Zhang Enlou

Weak chemical weathering during the Little Ice Age recorded by lake sediments

Low magnetic susceptibility, low Sr content and hence high Rb/Sr ratio in the lake sediment sequence indicate a weak chemical weathering process under arid and cold climate of the Little Ice Age in a single closed lake watershed. According to different geochemical behavior between rubidium and strontium in earth surface processes, variation of Rb/Sr ratios in the lake sediment sequence can be used as an effective geochemical proxy with definite climatic significance of chemical weathering in watershed. Unlike chemical weathering process in tropic zone and modern temperate-humid climate, concordant changes in both Sr content and magnetic susceptibility with δ18O values of Dunde ice core suggest that the weak chemical weathering was controlled by air temperature during the Little Ice Age maximum. After the Little Ice Age, chemical weathering intensity was controlled also gradually by precipitation with increasing in temperature.

The Medieval Warm Period in the Daihai Area

Based on the rubidium to strontium (Rb/Sr) ratio, carbonate (CaCO 3 ) content and organic carbon concentration (C org ) record in lake sediments from the Daihai Lake, Inner Mongolia, dated by AMS 14 C and 210 Pb, the climatic and environmental change process was reconstructed since the last 2,200 years, including two important climate events, i.e. the Medieval Warm Period and Little Ice Age. An increase Rb/Sr ratio but a decrease both CaCO 3 and C org correspond to a cold period, and vice versa. In this paper, we first suggest that there existed the Medieval Warm Period in the northern China during 900-1200a B.P. by a high resolution lake record characterized according to lower Rb/Sr ratios and higher CaCO 3 and C org concentrations in the sediments from a single watershed, which a warm and humid environment was demonstrated by significant increase of chemical weathering and by a progressive increase of biologic productivity, and by a high lake level. The Medieval Warm Period has not only an inner climatic fluctuant, but also the strongest chemical weathering during the last 2,200 years, which are indicated by a high resolution natural record in northern China. Furthermore, the Medieval Warm Period in northern China is contemporaneous with the worldwide event identified in the lakes, oceans, land mollusk sequences, polar ice cores, pollens and historical documents.

Environmental changes during early-middle Holocene from the sediment record of the Chaohu Lake, Anhui Province

A typical lake sediment core is obtained from the Chaohu Lake in the lower reaches of the Yangtze River, Anhui Province, China. The timing scale is constrained by AMS 14C dating method. Climate proxies such as pollen and grain size in the core are analyzed to reconstruct the environment changes at this site approximately between 9870 and 2170 cal. a BP. The results indicate that at the research area, the climate in the early-middle Holocene had evolved through 3 stages. From 9870 to 6040 cal. a BP, proxy records show a warm and dry climate with low water levels after the late-glacial period. During this stage, cool and dry events occurred at about 8910 and 6060–6030 cal. a BP. Then, between 6040 and 4860 cal. a BP, the climate was humid and vegtation was more flourishing in the Chaohu Lake Valley. The Holocene Optimum occurred at 5840–5500 cal. a BP in the Chaohu Lake, showing the best condition of water and heat. Elm Decline occurred at the period of 5380–4930 cal. a BP. Since 4860 cal. a BP, the climate was warm and dry through 2170 cal. a BP as shown in both pollen spectrum and grain-size histories. Two obvious dry events occurred in 3760 and 2170 cal. a BP, respectively. At 2170 cal. a BP, the water level of the Chaohu Lake reached the lowest as the lakebed possibly exposed. Such lake sediment observations are consistent with the historical records in this area.

Hard-water effect correction of lacustrine sediment ages using the relationship between ~(14)C levels in lake waters and in the atomosphere:the case of Lake Qinghai

On the basis of analyzing the nature and feature of hard-water effect,the hard-water effect calibration of sediment ~(14)C ages of Lake Qinghai is carried out using the relationship between ~(14)C levels in lake waters and the at- mosphere,in the light of marine reservoir correction.The variation of the hard water effect in Lake Qinghai back to 8000 aBP is also reconstructed.Its concluded that ~(14)C ages of the surface sediment in Lake Qinghai is less than 1549 aBP and the hard-water effect of Lake Qinghai increased evidently during 7000-1000 aBP.On the basis of analyzing the nature and feature of hard-water effect,the hard-water effect calibration of sediment ~(14)C ages of Lake Qinghai is carried out using the relationship between ~(14)C levels in lake waters and the at- mosphere,in the light of marine reservoir correction.The variation of the hard water effect in Lake Qinghai back to 8000 aBP is also reconstructed.Its concluded that ~(14)C ages of the surface sediment in Lake Qinghai is less than 1549 aBP and the hard-water effect of Lake Qinghai increased evidently during 7000-1000 aBP.

A 4000-year moisture evolution recorded by sediments of Lake Kusai in the Hoh Xil area, northern Tibetan Plateau

Lake Kusai lies in the Hoh Xil area, northern Tibetan Plateau. Based on the analyses of Total Organic Carbon content (TOC), Total Nitrogen content (TN) and Grain Size of sediments from Core KS-2006, this paper mainly analyzed the moisture variability in this area during the past 4000 years. The results showed that the moisture evolution of this area in the past 4000 years experienced the stages as follow: dry periods: 3900-3590 cal aBP, 3320-2630cal aBP, 1720-1420cal aBP and 1100-840cal aBP; humid periods: 3590-3320cal aBP, 2630-1720cal aBP, 1420-1100cal aBP and after 840cal aBP. Our results were generally consistent with the climatic changes inferred from the Oxygen Isotope, Dongge Cave, which indicates that the climate in northern Tibetan Plateau was also controlled by the Asian Monsoon. Additionally, Medieval Warm Period and Little Ice Age were well recorded in Lake Kusai as well.

Dust-storm Events in Daihai Lake Area, Inner Mongolia During the Past 400 Years: Evidence from Grain size Analysis of Lake Sediments

A 400 year dust storm history in Daihai Lake area, Inner Mongolia, Northern China, was reconstructed on the basis of high resolution grain size analysis of the lake sediments, in combination with reconstructed historical data of regional precipitation from tree ring, frequency of dust storms in China and historical documentary records. The Daihai Lake is located at a climatically transitional zone between semi arid and semi humid where front position and intensity of the East Asian summer monsoon and winter monsoon chiefly control the climate change. Both the median grain size and coarse grain content indicate that dust storm events were controlled mainly by the intensity of the summer and winter monsoon in the past 400 years. Major dust fall events are generally associated with dry and cold climate during the Little Ice Age, when the median grain size of the lake sediments is larger with high contents of coarse grain (100μm). At time of warm conditions after the Little Ice Age, dust storm events were limited, both the contents of coarse grain (100μm) and the median grain size also decreased. During the last one to two decades, however, dust storm events increased gradually, related partly with more desertificated soil by human activity.

Spectral absorption properties of chromophoric dissolved organic matter and particulate matter in Yunnan Plateau lakes

The Yunnan Plateau lakes with different lake types, located in Yunnan Province in southwestern China, are one of the five densest lake areas in China. The lakes, which are generally distributed at elevations from 1200m to 4000m above sea level, receive high intensity UV-B radiation. In this study, the spectral absorption properties of chromophoric dissolved organic matter(CDOM) and particulate matter are studied based on a wide investigation including 34 lakes in the Yunnan Plateau. The study will be helpful in the understanding of the ultraviolet radiation attenuation and the effects of increased UV-B radiation on lake ecosystems. A wide range of variability in CDOM absorption was found among the Yunnan Plateau lakes. CDOM absorption coefficient generally corresponded to the nutrient level with a significantly positive linear correlation between aCDOM(320) and total nitrogen concentration. Exponential model with constant parameters of background items could give the most approximate estimation of CDOM absorption spectra. A marked phytoplankton absorption peak around 675nm was only observed in Qiluhu, Tinghu and Xinyunhu with very high chlorophyll a concentration. High contribution of CDOM to the total absorption was found during the wavelength range lower than 600 nm especially in the ultraviolet range lower than 400nm. The contribution of CDOM to the total absorption was significantly higher in UVR range (350-400nm) than in PAR range (400-700nm) (ANOVA, P0.001). For the clear lake water with secchi disc: SD≥1.0, the contribution of CDOM absorption to the diffuse attenuation of ultraviolet radiation would increase, and CDOM absorption, with a large probability, would determine the attenuation depth of ultraviolet radiation.

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.

Temporal and spatial variations in sedimentary characteristics of Lake Lugu during the last hundred years and the influence factors analysis

Twelve short sediment cores were collected from Lake Lugu to analyze the particle size composition, magnetic suscepti⁃ bility (χlf), metal content and chemical index of alteration (CIA) in this study. Referencing to the 210Pb and 137Cs dating results, natural factor such as precipitation on the sedimentary characteristics, and human activity records such as agricultural expansion and deforestation, the temporal and spatial variations in sedimentary characteristics of Lake Lugu in recent 150 years and were dis⁃ cussed. The results indicated that the sediments of Lake Lugu were mainly (> 80% ) composed of clay and fine silt. The fine frac⁃ tions (such as clay) showed typical positive correlations with χlf and CIA. The sedimentary proxies showed similar vertical changes in the cores from different lake areas. Before 1920s, χlf and CIA remained low values and the grain size compositions were stable, suggesting weak human disturbance and stable particle input from the catchment. The clay content, χlf and CIA increased gradually since 1920s, which were more remarkable change since ca.1970, indicating enhanced erosion of well⁃weathered and fine⁃grained

TOC and Black Carbon records in sediment of Lake Yangzong, Yunnan Province under the influence of human activities during the past century

Multi⁃proxy, including total nitrogen (TN), total organic carbon (TOC), black carbon (BC) and its stable carbon iso⁃ tope (δCBC), were analyzed in the 40 cm sediment core from Lake Yangzong in Kunming city,Yunnan province. The sediment core was dated using 137Cs and 210Pb, and then we discussed the deposition processes of TOC and BC responding to human activi⁃ ties during the past 100 years in Lake Yangzong Basin. The results showed that:(1)the contents of TN,TOC and BC in the core of Lake Yangzong ranged from 0.76 to 5.07 mg / g, 4.95 to 42.65 mg / g, 0.69 to 11.16 mg / g, respectively, and the δCBC values ranged from -31.03‰ to -25.45‰ with a mean of -27.18‰; (2) the TN and TOC contents were both increased from the bottom to the top of the sediment core, and the molar ratio of TOC / TN reached a maximum in 1994 A.D. then gradually decreased, which indicated that the exogenous pollutants produced by human activities in the basin were continuously discharged into the Lake Yang⁃ zong, and led to an eutrophication of the lake; (3) the changes of BC and δCBC series in the sediment core in Lake Yangzong re⁃ flected the changes of local energy structure. With the rapid development of population growth and industrialization, carbon emis⁃