Hong-Chun Li

Transportation characteristics of δ 13 C in the plants-soil-bedrock-cave system in Chongqing karst area

Here we use an analytical method to determine δ13C in local plants and organic matter in the soils above Furong cave, Chongqing, China. We also monitored δ13C in dissolved inorganic carbon (DIC) of drip water, δ13C of active deposits under the drip waters, and the concentration of air CO2 (PCO2). Based on these, we preliminarily studied the transportation characteristics of stable carbon isotope (13C) in cave system of the subtropical karst area. The average δ13C value of 27 local plant samples, which belong to 16 families, was −32‰ and the weighted δ13C for surface dry biomass was −33‰. We found that for 54 soil samples collected from 5 soil profiles, δ13C of soil organic matters was −22‰, which could be attributed to the different transportation rates of stable carbon isotopes during the decomposition of plants and organic matters in soils. The relatively lighter 12C tended to transfer into gaseous CO2, which made the relatively heavier 13C concentrated in the soils. On the basis of monitoring of DIC-δ13C in drip waters from July 2009 to June 2010, we found that values in winter months were heavier and values in summer months were lighter in general, the reason of which was that in summer months, both the temperature and the humidity were comparatively higher, resulted in more CO2 with lighter δ13C generated from organic matters decomposition and plants respiration. The average DIC-δ13C value was −11‰, about 11‰ heavier than the δ13C of organic matters in soils, which proved that part of DIC in cave drip water was sourced from dissolution of inorganic carbonate (host rock, with heavier δ13C). As for the δ13C of active deposits at five drip water sites in Furong cave, they had almost the same variation with relatively light values. In other words, these active speleothems were deposited at equilibrium conditions for isotopic fractionation. These results suggest that the carbon isotopic information of speleothems could be used to track the evolution of local vegetation in certain situations.