the analysis of the main component distribution and chemical weathering coefficient in the soil weathering profiles,it is concluded that the soil profile of the Huangshan landscape watershed has experienced the weathering leaching process and the effect of desiliconization.The granite FRL profile has been moderate weathering,while the chemical weathering intensity of the TPH weatheringprofile is very strong.The chemical weathering intensity in the section has postivie correlation with the ratios of the LREE / HREE.The order of rare earth elements(?REE)in the soil profiles is: FRL profile> MK profile> TPH profile and the amount of them are mainly depended on their soil parent materials.The Ce in the three soil profiles shows a positive anomaly in the leaching layer,and the lower layer has negative one.The former should be related to the strong acidity of the soil surface.The abnormal of Eu in the three soil profiles is not obvious and can be attributed to the difference of weathering of granite.(2)Water samples in the watershed were sampled in order to define sources and distribution of dissolved REEs,and to describe the factors especially the mixed geological background how to govern their mobility.Results show that rock chemical weathering is the dominant source for REEs and atmospheric input is another important source.? REE in the headstreams was higher than that in middle and lower reaches,which is in agreement with the corresponding REE amount in the soila distributed from sourceto downstream.This is related to geology and presents typical lithologic control features.The contribution of rainfall to river of REE by quantitative calculation is 25% in wet season and the calculation is 17% in dry season,explaining that atmospheric deposition has a certain contribution to REE in the watershed.The source water enriches in HREE.However,the content in middle-lower reaches shows enrichment of LREE.The major reason for these changes is spatial variation of draining primary rocks from granites to sedimentary rocks.Most water samples show negative Ce and positive Eu anomalies,respectively.The former is fact of inheritance with primary rocks and the latter appears to be a result of preferential weathering of feldspar minerals such as plagioclase,orthoclase in granites.The Y/Ho ratios display seasonal and spatial fractionations which are linked to water/particle interactions and spatial changes of primary rocks,respectively.(3)The total amount of rare earth elements in the suspended particulate matters and sediments of the watershed show greatly spatial variations.The content of them inthe source was obviously lower than that of the middle and lower reaches of the source region.The phenonmenon was contrary to that observed in the river water and soil samples in profiles.The research showed that the difference of weathering of granite lead to the release of rare earth ion in the river to form complexes or precipitation.The suspended particulate matter and sediments was enriched in LREE,which is consistent with the enrichment of LREE in the rocks of the Huangshan granite-sandstone and shale in the area.The result indicated that the LREE enrichment in the suspended matter and sediments ware controlled by rock weathering.The Ce exhibits a significant positive anomaly in the suspended particulate matters and sediments,mainly due to the redox of Ce.Most of the Eu in sediments is positive which may be attributed to chemical weathering of Eu-rich K-feldspar or plagioclase.In addition,the distribution of ?Eu-(La / Yb)N in the river and soil profiles showed that the soil weathering profile and the dissolved river in the Huangshan landscape watershed were affected by rock weathering control.The sediments in the distribution of rare earth elements are related to the physical and chemical properties of water,redox conditions and other factors.