Impacts of Carbonate Buffering on Atmospheric Equilibration of CO<sub>2</sub>, δ<sup>13</sup>C<sub>DIC</sub>, and Δ<sup>14</sup>C<sub>DIC</sub> in Rivers and Streams
Matthew Winnick, Brian Saccardi
Abstract
Abstract Rivers and streams play an important role within the global carbon cycle, in part through emissions of carbon dioxide (CO 2 ) to the atmosphere. However, the sources of this CO 2 and their spatiotemporal variability are difficult to constrain. Recent work has highlighted the role of carbonate buffering reactions that may serve as a source of CO 2 in high alkalinity systems. In this study, we seek to develop a quantitative framework for the role of carbonate buffering in the fluxes and spatiotemporal patterns of CO 2 and the stable and radio‐ isotope composition of dissolved inorganic carbon (DIC). We incorporate DIC speciation calculations of carbon isotopologues into a stream network CO 2 model and perform a series of simulations, ranging from the degassing of a groundwater seep to a hydrologically‐coupled 5th‐order stream network. We find that carbonate buffering reactions contribute >60% of emissions in high‐alkalinity, moderate groundwater‐CO 2 environments. However, atmosphere equilibration timescales of CO 2 are minimally affected, which contradicts hypotheses that carbonate buffering maintains high CO 2 across Strahler orders in high alkalinity systems. In contrast, alkalinity dramatically increases isotope equilibration timescales, which acts to decouple CO 2 and DIC variations from the isotopic composition even under low alkalinity. This significantly complicates a common method for carbon source identification. Based on similar impacts on atmospheric equilibration for stable and radio‐ carbon isotopologues, we develop a quantitative method for partitioning groundwater and stream corridor carbon sources in carbonate‐dominated watersheds. Together, these results provide a framework to guide fieldwork and interpretations of stream network CO 2 patterns across variable alkalinities.