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

Abstract

Abstract The stable isotopic composition of dissolved inorganic carbon (δ 13 C‐DIC) was investigated as a potential tracer of streamflow generation processes at the Sleepers River Research Watershed, Vermont, USA. Downstream sampling showed δ 13 C‐DIC increased between 3–5‰ from the stream source to the outlet weir approximately 0·5 km downstream, concomitant with increasing pH and decreasing P CO 2 . An increase in δ 13 C‐DIC of 2·4 ± 0·1‰ per log unit decrease of excess P CO 2 (stream P CO 2 normalized to atmospheric P CO 2 ) was observed from downstream transect data collected during snowmelt. Isotopic fractionation of DIC due to CO 2 outgassing rather than exchange with atmospheric CO 2 may be the primary cause of increased δ 13 C‐DIC values downstream when P CO 2 of surface freshwater exceeds twice the atmospheric CO 2 concentration. Although CO 2 outgassing caused a general increase in stream δ 13 C‐DIC values, points of localized groundwater seepage into the stream were identified by decreases in δ 13 C‐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 CO 2 during the early spring growing season. Thus, in spite of effects from CO 2 outgassing, δ 13 C of DIC can be a useful indicator of groundwater additions to headwater streams and a tracer of carbon dynamics in catchments. Copyright © 2007 John Wiley & Sons, Ltd.