Search

International audience; The Indian monsoon and tectonic both control the Himalayan erosion. Tectonic generates uplift and maintain high elevation of the range and earthquakes weaken the mountain front. Monsoon seasonal intensity controls 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 is also tightly controlled by climate. The seasonality and intensity of the monsoon generate discharge high enough to ensure the efficiency of river transport of sand-rich sediments to the delta and shelf (Lupker et al. JGR, 2011). IODP Expedition 354 allowed to drill a series of seven sites along an E-W transect at 8 • N in the Bengal fan. This generated a comprehensive record of Himalayan erosion since 20 Ma. Bengal fan sediments are dominated by turbidites characterised by high accumulation rate and variable grain size raging from clayey silt to sand. Plio-Pleistocene turbidites reveal frequent channel levee deposition with massive sand deposition in inter-levee intervals. Overall, sand represents at least a third of the fan deposition. Turbidites are intercalated with hemipelagic sediments characterised by low accumulation rates, 20 to 50% of biogenic carbonates, fine grains and bioturbation. Turbidites have petrographic and geochemical signatures characteristic of sediments derived from the Himalaya such as the Ganga and Brahmaputra rivers that supply the fan (see A. Galy et al. this session). 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 river and northern fan sediments. This suggests that weathering and/or soil erosion are amplified in the modern time. Detrital carbonate concentration vary significantly with higher concentrations during the Miocene then falling during the Plio-Pleistocene. This suggests either a change in the exposition of Tethyan series in Himalaya or significantly lower chemical weathering during Miocene. Finally, clay composition is dominated by illite which also differs with the present Ganga which is rich in smectite. The record at 8 • N contrasts with the ODP Leg 116 record at 1 • S in the distal fan, which from 7 to 1 Ma shows a high degree of weathering associated to lower accumulation rates. The record at 8 • N indicates that these variations do not reflect changes in the continental source basin but rather reflect a change in the turbiditic export. The Bengal Fan record at 8 • N suggests that an erosion regime dominated by physical erosion able to drive large amount of sand and silt with low weathering was established at least 20 Ma ago and remained relatively stable in spite of the global climate variations. These observations suggest that erosion sustained by a highly seasonal climate able to ensure rapid transport of sediments such as the monsoon was already active 20 Ma ago.

International audience; En Himalaya, la mousson et la tectonique de convergence Inde-Asie contrôlent directement l'érosion. La tectonique engendre la haute élévation de la chaîne et les tremblements de terre qui fragilisent l'ensemble. L'intensité de la mousson contrôle la couverture glaciaire, les glissements de terrain, l'incision, la couverture végétale et l'export des sédiments vers la plaine. La mousson contrôle également l'intensité de l'érosion chimique. Dans la plaine, la saisonnalité et l'intensité de la mousson permettent d'atteindre des débits suffisamment élevés pour assurer l'efficacité du transport sédimentaire vers le delta. Ce transport rapide est également un facteur limitant de l'altération en réduisant le temps de séjour dans la plaine. Ces sédiments sont ensuite exportés vers la baie du Bengale ou ils alimentent le plus important système turbiditique de la planète. L'Expédition IODP 354 a foré le Cône du Bengale en 2015 et a restitué un enregistrement quasi continu de l'érosion de l'Himalaya au cours du Néogène et du Quaternaire (France-Lanord et al. 2016). Les sédiments sont principalement composés de turbidites issues du bassin du Gange-Brahmapoutre. L'Expedition 354 a foré un transect E-W de sept Sites sur 320 km à 8° N afin de couvrir autant que possible les déplacements de dépocentre. Le site le plus profond U1451, atteint la base du cône datant un début de sédimentation turbiditique vers 20 Ma sur le flanc de la ride Ninetyeast. Les sédiments présentent des analogies minéralogiques, géochimiques et isotopiques avec les sédiments modernes du Gange-Brahmapoutre. La variabilité interne révèle que les turbidites ne sont pas en permanence un mélange des deux fleuves. Au Quaternaire, il est évident que le Brahmapoutre était parfois la seule source de turbidites. La géochimie des éléments majeurs et des éléments traces montre des compositions relativement stables tout au long du Néogène et du Quaternaire. Ils révèlent un très faible régime d'altération sans variation significative dans le temps. Ces données comparées à celles des sédiments modernes du Gange et du Brahmapoutre impliquent que l'altération ou l'érosion des sols est amplifiée par la pression anthropique. Au plan paléoenvironnemental, les compositions isotopiques d'hydrogène et de carbone des cires foliaires datent l'expansion connue des plantes de type savane (C4) dans le bassin vers 6 Ma et montrent que depuis le Miocène les paléoprécipitations sont fortement appauvries en deutérium. Ce point implique que dès le Miocène, le régime de mousson indienne est établi, le Pléistocène étant marqué par une augmentation de la variabilité. Les paléoconcentrations en 10 Be du quartz ont pu être appliqués pour la première fois sur des sédiments marins détritiques pour déduire les vitesses d'érosion. Elles démontrent une stabilité depuis le Miocène supérieur qui implique que la mise en place de l'alternance glaciaire interglaciaire a eu un impact limité sur l'intensité de l'érosion en Himalaya.

International audience; In Himalaya, the monsoon precipitations and tectonic exert a primary control on erosion. Monsoon intensity acts on 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 limited by transport efficiency. IODP Expedition 354 drilled the Bengal fan (France-Lanord et al. 2016) generated a record of Himalayan erosion over the Neogene and Quaternary. Most of the sediments composing the fan are turbidites issued from the Ganga-Brahmaputra delta and northern Bengal shelf. Turbiditic deposition being discontinuous, the Expedition 354 drilled an E-W transect of seven sites in order to capture as much as possible the displacement of the depocenter. Turbiditic sediments show mineralogical, geochemical and isotopic characteristics which reveal a close analogy with the modern Ganga-Brahmaputra sediments. Internal variability reveals that turbidites are not permanently a mixing of both rivers. There are evidences during the Quaternary that Brahmaputra was for some periods the only source of the turbidites. 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 suggesting that weathering or soil erosion is amplified in the modern time. The record of detrital carbonate deposition shows variations with higher concentrations during the Miocene then falling concentrations during the Plio-Pleistocene. This suggests either a change in the exposition of Tethyan series in Himalaya or significantly lower chemical weathering during Miocene. Overall, the record of erosion at 8°N reveals a relatively stable erosion regime dominated by physical erosion. These observations combined with the reconstitution of paleo-precipitation δD (V. Galy et al., this session) suggest that since 20 Ma the Himalaya was exposed to a monsoonal climate i.e. a highly seasonal climate able to ensure high physical erosion.

International audience; In Himalaya, the monsoon precipitations and tectonic exert a primary control on erosion. Tectonic is generating uplift and maintain high elevation of the range and earthquakes fragilise the mountain front. Monsoon intensity controls the extension of the glacier cover, landslide activity, river incision, vegetation cover, and exports 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. IODP Expedition 354 drilled the Bengal fan (France-Lanord et al. 2016) and allows to assemble a comprehensive record of Himalayan erosion over the Neogene and Quaternary. Sediments are predominantly composed of turbidites generated from the Ganga-Brahmaputra shelf. Turbiditic deposition being discontinuous, the Expedition 354 drilled an E-W transect in order to cover as much as possible the displacement of the depocenter. Seven sites were drilled 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. The deepest Site U1451 on the Eastern end of the transect reach the base of the fan. This marks the onset of fan deposition on this flank of the Ninetyeast ridge around 20 Ma. Turbiditic sediments show mineralogical, geochemical and isotopic characteristics which reveal a close analogy with modern Ganga-Brahmaputra sediments. Internal variability reveals that turbidites are not permanently a mixing of both rivers. Rather, there are evidences during the Quaternary that Brahmaputra was for some periods the only source of the turbidites. 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. The record of detrital carbonate deposition shows variations with higher concentrations during the Miocene then falling concentrations during the Plio-Pleistocene. This suggests either a change in the exposition of Tethyan series in Himalaya or significantly lower chemical weathering during Miocene. This record contrasts with the ODP Leg 116 record at 1°S in the distal fan, which from 7 to 1 Ma shows an apparent high degree of weathering associated to lower accumulation rates. The record at 8°N indicates that these variations do not reflect changes in the continental source basin but rather reflect a change in the turbiditic export. Overall, the record of erosion at 8°N reveals that a relatively stable erosion regime dominated by physical erosion was established since Miocene. The modern period appears one with higher intensity of chemical erosion. These observations suggest that erosion sustained by a highly seasonal climate able to ensure rapid transport of sediments such as the monsoon was already active 20 Ma ago.

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; Stable isotopic studies (H and O) have been continuously performing from the major large rivers like Kosi and Narayani as well as their small tributaries to understand the water cycle from yearly isotopic record in relation with direct precipitation. Here we present a set of data, mostly from the southern Himalayan flank, including daily sampling of rivers in Narayanghat, Khudi, Marshyandi, Trisuli and Kosi as well as precipitation water data since the pre-Gorkha Earthquake (Mw 7.8) of 25 April 2015. For the precipitation at a given location, data are marked by strong seasonal contrast (e.g. Zhang et al. 2001, Gajurel et al. 2006) probably related to source of moisture and amount effect.Strong seasonality in oxygen isotopic composition of river water (4-5‰) in the catchment of Narayani River in the Himalaya in 2010 is recorded. In contrast, the composition evolves much more stable with a slight tendency to increase during the monsoon in 2015. The Kosi data set of 2015 shows a variation in δ18Osmow from -12‰ to -8‰, and for the δDSMOW from -80‰ to -50‰ (Fig. 1). The data set situated on line δD = 7.7 × δ18O + 7.7‰) is close to the GMWL. The seasonal trend in δ18O generates an offset particularly between June-July and August that influence on river isotopic composition. The river isotopic compositions are progressively depleted in heavy isotopes during the monsoon reflecting the seasonal evolution of precipitation, however, it is buffered by the ground water reservoir. At the beginning and end of the monsoon, the contrast between precipitation and ground water is large and precipitation events generate scattered river variations depending on direct runoff to ground water supply, however, such trend is not always observed particularly after 2015 Gorkha earthquake.

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.

International audience; Since the Oligocene, the Himalayan erosion drives one of the most important detrital flux to the oceans. More than 500 Mt of sediments are transferred annually from the Bangladesh delta to the shelf and submarine canyon, the Swatch of No Ground. From the canyon, turbidity current drive sediments along channel levees and disperse sediment over 3000 km south of the delta. On land, the Ganga river and its main tributaries export particulate carbonate resulting from the physical erosion of limestone, marls and marble present mainly in the Tethys Himalaya and lesser Himalaya. In spite of high runoff, these rivers reach carbonate saturation and export particulate carbonate to the ocean. The modern river system exports ≈ 3 wt% of calcite and dolomite, and comparable concentrations are observed in Holocene sediments from the Northern Bay of Bengal. Detrital carbonates are easily sorted from marine carbonates thanks to their low  18 O and high 87 Sr/ 86 Sr acquired during metamorphic stage. Their proportion increase in LGM sediments, likely reflecting weaker conditions of weathering during this period [1-2]. Recent IODP Exp. 354 on the Bengal Fan documented Himalayan derived turbiditic sedimentation at 8°N/≈3600 mbsl over the whole Neogene. There, large variations of detrital carbonate proportion are observed with maxima around 10 wt% reached during the upper Miocene. Analyses of the carbonate load also reveal the minor presence of biogenic carbonates, and the significant presence of diagenetic carbonates. They are characterized by high  18 O around-2‰ PDB, low  13 C ≈-8‰ and 87 Sr/ 86 Sr near seawater composition. Their occurrence and proportion are closely associated with the fine clay fraction of the sediments. The average concentration of this diagenetic component is 1.2 wt% and it can reach 4 wt% in clay-rich samples. Pore-water compositions suggest that they were not produced in situ. Rather they may be produced up-fan and incorporated in the tubiditic mixing. 1 Lupker, M. et al. 2013

International audience; In Himalaya, the monsoon precipitations exert a primary control on the erosion. From its intensity depends on the extension of the glacier cover, landslide activity, river incision and export of sediments towards the floodplain and oceanic basins. 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 of the monsoon allows to reach high flooding conditions generating efficient 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 confirm that Himalayan weathering is limited by transport. When looking at sedimentary record of erosion in the proximal Bengal fan, weathering as traced by K/Al or OH-/Al however appears lower during the LGM in spite of reduced discharge i.e. slower transport (Lupker et al. 2013). Reduced weathering intensity is, however, consistent with lower precipitation and temperature in the basin. Recent IODP Expedition 354 in the Bengal fan generated a comprehensive record of Himalayan erosion over the Neogene and Quaternary. Turbiditic sediments have clear Himalayan origin and close mineralogical and isotopic analogy with those of the modern Ganga-Brahmaputra river sediments. This long-term record also reveals that the chemical compositions of turbiditic sediments cored across the transect are relatively stable throughout the Neogene and also during Quaternary. Over the last 25 Ma it appears that weathering was weak, and lower than during modern conditions. This long-term record suggests that the Himalayan erosion has been controlled by rapid physical erosion, with transport processes efficient enough to prevent weathering of the sediment load. Lupker (2011) JGR 116(F4), F04012. Lupker (2013) EPSL 365: 243-252.

International audience

International audience

International audience

International audience

International audience

Based on the concept that sedimentary processes average large areas of exposed crust, sediment data have been widely used to estimate the average Pb isotopic composition of the upper continental crust. However, the possible effects of mineral sorting processes on sediment Pb isotopes have never been fully investigated. Here, we report Pb isotopic compositions of Himalayan river sediments as well as those of several grain-size fractions and mineral separates. We demonstrate that Pb isotopes of both bed loads and suspended loads are biased toward more radiogenic values than their source rocks due to a "heavy mineral effect" caused by mineral sorting during fluvial transport on continents. The sparse zircons, monazites and allanites present in all samples (

International audience; IODP Expedition 354 in the Bengal fan [1] generated a comprehensive record of Himalayan erosion over the Neogene and Quaternary. It documents the interplay between Himalayan tectonic and the monsoon. The Bengal fan is predominantly composed of detrital sediments originating from Himalayan rivers, and transported to the N. Bengal shelf. They supply turbidity currents able to spread sediments overs 3000 km. Turbiditic record at a given site being discontinuous, 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. In spite of the transect extension, a long absence of deposition is observed between 0.6 to 1.2 Ma indicating that turbiditic depocenter was derived more to the West for ca. 600 kyr. Bengal fan sediments have close mineralogical, geochemical and isotopic analogy with sediments of the modern Ganga-Brahmaputra rivers. Altogether they reveal mineralogical sorting associated to turbiditic transports that compares well with that of rivers in the floodplain and delta. Major and trace element geochemistry and clay compositions reveal relatively even and weak conditions of weathering throughout the 20-Ma record. Most variations are controlled by the origin of eroded sources ie. the proportions of sediments derived from Himalaya and Transhimalaya. Source tracers such as Sr-Nd isotopic compositions, and detrital carbonate compositions show organised variations with time. They also imply for some periods compositions with very high proportion of sediment derived from the Transhimalaya suggesting that the Ganga and Brahmaputra sediment delivery to the fan can be decoupled at some periods. Long term source variations imply that exposure to erosion of the different Himalayan formations has evolved as a result of the evolution of the thrusting structures. Data suggest that (1) a component derived from Transhimalayan formation was present even during Lower Miocene, (2) the Tethys Himalaya exposure to erosion was higher during Miocene than during Pliocene and Pleistocene, and (3) that the exhumation of the Lesser Himalaya was initiated around 8 Ma.

International audience; Global records of erosion have led to conjecture that the Quaternary climatic transition was linked to an acceleration of erosion on Earth (Zhang et al. 2001, Herman et al. 2013). However, the reality of this increase of the erosion rates remains debated (e.g. Willenbring and von Blanckenburg 2010). In order to implement the debate at the scale of a major orogen, we have applied to the Himalaya a distinct approach, which is not affected by the same possible bias than for previously developed methods. We present a new record of Himalayan erosion from IODP drilled cores in the Bengal fan. The fan mainly consists of sand-rich turbidites that originate from sediments eroded in the Himalaya and transported by the Ganga and the Brahmaputra since at least 20 Ma onwards. IODP Expeditions 353 and 354 (Clemens et al. 2016, France-Lanord et al. 2016) recently drilled 1 site at 14°N and a transect of seven sites at 8°N in the fan. These cores were dated by magnetostratigraphy and biostratigraphy. The provenance of Plio-Pleistocene sediments was traced using 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios from bulk silicates. Contrasted isotopic signatures characterize the Himalayan formations and the Transhimalaya, which results in distinct compositions for modern Ganga and Brahmaputra river sediments. Sr and Nd isotopic ratios range from 0.72 to 0.76 and from-17 to-13. They imply significant variations in the provenance mixture between Himalayan and Transhimalayan formations since 6 Ma. In particular, we notice a higher proportion of Transhimalayan provenance for the last 0.2 Ma. Past erosion rates (mean values over timescales of ~1 kyr) were traced using in-situ cosmogenic 10 Be paleo-concentrations in the quartz of the sandy 75-250 μm fraction (e.g. Charreau et al. 2011). 10 Be paleo-concentrations vary in a range similar to the concentrations measured on the Brahmaputra and Ganga modern sands (Lupker et al. 2012; 2017). Although paleo-concentrations appear rather variable before 2 Ma and steadier afterwards, the time-averaged signal is relatively constant over the whole Plio-Pleistocene period and highlights a steadiness of the Himalayan erosion rates. Even if dominant sources of sediments seem to fluctuate in the Brahmaputra-Ganga watershed, our dataset suggests that the Himalayan erosion may not have been sensitive to climate cooling at the Quaternary transition, and that its erosion patterns may be mainly controlled by regional tectonics. References Charreau J, Blard PH, Puchol N, Avouac JP, Lallier-Vergès E, Bourlès D, Braucher R, Gallaud A, Finkel R, Jolivet M, Chen Y, Roy P (2011) Paleo-erosion rates in Central Asia since 9 Ma: A transient increase at the onset of Quaternary glaciations? Earth Planet Sci Lett 304: 85-92. (2013) Worldwide acceleration of mountain erosion under a cooling climate. Nature 504: 423-426. Lupker M, Blard PH, Lavé J, France-Lanord C, Leanni L, Puchol N, Charreau J, Bourlès D (2012) 10Be-derived Himalayan denudation rates and sediment budgets in the Ganga basin. Earth Planet Sci Lett 333-334: 146-156.

International audience; Recent IODP Expedition 354 in the Bengal fan [1] generated a comprehensive record of Himalayan erosion over the Neogene and Quaternary. It documents the interplay between Himalayan tectonic and the monsoon. 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. In spite of the transect extension, a long absence of deposition is observed between 0.6 to 1.2 Ma indicating that turbiditic depocenter was derived more to the West for ca. 600 kyr. Turbiditic sediments have close mineralogical and isotopic analogy with sediments of the modern Ganga-Brahmaputra rivers. Major and trace element geochemistry show stable compositions throughout the Neogene and Quaternary with most variations controlled by the origin of eroded sources ie. the proportions of sediments derived from Himalaya and Transhimalaya. Although relatively stable, source tracers such as Sr-Nd isotopic compositions, and detrital carbonate compositions show organised variations with time. They imply that exposure to erosion of the different Himalayan formations has evolved as a result of the evolution of the thrusting structures. Data suggest that (1) a component derived from Tranhimalayan formation was present even during Lower Miocene, (2) the Tethys Himalaya exposure to erosion was higher during Miocene than during Pliocene and Pleistocene, and (3) that the exhumation of the Lesser Himalaya was initiated around 8 Ma. Coring of large sand samples also allowed to initiate cosmogenic 10 Be tracing of erosion rates at the scale of the Ganga-Brahmaputra basin. Relatively stable paleo-concentrations reveal relatively steady erosion rates in spite of the Pleistocene climatic changes. Clays and major and trace element geochemistry 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 suggesting that weathering or soil erosion is amplified in the modern time. Compositions are indeed controlled by source and weathering processes, but mineral sorting during turbiditic transport may also alter the geochemical signature of the sediments. Close attention has been given to evaluate the possible loss of clay size particles by large scale dispersion compared to silt and sand. It seems, however, that such processes cannot account for the differences observed with modern rive sediments. Low weathering of the sediments at 8°N indicates that erosion was dominated by physical processes and that transport is 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. Overall, the low weathering intensity suggests that comparable erosion regime prevailed since at least the Early Miocene.

International audience; Mineralogical and H, O, Sr, and Nd isotope compositions have been analyzed on a set of representative samples from the 17-m.y. section in ODP Leg 116 Holes 717C and 718C. Based on the mineralogical composition of the fraction 63 µm, the whole section can be subdivided into three major periods of sedimentation. Between 17.1 and 6 m.y., and between 0.8 m.y. to present, the sediments are characterized by sandy and silty turbiditic inputs with a high proportion of minerals derived from a gneissic source without alteration. In the fraction 63 µm contains quartz, muscovite, biotite, chlorite, and feldspars. The 6-to 0.8-m.y. period is represented by an alternation of sandy/silty horizons, muds, and calcareous muds rich in smectite, and kaolinite (50% to 85% of the fraction