ASSESSING THE TEMPORAL EVOLUTION OF DISSOLVED INORGANIC CARBON IN SURFACE WATERS THAT INTERACT WITH ATMOSPHERIC CO2(G)

Documenting the transformation of dissolved inorganic carbon (DIC) during the interaction of surface waters (e.g., rivers, lakes) with atmospheric CO_2(g) is vital to understanding carbon cycling. We exposed an artificial solution of NaHCO₃ and groundwater (potential source of surface water) and lake water samples to the atmosphere in a laboratory setting for 850 to 1000 hrs, until they attained chemical and isotopic equilibrium with atmospheric CO_2(g). All samples were prepared in duplicate and one set was agitated to simulate mixing in surface waters. The DIC concentrations of the NaHCO₃ samples increased with no C loss and the δ¹³C_DIC was enriched to a steady state for the mixed sample. We modeled the increase in the DIC concentrations as evaporation and the increases in the δ¹³C_DIC as equilibrium carbon isotopic exchange with atmospheric CO_2(g). The DIC concentrations in the mixed groundwater samples initially decreased due to CO_2(g) outgassing and the accompanying increases in δ¹³C_DIC was modeled as kinetic isotopic fractionation. After the initial decrease, the DIC concentrations increased continuously while the δ¹³C_DIC increased to a steady state. The increasing DIC concentrations was modeled as evaporation and the increasing δ¹³C_DIC as equilibrium carbon isotopic exchange with atmospheric CO_2(g). Overall, the unmixed samples showed similar temporal trends to the mixed samples, even though the samples did not achieve chemical and isotopic equilibrium with atmospheric CO_2(g). Both the mixed and unmixed lake samples showed only small increases in temporal DIC concentrations and a slight initial decrease, followed by a small increase in the δ¹³C_DIC during the experiment. The minor changes suggests that these samples were closer to chemical and carbon isotopic equilibrium with atmospheric CO_2(g). Our models based on the DIC concentrations and δ¹³C_DIC can be used to assess processes and their temporary trajectory during carbon cycling in surface waters with variable water residence times.