基于锶同位素的亚热带典型岩溶动力系统降雨条件下岩溶作用速率及其意义

摘要

Objectives:Karst dynamic system is a extremely open,sensitive and complex system.An important method on the research of dissolution rate and carbon cycle in karat dynamic system is hydrological and geochemical analysis under the influence of rainfall in a karst area.Strontium and its isotope (n(87Sr)/n(86Sr)) are important tools to trace dissolution rate,intensity and carbon sink in karst process.Achievements and studies were mainly on hydrogeological processes,groundwater solutes origin and anthropogenic activities through strontium research.However,very few studies have been conducted to distinguished the differences of dissolution rate and carbon sink flux triggered respectively by "old water" storage and fresh rainwater recharge in a karst dynamic system.Methods:Employing the high-resolution monitoring and sampling methods,a typical karst dynamic system in subtropical area,south China was chosen to research diel variations of groundwater Sr and n (87 Sr)/n(86Sr) in a karst spring and to calculate the dissolution rate in a storm rainfall.In-situ measurement of hydrochemical variables including temperature,pH and specific conductivity (Spc,25 ℃) were performed at S31 spring outlet using multi-parameters meters.HCO3-was titrated immediately in the field using a portable testing kit by Merck KGaA Co.(Germany).For continuous streamflow data,weir water stage was continuously monitored at 15-min time interval at S31 spring using a CTDP data sonde.Discharge was then calculated using the rectangular weir-discharge formula.Rainwater sample and filtered spring water samples were collected in pre-washed high density polyethylene (HDPE).Major anions were measured by an automated ion chromatography.Major cations were analyzed by ICP-OES (IRIS Intrepid Ⅱ XSP,Thermo Fisher Scientific,USA).In addition,the water samples (500 mL) and a bedrock sample (limestone) were collected for the determination of n (87 Sr)/n(86Sr).Isotope analyses were determined using a Finnigan MAT 261 multiple collector mass spectrometer.Partial pressure of CO2 (PCO2) and saturation index of calcite (SIc) were calculated by the program WATSPEC using hydrochemical data sets.Results:The results showed that both CO2 effect and dilution effect were the controlling mechanism in hydrochemical variation of spring water in the initial rainfall.Dilution effect became the main controlling mechanism with the spring discharge rise and rainfall continuance.The variations of Sr concentration in spring water were obviously impacted by dilution effect,while n(87Sr)/n(86Sr) variations were not almost impacted,which indicated that continuously karst processes in karst aquifer in rainfall event may maintained the almost stable n(87Sr)/n(86Sr) value of spring water.Conclusions:Results from the mass balance equation indicated that n(87Sr)/n(86Sr) shares originated from the limestone dissolution which triggered by fresh rainwater recharge was 24.3％ of total n(87Sr)/n(86Sr) flux in spring water,and the net dissolution rate of CaCO3 (limestone) and carbon sink flux was 0.136 mg/(cm2 · d) and 1.01 t CO2,respectively.These results preliminary distinguished the differences of dissolution rate and carbon sink flux triggered respectively by "old water" storage and fresh rainwater recharge in a karst dynamic system.These results also contribute to the exact calculation of dissolution rate and carbon sink flux that triggered by a rain event.%岩溶动力系统中微量元素及其同位素是指示岩溶动力系统运行过程及其环境意义的重要指标.本研究以我国南方亚热带地区典型岩溶动力系统为研究对象,利用高分辨率监测及采样方法研究了暴雨期昼夜尺度上岩溶泉水微量元素Sr及其同位素的变化特征,计算了暴雨条件下的岩溶作用速率.研究结果表明CO2效应和稀释效应是降雨初期泉水水化学变化的主要控制机制,随后随着流量的持续上升,稀释效应占主导地位.降雨所导致的稀释效应对泉水Sr含量变化影响非常明显,但对岩溶泉水n(87Sr)/n(86Sr)同位素的影响非常有限.降雨期岩溶动力系统内持续的岩溶作用,维持了泉水相对稳定的n(87Sr)/n(86Sr)值特征.通过质量平衡方程计算表明暴雨期内补给的雨水溶蚀灰岩来源的n(87Sr)/n(86Sr)只占泉水总输出n(87Sr)/n(86Sr)的24.3％,当期补给的雨水所引发的CaCO3净溶蚀速率为0.136 mg/(cm2·d),其所产生的碳汇通量为1.01 t CO2.这一结果初步确定了岩溶动力系统中“老水”和新近补给的雨水所导致的溶蚀速率差异和碳汇量差异,这为准确计算降雨诱发的岩溶溶蚀速率和碳汇量准确计算具有十分重要的意义.