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Despite numerous studies on Himalayan erosion, it is not known how the very high Himalayan peaks erode. Although valley floors are efficiently eroded by glaciers, the intensity of periglacial processes, which erode the headwalls extending from glacial cirques to crest lines, seems to decrease sharply with altitude1,2. This contrast suggests that erosion is muted and much lower than regional rock uplift rates for the highest Himalayan peaks, raising questions about their long-term evolution3,4. Here we report geological evidence for a giant rockslide that occurred around 1190 adin the Annapurna massif (central Nepal), involving a total rock volume of about 23 km3. This event collapsed a palaeo-summit, probably culminating above 8,000 m in altitude. Our data suggest that a mode of high-altitude erosion could be mega-rockslides, leading to the sudden reduction of ridge-crest elevation by several hundred metres and ultimately preventing the disproportionate growth of the Himalayan peaks. This erosion mode, associated with steep slopes and high relief, arises from a greater mechanical strength of the peak substratum, probably because of the presence of permafrost at high altitude. Giant rockslides also have implications for landscape evolution and natural hazards: the massive supply of finely crushed sediments can fill valleys more than 150 km farther downstream and overwhelm the sediment load in Himalayan rivers for a century or more.

The erosion and weathering of mountain ranges exert a key control on the long-term (105–106yr) cycling of carbon between Earth’s surface and crust. The net carbon budget of a mountain range reflects the co-existence of multiple carbon sources and sinks, with corresponding fluxes remaining difficult to quantify. Uncertain responses of these carbon fluxes due to the stochastic nature of erosional processes further complicate the extrapolation of short-term observations to longer, climatically relevant timescales. Here, we quantify the evolution of the organic and inorganic carbon fluxes in response to the 2015 Gorkha earthquake (Mw7.8) in the central Himalaya. We find that the Himalayan erosion acts as a net carbon sink due mainly to efficient biospheric organic carbon export. Our high-resolution time series encompassing four monsoon seasons before and after the Gorkha earthquake reveal that coseismic landslides did not significantly perturb large-scale Himalayan sediment and carbon fluxes. This muted response of the central Himalaya to a geologically frequent perturbation such as the Gorkha earthquake further suggests that our estimates are representative of at least interglacial timescales.

The Oligocene-Miocene boundary Asian climatic reorganization linked to the northward migration of the East Asian monsoon into subtropical China is a potentially important but poorly constrained atmospheric CO2consumption process. Here, we performed a first-order estimate of the CO2consumption induced by silicate chemical weathering and organic carbon burial in subtropical China related to this climatic reorganization. Our results show that an increase in long-term CO2consumption by silicate weathering varies from 0.06×1012to 0.87×1012mol yr−1depending on erosion flux reconstructions, with an ~50% contribution of Mg-silicate weathering since the late Oligocene. The organic carbon burial flux is approximately 25% of the contemporary CO2consumption by silicate weathering. The results highlight the significant role of weathering of the Mg-rich upper continental crust in East China, which would contribute to the rapid decline in atmospheric CO2during the late Oligocene and the Neogene rise in the seawater Mg content. If this climatic reorganization was mainly induced by the Tibetan Plateau uplift, our study suggests that the growth of the Himalayan-Tibetan Plateau can lead to indirect modification of the global carbon and magnesium cycles by changing the regional hydrological cycle in areas of East Asia that are tectonically less active.

Sediment accumulation rates and thermal trackers suggest a substantial and global increase in erosion rates over the past few million years. That increase is commonly associated with the impact of the Northern Hemisphere glaciation, but methodological biases have led researchers to debate this hypothesis. Here, we test whether Himalayan erosion rates increased by measuring beryllium-10 (10Be) in the sediment of the Bengal Bay seabed. Sediment originated from rocks that produced 10Be under the impact of cosmic rays during erosion near surface. Thus, the 10Be concentrations indicate erosion rates. The 10Be concentration of the Bengal Bay sediment depends on the contributions of the Ganga and Brahmaputra rivers. Their sediments have distinct 10Be concentrations because of distinct elevations and erosion in their drainage basins. Variable contributions could thus complicate erosion-rate calculation. We traced these contributions by a provenance study using the strontium (Sr) and neodymium (Nd) isotopic sediment compositions. Within uncertainties of ±30%, our reconstructed past erosion rates show no long-term increase for the past six million years. This stability suggests that climatic changes during the late Cenozoic have an undetectable impact on the erosion patterns in the Himalayas, at least on the ten thousand to million year timescales accounted for by our dataset.

Identifying the roles of erosional processes in the denudation of mountain ranges requires a better understanding of erosional sensitivity to climatic, topographic, or lithologic controls. We analyzed erosion in the Narayani River basin (draining central Nepal and presenting contrasted lithologic and geochemical signatures in its outcropping rocks and a wide variety of erosional processes and climatic conditions) to assess the relative contributions of erosion processes to the annual sediment export. By combining acoustic Doppler current profiler measurements with depth profiles and daily surface samplings of the suspended load, we propose a simplified model to precisely calculate sediment fluxes at the basin outlet. We estimate an equivalent erosion rate of 1.8−0.2+0.35mm/year for the year 2010, similar to the average value of 1.6−0.2+0.35mm/year estimated from 15 years of records and long‐term (~ky) denudation rates of 1.7 mm/year derived from cosmogenic nuclides. The stability of erosion is attributed to efficient buffering behavior and spatial integration in the drainage system. Strong relations between rainfall events and the sediment export suggest that the system is mainly supply limited. Combining physical calculation of sediment fluxes with grain size analyses and geochemical tracers (hydroxyl isotopic compositions, carbonate contents, and total organic carbon content), we estimate that glacial and soil erosion do not contribute more than 10% and a few percentage, respectively, to the total budget and are only detectable during premonsoon and early monsoon periods. During the monsoon, erosion by landslides and mass wasting events overwhelms the sediment budget, confirming the dominant role of these erosional processes in active mountain chains. Erosion in Central Nepal is supply limited and driven by monsoonal precipitation, which controls triggering of landslidesGlacial and soil erosion are minor in the annual sediment budget compared to landslidesCurrent erosion rates agree with long‐term erosion rates, suggesting efficient buffering of these drainage systems

Gaseous carbon dioxide (CO2) and radon‐222 release from the ground was investigated along the Main Central Thrust zone in the Nepal Himalayas. From 2200 CO2and 900 radon‐222 flux measurements near 13 hot springs from western to central Nepal, we obtained total CO2and radon discharges varying from 10−3to 1.6 mol s−1and 20 to 1600 Bq s−1, respectively. We observed a coherent organization at spatial scales of ≈ 10 km in a given region: low CO2and radon discharges around Pokhara (midwestern Nepal) and in the Bhote Kosi Valley (east Nepal); low CO2but large radon discharges in Lower Dolpo (west Nepal); and large CO2and radon discharges in the upper Trisuli Valley (central Nepal). A 110 km long CO2‐producing segment, with high carbon isotopic ratios, suggesting metamorphic decarbonation, is thus evidenced from 84.5°E to 85.5°E. This spatial organization could be controlled by geological heterogeneity or large Himalayan earthquakes. CO2and radon fluxes were measured near 13 hot springs in the Nepal HimalayasIntegrated CO2discharges are heterogeneous but coherently spatially organizedA 110 km long metamorphic CO2‐producing segment is clearly defined

The Syabru‐Bensi hydrothermal system (SBHS), located at the Main Central Thrust zone in central Nepal, is characterized by hot (30–62°C) water springs and cold (<35°C) carbon dioxide (CO2) degassing areas. From 2007 to 2011, five gas zones (GZ1–GZ5) were studied, with more than 1600 CO2and 850 radon flux measurements, with complementary self‐potential data, thermal infrared imaging, and effective radium concentration of soils. Measurement uncertainties were evaluated in the field. CO2and radon fluxes vary over 5 to 6 orders of magnitude, reaching exceptional maximum values of 236 ± 50 kg m−2d−1and 38.5 ± 8.0 Bq m−2s−1, with estimated integrated discharges over all gas zones of 5.9 ± 1.6 t  d−1and 140 ± 30 MBq d−1, respectively. Soil‐gas radon concentration is 40 × 103Bq m−3in GZ1–GZ2 and 70 × 103Bq m−3in GZ3–GZ4. Strong relationships between CO2and radon fluxes in all gas zones (correlation coefficient R= 0.86 ± 0.02) indicate related gas transport mechanisms and demonstrate that radon can be considered as a relevant proxy for CO2. CO2carbon isotopic ratios (δ13C from −1.7 ± 0.1 to −0.5 ± 0.1‰), with the absence of mantle signature (helium isotopic ratios R/RA< 0.05), suggest metamorphic decarbonation at depth. Thus, the SBHS emerges as a unique geosystem with significant deep origin CO2discharge located in a seismically active region, where we can test methodological issues and our understanding of transport properties and fluid circulations in the subsurface. The Syabru‐Bensi hydrothermal system (SBHS) shows large CO2and radon fluxesRadon gas is a proxy of its carrier gas CO2and may facilitate monitoringThe SBHS emerges as a unique medium‐scale system in a seismically active region

The Indus Delta is constructed of sediment eroded from the western Himalaya and since 20 ka has been subjected to strong variations in monsoon intensity. Provenance changes rapidly at 12–8 ka, although bulk and heavy mineral content remains relatively unchanged. Bulk sediment analyses shows more negative Ndand higher 87Sr/86Sr values, peaking around 8–9 ka. Apatite fission track ages and biotite Ar–Ar ages show younger grains ages at 8–9 ka compared to at the Last Glacial Maximum (LGM). At the same time δ13C climbs from –23 to –20‰, suggestive of a shift from terrestrial to more marine organic carbon as Early Holocene sea level rose. U–Pb zircon ages suggest enhanced erosion of the Lesser Himalaya and a relative reduction in erosion from the Transhimalaya and Karakoram since the LGM. The shift in erosion to the south correlates with those regions now affected by the heaviest summer monsoon rains. The focused erosion along the southern edge of Tibet required by current tectonic models for the Greater Himalaya would be impossible to achieve without a strong summer monsoon. Our work supports the idea that although long-term monsoon strengthening is caused by uplift of the Tibetan Plateau, monsoon-driven erosion controls Himalayan tectonic evolution. SUPPLEMENTARY MATERIAL: A table of the population breakdown for zircons in sands and the predicted Nd isotope composition of sediments based on the zircons compared to the measured whole rock value is available at http://www.geolsoc.org.uk/SUP18412

U activity ratios have been measured in the dissolved loads of selected rivers from the Himalayan range, in Central Nepal, and from the Bangladesh, as well as in some rain waters. A few European and Asian rivers have also been analyzed for their U activity ratios. The data confirm the negligible effect of rainwater on the budget of dissolved U in river waters. The results also indicate that rivers on each Himalayan structural unit have homogeneous and specific U isotope compositions: i) (234U/238U) activity ratios slightly lower than unity in the dissolved load of the streams draining the Tethyan Sedimentary Series (TSS); ii) values slightly higher than unity for waters from the High Himalaya Crystalline (HHC) and the Lesser Himalaya (LH); iii) systematically higher (234U/238U) activity ratios for waters from the Siwaliks. Thus, U activity ratios, in association with Sr isotopic ratios, can be used to trace the sources of dissolved fluxes carried by these rivers. Coupling of U with Sr isotope data shows (1) that the U carried by the dissolved load of the Himalayan rivers mainly originates from U-rich lithologies of the TSS in the northern formations of the Tibetan plateau; and (2) that the elemental U and Sr fluxes carried by the Himalayan rivers at the outflow of the highlands are fairly homogeneous at the scale of the Himalayan chain. Rivers flowing on the Indian plain define a different trend from that of the Himalayan rivers in the U-Sr isotopic diagram, indicating the contribution of a specific floodplain component to the U and Sr budgets of the Ganges and the Brahmaputra. The influence of this component remains limited to 10 to 15 percent for the U flux, but can contribute 35 to 55% of the Sr flux. The variations of the Sr and U fluxes of the Ganges-Brahmaputra river system in response to climatic variations have been estimated by assuming a temporary cut off of the chemical fluxes from high-altitude terrains during glacial episodes. This scenario would significantly decrease the dissolved U flux of the Ganges-Brahmaputra river system and increase its U activity ratio. Such a climatic dependence of the Himalayan U flux could induce a periodic variation of the mean U activity ratio of the world rivers on glacial-interglacial timescales

A new installation for the analysis of picomole quantities of nitrogen has been designed and constructed. It permits the simultaneous analysis of N and rare gases extracted from silicates by heating. The extraction procedure involves the use of a CO2laser as a heater, and the analysis is made by static mass spectrometry using a high sensitivity, high resolution mass spectrometer. Procedural blanks of 2 picomoles N2(60 picogram N) allow us to analyse routinely N in mg‐sized samples such as mantle‐derived and meteoritic minerals, and Lunar soil grains.

Fluid inclusions were studied in a quartz lens from the structurally highest unit of the Himalaya mountains in Nepal from a textural, geometrical, chemical and isotopic point of view. Six types of fluid inclusions were distinguished. One of these types consists of annular inclusions; this shape is attributed to a confining pressure increase in a non-isotropic stress field. Two successive stress fields were deduced from the orientation of the inclusion planes relative to the schistosity. The bulk composition of the fluid was dominated by CO2 (>84 mol%) and H2O. The composition remained constant during the whole history of the sample indicating that it was buffered by the carbonaceous host rock and/or that one single fluid was reworked in situ by decrepitation. Stable isotope of fluids and minerals indicate (1) that fluids were buffered by surrounding rocks for O and C and (2) that at least two types of water (metamorphic and meteoric) were involved. Finally, a P-T-t-e-s path is proposed for the sample, taking into account the southward thrusting along the Main Central Thrust, the northward tectonic denudation of the Himalaya mountains inducing tectonic burying below the Annapurna Range, and lastly, rapid uplift.

Clastic sediments in the Bengal Fan contain a Neogene history of erosion and weathering of the Himalaya. We present data on clay mineralogy, major element, stable and radiogenic isotope abundances from Lower Miocene-Pleistocene sediments from ODP Leg 116. Nd and Sr isotope data show that the Himalayan provenance for the eroded material has varied little since 17 Ma. However, from 7 to 1 Ma smectite replaces illite as the dominant clay, while sediment accumulation decreased, implying an interval of high chemical weathering intensity but lower physical erosion rates in the Ganges-Brahmaputra (GB) basin. O and H isotopes in clays are correlated with mineralogy and chemistry, and indicate that weathering took place in the paleo-Gangetic flood plain. The87Sr86Srratios of pedogenic clays (vermiculite, smectite) record the isotopic composition of Sr in the weathering environment, and can be used as a proxy for87Sr86Srin the paleo-GB basin. The Sr data from pedogenic clays shows that river87Sr86Srvalues were near 0.72 prior to 7 Ma, rose rapidly to ≥ 0.74 in the Pliocene, and returned to ≤ 0.72 in the middle Pleistocene. These are the first direct constraints available on the temporal variability of87Sr86Srin a major river system. The high87Sr86Srvalues resulted from intensified chemical weathering of radiogenic silicates and a shift in the carbonate-silicate weathering ratio. Modeling of the seawater Sr isotopic budget shows that the high river87Sr86Srvalues require a ca. 50% decrease in the Sr flux from the GB system in the Pliocene. The relationship between weathering intensity,87Sr86Srand Sr flux is similar to that observed in modern rivers, and implies that fluxes of other elements such as Ca, Na and Si were also reduced. Increased weathering intensity but reduced Sr flux appears to require a late Miocene-Pliocene decrease in Himalayan erosion rates, followed by a return to physically dominated and rapid erosion in the Pleistocene. In contrast to the view that increasing seawater87Sr86Srresults from increased erosion, Mio-Pliocene to mid-Pleistocene changes in the seawater Sr budget were the result of reduced erosion rates and Sr fluxes from the Himalaya.

Os isotopic compositions and Os and Re concentrations were measured in H2O2—H2SO4 leachates and bulk sediment samples from Holes 717C and 718C of ODP Leg 116 in the Bengal Fan. Os isotopic results indicate that, at the sediment surface, the leachable Os fraction is derived from seawater. In contrast, leachable Os from Ganges River sediments has 187Os/188Os ratios [1] much higher than the marine value. This difference suggests that the leachable radiogenic Os carried by the river sediments is completely released to the oceans prior to sediment deposition in the Fan. A simple calculation, assuming these sediments to be typical of those delivered by the Ganges-Brahmaputra river system, suggests that this process can account for a substantial part of the rise in the seawater Os isotopic ratio observed over the past 16 m.y.

Clastic sediments in the Bengal Fan contain a Neogene history of erosion and weathering of the Himalaya. We present data on clay mineralogy, major element, stable and radiogenic isotope abundances from Lower Miocene-Pleistocene sediments from ODP Leg 116. Nd and Sr isotope data show that the Himalayan provenance for the eroded material has varied little since > 17 Ma. However, from 7 to 1 Ma smectite replaces illite as the dominant clay, while sediment accumulation decreased, implying an interval of high chemical weathering intensity but lower physical erosion rates in the Ganges-Brahmaputra (GB) basin. O and H isotopes in clays are correlated with mineralogy and chemistry, and indicate that weathering took place in the paleo-Gangetic flood plain. The87Sr86Sr ratios of pedogenic clays (vermiculite, smectite) record the isotopic composition of Sr in the weathering environment, and can be used as a proxy for87Sr86Sr in the paleo-GB basin. The Sr data from pedogenic clays shows that river87Sr86Sr values were near 0.72 prior to 7 Ma, rose rapidly to ≥ 0.74 in the Pliocene, and returned to ≤ 0.72 in the middle Pleistocene. These are the first direct constraints available on the temporal variability of87Sr86Sr in a major river system. The high87Sr86Sr values resulted from intensified chemical weathering of radiogenic silicates and a shift in the carbonate-silicate weathering ratio. Modeling of the seawater Sr isotopic budget shows that the high river87Sr86Sr values require a ca. 50% decrease in the Sr flux from the GB system in the Pliocene. The relationship between weathering intensity,87Sr86Sr and Sr flux is similar to that observed in modern rivers, and implies that fluxes of other elements such as Ca, Na and Si were also reduced. Increased weathering intensity but reduced Sr flux appears to require a late Miocene-Pliocene decrease in Himalayan erosion rates, followed by a return to physically dominated and rapid erosion in the Pleistocene. In contrast to the view that increasing seawater87Sr86Sr results from increased erosion, Mio-Pliocene to mid-Pleistocene changes in the seawater Sr budget were the result of reduced erosion rates and Sr fluxes from the Himalaya.

We present a mineralogical and isotopic study of Early Miocene turbidites from the Bengal Fan, which are the oldest sediments recovered during ODP Leg 116. Clay fractions are dominated by illite and chlorite and have low δ18O (11–16‰). The clays are dominantly derived from a metamorphic precursor by physical erosion, and have undergone only minor low-Twater-rock exchange and chemical weathering. Sr and Nd isotope analysis of silt and clay fractions and coarse muscovite, biotite and feldspar fractions are remarkably similar to those of the presently exposed High Himalaya Crystalline (HHC) sequence. The data show that a close analogue of the HHC was already subaerially exposed to active erosion during Early Miocene. Rb-Sr data on the biotite separates has been used to evaluate the time elapsed between cooling through the closure temperature for Sr in biotite in the HHC (ca. 325°C) and final sedimentation in the Bengal Fan. Despite the large uncertainties in the initial 87Sr86Srvalues of sedimentary mixtures, our estimates of the elapsed time between cooling and deposition are short (4–14 Ma). Assuming a transport time scale (≤1 Ma), deduced exhumation rates are in the same range of modern values (1–3 mm/yr) since ca. 30 Ma. Given that (a) the rocks exposed at the surface, (b) the style of weathering, and (c) rates of exhumation were similar to the present, we infer that the factors controlling erosion of the Early Miocene Himalayan belt were also similar to the present. The data suggest that the Early Miocene Himalayan belt had significant relief and induced orographic effects on precipitation similar to the modern mountain range. It appears that the thrusting along the Main Central Thrust (MCT) or similar structures has uplifted metamorphic basement analogous to the present HHC at sufficiently high rate to maintain steep relief throughout Miocene time, despite high average erosion rates.