| 其他摘要 | The past is the key to understand the present and to predict the future. Therefore, acquiring continuous and high resolution natural records in recent 2000 years is one of the most significant issues in the paleoclimate research.
On the basis of precise 137Cs , 210Pb and 14C dating, we systematically discussed stable carbon and oxygen isotope composition of carbonate, stable carbon and oxygen isotope composition of sediments cellulose, carbon isotope composition of organic matter, and Li/Ca ratios of ostracod shells in Lake Chenghai, Lake Caohai and Lake Qinghai to deduce their paleoenvironmental implications. The main conclusions we have obtained are as follows:
1. An effective four processes to extract cellulose from lake sediments are established, including alkaline-washing by 5% NaOH, acid-washing by 5% HCl, bleaching by glacial acetic acid and sodium chlorite mixed solution and alkaline-washing by 17.5% NaOH solution. The result of infrared spectrum shows that spectrogram is consistent with standard α-cellulose, indicating that this procedure is simple and feasible, and also provides a good approach to study stable isotopes of lake sediments cellulose.
2. On the basis of multi-proxy methods, carbonate in Lake Chenghai and Caohai are identified to be authigenic, and organic matter in Lake Chenghai and Caohai are derived from aquatic algal and aquatic macrophytes, respectively. The carbonate content of Lake Chenghai and Caohai are both controlled by temperature variations.
3. The δ18O values of ostracod shells in Lake Qinghai, and of carbonate in Lake Chenghai and Lake Caohai mainly reflect the ratios of precipitation to evaporation. Humid periods with higher precipitation/evaporation ratios leads to lower δ18O values of carbonate, and vice versa. The δ13C values of carbonate in Lake Chenghai are both controlled by CO2 exchange between atmosphere and lake water and photosynthesis/respiration of aquatic plants. The larger range δ13C values of carbonate in Lake Caohai suggests the effect of photosynthesis/respiration of aquatic plants, and abnormal positive δ13C values indicates input of bacteria to carbon isotope fractionation of organic matter.
4. Based on the δ18O values of cellulose, we quantitatively reconstructed the δ18O values of lake water. According to δ18O values of cellulose and carbonate, temperature variations series have been reconstructed during the past 500 years in Lake Caohai. The result shows that there were four coldest intervals at Lake Caohai during the past 500 years, namely 1550-1610AD, 1670-1730AD, 1770-1870AD and 1890-1920AD coldest periods. The former three coldest intervals occurred during the Little Ice Age. Such a relationship suggests that co-analysis of δ18O values of cellulose and carbonate is one of the most effective approaches to reconstruct temperature variations, and also provides the evidence of the existence of Little Ice Age in Southwestern China.
5. In different lakes, δ13C values of organic matter have different response to lake productivity. As to lakes dominated by aqutic macrophytes such as Lake Caohai, organic matter becomes enriched in 13C with increased productivity. As to lakes dominated by aquatic algal such as Lake Chenghai, δ13C values of organic matter decrease with increased productivity, and the degradation of aquatic algal is the main factor of the decrease of δ13C values of organic matter with increased productivity .
6. The positive correlation between δ13C values of carbonate and organic matter in Lake Chenghai and the negative correlation between δ13C values of carbonate and organic matter in Lake Caohai suggests that lake productivity is not the only primary factor controlling the correlation between δ13C values of carbonate and organic matter, and the magnitude of lakes and the types of aquatic plants may play an important role. Carbon isotope fractionation between carbonate and organic matter (△δ13C) is an effective indicator of lake productivity, even though in the Lake Caohai where bacteria participates in the carbon isotope fractionation of organic matter.
7. Comparisons between variations of single species Eucypris inflata Li/Ca ratios from Lake Qinghai sediments and temperature variations inferred from meteorologic records and tree ring widths in adjacent regions Dulan and Qilianshan have revealed that Li/Ca ratios of ostracod shells are negatively correlated with temperature variations. Higher temperature corresponds with lower Li/Ca ratios, and vice versa. Therefore, Li/Ca ratio of ostracod shells is an effective indicator of palaeotemperature variations.
8. Variations of precipitation and temperature are synchronous with variations of solar irradiance reconstructed from the atmospheric 14C concentration in tree rings and variations of solar activity reconstructed from the 10Be concentration in ice cores, suggesting that, on decadal scales solar activity may be responsible for the synchronous variations of precipitation and temperature at Lake Qinghai during the past 800 years. |
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