珠江流域水化学组成的时空变化特征及对岩石风化碳汇估算的意义

基于水-岩-气-生相互作用的碳酸盐风化碳汇模型表明,陆地碳酸盐风化碳汇是大气CO_2汇的重要组成部分;然而,碳酸盐风化碳汇的过程、机制和控制因素仍有待进一步的研究。本文采用野外监测、现场滴定和样品室内测试相结合的方法,对珠江流域支干流分旱季和雨季进行了4次野外监测取样,研究其水化学组成的时空变化特征。结果表明:珠江流域支干流的水化学组成受流域岩石风化、气候和水生光合作用的共同影响,并具有明显的时空变化特征。空间上,南盘江下游双江口段至红水河上游蔗香段,水流较慢,水质清澈,水生光合作用强烈,电导率、HCO_3~-]和Ca~(2+)]的空间变化主要受水生光合作用控制;从红水河上游蔗香段至肇庆西江大桥监测点,反映的是流域岩石风化对水化学空间变化的影响。季节变化上,西江的电导率、HCO_3~-]和Ca~(2+)]呈现出夏季低、冬春季高的特征,主要反映稀释效应的控制。西江干流河水的溶解氧具有明显的季节变化,在上述南盘江下游双江口段至红水河上游蔗香段,DO是夏季高冬季低,其他监测点是冬季高而夏季低;DO的季节变化受水生光合作用强度的控制。在南盘江下游双江口段至红水河上游蔗香段,温度是光合作用强度的限制因子,而其他监测点的光合作用限制因子为浊度影响的光照。造成上述差异的原因是监测点的水文环境不同。通过对梧州水文站流量和HCO_3~-]变化的分析发现,HCO_3~-]的季节变化幅度相对流量小得多,显示在西江流域中,HCO_3~-也存在化学稳定性行为。因此在西江流域中,流量变化是岩溶碳汇通量变化的主控因子。研究还发现,西江流域中具有强烈的生物碳泵效应,由内源有机碳形成的碳汇通量约占传统计算模式碳汇(溶解无机碳-DIC)通量的40%。因此,在估算珠江流域碳酸盐风化碳汇时,必须考虑水生生态系统光合生物对DIC的利用形成的有机碳的贡献。

Spatial and Seasonal Variations of Hydrochemistry of the Peral River and Implications for Estimating the Rock Weathering-related Carbon Sink

The carbon sink produced by carbonate weathering based on the H₂O-carbonate-CO₂-aquatic phototroph interaction on land, is a significant component of CO₂ sink. However, the process, mechanisms and controlling factors of such a carbon sink still need to be researched. A method of combining on-site monitoring and in-situ titrating with measurement of samples in laboratory was used in present study to investigate the spatial and seasonal variations of hydrochemistry in the main stream and tributary of the Pearl River. It is found that the hydrochemical compositions of river water were controlled jointly by the rock weathering, climate and aquatic primary production in the basin. For Xijiang River, the spatial variations of hydrochemistry mainly reflected the control of the rock weathering and utilization of DIC by aquatic photosynthesis. The hydrochemistry at all sites displayed similar seasonal variations, lower in summer and higher in spring and winter, but with much less variations than the river discharge. The lower concentrations of HCO₃^- and Ca²⁺ in summer were mainly due to the dilution effect. The concentrations of dissolved oxygen also showed seasonal variations and reflected the seasonal variations of aquatic photosynthetic intensity. At sampling sites with slow flowing clean water, the values of DO were higher in summer and lower in winter, showing the temperature was the controlling factor of aquatic photosynthesis. At other sampling sites, DO showed reverse variations and the light intensity determined by turbidity was the limiting factor of aquatic photosynthesis. The hydrological environment might be the reason for the difference. It was found that the chemostatic behavior ofHCO₃^-] also existed in Xijiang River, indicating the variations in discharge played a more important role in controlling the variations in the carbon sink flux than the variation inHCO₃^-].It was also found that the autochthonous organic carbon sink produced by biological carbon pump was about 40% of the dissolved inorganic carbon sink calculated by the traditional model. Therefore, autochthonous organic carbon sink must be considered in the assessment of total rock weathering-related carbon sink in the Peal River.