Tracing source and transformation of carbon in an epikarst spring-pond system by dual carbon isotopes (C-13-C-14): Evidence of dissolved CO2 uptake as a carbon sink

摘要

delta C-13 and delta C-14 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 (DC)-C-14 and delta C-13 values of DIC for the spring, midstream pond (MP) and downstream pond (DP) are -26 +/- 36 parts per thousand and -13 +/- 2 parts per thousand, 6 +/- 56 parts per thousand and -12 +/- 3 parts per thousand, and 0 +/- 64 parts per thousand and -9 +/- 2 parts per thousand, respectively. The carbon source for the DIC is mainly from biogenic CO2 rather than the dissolution of limestone rock as the delta C-14 and delta C-13 of limestone are about 1000% and 2%, respectively. The enrichment trend of (DCDIC)-C-14 and delta C-13(DIC) from the spring to the DP indicates CO2 exchange between atmospheric CO2 and DIC, because (DC)-C-14 and delta C-13 values of atmospheric CO2 are ca. 50 parts per thousand and -8 parts per thousand, respectively. The average (DCPOC)-C-14 values in the spring, MP and DP were -325 parts per thousand, -123 parts per thousand and -158 parts per thousand, respectively, which are all lower than these of the DIC in each reservoir. The lower (DC)-C-14 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 (DCPOC)-C-14 and evidence of growth in EDS/SEM analyses. Furthermore, the (DC)-C-14 values of the submerged aquatic plants range from -153 parts per thousand to -26 parts per thousand, reflecting that the aquatic plants used DIC for photosynthesis. The (DC)-C-14 value of an emergent plant which uses atmospheric CO2 during photosynthesis is 52.5 +/- 0.3%, equivalent to the atmospheric (DC)-C-14. Seasonal variations of (DCDIC)-C-14 and delta C-13(DIC) are influenced by soil CO2 input, primary productivity in the ponds, and CO2 exchange; hydrochemical condition show lower (DC)-C-14 values but higher delta C-13 values in cold/dry season, and vice versa in summer rainy season. A simple mass balance calculation indicates similar to 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 C-14 dating on aquatic plant remains, regional and perhaps global carbon budgets, and the different behaviors of C-13 and C-14 in karst systems.