珠江水系桂平、高要和清远站河水化学组成的季节变化及对化学风化研究的意义

文章报道了珠江水系桂平、高要和清远3个站河水主要离子及部分微量元素近一年持续观测数据。结果显示,河水中的Cl-主要通过夏季风降雨从海洋输送而来,Cl-/Na比值呈现季风雨季较高的特点,K含量则与Rb含量显著相关,指示河水中K主要是由硅酸盐化学风化贡献,而 Rb/K 比值与水温呈现负相关的关系,暗示植物分解对河水中K的贡献不大。3个站点中河水的主要离子及微量元素均主要来自碳酸盐的化学风化的贡献,相对而言桂平站碳酸盐化学风化的贡献最大,高要站次之,而清远站则来自硅酸盐化学风化的贡献较大。流域的化学风化存在明显的季节性变化,硅酸盐的化学风化主要呈现夏秋季节较强,冬季相对较弱的特点,而碳酸盐的化学风化也呈现显著的季节波动,但3个站的变化样式并不一致,可能存在更复杂的控制因素。

SEASONAL VARIATIONS OF THE RIVER WATER CHEMICAL COMPOSITIONS AT GUIPING,GAOYAO AND QINGYUAN STATIONS OF THE PEARL RIVER SYSTEM

Chemical compositions of river dissolved loads provide a mean to estimate regional chemical weathering rates,which links to calculate atmospheric CO2 consumption by continental weathering.Such investigations have broadly carried out in many of the China river systems.Most of these estimations,however,are based on the observations handled at limited time spans,and time series observations covering several seasons at definite locations are scarce.Considering that seasonal climatic change is significant in most of the Chinese river systems,time-series observations provide more comprehensive results and help to achieve to more accurate estimation on CO2 consumption.We have conducted ca.1-year observations at Guiping,Gaoyao and Qingyuan stations of the Pearl River system,and the main ions(F-,Cl-,NO-3,SO2-4,Na+,K+,Mg2+ and Ca2+)and some selected trace elements(Rb and Sr)of the river water were reported here. Cl-can be used to evaluate the contributions from atmospheric inputs to the river water,and the results indicate that the mean Cl-/Na are 0.72±0.12,0.75±0.17 and 0.66±0.20 in Guiping,Gaoyao and Qingyuan stations,respectively,and high Cl-/Na ratios generally occur during rainy seasons,suggesting that Cl-in the river water was mainly from the sea carrying by the summer monsoon precipitation.K/Rb ratios in the river water were 323±73,518±91 and 822±91 for Guiping,Gaoyao and Qingyuan stations,respectively,which were significantly higher than the mean K/Rb of terrestrial continent(ca.260).However,K concentrations were highly correlated to those of the Rb,with higher K/Rb ratios during winters and lower K/Rb ratios during summers.This suggests that K was mainly contributed from silicate weathering,and the contribution from decaying of plants was negligible.Carbonate weathering significantly contributed to the river water at all these stations,and the maximum contribution was observed at Guiping Station.The maximum contribution from silicate weathering was observed at Qingyuan Station,and the situations for Gaoyao Station were between them.Significant seasonal variations of the silicate and carbonate weathering were implied by the large ventilations of Ca/Na,Mg/Na,Rb/Na and Sr/Na ratios of the ca.1-year observations at these stations.Similar trends of Rb/Na variations were observed in all these stations,with higher Rb/Na occurring during summers and autumns and lower during winters,suggesting that silicate weathering was enhanced during summers and autumns,and relatively weaken during winters.Warm and humid climate,as well as enhanced organic material decays during summer and autumn favors chemical weathering.While the variations of carbonate weathering were different in these stations,suggesting more complicated controls for carbonate weathering.