INVESTIGATING THE EFFECTS OF PRECIPITATION ON CARBON EVOLUTION IN SURFACE WATER THAT INTERACT WITH ATMOSPHERIC CO2(G): SIGNIFICANCE OF STABLE CARBON ISOTOPE RATIOS

We performed laboratory studies in which we exposed solutions of NaHCO₃, lake water and river water samples diluted with snow melt at 25, 50 and 75% proportions and undiluted samples to the laboratory air from 0 to 1000 hours. We aim to trace the evolution of carbon in the samples as they interact with atmospheric CO_2(g) and determine the effect of precipitation on carbon evolution. The samples were agitated by aquarium pumps to simulate turbulence and mixing in surface waters. We made concentration measurements of alkalinity and dissolved inorganic carbon (DIC) and measured the stable carbon isotope ratio (δ¹³C) of DIC. In all samples the alkalinity and DIC concentrations were lowest at 25% mixture; followed by 50% and 75% and the highest concentrations were measured in the undiluted samples. The highest δ¹³C values where recorded in the 25%, followed by the 50%, 75% and the undiluted samples for the NaHCO₃ and river samples but for the lake samples, the lowest δ¹³C were recorded for the 25% samples, followed by the 50%, 75% and the undiluted samples. The alkalinity and DIC concentrations of all three solutions mirrored each other with no apparent change over 100 hours of exposure but increased over 110% for the NaHCO₃ and river samples and 32% for the lake samples from 100 to 1000 hours. Overall, the δ¹³C of DIC increased for the NaHCO₃ and river samples over time in all mixtures and established a shift of about 16.0 per mil for the NaHCO₃ sample and 7.0 per mil for the river sample. In the lake water, the undiluted samples δ¹³C of DIC stayed at 4.0 ± 1.0 per mil from 0 to 1000 hours. Though dilution yielded slightly different δ¹³C of DIC values at the start of the experiment, they all culminated to a constant value after about 250 hours of exposure. The overall increase in alkalinity and DIC in all samples was due to evaporative concentration, as water evaporated from the samples over time. This suggests that DIC loss to the atmospheric reservoir from the conversion of CO_2(aq) to CO_2(g) was negligible. The faster recovery rate of the δ¹³C of DIC of the diluted samples to constant values over time shows that precipitation could drive a surface water system faster to equilibrium and that the dilution proportion could have an effect on the rate of carbon exchange between the sample DIC and atmospheric CO_2(g).