Dominik J. Weiss

Geochemistry of the peat bog at Etang de la Gruère, Jura Mountains, Switzerland, and its record of atmospheric Pb and lithogenic trace metals (Sc, Ti, Y, Zr, and REE) since 12,370 14C yr BP

A 650 cm core from a Swiss bog represents 12,370 14C years of peat accumulation and provides the first complete record of atmospheric Pb deposition for the entire Holocene. Excess, non-atmospheric Sr in the peat was calculated by normalizing Sr/Sc to crustal abundance; this was used to differentiate between the ombrogenic section of the bog in which inorganic solids are supplied exclusively by atmospheric deposition and the minerogenic zone where mineral-water interactions contribute metals to the peat. While sediment dissolution contributes significantly to the Sr inventory of the minerogenic section of the peat profile, there is no measurable effect of this process on the Pb burden. Isotopic analyses (204Pb, 206Pb, 207Pb, 208Pb) show that effectively all of the Pb in the peat profile was supplied exclusively from the atmosphere. To separate natural and anthropogenic Pb, Sc was selected over Ti, Y, Zr, Hf and REE as a conservative, reference element which is supplied by soil dust aerosols derived from rock weathering. Enrichment factors (EF) were calculated using the Pb/Sc ratio in the peat samples, normalized to the “natural, background” Pb/Sc which is found in peats dating from 8030 to 5320 14C yr BP. The results show that anthropogenic source have dominated the supply of atmospheric Pb to the peat core continuously since 3000 14C yr BP. The aerosols supplied to the bog can be divided into 3 classes: a) Pre-Anthropogenic (older than 3000 14C yr BP with Pb EF 1.194); b) Pre-Industrial (dating from 3000–240 14C yr BP, with Pb EF ≥ 2 but 20 and 206Pb/207Pb < 1.179). Elevated soil dust fluxes are observed at 5320, 8230 and 10,590 14C yr BP; the latter corresponds to the Younger Dryas. Aluminum, Zr, Hf, and REE/Sc ratios also are elevated at the same depths, suggesting differences in particle size, wind strength, or source regions. Pre-Anthropogenic aerosols deposited since 8230 14C yr BP reveal Pb/Sc which is significantly higher, and 206Pb/207Pb which is less radiogenic, than during the early Holocene. While the trend toward increasing Pb/Sc could be due to chemical weathering and soil development, this could not explain the shift in Pb isotopic composition. The changes which took place at 8230 14C BP, therefore, may be related to a large scale climatic reorganization which, at present, is poorly understood.

The geochemistry of major and selected trace elements in a forested peat bog, Kalimantan, SE Asia, and its implications for past atmospheric dust deposition

Abstract Biogeochemical processes in a forested tropical peat deposit and its record of past atmospheric dust deposition were assessed using the vertical distribution of lithophilic and plant essential elements in a dated core profile from Borneo, SE Asia. Peat formation started ∼22,120 14C yr before present (BP), and Ca/Mg mass ratios of the solid peat and very low ash contents indicate a strongly ombrotrophic character throughout the deposit, implying that most of the inorganic fraction has been supplied exclusively by atmospheric inputs. Concentration profiles of Mn, Sr, and Ca suggest a very minor influence of chemical diagenesis in the underlying sediments. Silicon, Ca, Mg, P, S, and K show a strong and extended zone of enrichment in the top 200 cm of the profile, indicating that biological accumulation mechanisms are much more extensive than in temperate peat bogs. In the lower core sections, where the element distribution is dominated solely by past atmospheric deposition, average Al/Ti ratios are similar to the upper continental crust (UCC), whereas Fe is slightly enriched and Si is strongly depleted: this condition favors highly weathered tropical soil dust as the main inorganic mineral source. Significant correlation of Al, Fe, Si, S, Ca, and Ti with the lithophilic elements Y and Zr suggests that the distribution of these elements is controlled by sources of atmospheric mineral dust. The Ca/Mg, Ca/K, and Mg/K ratios of the collected rainwater samples are similar to the global average of continental rainwater and suggest a continental character for the site. This is supported by the similarity of the average concentration of Br, Mg, Ca, and S to that in temperate continental and maritime bogs in Switzerland and Scotland. The concentration profiles of Si, Fe, Al, and Ti show distinct peaks within the profile, implying enhanced dust deposition, reduced rates of peat accumulation, or possibly both owing to climatic changes during the Holocene. Enhanced dust deposition between ∼10,830 and 9060 14C yr BP is tentatively interpreted as a Younger Dryas–like event with dust fluxes of ∼10.8 mg/m2/yr. The variations in Al/Ti and Fe/Ti profiles suggest that mineral dust sources have been changing constantly during the Holocene, with local sources being dominant between ∼7820 and 9500 14C yr BP and long-range transport (derived most likely from China) being important during the late Pleistocene and early Holocene and from ∼7820 14C yr BP to the present.

Sphagnum mosses as archives of recent and past atmospheric lead deposition in Switzerland

Sphagnum mosses received from a herbarium and collected recently from a peat bog surface, were used to assess the isotopic character of past and recent atmospheric Pb deposition in Switzerland and to constrain possible Pb sources. Lead removed from the moss surface was isotopically similar to that measured in the corresponding solid plant, suggesting that neither preservative actions for the herbarium samples nor dust had affected the isotopic composition of the samples. The addition of HCl to aqueous extracts to remove surface particles from the plants released more Pb compared to H2O alone. The changes in isotope ratios between Sphagnum collected during the past c. 130 yr were significantly greater than the small fluctuations between and among species collected at any one time. Three isotope ratio plots and emission inventories indicated that the most likely source of atmospheric Pb was coal-burning at the turn of the century, fly ash from waste incineration until approximately 1950, and gasoline combustion after that. The pollution record derived from the Sphagnum plants is in good agreement with other archives from Switzerland (peat, sediment, ice) and with other herbarium records in Europe.

Stable isotopes of Cu and Zn in higher plants: evidence for Cu reduction at the root surface and two conceptual models for isotopic fractionation processes

Recent reports suggest that significant fractionation of stable metal isotopes occurs during biogeochemical cycling and that the uptake into higher plants is an important process. To test isotopic fractionation of copper (Cu) and zinc (Zn) during plant uptake and constrain its controls, we grew lettuce, tomato, rice and durum wheat under controlled conditions in nutrient solutions with variable metal speciation and iron (Fe) supply. The results show that the fractionation patterns of these two micronutrients are decoupled during the transport from nutrient solution to root. In roots, we found an enrichment of the heavier isotopes for Zn, in agreement with previous studies, but an enrichment of isotopically light Cu, suggesting a reduction of Cu(II) possibly at the surfaces of the root cell plasma membranes. This observation holds for both graminaceous and nongraminaceaous species and confirms that reduction is a predominant and ubiquitous mechanism for the acquisition of Cu into plants similar to the mechanism for the acquisition of iron (Fe) by the strategy I plant species. We propose two preliminary models of isotope fractionation processes of Cu and Zn in plants with different uptake strategies.

Comparative study of the temporal evolution of atmospheric lead deposition in Scotland and eastern Canada using blanket peat bogs

The temporal evolution of atmospheric lead deposition and its possible sources were assessed in eastern Canada and in western Scotland, using blanket peat bogs as geochemical archives. Short cores were taken from two remote sites located close to the sea. Significant lead enrichments in the upper layers at both sites reflect the increasing emission of lead into the atmosphere due to anthropogenic activities during the last century. At the Scottish site, a region under aeolian influence from Europe, anthropogenic derived lead could be recognized by the distinctive unradiogenic composition (206Pb/207Pb ratios down to approximately 1.115), being clearly different from the pre-industrial values (206Pb/207Pb approximately 1.166). In contrast, the lead pollution in eastern Canada (influenced by North American sources) is identified by a more radiogenic lead isotope composition (206Pb/207Pb ratios up to approximately 1.199) compared to preindustrial values (206Pb/207Pb approximately 1.161). Emission inventories and isotope characteristics suggest that industrial (coal burning, mining) and traffic (leaded gasoline) outputs are the most likely sources during the first and the second half of the 20th century, respectively, in both, western Scotland and eastern Canada alike. The Scottish record is in line with previous studies of past atmospheric lead deposition. However, the Canadian deposit suggests that the wind derived, pre-industrial lead, is less radiogenic as previously implied using sediment archives. These results are thus the first to report pre-industrial lead isotope ratios and concentrations of atmospheric derived aerosols in North America.

Archives of Atmospheric Lead Pollution

Environmental archives such as peat bogs, sediments, corals, trees, polar ice, plant material from herbarium collections, and human tissue material have greatly helped to assess both ancient and recent atmospheric lead deposition and its sources on a regional and global scale. In Europe detectable atmospheric lead pollution began as early as 6000 years ago due to enhanced soil dust and agricultural activities, as studies of peat bogs reveal. Increased lead emissions during ancient Greek and Roman times have been recorded and identified in many longterm archives such as lake sediments in Sweden, ice cores in Greenland, and peat bogs in Spain, Switzerland, the United Kingdom, and the Netherlands. For the period since the Industrial Revolution, other archives such as corals, trees, and herbarium collections provide similar chronologies of atmospheric lead pollution, with periods of enhanced lead deposition occurring at the turn of the century and since 1950. The main sources have been industry, including coal burning, ferrous and nonferrous smelting, and open waste incineration until c .1950 and leaded gasoline use since 1950. The greatest lead emissions to the atmosphere all over Europe occurred between 1950 and 1980 due to traffic exhaust. A marked drop in atmospheric lead fluxes found in most archives since the 1980s has been attributed to the phasing out of leaded gasoline. The isotope ratios of lead in the various archives show qualitatively similar temporal changes, for example, the immediate response to the introduction and phasing out of leaded gasoline. Isotope studies largely confirm source assessments based on lead emission inventories and allow the contributions of various anthropogenic sources to be calculated.

Evidence for the mechanisms of zinc uptake by rice using isotope fractionation.

In an earlier study, we found that rice (Oryza sativa) grown in nutrient solution well-supplied with Zn preferentially took up light (64)Zn over (66)Zn, probably as a result of kinetic fractionation in membrane transport processes. Here, we measure isotope fractionation by rice in a submerged Zn-deficient soil with and without Zn fertilizer. We grew the same genotype as in the nutrient solution study plus low-Zn tolerant and intolerant lines from a recombinant inbred population. In contrast to the nutrient solution, in soil with Zn fertilizer we found little or heavy isotopic enrichment in the plants relative to plant-available Zn in the soil, and in soil without Zn fertilizer we found consistently heavy enrichment, particularly in the low-Zn tolerant line. These observations are only explicable by complexation of Zn by a complexing agent released from the roots and uptake of the complexed Zn by specific root transporters. We show with a mathematical model that, for realistic rates of secretion of the phytosiderophore deoxymugineic acid (DMA) by rice, and realistic parameters for the Zn-solubilizing effect of DMA in soil, solubilization and uptake by this mechanism is necessary and sufficient to account for the measured Zn uptake and the differences between genotypes.

Kandelia obovata (S, L) Yong tolerance mechanisms to Cadmium: Subcellular distribution, chemical forms and thiol pools

In order to explore the detoxification mechanisms adopted by mangrove under cadmium (Cd) stress, we investigated the subcellular distribution and chemical forms of Cd, in addition to the change of the thiol pools in Kandelia obovata (S., L.) Yong, which were cultivated in sandy culture medium treated with sequential Cd solution. We found that Cd addition caused a proportional increase of Cd in the organs of K. obovata. The investigation of subcellular distribution verified that most of the Cd was localized in the cell wall, and the lowest was in the membrane. Results showed sodium chloride and acetic acid extractable Cd fractions were dominant. The contents of non-protein thiol compounds, Glutathione and phytochelatins in K. obovata were enhanced by the increasing strength of Cd treatment. Therefore, K. obovata can be defined as Cd tolerant plant, which base on cell wall compartmentalization, as well as protein and organic acids combination.

Measurement of zinc stable isotope ratios in biogeochemical matrices by double-spike MC-ICPMS and determination of the isotope ratio pool available for plants from soil

AbstractAnalysis of naturally occurring isotopic variations is a promising tool for investigating Zn transport and cycling in geological and biological settings. Here, we present the recently installed double-spike (DS) technique at the MAGIC laboratories at Imperial College London. The procedure improves on previous published DS methods in terms of ease of measurement and precisions obtained. The analytical method involves addition of a 64Zn–67Zn double-spike to the samples prior to digestion, separation of Zn from the sample matrix by ion exchange chromatography, and isotopic analysis by multiple-collector inductively coupled plasma mass spectrometry. The accuracy and reproducibility of the method were validated by analyses of several in-house and international elemental reference materials. Multiple analyses of pure Zn standard solutions consistently yielded a reproducibility of about ±0.05‰ (2 SD) for δ66Zn, and comparable precisions were obtained for analyses of geological and biological materials. Highly fractionated Zn standards analyzed by DS and standard sample bracketing yield slightly varying results, which probably originate from repetitive fractionation events during manufacture of the standards. However, the δ66Zn values (all reported relative to JMC Lyon Zn) for two less fractionated in-house Zn standard solutions, Imperial Zn (0.10 ± 0.08‰: 2 SD) and London Zn (0.08 ± 0.04‰), are within uncertainties to data reported with different mass spectrometric techniques and instruments. Two standard reference materials, blend ore BCR 027 and ryegrass BCR 281, were also measured, and the δ66Zn were found to be 0.25 ± 0.06‰ (2 SD) and 0.40 ± 0.09‰, respectively. Taken together, these standard measurements ascertain that the double-spike methodology is suitable for accurate and precise Zn isotope analyses of a wide range of natural samples. The newly installed technique was consequently applied to soil samples and soil leachates to investigate the isotopic signature of plant available Zn. We find that the isotopic composition is heavier than the residual, indicating the presence of loosely bound Zn deposited by atmospheric pollution, which is readily available to plants. FigureZinc isotope ratio pools of bulk soil and the associated acid leach (estimated plant available pool) as measured by double-spike MC-ICPMS. δxZnLyon-JMC=(Rsample/RJMC-Lyon -1)x103, where Rsample and RJMC-Lyon denote the xZn/64Zn isotope ratio of the sample and standard (JMC-Lyon), respectively, and where x denotes either 66 or 68.

Adsorption kinetics, capacity and mechanism of arsenate and phosphate on a bifunctional TiO2–Fe2O3 bi-composite

Mixed oxide TiO(2)-Fe(2)O(3) bi-composites have been recognised as efficient and economical sorbents with great promise for arsenic removal from groundwater. In this study, we use a fast, simple and inexpensive synthesis method for this type of bi-composite and assess its adsorption performance. The kinetics of arsenate and phosphate adsorption onto the bi-composite are determined, demonstrating rapid and stable uptake of both oxy-anions over several days and with improved performance compared to the widely used TiO(2) sorbent. A modified pseudo-second order rate equation is introduced, which allows the adsorption kinetics to be modelled as two simultaneous, parallel reaction pathways with separate kinetic parameters. This equation reproduces the experimental observations accurately across a wide range of timescales from minutes to days. Our experimental data agrees with previous interpretations of the adsorption mechanism including the formation of mono-dentate and bi-dentate inner-sphere surface complexes. The arsenate and phosphate uptake capacities of the bi-composite are reported. Equilibrium studies were conducted between pH 5 and 9 and interpreted within the Langmuir, Freundlich and Dubinin-Radushkevich isotherm models.