2021 |
Blard, P. H. "Cosmogenic 3He in terrestrial rocks: A review." Chemical Geology 586 (2021): 120543.
Résumé : This review article summarizes the state of the art of cosmogenic 3He (3Hec), with a focus on the most efficient methods and strategies for measuring this cosmogenic noble gas in the terrestrial minerals. Our ability to accurately and precisely measure cosmogenic 3He in minerals is mainly constrained by the level of the noncosmogenic 3He background (i.e., magmatic, radiogenic, nucleogenic, and atmospheric 3He), and thus by the geological characteristics of the samples. Constructing 3He vs 4He isochrons by analyzing several aliquots from the same surface sample constitutes a useful and overlooked method. Isochrons can be applied to mineral samples with homogeneous radiogenic 4He concentration and variable magmatic helium concentrations. This method also allows the direct and joint determination of the cosmogenic 3He concentration and the magmatic 3He/4He ratio, precluding the often-complicated step of vacuum crushing. I perform numerical modeling to explore the impact of the non-cosmogenic 3He components on the final uncertainties and detection limits of 3Hec measurement. Reducing the magmatic component by selecting phenocrysts in the 100–500 μm size fraction improves the precision of cosmogenic 3He analyses. Moreover, it is important to measure U, Th, and Li concentrations in the analyzed minerals and their host rocks to ensure proper corrections for radiogenic 4He and nucleogenic 3He, improving both the accuracy and precision of the method. I discuss the most important aspects of analytical techniques, including the key parameters of noble gas mass spectrometry that result in accurate and precise helium isotopic measurements. I also review 3Hec production rates and their spatial variability : the global database of absolute calibration sites yields a world-wide average 3Hec production rate in olivine and pyroxene of 124 ± 11 at g-1 yr- 1 using the LSD scaling and the online CREp calculator (https://crep.otelo.univ-lorraine.fr/#/). Cross-calibrations against 10Be indicate that the ratio of the production rate of 3Hec in olivine/pyroxene to that of 10Be in quartz is 33 ± 2 and increases by less than 7% between sea level and 5000 m elevation. This important observation demonstrates that 3He in olivine/pyroxene and 10Be in quartz are accurately scaled with existing models and can hence be considered as synchronized chronometers. However, 3Hec/10Be cross-calibrations based on 3Hec in accessory minerals (zircon, garnet, kyanite, apatite) yield unexpectedly high 3He/10Be production ratios of 40–60 above 3000 m elevation. As the capture of cosmogenic thermal neutrons by 6Li is unlikely to explain this excess, other mechanisms are required, such as 3Hec inherited from previous exposure episodes, unrecognized specific reaction pathways, or the impact of snow cover. New cross-calibration data obtained by measuring 3Hec against other cosmogenic nuclides (14C, 36Cl, 21Ne, 53Mn) in different settings will advance our understanding of cosmogenic nuclide production rates and improve the accuracy and precision of applications relying on cosmogenic 3He. Other improvements could extend the applicability of the 3He geoscientific toolbox ; for example, coupling 3He with radioactive cosmogenic nuclides (10Be, 14C, 36Cl, 53Mn) will allow paleoaltimetry or the determination of burial ages or paleo-depths in intermediate and mafic terrains
Mots-clés : Cycla teer magma
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Bufe, A., N. Hovius, R. Emberson, J. K. C. Rugenstein, A. Galy, H. J. Hassenruck-Gudipati, and J. M. Chang. "Co-variation of silicate, carbonate and sulfide weathering drives CO2 release with erosion." Nature Geoscience 14 (2021): 211–216.
Résumé : Global climate is thought to be modulated by the supply of minerals to Earth’s surface. Whereas silicate weathering removes carbon dioxide (CO2) from the atmosphere, weathering of accessory carbonate and sulfide minerals is a geologically relevant source of CO2. Although these weathering pathways commonly operate side by side, we lack quantitative constraints on their co-variation across erosion rate gradients. Here we use stream-water chemistry across an erosion rate gradient of three orders of magnitude in shales and sandstones of southern Taiwan, and find that sulfide and carbonate weathering rates rise with increasing erosion, while silicate weathering rates remain steady. As a result, on timescales shorter than marine sulfide compensation (approximately 106–107 years), weathering in rapidly eroding terrain leads to net CO2 emission rates that are at least twice as fast as CO2 sequestration rates in slow-eroding terrain. We propose that these weathering reactions are linked and that sulfuric acid generated from sulfide oxidation boosts carbonate solubility, whereas silicate weathering kinetics remain unaffected, possibly due to efficient buffering of the pH. We expect that these patterns are broadly applicable to many Cenozoic mountain ranges that expose marine metasediments.
Mots-clés : Cycla
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Cavalazzi, B., L. Lemelle, A. Simionovici, S. L. Cady, M. J. Russell, E. Bailo, R. Canteri, E. Enrico, A. Manceau, A. Maris et al. "Cellular remains in a 3.42-billion-year-old subseafloor hydrothermal environment." Science Advances 7, no. 29 (2021): eabf3963.
Résumé : Subsurface habitats on Earth host an extensive extant biosphere and likely provided one of Earth’s earliest microbial habitats. Although the site of life’s emergence continues to be debated, evidence of early life provides insights into its early evolution and metabolic affinity. Here, we present the discovery of exceptionally well-preserved, 3.42-billion-year-old putative filamentous microfossils that inhabited a paleo-subseafloor hydrothermal vein system of the Barberton greenstone belt in South Africa. The filaments colonized the walls of conduits created by low-temperature hydrothermal fluid. Combined with their morphological and chemical characteristics as investigated over a range of scales, they can be considered the oldest methanogens and/or methanotrophs that thrived in an ultramafic volcanic substrate.
Mots-clés : Cycla
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Christ, A. J., P. R. Bierman, J. M. Schaefer, D. Dahl-Jensen, J. P. Steffensen, L. B. Corbett, D. M. Peteet, E. K. Thomas, E. J. Steig, T. M. Rittenour et al. "A multimillion-year-old record of Greenland vegetation and glacial history preserved in sediment beneath 1.4 km of ice at Camp Century." PNAS 118, no. 13 (2021): e-2021442118.
Résumé : Understanding the history of the Greenland Ice Sheet (GrIS) is critical for determining its sensitivity to warming and contribution to sea level; however, that history is poorly known before the last interglacial. Most knowledge comes from interpretation of marine sediment, an indirect record of past ice-sheet extent and behavior. Subglacial sediment and rock, retrieved at the base of ice cores, provide terrestrial evidence for GrIS behavior during the Pleistocene. Here, we use multiple methods to determine GrIS history from subglacial sediment at the base of the Camp Century ice core collected in 1966. This material contains a stratigraphic record of glaciation and vegetation in northwestern Greenland spanning the Pleistocene. Enriched stable isotopes of pore-ice suggest precipitation at lower elevations implying ice-sheet absence. Plant macrofossils and biomarkers in the sediment indicate that paleo-ecosystems from previous interglacial periods are preserved beneath the GrIS. Cosmogenic 26Al/10Be and luminescence data bracket the burial of the lower-most sediment between <3.2 ± 0.4 Ma and >0.7 to 1.4 Ma. In the upper-most sediment, cosmogenic 26Al/10Be data require exposure within the last 1.0 ± 0.1 My. The unique subglacial sedimentary record from Camp Century documents at least two episodes of ice-free, vegetated conditions, each followed by glaciation. The lower sediment derives from an Early Pleistocene GrIS advance. 26Al/10Be ratios in the upper-most sediment match those in subglacial bedrock from central Greenland, suggesting similar ice-cover histories across the GrIS. We conclude that the GrIS persisted through much of the Pleistocene but melted and reformed at least once since 1.1 Ma.
Mots-clés : Cycla
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Dussubieux, L., C. Cloquet, and T. O. Pryce. "Isotope analysis and its applications to the study of ancient Indian glass." In Ancient Glass of South Asia. Springer, 2021.
Résumé : The use of lead, strontium and neodymium isotope signatures in ancient glass is developing rapidly and allows the identification of the geological origin of the different raw materials entering the composition of the glass. This approach has been recently applied to glass manufactured in India to define the possible regions of production. Three glasses were investigated : the m-Na-Al 1, m-Na-Al 2 and m-Na-Al 3 glasses. The strontium and neodymium isotopic composition of the m-Na-Al 1 glass is supporting archaeological evidences suggesting that this glass was produced in Sri Lanka and in south-eastern India. In contrast, although it was assumed that the m-Na-Al 2 glass was produced in Maharashtra, the strontium and neodymium signature of this glass is incompatible with this hypothesis. Isotopic data and historical evidence are pointing instead towards an origin from Uttar Pradesh. The isotopic signatures of the m-Na-Al 2 and 3 glasses are fairly similar. The m-Na-Al 3 glass was discovered in Kopia, eastern Uttar Pradesh, where evidence suggests that glassmaking and glass working took place on-site. The raw material available around the site has an isotopic signature different from the glass showing that ingredients for glass production were brought from elsewhere or that the glass itself was imported to the site to be turned into finished objects. Lead isotope signatures were considered for glass samples containing lead-based or lead-containing colouring ingredients. They indicate two possible provenances for the lead : Thailand and north-western South Asia.
Mots-clés : Cycla
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Laurent, D., C. Durlet, G. Barré, P. Sorriaux, P. Audra, P. Cartigny, C. Carpentier, G. Paris, P. Collon, T. Rigaudier et al. "Epigenic vs. hypogenic speleogenesis governed by H2S/CO2 hydrothermal input and Quaternary icefield dynamics (NE French Pyrenees)." Geomorphology 387 (2021): 107769.
Résumé : Hypogenic caves, linked to carbonate rock dissolution due to CO2- and H2S-rich ascending deep waters, represent more than 10% of karstic networks worldwide; a proportion that increases as these systems are better constrained. However, interaction between hypogenic and epigenic processes is still poorly understood, especially since the subsequent invasion of surface water often obliterates the morphological and mineral markers of hypogenic activities. The Ariège Valley (French Pyrenean foothills) hosts significant karstic networks epigenically reworked by several episodes of glacier meltwater penetration during the successive coverage of Quaternary icefields. Among these karstic systems, the Vapeur and Ermite caves were probably initiated by a hypogenic component during the Miocene. In particular, multiple-S, Sr, H, C, and O isotopes of thermo-mineral waters and calcite-sulfate speleothems confirm that hydrothermal fluids reached the caves, and subsequently interacted with Quaternary glacial epigenic phases. Deep fluids conveyed CO2 and H2S, both produced from the thermochemical reduction of Triassic evaporites at depth. H2S oxidation and CO2 hydration in the cave atmosphere, above the water table, created sulfuric and carbonic acids responsible for an intense karstification. Interpretation of isotopic data, together with a geomorphological, mineralogical and textural study of cave minerals, allow us to propose a speleogenetic model in which the respective impact of epigenic and hypogenic processes was driven by base-level changes during successive Quaternary glacial/interglacial epochs: (i) during glacial periods, invasion of glacier meltwater within the karst led to the dilution of the hydrothermal water, and was responsible for an “epigenic mechanical-dominant” speleogenesis through water-related abrasion; (ii) interglacial epochs were marked by base-level drops and the establishment of a vadose domain in caves, favoring the widening of karstic conduits through carbonic and sulfuric acid condensation-corrosion during thermal water degassing. This “hypogenic chemical-dominant” speleogenesis was active until a new advance of glaciers, and this cycle occurred several times.
Mots-clés : Cycla
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Le Meur, C., M. Cadet, N. V. Doan, D. N. Trien, C. Cloquet, P. Dillmann, A. Thote, and T. O. Pryce. "Typo-technological, elemental and lead isotopic characterization and interpretation of Đông Sơn miniature drums." Journal of Archaeological Science: Reports 38 (2021): 103017.
Résumé : Copper-base drums are among the most iconic artefacts of mid-late 1st millennium BC Southeast Asia and southern China, and more specifically of the Vietnamese Đˆong Sơn culture and the Yunnanese Dian culture. The wide distribution of these drums, in public museums and private collections, renders their technical and stylistic
study difficult, and their comparison complex. In this paper, we focus on a copper-base miniature drum assemblage, discovered in a tightly delimited area of northern Vietnam, associated to the Đˆong Sơn culture. This multi-disciplinary study incorporates morphological, stylistic, technological, elemental and lead isotopic analysis of a large proportion of a regional drum assemblage for the first time. With this combined approach was intended to improve our knowledge of the people who produced and consumed miniature drums; a class which, until now, has not aroused much interest. The present paper details the archaeometric results of this original analysis, based on 25 samples from the National Museum of History (Hanoi). The results indicate a wide variety of alloy types was used, alongside a range of raw material sources, both with weak correspondence to morpho-stylistic classification.
We propose these data evidence Đˆong Sơn drum miniature production was practiced in a decentralised manner, using materials at hand, but questions concerning their emergence and their link to burial practices seen in contemporary southern China remains unsolved from an archaeometallurgical point of view but will be the
subject of another paper (Le Meur et al. (forthcoming).
Mots-clés : Cycla
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Mariotti, A., P. H. Blard, J. Charreau, S. Toucanne, S. J. Jorry, S. Molliex, D. L. Bourlès, G. Aumaître, and K. Keddadouche. "Nonlinear forcing of climate on mountain denudation during glaciations." Nature Geoscience 14 (2021): 16–22.
Résumé : Denudation is one of the main processes that shapes landscapes. Because temperature, precipitation and glacial extents are key factors involved in denudation, climatic fluctuations are thought to exert a strong control on this parameter over geological timescales. However, the direct impacts of climatic variations on denudation remain controversial, particularly those involving the Quaternary glacial cycles in mountain environments. Here we measure in situ cosmogenic 10Be concentration in quartz in marine turbidites of two high-resolution cores collected in the Mediterranean Sea, providing a near-continuous (temporal resolution of 1–2 kyr) reconstruction of denudation in the Southern Alps since 75 kyr ago (ka). This high-resolution palaeo-denudation record can be compared with well-constrained climatic variations over the last glacial cycle. Our results indicate that total denudation rates were approximately two times higher than present during the Last Glacial Maximum (26.5–19 ka), the glacial component of the denudation rates being 1.5+0.9−1.0 mm yr−1. However, during moderately glaciated times (74–29 ka), denudation rates were similar to those today (0.24 ± 0.04 mm yr−1). This suggests a nonlinear forcing of climate on denudation, mainly controlled by the interplay between glacier velocity and basin topography. Hence, the onset of Quaternary glaciations, 2.6 million years ago, did not necessarily induce a synchronous global denudation pulse.
Mots-clés : Cycla teer
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Mitra, A., I. S. Sen, S. K. Pandey, V. Velu, L. Reisberg, M. Bizimis, and S. Nizam. "Lead isotope evidence for enhanced anthropogenic particle transport to the Himalayas during summer months." Environmental Science & Technology 55 (2021): 13697–13708.
Résumé : The Indo-Gangetic Plain (IGP) is one of the most highly polluted regions of the world, yet the temporal pattern of transport of anthropogenic aerosols from this region to the Himalayas is poorly constrained. On the basis of the seasonal variation of planetary boundary layer heights, air mass back trajectory analysis, and year-long time-series data for 208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb, and 143Nd/144Nd from aerosols collected over a high-altitude station, we demonstrate that anthropogenic Pb transport to the glacierized catchment has a seasonal pattern. The Pb isotope data reveal that during winter, the thinned boundary layer traps up to 10 ± 7% more coal-derived Pb in the IGP. In contrast, in nonwinter months, a thicker boundary layer and enhanced subtropical westerly winds result in efficient Pb transport to the Himalayas. As Pb isotope ratios are robust conservative chemical tracers and Pb is predominantly derived from anthropogenic sources, these observations suggest that enhanced transport of anthropogenic aerosols to the glacierized catchment of the Himalayas coincides with higher near-surface temperatures in the summer, creating positive feedback that enhances melting. Our results further suggest that >50% of Pb in the Himalayan aerosols originates from the resuspension of historic Pb derived from phased out leaded gasoline, highlighting the importance of legacy Pb stored in the Indo- Gangetic Plains
Mots-clés : Cycla
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Pasava, J., L. Ackerman, J. Zak, F. Veselovsky, R. A. Creaser, M. Svojtka, B. Luais, O. Pour, O. Sebek, J. Trubac et al. "Elemental and isotopic compositions of trench-slope black shales, Bohemian Massif, with implications for oceanic and atmospheric oxygenation in early Cambrian." Palaeogeography, Palaeoclimatology, Palaeoecology 564 (2021): 110195.
Résumé : This study examines a lower Cambrian pyrite-bearing black shale–graywacke succession in the Czech Republic interpreted as infill of a deep-marine, extensional trench-slope basin on top of an accretionary wedge that developed during subduction of an oceanic plate beneath the northern margin of Gondwana. The new U–Pb detrital zircon geochronology of the graywacke constrains the maximum age of deposition to 533 +5/−6 Ma, whereas the Re–Os dating of pyrite yielded a younger age of 507 ± 16 Ma. Distribution of major and trace elements and calculated enrichment factors (EF) indicate that this succession was deposited under changing redox conditions over a short time span and that the basin was presumably controlled by tectonic subsidence and varying supply of terrigenous arc-derived material. This depositional setting is reflected by largely variable EFs and δ98Mo and δ53Cr values. Euxinic conditions detected at the base of the sampled black shale interval are documented by the highest values of EFs of redox-sensitive metals (e.g., Mo, U, V, Ni, Co, As) and also the Corg/P (>1000) and DOPT (>0.7) values. Black shales show lower δ56Fe values due to the excess of authigenic pyrite-Fe with a mean δ56Fe value of −0.02‰ over detrital Fe in graywacke with a mean δ56Fe value of +0.25‰. However, the Fe isotopic signatures of the black shales are not consistent with iron shuttling, mixing of authigenic and detrital sources, or hydrothermal metal enrichment. Instead, they most likely resulted from partial oxidation of pyrite through the syndepositional oxidizing hydrothermal fluids (Si–Ba enrichment), which resulted in precipitation of isotopically heavy Fe-oxyhydroxides. We propose that our maximal recorded δ98Mo value (+ 0.98‰) might represent the best estimate for the ancient local seawater Mo composition at around 533 Ma and argues against deep-ocean oxygenation in the early Cambrian. On the other hand, the Se/Co ratios of synsedimentary pyrite indicate a mean atmosphere O2 value of ~27%.
Mots-clés : Cycla
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Animators : Christophe Cloquet & Guillaume Paris
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