Assessing the temporal evolution of dissolved inorganic carbon in waters exposed to atmospheric CO2(g): A laboratory approach

Abstract

Summary 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 for understanding carbon cycling. Investigations that mimic the continuum of changes in DIC concentrations and stable carbon isotope ratio of DIC (δ 13 C DIC ) to equilibrium with atmospheric CO 2(g) are difficult to conduct in natural settings because of multiple processes that occur in the water column, the interaction between water and sediments or rocks in stream channels and lake beds, as well as the variability in water residence times. Thus, laboratory simulations of the spectrum of DIC transformation provide insights which reduce the ambiguity in describing the mechanisms that control the behavior of DIC during surface water-atmospheric CO 2(g) interaction. To test how surface water-atmospheric CO 2(g) interaction affects DIC concentrations and δ 13 C DIC , we used three types of samples: (1) we prepared an artificial solution using NaHCO 3 where the DIC concentration is near chemical equilibrium and the δ 13 C DIC is far from isotopic equilibrium with atmospheric CO 2(g) , (2) natural groundwater where the DIC concentration and the δ 13 C DIC are both sufficiently far from chemical and isotopic equilibrium with atmospheric CO 2(g) and (3) lake water where the DIC concentration and the δ 13 C DIC are near chemical and isotopic equilibrium with atmospheric CO 2(g) . These samples allowed us to ascertain when only chemical or isotopic changes are occurring, or when both chemical and isotopic changes are occurring. The NaHCO 3 solution was prepared by dissolving ∼6 g of laboratory grade NaHCO 3 salt in 20 L of deionized water. Groundwater was collected from Stillwater, Oklahoma (36°08′22.20″N, 97°03′22.66″W) and lake water was collected from Lake McMurtry, Stillwater, Oklahoma (36°10′49.37″N, 97°10′52.9″W). The solution of NaHCO 3 , and groundwater (potential source of surface water) and lake water samples were exposed to the atmosphere in a laboratory setting for 850–1000 h until their DIC 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 NaHCO 3 samples increased without C loss and the δ 13 C DIC was enriched to a steady state for the mixed sample. The increase in the DIC concentrations was modeled as evaporation and not as CO 2(g) invasion since the pCO 2 was higher than atmospheric throughout the experiment. The enrichment in the δ 13 C DIC was modeled as equilibrium carbon isotopic exchange with atmospheric CO 2(g) . The DIC concentrations in the mixed groundwater sample initially decreased due to CO 2(g) outgassing and the accompanying enrichment in δ 13 C DIC was modeled as kinetic isotopic fractionation. After the initial decrease, the DIC concentrations increased continuously while the δ 13 C DIC was enriched to a steady state. Overall, the unmixed groundwater sample showed similar temporal δ 13 C DIC trends to the mixed groundwater sample, even though the unmixed sample did not achieve 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 enrichment in the δ 13 C DIC during the experiment. The minor changes suggest that the lake samples were closer to chemical and carbon isotopic equilibrium with atmospheric CO 2(g) . The results of this study would apply in settings where the predominant process controlling carbon cycling is the interaction between the surface water DIC and atmospheric CO 2(g) .