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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 ad in 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.Observations and samples from the central Himalayas show that a giant rockslide occurring around 1190 ad in the Annapurna massif led to the collapse of an elevated palaeo-summit, illustrating the episodic mode of erosion of the glaciated high relief by mega-rockslides. [ABSTRACT FROM AUTHOR]

Substantial terrestrial gas emissions, such as carbon dioxide (CO2), are associated with active volcanoes and hydrothermal systems. However, while fundamental for the prediction of future activity, it remains difficult so far to determine the depth of the gas sources. Here we show how the combined measurement of CO2 and radon-222 fluxes at the surface constrains the depth of degassing at two hydrothermal systems in geodynamically active contexts: Furnas Lake Fumarolic Field (FLFF, Azores, Portugal) with mantellic and volcano-magmatic CO2, and Syabru-Bensi Hydrothermal System (SBHS, Central Nepal) with metamorphic CO2. At both sites, radon fluxes reach exceptionally high values (> 10 Bq m−2 s−1) systematically associated with large CO2 fluxes (> 10 kg m−2 day−1). The significant radon‒CO2 fluxes correlation is well reproduced by an advective–diffusive model of radon transport, constrained by a thorough characterisation of radon sources. Estimates of degassing depth, 2580 ± 180 m at FLFF and 380 ± 20 m at SBHS, are compatible with known structures of both systems. Our approach demonstrates that radon‒CO2 coupling is a powerful tool to ascertain gas sources and monitor active sites. The exceptionally high radon discharge from FLFF during quiescence (≈ 9 GBq day−1) suggests significant radon output from volcanoes worldwide, potentially affecting atmosphere ionisation and climate. [ABSTRACT FROM AUTHOR]

The International Ocean Discovery Program (IODP) conducted a series of expeditions between 2013 and 2016 that were designed to address the development of monsoon climate systems in Asia and Australia. Significant progress was made in recovering Neogene sections spanning the region from the Arabian Sea to the Sea of Japan and southward to western Australia. High recovery by advanced piston corer (APC) has provided a host of semi-continuous sections that have been used to examine monsoonal evolution. Use of the half-length APC was successful in sampling sand-rich sediment in Indian Ocean submarine fans. The records show that humidity and seasonality developed diachronously across the region, although most regions show drying since the middle Miocene and especially since ∼ 4 Ma, likely linked to global cooling. A transition from C 3 to C 4 vegetation often accompanied the drying but may be more linked to global cooling. Western Australia and possibly southern China diverge from the general trend in becoming wetter during the late Miocene, with the Australian monsoon being more affected by the Indonesian Throughflow, while the Asian monsoon is tied more to the rising Himalaya in South Asia and to the Tibetan Plateau in East Asia. The monsoon shows sensitivity to orbital forcing, with many regions having a weaker summer monsoon during times of northern hemispheric Glaciation. Stronger monsoons are associated with faster continental erosion but not weathering intensity, which either shows no trend or a decreasing strength since the middle Miocene in Asia. Marine productivity proxies and terrestrial chemical weathering, erosion, and vegetation proxies are often seen to diverge. Future work on the almost unknown Paleogene is needed, as well as the potential of carbonate platforms as archives of paleoceanographic conditions. [ABSTRACT FROM AUTHOR]

Substantial terrestrial gas emissions, such as carbon dioxide (CO2), are associated with active volcanoes and hydrothermal systems. However, while fundamental for the prediction of future activity, it remains difficult so far to determine the depth of the gas sources. Here we show how the combined measurement of CO2 and radon-222 fluxes at the surface constrains the depth of degassing at two hydrothermal systems in geodynamically active contexts: Furnas Lake Fumarolic Field (FLFF, Azores, Portugal) with mantellic and volcano-magmatic CO2, and Syabru-Bensi Hydrothermal System (SBHS, Central Nepal) with metamorphic CO2. At both sites, radon fluxes reach exceptionally high values (> 10 Bq m−2 s−1) systematically associated with large CO2 fluxes (> 10 kg m−2 day−1). The significant radon‒CO2 fluxes correlation is well reproduced by an advective–diffusive model of radon transport, constrained by a thorough characterisation of radon sources. Estimates of degassing depth, 2580 ± 180 m at FLFF and 380 ± 20 m at SBHS, are compatible with known structures of both systems. Our approach demonstrates that radon‒CO2 coupling is a powerful tool to ascertain gas sources and monitor active sites. The exceptionally high radon discharge from FLFF during quiescence (≈ 9 GBq day−1) suggests significant radon output from volcanoes worldwide, potentially affecting atmosphere ionisation and climate. [ABSTRACT FROM AUTHOR]

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 CO2 consumption process. Here, we performed a first-order estimate of the CO2 consumption 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 CO2 consumption by silicate weathering varies from 0.06×1012 to 0.87×1012 mol yr−1 depending 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 CO2 consumption 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 CO2 during 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We investigate chronology and age uncertainty for the middle to upper Pleistocene lower Bengal Fan using a novel age‐depth modeling approach that factors lithostratigraphic, magnetostratigraphic, biostratigraphic, cyclostratigraphic, and seismic stratigraphic constraints, based on results from the International Ocean Discovery Program Expedition 354 Bengal Fan and analysis of the GeoB97‐020/027 seismic line. The initial chronostratigraphic framework is established using regionally extensive hemipelagic sediment units, and only age‐depth models of fan deposits that respect the superposition of channel‐levee systems between sites are accepted. In doing so, we reconstruct signals of regional sediment accumulation rate and lithogenic sediment input through the perspective of a two‐dimensional ~320 km transect at 8°N that are consistent with more distal and more ambiguous regional records. This chronology allows us to discuss the depositional history of the middle to upper Pleistocene lower Bengal Fan within the context of sea level, climate, and tectonic controls. We hypothesize, based on the timing of accumulation rate changes, that progradation and intensification of the Bengal Fan's channel‐levee system at 8°N was largely driven by increases in sea level amplitude during this time. However, it is also possible this progradation was influenced by changes in Pleistocene climate and increased Himalayan erosion rates, driving greater sediment flux to the fan. Plain Language Summary: Deep sea fans are sediment deposits in the ocean that often form near river systems offshore continental margins. The largest of these, the Bengal Fan in the northern Indian Ocean, contains the most complete record of materials eroded from the Himalayan Mountains and can be used to study the climate and tectonic history of the region. Sediments are moved from the river mouth to the fan in a series of ever‐changing channels that distribute sediments across the fan surface, making it impossible to obtain a complete and continuous record of Himalayan erosion at any one location. International Ocean Discovery Program Expedition 354 drilled a series of seven locations in a transect across the fan to capture a more complete record of where sediment was deposited over the last 1.25 Myr, a time characterized by major changes in Earth's climate system. Here we discuss statistics of sediment deposition from a computer model constrained by observations from those seven sites. The results indicate that the Bengal Fan grew rapidly during a time when global sea level changes, caused by the growth and decay of continental ice sheets, became more intense. Key Points: A novel regional chronostratigraphic model is used to investigate evolution of the lower Bengal Fan over the last 1.25 MyrSeven IODP site chronologies are constrained by lithostratigraphic, magnetostratigraphic, biostratigraphic, cyclostratigraphic, and seismic stratigraphic constraintsGrowth and intensification of channel‐levee systems at 8°N correspond to increases in the amplitude of Pleistocene sea level changes [ABSTRACT FROM AUTHOR]

The Ganges-Brahmaputra (G-B) River system transports over a billion tons of sediment every year from the Himalayan Mountains to the Bay of Bengal and has built the world's largest active sedimentary deposit, the Bengal Fan. High sedimentation rates drive exceptional organic matter preservation that represents a long-term sink for atmospheric CO2. While much attention has been paid to organic-rich fine sediments, coarse sediments have generally been overlooked as a locus of organic carbon (OC) burial. However, International Ocean Discovery Program Expedition 354 recently discovered abundant woody debris (millimeter- to centimeter-sized fragments) preserved within the coarse sediment layers of turbidite beds recovered from 6 marine drill sites along a transect across the Bengal Fan (~8°N, ~3,700-m water depth) with recovery spanning 19 My. Analysis of bulk wood and lignin finds mostly lowland origins of wood delivered episodically. In the last 5 My, export included C4 plants, implying that coarse woody, lowland export continued after C4 grassland expansion, albeit in reduced amounts. Substantial export of coarse woody debris in the last 1 My included one wood-rich deposit (~0.05 Ma) that encompassed coniferous wood transported from the headwaters. In coarse layers, we found on average 0.16 weight % OC, which is half the typical biospheric OC content of sediments exported by the modern G-B Rivers. Wood burial estimates are hampered by poor drilling recovery of sands. However, high-magnitude, low-frequency wood export events are shown to be a key mechanism for C burial in turbidites. [ABSTRACT FROM AUTHOR]

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+0.35 mm/year for the year 2010, similar to the average value of 1.6−0.2+0.35 mm/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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Located at the southern edge of the eastern Himalayan syntaxis, the Central Myanmar Basin ( CMB) is divided into several Tertiary sub-basins that have been almost continuously filled since the Indo-Asia collision. They are currently drained by the Irrawaddy River, which flows down the eastern Tibetan Plateau and the Sino-Burman Ranges. Tracing sediment provenance from the CMB is thus critical for reconstructing the past denudation of the Himalayan-Tibetan orogen; it is especially relevant since a popular drainage scenario involves the capture of the Tsangpo drainage system in Tibet by a precursor to the Irrawaddy River. Here, we document the provenance of sediment samples from the Minbu Sub-Basin at the southern edge of the CMB, which is traversed by the modern stream of the Irrawaddy River. Samples ranging in age from middle Eocene to Pleistocene were investigated using Nd isotopes, trace element geochemistry and sandstone modal compositions. Our data provide no evidence of a dramatic provenance shift; however, sandstone petrography, trace element ratios and isotopic values display long-term trends indicating a gradual decrease of the volcanic input and its replacement by a dominant supply from the Burmese basement. These trends are interpreted to reflect the progressive denudation of the Andean-type volcanic arc that extended onto the Burmese margin, along the flank of the modern Sino-Burman Ranges, where most of the post-collisional deformation of central Myanmar is located. Though our results do not exclude an ephemeral or diluted contribution from a past Tsangpo-Irrawaddy connection, sedimentation rates suggest that this hypothesis is unlikely before the development of a stable Tsangpo-Brahmaputra River in the Miocene. These results thus suggest that the central Myanmar drainage basin has remained restricted to the Sino-Burman Ranges since the beginning of the India-Asia collision. [ABSTRACT FROM AUTHOR]

A sedimentological and geochemical study was carried out to explore the origin of arsenic contamination in sediments in Nawalparasi district, in the western Terai of Nepal. The investigation tools include major, trace and rare earth element analyses of core sediments, as well as C datings, and O, C isotopic analyses on mollusk shells. The results show that black schists from the Lesser Himalaya highly contributed to the detrital input in Parasi during the Pleistocene-Holocene transition because of focused erosion related to rapid uplift and high rainfall along the Main Central Thrust zone. In addition, aquifer silts, sands, and most of the brown clays underwent a certain degree of chemical weathering and physical reworking, and show possible inputs from the Siwaliks during the Late Holocene. A possible correlation between late Quaternary climate regimes and the concentration of arsenic in sediments is suspected, with arsenic preferentially concentrated during the drier periods of the last 25 kyr BP. The process of arsenic eluviations in sandy and silty sediments can explain the lower arsenic concentrations in sediments during humid periods. During the drier periods, seasonal precipitation was smaller and temperature was lower, leading to wet (less evaporative) soils in swampy environments. This environment favoured the development of aquatic plants and bacteria growing within in the moist land areas, enhancing the strong weathering of initially suspended load particles (micas and clays), which were preferentially deposited in quiet hydraulic environments. These sorting and weathering processes presumably allowed the arsenic to be concentrated in the finest sediment fraction. [ABSTRACT FROM AUTHOR]

Wells located in the Colombian Andean foreland often produce biodegraded hydrocarbons and relatively fresh water (total dissolved solids concentrations of 2000 mg l−1 dominate across most of the basin). The ratio of water to hydrocarbon in the produced fluids is high and has often been interpreted by explorationists and simulation engineers as being due to massive meteoric water invasion. To challenge this hypothesis, previously published and existing data are compiled here, along with 68 new water samples collected from both the surface (rivers) and subsurface (production wells), which are analyzed for major elements, salinity, and stable isotope compositions. The data indicate a mixed origin for the water, which involves formation water, meteoric water, and a third source that we interpret as being related to diagenetic processes (such as clay dehydration). Analyses of the variation of the parameters from north to south and west to east allow us to define the mixtures as coming from waters of different origins and make it possible to determine the relative contribution of each source. [ABSTRACT FROM AUTHOR]

This study examines the lateral distribution of hydromorphy in the fine-grained alluvial deposits of the Eocene Pondaung Formation, central Myanmar. Through detailed outcrop analysis and using a combined sedimentological and pedological approach, this study proposes a reconstruction of Pondaung overbank floodplain palaeoenvironments. The variations of hydromorphic features in the different overbank sub-environments are then discussed and used to build a model of hydromorphic variability in alluvial deposits. Two main architectural associations with distinctive lithofacies and pedogenic features were identified, corresponding to different sub-environments: heterolithic deposits and extensive mudstone successions. The heterolithic deposits display variegated fine-grained lithofacies and contain poorly developed palaeosols with gley and vertic features, which are interpreted to reflect a seasonal wetlands landscape, developed in actively aggrading avulsion belts. Extensive mudstone successions with Vertisols that locally exhibit mukkara-style pseudogley features are interpreted to represent a distal open-forested environment. The palaeosols of both sub-environments display dense local hydromorphic variations they are also characterized by a gradual shift from gley-dominated to pseudogley-dominated features with increasing distance from the avulsion belt. The clay-dominated lithology of the floodplain parent material, which forms numerous subsurface permeability barriers, is shown to have acted as a fundamental control in limiting water-table dynamics in coarse-grained parts of the succession, thereby favouring hydromorphic variability. Palaeosol sequences of the Pondaung Formation contrast with the soil-landscape associations described in other studies and provide an alternative model with which to account for the hydromorphic variability in poorly drained, alluvial soils. The model proposed as an outcome of this study demonstrates that hydromorphic variations can be dramatic in floodplains where permeability barriers are numerous. Further, the model stresses the importance of undertaking detailed lateral palaeosol analyses prior to making interpretations regarding hydromorphic variability. [ABSTRACT FROM AUTHOR]

To determine whether the diversity of phenanthrene-degrading bacteria in an aged polycyclic aromatic hydrocarbon (PAH) contaminated soil is affected by the addition of plant root exudates, DNA stable isotope probing (SIP) was used. Microcosms of soil with and without addition of ryegrass exudates and with C-labelled phenanthrene (PHE) were monitored over 12 days. PHE degradation was slightly delayed in the presence of added exudate after 4 days of incubation. After 12 days, 68% of added PHE disappeared both with and without exudate. Carbon balance using isotopic analyses indicated that a part of the C-PHE was not totally mineralized as CO but unidentified C-compounds (i.e. C-PHE or C-labelled metabolites) were trapped into the soil matrix. Temporal thermal gradient gel electrophoresis (TTGE) analyses of 16S rRNA genes were performed on recovered C-enriched DNA fractions. 16S rRNA gene banding showed the impact of root exudates on diversity of PHE-degrading bacteria. With PHE as a fresh sole carbon source, Pseudoxanthomonas sp. and Microbacterium sp. were the major PHE degraders, while in the presence of exudates, Pseudomonas sp. and Arthrobacter sp. were favoured. These two different PHE-degrading bacterial populations were also distinguished through detection of PAH-ring hydroxylating dioxygenase (PAH-RHD) genes by real-time PCR. Root exudates favoured the development of a higher diversity of bacteria and increased the abundance of bacteria containing known PAH-RHD genes. [ABSTRACT FROM AUTHOR]

Continental erosion controls atmospheric carbon dioxide levels on geological timescales through silicate weathering, riverine transport and subsequent burial of organic carbon in oceanic sediments. The efficiency of organic carbon deposition in sedimentary basins is however limited by the organic carbon load capacity of the sediments and organic carbon oxidation in continental margins. At the global scale, previous studies have suggested that about 70 per cent of riverine organic carbon is returned to the atmosphere, such as in the Amazon basin. Here we present a comprehensive organic carbon budget for the Himalayan erosional system, including source rocks, river sediments and marine sediments buried in the Bengal fan. We show that organic carbon export is controlled by sediment properties, and that oxidative loss is negligible during transport and deposition to the ocean. Our results indicate that 70 to 85 per cent of the organic carbon is recent organic matter captured during transport, which serves as a net sink for atmospheric carbon dioxide. The amount of organic carbon deposited in the Bengal basin represents about 10 to 20 per cent of the total terrestrial organic carbon buried in oceanic sediments. High erosion rates in the Himalayas generate high sedimentation rates and low oxygen availability in the Bay of Bengal that sustain the observed extreme organic carbon burial efficiency. Active orogenic systems generate enhanced physical erosion and the resulting organic carbon burial buffers atmospheric carbon dioxide levels, thereby exerting a negative feedback on climate over geological timescales. [ABSTRACT FROM AUTHOR]

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[1] Sources and fluxes of particulate organic carbon (OC) in the Ganga-Brahmaputra river system were estimated from 13C/12C ratios of bedrocks, soils, bank and suspended river sediments from a Himalayan watershed (Narayani, Nepal), and the Bangladesh floodplain. In the watershed of the Narayani, OC δ13C values for bank and suspended sediments have similar values and narrow ranges, with a mean of about -24.2‰ at the base of the Himalayas. On the Bangladesh floodplain, bank and suspended sediments are enriched in 13C (δ13C about -22.5‰) relative to those collected at the base of the Himalayas. The OC exported at the base of the Himalayan range is estimated to include about 15% from the 13C enriched C4 biomass while the C4 contribution is about 25% of the OC exported by the Ganga and Brahmaputra. A calculated total OC flux of 0.65 × 1012 molC yr-1 is exported to the ocean or trapped in the plain. In sediments of the Ganga and Brahmaputra, the clay-size fraction is depleted in 13C (<2.6‰) relative to the bulk sediment. Possible explanations are that either the organic matter associated with clays is largely inherited from the Himalayan watersheds, or, and considered more likely, the clays and coarser sediments sample different OC pools, possibly with different ages (the growth of C4 crops has dramatically increased since the mid twentieth century), on the floodplain. The OC budget of the actual Ganga-Brahmaputra system is broadly comparable to that derived from the Quaternary sediments of the Bengal fan that represents about 10% of the global OC contribution to the continental margins. [ABSTRACT FROM AUTHOR]

Aquitards-layers of rock having low permeability-have been suggested as potential long-term reservoirs for toxic materials such as nuclear or chemical waste. But information about the isolation properties of aquitard layers is essential to evaluate whether they can indeed be used safely as reservoirs. Here we investigate the long-term mobility of groundwaters between two aquifers surrounding an aquitard layer in the eastern recharge area of the Paris basin, France, using helium isotopes as a geochemical tracer. The deeper Trias sandstone aquifer, which lies above the crystalline basement, accumulates radiogenic 4He and primordial 3He from large regions of the crust and mantle at rates comparable to the degassing of the whole crust and of mid-ocean ridges. We show that the overlying carbonate Dogger aquifer, which is separated from the Trias aquifer by an aquitard layer consisting of a ~600?m succession of shales and clays, is stagnant and has been extremely well isolated from the Trias over the past several million years. This finding, together with previous studies at the centre of the Paris basin, shows that diffusive mass transfer across aquitards is negligible and that cross-formational flow in basins takes place preferentially in faulted areas. [ABSTRACT FROM AUTHOR]

Utilizing a novel data set of integrated high‐resolution multichannel seismic data with IODP Expedition 354 drilling results, a Middle to Late Pleistocene stratigraphy for the lower Bengal Fan is developed. The study reveals a high lateral and temporal variability of deposition expressed by lateral shifts (often exceeding 100 km) between successive channel‐levee systems (CLSs), which occurred on average every ~15 kyr independent from sea‐level changes. The CLSs are embedded in sheeted sediments deposited out of unchannelized turbidity currents, which represent almost two thirds of the lower Bengal Fan sediments. On 100‐kyr timescales, CLSs and sheeted/unchannelized sediments build up subfans, which alternately occupied the western and the eastern Bengal Fan, while the remaining area was draped by ~10 to 20 m‐thick layers of background/hemipelagic sediments. Three subfans have been reconstructed: Subfan B (1.24–0.68 Ma) formed concurrently with the Middle Pleistocene Hemipelagic Layer, Subfan C (0.68–0.25 Ma) covered the entire study area, and Subfan D (0.25 Ma to recent) deposited concomitant with the Late Pleistocene Hemipelagic Layer. The continuous succession of subfans indicates an uninterrupted fan activity independent from sea‐level cycles at least since the Middle Pleistocene. This remarkable independent behavior in terms of sediment supply has not been observed at the Amazon Fan but is in agreement with observations from the Congo Fan. Finally, the analysis of a complete cross section through the lower Bengal Fan reveals that almost half of the sediment represents sands, indicating that the lower Bengal Fan may not generally be classified as "mud rich" (≤30% sand). Plain Language Summary: The northern Indian Ocean receives large amounts of material eroded by wind and rain onshore. Most of the sediment comes from the Himalayan mountain range and contains important information about the evolution of the Himalaya as well as the past Asian climate. After erosion, it is transported by large rivers, such as the Ganges and Brahmaputra, to the coast. Underwater, the sediment moves even farther southwards within river‐like features, the so‐called channel‐levee systems. Eventually, the material is deposited within the submarine Bengal Fan, a large sediment body covering most parts of the Bay of Bengal. In this study, we combine the results of a large underwater drilling campaign (IODP Expedition 354) with acoustic images of the subsurface from the lower Bengal Fan in order to develop a profound understanding of the timing and location of channel‐levee activity and sediment deposition. This knowledge will eventually help scientists to understand the past climate in the Himalaya region. We show that the location of sediment deposition is highly variable and can move more than 100 km laterally. These variations are investigated in detail for the last 1 million years. Key Points: Sedimentation on the Bengal Fan is strongly influenced by significant depocenter relocations on different time and spatial scalesA novel stratigraphy along IODP Expedition 354 drillings reveals a sea‐level independent interplay of hemipelagic and turbiditic depositionThe lower Bengal Fan has a sand content of ~44% despite its classification as a mud‐rich fan [ABSTRACT FROM AUTHOR]

Presents research which showed that Neogene CO2 consumption from the net burial of organic carbon during Himalayan sediment deposition was two to three times that resulting from the weathering of Himalayan silicates. Impact of weathering and erosion on long-term ocean-atmosphere budget of carbon dioxide; Weathering of tectonically uplifted areas in Indian-Asia collision zone; Chemistry of Neogene sediments; Effect of Neogene Himalayan erosion.

The Bangladesh lowlands are traversed by the largest sediment flux on the planet. Detritus generated mostly in Himalayan highlands and conveyed through the Ganga–Brahmaputra rivers and Meghna estuary reaches the Bay of Bengal, where it forms a composite deltaic system. This study integrates the vast existing database on Ganga–Brahmaputra sediments of all grain sizes from clay to sand with new petrographic, mineralogical, and geochemical data on estuarine and shallow-marine sands. A large spectrum of compositional signatures was used to: (i) assess the relative supply of the Ganga and Brahmaputra rivers to estuarine and shelfal sediments; (ii) define the compositional variability of estuarine sediments and the impact exerted by hydraulic sorting and climate-related chemical weathering on provenance signals; (iii) define the compositional variability of shelf sediments and the potential hydrodynamic segregation of fast-settling heavy minerals in coastal environments and of slow-settling platy micas on low-energy outer-shelf floors; (iv) consider the potential additional mud supply from the western subaerial part of the delta formerly built by the Ganga River; and (v) draw a preliminary mineralogical comparison between fluvio-deltaic sediments and turbidites of the Bengal–Nicobar deep-sea fan, thus tracing sediment dispersal across the huge sedimentary system extending from Tibet to the equatorial Indian Ocean. All investigated mineralogical and geochemical parameters, as well as Sr and Nd isotope ratios and clay–mineral assemblages, showed a clear prevalence in sediment supply from the Brahmaputra (60–70%) over the Ganga (30–40%). Heavy-mineral suites and Sr and Nd isotope fingerprints of Bengal shelf sediments are nearly identical to those of the Brahmaputra River and Meghna estuary, also because the Brahmaputra carries almost twice as many Ca-plagioclase grains and heavy minerals including epidote than the Ganga, and these minerals control the large majority of the Sr and Nd budgets. The experience gained in modern settings can be directly extrapolated only to the recent past, because sediments older than the late Pleistocene and buried more than a few hundred meters begin to lose less durable ferromagnesian minerals by selective chemical dissolution, which makes quantitative estimates progressively less robust in more deeply buried older strata. [ABSTRACT FROM AUTHOR]

This article illustrates a multi-technique frontier approach for the provenance study of silt-size sediments. The mineralogical composition of low-density and heavy-mineral fractions of four samples of fine to very coarse silt deposited on the Bengal shelf was analyzed separately for six different grain-size classes by combining grain counting under an optical microscope, Raman spectroscopy, and X-ray diffraction. The geochemical composition was determined on both bulk-sediment samples and on their <5-μm classes. Such a "multiple-window" approach allowed capturing the full mineralogical information contained in each sample, as well as the size-dependent intra-sample variability of all compositional parameters. The comparison between grain-size distributions obtained by different methods highlighted a notable fallacy of laser granulometry, which markedly overestimated the size of the finest mode represented by fine silt and clay. As a test case, we chose to investigate sediments of the Bengal shelf, where detritus is fed from the Meghna estuary, formed by the joint Ganga and Brahmaputra Rivers and representing the largest single entry point of sediment in the world's oceans. The studied samples show the typical fingerprint of orogenic detritus produced by focused erosion of collision orogens. Bengal shelf silt is characterized by a feldspatho-quartzose (F-Q) composition with a Q/F ratio decreasing from 3.0 to 1.7 with increasing grain size, plagioclase prevailing over K-feldspar, and rich transparent-heavy-mineral assemblages including mainly amphibole with epidote, and minor garnet and pyroxene. Such a detrital signature compares very closely with Brahmaputra suspended load, but mineralogical and geochemical parameters, including the anomalous decrease of the Q/F ratio with increasing grain size, consistently indicate more significant Ganga contribution for cohesive fine silt. The accurate quantitative characterization of different size fractions of Bengal shelf sediments represents an essential step to allow comparison of compositional signatures characterizing different segments of this huge source-to-sink system, from fluvial and deltaic sediments of the Himalayan foreland basin and Bengal shelf to the Bengal Fan. [ABSTRACT FROM AUTHOR]

Fluid-earthquake interplay, as evidenced by aftershock distributions or earthquake-induced effects on near-surface aquifers, has suggested that earthquakes dynamically affect permeability of the Earth’s crust. The connection between the mid-crust and the surface was further supported by instances of carbon dioxide (CO2) emissions associated with seismic activity, so far only observed in magmatic context. Here we report spectacular non-volcanic CO2 emissions and hydrothermal disturbances at the front of the Nepal Himalayas following the deadly 25 April 2015 Gorkha earthquake (moment magnitude Mw = 7.8). The data show unambiguously the appearance, after the earthquake, sometimes with a delay of several months, of CO2 emissions at several sites separated by > 10 kilometres, associated with persistent changes in hydrothermal discharges, including a complete cessation. These observations reveal that Himalayan hydrothermal systems are sensitive to co- and post- seismic deformation, leading to non-stationary release of metamorphic CO2 from active orogens. Possible pre-seismic effects need further confirmation. Earthquakes rarely affect hydrothermal systems in non-magmatic context. Here the authors report outbursts of CO2 and hydrothermal disturbances triggered by the 2015 Nepal earthquake, revealing high sensitivity of Himalayan hydrothermal systems to co-, post- and possibly pre- seismic deformation. [ABSTRACT FROM AUTHOR]