2021 |
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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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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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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Reisberg, L., and C. Zimmermann. "Optimisation of 186Os/188Os measurements by N-TIMS using amplifiers equipped with 1013 Ω resistors." Geostandards and Geoanalytical Research (2021).
Résumé : Analysis of 186Os/188Os variations in most natural samples is hampered by their low Os mass fractions (usually < 100 pg g-1), requiring Os separation from up to a kilogram of powder to obtain high precision measurement results. Recently developed amplifiers equipped with 1013 Ω feedback resistors improve the signal to noise ratio by an order of magnitude, allowing acquisition of high precision data (2SE < 50 ppm) from 186Os16O3 – ion beams of 40 fA and thus analysis of less material ( 10–500 g). Factors controlling the uncertainty of 186Os/188Os N-TIMS measurement results, using a mixed array of 1013 and 1011 Ω amplifiers, include isobaric interferences, baseline variability, gain calibration, decay time corrections and oxygen isotope composition variations. Isobaric interferences present the greatest challenges, but these can be largely overcome by using double Pt filaments and periodically monitoring interferences during each measurement. The optimised procedures produce 186Os/188Os ratios with precisions close to the theoretical limits imposed by counting statistics and Johnson–Nyquist electronic noise for ion beams of 10–70 fA. The intermediate measurement precision (85 ppm ; 2s) approaches theoretically expected values ( 50–80 ppm). This precision is sufficient to allow 186Os/188Os analyses of crustal rocks, for which relatively large variations are expected.
Mots-clés : Cycla
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2020 |
Barkan, Y., G. Paris, S. M. Webb, J. F. Adkins, and I. Halevy. "Sulfur isotope fractionation between aqueous andcarbonate-associated sulfate in abiotic calcite and aragonite." Geochimica et Cosmochimica Acta 280 (2020): 317–339.
Résumé : Sulfate (SO42-) incorporated into calcium carbonate minerals enables measurements of sulfur (S) isotope ratios in carbon-ate rocks. This Carbonate Associated Sulfate (CAS) in marine carbonate minerals is thought to faithfully represent the S iso-tope composition of the seawater sulfate incorporated into the mineral, with little or no S isotope fractionation in the process.However, comparison between different calcifying species reveals both positive and negative S isotope fractionation betweenCAS and seawater sulfate, and a large range of S isotope ratios can be found within a single rock sample, depending on thecomponent measured. To better understand the isotopic effects associated with sulfate incorporation into carbonate minerals,we precipitated inorganic calcite and aragonite over a range covering more than two orders of magnitude of sulfate concen-tration and precipitation rate. Coupled measurements of CAS concentration, S isotope composition and X-ray absorptionnear-edge spectra (XANES) permit characterization and explanation of the observed dependence of S isotope fractionationbetween CAS and aqueous sulfate (CAS-SO42-isotope fractionation) on sulfate concentration and precipitation rate. In arag-onite, the CAS-SO42isotope fractionation is 1.0 ± 0.3‰and independent of the sulfate (and CAS) concentration. In contrast,the CAS-SO42-isotope fractionation in calcite covaries strongly with the sulfate concentration and weakly with the precipi-tation rate, between values of 1.3 ± 0.1 and 3.1 ± 0.6‰. We suggest that the correlation between aqueous sulfate concentra-tion and CAS-SO42-isotope fractionation in calcite reflects a dependence of the equilibrium S isotope fractionation on theconcentration of CAS, through the effect of the sulfate impurity on the carbonate mineral’s energetic state.Ó2020 Elsevier Ltd. All rights reserved.
Mots-clés : Cycla
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Georgiev, S. V., H. J. Stein, G. Yang, J. L. Hannah, M. E. Böttcher, K. Grice, A. I. Holman, S. Turgeon, S. Simonsen, and C. Cloquet. "Late Permian–Early Triassic environmental changes recorded by multi-isotope (Re-Os-N-Hg) data and trace metal distribution from the Hovea-3 section,Western Australia." Gondwana Research 88 (2020): 353–372.
Résumé : The temporal coincidence between the Late Permian mass extinction (LPME) and the emplacement of Siberian Trap basalts suggests a causal link between the two events. Here, we discuss stratigraphic changes of organic and inorganic (including isotopic) geochemical properties of marine sediments across the Permian–Triassic boundary (PTB) in the Hovea-3 core, Western Australia, a key PTB section in the southern Neo-Tethys ocean. These data are compared with published data from the Meishan section, southern China, and from the Opal Creek section in western Canada, providing a view of Tethys and Panthalassa changes at the PTB. Trace metal and N-isotopic data, together with organic matter properties suggest that anoxic conditions were established prior to the LPME, intensified close to the LPME, and continued with photic-zone euxinia into the Early Triassic. For the Hovea-3 section, Re-Os ages confirm Changhsingian (253.5 ± 1.4 Ma) deposition of the dated interval sampled immediately below the stratigraphic level characterized by major lithological and isotopic changes. Evaluation of Re-Os, N, and Hg elemental and isotopic data for Hovea-3 suggests that anoxic conditions in the latest Permianwere generally unrelated to direct magmatic contributions. Amajor increase in the initial Os isotopic ratio of Lower Triassic shales suggest an 8× increase in the Early Triassic continental runoff, based on moderately conservative assumptions for end-members contributing Os to the Permian–Triassic ocean. Comparison to other PTB sections confirms a global signal of increasing Re/Os ratios in the Late Permian, and major and long-lived changes in the isotopic composition of the post-extinction ocean. A distinct peak in Hg concentrations carrying a volcanic isotopic signature, also identified in other PTB sections, likely represents a major pulse of Siberian Trap volcanism. This Hg peak in the Hovea-3 section, however, is detected above the stratigraphic level containing multiple other widely recognized and more permanent geochemical changes. Therefore, direct volcanic inputs to the Permian–Triassic Ocean likely post-date the LPME in thisWestern Australian section.
Mots-clés : Cycla
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Han, W., E. Appel, A. Galy, W. Rösler, X. Fang, X. Zhu, J. Vandenberghe, J. Wang, A. Berger, S. Lü et al. "Climate transition in the Asia inland at 0.8–0.6 Ma related to astronomically forced ice sheet expansion." Quaternary Science Reviews 248 (2020): 106580.
Résumé : Knowing the evolution history of the climate systems in the Asian inland dominated by either the Westerlies or the Asian monsoon, and understanding their associated driving mechanisms are crucial for assessing future trends of climate and environmental conditions in this region, but both the evolution and mechanisms are still under debate. In this study, we present a comparative analysis of massive data from an accurately dated drill core retrieved from the Westerlies controlled western Qaidam Basin (QB), with records from the Chinese Loess Plateau (CLP) dominated by the East Asia summer monsoon (EASM), to track the time and frequency domain evolution patterns and dynamic changes of the QB and the CLP systems. The results infer two main conclusions. First, a critical transition in the evolution of Qaidam paleolake occurred at 0.8–0.6 Ma, characterized by striking changes in proxy variations and a system shift from periodic variations to more irregular fluctuations after 0.6 Ma. Second, a similar evolution pattern prevailed in the Qaidam paleolake region and in the EASM-dominated CLP between ∼2.7 and ∼1.2 Ma, but a divergence of both systems started at ∼1.2 Ma and fully established after 0.6 Ma, when largely fluctuating climate conditions in the QB with a distinct drying trend was accompanied by synchronous largely fluctuated EASM with an increasing trend after 0.6 Ma. We suggest that ice sheet expansion in the Northern Hemisphere, promoted by co-occurrence of low obliquity amplitudes and low eccentricity, drove both systems across a threshold at ∼1.2 Ma, and the internal forcing due to glaciation disturbed the previous response of both systems to solar insolation and led to the divergence of two systems. At ∼0.9–0.8 Ma, a node of the 1.2-Myr obliquity cycle co-occurred with an eccentricity minimum, which together with coeval decrease in atmospheric CO2 concentration, could have facilitated a striking expansion of ice sheets. The resulting more equatorial and zonally oriented northern hemisphere westerly jet could serve as key factor leading to the final collapse of Qaidam paleolake after ∼0.6 Ma, and the divergence of the QB and the CLP systems.
Mots-clés : Cycla
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Labidi, J., P. H. Barry, D. V. Bekaert, M. W. Broadley, B. Marty, T. Giunta, O. Warr, B. S. Sherwood, T. P. Fischer, G. Avice et al. "Hydrothermal 15N15N abundances constrain the origins of mantle nitrogen." Nature 580 (2020): 367–371.
Résumé : Nitrogen is the main constituent of the Earth’s atmosphere, but its provenance in the Earth’s mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth’s accretion versus that subducted from the Earth’s surface is unclear1,2,3,4,5,6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15N15N isotopologue of N2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle δ15N (the fractional difference in 15N/14N from air), N2/36Ar and N2/3He. Our results show that negative δ15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen7,8,9,10, in fact represent dominantly air-derived N2 that experienced 15N/14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15N15N data allow extrapolations that characterize mantle endmember δ15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly increased δ15N and N2/36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has δ15N values substantially greater than that of the convective mantle, resembling surface components12,13,14,15, its N2/36Ar and N2/3He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume δ15N values may both be dominantly primordial features.
Mots-clés : magma cycla
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Animators : Christophe Cloquet & Guillaume Paris
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