亚热带典型岩溶区地表溪流溶解无机碳昼夜变化特征及其影响因素

DIEL VARIATION AND INFLUENCE FACTORS OF DISSOLVED INORGANIC CARBON IN A SURFACE CREEK FED BY A KARST SUBTERRANEAN STREAM IN SUBTROPICAL AREA,SW CHINA

岩溶水体溶解无机碳(DIC)的转化过程是评价岩溶碳汇稳定性及效应的重要指标之一,为了解其在岩溶地下水补给的地表溪流中的变化特征和影响因素,于2013年7月8~14日对广西融安县官村地下河水补给的地表溪流进行7昼夜定位连续监测,利用高分辨率自动监测仪器每15分钟测定pH、溶解氧(DO)、水温(T)、电导率(SpC)等参数,人工每2小时采集水样用于测定常规离子和δ^(13)C_(DIC)等指标。结果表明溪流水体T,pH,DO,SpC,DIC(以HCO_3^-表示),Ca^(2+),δ^(13)C_(DIC),方解石饱和指数(SIc)以及水体二氧化碳分压(pCO_2)等水化学指标均表现出明显的昼夜变化过程。SpC,DIC,Ca^(2+)和pCO_2等指标白天降低,夜间升高;T,pH,DO,δ^(13)C_(DIC)和SIc等指标白天升高,夜间降低。SIc变化范围为0.70~0.89,表明溪流中可能发生了碳酸盐的沉积。通过热力学参数计算表明水温(昼夜变化幅度为5.8℃)对水体pCO_2的昼夜变化影响率为27.48%~54.88%。反映水体水生植物新陈代谢过程(光合作用和呼吸作用过程)的水体DO指标和水体δ^(13)C_(DIC)均在白天上升,夜间下降,而δ^(13)C_(DIC)和水体DIC呈明显负相关关系(R^2=0.71),这也证明溪流水体DIC的变化过程受到水生植物新陈代谢过程的影响,白天水生植物以光合作用为主,消耗DIC,释放O_2,水体δ^(13)C_(DIC)上升;晚上以呼吸作用为主,消耗O_2,释放CO_2,增加DIC,水体δ^(13)C_(DIC)下降,其对水体pCO_2的昼夜变化影响率为45.12%~72.52%。通过研究表明溪流溶解无机碳变化过程受到物理因素(太阳辐射和水温等)和生物作用(呼吸作用和光合作用)的共同影响,这为进一步了解岩溶地下水出露地表后溶解无机碳转化过程及岩溶碳汇稳定性评价提供了依据。

Guancun surface creek is located in Guancun Village,Daliang Town,Rong＇an County,Guangxi Zhuang Autonomous Region,SW China,which is fed by Guancun karst subterranean stream.Guancun karst subterranean stream is about 60km away from Liuzhou City.Its outlet situated in 24°52′5.34″N and 109°20′3.41″E with an altitude of about 160m above sea level.Its drainage area is about 30.5km2 which is dominated by typical karst peakcluster in geomorphy.The geological stratum is main limestone that develops in Rongxian Formation （D3r） of upper Devonian.The annual mean temperature and rainfall in the study area are about 20℃ and 1750mm,respectively.Rain season is from March to August and dry season is from September to February in study area.The Guancun surface creek which flow length is about 1.35km with low gradient （＜2m/km） and slow flow velocity （0.2m/s） is directly recharged from the Guancun subterranean stream.Flourishing submerged plants grow in the creek channel.The sampling site is located at the surface creek mouth.High resolution data logger and high-frequency autosampler was employed to collect water samples at sampling site during July 8～14th,2013.The high-resolution data of pH,dissolved oxygen （DO）,water temperature （T） and specific conductivity （SpC） were measured at 15min interval.Water samples for analyzing hydrochemical character and δ13CDIC were collected at 2 hr interval.Solar radiation and climate data were collected by a field weather station.Partial pressure of carbon dioxide （pCO2） and calcite saturation index （SIc）were calculated by a thermodynamic model （WATSPEC software）.The result indicated that obviously diurnal cycle variations were observed in T,pH,DO,SpC,DIC,Ca2＋,δ13CDIC,SIc and pCO2 in the creek.SpC,DIC,Ca2＋ and pCO2 decreased during the daylight and increased at night.Conversely,T,pH,DO,δ13CDIC and SIc increased during the daylight and decreased at night.The variation of SIc（range from 0.70 to 0.89） indicated that carbonate precipitation may occur in the creek.By calculating the thermodynamic parameters,the ratio of influence of water temperature on the pCO2 variations in water was 27.48％ ～54.88％ in study area.DO reflecting the metabolism processes （photosynthesis and respiration） of subaquantic plant in the creek and δ13CDIC increased both in daylight and decreased at night,moreover,δ13CDIC also showed a negative correlation with DIC （R2=0.71）.The variation characteristics of δ13CDIC and DO indicated that the creek was obviously influenced by subaquatic plant metabolism.Subaquatic plants consumed DIC and released O2 in daylight due to photosynthesis.Hence,DIC decreased and DO increased in water body in daylight.Creek was mainly influenced by respiration at night,which caused the increase of DIC and decrease of DO content.The ratio of influence of subaquatic plants metabolism on the pCO2 variations in water was 45.12％ ～72.52％ in study area.The results show that DIC variation of the karst surface creek is the interactively influenced by physical factors （solar radiation,T）,biogenic process （subaquatic plants photosynthesis and respiration）.The results will contribute to clearer exposition of DIC fate which is discharged from karst aquifer and to answer the query about the stability of karst carbon sink at some extent.