珠江流域的化学侵蚀

Chemical weathering in Zhujiang River Drainage

是流域化学侵蚀过程的主要输出物质。在珠江的马口站、河口站两个断面进行了一个完整水文年(包括4个水文季节)的采样,用容量法对78组样品进行了含量分析。结果表明,珠江径流的含量在一个水文年中发生约36%的变化,汛期含量大于枯水期含量。径流含量与悬浮物含量之间存在相同的变化趋势。在同一水文季节,西江马口站断面径流含量较为均一,而北江河口站断面各采样点的含量相差较大。西江和北江流域侵蚀通量分别为1103×103mol/km2·a和1289×103mol/km2·a。影响珠江流域侵蚀通量的主要因素是流域内碳酸盐岩的出露面积和径流深度过度的土地开垦和酸雨等现代环境问题也是造成流域化学侵蚀强度增加的主要原因。

The chemical weathering processes on the continental carbonate and silicate minerals consume a large amount of carbon dioxide both from atmosphere and soil air. This gaseous substance, which is the dominating greenhouse gases in the atmosphere, was transformed into fluid substance and transported into the ocean by rivers in the form of bicarbonate ion. In this aspect, the weathering and transportation of bicarbonate ion in river system stand for an important linkage in the global carbon cycle. The range of global climate change in the geological history was weakened through this kind of negative feedback mechanism, i.e., the higher the atmospheric temperature is, the intense the chemical weatheringprocess will be. However, an intense chemical weathering process will consume much greater amount of carbon dioxide, which can drop down the atmospheric temperature. The Xijiang and Beijiang river drainage areas, which buildup most of the area of the Zhujiang (Pearl) river drainage, located in the typical sub tropical region in South China, are about 353 120 and 46 710 km2 in area, respectively. The discharge is about 230×109 m3/a for Xijiang River, and 51×109 m3/a forBeijiang River. The two drainage areas, characterized by the superposition temporarily of the high atmospheric temperature and plenty of precipitation, are theonly areas of highest yield of biomass in mid low latitude zone in Northern Hemisphere. The carbonate rocks distribute widely in the two drainage basins. Redregolith crust and limestone red earth are the main soil forming material in those areas. The surficial geochemistry process taken place in the two drainage areas is intense due to the high degree of plant coverage, and due to the plentyof precipitation and a high atmospheric temperature. On the other hand, the mechanical erosion is also intense due to the precipitous topography, and due to high population density, which lead to a high ratio of cultivated area. Xijiang River and Beijiang River converge nearby Sanshui City. The Makou and the Hekou hydrographic gauge stations are situated at the upper reaches of the converge point, and are out of the influence of the tidal current. The drainage area controlled by the two stations is close to their maximum drainage area. Those two hydrographic gauge stations were chosen as the sampling sections in this study. Three sampling vertical lines were arranged in every section, locatingat right channel, middle channel and left channel, respectively. Three samplingpositions were chosen in each sampling vertical line, locating 0.5 m under the water surface, and at the middle water depth, and 0.5 m above the river bottom, respectively. Samples were obtained in 24 July and 21 October 1997 (water levelsare 6.0 m and 2.0 m above the Pearl River basic level, respectively), and in 24February and 24 April 1998 (water levels are 0.3 m and 0.45 m above the Pearl River basic level, respectively). The sample is about 6 L in volume loaded in a plastic bucket with a cover. The four sampling seasons covered a whole hydrological year. A supplement sampling was performed in 23 July 1998, which only collected samples in the middle channel of the three sampling positions. The concentrations of bicarbonate ion of all 78 samples were determined with the method of volumetric analysis, i.e., dilute hydrochloric acid titration method. The analytical results demonstrated that the concentration of riverine bicarbonate ion is 36% higher in the high water level period than in the low water level in Zhujiang river drainage. There is the same variation tendency between the concentrations of riverine bicarbonate ion and that of the suspended grain substance. In the same hydrological seasons, the concentrations of riverine bicarbonate ion in Makou gauge station are comparably uniform. However, the concentrations of riverine bicarbonate ion in Hekou gauge station section are quite different between different samples. It can be seen from the correlation analysis between the concentrations of riverine bicarbonate ion and suspended subs