Search

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]

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]

Abstract: Continental chemical weathering is central in Earth''s surface biogeochemical cycles as it redistributes elements across reservoirs such as the crust and the oceans. However the evolution of weathering through time and its response to external forcing such as changes in climate remain poorly constrained. In this work, a composite sediment record from the Bay of Bengal is used to document the evolution of chemical weathering in the Himalayan system (Himalayan range and Indo-Gangetic floodplain), the world largest sediment conveyor to the oceans, since the Last Glacial Maximum (LGM). The degree of weathering of the sediments is documented using mobile to immobile ratios such as K/Si and H2O+/Si as well as detrital calcite abundance. Robust weathering proxies are derived by correcting the chemical composition of sediment for sorting effects that occur during transport and deposition. The Bay of Bengal record is also further compared to the chemical composition of modern river sediments from the Ganga and Brahmaputra basin. Weathering proxies all indicate that the sediments exported by the Ganga and Brahmaputra Rivers became increasingly weathered over the past ∼21kyr, whereas, Sr, Nd and major elements suggest a constant sediment provenance in the system over the last 21kyr. These changes in the degree of weathering of the sediments show that the weathering flux exported by the system to the Indian Ocean during the LGM was significantly lower than at present and demonstrate that chemical weathering in continental scale basins such as the Ganga and Brahmaputra responds to Late Quaternary climate changes. [Copyright &y& Elsevier]

Abstract: 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 mol/yr of carbonate bound Ca and Mg to the dissolved load, respectively. Silicate weathering releases (53±18)×109 and (42±13)×109 mol/yr of Na and K while the release of silicate Mg and Ca is substantially lower, between ca. 0 and 20×109 mol/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. [Copyright &y& Elsevier]

Abstract: Modern erosion of the Himalaya, the world''s largest mountain range, transfers huge dissolved and particulate loads to the ocean. It plays an important role in the long-term global carbon cycle, mostly through enhanced organic carbon burial in the Bengal Fan. To understand the role of past Himalayan erosion, the influence of changing climate and tectonic on erosion must be determined. Here we use a 12Myr sedimentary record from the distal Bengal Fan (Deep Sea Drilling Project Site 218) to reconstruct the Mio-Pliocene history of Himalayan erosion. We use carbon stable isotopes (δ 13C) of bulk organic matter as paleo-environmental proxy and stratigraphic tool. Multi-isotopic — Sr, Nd and Os — data are used as proxies for the source of the sediments deposited in the Bengal Fan over time. δ 13C values of bulk organic matter shift dramatically towards less depleted values, revealing the widespread Late Miocene (ca. 7.4Ma) expansion of C4 plants in the basin. Sr, Nd and Os isotopic compositions indicate a rather stable erosion pattern in the Himalaya range during the past 12Myr. This supports the existence of a strong connection between the southern Tibetan plateau and the Bengal Fan. The tectonic evolution of the Himalaya range and Southern Tibet seems to have been unable to produce large re-organisation of the drainage system. Moreover, our data do not suggest a rapid change of the altitude of the southern Tibetan plateau during the past 12Myr. Variations in Sr and Nd isotopic compositions around the late Miocene expansion of C4 plants are suggestive of a relative increase in the erosion of High Himalaya Crystalline rock (i.e. a simultaneous reduction of both Transhimalayan batholiths and Lesser Himalaya relative contributions). This could be related to an increase in aridity as suggested by the ecological and sedimentological changes at that time. A reversed trend in Sr and Nd isotopic compositions is observed at the Plio-Pleistocene transition that is likely related to higher precipitation and the development of glaciers in the Himalaya. These almost synchronous moderate changes in erosion pattern and climate changes during the late Miocene and at the Plio-Pleistocene transition support the notion of a dominant control of climate on Himalayan erosion during this time period. However, stable erosion regime during the Pleistocene is suggestive of a limited influence of the glacier development on Himalayan erosion. [Copyright &y& Elsevier]

[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]

Determining the rate of CO 2 consumption associated with the Himalayan uplift is an essential prerequisite to understanding climate evolution throughout the Cenozoic. The riverine fluxes of dissolved calcium can be used to quantify uptakes of atmospheric CO 2 by chemical weathering, but this approach requires deciphering the silicate-derived Ca flux from the generally dominant carbonate-derived Ca flux. Here we present high-precision radiogenic calcium (40Ca) analyses of bank sediments and dissolved loads from a network of rivers in central Nepal Himalaya and Bangladesh, to constrain the sources and relative contributions of carbonate and silicate-derived calcium to the dissolved loads of Himalayan rivers. Calcium isotope analyses were performed in multidynamic mode by thermal ionization mass spectrometry, yielding an external precision of ±0.4 ε-units (2 S.D.). Our results show that silicate catchments exposed in the Himalayan range have variably radiogenic ε40Ca compositions relative to seawater, ranging from +0.7 in the TSS to +14 in the LH. In contrast, sedimentary carbonates, including metamorphosed dolomites with variably radiogenic 87Sr/86Sr, exhibit uniform ε40Ca identical to modern seawater. The homogeneous ε40Ca composition of sedimentary carbonates confirms the relative resistance of radiogenic Ca signatures to post-depositional alteration and provides a robust baseline against which the contribution of silicate weathering to the riverine Ca flux can be evaluated. Dissolved load compositions of rivers draining LH catchments exhibit ε40Ca values ranging from 0 to +11, reflecting the relative contribution of unradiogenic carbonates and highly radiogenic metapelitic units of the LH sequence. HHC rivers show a more moderate range, from 0.5 to +2, reflecting the less radiogenic composition of HHC paragneisses. Himalayan front rivers draining all three lithotectonic units of the Himalayan range exhibit moderately radiogenic ε40Ca, ranging from +0.5 to +1.4 and overall similar to the average composition of the Ganga mainstream in Bangladesh. Using an approach combining conventional alkalinity budgets with isotopic mass balance calculations, we show that the dissolved silicate Ca flux of major Himalayan front rivers is primarily derived from the weathering of metapelitic rocks of the lesser Himalaya. In contrast, regions of the high range that generate the bulk of the detrital flux to the Gangetic floodplain represent a subordinate source of silicate-derived Ca. This result demonstrates that the chemical weathering of Ca-silicates in the Himalayan system is decoupled from physical erosion. The extensive erosional activity observed in the high range may thus play a limited role in promoting CO 2 consumption and global cooling by silicate weathering. [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.

An unambiguous Neogene and Quaternary sequence of clastic sedimentation in the middleBengal fan has been retrieved by IODP Expedition 354. The Sr-Nd isotopic fingerprint ofsilicate fraction, turbiditic sediments have clear Himalayan origin. The turbidites&#x2019; chemicalcomposition showed virtually no differences with the modern Himalayan riversediments, with no significant variation through time1. In details, however, theisotope geochemistry defines a systematic consistent with subtle variations in thecomposition of the sources of the sediments. Mineralogical effects, such as nuggetseffects and the predominance of some heavy minerals in the isotopic budget ofsome elements2 and sedimentological sorting3 are also prone to modify the Sr-Ndisotopic fingerprint of deep-sea turbidites. In the Bay of Bengal, such effects generateanti-correlated changes but the extend on ɛNd is rather small, at most 1ɛg???? betweencoarsest and fine layers within the same tubidite. The analysis of 75 samples ofcoarse silt across the transect cored by Exp. 354 at 8˚ N and U1444 hole from byExp. 353 at 14˚ N define temporal changes that are insensitive to the samplinglocation and attribute to changes in the tectonic regime in the Himalayan range.In particular, the activation of the MBT and associated exhumation of the LesserHimalayan sequence (LHS) is marked by a drop in the ɛNd by up to 2.1ɛg???? startingaround 7.6Ma. The rise in the proportion of LHS material is then stopped around6.0Ma, likely to correspond to the activation of the MCT1 in the range. For theolder period, only one record from U1451 is available and might be more subject tochanges in the sediment supply within the Bengal Fan. Nevertheless, the middleMiocene is dominated by material derived from the high Himalayan crystalline seriesrelated to exhumation along the MCT with rising contribution through time (until∼10Ma) of the Trans-Himalaya batholith and the significant erosion of the southernTibet. 1) France-Lanord, C., Spiess, V., Klaus, A., Schwenk, T., Expedition 354Scientists. (2016). http://doi.org/10.14379 /iodp.proc.354.2016. 2) Garçon et al.(2014) https://doi.org/10.1016/j.chemgeo.2013.11.018. 3) McLennan et al. (1989)https://doi.org/10.1038/337547a0 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Enhanced silicate weathering induced by the uplift of the Himalayan-Tibetan Plateau (HTP) has been considered as the major cause of p CO 2 decline and Cenozoic cooling. However, this hypothesis remains to be validated, largely due to the lack of a reliable reconstruction of the HTP weathering flux. Here, we present a 37-million-year record of the difference in the seawater radiogenic neodymium isotopic composition (Δ ε Nd) of Ocean Drilling Program (ODP) sites and Fe-Mn crusts between the northern and central Indian Ocean, which indicates the contribution of regional weathering input from the South Asian continent to the Indian Ocean. The results show a long-term increase in Δ ε Nd and thus provide the first critical evidence of enhanced South Asian weathering input since the late Eocene. The evolution coincided well with major pulses of surface uplift in the HTP and global climatic transitions. Our foraminiferal ε Nd record suggests that tectonic uplift and silicate weathering in South Asia, especially in the Himalayas, might have played a significant role in the late Cenozoic cooling. [ABSTRACT FROM AUTHOR]

40Ar/39Ar ages from detrital muscovites have been analyzed from six modern rivers in central and western Nepal; the size of the drainage basins associated with these samples ranges from a few square kilometers to >40,000 km2. These data, when combined with previously published ages of detrital muscovites from other modern rivers in the region, suggest that a good correspondence between normalized age and normalized topography (the comparison of t* and z*) is rare, due to either nonuniform rates of passage through the ~400 °C isotherm or subsequent faulting in the drainage area. The closure temperature of Ar in muscovite is perhaps too high to make meaningful comparisons to modern topography in tectonic analysis of active orogens. The distribution of 40Ar/39Ar ages from detrital muscovites from the Karnali basin in western Nepal is much older than that for the Narayani basin in central Nepal. The Karnali muscovites, when combined with previously published muscovites from the Siwalik Group in western Nepal and zircon fission track ages from modern and ancient samples from the region, suggest a thermal history for western Nepal consistent with vigorous tectonics (and attendant erosion) before the middle Miocene but a significant diminution in the rate of erosion since ca. 10 Ma. 40Ar/39Ar ages of detrital muscovites from the Narayani basin in central Nepal suggest a markedly different history with an acceleration of the rate of erosion since ca. 10 Ma and reactivation of major faults; this is consistent with the abundant bedrock data from the Narayani basin. The strong difference in the erosional history of the adjacent Karnali and Narayani basins, as evidenced by the 40Ar/39Ar ages from detrital muscovites, is not likely to have been due to variations in climate, but rather due to strain partitioning within the Himalaya during and after the Miocene. [ABSTRACT FROM AUTHOR]

Abstract: The neodymium and hafnium isotopic compositions of most crustal and mantle rocks correlate to form the “Terrestrial Array”. However, it is now well established that whereas coarse detrital sediments follow this trend, fine-grained oceanic sediments have high Hf ratios relative to their Nd isotopic ratios. It remains uncertain whether this “decoupling” of the two isotopic systems only occurs in the oceanic environment or if it is induced by sedimentary processes in continental settings. In this study, the hafnium and neodymium isotopic compositions of sediments in large rivers is expressly used to constrain the behavior of the two isotopic systems during erosion and sediment transport from continent to ocean. We report major and trace element concentrations together with Nd and Hf isotopic compositions of bedloads, suspended loads and river banks from the Ganges River and its tributaries draining the Himalayan Range i.e. the Karnali, the Narayani, the Kosi and the Marsyandi Rivers. The sample set includes sediments sampled within the Himalayan Range in Nepal, at the Himalayan mountain front, and also downstream on the floodplain and at the outflow of the Ganges in Bangladesh. Results show that hydrodynamic sorting of minerals explains the entire Hf isotopic range, i.e. more than 10 ε Hf units, observed in the river sediments but does not affect the Nd isotopic composition. Bedloads and bank sediments have systematically lower ε Hf values than suspended loads sampled at the same location. Coarse-grained sediments lie below or on the Terrestrial Array in an ε Hf vs. ε Nd diagram. In contrast, fine-grained sediments, including most of the suspended loads, deviate from the Terrestrial Array toward higher ε Hf relative to their ε Nd, as is the case for oceanic terrigenous clays. The observed Nd–Hf decoupling is explained by mineralogical sorting processes that enrich bottom sediments in coarse and dense minerals, including unradiogenic zircons, while surface sediments are enriched in fine material with radiogenic Hf signatures. The data also show that Nd–Hf isotopic decoupling increases with sediment transport in the floodplain to reach its maximum at the river mouth. This implies that the Nd–Hf isotopic decoupling observed in worldwide oceanic clays and river sediments is likely to have the same origin. Finally, we estimated the Nd–Hf isotopic composition of the present-day mantle if oceanic sediments had never been subducted and conclude that the addition of oceanic sediments with their anomalous Nd–Hf isotopic compositions has slowly shifted the composition of the Earth’s mantle towards more radiogenic Hf values through time. [Copyright &y& Elsevier]

Reconstructing the timing of mountain range uplift and the evolution of high-altitude plateaus is important when attempting to understand potential feedbacks between tectonics and climate at geological timescales. This requires proxies that are able to accurately reconstruct elevation during different time periods in the past. Often, the sensitivity of climatic parameters to elevation gradients, recorded in geological archives such as soils, is used to estimate paleoelevations. However, most proxies reflect an indirect response to elevation change, adding uncertainties to reconstructions. In this study, we aim to identify those sources of uncertainty with respect to elevation reconstructions and test if the combined application of two such proxies, i.e., stable isotopes (δ2H) of plant waxes in modern soils and surface waters and bacterial membrane lipids (brGDGTs) in soils, which can potentially reduce uncertainties in the estimation of (paleo-) elevation. We performed this study in four Himalayan catchments (from west to east: Sutlej, Alaknanda, Khudi, and Arun), of which each individual catchment is subject to a unique precipitation regime, relative influences of moisture sources, and vegetation cover. In total, we analyzed 275 surface water samples, 9 precipitation samples, 131 xylem water samples, and 60 soil samples, which were collected between 2009 and 2014. The following key observations were made: Soil n C 31 -alkane δ2H values (δ2H wax) in the Sutlej, Alaknanda, Khudi, and Arun generally record surface water δ2H values, confirming that the first-order control on the plant wax isotopic signature is precipitation δ2H and, therefore, the elevation in orogenic settings. We identified aridity as the factor that introduces scatter to this relationship. BrGDGT-derived Mean Annual Temperature (MAT) correlates in a statistically significant manner with sample site elevation and a 14-year annual average of remotely sensed land-surface temperature, showing that the main process influencing the brGDGT distribution is the adiabatic cooling of air. In an effort to combine these proxies to improve uncertainties in elevation reconstruction, elevations were inferred from both the δ2H wax and brGDGT distributions. Arid, high elevation sites appear to underestimate actual sample site elevations using δ2H values while sites subject to high (>23–25 °C) annual temperatures overestimate the actual sample site elevation using brGDGT distributions. Elevations inferred from both proxies under such paleoclimatic conditions should be interpreted with caution. Elevations derived from the brGDGT distribution appear to most accurately reconstruct elevation. However, we show that the difference in elevation between the two proxies, described by the proposed Δ Elevation parameter, can provide information on the hydrological conditions of the soil's depositional environment. In conclusion, we emphasize that knowledge of the sample site's climatic conditions are essential to reconstruct elevation from paleoarchives. In particular, knowledge of moisture availability and annual air temperatures are important, as these have been found to cause the largest scatter in the observed data. [ABSTRACT FROM AUTHOR]

At geological time scales, the role of continental erosion in the organic carbon (OC) cycle is determined by the balance between recent OC burial and petrogenic OC oxidation. Evaluating its net effect on the concentration of carbon dioxide and dioxygen in the atmosphere requires the fate of petrogenic OC to be assessed. Here, we report a multiscale (nanometer to micrometer) structural characterization of petrogenic OC in the Himalayan system. We show that graphitic carbon is preserved and buried in marine sediments, while the less graphitized forms are oxidized during fluvial transport. Radiocarbon dating indicates that 30 to 50% of the carbon initially present in the Himalayan rocks is conserved during the erosion cycle. Graphitization during metamorphism thus stabilizes carbon in the crust over geological time scares. [ABSTRACT FROM AUTHOR]

For documenting recent or Late Quaternary erosion rates at the scale of a small watershed, or even an entire mountain range, the use of in-situ terrestrial cosmogenic nuclides (TCN), such as 10 Be, in river sediments has become widespread over the last decade. In mountainous settings, however, landslides may induce a two-fold complication in the cosmogenic nuclide budget. First, they may episodically deliver large amounts of sediment with low TCN concentrations to the river channel. Second, they may generate a grain-size-concentration dependence in these sediments. However, studies that have explored grain-size dependence in landslide-dominated areas have reached differing conclusions and the underlying mechanisms remain poorly understood. The present study focuses on the Khudi Khola river basin, a small drainage basin in the central Himalayas. The area is characterized by steep slopes, heavy rainfall, and high sediment production rates. Importantly, the watershed contains a large area of active landsliding. In-situ 10 Be was analyzed in sediments of grain sizes ranging from 75 μm to ~ 4 cm, collected from five locations: upstream, downstream, and in the small tributaries which drain the landslides. In all sampling locations, including those situated upstream of the landslides, the finest grains (75–250 μm) are two to four times more concentrated in 10 Be than the largest clasts (> 4.7 mm). We developed a numerical slope model which includes a low level of background soil erosion and episodic denudation from large mass-wasting events. We then introduced a coarsening of average grain-size with soil-depth to the model. Our simulation shows that the data available for this valley are compatible with a scenario in which landslides deliver coarse grains to the channel that are less concentrated in TCN and have deeper provenance. Best-fit estimates of the model parameters suggest recurrence times of 3000 to 9000 years for a major landslide on the same slope, and mean long-term landslide denudation rates of 1.7 to 4.7 mm yr − 1 . A negative correlation between TCN concentration and grain-size is also observed directly upstream and 5-km downstream of the landsliding area, suggesting that nearly all of the sediment carried by the river is derived from ancient landslides while almost no sediment is supplied by erosion of soil-mantled slopes. Despite these complications, apparent denudation rates directly derived from the TCN concentrations, and from previously published data in the region, are all on the same order of magnitude, around ~ 2 mm yr − 1 . However, a factor-of-three variability remains between the different studies, depending on the location, the year of sampling and the analyzed grain size fraction. Thus, although TCN-derived average denudation rates provide a useful first order estimate, even in fast-eroding terrains dominated by landslides, caution should be applied when using quantitative results in such settings. [ABSTRACT FROM AUTHOR]

Abstract: The Himalayas represent the archetype of mountain building due to active continental collision and are considered in many studies as the locus of intense interactions between climate, denudation and tectonics. Estimates of modern denudation rates across the entire range remain, however, relatively sparse. In this study, in situ-produced cosmogenic 10Be concentrations were measured in detritic quartz in order to determine basin-scale denudation rates for the central part of the Himalayan range. River sand was sampled over several years in the main trans-Himalayan rivers, from the Himalayan front to the Ganga outlet in Bangladesh. The calculated 10Be denudation rates of the trans-Himalayan river basins range from 0.5 to 2.4mmyr−1 (average 1.3mmyr−1) and vary by up to a factor of 3 between sampling years. These denudation rates strongly contrast with the 0.007mmyr−1 denudation rate of southern tributary basins draining the Indian craton. This work also shows that in the Ganga basin, no systematic evolution of average 10Be concentrations is observed during floodplain transfer, implying that distal samples can be used to estimate the integrated denudation rate of the whole central Himalayan range. Samples from the Ganga in Bangladesh display remarkably low variability in 10Be concentration, implying an average Himalayan denudation rate of 1.0–1.1mmyr−1. However, within the floodplain, several samples suggest a recent perturbation of sediment transport dynamics with a recent increase in the relative sediment contribution from southern tributaries. The Himalayan sediment flux, deduced from the 10Be denudation rate of the range, is 610±230Mtyr−1. This flux is consistent, within uncertainty, with sediment fluxes derived from sediment gauging. The similarity of the two flux estimates suggests that Himalayan erosion fluxes have remained stable over the last centuries, even if the large uncertainties associated with each method hamper more precise assessments. [Copyright &y& Elsevier]

Abstract: Gas discharges have been identified at the Syabru–Bensi hot springs, located at the front of the High Himalaya in Central Nepal, in the Main Central Thrust zone. The hot spring waters are characterized by a temperature reaching 61 °C, high salinity, high alkalinity and δ 13C varying from +0.7‰ to +4.8‰. The gas is mainly dry carbon dioxide, with a δ 13C of −0.8‰. The diffuse carbon dioxide flux, mapped by the accumulation chamber method, reached a value of 19000 g m−2day−1, which is comparable with values measured on active volcanoes. Similar values have been observed over a two-year time interval and the integral around the main gas discharge amounts to 0.25±0.07 mol s−1, or 350±100 ton a−1. The mean radon-222 concentration in spring water did not exceed 2.5 Bq L−1, exponentially decreasing with water temperature. In contrast, in gas bubbles collected in the water or in the dry gas discharges, the radon concentration varied from 16000 to 41000 Bq m−3. In the soil, radon concentration varied from 25000 to more than 50000 Bq m−3. Radon flux, measured at more than fifty points, reached extreme values, larger than 2 Bq m−2s−1, correlated to the larger values of the carbon dioxide flux. Our direct observation confirms previous studies which indicated large degassing in the Himalaya. The proposed understanding is that carbon dioxide is released at mid-crustal depth by metamorphic reactions within the Indian basement, transported along pre-existing faults by meteoric hot water circulation, and degassed before reaching surface. This work, first, confirms that further studies should be undertaken to better constrain the carbon budget of the Himalaya, and, more generally, the contribution of mountain building to the global carbon balance. Furthermore, the evidenced gas discharges provide a unique natural laboratory for methodological studies, and appear particularly important to study as a function of time, especially in relation to the seismic activity. For this purpose, the observed high radon-222 flux is a particularly interesting asset. Indeed, while the relationship between radon and carbon dioxide needs to be better understood, radon measurements, using the available radon sensors, constitute a powerful tool for robust and cost effective long term monitoring. [Copyright &y& Elsevier]