2019 |
Balco, G., P. H. Blard, D. L. Shuster, J. O. H. Stone, and L. Zimmermann. "Cosmogenic and nucleogenic 21Ne in quartz in a 28-meter sandstone core from the McMurdo Dry Valleys, Antarctica." Quaternary Geochronology 52 (2019): 63–76.
Résumé : We measured concentrations of Ne isotopes in quartz in a 27.6-m sandstone core from a low-erosion-rate site at 2183m elevation at Beacon Heights in the Antarctic Dry Valleys. Surface concentrations of cosmogenic 21Ne indicate a surface exposure age of at least 4.1 Ma and an erosion rate no higher than ca. 14 cm Myr−1 21Ne concentrations in the upper few centimeters of the core show evidence for secondary spallogenic neutron escape effects at the rock surface, which is predicted by first-principles models of cosmogenic-nuclide production but is
not commonly observed in natural examples. We used a model for 21Ne production by various mechanisms fit to the observations to distinguish cosmic-ray-produced 21Ne from nucleogenic 21Ne produced by decay of trace U and Th present in quartz, and also constrain rates of subsurface 21Ne production by cosmic-ray muons. Core samples have a quartz (U-Th)/Ne closure age, reflecting cooling below approximately 95 °C, near 160 Ma, which is consistent with existing apatite fission-track data and the 183 Ma emplacement of nearby Ferrar dolerite intrusions. Constraints on 21Ne production by muons derived from model fitting are consistent with a previously proposed value of 0.79 mb at 190 GeV for the cross-section for 21Ne production by fast muon interactions, but indicate that 21Ne production by negative muon capture is likely negligible.
Mots-clés : Cycla teer
|
|
Gou, L. F., Z. Jin, A. Galy, H. Sun, L. Deng, and Y. Xu. "Effects of cone combinations on accurate and precise Mg‐isotopic determination using multi‐collector inductively coupled plasma mass spectrometry." Rapid Communications in Mass Spectrometry 33 (2019): 351–360.
Résumé : Rationale
High‐precision determination of magnesium (Mg) isotopes can now be routinely achieved by multi‐collector inductively coupled plasma mass spectrometry (MC‐ICP‐MS). The analytical sensitivity and instrumental mass discrimination behavior of this method are, however, sensitive to the types of sample and skimmer cones used in these measurements, so it is important that these parameters should be investigated.
Methods
Using the sample‐standard‐bracketing method in the wet‐plasma mode, four available combinations of sample and skimmer cones [Jet sample cone + H skimmer cone (Jet + H), standard sample cone + H skimmer cone (Standard + H), standard sample cone + X skimmer cone (Standard + X), and Jet sample cone + X skimmer cone (Jet + X)] were systematically investigated for peak shape, sensitivity, mass discrimination, accuracy, and precision in Mg‐isotopic ratio determination using a Neptune plus MC‐ICP mass spectrometer.
Results
The results showed that different cone combinations do not affect peak shapes but would significantly change the sensitivities for Mg‐isotopic determinations. Compared with using the Standard + H, the sensitivities of Mg‐isotopic determinations were enhanced by approximately a factor of 1.3, 1.4, and 1.9 by using the Standard + X, the Jet + H, and the Jet + X combinations, with the most stable mass discrimination behaviors obtained by the Jet + H. The instrumental mass fractionation slope for any combination of a modified cone geometry (i.e. Standard + X, Jet + X, and Jet + H) is 0.500, while it is 0.510 for the Standard + H. In addition, the mass discrimination behavior is related to Mg concentrations once the combination is set, indicating the necessity of concentration match during Mg‐isotopic determination.
Conclusions
The precision and accuracy of the Jet + H combination are better than those of the other combinations, and this is further supported by the validation of the Mg‐isotope data for four international reference materials: Cambridge‐1, NASS‐6, AGV‐2, and BHVO‐2. As the Jet + H combination also provides a high signal, this combination gives the most robust strategy for the highly precise and accurate determination of Mg isotopes.
Mots-clés : Cycla
|
|
Gou, L. F., Z. Jin, P. A. E. Pogge von Strandmann, G. Li, Y. X. Qu, J. Xiao, L. Deng, and A. Galy. "Li isotopes in the middle Yellow River: Seasonal variability, sources and fractionation." Geochimica et Cosmochimica Acta 248 (2019): 88–108.
Résumé : To evaluate the roles of climate and hydrology in continental-scale silicate weathering, we applied Li isotopes to the Yellow River and systematically investigated seasonal Li flux, Li isotopic compositions and potential sources. We collected samples from the middle reaches of the Yellow River weekly over the full hydrological year of 2013. We find that the dissolved Li is mainly derived from silicates and evaporites in the arid to semi-arid Yellow River basin. Silicate weathering of loess during the monsoonal season dominates the Li flux in the middle reaches of the Yellow River, with a positive relationship between dissolved Li flux and physical erosion rate. Evaporite contribution for riverine Li was relatively constant in the middle reaches of the Yellow River but slightly increased after the storm event, with an average proportion of ∼25%, which might represent the proportion of evaporite contribution to global oceans. Seasonal variations in the riverine Li isotopic compositions are dominantly controlled by temperature with a fractionation gradient as −0.182‰ per °C over the full year with deviations likely driven by re-dissolution of suspended particulate matter, extreme hydrological events, and groundwater contribution. Temperature dependent δ7Li value variation of river water inputted into oceans indicates that Cenozoic climate cooling itself may be able to explain ∼2‰ of the 9‰ rise of Cenozoic seawater δ7Li value (Misra and Froelich, 2012). The seasonal variation in riverine Li isotopes highlights that erosion and weathering of loess may provide valuable clues on secular chemical weathering and seawater δ7Li variation spanning a range of time scales.
Mots-clés : Cycla
|
|
Ruan, X., Y. Yang, A. Galy, X. Fang, Z. Jin, F. Zhang, R. Yang, L. Deng, Q. Meng, C. Ye et al. "Evidence for early (≥12.7 Ma) eolian dust impact on river chemistry in the northeastern Tibetan Plateau." Earth and Planetary Science Letters 515 (2019): 79–89.
Résumé : As one of the largest dust sources on the Earth's surface, dryland in Central Asia gives rise to thick eolian deposits over East Asia (e.g., the Chinese Loess Plateau, CLP) and significantly influences the regional hydrochemistry in the downwind drainage areas. However, the formation of thick eolian dust deposits requires not only climatic prerequisites for dust emission and transport but also climatic and topographic conditions favourable for deposition and accumulation. The scarcity of widespread eolian deposition around the CLP before 7-8 Ma hinders a full understanding of the processes and mechanisms of Central Asian aridification. The deposition of eolian dust also impacts the hydrogeochemistry of fluvial systems and the precipitation of authigenic phases in continental sedimentary systems could be an archive for studying eolian dust dynamics when pure eolian deposits are scarce. Here, we present the Ca-Mg-Sr concentrations and 87Sr/86Sr isotope compositions of bulk carbonates in a new fluvial sequence (12.7-4.8 Ma) of the Xining Basin. The Mg/Ca and Sr/Ca ratios of the carbonate describe a power law relationship with a power coefficient of ∼0.8, lower than the coefficient characteristic of prior calcite precipitation (PCP). An input of eolian dust with the dissolution of Mg-rich carbonate is likely responsible for the deviation from a pure PCP process. The bulk carbonates also show a general decrease of 87Sr/86Sr ratios from 12.7 to 4.8 Ma, with a transition around 8.6 Ma revealed by lower Sr/Mg ratios. The comparison of these proxies to a previously reported fluvial section (12.2-5.1 Ma) in the Linxia Basin, ∼200 km to the southeast, shows that the 87Sr/86Sr ratios of the bulk carbonates and water-soluble salts in the Linxia Basin are around 0.7098, which is 0.0018 lower than those in the Xining Basin before 8.6 Ma, but shows a significant rise between 8.6 and 7.0 Ma. The two basins share the same range of carbonate 87Sr/86Sr ratios when sediments are younger than 7 Ma. For the last 7 Myrs, the evolution of the 87Sr/86Sr ratios in bulk carbonates of fluvial sediments and Pliocene-Quaternary eolian deposits found in the Xining Basin are similar to those in typical eolian red clays/loess-palaeosol sequences on the CLP. These results suggest a transition of the hydrochemical regime at 8.6 Ma in the Linxia Basin from a catchment only influenced by the weathering of its bedrock to one significantly impacted by eolian dust input. In the Xining Basin, the carbonate elemental and 87Sr/86Sr ratios are consistent with a hydrochemistry more impacted by the presence of the eolian dust. There, the dust input occurred earlier, at ≥12.7 Ma, though it has strengthened since 8.6 Ma. The eolian dust impact on fluvial systems in the Xining Basin was much earlier than in the Linxia Basin and also preceded the initial accumulation of widespread eolian red clays on the CLP (7-8 Ma), suggesting a temporally propagating and spatially stepwise expansion of eolian dust delivery across the Asian inland during the late Cenozoic.
Mots-clés : Cycla
|
|
Yang, R., Y. Yang, X. Fang, X. Ruan, A. Galy, C. Ye, Q. Meng, and W. Han. "Late miocene intensified tectonic uplift and climatic aridification on the northeastern Tibetan plateau: Evidence from clay mineralogical and geochemical records in the Xining Basin." Geochemistry Geophysics Geosystems G3 20 (2019).
Résumé : The uplift of the Tibetan Plateau (TP) during the late Cenozoic is thought to be one of crucial factors controlling Asian climate. However, the complex interaction between tectonics and climate change is still unclear. Here we present the first record of clay mineralogy and elemental geochemistry covering ~12.7–4.8 Ma in a fluvial‐lacustrine sequence in the Xining Basin. Geochemical provenance proxies (Th/Sc, Zr/Sc, and Cr/Zr) in the <2‐μm fraction show a significant provenance change at ~8.8 Ma. Silicate‐based weathering indexes (CIA, CIW, and PIA) displayed coeval changes with provenance but discrepant changes with regional climate. Since the clay mineralogy exhibits significant change at ~7.8 Ma uncorrelated with modifications in provenance, it can be employed to reveal regional climate change. The rise in illite and associated decrease in the sum of smectite and illite/smectite mixed layers reflect gradual and slow aridification since ~12.7 Ma with intensified drying since ~7.8 Ma until approaching the modern climate status. Our results, together with other regional climatic and tectonic records, clearly illustrate that accelerated uplift of the northeastern TP since ~8–9 Ma has mainly modulated the regional erosion, weathering, transportation, and sedimentation and amplified the global cooling and drying trend toward the regional climate of modern conditions. Our study suggests that in the tectonically active northeastern TP, a comprehensive mineralogical and geochemical investigation of the fine‐grained fraction of the basin sediments could retrieve the interactions between tectonics and climate behind the complex change in exhumed lithology and sedimentary routing systems.
Mots-clés : Cycla
|
|
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
|
|
Boucher, C., T. Lan, B. Marty, P. G. Burnard, T. P. Fischer, D. Ayalew, J. Mabry, J. M. de Moor, M. E. Zelenski, and L. Zimmermann. "Atmospheric helium isotope composition as a tracer of volcanic emissions: A case study of Erta Ale volcano, Ethiopia." Chemical Geology 480 (2018): 3–11.
Résumé : The composition of atmospheric helium is generally considered to be constant (3He/4He = 1.39 × 10− 6) on a large spatial scale. However, local variations may arise in tectonically active areas due to focussed degassing of one of its two isotopes, for example degassing of mantle-derived 3He or crustal-derived 4He. If detected, such variations have the potential to trace open conduit degassing of magmatic bodies and/or diffusive emissions from volcanic and/or crustal sources. Here, we test the possibility of detecting such variations in the Afar region of north-eastern Ethiopia, which is located over a well-developed rift system. Special attention was paid to the Erta Ale volcanic edifice, where both magma lake activity and strong degassing occur. We conducted high-precision 3He/4He ratio measurements of air samples from this region using an analytical facility at the Centre de Recherches Pétrographiques et Géochimiques (CRPG), Nancy (France) that was specially designed for high-precision noble gas analyses.
Within the precision of our measurements (0.2–0.4%, 95% confidence interval), the helium isotopic compositions of air from both the Afar rift zone and the crater zone of Erta Ale are similar to the composition of air collected at Brabois Park in Villers-les-Nancy, France (labelled here as RBB). An additional air sample collected in a large tank (500 cm3) in Afar in 2015 permitted replicate analysis (n = 8) and improved precision. The 3He/4He ratio of this additional sample was also identical to RBB within 0.19% (95% confidence interval, CI). However, a clear excess of 3He (1.32 ± 0.64%, 95% CI, relative to RBB based on a weighted mean of two samples) was detected in air collected above the active lava lake located in the central pit crater of Erta Ale volcano. Such excess requires a 3He flux of 0.15 ± 0.09 mol/yr from the crater lava lake to be sustained. A similar 3He flux of 0.12 ± 0.06 mol/yr is calculated from SO2 flux measurements and fumerolic gas data obtained during the same field trip. At several sites in the rim of the Northern crater, we conducted soil flux measurements using an accumulation chamber. Both the CO2 contents and the helium isotope ratios increased over time within the chamber, allowing us to evaluate the soil CO2 and 3He fluxes outside the lava lake area. These fluxes were found to be minor ( 1%) compared to the lava lake flux. The CO2/3He ratio of (3.1 ± 0.7) × 109 of soil gases is comparable to that of the high temperature (1084 °C) fumaroles sited in the north pit crater. Using this ratio and our estimated 3He flux, we determined a CO2 flux of 4.6 ± 3.0 × 108 mol/yr for the lava lake, which is about 105 times lower than the global volcanic subaerial CO2 flux. Based on this pilot study, we suggest that 3He excesses in air could provide another means with which to evaluate the fluxes of CO2 and other volatile species in specific environments, such as highly active volcanic areas.
Mots-clés : Cycla
|
|
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
|
|
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
|
|
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
|
|
Animators : Christophe Cloquet & Guillaume Paris
RSS 2.0