岩溶水库热结构变化对水体溶解无机碳及其同位素的影响过程

Influence process of thermal structure variations of a karst water reservoir on dissolved inorganic carbon and its stable carbon isotope

以中国南方亚热带地区典型的地下水补给型水库——大龙洞水库为对象,于2018年1月、4月、7月、10月、12月分别在上、中、下游三个监测点进行采样,探究水库热结构变化对于水体无机碳及其同位素的影响过程及机理。结果表明:(1)大龙洞水库水体在一个水文年中呈现周期性的混合期—分层期—混合期的热结构变化,4月热分层开始显现,7月逐渐显著呈现完整的热分层,10月以后热分层逐渐消失,水体逐渐实现混合;(2)水体热分层是溶解无机碳(DIC)浓度与碳稳定同位素(δ^(13)C_(DIC))值变化的主要驱动力。表水层中DIC主要受水—气界面二氧化碳脱气、水生生物光合作用控制,其DIC浓度与δ^(13)C_(DIC)值分别为3.22 mmol·L^(−1)和−9.15‰;温跃层中DIC主要受有机质降解过程影响,其DIC浓度与δ^(13)C_(DIC)值分别为3.43 mmol·L^(−1)和−9.70‰;底水层中DIC主要受碳酸盐沉淀过程影响,其DIC浓度与δ^(13)C_(DIC)值分别为4.32 mmol·L^(−1)和−11.89‰;(3)三种过程伴随水库热结构的变化而变化,驱动DIC浓度及其同位素的变化梯度G(DIC)与G(δ^(13)C_(DIC))的变化,表现为底水层<表水层<温跃层。热分层结束进入混合期后,DIC浓度与δ^(13)C_(DIC)值的时空差异均逐渐消失,最终表现出DIC浓度与δ^(13)C_(DIC)值的均一化。

Dalongdong Reservoir,a typical groundwater recharge reservoir in karst area of southwest China,is located at 23°30′01″−23°40′08″N and 108°30′02″−108°36′04″E in Shanglin county,Nanning City,Guangxi.This large reservoir was built mainly for irrigation and power generation by blocking caves,sinkholes and some fissures in a karst valley in 1958.It covers a rainwater collecting area of 310 km2,totaling storage capacity of 151 million m3.Approximately 95%of the recharge water comes from two karst subterranean streams.The principal strata in the reservoir catchment are carbonate rocks of Carboniferous(C1)and Devonian(D2d3)era.Influenced by the subtropical monsoon climate,the average temperature in the reservoir area is 21℃and the average rainfall is 1,837.3 mm.The rainy season in this area is from April to September.Samples were collected respectively by the plankton method at the upper,middle and downstream monitoring sites in January,April,July,October and December,2018.Based on DIC concentration,δ^(13)C_(DIC) value,water temperature,DO,SIC,partial pressure of carbon dioxide,carbon dioxide diffusion flux and other indicators,the thermal structure change on DIC concentrations and isotope composition δ^(13)C_(DIC) in each water layer of reservoir and its influencing factors are systematically studied.The gradient differences of DIC concentrations and isotopic composition δ^(13)C_(DIC) in different water layers and their causes are also discussed.In order to provide scientific and technological support for accurate assessment of carbon budget of land and water,the carbon cycle process of karst reservoirs was deeply revealed.The results show that Dalongdong reservoir presents periodic thermal structure changes in the mixing stage−stratification stage−in a hydrological year.The thermal stratification begins to appear in April,gradually presents complete thermal stratification in July,and gradually disappears after October.Besides,thermal stratification is an important driving force of DIC concentration and δ^(13)C_(DIC) value.After the formation of thermal stratification,DIC concentrations in the upper,middle and lower reaches show the same characteristics as those of the δ^(13)C_(DIC) values at the same layer,and DIC concentrations increase with depth,while δ^(13)C_(DIC) values become negative.DIC concentrations and δ^(13)C_(DIC) values are respectively 3.22 mmol·L^(−1) and−9.15‰in surface water,3.43 mmol·L^(−1) and−9.70‰in thermocline,and 4.32 mmol·L^(−1) and−11.89‰in bottom water.After thermal stratification,the vertical,horizontal and seasonal variations of DIC concentrations and δ^(13)C_(DIC) values gradually disappear,and finally show their homogenization.Finally,carbon dioxide degassing and photosynthesis dominate the water-air interface in the surface water layer.Degradation of organic matter is the main process in thermocline,and carbonate precipitation can be mostly found in the bottom water layer.The change of these three processes with the reservoir thermal structure may drive the variation of DIC concentrations and isotope gradient G(DIC)and G(δ^(13)C_(DIC)),which shows the characteristics of bottom water layer<surface water layer<thermocline layer.