Zn isotope fractionation in a pristine larch forest on permafrost-dominated soils in Central Siberia.

Stable Zn isotopes fractionation was studied in main biogeochemical compartments of a pristine larch forest of Central Siberia developed over continuous permafrost basalt rocks. Two north- and south-oriented watershed slopes having distinctly different vegetation biomass and active layer depth were used as natural proxy for predicting possible future climate changes occurring in this region. In addition, peat bog zone exhibiting totally different vegetation, hydrology and soil temperature regime has been studied. The isotopic composition of soil profile from Central Siberia is rather constant with a δ(66)Zn value around 0.2‰ close to the value of various basalts. Zn isotopic composition in mosses (Sphagnum fuscum and Pleurozium schreberi) exhibits differences between surface layers presenting values from 0.14 to 0.2‰ and bottom layers presenting significantly higher values (0.5 - 0.7‰) than the underlain mineral surface. The humification of both dead moss and larch needles leads to retain the fraction where Zn bound most strongly thus releasing the lighter isotopes in solution and preserving the heavy isotopes in the humification products, in general accord with previous experimental and modeling works [GCA 75:7632-7643, 2011]. The larch (Larix gmelinii) from North and South-facing slopes is enriched in heavy isotopes compared to soil reservoir while larch from Sphagnum peatbog is enriched in light isotopes. This difference may result from stronger complexation of Zn by organic ligands and humification products in the peat bog compared to mineral surfaces in North- and South-facing slope. During the course of the growing period, Zn followed the behavior of macronutrients with a decrease of concentration from June to September. During this period, an enrichment of larch needles by heavier Zn isotopes is observed in the various habitats. We suggest that the increase of the depth of rooting zone, and the decrease of DOC and Zn concentration in soil solution from the root uptake zone with progressively thawing soil could provoke heavy isotopes to become more available for the larch roots at the end of the vegetative season compared to the beginning of the season, because the decrease of DOC will facilitate the uptake of heavy isotope as it will be less retained in strong organic complexes.

Multielementary (Cd, Cu, Pb, Zn, Ni) Stable Isotopic Exchange Kinetic (SIEK) method to characterize polymetallic contaminations.

A new method is proposed to precisely and simultaneously quantify the exchangeable pool of metals in soils and to describe its reactivity at short- and long-term. It is based on multielementary Stable Isotopic Exchange Kinetics (multi-SIEK), first validated by a comparison between two monoelementary radioactive ((109)Cd*, (65)Zn*) IEK experiments, a mono- ((106)Cd) and multi- ((62)Ni, (65)Cu, (67)Zn, (106)Cd, (204)Pb) SIEK. These experiments were performed on a polluted soil located near the Zn smelter plant of Viviez (Lot watershed, France). The IEK results obtained for Cd and Zn were consistent across the experiments. (109)Cd*, (65)Zn* IEK, and multi-SIEK were then applied on 3 non- and moderate impacted soils that also provided consistent results for Cd and Zn. Within these experimental conditions, it can be concluded that no competition occurs between Cd, Zn, and the other metals during SIEK. Multi-SIEK results indicate that the isotopically exchangeable pool of Ni, Zn, and Cu are small (E(Ni), E(Zn), and E(Cu) values up to 17%) whatever the pollution degree of the soils considered in this study and whatever the duration of the interaction. On the contrary, Cd displays the highest E values (from 35% to 61% after 1 week), and E(Pb) displays a maximum value of 26% after 1 week. The multi-SIEK provides useful information on metal sources and reactivity relationship. Ni would be located in stable pedogenic phases according to its very low enrichment factor. The low E(Zn) and E(Cu) are consistent with location of Zn and Cu in stable phases coming from tailings erosion. Though Pb enrichments in soils may also be attributed to tailings particles, its larger exchangeable pool suggests that the Pb-bearing phases are more labile than those containing Zn and Cu. The high mobility of Cd in upstream soils indicates that it has been mostly emitted as reactive atmospheric particles during high temperature ore-treatment.

Chemical composition of suspended sediments in World Rivers: New insights from a new database.

The aim of this paper is to present a new database on the chemical composition of suspended matter in World Rivers, together with the associated elemental fluxes. There is a lack of any recent attempt in the literature to update the pioneering work of Martin and Meybeck [Martin, J.-M., Meybeck, M., 1979. Elemental mass balance of material carried by major world rivers. Mar. Chem. 7, 173-206.] and Martin and Whitfield [Martin, J.-M., Whitfield, M., 1983. The significance of the river input of chemical elements to the ocean. Trace metals in sea water Wong, Boyle, Bruland, Burton, Goldberg (Eds) Plenum Publishing Corporation.] regarding the worldwide average major and trace element chemistry of riverine particulate matter. Apart from compiling a new database on particulate matter, this paper also aims to give a "snap-shot" of elemental fluxes for each continent. This approach should allow us to obtain new insights on weathering conditions in different environments and assess the influence of human activities on natural geochemical cycles. Finally, this study demonstrates the large uncertainties currently associated with estimating the flux of sediments transported by rivers. By comparing the riverine suspended sediment fluxes of some metals (Cd, Zn, Ni, Cu, Cr and Pb) given in this study with estimates of the anthropogenic fluxes of these metals to the atmosphere, soils and waters (natural ecosystems) [Nriagu, J.O., 1988. A silent epidemic of environmental poisoning. Environ. Pollut. 50, 139-161.], we can see that riverine fluxes are similar to anthropogenic fluxes. This casts light on the effect of human activities on the cycles of trace elements at the Earth's surface.

The influence of the Amazonian floodplain ecosystems on the trace element dynamics of the Amazon River mainstem (Brazil).

The purpose of this paper is to forecast the role of riverine wetlands in the transfer of trace elements. One of the largest riverine wetlands in the world is the floodplain (várzea) of the Amazon River and its tributaries (Junk and Piedade, 1997). The central Amazon wetlands are constituted by a complex network of lakes and floodplains, named várzeas, that extend over more than 300,000 km2 (Junk, W.J., The Amazon floodplain--a sink or source for organic carbon? In Transport of Carbon and Minerals in Major World Rivers, edited by E.T. Degens, S. Kempe, R. Herrera, SCOPE/UNEP; 267-283, 1985.) and are among the most productive ecosystems in the world due to the regular enrichment in nutrients by river waters In order to understand if the adjacent floodplain of Amazon River have a significant influence on the trace element concentrations and fluxes of the mainstem, the concentrations of selected elements (i.e., Al, Mn, Fe, Co, Cu, Mo, Rb, Sr, Ba, and U) have been measured in the Amazon River water (Manacapuru Station, Amazonas State, Brazil) and in lake waters and plants (leaves) from a várzea(Ilha de Marchantaria, Amazonas State, Brazil) during different periods of the hydrological cycle. Four plant species (two perennial species: Pseudobombax munguba and Salix humboldtiana, and two annual herbaceous plants: Echinochloa polystachya and Eichhornia crassipes) were selected to represent the ecological functioning of the site. Time series obtained for dissolved Mn and Cu (<0.20 microm) in Amazon River water could not be explained by tributary mixing or instream processes only. Therefore, the contribution of the waters transiting the floodplains should be considered. These results suggest that the chemical composition of the waters draining these floodplains is controlled by reactions occurring at sediment-water and plant-water interfaces. Trace elements concentrations in the plants (leaves) vary strongly with hydrological seasonality. Based on the concentration data and the biological productivity of floodplain ecosystems, a first order approximation of trace element storage (permanent or temporary) in the vegetation of these floodplains was made. It was found that floodplain-mainstem elemental fluxes make a significant contribution to the dissolved flux of the Amazon River. This study is part of the Brazilian_French joint research program Hybam (Hydrology and Geochemistry of the Amazonian Basin).