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International audience; To better constrain late Neogene denudation of the Himalayas, we analysed in situ 10Be concentrations in 17 Neogene sediment samples of the Surai section (central Nepal) and two modern sediment samples of the Rapti River. We first refined the depositional ages of the Surai section from 36 new paleomagnetic analyses, five 26Al/10Be burial ages, and, based on the Dynamic Time Warping algorithm, 104 automatically calculated likely magnetostratigraphic correlations. We also traced changing sediment sources using major element and Sr‐Nd isotopic data, finding at 4–3 Ma a switch from a large, trans‐Himalayan river to a river draining only the Lesser Himalaya and Siwalik piedmont, increasing the contribution of recycled sediments at that time. 10Be concentrations in Neogene sediments range from (1.00 ± 0.36) to (5.22 ± 0.98) × 103 at g–1 and decrease with stratigraphic age. Based on a flood plain transport model, our refined age model, and assuming a drainage change at 4–3 Ma, we reconstructed 10Be concentrations at the time of deposition. Assuming cosmogenic production rates similar to those of the modern basins, we calculated palaeodenudation rates of 0.9 ± 0.5 to 3.9 ± 2.7 mm a–1 from ca. 6 to 3 Ma in the palaeo‐Karnali basin and 0.6 ± 0.2 to 1.6 ± 0.8 mm a–1 since ca. 3 Ma in the palaeo‐Rapti basin. Given the uncertainties and similar modern values of ~2 mm a–1, the palaeo‐Karnali denudation rates may have been steady at ~1.7 ± 0.3 mm a–1 for the last ca. 6 Ma. A transient acceleration of the denudation in the palaeo‐Rapti basin of ~1.5 mm a–1 since ca. 1.5 Ma was likely due to the reworking of older, 10Be‐depleted Siwalik sediments in the foreland. If true, this steadiness of the denudation rates may suggest that Quaternary glaciations did not largely affect Himalayan denudation.

International audience

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.

International audience; Climate variability and glacial extension may have led to a global increase of erosion rates at the Quaternary transition (Zhang et al. 2001). This potential increase may have impacted mountain building and also climate through feedback effects. However, the existence of strengthened erosion during the Quaternary is debated, particularly for the Himalaya. While the high relief of the range drives monsoonal precipitations that produce a major erosional flux on Earth (Milliman and Syvitski 1992), the existing records yield contrasting results. Addressing the question of Quaternary accelerated erosion both requires the application of new approaches and investigation of new archives. Here, we present a new Siwalik record. The sections are located in the central Himalaya, South of Chitwan Dun, where local rivers have entrenched the Himalayan sediments exhumed by the uplifting Siwalik fold. The sections expose sediments of a paleo-fan that we assume to have been fed by the Narayani-Gandak river, one of the major Transhimalayan rivers. The studied sections present a cumulated thickness of ~4000 m and consist in almost continuous sandy to fine fractions of sedimentary rocks. The depositional ages were constrained by a magnetostratigraphy analysis and range from ca. 8 Ma to < 0.8 Ma. To determine the paleo-erosion, we quantified the in-situ 10 Be cosmogenic nuclide concentrations in the 125-250 μm quartz fraction of the sedimentary rocks (e.g. Charreau et al. 2011). By measuring 36 Cl in feldspar, we checked that the contribution of recent exposure to 10 Be concentration was minor. We computed paleo-concentrations, which range from 5000 to 40'000 at/g and average 10'000 at/g. We then analysed the provenance stability and potential recycling with Sr-Nd isotopes and major elements respectively. Major elements show a transition at ca. 1 Ma, with SiO 2 increasing from 75 to > 85% and Na/Al decreasing from 20% to < 10%. Na depletion in young sediments reflects an increase of weathering and may be indicative of a transition from direct deposition of sediments from the Himalaya to recycling of proximal sources, i.e. from Siwalik fold denudation. The εNd and 87 Sr/ 86 Sr isotopic signatures of the sediments range from-19.5 and-17.5, and from 0.75 to 0.78 respectively. These signatures are similar to modern Narayani-Gandak sands (-19 and 0.75 on average respectively). They are interpreted as resulting from a rather steady mix of Himalayan formations, with 50 to 90% High Himalayan contributions. Based on these results we assumed the cosmogenic production rates during the past were similar to the present value across the Narayani watershed at least until 1 Ma. The derived paleo-erosion rates of the Narayani watershed range from 0.5 to 4 mm/yr, around an average of 1.8 mm/yr. This corresponds to an average time scale of 1 kyr. The average paleo-erosion rate is similar to modern values (Lupker et al. 2012). However, the paleo-concentrations and the paleo-erosion rates display significant dispersion, with large (+100%) fluctuations of at high frequency (< 0.3 Ma). This dispersion could highlight the naturally stochastic character of erosion even for a large watershed in a range apparently dominated by tectonic processes. References

International audience; In Himalaya, the monsoon precipitations exert a primary control on erosion. From its intensity depends the extension of the glacier cover, landslide activity, river incision, vegetation cover, and export of sediments towards the floodplain. The monsoon also exerts a primary control on the chemical erosion as weathering rates are clearly dependent upon river discharge. In the floodplain, sediment export also is tightly controlled by climate. The seasonality and intensity of the monsoon allow to reach discharge high enough to ensure the efficiency of river transport to the delta (Lupker et al. 2011). This efficient transport also acts as a limiting factor for weathering as it reduces residence time in the floodplain. The comparison between the Ganga with a large floodplain and the Brahmaputra with a narrow floodplain and comparatively lower residence time confirms that Himalayan weathering is limited by transport. Recent IODP Expedition 354 in the Bengal fan (France-Lanord et al. 2016) generated a comprehensive record of Himalayan erosion over the Neogene and Quaternary. It documents the interplay between Himalayan tectonic and climate. The Bengal fan is predominantly composed of detrital turbiditic sediments originating from Himalayan rivers, and transported through the delta and shelf canyon, supplying turbidity currents loaded with a wide spectrum of grain sizes. Turbiditic deposition makes that record at a given site is discontinuous which was the reason for an E-W transect approach. Expedition 354 drilled seven sites along a 320 km E-W transect at 8°N allowing the restitution of an almost complete record of Himalayan erosion at the scale of the Neogene. Turbiditic sediments have clear Himalayan origin and close mineralogical and isotopic analogy with those of the modern Ganga-Brahmaputra river sediments. Major and trace element geochemistry show relatively stable compositions throughout the Neogene and Quaternary. They reveal a very weak regime of chemical weathering with no significant variation through time. Concentrations in mobile elements such as Na and K relative to Al are significantly higher than in modern sediments and suggest that weathering or soil erosion is amplified in the modern time. Low weathering of the sediments at 8°N indicates that erosion was dominated by physical processes and that transport was rapid enough to prevent evolution of particles in the floodplain. In the modern Himalaya, low weathering is achieved primarily by landslides and rapid transfer through the floodplain, i.e. limited recycling of sediment deposited in the floodplain. Both processes are favoured by the seasonality and the intensity of the monsoon. On the organic side, carbon isotopic analyses of organic matter in turbidites clearly reveal its continental origin. The hydrogen isotopic composition of leaf-wax compounds reveals low δD since Early Miocene, indicative of significant amount effect i.e. Monsoon. Overall, the low weathering intensity suggests that comparable erosion regime was established since at least Early Miocene. References France-Lanord C, Spiess V, Klaus A, Schwenk T, Expedition 354 Scientists (Eds.) (2016) Proceedings of the International Ocean Discovery Program, 354. IODP.

La mesure quantitative des températures passées a été initiée sur les bases de la physique atomique dès 1947 par Harold C. Urey (Nobel de chimie pour la découverte du deutérium*). Elle repose sur l’enregistrement de la température de formation d’une phase constitutive d’archives naturelles (sédiments*, biominéraux, glace) par une composition isotopique ou chimique. Cette quantification est obtenue à partir de la mesure très précise d’une grandeur physico-chimique ou isotopique qui varie de ma...

Qu'est ce que l'effet de serre ? Le rôle de l'homme sur le climat est-il détectable et comment ? Comment mesure-t-on la fonte de la banquise, le recul des glaciers de montagne ou bien encore l'élévation du niveau de la mer ? Comment les chercheurs font-ils pour modéliser un système aussi complexe que la planète terre ? Quelles données permettent de décrire et modéliser les climats passés ? Comment s'y prend-on pour prévoir l'évolution à venir du climat ? À l'écart de la polémique médiatique, Catherine Jeandel et Rémy Mosseri ont mobilisé plus d'une centaine de contributeurs qui livrent ici un panorama large des méthodes et outils mis en oeuvre pour étudier notre climat et son avenir. Ils montrent que, pour résoudre cette question extraordinairement complexe, une approche pluridisciplinaire est plus que jamais nécessaire, à la croisée de l'expérimentation, de l'observation, de la simulation et de la théorie. Un livre majeur.

The Tsangpo-Brahmaputra River drains the eastern part of the Himalayan range and flows from the Tibetan Plateau through the eastern Himalayan syntaxis downstream to the Indo-Gangetic floodplain and the Bay of Bengal. As such, it is a unique natural laboratory to study how denudation and sediment production processes are transferred to river detrital signals. In this study, we present a new 10Be data set to constrain denudation rates across the catchment and to quantify the impact of rapid erosion within the syntaxis region on cosmogenic nuclide budgets and signals. The measured 10Be denudation rates span around 2 orders of magnitude across individual catchments (ranging from 0.03 to > 4 mm yr−1) and sharply increase as the Tsangpo-Brahmaputra flows across the eastern Himalaya. The increase in denudation rates, however, occurs ∼ 150 km downstream of the Namche Barwa–Gyala Peri massif (NBGPm), an area which has been previously characterized by extremely high erosion and exhumation rates. We suggest that this downstream lag is mainly due to the physical abrasion of coarse-grained, low 10Be concentration, landslide material produced within the syntaxis that dilutes the upstream high-concentration 10Be flux from the Tibetan Plateau only after abrasion has transferred sediment to the studied sand fraction. A simple abrasion model produces typical lag distances of 50 to 150 km compatible with our observations. Abrasion effects reduce the spatial resolution over which denudation can be constrained in the eastern Himalayan syntaxis. In addition, we also highlight that denudation rate estimates are dependent on the sediment connectivity, storage, and quartz content of the upstream Tibetan Plateau part of the catchment, which tends to lead to an overestimation of downstream denudation rates. While no direct 10Be denudation measurements were made in the syntaxis, the dilution of the upstream 10Be signal, measured in Tsangpo-Brahmaputra sediments, provides constraints on the denudation rates in that region. These denudation estimates range from ca. 2 to 5 mm yr−1 for the entire syntaxis and ca. 4 to 28 mm yr−1 for the NBGPm, which is significantly higher than other large catchments. Overall, 10Be concentrations measured at the outlet of the Tsangpo-Brahmaputra in Bangladesh suggest a sediment flux between 780 and 1430 Mt yr−1 equivalent to a denudation rate between 0.7 and 1.2 mm yr−1 for the entire catchment., Earth Surface Dynamics, 5 (3), ISSN:2196-632X, ISSN:2196-6311

International audience

International audience