Sources and Dynamics of Inorganic Carbon within the Upper Reaches of the Xi River Basin, Southwest China.

Junyu Zou,Varenyam Achal

doi:10.1371/journal.pone.0160964

Abstract

The carbon isotopic composition (δ13C) of dissolved and particulate inorganic carbon (DIC; PIC) was used to compare and analyze the origin, dynamics and evolution of inorganic carbon in two headwater tributaries of the Xi River, Southwest China. Carbonate dissolution and soil CO2 were regarded as the primary sources of DIC on the basis of δ13CDIC values which varied along the Nanpan and Beipan Rivers, from −13.9‰ to 8.1‰. Spatial trends in DIC differed between the two rivers (i.e., the tributaries), in part because factors controlling pCO2, which strongly affected carbonate dissolution, differed between the two river basins. Transport of soil CO2 and organic carbon through hydrologic conduits predominately controlled the levels of pCO2 in the Nanpan River. However, pCO2 along the upper reaches of the Nanpan River also was controlled by the extent of urbanization and industrialization relative to agriculture. DIC concentrations in the highly urbanized upper reaches of the Nanpan River were typical higher than in other carbonate-dominated areas of the upper Xi River. Within the Beipan River, the oxidation of organic carbon is the primary process that maintains pCO2 levels. The pCO2 within the Beipan River was more affected by sulfuric acid from coal industries, inputs from a scenic spot, and groundwater than along the Nanpan River. With regards to PIC, the contents and δ13C values in the Nanpan River were generally lower than those in the Beipan River, indicating that chemical and physical weathering contributes more marine carbonate detritus to the PIC along the Beipan River. The CO2 evasion flux from the Nanpan River was higher than that in the Beipan River, and generally higher than along the middle and lower reaches of the Xi River, demonstrating that the Nanpan River is an important net source of atmospheric CO2 in Southwest China.

Highlights

During the past two decades there has been increasing interest in biogeochemical processing of dissolved inorganic carbon (DIC) in freshwater riverine ecosystems at the global, regional, and local scale [1,2]
The cited results indicate that the observed variations of δ13CDIC values may be influenced by multiple factors, including soil CO2 produced by root respiration and microbiologic degradation, dissolution of carbonate rock, isotopic exchange with the atmosphere by degassing of CO2, the involvement of sulfuric acid derived from the dissolution of evaporates, the oxidation of sulfuric minerals and coal-containing strata, and various types of anthropogenic inputs
Urbanization and industrialization had a strong influence on the pCO2 and the formation of DIC in the upper reaches of the Nanpan River characterized by silicate bedrock

Summary

Introduction

During the past two decades there has been increasing interest in biogeochemical processing of dissolved inorganic carbon (DIC) in freshwater riverine ecosystems at the global, regional, and local scale [1,2]. At the local to regional scale, inorganic carbon is an important factor controlling the buffering of stream waters against changes in pH, and the speciation and solubility of dissolved constituents (e.g., trace metals) [17] as well as the kinetics of chemical reactions within the water column. As such, it influences the geochemical nature of the aquatic environment. The impacts of anthropogenic activities on DIC needs to be urgently documented to better understand their influence on chemical weathering processes and the carbon cycle within headwater tributaries of the Xi River. Along the channels, and to estimate the fluxes of CO2 outgassing along the Nanpan and Beipan Rivers

Geographic and Hydrologic Settings

Carbon isotopic composition of dissolved inorganic carbon in the rivers

Variation in pCO2 and the controlling factors

Findings

CO2 evasion to the atmosphere

Conclusions