C and O isotope compositions of modern fresh-water mollusc shells and river waters from Himalaya and Ganga plain

The aim of this paper is to unfold the relationship between O and C isotope compositions of modern fresh-water mollusc shells and water in order to refine the basis of interpretation for paleoenvironnemental reconstruction in the sub-Himalayan river basins. Large number of mollusc shells and associated host water from both running water and closed body of water were analysed including intra-shell variability in a few cases. The O isotopic compositions of river waters in the Himalayas and Ganga plain have a large range, from -18‰ in the north of the High range up to -8 to -4 ‰ in the Ganga plain. d18O of rivers are also seasonally variable, especially in foothills rivers whereas the seasonal contrast is smoothed out for main Himalayan rivers having large catchments. O isotopic compositions of bulk shells (Aragonite) vary between -15 and -5 ‰. Average d18OAra values are consistent with precipitation at equilibrium with host waters at a temperature range of 20-25°C suggesting that shell growth may be favoured during non-monsoon conditions. Shells collected along main Himalayan rivers have d18O values uniformly distributed within -11 and -6 ‰ reflecting the minimal seasonal contrast shown by these rivers. In contrast, O isotopic compositions of shells from foothills rivers vary only by 4‰. This shows that, depending on the type of river where the molluscs grow, the information in term of d18O amplitude will be different for identical climatic conditions. In closed or pond water bodies significant enrichment in 18O due to evaporation is observed. The C isotopic compositions of river dissolved inorganic carbon (DIC) decrease downstream from 0 to -10 ‰ reflecting input of soil derived alkalinity and plant productivity in the river. d13C of shells are systematically lower than compositions calculated for equilibrium with river DIC indicating that in addition to DIC, a significant fraction of carbon is derived from metabolic sources. Intra-shell d13C are stable compared to the seasonal variability of DIC suggesting that the pool of organic carbon changes throughout year

Discovery of sediment indicating rapid lake-level fall in the late Pleistocene Gokarna Formation, Kathmandu Valley, Nepal: implication for terrace formation

Sediment indicating a rapid fall in lake level has been discovered in the late Pleistocene Gokarna Formation, Kathmandu Valley, Nepal. The indicator is observed along a widely traceable erosional surface in this formation, and is characterized by (1) gently inclined (ca. 10°) tabular cross-stratified sand beds of delta front origin consisting of coarser material and showing gradual decrease in elevation of its top to the progradation direction, (2) an antidune cross-laminated sand bed that interfingers with the delta front deposit, and (3) an approximately 5 m-deep erosional depression filled with convolute laminated sand beds and recognized at a location distal to that where deposits (1) and (2) were found. The early phase of rapid lake level fall caused minor erosion of the delta plain deposits by fluvial processes, introducing a higher rate of progradation of the delta front and resulting in the accumulation of deposit (1). The delta emerged as dry land due to further lowering of the lake level. The antidune cross-laminated sand bed shows evidence of having accumulated from a high-velocity stream that may have formed as the lake water drained from the delta front during the lowering of lake level. When the lake level fell below the level of the topographic high created by delta accumulation, incised valleys may have formed and part of them may have been filled with sediment at that time. The rapid fall in lake level is interpreted to have been the result of lake-wall failure, which would have occurred at the gorge outlet as the only discharge path for the basin. The initial rise of lake level causing accumulation of terrace sediments may have been due to the formation of a plug at this outlet, attributable to mass movement along the gorge

A case study using 2019 pre-monsoon snow and stream chemistry in the Khumbu region, Nepal

This case study provides a framework for future monitoring and evidence for human source pollution in the Khumbu region, Nepal. We analyzed the chemical composition (major ions, major/trace elements, black carbon, and stable water isotopes) of pre-monsoon stream water (4300–5250 m) and snow (5200–6665 m) samples collected from Mt. Everest, Mt. Lobuche, and the Imja Valley during the 2019 pre-monsoon season, in addition to a shallow ice core recovered from the Khumbu Glacier (5300 m). In agreement with previous work, pre-monsoon aerosol deposition is dominated by dust originating from western sources and less frequently by transport from southerly air mass sources as demonstrated by evidence of one of the strongest recorded pre-monsoon events emanating from the Bay of Bengal, Cyclone Fani. Elevated concentrations of human-sourced metals (e.g., Pb, Bi, As) are found in surface snow and stream chemistry collected in the Khumbu region. As the most comprehensive case study of environmental chemistry in the Khumbu region, this research offers sufficient evidence for increased monitoring in this watershed and surrounding areas

Predominant floodplain over mountain weathering of Himalayan sediments (Ganga basin)

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 84 (2012): 410-432, doi:10.1016/j.gca.2012.02.001.We present an extensive river sediment dataset covering the Ganga basin from the Himalayan front downstream to the Ganga mainstream in Bangladesh. These sediments were mainly collected over several monsoon seasons and include depth profiles of suspended particles in the river water column. Mineral sorting is the first order control on the chemical composition of river sediments. Taking into account this variability we show that sediments become significantly depleted in mobile elements during their transit through the floodplain. By comparing sediments sampled at the Himalayan front with sediments from the Ganga mainstream in Bangladesh it is possible to budget weathering in the floodplain. Assuming a steady state weathering regime in the floodplain, the weathering of Himalayan sediments in the Gangetic floodplain releases ca. (189 ± 92)109 and (69 ± 22)109 moles/yr of carbonate bound Ca and Mg to the dissolved load, respectively. Silicate weathering releases (53 ± 18)109 and (42 ± 13)109 moles/yr of Na and K while the release of silicate Mg and Ca is substantially lower, between ca. 0 and 20109 moles/yr. Additionally, we show that sediment hydration, [H2O+], is a sensitive tracer of silicate weathering that can be used in continental detrital environments, such as the Ganga basin. Both [H2O+] content and the D/H isotopic composition of sediments increases during floodplain transfer in response to mineral hydrolysis and neoformations associated to weathering reactions. By comparing the chemical composition of river sediments across the floodplain with the composition of the eroded Himalayan source rocks, we suggest that the floodplain is the dominant location of silicate weathering for Na, K and [H2O+]. Overall this work emphasizes the role of the Gangetic floodplain in weathering Himalayan sediments. It also demonstrates how detrital sediments can be used as weathering tracers if mineralogical and chemical sorting effects are properly taken into account.This work was supported by INSU program “Relief de la Terre” and ANR Calimero. Valier Galy was supported by the U.S. National Science Fundation (Grant OCE-0851015)

ETUDE SEDIMENTOLOGIQUE ET GEOCHIMIQUE (ISOTOPES STABLES) DES BASSIN SYN-OROGENIQUES DE L'HYMALAYA DU NEPAL (SIWALIKS ET BASSIN DE KATHMANDU)

The work deals with paleo-environmental study of the Siwalik foreland basin (upper Miocene) and the intermountain Kathmandu basin (Plio-Pleistocene). O and C composition of modern mollusks and waters of Ganga Plain and intermountain basins were realized as a reference for paleo-environmental study.The Δ18O of rivers varies from -18‰ in north of Himalaya to -4‰ in the plain. The Δ18Ocar of modern shell show an equilibrium fractionation with the river waters (20 to 25°C). Both carbonate and river water values are different according to catchment basin extension. The Δ13CDIC of rivers are higher than Δ13Ccar due to organic carbon incorporation in shell.In the Siwaliks, Δ18Ocar varies from -16.4 to 0.6‰. For the Late Miocene, the Δ18Ocar values are lower than Pliocene and modern values. Therefore, rivers running through the Siwalik plain should show different geometry. The Δ13C show a sharp increase up to 0‰ around 5 Ma, which seems to be linked to the contemporaneous evolution of C4 plant.The Kathmandu Basin (3-0 Ma) is a transported basin and characterized by fluvio-lacustrine sediments. The evolution of lake is controlled by tectonic, landslide and debris-flow.The Δ18Ocar older than 1 Ma vary from -11 to -4‰ and are compatible with river water (-10.1 to -7‰) as well as monsoon water (-18 to 2‰) in Kathmandu. In contrast Δ18Ocar of Gokarna Formation ( Pour l'actuel, le Δ18Ow des rivières varie de -18 à -4‰. Le Δ18Ocar montre que les carbonates sont proches de l'équilibre isotopique avec les eaux dans lesquels ils sont formés (20 à 25°C). Le Δ13Ccar est appauvri par rapport au DIC des rivières, impliquant qu'une part de carbone organique est utilisée pour la synthèse des coquilles. Dans les Siwaliks (16-1 Ma), le Δ18Ocar varie de -16.4 à 0.6‰. Les valeurs anté-6Ma sont très inférieures aux valeurs pliocènes et actuelles de la plaine du Gange. La plaine miocène sup. était donc drainée par des rivières de configuration différente des rivières modernes. Le Δ13C augmente vers 5 Ma jusqu'à environ 0‰, évolution qui semble en rapport avec l'explosion de la flore C4. Le bassin de Kathmandu (3-0 Ma) est caractérisé par une sédimentation fluvio-lacustre. La profondeur du lac montre des fluctuations importantes, contrôlées par la tectonique, les glissements et débris-flow. Pour les périodes pos-50 Ka, la géochimie des isotopes de l'O des mollusques montre des périodes pour lesquelles les valeurs Δ18Ocar sont très élevées (-2.4 à 8‰), compatibles avec les eaux de précipitation d'hiver à Kathmandu (-6.5 à 7.5‰). Ces valeurs sont généralement associées à des diatomées. Deux processus semblent se combiner durant ces épisodes: une diminution des précipitations d'été et une forte évaporation, qui augmentent le 18O

Frontal and piggy-back seismic ruptures in the external thrust belt of western Nepal.

International audiencehis paper gives structural and morphological evidence for meter-scale episodic displacement pulses along the MFT and MDT during a complex sequence of thrusting in the sub-Himalayan fold and thrust belt. The studied cross-section is located in Western Nepal at a latitude of E 82° 20′. Along this section, the Main Dun Thrust (MDT) is in a piggy-back setting and comprises three splays that are spaced by less than 200 m. The splays display the following sequence of motion: (1) the medium splay was active; (2) the internal splay was active and had a dip-slip displacement of not, vert, similar3 m that could be the surface rupture of an earthquake; (3) a period of quiescence occurred for all the splays of the MDT; (4) the external splay is active. The motion along the medium and internal thrust predates not, vert, similar70 kyr whereas motion along the external splay postdates 5468–5214 yr before J.C. The Main Frontal Thrust (MFT) is the most external Himalayan thrust. Ten encased strath terraces are found at its hangingwall. Sudden not, vert, similar1.5 m uplift events could have induced their abandonment and could have been linked to not, vert, similar2 m slip events along the MFT, a value close to the lower bound inferred for co-seismic slip along the basal detachment (MHT) during Mnot, vert, similar8 earthquakes. From the offset of a terrace level, a displacement of not, vert, similar8 m is inferred between 1224–1280 yr after J.C. and 1828–1883 yr after J.C.; it could be linked to a succession of several seisms along the MFT. This study suggests that: (a) several thrusts, that branch off the basal décollement, are active faults; (b) great earthquakes, that occur along the outer part of the basal décollement of the Himalayan thrust belt, reach the surface at different location; (c) only one thrust moves during one earthquake; (d) the sequence of thrusting is successively out-of-sequence and in sequence and changes at an intermediate time-scale between earthquake cycle and finite geometry of the thrust system

Significance of the clay mineral distribution in fluvial sediments of the Neogene to Recent Himalayan Foreland Basin (west-central Nepal)

Clay mineral assemblages of the Neogene Himalayan foreland basin are studied to decipher their significance with respect to tectonic and climate processes. Fluvial deposits of the Siwalik Group (west-central Nepal), and sediment of the Ganga River drainage system were analysed for clay mineralogy. The observed clay mineral assemblages are mainly composed of illite (dominant), chlorite, smectite and kaolinite. Illite and chlorite are chiefly of detrital origin, derived from Himalayan sources. Kaolinite and smectite are authigenic, and mainly developed within pore space and as coating of detrital particles. With increasing burial, diagenetic processes affected the original clay mineral signature. Illitisation of smectite and kaolinite occurred below 2500 and 3500m depth, respectively. Therefore, illite in the lower parts of the Siwalik Group consists of a mixture of inherited illite and illitised smectite and kaolinite, as suggested by illite crystallinity. Detrital grains that make up the framework of the Siwalik Group sandstones mainly consist of quartz, feldspar and lithic fragments, which are principally of sedimentary and metamorphic origin. Lithoclast content increases over time at the expense of quartz and K-feldspar in response to uplift and erosion of the Lesser Himalaya Series since about 11-10Ma. Despite mainly felsic source rocks, dominantly physical erosion processes in the Himalayan belt, and high-energy fluvial depositional systems, smectite is abundant in th

High K and Ca Chemical Erosion Triggered by Physical Erosion in a Watershed of the High Himalaya of Nepal

International audienceThe Khudi river in Nepal is an example of a basin undergoing intense physical erosion by landslide under very wet monsoonal conditions. Although under such a regime, dissolved element concentrations are expected to during the monsoon, we observe marked increases in dissolved K and Ca during flood events. These peaks in K and Ca concentrations are well correlated with increases of suspended load by an order of magnitude. The data suggest that release of K and Ca is enhanced by physical erosion and rock disaggregation events during sediment transfer