A laboratory study investigating the effects of dilution by precipitation on dissolved inorganic carbon and stable isotope evolution in surface waters

Abstract

Surface waters are a major pathway of carbon cycling between the atmosphere and the earth's surface. Yet studies describing water column processes that affect carbon cycling do not consider the effects of dilution by precipitation. In this study, we conducted a laboratory experiment in which we prepared undiluted (100%) and snowmelt diluted 25, 50, and 75% by volume of samples of NaHCO₃ solution and lake and river water and then exposed them to the laboratory atmosphere for up to 1000 h. We aim to determine how dilution by precipitation followed by water-atmosphere CO_2(g) interaction affects DIC and δ¹³C_DIC evolution. Dilution resulted in decreased pH, solutes, and DIC concentrations according to the dilution proportion. The decreased pH perturbed the carbonate equilibrium resulting in CO_2(g) outgassing. In all the samples, there was continuous enrichment in the δ¹³C_DIC composition. Isotopic evolution by CO_2(g) loss in the > 50% snowmelt-diluted samples lasted for about 10 h, while it took about 400 h for the diluted samples to evolve to a similar isotopic composition as the undiluted samples. Our laboratory results suggest that the effects of precipitation dilution should not be ignored in studying DIC evolution in surface waters during periods where precipitation dilution exceeds 50% by volume.