2021 |
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. "Prochainement au CRPG : Jeudi 11 mars 13:30 Simon Daout InSAR Monitoring of Climatic Forcing and Tectonic Deformation in N-E Tibet Lundi 15 mars 13:00 Présentations nouv. doctorants M. Barthez + D. Thomassin + G. Pinto Jeudi 18 mars 13:30 Christelle Wauthier Recrutement : Assistant Professor Position Starting date 1 September 2021 Recrutement d’un Poste de Maître des Conférences Date de début : 1er septembre 2021 Publications récentes : Optimisation of 186Os/188Os measurements by N-TIMS using amplifiers equipped with 1013 Ω (...) Reisberg, L. ; Zimmermann, C., GGR When zircon drowns : Elusive geochronological record of water-fluxed orthogneiss melting in the (...) Couzinié, S. ; Bouilhol, P. ; Laurent, O. ; Marko, L. ; Moyen, J.F., Lithos Nonlinear forcing of climate on mountain denudation during glaciations Mariotti, A. ; Blard, P.H. ; Charreau, J. ; Toucanne, S. ; Jorry, S.J. ; Molliex, S. ; Bourlès, (...) Flower-shaped trapiche ruby from Mong Hsu, Myanmar : A revised growth mechanism Pignatelli, I. ; Giuliani, G. ; Morlot, C. ; Cathelineau, M. ; Liu, Shang I (Edward), J. of (...) Analyse des gaz rares par spectrométrie de masse statique. Théorie et instrumentation Zimmermann, L. ; Bekaert, D., Techniques de l’ingénieur Accueil du site > Publications > Optimisation of 186Os/188Os measurements by N-TIMS using amplifiers (...) 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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Lénard, S., J. Cruz, C. France-Lanord, J. Lavé, and B. T. Reilly. "Data report : calcareous nannofossils and lithologic constraints on the age model of IODP Site U1450, Expedition 354, Bengal Fan." Proceedings of the International Ocean Discovery Program 354 (2020): 1–5.
Résumé : International Ocean Discovery Program (IODP) Expedition 354 Site U1450 was drilled at the center of a transect of 7 sites across the Bengal Fan at 8°N where long-term accumulation rates are highest. Site U1450 primarily consists of sandy and silty-sandy turbidites deposited at a rate higher than 20 cm/ky. During periods when the depocenter shifted away from Site U1450, calcareous clay hemipelagic sediment was deposited at lower accumulation rates around 1–2 cm/ky. Dating of the lower Pleistocene and Pliocene sequences is hindered by the scarcity of microfossils in turbidites and the restriction of paleomagnetic data to the upper 190 m. This report presents the identification of new calcareous nannofossils collected from hemipelagic and turbiditic intervals between 218 and 687 m core depth below seafloor, Method A (CSF-A). These data are consolidated in a statistical age model that is constrained with ranges of plausible accumulation rates for the distinct lithologies. The age probability model ranges from 1.2 to 7.3 Ma for depths from 175.8 to 812.0 m CSF-A. Depending on constraints, 2σ uncertainties are around ±0.2 and 0.4 Ma.
Mots-clés : Cycla teer
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Martin, L. C. P., P. H. Blard, J. Lavé, V. Jomelli, J. Charreau, T. Condom, and M. Lupker. "Antarctic-like temperature variations in the Tropical Andes recorded by glaciers and lakes during the last deglaciation." Quaternary Science Reviews 247 (2020): 106542.
Résumé : The respective impacts of Northern and Southern Hemispheric climatic changes on the Tropics during the last deglaciation remain poorly understood. In the High Tropical Andes, the Antarctic Cold Reversal (ACR, 14.3–12.9 ka BP) is better represented among morainic records than the Younger Dryas (12.9–11.7 ka BP). However, in the Altiplano basin (Bolivia), two cold periods of the Northern Hemisphere (Heinrich Stadial 1a, 16.5–14.5 ka BP, and the Younger Dryas) are synchronous with (i) major advances or standstills of paleoglaciers and (ii) the highstands of giant paleolakes Tauca and Coipasa.
Here, we present new cosmic ray exposure (CRE) ages from glacial landforms of the Bolivian Andes that formed during the last deglaciation (Termination 1). We reconstruct the equilibrium line altitudes (ELA) associated with each moraine and use them in an inverse algorithm combining paleoglaciers and paleolake budgets to derive temperature and precipitation during the last deglaciation.
Our temperature reconstruction (ΔT relative to present day) yields a consistent regional trend of progressive warming from ΔT = −5 to −2.5 °C during 17–14.5 ka BP, followed by a return to colder conditions around −4 °C during the ACR (14.5–12.9 ka BP). The Coipasa highstand (12.9–11.8 ka BP) is coeval with another warming trend followed by ΔT stabilization at the onset of the Holocene (ca. 10 ka BP), around −3 °C. Our results suggest that, during the last deglaciation (20–10 ka BP) atmospheric temperatures in the Tropical Andes mimicked Antarctic variability, whereas precipitation over the Altiplano was driven by changes in the Northern Hemisphere.
Mots-clés : Cycla
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Märki, L., M. Lupker, A. P. Gajurel, H. Gies, N. Haghipour, S. Gallen, C. France-Lanord, J. Lavé, and T. Eglinton. "Molecular tracing of riverine soil organic matter from the Central Himalaya." Geophysical Research Letters 47, no. 16 (2020): e2020GL087403.
Résumé : The isomer distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils has been shown to correlate to the local mean annual temperature. Here, we explore the use of brGDGT distributions as proxy for the elevation at which soil organic carbon is preferentially mobilized in the Central Himalaya. Soil brGDGT distributions collected along an altitudinal profile, spanning elevations from 200 to 4,450 m asl, are linearly correlated to elevation. We use this calibration to trace the provenance of soil organic matter in suspended sediments of rivers draining the Himalaya. BrGDGT distributions of fluvial sediments reflect the mean elevation of the soil cover in most catchments. Inverse modeling of the brGDGT data set suggests similar relative contribution to soil organic carbon mobilization from different land covers within a factor 2. We conclude that riverine soil organic carbon export in the Himalaya mostly occurs pervasively and is at the catchment scale insensitive to anthropogenic perturbations.
Mots-clés : Cycla teer
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Animators : Christophe Cloquet & Guillaume Paris
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