2018 |
Bickle, M. J., H. J. Chapman, E. Tipper, A. Galy, C. De La Rocha, and T. Ahmad. "Chemical weathering outputs from the flood plain of the Ganga." Geochimica et Cosmochimica Acta 225 (2018): 146–175.
Résumé : Transport of sediment across riverine flood plains contributes a significant but poorly constrained fraction of the total chemical weathering fluxes from rapidly eroding mountain belts which has important implications for chemical fluxes to the oceans and the impact of orogens on long term climate. We report water and bedload chemical analyses from the Ganges flood-plain, a major transit reservoir of sediment from the Himalayan orogen. Our data comprise six major southern tributaries to the Ganga, 31 additional analyses of major rivers from the Himalayan front in Nepal, 79 samples of the Ganga collected close to the mouth below the Farakka barrage every two weeks over three years and 67 water and 8 bedload samples from tributaries confined to the Ganga flood plain. The flood plain tributaries are characterised by a shallow d18O – dD array, compared to the meteoric water line, with a low dDexcess from evaporative loss from the flood plain which is mirrored in the higher dDexcess of the mountain rivers in Nepal. The stable-isotope data confirms that the waters in the flood plain tributaries are dominantly derived from flood plain rainfall and not by redistribution of waters from the mountains. The flood plain tributaries are chemically distinct from the major Himalayan rivers. They can be divided into two groups. Tributaries from a small area around the Kosi river have 87Sr/86Sr ratios >0.75 and molar Na/Ca ratios as high as 6. Tributaries from the rest of the flood plain have 87Sr/86Sr ratios <-0.74 and most have Na/Ca ratios <1. One sample of the Gomti river and seven small adjacent tributaries have elevated Na concentrations likely caused by dissolution of Na carbonate salts. The compositions of the carbonate and silicate components of the sediments were determined from sequential leaches of floodplain bedloads and these were used to partition the dissolved cation load between silicate and carbonate sources. The 87Sr/86Sr and Sr/Ca ratios of the carbonate inputs were derived from the acetic-acid leach compositions and silicate Na/Ca and 87Sr/86Sr ratios derived from silicate residues from leaching. Modelling based on the 87Sr/86Sr and Sr/Ca ratios of the carbonate inputs and 87Sr/86Sr ratios of the silicates indicates that the flood plain waters have lost up to 70% of their Ca (average 50%) to precipitation of secondary calcite which is abundant as a diagenetic cement in the flood plain sediments. 31% of the Sr, 8% of the Ca and 45% of the Mg are calculated to be derived from silicate minerals. Because of significant evaporative loss of water across the flood plain, and in the absence of hydrological data for flood plain tributaries, chemical weathering fluxes from the flood plain are best calculated by mass balance of the Na, K, Ca, Mg, Sr, SO4 and 87Sr/86Sr compositions of the inputs, comprising the flood plain tributaries, Himalayan rivers and southern rivers, with the chemical discharge in the Ganga at Farakka. The calculated fluxes from the flood plain for Na, K, Ca and Mg are within error of those estimated from changes in sediment chemistry across the flood plain (Lupker et al., 2012, Geochemica Cosmochimica Acta). Flood plain weathering supplies between 41 and 63% of the major cation and Sr fluxes and 58% of the alkalinity flux carried by the Ganga at Farakka which compares with 24% supplied by Himalayan rivers and 18% by the southern tributaries.
Mots-clés : Cycla
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Burke, A., T. M. Present, G. Paris, E. C. M. Rae, B. H. Sandilands, J. Gaillardet, B. Peucker-Ehrenbrink, W. W. Fischer, J. W. McClelland et al. "Sulfur isotopes in rivers: Insights into global weathering budgets, pyrite oxidation, and the modern sulfur cycle." Earth and Planetary Science Letters 496 (2018): 168–177.
Résumé : The biogeochemical sulfur cycle is intimately linked to the cycles of carbon, iron, and oxygen, and plays an important role in global climate via weathering reactions and aerosols. However, many aspects of the modern budget of the global sulfur cycle are not fully understood. We present new δ34S measurements on sulfate from more than 160 river samples from different geographical and climatic regions—more than 46% of the world’s freshwater flux to the ocean is accounted for in this estimate of the global riverine sulfur isotope budget. These measurements include major rivers and their tributaries, as well as time series, and are combined with previously published data to estimate the modern flux-weighted global riverine δ34S as 4.4 ±4.5°/°°(V-CDT), and 4.8 ±4.9°/°°when the most polluted rivers are excluded. The sulfur isotope data, when combined with major anion and cation concentrations, allow us to tease apart the relative contributions of different processes to the modern riverine sulfur budget, resulting in new estimates of the flux of riverine sulfate due to the oxidative weathering of pyrites (1.3 ±0.2TmolS/y) and the weathering of sedimentary sulfate minerals (1.5 ±0.2 Tmol S/y). These data indicate that previous estimates of the global oxidative weathering of pyrite have been too low by a factor of two. As pyrite oxidation coupled to carbonate weathering can act as a source of CO2to the atmosphere, this global pyrite weathering budget implies that the global CO2weathering sink is overestimated. Furthermore, the large range of sulfur isotope ratios in modern rivers indicates that secular changes in the lithologies exposed to weathering through time could play a major role in driving past variations in the δ34S value of seawater.
Mots-clés : Cycla
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Mariet, A. L., F. Monna, F. Gimbert, C. Bégeot, C. Cloquet, S. Belle, L. Millet, D. Rius, and A. V. Walter-Simonnet. "Tracking past mining activity using trace metals, lead isotopes and compositional data analysis of a sediment core from Longemer Lake, Vosges Mountains, France." Journal of Paleolimnology (2018).
Résumé : A 157-cm-long sediment core from Longemer Lake in the Vosges Mountains of France
spans the past two millennia and was analyzed for trace metal content and lead isotope composition. Trace metal accumulation rates highlight three main
input phases: Roman Times (cal. 100 BC–AD 400), the Middle Ages (cal. AD 1000–1500), and the twentieth century. Atmospheric contamination displays
a pattern that is similar to that seen in peat bogs from the region, at least until the eighteenth century. Thereafter, the lake sediment record is more precise
than peat records. Some regional mining activity, such as that in archaeologically identified eighteenth-century mining districts, was detected from the lead
isotope composition of sediment samples. Compositional data analysis, using six trace metals (silver, arsenic, cadmium, copper, lead and zinc), enabled us
to distinguish between background conditions, periods of mining, and of other anthropogenic trace metal emissions, such as the recent use of leaded gasoline.
Mots-clés : Cycla
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Mariet, A. L., A. V. Walter-Simonnet, F. Gimbert, C. Cloquet, and C. Bégeot. "High-temporal resolution landscape changes related to anthropogenic activities over the past millennium in the Vosges Mountains (France)." Ambio (2018).
Résumé : Iron mining activities in the Bruche valley (Vosges Mountains, France) date historically from the Roman period to the mid-nineteenth century. The geochemical and palynological study of a core from the peat bog of Le Champ du Feu allows highlighting impacts of these activities over the past millennium. Trace metal
contamination is recorded for lead (Pb), arsenic, zinc, and antimony during the Middle Ages, the sixteenth century, and from cal. AD 1750–1900, with several sources distinguished by Pb isotope analyses. Forest exploitation is attested by the palynological analysis of the core, with exploitation of Fagus for smelting processes and cutting of Abies for agro-pastoralism. This approach highlights
several patterns of contamination, corresponding to the mixing sources and the contamination intensity, which can be linked to the pollen assemblage zones. Hence,
anthropogenic activities such as mining and farming led to long-term modification of the landscape composition in this mountainous area
Mots-clés : Cycla
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Schulze-Makuch, D., D. Wagner, S. Kounaves, K. Mangelsdorf, K. G. Devine, J. P. de Vera, P. Schmitt-Kopplin, H. P. Grossart, V. Parro, M. Kaupenjohann et al. "Transitory microbial habitat in the hyperarid Atacama Desert." PNAS (2018).
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Zelano, I., C. Cloquet, F. Fraysse, S. Dong, N. Janot, G. Echevarria, and E. Montargès-Pelletier. "The influence of organic complexation on Ni isotopic fractionation and Ni recycling in the upper soil layers." Chemical Geology 483 (2018): 47–55.
Résumé : The quantification of Ni isotopic fractionation induced by Ni binding to organic acids is a preliminary step to better constrain the mechanisms determining Ni isotopic fingerprint observed in surface soils, waters and plants, as well as the contribution of metal recycling during plant litter degradation. In this study, Ni isotopic fraction induced by reaction with small organic acids, e.g. citric and oxalic acids, and with soil purified humic acids (PHA) was investigated at different Ni-L ratio and pH conditions. The Donnan Membrane Technique was used to separate Ni bound to organic ligands from the free metal. Obtained results highlighted that Ni binding with carboxylic groups produces, in the adopted experimental conditions, a Δ60Nibond-free < 0.2‰. This value is not high enough to justify neither metal fractionation previously observed between soil and hyperaccumulators, nor the fractionation between different plant parts, e.g. roots and leaves. In parallel, leaf degradation experiments of two hyperaccumulating plants, where Ni is mainly present as Ni-citrate, were performed to simulate litter decomposition and to highlight the contribution of plants on Ni isotopic composition in surface soils and waters. In the case of the hyperaccumulator Alyssum murale, the degradation process did not induce any observable fractionation. On the contrary, during Rinorea bengalensis degradation experiment, a fractionation between Ni leached out in the first 10 days and between 10 and 30 days was observed (Δ60Ni10–30day=0.20 ± 0.05‰). The observed fractionation evidenced a heterogeneous distribution of Ni within the leaves, and/or distinct chemical bonding to the leaf cells, and finally suggested the influence of the chemical bonding on Ni isotopic signature. Although a precise quantification of plant contribution on Ni isotopic signature in surface soils and waters is still not reached, our results produced important progress to elucidate the role of organic matter in regulating Ni isotopic fingerprint in surface layers.
Mots-clés : Cycla
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Zhang, Y., H. Wen, C. Zhu, H. Fan, and C. Cloquet. "Cadmium isotopic evidence for the evolution of marine primary productivity and the biological extinction event during the Permian-Triassic crisis from the Meishan section, South China." Chemical Geology 481 (2018): 110–118.
Résumé : Cadmium (Cd) isotopic compositions in seawater are important parameters for tracing the biogeochemical cycling of Cd in modern oceans and may be used as a direct proxy for nutrient utilization by phytoplankton and an indirect proxy for primary productivity. In this study, we measured the stable Cd isotope ratios (δ114Cd) of
marine sedimentary carbonates from the Meishan section in Changxing County of Zhejiang Province, South China, the Global Stratotype Section and Point (GSSP) of the Permian-Triassic Boundary (PTB). The results suggest that the inferred marine primary productivity recorded in the marine carbonates from the Late Permian
(i.e., before the mass extinction) was high and relatively stable (the δ114/110Cd values of carbonates range from +0.31‰to +0.60‰; after correction for salinity-controlled fractionation into inorganic calcite, the δ114/110Cd values of palaeo-seawater range from +0.47‰to +0.79‰). During the mass extinctions, the δ114/110Cd values of carbonates abruptly decreased at the beginning of each episode of mass extinction, suggesting that the primary productivity of the palaeo-ocean declined before each episode of extinction events at the PTB. We suggest that the mass extinction events at the PTB in the Meishan section could be a ripple effect due to the destruction of primary producers at the base of the food chain in the palaeo-ocean ecosystem.
Mots-clés : Cycla
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2017 |
Archer, C., M. B. Andersen, C. Cloquet, T. M. Conway, S. Dong, M. Ellwood, R. Moore, J. Nelson, M. Rehkämper, O. Rouxel et al. "Inter-calibration of a proposed new primary reference standard AA-ETH Zn for zinc isotopic analysis." Journal of Analytical Atomic Spectrometry 32 (2017): 145–149.
Résumé : We have prepared a large volume of pure, concentrated and homogenous zinc standard solution. This new standard solution is intended to be used as a primary reference standard for the zinc isotope community, and to serve as a replacement for the nearly exhausted current reference standard, the so-called JMC-Lyon Zn. The isotopic composition of this new zinc standard (AA-ETH Zn) has been determined
through an inter-laboratory calibration exercise, calibrated against the existing JMC-Lyon standard, as well as the certified Zn reference standard IRMM-3702. The data show that the new standard is isotopically indistinguishable from the IRMM-3702 zinc standard, with a weighted d66/64Zn value of 0.28 °/°° 0.02& relative to JMC-Lyon. We suggest that this new standard be assigned a d66/64Zn value of
+0.28°/°° for reporting of future Zn isotope data, with the rationale that all existing published Zn isotope data are presented relative to the JMC-Lyon standard. Therefore our proposed presentation allows for a direct comparison with all previously published data, and that are directly traceable to a certified
reference standard, IRMM-3702 Zn. This standard will be made freely available to all interested labs through contact with the corresponding author.
Mots-clés : Cycla
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Bickle, M., N. Kampman, H. Chapman, C. Ballentine, B. Dubacq, A. Galy, T. Sirikitputtisak, O. Warr,, M. Wigley et al. "Rapid reactions between CO2, brine and silicate minerals during geological carbon storage: Modelling based on a field CO2 injection experiment." Chemical Geology 468 (2017): 17–31.
Résumé : The dissolution of CO2 into formation brines and the subsequent reactions of the CO2-charged brines with reservoir minerals are two key processes likely to increase the security of geological carbon-dioxide storage. These processes will be dependent on the permeability structure and mineral compositions of the reservoirs, but there is limited observational data on their rates. In this paper we report the cation and anion concentrations and Sr, oxygen and carbon isotopic compositions of formation waters from four extraction wells sampled at surface,
over ~6 months after commencement of CO2 injection in a five spot pattern for enhanced oil recovery at the Salt Creek field, Wyoming. Sampled fluids, separated from the minor oil component, exhibit near-monotonic increases in alkalinity and concentrations of cations but little change in Cl and Br concentrations and oxygen and deuterium isotope ratios. The increases in alkalinity are modelled in terms of reaction with reservoir calcite and silicate minerals as the changes in fluid chemistry and Sr-isotopic compositions are inconsistent with simple addition of injected fluids sampled over the course of the experiment. The reservoir mineral chemical and isotopic compositions are characterised by sampling core as well as surface exposures of the Frontier Formation elsewhere in Wyoming. The evolution of the fluid chemistries reflects extensive dissolution of both carbonate
and silicate minerals over the course of the six months sampling implying rapid dissolution of CO2 in the formation waters and reaction of CO2-bearing brines with formation minerals. Rates of CO2 diffusion into the brines and advection of CO2 charged brines in the reservoir are sufficiently slow that, if present, calcite should react to be close to equilibrium with the fluids. This allows estimation of the CO2 partial pressures in the sampled fluids and comparison with the thermodynamic driving force for the relatively rapid average plagioclase dissolution rates of ~10−12 mol·m−2·s−1.
Mots-clés : Cycla
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Emberson, R., A. Galy, and N. Hovius. "Combined effect of carbonate and biotite dissolution in landslides biases silicate weathering proxies." Geochimica et Cosmochimica Acta 213 (2017): 418–434.
Résumé : Long-term estimates of the dissolution of silicate rock are generally derived from a range of isotopic proxies, such as the radiogenic strontium isotope ratio (87Sr/86Sr), which are preserved in sediment archives. For these systems to fairly represent silicate weathering, the changes in isotopic ratios in terrestrial surface waters should correspond to changes in the overall silicate dissolution. This assumes that the silicate mineral phases that act as sources of a given isotope dissolve at a rate that is proportional to the overall silicate weathering. Bedrock landsliding exhumes large quantities of fresh rock for weathering in transient storage, and rapid weathering in these deposits is controlled primarily by dissolution of the most reactive phases. In
this study, we test the hypothesis that preferential weathering of these labile minerals can decouple the dissolution of strontium sources from the actual silicate weathering rates in the rapidly eroding Western Southern Alps (WSA) of New Zealand. We find that rapid dissolution of relatively radiogenic calcite and biotite in landslides leads to high local fluxes in strontium with isotopic ratios that offer no clear discrimination between sources. These higher fluxes of radiogenic strontium are in contrast to silicate weathering rates in landslides that are not systematically elevated. On a mountain belt scale, radiogenic strontium fluxes are not coupled to volumes of recent landslides in large (>100 km2) catchments, but silicate weathering fluxes are. Such decoupling is likely due first to the broad variability in the strontium content of carbonate minerals, and second to the combination of radiogenic strontium released from both biotite and carbonate in recent landslides. This study supports previous work suggesting the limited utility of strontium isotopes as a system to study silicate weathering in the WSA. Crucially however, in settings where bedrock landsliding is a dominant erosive process there is potential for both random and systematic bias in isotope proxies if the most reactive phases exposed for dissolution by landslides disproportionately contribute to the proxy of choice. This clearly suggests that the isotopic composition of marine Sr is a proxy for periods of rapid mountain
uplift and erosion rather than for the associated enhanced silicate weathering.
Mots-clés : Cycla
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Animators : Christophe Cloquet & Guillaume Paris
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