engleski

Carbon isotopes as tracers of processes in the karst waters

Dissolved inorganic carbon (DIC) in spring waters of karst region originates mainly from dissolution of carbonate minerals by CO2, generated from root respiration and decomposition of organic matter in the catchment area. In downstream flow the processes of photosynthesis and isotope exchange of carbon between atmospheric CO2 and DIC can play important role in the 14C and 13C composition of DIC. We used the Plitvice Lakes, Croatia, as a case study area for carbon isotope distribution in karst springs, lakes and tributary waters. PlitviceLakesconsist of 16 karst lakes at different altitudes connected by waterfalls and streams and characterized by intense calcium carbonate precipitation in form of tufa and lake sediments. Measurements of 14C and δ13C of DIC have been made on 15 points downstream from springs including lakes and tributaries along a distance of approximately 12 km. In two biggest lakes, Kozjak and Prošće, the 14C and δ13C in DIC were measured along the water depth profile down to 10 and 30 m water depth. Additionally, a14C and δ13C of dissolved organic carbon (DOC) were measured at 4 points. 14C DIC values increase from 60 and 80 pMC in the two main springs to 85 pMC in the Korana River which issues from the lakes. δ13C values range from -12.5 ‰ in springs to -9 ‰ in Korana River. The largest change/increase of both values was noticed at the waterfalls area showing stronger influence of exchange process between DIC and atmospheric CO2. In the lake areas slight decrease or steady values of 14C and δ13C indicate that biological component/photosynthesis dominates. Higher 14C values of DIC and more negative values δ13C for two tributaries indicate higher influence of organic matter decomposition. 14C values in the water depth profile showed significant increase of 4 pMC from the surface to the bottom at the thermocline level and at the same time a decrease of δ13C. 14C values of DOC are 1 - 3 pMC higher than those of DIC and δ13C values of DOC range from -24 to -21‰ for the lakes and from -28 to ‑26‰ for tributaries. The results of 14C and δ13C of DIC of Plitvice Lakes waters measured in 2011 were compared with the values measured 30 years ago (Srdoč et al, 1986). A decrease of between 3 and 9 pMC in the lakes and streams and less then 2 pMC in springs can be seen in the 2011 data in comparison with the 1981 data. The 2011 14C values are slightly higher (2 - 4 pMC) than those predicted by a prebomb-semiempiric model (Srdoč et al., 1986). No difference in δ13C values was observed in the last 30 years. References Srdoč D, Krajcar Bronić I, Horvatinčić N, Obelić B. 1986. Radiocarbon 28:515-521.

carbon isotopes ; DIC ; DOC ; karst lakes

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