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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Dellinger, M., A. J. West, G. Paris, J. F. Adkins, P. A. E. Pogge von Strandmann, C. V. Ullmann, R. A. Eagle, P. Freitas, M. L. Bagard, J. B. Ries et al. "The Li isotope composition of marine biogenic carbonates: Patterns and mechanisms." Geochimica et Cosmochimica Acta 236 (2018): 315–335.
Résumé : Little is known about the fractionation of Li isotopes during formation of biogenic carbonates, which form the most promising geological archives of past seawater composition. Here we investigated the Li isotope composition (d7Li) and Li/Ca ratios of organisms that are abundant in the Phanerozoic record: mollusks (mostly bivalves), echinoderms, and brachiopods. The measured samples include (i) modern calcite and aragonite shells from various species and natural environ-
ments (13 mollusk samples, 5 brachiopods and 3 echinoderms), and (ii) shells from mollusks grown under controlled conditions at various temperatures. When possible, the mollusk shell ultrastructure was micro-sampled in order to assess
intra-shell heterogeneity. In this paper, we systematically characterize the influence of mineralogy, temperature, and biological processes on the d7Li and Li/Ca of these shells and compare with published data for other taxa (foraminifera and corals).
Aragonitic mollusks have the lowest d7Li, ranging from +16 to +22‰, echinoderms have constant d7Li of about +24‰, brachiopods have d7Li of +25 to +28 ‰, and finally calcitic mollusks have the largest range and highest d7Li values, ranging from +25‰ to +40‰. Measured brachiopods have similar d7Li compared to inorganic calcite precipitated from seawater (d7Li of +27 to +29‰), indicating minimum influence of vital effects, as also observed for other isotope systems and making them a potentially viable proxy of past seawater composition. Calcitic mollusks, on the contrary, are not a good archive for seawater paleo–d7Li because many samples have significantly higher d7Li values than inorganic calcite and display large interspecies variability, which suggests large vital effects. In addition, we observe very large intra-shell variability, in particular for mixed calcite-aragonite shells (over 20‰ variability), but also in mono-mineralic shells (up to 12
‰ variability). Aragonitic bivalves have less variable d7Li (7‰ variability) compared to calcitic mollusks, but with significantly lower d7Li compared
to inorganic aragonite, indicating the existence of vital effects. Bivalves grown at various temperatures show that temperature has only a minor influence on fractionation of Li isotopes during shell precipitation. Interestingly, we observe a strong correlation (R2= 0.83) between the Li/Mg ratio in bivalve Mytilus edulis
and temperature, with potential implications for paleo-temperature reconstructions.
Finally, we observe a negative correlation between the d7Li and both the Li/Ca and Mg/Ca ratio of calcite mollusks, which we relate to biomineralization processes. To explain this correlation, we propose preferential removal of 6Li from the calci-
fication site of calcite mollusks by physiological processes corresponding to the regulation of the amount of Mg in the calcifying medium. We calculate that up to 80% of the initial Li within the calcification site is removed by this process, leading to high d7Li and low Li/Ca in some calcite mollusk specimens. Collectively, these results suggest that Mg (and thus [Li]) is strongly biologically controlled within the calcifying medium of calcite mollusks.
Overall, the results of this study show that brachiopods are likely to be suitable targets for future work on the determination of paleo-seawater Li isotope composition—an emerging proxy for past weathering and hydrothermal processes.
Mots-clés : Cycla
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Fan, H., H. Wen, C. Xiao, T. Zhou, C. Cloquet, and X. Zhu. "Zinc geochemical cycling in a phosphorus-rich ocean during the early Ediacaran." Journal of Geochemical Research: Oceans 123, (2018): 5248–5260.
Résumé : During the early Ediacaran, there was a large influx of phosphorus into the oceans and a resultant high phosphorus concentration in seawater, where multicellular eukaryotes may have been the primary type of marine productivity. The eukaryotes could play a critical role in regulating Zn cycling and isotopes. To establish Zn geochemical cycling patterns in the phosphorus-rich ocean, this study investigates Zn isotopic signatures of shallow water phosphorite that contains phosphatized microfossils (Weng’an biota) and deep-water shale from the Doushantuo Formation. Our results indicate that phosphorite commonly preserves heavier Zn isotope composition, with an average of 0.80‰. The positive δ66Zn values in phosphorites may be ascribed to Zn isotope fractionation associated with the complexation of Zn with phosphate and the adsorption of isotopically heavy Zn onto Fe-Mn oxides and organism’s surfaces. We argue that phosphorite may represent an important sink of isotopically heavy Zn in a phosphorus-rich ocean during Earth history. Meanwhile, deep-water organic-rich shale shows an enrichment of isotopically light Zn with an average of 0.23‰, which may be attributed to sulfide precipitation in mid-depth environment. The organic-rich shale may represent an isotopically light Zn sink. In addition, the highest δ66Zn value (0.45‰) in a euxinic black shale may indicate that Zn isotope signal of anoxic deep water is similar to that of modern deep seawater. If that is the case, it suggests that Zn geochemical cycling in the early Ediacaran oceans was similar to that of modern oceans
Mots-clés : Cycla
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Girault, F., L. B. Adhikari, C. France-Lanord, P. Agrinier, B. P. Koirala, M. Bhattarai, S. S. Mahat, C. Groppo, F. Rolfo, L. Bollinger et al. "Persistent CO2 emissions and hydrothermal unrest following the 2015 earthquake in Nepal." Nature Communications 9, no. 2956 (2018).
Résumé : Fluid–earthquake interplay, as evidenced by aftershock distributions or earthquake-induced effects on near-surface aquifers, has suggested that earthquakes dynamically affect permeability of the Earth’s crust. The connection between the mid-crust and the surface was further supported by instances of carbon dioxide (CO2) emissions associated with seismic activity, so far only observed in magmatic context. Here we report spectacular non-volcanic CO2 emissions and hydrothermal disturbances at the front of the Nepal Himalayas following the deadly 25 April 2015 Gorkha earthquake (moment magnitude Mw = 7.8). The data show unambiguously the appearance, after the earthquake, sometimes with a delay of several months, of CO2 emissions at several sites separated by > 10 kilometres, associated with persistent changes in hydrothermal discharges, including a complete cessation. These observations reveal that Himalayan hydrothermal systems are sensitive to co- and postseismic deformation, leading to non-stationary release of metamorphic CO2 from active orogens. Possible pre-seismic effects need further confirmation.
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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Piani, L., and Y. Marrocchi. "Hydrogen isotopic composition of water in CV-type carbonaceous chondrites." Earth and Planetary Science Letters 504 (2018): 64–71.
Résumé : Among the different groups of carbonaceous chondrites, variable concentrations of hydrous minerals and organic matter are observed that might be related to the time and/or place of formation of their asteroidal parent bodies. However, the precise distribution of these volatile-bearing components between chondrite groups and their chemical and isotopic compositions remain fairly unknown. In this study, we used a novel secondary ion mass spectrometry analytical protocol to determine the hydrogen isotopic composition of water-bearing minerals in CV-type carbonaceous chondrites. This protocol allows for the first time the D/H ratio of CV chondrite hydrous minerals to be determined without hindrance by hydrogen contributions from adjacent organic material. We found that water in the altered CV chondrites Kaba, Bali, and Grosnaja has an average D/H ratio of D/HCV-water=[144+8−21] ×10−6(or δDCV-water=−77+54−131°/°°, 2σ), significantly higher than water in most CM-type carbonaceous chondrites (D/HCM-water=[101 ±6] ×10−6or δDCM-water=−350 ±40°/°°, 2σ). We show that because organic matter in CV chondrites is depleted in deuterium compared to that in CM chondrites, such differences could result from isotopic exchange between water and organics. Another possibility is that the CM and CV parent bodies sampled different reservoirs of water ice and organics characterized by variable isotopic compositions due to their different time and/or place of accretion.
Mots-clés : Cycla
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Rennie, V. C. F., G. Paris, A. L. Sessions, S. Abramovich, A. V. Turchyn, and J. F. Adkins. "Cenozoic record of δ 34 S in foraminiferal calcite implies an early Eocene shift to deep-ocean sulfide burial." Nature Geoscience 11, no. 10 (2018): 761–765.
Résumé : Understanding the changes in, and drivers of, isotopic variability of sulfur in seawater sulfate (δ34SSO4-sw) over geological time remains a long-standing goal, particularly because of the coupling between the biogeochemical sulfur and carbon cycles. The early Cenozoic has remained enigmatic in this regard, as the existing seawater sulfate isotopic records appear to be decoupled from the well-defined carbon isotope composition of the ocean. Here, we present a new Cenozoic record of sulfur isotopes, using carbonate-associated sulfate hosted in the calcite lattice of single-species foraminifera. The vastly improved stratigraphy
afforded by this record demonstrates that carbon and sulfur cycles, as recorded by their isotopes, are not fully decoupled in the early Cenozoic. With a model driven by partial coupling of the carbon and sulfur cycles, we demonstrate that a change in sulfur isotopic fractionation of the pyrite burial flux best explains the large increase in δ34SSO4-sw ~53 million years ago (Ma) and the subsequent long steady state. We suggest that the locus of pyrite burial changed from shallow epicontinental seas and shelf environments to more open-ocean sediments around 53 Ma. Loss of extensive shelf environments corresponds to Cretaceous–Palaeogene sea-level changes and tectonic reorganization, occurring as the Himalayan arc first collided with Asia.
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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Animators : Christophe Cloquet & Guillaume Paris
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