Environmental Record In Sediments Over Past Hundred Years And Effect By Anthropogenic Activities Of Lake Qinghai, China

Lakes are one of the most important parts of the terrestrial ecosystem,which plays an important role in regional climate change,regional ecosystem security and regional economic development.Lake sediments were well recorded the environmental changed and human activities during the past time.Radioactive isotopes as the natural tracers,which can trace the source of sediments,weathering level of the source region,and the process of sediments transportation.Organic matter can reflect the initial productivity of lakes,trace the source of sediments and the intensity of human activities.Trace and major elements and heavy metals of lake sediments can trace the natural processes and human activities due to they were persistence in the environment.Therefore,eight suite sediment cores were collected in Lake Qinghai,to observe the grain size,radionuclides,organic matter,trace,and major elements,and heavy metals.The recorded composite tracers in lake sediment were evaluated to elucidate the source and the transportation process of sediment in different spatial and temporal scales.The radiochronology based on ¹³⁷Cs in Lake Qinghai to evaluate the challenge in a saltwater lake environment.The climate changed was also conducted to reconstructed in Lake Qinghai during the past100 years.Meanwhile,trace elements and heavy metals were studied to evaluate they were responded to the development pattern and intensity of the catchment and national economies during the past 100 years,and effect to the lake ecosystem.(1)The positive ²²⁸Th_ex values in the upper 1 cm sediments suggested that these sediments were young in the lake,and the sediment may have been resuspended or/and mixed or/and transported in the top 1 cm.The ²²⁸Ra/²²⁶Ra were more than 1 and the²²⁶Ra/²³⁸U were less than 1 in the sediment and soils,support that the weathering region is accumulating Th relative to Ra and U in the watershed,and indicated that the removal of uranium relative to its daughters during erosion process.The inventory of ¹³⁷Cs was1470±33 Bq.m^-2in the background soils of Lake Qinghai,which was similar with others regions where have the same latitude and similar amount of precipitation,and the atomic ratio of ²⁴⁰Pu/²³⁹Pu were 0.174-0.176 and the activity ratio of ²⁴¹Pu/²³⁹Pu were 2.81-3.04 in the sediments of Lake Qinghai from 1950 to 2007 indicated that the¹³⁷Cs accumulation peak in the sediments of Lake Qinghai was the global nuclear explosion in 1963 rather than the Chernobyl nuclear power plant explosion in 1986.Due to the ¹³⁷Cs diffusion in the sediments of Lake Qinghai,the accumulation peak of¹³⁷Cs in 1963 become flat peak in QHH04,and also indicated that it was unsuitable to choose 1952 as ¹³⁷Cs started to record in the sediment of Lake Qinghai.So it must combine to the radiochronology of ²¹⁰Pb_exwhen used the radiochronology of ¹³⁷Cs in Lake Qinghai.(2)The median size was less than 25?m in sediment cores which were located in the north part of Lake Qinghai,and the median size was more than 25?m in Z04 and Z07,indicated that the sedimentation processed were dominated by river input in the North of Lake Qinghai and the dust input dominated the sedimentation processed in Lake Gahai and Lake Ehai.The values of?¹³C_org and?¹⁵N in the sediment cores were-27.7‰to-26.5‰and 6.77‰to 7.58‰,respectively,indicated that the main source of TOC in the sediments was from authigenic algae of the lake and the main source of TN was from plankton and DIN in the sediments.The correlation of Fe,Mn,Al,Rb and Ti indicated that they were from detrital material in the sediment cores of Lake Qinghai.Ca and Sr has similarly processed during sediment depositing.The resulted of correlation relationship,PCA,FA,and CA for Cu,Cr,Pb,Zn,As,Cd and Al indicated that the source of Cu,Cr,Pb,Zn and As have from the natural background source in the sediment core during past 100 years,while the Cd was main control by anthropogenic sources.The flux of sediment was 1.66×10⁶ ton.a^-1 in the main lake calculated by mass accumulation rate,The mass balance showed that the contribution of the sediments from rivers,authigenic carbonates,lacustrine organism,and atmospheric deposition were 45.18%,22.44%,3.02%and 29.36%,respectively.(3)In Z04,the concentration of Rb was increased from 16.3 to 14.7?g.g^-1,while Sr was decreased from 650 to 820?g.g^-1 form 1884 to 2008.The variation of Rb vs clay and fine silt and Sr vs coarse silt and sand have low correlation from 1915 to 1973,while the component with more 16?m of grain size.While,the trend of Rb vs clay and fine silt and Sr vs coarse silt and sand have good correlation(R=0.869,P<0.001 and R=0.944,P<0.001,respectively)after 1973,also the Rb/Sr ratio was decreased trend in this period,indicate that the climate becomes drier in the Lake Qinghai catchment during the past 100 years.(4)The values of EF(Ti),EF(P),EF(Pb),EF(Zn),EF(As)and EF(Cd)more than1 in QH01,QH02,QH07 and Z04 from 1950 to 2010,suggest that the anthropogenic activities were increased in this period.The variation of TOC,TN,TOC/TN and EF(Ti),EF(P),EF(Pb),EF(Zn),EF(As)and EF(Cd)were influenced by the development of the economy in the catchment and the country.Therefore,the intensity and pattern of anthropogenic activities information in the regional and the whole country can well de recorded by lake sediments.The maximum value of EF(P)was 1.55 indicate that Lake Qinghai is evolving into a eutrophic lake.The maximum value of I_geo-Cd was 1.75 in sediment cores supported that Cd was moderate pollution stage in the surface sediment of Lake Qinghai.The potential ecological risk index was increased in all sediment cores after 1950,and the values were closed to 150(184 for QH07)in the top of sediment cores,indicate that the potential ecological risk index was close or get into the moderate ecological risk effect by anthropogenic activities.In summary,this study creatively utilized the radionuclides,major and trace elements,grain size,and heavy metals geochemical methods to multi-tracing the source of sediment in Lake Qinghai,evaluate the ¹³⁷Cs applicability for radiochronology in a saltwater lake and understanding of the sediment dynamic process in the lake.This study also evaluated the budge of mode sediment and reconstruction the climate change in the Lake Qinghai catchment during the past 100 years.This study investigated the organic matter,trace element,and heavy metals recorded in the sediment cores to respond the anthropogenic activities information in the regional and the whole country during the past 100 years and evaluate the development trend of lake pollution and ecological risk in Lake Qinghai.These results provide basic data for protecting the ecology of Lake Qinghai.This study offers the complementary information not only helps to understand the biogeochemical processes during the sediment deposit in the plateau lake but also give the references for studies the anthropogenic activities recorded in other lakes on the plateau which were no history recorded data of the human activity.