Carbon isotope fractionation of dissolved inorganic carbon (DIC) due to outgassing of carbon dioxide from a headwater stream

The stable isotopic composition of dissolved inorganic carbon (δ¹³C-DIC) was investigated as a potential tracer of streamflow generation processes at the Sleepers River Research Watershed, Vermont, USA. Downstream sampling showed δ ¹³C-DIC increased between 3-5‰ from the stream source to the outlet weir approximately 0·5 km downstream, concomitant with increasing pH and decreasing PCO₂. An increase in δ¹³C-DIC of 2.4 ± 0·1‰ per log unit decrease of excess PCO₂ (stream PCO₂ normalized to atmospheric PCO₂) was observed from downstream transect data collected during snowmelt. Isotopic fractionation of DIC due to CO₂ outgassing rather than exchange with atmospheric CO₂ may be the primary cause of increased δ ¹³C-DIC values downstream when PCO₂ of surface freshwater exceeds twice the atmospheric CO₂ concentration. Although CO₂ outgassing caused a general increase in stream δ¹³C-DIC values, points of localized groundwater seepage into the stream were identified by decreases in δ¹³C-DIC and increases in DIC concentration of the stream water superimposed upon the general downstream trend. In addition, comparison between snowmelt, early spring and summer seasons showed that DIC is flushed from shallow groundwater flowpaths during snowmelt and is replaced by a greater proportion of DIC derived from soil CO₂ during the early spring growing season. Thus, in spite of effects from CO₂ outgassing, δ¹³C of DIC can be a useful indicator of groundwater additions to headwater streams and a tracer of carbon dynamics in catchments.