Xia Weilan

Sedimentary geochemical evidence for recent eutrophication of Lake Chenghai, Yunnan, China

Geochemical anomalies and stable isotope ratios (δ18O, δ13C) in authigenic carbonates and organic matter (δ13C) from a 660-year sediment core from Lake Chenghai, southern China, provide a continuous history of recent lake eutrophication. The multi-proxy geochemical and isotopic record can be divided into a three-part history of contrasting limnological development, including: (1) a clear-water, oligotrophic open lake system (1340 and 1690 AD); (2) an environmentally unstable, hydrologically closed, oligotrophic lake system (1690–1940 AD); and (3) an increasingly eutrophic, closed lake system marked by higher organic matter, nitrogen, CaCO3, and pigment concentrations, and lower δ18O and δ13C values in authigenic calcite (1940–1999 AD). The unanticipated lowering of δ18O and δ13C of authigenic calcite during eutrophication is thought to be the result of disequilibrium water–carbonate fractionation of oxygen and carbon isotopes during periods of elevated primary production, pH, and [CO32−] activities in the water column. The recent eutrophication of Lake Chenghai indicated by these geochemical proxies is essentially simultaneous with large-scale human migration and the application of agricultural fertilizers in the catchment area during the 20th century.

Source Analysis of Heavy Metals in Surface Sediments of Lake Taihu

Heavy metals of natural and anthropogenic sources are mixed in lake sediments, and it is dif- ficult to discern. Based on linear regression analysis results and element chemical properties, Al and V are selected as the concentration normalizing elements. The natural and anthropogenic sources contributing to the elemental concentrations in core MS of Lake Taihu are analyzed in terms of element--to--Al ratio versus V--to--Al ratio. Results show that heavy metals were from natural sources before mid 1920s. The heavy metals, such as Al, Fe, Mn, V, Cr, Pb, Cu, Zn and As, were from changed natural sources, and Hg originated from human pollution partly between mid 1920s and 1970s. The heavy metals, such as Pb, Cu, Zn, Hg, As, are contaminated by human pollution since mid 1970s. The source characters of heavy metals are correspondent with the hu- man activities and economic development in Lake Taihu drainage basin.

Environmental variation in central Tibetan Plateau in the last 200 years

Through the analyses on sediment lithology,210Pb and137Cs dating, carbonate content, Sr/Ca and Mg/Ca of Ostrocod, and carbon and oxygen stable isotope of carbonate of gravity core from Cuoe Lake in the Naqu area, Tibetan Plateau, the paleolimnology was studied to investigate the environmental variation in the central Tibetan Plateau in the last 200 years. It is inferred that the environmental variation has undergone two stages. The climate was arid in the early 100 years, while it became warmer and more humid in the later 100 years, with a period of 20 years for humidity change.

Geochronology of recent lake sediments from Longgan Lake, middle reach of the Yangtze River, influenced by disturbance of human activities

The impacts of human activities in Longgan Lake region on the distribution of 210Pb in lake sediment made it difficult to establish geochronological sequence. However, environmental proxies recorded these human activities and could be used as time marker to check 210Pb dating results. Based on the analysis of 210Pbexc distribution in cores LS-1 and LGL-1, and in combination with the human activities records in grain-size and element concentration, the sedimentation rates in the upper layers of these two cores were determined to be 0.19 and 0.23 cm/a respectively. The MAR (mass accumulation rates) of these two cores demonstrated that natural condition, such as precipitation and runoffs, led to the variation of MAR before human disturbance, and human activities controlled the sedimentary processes afterwards, which obviously reduced the particles influx into the lakes. During 1940s when human activity had not changed the water system within the catchment, 210Pb activity was abnormally strong in lakes along middle to lower reaches of the Yangtze River, which might be attributed to less precipitation and can be used as a time marker for geochronological research.

Sedimentary flux of Lake Taibai, Hubei Province and correlations with precipitation and human activities in its catchment during the last century

Time-depth correlation of the sediment core from Lake Taibai,Hubei Province was established based on the ~(210)Pb intensity and CRS model.The sedimentary flux variation and its correlations with the summer precipitation and human activities during the last century in Lake Taibai catchment were discussed.There were six periods with high sedimentary flux.The former four periods,1900-1920,1928,1937-1942 and 1953-1954 AD,were cor- respondent with the four phases with a higher precipitation in summer,which were 1900-1920,1931,1938- 1939 and 1954 AD.The high sedimentary flux was resulted from more material input by stronger runoff.After three reservoirs were built in the upstream region of Lake Taibai inflow rivers in 1958-1963 AD,there was a lower aver- age sedimentary flux for Lake Taibai due to the regulation of the reservoirs to the flood and silts,and the controlling function of precipitation on sedimentary flux became weakened.The high sedimentary flux during 1958-1970 AD was mainly related to the land reclamation around the Lake,resulting lake area decreased.The high sedimentary flux during 1983-1993 AD was induced by the soil erosion with the rapid development of cultivation after the change of agricultural production patterns.By the contrastive analysis of sedimentary flux with precipitation and hu- man activities,the accuracy of the timescale of Lake Taibai sediment can be reliable,which also indicated the e- nough stable sedimentary environment.Thus,the sedimentary proxies can be used to reconstruct the environment evolution process in Lake Taibai catchment.

Isotopic Tracing of Trophication Processes over the Last Millennia on Lake Chenghai

Multi-parameter studies (stable isotopes in carbonate and organic matter, pigment, organic carbon and nitrogen contents) from a 660-yr continuous sediment core from Lake Chenghai, a closed, eutropic lake in southern China, provide information on lake historical eutrophication. During the last 660 years, great changes have taken place in productivity and eutrophication of Lake Chenghai in response to human activities. In 1690, the productivity of the lake began to increase as Lake Chenghai became closed from agriculture in the lake’s watershed. In 1942, Lake Chenghai evolved to eutrophic state, marked by an increase in organic carbon, nitrogen, CaCO3, pigment contents and obvious negative values of stable isotopes, which is more or less simultaneous with the large-scale population immigration during the period. In 1984, intensive human activities induced modem lacustrine productivity and eutrophic level. Human-induced trophic changes during the past few decades have affected the Lake Chenghai ecosystem to such an extent that it has never experienced before in the last 660 years.

Weathering and climate change since 1820 AD in Hoh Xil, Qinghai-Zizang Plateau, China—Evidence from element geochemistry of lake sediments of Gulug Co Lake

Element geochemistry of lake sediments has been widely used to detect climate change because element composition and ratios can reflect the weathering degree in the source area. Given the element composition of lake sediments from Gulug Co Lake, Hoh Xil, Qinghai-Xizang Plateau, chemical index of alteration (CIA), index of composition variability (ICV) and other element ratios have been used to establish the weathering sequence of this area since 1820 AD. The weathering is so weak that the element composition change is more sensitive to climate change and autochthonous processes. From 1820 to 1984 AD, there were two drier periods with a wetter interval from 1870 to 1945 AD. After 1984 the weather showed a tendency of becoming wet.

A Preliminary Study on Forming Age of the Nansihu Lake

Tha Nansihu Lake has been strongly influenced by both of the Yellow Rivers overflow and human activities. Its forming age has not been determined for a long time. By the two sedimentary cores in the Nansihu Lake, this paper analyzed the environmenntal indexes of pigment, organic δ13C and C/N. The ages of the low parts in the cores are measured by 14C and sedimentation rate of the cores are determined by both of the 210Pb and 137Cs. Combined with the distribution of pigment, C/N and organic δ13C, it can be determined that the forming age of the Nansihu lake is 2450aBP.

Sedimentation Characterization and Reconstruction of Time-scale of Typical Profile in Xianghai Marsh Wetlands, Jilin Province

Cumulated sediments during marsh wetlands development really recorded the region environment change and marsh processes. Here we reported a case study on the Xianghai marsh wetlands at downstream of an anourous river-Huolin River, Jilin Province. By analysis of sediment layer structure, sediment bulk density, and sedimentation rate in the typical sedimentation profiles, the following results are revealed: The layer relationship among marsh sediments could reveal the marsh formation and its environment change; The riverine marsh might receive much more mineral sediments; the mark of river shifting near 1880-1885AD in the research field was discovered; That the sedimentation rates of Xianghai marsh wetlands were similar to typical estuarine deltas indicated the property of trailing end of estuarine; The cause of suddenly accelerating of the sedimentation rates in Xianghai marsh wetlands in recent 50 years, might related with the human activities strengthen , the plants destroy, and top soil strictly lose after liberation. Furthermore, on time scale, the suddenly accelerating of the sedimentation rates had much better coupling with the mining period of time of large open-cast coal located on upstream of the Huolin River catchments.

Climatic Signals Recorded in Stable Isotope and Trace Elements of Shells in Xincuo Lake, Eastern Tibetan Plateau

Here we displayed the results of trace elements and stable isotope in shell Gyraulus sibirica continuously preserved in the sediments of Xincuo Lake, Eastern Tibetan Plateau, in the past 50 years. By coupling these indexs and instrumental meteorological data in its basin, we probed quantitatively modern climatic signals recorded in these indexes to build relative function relations. The results showed that gastropod shells in Xincuo Lake were formed in summer half year, so that trace elements, stable isotope in shells just recorded the climatic signals of the summer half year instead of those of the whole year, Mg/Ca ratio and δ 18O proxy had a certain correlativity with summer half year air temperature, in which δ 18O proxy was more sensitive with variability of δ 18O to temperature d δ 18O/dT 1 64‰/℃; As to Sr/Ca ratio and δ 13C proxy, especially Sr/Ca ratio, there had a close correlation with summer half year precipitation, in which the variability of Sr/Ca to precipitation d (Sr/Ca)/dP is -0.045/mm.