Carbon Cycling Dynamics Inferred from Carbon Isotope Sourcing in a Mid-Latitude Karst-Influenced River

As ever-increasing levels of carbon dioxide alter the chemistry of the Earth’s atmosphere, understanding the global carbon cycle becomes increasingly important. A particularly important component is the riverine carbon cycle, as rivers are the primary conduits for dissolved inorganic carbon from terrestrial watersheds to ocean basins. Stable carbon isotopes (13C/12C) were collected weekly and input into the mixing model IsoSource to delineate seasonal carbon sourcing along two nested basins in the upper Green River System, Kentucky. In the more siliciclastic upstream catchment, dissolved inorganic carbon (DIC) was primarily derived from soil respiration (34%). Groundwater dissolving carbonate bedrock and carbonate dissolution/precipitation reactions contributed 31% and 11%, respectively. The more carbonate-dominated downstream catchment also was influenced greatly by soil respiration (35%). Due to the more pronounced levels of carbonate bedrock, carbonate reactions contributed double that of the upstream catchment (20%), with groundwater contributing 22%. Seasonally, the upstream basin gathered most DIC from soil respiration from late spring to winter. Early spring precipitation and still limited photosynthesis caused the primary carbon sourcing to shift to groundwater. Downstream, the primary source throughout the entire study period was soil respiration. Collectively, this study provides insight into the carbon cycling process in a mid-latitude, karstic river using carbon isotope sourcing to aid in the quantification of global carbon flux in the critical zone.