SPATIAL AND TEMPORAL VARIATION OF DISSOLVE INORGANIC CARBON (DIC) AND CO2 DEGASSING FROM A KARST STREAM

Riverine fluxes of carbon dioxide to the atmosphere have recently been included in assessments of regional and global carbon budgets. However, understanding of carbon dynamics in karstic watersheds remains challenging due to coupling of biogeochemical processes at the surface with dissolution reaction in the subsurface. To address this problem, we measured dissolved inorganic carbon (DIC) concentrations and its δ¹³C isotopic signature and calculated partial pressure of CO₂ (PCO₂) over two years in the Santa Fe River, which flows across a partially confined karst aquifer in North Florida. These data allow us to assess the sources and cycling of inorganic carbon, river fluxes of CO₂ and their spatio-temporal variations. Isotopic compositions of DIC (δ¹³C_DIC) vary between -7.89 and -18.46 %ₒ and divide the river into two distinct sections: (1) highly variable δ¹³C_DIC values in upper Santa Fe River which flows across the confining Hawthorn Group rocks, and (2) ¹³C–enriched DIC in the Lower Santa Fe River which flows across the unconfined Floridan Aquifer. The isotopically light samples in the upper watershed reflects soil respiration as a major source of inorganic carbon in the river and the isotopically heavy DIC in the lower basin reflects increased influence of carbonate dissolution as the source of carbon in the river water. The lower variability of δ¹³C_DIC in the upper than the lower watershed suggests that the distribution of DIC in the river water is primarily controlled by hydrogeological settings of the basin. Further, we estimate that the river is supersaturated with CO₂ with respect to atmospheric CO₂, emitting 1010 g C m^-2 yr^-1 during base flow and 5670 g C m^-2 yr^-1 during high flow on average. The elevated CO₂ fluxes result from soil CO₂ flushed to the river from storm interflow. The CO₂ fluxes are about 3-fold less from the lower than the upper watershed, reflecting consumption of CO₂ by carbonate dissolution reactions. This inference is supported by negative correlation between logPCO₂ and calcite saturation index. Elevated productivity in the headwaters of the Santa Fe River gives it a higher average carbon flux than the reported average flux rate for other streams in the US, even though the flux is reduced by dissolution reactions as it flows across the karst portion of the watershed.