Abstract

Carbon is a major source of energy within the Critical Zone, which spans from the uppermost level of vegetation to the lowest level of groundwater, where the atmosphere, biosphere, hydrosphere and lithosphere interact. This study aims to better understand the sources and cycling of carbon within the Critical Zone, by investigating two headwater catchments in the Jemez River Basin (JRB), NM as part of the JRB-SCM Critical Zone Observatory, using carbon isotopes of dissolved inorganic carbon (DIC), DIC and dissolved organic carbon (DOC) concentrations, and stream discharge. The two catchments, La Jara and Redondo Creeks, drain a resurgent volcanic dome composed primarily of rhyolitic tuff and ash. During the peak of water input during the snowmelt season, stream waters in La Jara and Redondo Creeks have δ13C-DIC values ranging from -17.5‰ to -14.3‰, which are close to those of soil organic matter within the catchment (-18.1‰) with some enrichment. This change correlates with the flushing of shallow soil waters by snowmelt, which creates a spike in the amount of DOC within stream waters. As input from snowmelt decreases, the ratio of DIC to DOC within stream waters increases along with the δ13C-DIC values of surface waters (up to -7.3‰ to -2.3‰). This increase in concentration of DIC and δ13C-DIC values suggests an increase in input from longer transit time groundwater and/or in-stream microbial cycling of carbon. Spring waters contributing to the streams have δ13C-DIC values from -10.7‰ to -3.3‰. The beginning of the monsoon season causes a brief influx of DOC (lowered DIC/DOC ratios) and decrease in δ13C-DIC values within surface waters, indicating an increase in carbon originating from shallow soil flow paths. The very positive δ13C-DIC values of stream waters and springs (up to -2.3‰) may be explained by invasion of atmospheric CO2, microbial fractionation of carbon, and/or inputs of mantle CO2. Surface waters influenced by snowmelt have PCO2 values lower than the atmosphere; invasion of atmospheric CO2 (-8‰) would cause δ13C-DIC values of stream waters to become more positive (up to ~0‰). Respiration of carbon along groundwater flowpaths or within surface waters may also cause an enrichment in 13C as microorganisms and plants preferentially remove 12C; typical enrichment values are on the order of a few per mil. In addition, volcanic fumaroles have been reported in the area, which may contribute magmatic CO2 with δ13C values of -5.5‰ to -2.5‰. Determining what controls the carbon isotope value and DIC/DOC ratios in JRB stream waters will help constrain carbon sources and cycling within the Critical Zone.

Citation

Ray, J., McIntosh, J.C., Perdrial, J.N., Brooks, P.D., Chorover, J., Rasmussen, C., Meixner, T. (2010): Sources and Cycling of Carbon in Two Semi-Arid Catchments, Valles Caldera Preserve, NM: Insights From Carbon Isotopes . AGU Fall Meeting (Poster) Abstract B21D-0340..