Faxiang Tao

The Amazon River: behaviour of metals (Fe, Al, Mn) and dissolved organic matter in the initial mixing at the Rio Negro/Solimoes confluence

We studied the changes in major elements and organic carbon concentrations during the initial stage of the mixing of the black (Rio Negro) and the White (Rio Solimoes) waters in the Amazon River basin to understand the geochemical processes that could control the redistribution between particulate and dissolved fractions. Water samples were collected at six stations including the Rio Negro and the Rio Solimoes and four stations downstream from the confluence. The relative contributions of the two tributaries were determined using a triple tracer approach (d 18 O, dD, Cl � ). Particulate (>0.2 Am) and dissolved (<0.2 Am) concentrations of major elements (Ca, Mg, Fe, Al, Si) and organic carbon (POC and DOC) were measured. Major elements in the particulate fraction were found to have a nonconservative behaviour in the initial stage of the mixing due to mineral removal. In the dissolved fraction, only the DOC, Fe, and Mn behaved nonconservatively. The Fe losses could be due to preferential removal of Fe bound to N-rich organic matter (OM) and/or to preferential removal of Fe oxyhydroxides. The increasing dissolved manganese content in the dissolved phase is explained by a reductive dissolution of manganese oxides due to massive inputs of phenolic-rich OM from the Rio Negro. The amount of DOC removed from the water column in the initial stage of the mixing would represent 4% of the total annual DOC flux of the Amazon River at the reference gauging station of Obidos. D 2002 Elsevier Science B.V. All rights reserved.

The spatial distribution and emission of nitrous oxide (N2O) in a large eutrophic lake in eastern China: anthropogenic effects.

The emission of N(2)O from China is globally significant, but relatively few direct observations have been made in many of the fresh water environments most likely to be important sites of N(2)O production. In this paper, N(2)O saturations were examined in the ecologically heterogeneous, eutrophied, Lake Taihu, as well as in surrounding rivers in eastern China. The emissions of N(2)O were estimated and compared with those from other landscapes within the Lake Taihu drainage basin. We found that anthropogenically-enhanced inorganic N inputs act as a limited primary control on the spatial distribution of N(2)O saturations in heavily eutrophied parts of the lake only and that overall, lake N(2)O production and emission are not raised as significantly as expected due to high N inputs. In comparison, the heavily eutrophied river network is an important fraction of the local N(2)O budget, and when considered together with emissions of N(2)O from the lake, constitute a major (10-50% depending on season) fraction of total N(2)O emissions from the Lake Taihu drainage basin.

Sensitivity of chemical weathering and dissolved carbon dynamics to hydrological conditions in a typical karst river

To better understand the mechanisms that hydrological conditions control chemical weathering and carbon dynamics in the large rivers, we investigated hydrochemistry and carbon isotopic compositions of dissolved inorganic carbon (DIC) based on high-frequency sampling in the Wujiang River draining the carbonate area in southwestern China. Concentrations of major dissolved solute do not strictly follow the dilution process with increasing discharge, and biogeochemical processes lead to variability in the concentration-discharge relationships. Temporal variations of dissolved solutes are closely related to weathering characteristics and hydrological conditions in the rainy seasons. The concentrations of dissolved carbon and the carbon isotopic compositions vary with discharge changes, suggesting that hydrological conditions and biogeochemical processes control dissolved carbon dynamics. Biological CO2 discharge and intense carbonate weathering by soil CO2 should be responsible for the carbon variability under various hydrological conditions during the high-flow season. The concentration of DICbio (DIC from biological sources) derived from a mixing model increases with increasing discharge, indicating that DICbio influx is the main driver of the chemostatic behaviors of riverine DIC in this typical karst river. The study highlights the sensitivity of chemical weathering and carbon dynamics to hydrological conditions in the riverine system.

On the necessity of organic solvent extraction for carbon isotopic analysis of α-cellulose: implications for environmental reconstructions

α-cellulose is widely used as a target substance for isotope ratio analysis in environmental reconstructions. Its preparation includes three basic steps: organic solvent extraction, delignification and alkaline hydrolysis. Recent works have suggested omission of the first step. We have made a detailed comparison in carbon isotope ratio of α-cellulose with or without organic solvent extraction using 32 consecutive tree ring and 30 subfossil peat samples. These samples were exposed to three different chemical treatments: with organic solvent extraction as the first step (CellOE), without organic solvent extraction (CellNOE), and with organic solvent extraction as the final step (CellNOE/OE). The third treatment is used to test if organic extractives can be completely removed or if their solubility in organic solvents has been altered by delignification and alkaline hydrolysis. In tree rings and peat, δ13CC ell NOE was always significantly different from δ13CC ell OE, but the trends were not the same. In tree rings, δ13CC ell NOE was always more negative than δ13CC ell OE by −0.31 ∼ −0.01‰. In contrast, δ13CC ell NOE in peat could be more negative or more positive than δ13CC ell OE by −3.08 ∼ 0.27‰. The third chemical treatment resulted in different patterns. For tree rings, δ13CC ell NOE/OE was still more negative than δ13CC ell OE by −0.36 ∼ −0.08‰. However, the differences between δ13CC ell NOE/OE and δ13CC ell OE for peat varied in a more narrow range from −0.58 to 0.61‰, compared to the differences between δ13CC ell NOE and δ13CC ell OE. These results exposed a complex chemical evolution behaviour and an incomplete removal of lipids during delignification and alkaline hydrolysis. The mean value, long-term trend and seesaw patterns for a tree ring or peat CellNOE series were significantly different from those for a CellOE series, indicating that omission of organic solvent extraction will lead to a biased inference of past environmental conditions.

The Response of Carbon Geochemistry to Hydrological Events within an Urban River, Southwestern China

Natural and anthropogenic impacts on dissolved inorganic carbon (DIC) within an urban river, Nanming River in southwestern China, were investigated using hydrochemistry and carbon isotopic compositions of dissolved inorganic carbon (δ13CDIC). Because of the anthropogenic inputs, generally, the TDS values and major ionic compositions showed an increasing trend along the mainstream. The TDS values and most of the dissolved solutes compositions showed a dilution effect during storms, but the dilution effect did not strictly follow the theoretical dilution curve. Lighter δ13CDIC values in the river after a rainstorm reflected the influx of rain water with biological CO2 during the rain event. Meanwhile, the negative relationship between δ13CDIC values and dissolved inorganic carbon concentrations in the mainstream at different sampling campaigns suggested significant degradation of organic matter in the riverine channels. The variabilities of DIC in an urban river were mainly impacted by biological activities and infiltration of soil carbon dioxide. This study demonstrated that hydrological events and anthropogenic inputs are the main controls on the variations of dissolved solutes compositions and the DIC dynamics for an urban river.

Nitrate dynamics during impoundment and flood periods in a subtropical karst reservoir: Hongfeng Lake, Southwestern China

Nitrogenous species, particularly nitrate, are some of the most significant contaminants in freshwater rivers and lakes in China, posing a significant threat to human and ecosystem health. To identify the major nitrate sources and transformation processes in a subtropical karst lake (Hongfeng, HF) in Southwest China, two sampling campaigns involving three lake profiles were conducted during the impoundment period (April) and flood period (August). Hydro-chemistry parameters, concentration of nitrogenous species, and dual isotopes of nitrate were analyzed. Nitrate was the major nitrogenous species in HF lake with higher proportion in April than August. The concentrations of NH4+, NO2- and dissolved organic nitrogen were below the detection limit in April, while NH4+ increased with depth in August, which may be a result of mineralization. Nitrification was detected during the impoundment period and from the surface to a depth of 10 m during the flood period. Denitrification was detected in the lake bottom waters during the flood period with isotopic fractionation of -10.7‰ for δ15N-NO3- and -4.7‰ for δ18O-NO3-. The Rayleigh distillation demonstrated denitrification showed spatial variation (53% and 89% of nitrate in the southern and northern region of the lake, respectively). The Bayesian modelling results suggest that organic nitrogen degradation and sediment make the largest contributions to nitrate (51% and 38% in April and 33% and 24% in August, respectively) to the lake. Tributaries contributed more nitrate in August (37%) than in April (10%). The results highlight that modified endogenous nitrogen contributed a high proportion of nitrate sources within the lake system during the two periods.