A dataset of river chemistry and carbon isotopes for the lower Xijiang River system

Sample collection and processing For simultaneous measurements of Δ14C and δ13C of riverine dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), particualte organic carbon (POC), outgassed CO2, and submerged plants, multiple rounds of sampling were performed at the outlets of the Xijiang, Guijiang, Hejiang, and Luoding Basins in both the dry (December and January) and wet seasons (June) over the period 2012–2014. To explore the longitudinal variations in the 14C content of riverine carbon, water samples were further collected along the Xijiang and Guijiang Basins in June 2016. At each sampling site, water samples were taken at 0.5–1-m depth below the water surface on the central line of the river. Submerged plant (Vallisneria natans) samples were collected from the four selected rivers for carbon isotopic analysis.\tFor outgassed CO2 sample collection, a closed-loop sampling system was designed by linking a glass bottle filled with a mixed solution of SrCl2 and NH4OH (pH>10) with a floating chamber submerged 3-5 cm below the water level, which allowed the evaded CO2 to be circulated in the system by a pump and trapped as precipitation (SrCO3) in the bottle. Prior to sampling, the background CO2 in the chamber and tubing was first removed by pumping gas from the chamber. Depending on the CO2 evasion rate, 1–4 h was required to obtain sufficient carbon for δ13C and Δ14C measurements. Replicated sampling of the evaded CO2 was performed at two different sites under various hydrological conditions to test the reliability of the sample collection system. At each site, two samples were successively collected with the same scrubbing time but different sampling times of 1 and 2 h, respectively. The results of these replicated samples have no significant difference, suggesting that there was no atmospheric contamination during sampling.In In situ and lab measurements\tThe pH and water temperature were measured on site. The alkalinity, TSS, major ions, DOC, and POC were analysed strictly in accordance with laboratory quality requirements. DIC, pCO2 and the buffer factor, or Revelle factor, were determined based on the measured alkalinity, pH and water temperature using the program CO2SYS. Most of the samples for Δ14C and δ13C analyses were tested at Beta Lab (USA), and a small portion were tested at the Institute of Heavy Ion Physics, Peking University (China). The 13C contents are reported using the conventional delta notation (δ) relative to the Vienna Pee Dee Belemnite (VPDB) standard in permil. The 14C results are expressed as Δ14C (‰) and conventional 14C age (B.P.). The report of radiocarbon results includes normalization to a δ13C value of -25‰.