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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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Palacios, D., C. R. Stokes, F. M. Philipps, J. J. Clague, J. Alcala-Reygosa, N. Andres, I. Angel, and P. H. et al. Blard. "The deglaciation of the Americas during the Last Glacial Termination." Earth-Science Reviews 203 (2020).
Résumé : This paper reviews current understanding of deglaciation in North, Central and South America from the Last Glacial Maximum to the beginning of the Holocene. Together with paleoclimatic and paleoceanographic data, we compare and contrast the pace of deglaciation and the response of glaciers to major climate events. During the Global Last Glacial Maximum (GLGM, 26.5-19 ka), average temperatures decreased 4° to 8°C in the Americas, but precipitation varied strongly throughout this large region. Many glaciers in North and Central America achieved their maximum extent during the GLGM, whereas others advanced even farther during the subsequent Heinrich Stadial 1 (HS-1). Glaciers in the Andes also expanded during the GLGM, but that advance was not the largest, except on Tierra del Fuego. HS-1 (17.5-14.6 ka) was a time of general glacier thickening and advance throughout most of North and Central America, and in the tropical Andes; however, glaciers in the temperate and subpolar Andes thinned and retreated during this period. During the Bølling-Allerød interstadial (B-A, 14.6-12.9 ka), glaciers retreated throughout North and Central America and, in some cases, completely disappeared. Many glaciers advanced during the Antarctic Cold Reversal (ACR, 14.6-12.9 ka) in the tropical Andes and Patagonia. There were small advances of glaciers in North America, Central America and in northern South America (Venezuela) during the Younger Dryas (12.9-11.7 ka), but glaciers in central and southern South America retreated during this period, except on the Altiplano where advances were driven by an increase in precipitation. Taken together, we suggest that there was a climate compensation effect, or ‘seesaw’, between the hemispheres, which affected not only marine currents and atmospheric circulation, but also the behavior of glaciers. This seesaw is consistent with the opposing behavior of many glaciers in the Northern and Southern Hemispheres.
Mots-clés : Cycla
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Paris, G., W. W. Fischer, J. E. Johnson, S. M. Webb, T. M. Present, A. L. Sessions, and J. F. Adkins. "Deposition of sulfate aerosols with positive D33S in the Neoarchean." Geochimica et Cosmochimica Acta 285 (2020): 1–20.
Résumé : Anomalous sulfur isotope compositions present in Archean rocks have been intensely scrutinized over the last 20 years because they record key aspects of Earth’s atmospheric composition prior to the appearance of free molecular oxygen ca. 2.3 billion years ago. These isotopic compositions can be described as mass anomalous fractionations (MAF) and are produced in the atmosphere as UV light interacts with SO2 molecules. Most interpretations suggest that atmospheric processes generate a reduced S-phase with a positive (33S-enriched) MAF signature, as measured in pyrites, and an oxidized S-phase with a negative anomaly, as measured in bedded barite deposits. However, recent data for carbonate-associated sulfate (CAS) — a direct proxy for the isotopic composition of sulfur from seawater sulfate — in Neoarchean rocks showed no such negative values, but rather the opposite. To understand if the positive MAF anomalies we measured in Neoarchean CAS reflect secondary processes (diagenetic, metamorphic, handling) instead of original signals of Archean seawater sulfate, we collected additional sample suites with various degrees of preservation and metamorphic alteration across the Campbellrand-Malmani platform in South Africa. Results illustrate that within this comprehensive suite, less-altered samples all contain positive MAF values while secondary processes tend to either remove CAS from the sample and/or decrease the 33S-enrichment. This positive MAF signal in sulfate is therefore reasonably interpreted as a primary depositional origin, and implies that the assumption that sulfate always carries a negative MAF anomaly throughout the Archean rock record needs to be reconsidered. Our CAS observations suggest that future experiments and calculations should also consider atmospheric and/or sulfur cycling processes that can produce oxidized sulfur with a positive MAF signature.
Mots-clés : Cycla
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van der Veen, I., F. Peterse, J. Davenport, B. Meese, B. Bookhagen, C. France-Lanord, A. Kahmen, H. J. Hassenruck-Gudipati, A. Gajurel, M. R. Strecker et al. "Validation and calibration of soil δ2H and brGDGTs along (E-W) and strike (N-S) of the Himalayan climatic gradient." Geochimica et Cosmochimica Acta 290 (2020): 408–423.
Résumé : Reconstructing the timing of mountain range uplift and the evolution of high-altitude plateaus is important when attempting to understand potential feedbacks between tectonics and climate at geological timescales. This requires proxies that are able to accurately reconstruct elevation during different time periods in the past. Often, the sensitivity of climatic parameters to elevation gradients, recorded in geological archives such as soils, is used to estimate paleoelevations. However, most proxies reflect an indirect response to elevation change, adding uncertainties to reconstructions. In this study, we aim to identify those sources of uncertainty with respect to elevation reconstructions and test if the combined application of two such proxies, i.e., stable isotopes (δ2H) of plant waxes in modern soils and surface waters and bacterial membrane lipids (brGDGTs) in soils, which can potentially reduce uncertainties in the estimation of (paleo-) elevation. We performed this study in four Himalayan catchments (from west to east: Sutlej, Alaknanda, Khudi, and Arun), of which each individual catchment is subject to a unique precipitation regime, relative influences of moisture sources, and vegetation cover. In total, we analyzed 275 surface water samples, 9 precipitation samples, 131 xylem water samples, and 60 soil samples, which were collected between 2009 and 2014.
The following key observations were made: Soil nC31-alkane δ2H values (δ2Hwax) in the Sutlej, Alaknanda, Khudi, and Arun generally record surface water δ2H values, confirming that the first-order control on the plant wax isotopic signature is precipitation δ2H and, therefore, the elevation in orogenic settings. We identified aridity as the factor that introduces scatter to this relationship. BrGDGT-derived Mean Annual Temperature (MAT) correlates in a statistically significant manner with sample site elevation and a 14-year annual average of remotely sensed land-surface temperature, showing that the main process influencing the brGDGT distribution is the adiabatic cooling of air.
In an effort to combine these proxies to improve uncertainties in elevation reconstruction, elevations were inferred from both the δ2Hwax and brGDGT distributions. Arid, high elevation sites appear to underestimate actual sample site elevations using δ2H values while sites subject to high (>23–25 °C) annual temperatures overestimate the actual sample site elevation using brGDGT distributions. Elevations inferred from both proxies under such paleoclimatic conditions should be interpreted with caution. Elevations derived from the brGDGT distribution appear to most accurately reconstruct elevation. However, we show that the difference in elevation between the two proxies, described by the proposed ΔElevation parameter, can provide information on the hydrological conditions of the soil’s depositional environment. In conclusion, we emphasize that knowledge of the sample site’s climatic conditions are essential to reconstruct elevation from paleoarchives. In particular, knowledge of moisture availability and annual air temperatures are important, as these have been found to cause the largest scatter in the observed data.
Mots-clés : Cycla
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