Christian France-Lanord

Weathering processes in the Ganges-Brahmaputra basin and the riverine alkalinity budget

Abstract We present river chemistry data for a network of rivers draining the western and central Nepal Himalaya. Our sampling locations cover the system from the sources of rivers in Tibet to the Gangetic plain. Water samples were collected throughout the year, including the monsoon season, for rivers in Nepal and for the Ganges and Brahmaputra in Bangladesh. Rivers draining the North Himalaya are characterized by low discharge under a cold and arid climate. Main stream waters have δ 13 C DIC near 0‰ and high [SO42−]/([SO42−]+[HCO3−]) ratios (XSO4) with values around 40 Eq% and high [Cl−]. Ca is the dominant cation (Ca2+/∑cations=55 to 75 Eq%, after correction of sodium by chloride). Dissolved sulfate is produced in waters from the Tethian Sedimentary Series whereas chlorine is related to thermal waters. Dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution. δ 13 C DIC values of up to 3.9‰ show that metamorphic CO2 is an important weathering agent. Rivers of the North Himalayan basins have about 50% of their alkalinity derived from carbonate dissolution, 20% from biogenic activity and 30% from metamorphic CO2. On the south flank of the Himalaya, rivers are more depleted in 13 C and have, on average, lower XSO4. Most rivers have δ 13 C DIC and XSO4 values compatible with a simple mixing between soil CO2 and sulfate derived from sulfide oxidation. During the monsoon, discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced. XSO4 and δ 13 C decrease while Ca/∑cations increases, implying enhanced dissolution of pedogenic calcite. As a whole, the G–B riverine flux of alkalinity derived from silicate weathering is around 2.7×1011 mol/year, modest at the global scale. Sulphuric acid controls 20–30% of the weathering reaction in the Brahmaputra and 6 to 9% in the Ganges. Na+ and K+ balance 60 to 65% of the silicate-derived alkalinity flux, and the long term CO2 consumption by Ganges and Brahmaputra is near 6.4×1010 mol/year. The flux of metamorphic CO2 converted to alkalinity via weathering reactions is ca. 1×1010 mol/year.

Crustal generation of the Himalayan leucogranites

Abstract Detailed studies of the Himalayan two-mica leucogranites, such as the Manaslu pluton, indicate that they have very uniform mineralogical, petrological and structural characteristics. One can relate their occurrence to the thickest zones of the underlying Tibetan Slab. In these zones, migmatization attains its greatest development and vertical extension. The granite is emplaced at first along the main disharmonic plane above the Main Central Thrust (MCT), at the top of the Tibetan Slab (infrastructure). Ductile deformation of the granite is variable; the granite being syn-to late-kinematic with regard to the functioning of the MCT. Major elements are very homogeneous (except for Na and K) implying that P-T conditions of melting were relatively uniform. The melted material was of a similar composition over a vast volume, and the percentage of melting was small (10–15%). Trace elements are highly variable. Some are characteristic for very evolved material (Ta, Rb, Cs, U) or show the link with the Tibetan Slab (Ba-Sr), whilst others are problematic (Th, REE). REE and Th abundances, being much less in the granite than in the Tibetan Slab, imply that they have been extracted during one of the main stages of formation, possibly by monazite. Radiogenic (Pb, Sr, Nd) and stable (O) isotopes are consistent with the origin of the granite from the Tibetan Slab. However, the heterogeneous Sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution. H-isotope data indicate that magmatic compositions of the main body of the Manaslu granite have been preserved. Late or post-magmatic alterations are extremely local in the main pluton. To the north, another belt of two-mica granites occurs whose characteristics are very similar to the High Himalaya belt. They were probably generated in a similar way during this recent intracontinental evolution.

Higher erosion rates in the Himalaya: Geochemical constraints on riverine fluxes

The modern erosion rate of continental-scale mountains is difficult to estimate and is usually based on measurement of the suspended load flux of rivers combined with assumptions about river bedload transport and sedimentation in flood plains. These two parameters are very difficult to measure directly in continental-scale basins. In this paper we examine the chemical composition of the suspended load, bedload, and dissolved load of the Ganga and Brahmaputra Rivers and compare them with the average composition of Himalayan source rocks. A mass-balance equation of erosion fluxes shows that a Si-rich component is needed in addition to suspended and dissolved load fluxes to account for the composition of the source rock. It corresponds to bedload sediment and flood-plain deposits, which are enriched in quartz by mineral sorting during transport. The combined budget of Si, Al, and Fe in the river system allows us to estimate this Si-rich flux. By this method, the total Himalayan erosion is estimated to be twice the measured flux of suspended load. The comparison between the Brahmaputra and the Ganga shows that the eastern Himalaya has a higher erosion rate (2.9 mm/yr) than the western Himalaya (2.1 mm/yr). This is likely the result of the higher runoff in the Brahmaputra basin. The intensity of the monsoon acts as a first-order control of the erosion rate in the range.

Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system

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.

Neogene Himalayan weathering history and river87Sr86Sr: impact on the marine Sr record

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

The strontium isotopic budget of Himalayan rivers in Nepal and Bangladesh

Himalayan rivers have very unusual Sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic. We present Sr, O, and C isotopic data from river and rain water, bedload, and bedrock samples for the western and central Nepal Himalaya and Bangladesh, including the monsoon season. Central Himalayan rivers receive Sr from several sources: carbonate and clastic Tethyan sediments, High Himalayan Crystalline (HHC) gneisses and granitoids with minor marbles, carbonates and metasediments of the Lesser Himalaya (LH), and Miocene-Recent foreland basin sediment from the Siwaliks group and the modern flood plain. In the Tethyan Himalaya rivers have dissolved [Sr] ≈ 6 μmol/l and 87Sr/86Sr ≈ 0.717, with a large contribution from moderately radiogenic carbonate. Rivers draining HHC gneisses are very dilute with [Sr] ≈ 0.2 μmol/l and 87Sr/86Sr ≈ 0.74. Lesser Himalayan streams also have low [Sr] ≈ 0.4 μmol/l and are highly radiogenic (87Sr/86Sr ≥ 0.78). Highly radiogenic carbonates of the LH do not contribute significantly to the Sr budget because they are sparse and have very low [Sr]. In large rivers exiting the Himalaya, Sr systematics can be modeled as a mixture between Tethyan rivers, where slightly radiogenic carbonates (mean 87Sr/86Sr ≈ 0.715) are the main source of Sr, and Lesser Himalaya waters, where extremely radiogenic silicates (>0.8) are the main source of Sr. HHC waters are less important because of their low [Sr]. Rivers draining the Siwaliks foreland basin sediments have [Sr] ≈ 4 μmol/l and 87Sr/86Sr ≈ 0.725. Weathering of silicates in the Siwaliks and the flood plain results in a probably significant radiogenic (0.72–0.74) input to the Ganges and Brahmaputra (G-B), but quantification of this flux is limited by uncertainties in the hydrologic budget. The G-B in Bangladesh show strong seasonal variability with low [Sr] and high 87Sr/86Sr during the monsoon. Sr in the Brahmaputra ranges from 0.9 μmol/l and 0.722 in March to 0.3 μmol/l and 0.741 in August. We estimate the seasonally weighted flux from the G-B to be 6.5 × 108 mol/yr with 87Sr/86Sr = 0.7295.

Evolution of the Himalaya since Miocene time: isotopic and sedimentological evidence from the Bengal Fan

Abstract We report Sr, Nd, O, and H isotopic data and clay mineral abundances for turbidite sediments recovered in ODP Leg 116 cores from the Bengal Fan at 1°S. The samples studied cover the period between c. 17 Ma and the present. We also present new and compiled data on the isotopic compositions of potential source regions for the Bengal Fan sediments. ɛNd(0) values in the Bengal Fan sediments (all samples) define a narrow range about − 16.0. 87Sr/86Sr values (all samples) are also in a narrow range near 0.741. δ18O values in quartz separates define a narrow range at +12.8±0.5‰. Coarse biotite-chlorite separates give δ18O = 3.6−5.6‰. Combined δ18O values of quartz and biotites indicate a metamorphic source. Clay mineral abundances define two clay facies: an illite-chlorite-rich assemblage (IC) and a smectite-kaolinite-rich assemblage (SK). δ18O in the IC clay fractions is 11.5–15‰, while SK clays are 18.2–22.6‰. The narrow range of isotopic values throughout the deposition history implies that the source of the Bengal Fan sediments has not changed since the early Miocene, despite changes in sedimentation rate, sedimentary facies, tectonic history and climactic regime. The difference between δ18O in the IC and SK clay fractions represents different alteration histories of the same source material. The SK clays appear to have been altered at low T in the Indo-Gangetic Plain, while the IC clays and coarse fractions preserve metamorphic signatures. The narrow range of the Sr values, despite wide variation in Rb/Sr ratio, also argues for a source that underwent isotopic homogenization shortly before erosion and deposition of the sediment. The source that meets these criteria is the High Himalayan Crystalline series (HHC) or a close analogue, although subordinate contributions (probably <20%) from the Lesser Himalaya (LH) and Tibetan Sedimentary Series (TSS) are possible. A model in which the HHC are exposed to erosion since the early Miocene on the south flank of the orogen by thrusting along the MCT, while the TSS is simultaneously removed by northward-directed normal faulting satisfies the constraints above. The results of this study require that the Himalaya have been a significant topographic feature since at least the early Miocene. Independent evidence supports this contention. Variations in the sedimentation style in the Bengal Fan since that time appear to represent a combination of factors, including tectonic activity and the coupled effects of climate and sea-level changes.

pH control on oxygen isotopic composition of symbiotic corals

Abstract Boron, carbon and oxygen isotopic compositions were determined at the micrometre scale by high-resolution ion microprobe in a sample of modern coral (massive hermatypic coral, Porites lutea). The ion probe data show for B and O much larger isotopic variations at the micrometre scale than those measured at the millimetre scale by conventional techniques: δ18OPDB values range from −10.6±0.9‰ to −0.2±0.5‰ and δ11B values range from +18.6±1.5‰ to +30.6±1.6‰. By contrast, δ13C values show the same range of variations, from −4.6±0.65‰ to −2.2±0.67‰ at the micrometre and millimetre scales. The range of δ11B values indicates that significant pH variations, from ≈7.1 to ≈9.0, are present at the sites of calcification. The largest δ18O variations correspond to the highest δ11B values, i.e. to the highest pHs. This measurement of pH allows modelling the oxygen isotopic fractionation occurring during aragonite precipitation. Taking into account the rate of O isotopic equilibrium between dissolved carbonate species (H2CO3, HCO3− and CO32−) and water via the two reactions of hydration and hydroxylation, the full range of δ18O values measured at the micrometre scale can be modelled for residence times of dissolved carbonates in the calcifying fluid ranging between ≈1 h and at maximum ≈12 h. The pH controls the δ18O of the growing carbonate through the relative fractions of dissolved carbonate species and through the kinetics of their isotopic equilibration with water via hydration and hydroxylation. The so-called ‘vital effect’ systematically observed for δ18O in corals can thus be understood as representing an average of rapid pH variations due to coral biology during coral growth. Selectively measuring δ18O values in the zones of coral skeletons that have low δ11B values (i.e. formed at low pH) should significantly improve the quality of palaeoclimatic reconstructions based on δ18O values.

Sustained sulfide oxidation by physical erosion processes in the Mackenzie River basin: Climatic perspectives

The chemical weathering of rocks with sulfuric acid is usually not considered in reconstructions of the past evolution of the carbon cycle, although this reaction delivers cations and alkalinity to the ocean without involvement of atmospheric CO 2 . The contribution of sulfuric acid as a weathering agent is still poorly quantifi ed; the identifi cation of riverine sulfate sources is diffi cult. The use of δ 34 S and δ 18 O of dissolved sulfate allows us to demonstrate that most of the sulfate in surface waters of the Mackenzie River system, Canada, derives from pyrite oxidation (85% ± 5%) and not from sedimentary sulfate. The calculated fl ux of pyrite-derived sulfate is 0.13 ◊ 10 12 mol/yr, corresponding to 20%‐27% of the estimated global budget. This result suggests that the modern global ocean delivery of sulfi de-derived sulfate, and thus chemical weathering with sulfuric acid, may be signifi cantly underestimated. A strong correlation between sulfi de oxidation rates and mechanical erosion rates suggests that the exposure of fresh mineral surfaces is the rate-limiting factor of sulfi de oxidation in the subbasins investigated. The chemical weathering budget of the Mackenzie River shows that more than half of the dissolved inorganic carbon discharged to the ocean is ancient sedimentary carbon from carbonate (62%) and not atmospheric carbon (38%). The subsequent carbonate precipitation in the ocean will thus release more CO 2 in the atmosphere-ocean system than that consumed by continental weathering, typically on glacial-interglacial time scales.

Tracing the distribution of erosion in the Brahmaputra watershed from isotopic compositions of stream sediments

Bank sediments and suspended loads of the Brahmaputra River and its important tributaries were collected from the Himalayan front to Bangladesh along with most of the important tributaries. Chemical and isotopic compositions of the sediments are used to trace sediment provenance and to understand erosion patterns in the basin. Overall isotopic compositions range from 0.7053 to 0.8250 for Sr and ϵNd from −20.5 to −6.9. This large range derives from the variable proportions of sediments from Himalayan formations with high Sr isotopic ratios and low ϵNd, and Transhimalayan plutonic belt with lower Sr isotopic ratios and higher ϵNd. The latter are exposed to erosion in the Tsangpo and in the eastern tributary drainages. Overall erosion of the Himalayan rocks is dominant, representing ca 70% of the detrital influx. Compositions of the Brahmaputra main channel are rather stable between 0.7177 and 0.7284 for Sr and between −14.4 and −12.5 for ϵNd throughout its course in the plain from the Siang-Tsangpo at the foot of the Himalayan range down to the delta. This stability, despite the input of large Himalayan rivers suggests that the Siang-Tsangpo River represents the major source of sediment to the whole Brahmaputra. Geochemical budget implies that erosion of the Namche Barwa zone represents about 45% of the total flux at its outflow before confluence with the Ganga from only 20% of the mountain area. Higher erosion rates in the eastern syntaxis compared to the other Himalayan ranges is related to the rapid exhumation rates of this region, possibly triggered by higher precipitation over the far-eastern Himalaya and the high incision potential of the Tsangpo River due to its very high water discharge.