Isotopic composition of stable carbon and carbon dioxide concentration of atmosphere in streambeds near Tombstone, Arizona

Abstract

Gas samples were taken at approximately a meter deep about every 30 days for a year at three sites from ephemeral streambeds of the Walnut Gulch Watershed near Tombstone, Arizona. The streambeds are composed of sands and gravels of volcanic or granitic origin and free of vegetation. Two of the sites are underlain by a conglomeritic layer from 90 cm below streambed surface to some unknown depth. The data from these two sites had CO₂ concentration values ranging from .39% vol. to .02% vol. and δ¹³C values ranging from -9.07%. to -19.02%0. The third site has no evidence of a conglomeritic layer near the streambed surface. The CO₂ concentration values ranged from .94% vol. to .29% vol. and δ¹³C values ranged from -17.53%. to -20.75%.. All δ¹³C values are with respect to PDB. Changes of CO₂ concentration and δ¹³C values were related to flood events, physical characteristics of the streambed and banks, type of bank vegetation, season, and fractionation of ¹³C and ¹²C between gaseous CO₂ and bicarbonate. Atmospheric CO₂ contributed significantly to the streambed atmosphere following flood events; the relative importance of atmospheric CO₂ diminished as a function of the ease with which root-respired CO₂ recharged a site.