Tracing source and transformation of carbon in an epikarst spring-pond system by dual carbon isotopes (13C14C): Evidence of dissolved CO2 uptake as a carbon sink

δ13C and D14C measurements on dissolved inorganic carbon (DIC), particulate organic carbon (POC) and aquatic plants from a karst spring and two spring-fed ponds in Laqiao, Maolan Township, Libo County, southeastern Guizhou of China in January, July and October of 2013 have been carried out to understand the roles of aquatic photosynthesis through DIC uptake in surface karst waters. The mean D14C and δ13C values of DIC for the spring, midstream pond (MP) and downstream pond (DP) are −26 ± 36‰ and −13 ± 2‰, 6 ± 56‰ and −12 ± 3‰, and 0 ± 64‰ and −9 ± 2‰, respectively. The carbon source for the DIC is mainly from biogenic CO2 rather than the dissolution of limestone rock as the D14C and δ13C of limestone are about −1000‰ and 2‰, respectively. The enrichment trend of D14CDIC and δ13CDIC from the spring to the DP indicates CO2 exchange between atmospheric CO2 and DIC, because D14C and δ13C values of atmospheric CO2 are ca. 50‰ and −8‰, respectively. The average D14CPOC values in the spring, MP and DP were −325‰, −123‰ and −158‰, respectively, which are all lower than these of the DIC in each reservoir. The lower D14C values of the POC may be caused by older soil carbon from surface runoff and dust fall. More aquatic algae were formed through photosynthesis in the stream ponds, especially in summer, shown by strongly increased D14CPOC and evidence of growth in EDS/SEM analyses. Furthermore, the D14C values of the submerged aquatic plants range from −153‰ to −26‰, reflecting that the aquatic plants used DIC for photosynthesis. The D14C value of an emergent plant which uses atmospheric CO2 during photosynthesis is 52.5 ± 0.3‰, equivalent to the atmospheric D14C. Seasonal variations of D14CDIC and δ13CDIC are influenced by soil CO2 input, primary productivity in the ponds, and CO2 exchange; hydrochemical condition show lower D14C values but higher δ13C values in cold/dry season, and vice versa in summer rainy season. A simple mass balance calculation indicates ~90% of carbon for the spring DIC is from biogenic CO2, with higher contribution in summer due to higher productivity. Although this simple calculation may overestimate the biogenic CO2, it indicates that organic decomposition is a major carbon source for DIC in the karst hydrological system. The results of the present study have implications for 14C dating on aquatic plant remains, regional and perhaps global carbon budgets, and the different behaviors of 13C and 14C in karst systems.