Seasonal and Spatial Variations of δ13CDIC Values in the Mun River, Northeast Thailand

As an important part of the global carbon cycle, dissolved inorganic carbon (DIC) concentration and its stable carbon isotopic composition (delta C-13(DIC)) have been used to constrain the sources of DIC in rivers. In this study, we systematically investigated the water chemistry, DIC contents, and delta C-13(DIC) values in a tropical agricultural river in northeast Thailand. The water temperature ranged from 20.3 to 31.3 degrees C, and water pH values ranged from 6.4 to 8.4, with seasonal variations. Based on the major ion compositions, the hydro-chemical type of the Mun River water was a unique Na-Ca-Cl-HCO3 type, controlled by evaporite and silicate weathering. Seasonal variation of DIC concentrations and its carbon isotopic composition was obvious; DIC and delta C-13(DIC) were significantly lower in the wet season (135 to 3146 mu mol/L and -31.0 parts per thousand to -7.0 parts per thousand) compared to the dry season (185 to 5897 mu mol/L and -19.6 parts per thousand to -2.7 parts per thousand). A high level of C-12-enriched DIC/CO2 from soil respiration and organic matter oxidation may cause the low pH values, delta C-13(DIC) values, and high partial pressure of CO2 (pCO(2)) in the middle and lower reaches during the wet/rainy season compared to the dry season. This may be responsible for the seasonal and spatial variations of DIC concentrations and delta C-13(DIC) values in the Mun River. According to the relationship between pCO(2) and delta C-13(DIC) values, CO2 outgassing may be more significant in the dry season, due to the greater influx of groundwater with higher pCO(2) levels; and the rapid CO2 diffusion into the atmosphere will continuously increase the delta C-13(DIC) values and decrease pCO(2) levels. These results show that riverine biologic effects and CO2 outgassing play important roles in the DIC and delta C-13(DIC) evolution of this typical agriculturally-dominated watershed.

Automated summary

This study investigated the water chemistry, DIC contents, and delta C-13(DIC) values in a tropical agricultural river in northeast Thailand. The results show significant seasonal variations in the DIC concentrations and delta C-13(DIC) values, with the wet season having lower values. Riverine biologic effects and CO2 outgassing play important roles in the evolution of DIC and delta C-13(DIC) in this agriculturally-dominated watershed.