Julien Bouchez

A Rouse‐based method to integrate the chemical composition of river sediments: Application to the Ganga basin

[1] The Ganga River is one of the main conveyors of sediments produced by Himalayan erosion. Determining the flux of elements transported through the system is essential to understand the dynamics of the basin. This is hampered by the chemical heterogeneity of sediments observed both in the water column and under variable hydrodynamic conditions. Using Acoustic Doppler Current Profiler (ADCP) acquisitions with sediment depth profile sampling of the Ganga in Bangladesh we build a simple model to derive the annual flux and grain size distributions of the sediments. The model shows that ca. 390 (±30) Mt of sediments are transported on average each year through the Ganga at Haring Bridge (Bangladesh). Modeled average sediment grain size parameters D 50 and D 84 are 27 (±4) and 123 (±9) mm, respectively. Grain size parameters are used to infer average chemical compositions of the sediments owing to a strong grain size chemical composition relation. The integrated sediment flux is characterized by low Al/Si and Fe/Si ratios that are close to those inferred for the Himalayan crust. This implies that only limited sequestration occurs in the Gangetic floodplain. The stored sediment flux is estimated to c.a. 10% of the initial Himalayan sediment flux by geochemical mass balance. The associated, globally averaged sedimentation rates in the floodplain are found to be ca. 0.08 mm/yr and yield average Himalayan erosion rate of ca. 0.9 mm/yr. This study stresses the need to carefully address the average composition of river sediments before solving large‐scale geochemical budgets. Citation: Lupker, M., C. France‐Lanord, J. Lave, J. Bouchez, V. Galy, F. Metivier, J. Gaillardet, B. Lartiges, and J.-L. Mugnier (2011), A Rouse‐based method to integrate the chemical composition of river sediments: Application to the Ganga basin,

Oxidation of petrogenic organic carbon in the Amazon floodplain as a source of atmospheric CO2

The two long-term sources of atmospheric carbon are CO2 degassing from metamorphic and volcanic activity, and oxidation of organic carbon (OC) contained in sedimentary rocks, or petrogenic organic carbon (OCpetro). The latter fl ux is still poorly constrained. In this study, we report particulate organic carbon content and 14C activity measurements in Amazon River sediments, which allow for estimates of the OCpetro content of these sediments. A large decrease of OCpetro content in riverine sediments is observed from the outlet of the Andes to the mouth of the large tributaries. This loss reveals oxidation of OCpetro during transfer of sediments in the floodplain, and results in an escape of ~0.25 Mt C/yr to the atmosphere, which is on the same order of magnitude as the CO2 consumption by silicate weathering in the same area. Raman microspectroscopy investigations show that graphite is the most stable phase with respect to this oxidation process. These results emphasize the significance of OCpetrooxidation in large river floodplains in the global carbon cycle.

Modeling novel stable isotope ratios in the weathering zone

When rock is converted to weathering products, the involved processes can be fingerprinted using the stable isotope ratios of metals (for example Li, Mg, Ca, Fe, Sr) and metalloids (B, Si). Here we construct a framework for interpreting these “novel” stable isotope ratios quantitatively in the compartments of the weathering zone in a geomorphic context. The approach is applicable to any novel stable isotope system and is based on a simple steady-state mass balance model that represents the weathering zone from the scale of a soil column to that of entire continents. Our model is based on the assumption that the two main processes associated with isotope fractionation are formation of secondary precipitates such as clays, and uptake of nutrients by plants. The model results show that the isotope composition of a given element in the weathering zone compartments depends on (1) the ratio between the release flux to water through primary mineral dissolution and the erosion flux of isotopically fractionated solid material, consisting of secondary precipitates and organic matter; (2) the isotope fractionation factors associated with secondary mineral precipitation and uptake by plants. A relationship is established between isotope ratios, isotope fractionation factors, and indexes for chemical weathering [such as chemical depletion fractions (CDF) and elemental mass transfer coefficients (τ)] derived from simple elemental concentration measurements. From this relationship, isotope fractionation factors can be calibrated from chemical and isotope data measured on field material. Furthermore, we show how the ratio of solid export to dissolved export of a given element from the weathering system can be estimated from the comparison of the isotope composition between bedrock, water, and sediment. This calculation can be applied to samples from soils, from rivers, and from the sedimentary record, and does not require knowing the isotope fractionation factors involved in the reactions. Finally, we apply the model to the oceanic Li isotope record reconstructed from marine carbonate sediments in order to discuss changes in global geomorphic regimes through the Cenozoic.

Erosion of organic carbon in the Arctic as a geological carbon dioxide sink

Soils of the northern high latitudes store carbon over millennial timescales (thousands of years) and contain approximately double the carbon stock of the atmosphere. Warming and associated permafrost thaw can expose soil organic carbon and result in mineralization and carbon dioxide (CO2) release. However, some of this soil organic carbon may be eroded and transferred to rivers. If it escapes degradation during river transport and is buried in marine sediments, then it can contribute to a longer-term (more than ten thousand years), geological CO2 sink. Despite this recognition, the erosional flux and fate of particulate organic carbon (POC) in large rivers at high latitudes remains poorly constrained. Here, we quantify the source of POC in the Mackenzie River, the main sediment supplier to the Arctic Ocean, and assess its flux and fate. We combine measurements of radiocarbon, stable carbon isotopes and element ratios to correct for rock-derived POC. Our samples reveal that the eroded biospheric POC has resided in the basin for millennia, with a mean radiocarbon age of 5,800 ± 800 years, much older than the POC in large tropical rivers. From the measured biospheric POC content and variability in annual sediment yield, we calculate a biospheric POC flux of teragrams of carbon per year from the Mackenzie River, which is three times the CO2 drawdown by silicate weathering in this basin. Offshore, we find evidence for efficient terrestrial organic carbon burial over the Holocene period, suggesting that erosion of organic carbon-rich, high-latitude soils may result in an important geological CO2 sink.

A fully automated direct injection nebulizer (d-DIHEN) for MC-ICP-MS isotope analysis: application to boron isotope ratio measurements

This work presents a fully automated setup for using direct injection nebulization as an introduction system for solution measurements by MC-ICP-MS, here applied to boron isotopes in pure boric acid solutions and natural samples. In this setup, a direct injection nebulizer (d-DIHEN) is plugged into the plasma torch without any spray chamber, and an automated 6-port valve interfaces the nebulizer and the autosampler. The advantages of a d-DIHEN for boron isotope ratio measurements are high sensitivity and short washout times, allowing for sample–standard bracketing (SSB) measurements at a higher rate than spray chambers. The measurement of boron isotopes by MC-ICP-MS at an unprecedented sub 0.1‰ repeatability level (2 standard deviation = 2SD) was achieved for pure boric acid solutions. The improved precision is allowed by a better stability of the introduction system with continuous operation of the peristaltic pump (which was manually switched off between samples before automation) and due to the possibility of multiple analyses of the same sample solution. However, such a good repeatability was not systematically obtained for boron isotopes SSB measurements of natural samples (in fine 2SD are between 0.02 and 0.5‰). Boron from natural samples has to be extracted before isotope analysis, with one to four steps depending on the sample type. Repeated analyses of boron independently separated up to ten times from the same sample lead to an external reproducibility no better than 0.2‰. Boron chemical separation from the samples prior to MC-ICP-MS analyses seems to remain the main limitation to precise measurements of boron isotope ratios.

How accurate are rivers as gauges of chemical denudation of the Earth surface

Examination of the behavior of oxygen and hydrogen during weathering reactions shows that river dissolved load, although widely used, is an imperfect tracer of chemical denudation. At the current state of knowledge, none of the metrics for river total dissolved loads (such as the silicate-derived total dissolved solids, TDS sil = Ca 2+ + Mg 2+ + Na + + K + + SiO 2 , converted or not to equivalent oxides) account, in a mechanistic manner, for the transfer of oxygen and hydrogen between the solid and fluid phase during weathering reactions. We assess that chemical denudation derived from TDS sil will significantly overestimate the true chemical denudation for weathering of Ca-feldspar to kaolinite, whereas weathering of water-rich sedimentary rocks will be characterized by an underestimation of chemical denudation by TDS sil . For a handful of field sites, we estimate that the bias is lower than ±10%. The sign and extent of the bias depends on the nature of bedrock and on weathering conditions. Altogether, our analysis questions the broadly accepted concept of chemical denudation rate.

A test of the cosmogenic 10Be(meteoric)/9Be proxy for simultaneously determining basin‐wide erosion rates, denudation rates, and the degree of weathering in the Amazon basin

We present an extensive investigation of a new erosion and weathering proxy derived from the 10Be(meteoric)/9Be(stable) ratio in the Amazon River basin. This new proxy combines a radioactive atmospheric flux tracer, meteoric cosmogenic 10Be, with 9Be, a trace metal released by weathering. Results show that meteoric 10Be concentrations ([10Be]) and 10Be/9Be ratios increase by >30% from the Andes to the lowlands. We can calculate floodplain transfer times of 2–30 kyr from this increase. Intriguingly however, the riverine exported flux of meteoric 10Be shows a deficit with respect to the atmospheric depositional 10Be flux. Most likely, the actual area from which the 10Be flux is being delivered into the mainstream is smaller than the basin-wide one. Despite this imbalance, denudation rates calculated from 10Be/9Be ratios from bed load, suspended sediment, and water samples from Amazon Rivers agree within a factor of 2 with published in situ 10Be denudation rates. Erosion rates calculated from meteoric [10Be], measured from depth-integrated suspended sediment samples, agree with denudation rates, suggesting that grain size-induced variations in [10Be] are minimized when using such sampling material instead of bed load. In addition, the agreement between erosion and denudation rates implies minor chemical weathering intensity in most Amazon tributaries. Indeed, the Be-specific weathering intensity, calculated from mobilized 9Be comprising reactive and dissolved fractions that are released during weathering, is constant at approximately 40% of the total denudation from the Andes across the lowlands to the Amazon mouth. Therefore, weathering in the Amazon floodplain is not detected.

Riverine dissolved lithium isotopic signatures in low‐relief central Africa and their link to weathering regimes

The isotopic composition of dissolved lithium (δ7Li) near the Congo River mouth varied from 14‰ to 22‰ in 2010 and was negatively correlated to discharge. From the relationship between dissolved δ7Li and strontium isotopes, we suggest that this large variation is due to mixing of waters from two contrasting continental weathering regimes. One end-member (high δ7Li ≈ 25‰) represents waters sourced from active lateritic soils covering the periphery of the basin (Li highly sequestered into secondary mineral products) and another representing blackwater rivers (low δ7Li ≈ 5.7‰) derived from the swampy central depression where high organic matter content in water leads to congruent dissolution of the Tertiary sedimentary bedrock. This suggests that the lithium isotopic signature of tropical low-relief surfaces is not unique and traces the long-term, large-scale vertical motions of the continental crust that control geomorphological settings. This evolution should be recorded in the oceanic secular δ7Li curve.

Tracing weathering regimes using the lithium isotope composition of detrital sediments.

Lithium (Li) isotopes are a promising tracer of chemical weathering processes for both modern and ancient times. In order to improve the use of Li isotopes in the sedimentary record, here we calibrate the relationship between weathering intensity and detrital Li isotope composition (δ 7 Li) using the fine fraction of modern large river sediments. Through independent estimates for sediment provenance to calculate the Li isotope signature of the rock from which the sediments derive through weathering, we show that source rock variability (in particular the relative contribution of sedimentary versus igneous rocks) must be corrected for before using Li isotopes as a weathering proxy. We also show that for rivers draining mountain ranges, the contribution to river sediments of particles derived from sedimentary rocks is correlated to their Li/Al ratio, making it possible to use Li contents to estimate the average source rock composition. Once corrected for bedrock variability, the Li isotope signature defines a negative relationship with the weathering intensity (ratio between silicate weathering rate and total denudation rate), with highest Li isotope fractionation for the highest weathering intensity. Altogether, we propose a set of new relationships between weathering, erosion, provenance, and Li isotopes that can be used to quantify present-day and paleo-weathering using detrital sediment.

River Mixing in the Amazon as a Driver of Concentration-Discharge Relationships

Large hydrological systems aggregate compositionally different waters derived from a variety of pathways. In the case of continental-scale rivers, such aggregation occurs noticeably at confluences between tributaries. Here we explore how such aggregation can affect solute concentration-discharge (C-Q) relationships and thus obscure the message carried by these relationships in terms of weathering properties of the Critical Zone. We build up a simple model for tributary mixing to predict the behavior of C-Q relationships during aggregation. We test a set of predictions made in the context of the largest worlds river, the Amazon. In particular, we predict that the C-Q relationships of the rivers draining heterogeneous catchments should be the most “dilutional” and should display the widest hysteresis loops. To check these predictions, we compute 10 day-periodicity time series of Q and major solute (Si, Ca2+, Mg2+, K+, Na+, Cl-, SO42-) C and fluxes (F) for 13 gauging stations located throughout the Amazon basin. In agreement with the model predictions, throughout the Amazon Basin C-Q relationships of most solutes shift from fairly a “chemostatic” behavior (nearly constant C) at the Andean mountain front and in pure lowland areas, to more “dilutional” patterns (negative C-Q relationship) towards the system mouth. More prominent C-Q hysteresis loops are also observed at the most downstream stations. Altogether, this study suggests that mixing of water and solutes between different flowpaths exerts a strong control on C-Q relationships of large-scale hydrological systems.