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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]

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]

Abstract: We use the evolution of river sediment characteristics and sedimentary Corg from the Himalayan range to the delta to study the transport of Corg in the Ganga–Brahmaputra system and especially its fate during floodplain transit. A detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers. We study the relationships between sediment characteristics (mineralogy, grain size, specific area) and Corg content in order to evaluate the controls on Corg loading. Contributions of C3 and C4 plants are estimated from Corg stable isotopic composition (δ13Corg). We use the evolution of δ13Corg values from the Himalayan range to the delta in order to study the fate of Corg during floodplain transit. Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In spite of 4–5 times higher specific area, Ganga sediments have similar Corg content, grain size and mineralogy as Brahmaputra sediments, indicating that specific area does not exert a primary control on Corg loading. The general correlation between the total Corg content and Al/Si ratio indicates that Corg loading is mainly related to: (1) segregation of organic particles under hydrodynamic forces in the river, and (2) the ability of mineral particles to form organo-mineral aggregates. Bed and suspended sediments have distinct δ13Corg values. In bed sediments, δ13Corg values are compatible with a dominant proportion of fossil Corg derived from Himalayan rocks erosion. Suspended sediments from Himalayan tributaries at the outflow of the range have low δ13Corg values (−24.8‰ average) indicating a dominant proportion of C3 plant inputs. In the Brahmaputra basin, δ13Corg values of suspended sediments are constant along the river course in the plain. On the contrary, suspended sediments of the Ganga in Bangladesh have higher δ13Corg values (−22.4‰ to −20.0‰), consistent with a significant contribution of C4 plant derived from the floodplain. Our data indicate that, during the plain transit, more than 50% of the recent biogenic Corg coming from the Himalaya is oxidised and replaced by floodplain Corg. This renewal process likely occurs during successive deposition-erosion cycles and river course avulsions in the plain. [Copyright &y& Elsevier]

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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Abstract: The aim of this paper is to unfold the relationship between the 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 Himalaya 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. δ18O of rivers are also seasonally variable, especially in foothills rivers whereas the seasonal contrast is smoothed out for the Main Himalayan rivers having large catchments. O isotopic compositions of bulk shells (Aragonite) vary between −15‰ and −5‰. Average δ18OAra 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 the Main Himalayan rivers have δ18O 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 δ18O 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. δ13C 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 δ13C are stable compared to the seasonal variability of DIC suggesting that the pool of organic carbon changes throughout year. [Copyright &y& Elsevier]

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]

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]

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]

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]

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’ 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]