【作者】 杨桂芳；

【导师】 陈中原；

【作者基本信息】 华东师范大学 ， 自然地理学， 2005， 博士

【摘要】 综合CaCO₃含量、碳酸盐碳、氧同位素资料,并结合研究区的区域特征,对兰州红咀寺剖面有机碳同位素气候指示意义进行了研究。结果表明,在黄土高原西缘有机碳同位素波动反映C₃/C₄植物比例改变,指示温度变化。δ¹³C_org值偏正时,表明C₄植物含量较大,反映温度较高。有机碳同位素和CaCO₃含量、碳酸盐碳、氧同位素共同揭示了兰州地区末次盛冰期气候较全新世明显偏低,在11.5ka BP,13.9ka BP存在两次明显的降温;进入全新世以后,兰州气候开始逐渐变暖,但仍存在8.8ka BP、5.0ka BP和3.2ka BP的三次突然升温和8.0ka BP、6.8ka BP和5.3ka BP的降温事件,说明了兰州地区气候变化的复杂性和不稳定性。更多还原

【Abstract】 This study examines the characteristics of sediment rating parameters recorded at various hydrological gauging stations in the Yangtze Basin, in relation to their controls. Our findings indicate that the distribution of parameters is closely associated with river channel morphology of the selected reaches. High b-value (>1.600) versus low log(a)-value (<-4.000) occur in the upper rock-confined valleys and characterize high unit-stream power, mostly controlled by steep gradients. Low b-value (<0.900, in general) and high log(a)-value (>-1.000) occur in the middle and lower Yangtze Basin associated with low-slope meandering reaches, and are taken to imply channel-bed aggradation of these low-stream power reaches. Of note are the higher b-value (0.900-1.600) versus lower log(a)-value (-4.000 - -1.000) that characterize the reaches between Yichang and Xinchang, immediately below the Three-Gorges valley. These values indicate channel erosion and bed instability resultant on changes in channel gradient from the upstream steep valley to downstream low slope flood-plain settings. Accompanying these changes are differences in channel morphology, i.e. upstream confined, V-shaped and downstream broad U-shaped channel patterns. There has been an apparent increase in channel bed erodibility of the middle and lower Yangtze in the past 40 years. This inference is based on sediment rating parameters from various hydrological gauging stations that record increasing b-value versus decreasing log(a)-value over that time. Analysis of the sediment load data also reveals a good correlation between the changes in sediment rating curve parameters and reduction of annual sediment budget (4.70×10~8 t/a to 3.50×10~8 t/a, from 1950s to 1990s), largely due to artificial damming of the Yangtze and subsequent sediment load depletion through siltation in the large-scale Dongting Lake located in the middle Yangtze reach. Short-term deviations from the general trends in the sediment rating parameters are related to hydroclimatic events. Extreme low b-values versus high log(a)-values signify the major flood years, while the opposite indicate drought events. When compared with riversfrom other climate settings, it is evident that wide range of values of the Yangtze rating parameters can reflect the huge discharge driven by the monsoon precipitation of the eastern China.更多还原