Tracing solute sources and carbon dynamics under various hydrological conditions in a karst river in southwestern China.

Understanding the mechanisms that lead various hydrological conditions to influence solute and carbon dynamics in karst rivers is a crucial issue. In this study, high-frequency sampling and analyses of water chemistry and ẟ13CDIC were conducted from October 2013 to September 2014 in a typical karst river, the Beipan River in southwestern China. The major ions (such as Ca2+, Mg2+, HCO3-, K+, SO42-, Na+, and Cl-) in the river are mainly from the weathering of carbonates and silicates and present temporal hydrological variabilities. Sr and U are mainly derived from carbonate weathering and show chemostatic behaviors responding to increasing discharge, similar to carbonate-sourced ions Ca2+, Mg2+, and HCO3-. Silicate weathering is the primary source of Al and Li, which show significant dilution effects similar to those of Na+ responding to high discharge. Meanwhile, most dissolved trace elements (such as Zn, Cu, Ba, Sb, Mn, Mo, and Pb) are strongly impacted by anthropogenic overprints and also exhibit a significant seasonal variability, which may be related with mining activities in the investigated area. A simultaneous increase of ẟ13CDIC and decrease in ∆DIC contents and pCO2 values suggests that photosynthesis is the primary control on riverine DIC variability during the high-flow season. Besides, the pCO2 values display significant chemostatic behaviors owing to the influx of biological CO2, which is produced by microbiological activities and ecological processes, and enhanced by monsoonal climatic conditions. A two-dimensional endmember mixing model demonstrates that carbonate weathering (averaging 62%) along with biological carbon (averaging 38%) are main sources to the riverine DIC, with temporal variability. Consequently, these results show that carbonate weathering and involved plant photosynthesis are the dominant processes controlling the riverine DIC contents under high discharge and temperature conditions. This work provides insight into the crucial influence of hydrological variability on solute sources and carbon dynamics under monsoonal climate for the karst rivers.

Pretreatment method for the analysis of phosphate oxygen isotope (δ18OP) of different phosphorus fractions in freshwater sediments.

Accurate measurement of the oxygen isotopic composition of dissolved phosphate (δ18OP) of different phosphorus (P) fractions in lacustrine sediments is very difficult because of the influence of large amounts of impurities. In this study, we developed a five-step method for obtaining high purity Ag3PO4 sample for the analysis of δ18OP of different P fractions in freshwater sediments. Sedimentary P was divided into NaHCO3-P, NaOH-P and HCl-P by chemical sequential extraction. Pretreatment procedures for different sedimentary P fractions were improved in the following respects: 1) abandonment of the magnesium-induced coprecipitation method to avoid the introduction of impurity ions, such as Mg2+ and Cl-, and reduce the loss of P; 2) use of a small amount of non-phosphate activated carbon powder to efficiently remove organic matter in extracts of NaHCO3-P and NaOH-P, and reduce the loss of P; 3) adjustment of the HCl-P extract pH to 4 in order to form Fe(OH)3-PO43- coprecipitate, thereby removing most of metals and Cl-. This method reduces the pretreatment steps, simplifies the operation and increases the recovery of phosphate (90.98%-96.69%). The high purity Ag3PO4 sample can be obtained and the repeatability and accuracy of measured δ18OP is better than 0.3‰, demonstrating high reliability and accuracy. This new method was used to analyze the δ18OP of different P fractions in sediments of a eutrophic lake in southwestern China. The preliminary results indicated that the δ18OP in the sediments can be used to identify different P sources, and provide new insights into sedimentary P cycling. The method established in this study provides a powerful tool for investigating the sources and biogeochemical cycle of P in freshwater sediments.

[High-resolution Distribution Characteristics of Phosphorous, Iron and Sulfur Across the Sediment-Water Interface of Aha Reservoir].

In situ two-dimension, high-resolution distribution of phosphorus(P), iron(Fe), sulfur(S) from the sediment-water interface was investigated in different areas of Aha Reservoir, a typical sub-deep water lake, using diffusive gradients in thin films(DGT) technique, combining with water chemistry and sedimentary phosphorus speciation analysis, aimed to discuss the distribution and control factors of P-Fe-S in sediments. DGT-P ranged from 0.00 mg·L-1 to 0.43 mg·L-1. DGT-Fe rangd from 0.00 mg·L-1 to 2.83 mg·L-1. DGT-S ranged from 0.00 mg·L-1 to 0.10 mg·L-1. There was no significant correlation among the concentrations of P, Fe, S in the sediment pore water of Aha Reservoir, which is markedly different from the results of previous studies. We hypothesize that the high ratios of Fe/P and Fe/S induced the direct reaction of surplus Fe(Ⅲ) with S2-, yielding FeS and/or FeS2, while P was firmly fixed in sediments by Fe(Ⅲ). NaOH-SRP(ranging between 192 and 604 mg·kg-1) and BD-P(ranging between 143 and 524 mg·kg-1) were the two major fractions of P in sediments. Since the environment for the bottom layer of water in Aha Reservoir is anoxic, the geochemical cycle of Fe and S potentially has a massive impact on the activation and migration of sedimentary P.