淡水碳酸盐湖泊中CaCO_(3)-CO^(2-)_(3)-HCO^(-)_(3)-CO_(2)化学平衡对CO_(2)的缓冲作用——以贵州百花湖为例

Buffering effect of chemical equilibrium of CaCO^(-)_(3)-CO^(2-)_(3)-HCO^(-)_(3)-CO_(2) on CO_(2) in freshwater carbonate lake:A case study of Baihua lake,Guizhou

水体吸收的CO_(2)转变为HCO^(-)_(3),构成了碳酸盐水化学系统对CO_(2)气体的缓冲,通常用Revelle因子(R)表征。陆地淡水系统释放的CO_(2)是全球碳循环的重要组成部分,一方面,湖泊水体释放的CO_(2)是来源于流域碳酸盐风化产物的输入,另一方面,碳酸盐的缓冲作用也是调节内陆水体CO_(2)释放的重要因素,这两个结论看似是矛盾的。为了揭示碳酸盐循环对水体CO_(2)的影响与缓冲机制,本研究选取一个碳酸盐岩地区的季节性分层湖泊(百花湖),分析Revelle因子变化,并与非碳酸盐湖泊进行比较。结果发现,碳酸盐岩湖泊Revell因子平均为20.1±8.1(8.0~50.0),大于表层海水的10.0(8.0~15.0),也远大于非碳酸盐地区湖泊的3.9±3.9,较高的Revelle因子意味着对CO_(2)的缓冲能力更弱。Revelle因子最大值46.4出现在夏季分层期的中部斜温层,对应的无机碳浓度为2.1 mmol·L^(-1)、pH为8.38、总无机碳与碱度比接近1.0、CO_(2)/CO_(2)3-等于1.0。实际观测与理论分析结果完全吻合,表明碳酸盐化学平衡是控制湖泊Revelle因子变化的主要因素。低pH的非碳酸盐岩系统可以溶解碳酸盐矿物,使pH升高,碱度增加,导致Revelle因子升高,在碳酸盐溶解达到平衡时Revelle因子升至最大。其后,无论是光合作用导致的碳酸盐沉淀还是呼吸作用导致的碳酸盐溶解,Revelle因子都会降低,新陈代谢导致碳酸盐系统的CO_(2)缓冲能力增强。

CO_(2) can be absorbed by water and transformed into HCO^(-)_(3).This process is well known as the buffering of carbonate system,which can be quantified using the Revelle factor(R).CO_(2) released by terrestrial freshwater systems is an important component of the global carbon cycle.On the one hand,CO_(2) released by lake water is from the input of carbonate weathering products in the basin;on the other hand,the buffering effect of carbonate is an important factor regulating CO_(2) release of inland water.These two conclusions seem to be contradictory.In order to reveal the effect of carbonate cycling on CO_(2) in freshwater lakes,in this study,we investigated the variation of Revelle factor R in a stratified lake(Baihua lake)located in carbonate area,and compared to the data obtained from lakes from non-karst areas.The results show that the average of Revelle factor in the lake water is 20.1±8.1(8.0-50.0),which is larger than 10.0(8.0-15.0)in the surface sea water and also much larger than 3.9±3.9 in lakes in non-karst areas.A higher Revelle factor means a weaker CO_(2) buffering capacity.The maximum value of R,46.4,occurred in the thermocline in summer,where corresponding variations of inorganic carbon concentration(2.1 mmol·L^(-1)),ratio of total inorganic carbon to alkalinity(1.0),ratio of CO_(2)/CO_(3)(1.0)and pH(8.38)are well coincident with the theoretical values.This consistency indicates that chemical equilibrium of carbonate is primary factor controlling Revelle factor of lake water.Lake waters with lower pH in non-karst areas could dissolve carbonate and then raise pH and alkalinity,thus elevate the Revelle factor to the maximum value while dissolution of carbonate reaches an equilibrium.Thereafter,Revelle factor will decrease and the buffering capability resulted by the metabolism increase,regardless of carbonate precipitation due to photosynthesis or carbonate dissolution during respiration.