Luc Bastian
email: bastian@geoazur.unice.fr

Nathalie Vigier

Marie Revel

Gezahegn Yirgu

Dereje Ayalew

Raphaël Pik

Chemical erosion rates in the upper Blue Nile Basin and related atmospheric CO2 consumption

Keywords: Ethiopian Traps, silicate weathering rate, lithium isotopes, CO2 consumption, basalt, African monsoon

Silicate weathering of basaltic rocks constitutes a non-negligible sink of atmospheric CO2 but the role it plays in the regulation of past and future global climate is still matter of debate. In this study, silicate weathering rates for various sub-basins of the Ethiopian Traps, emplaced 30 million years ago, and the corresponding atmospheric CO2 consumption rates are evaluated. For this, major and trace elements were measured in the dissolved phases and in the sedimentary particles carried and deposited by the main rivers flowing through this steep region. Lithium isotopes and major elements were also measured in the extracted clay fractions in order to infer complementary information on weathering processes in this region. Clay δ 7 Li values correlate positively with Mg/Ti ratios, and are best explained by varying ratios of leaching versus clay formation rate.

Although located in a region annually submitted to monsoon, average silicate weathering rate (16.1 tons/km 2 /year) and CO2 consumption rate (0.65x10 12 mol/year) are estimated to be low when compared to other basaltic regions such as the Deccan Traps, and volcanically active islands of the tropical zone. This is surprising since the concentrations of Total Dissolved Solids of the Ethiopian rivers are among the highest ones. With a 2D rainfall model that takes into account the detailed topography of the region, annual occurrence of the Monsoon, and monitoring station data, we show that runoff intensity is a key parameter that explains this difference. We determine that, at present, the weathering of the Ethiopian Traps plays a negligible role in the carbon cycle. However, simple calculations, which integrate recent knowledge on African climate variations and on weathering controls, illustrate that during the African Humid Period (14-8 kyr), a significant increase of Monsoon precipitation may have resulted in much higher weathering rates and related CO2 consumption (0.91-1.5x10 12 mol/year). This study therefore evidences the potential importance of this region in the past, and the need to quantify more precisely the variations of the monsoon intensity and its impact on tropical watersheds for reconstructing past CO2 levels.

Introduction

Chemical and physical erosion, which describe respectively mineral dissolution and soil formation, rock cracking and particle transport are key processes in landscape evolution.

Various parameters can control weathering (chemical erosion) at a watershed scale, among them lithology, tectonics, relief, soil thickness, age, vegetation cover and climate parameters [START_REF] Bayon | Environmental Hf-Nd isotopic decoupling in World river clays[END_REF][START_REF] Bouchez | Modeling novel stable isotope ratios in the weathering zone[END_REF][START_REF] Brady | Direct measurement of the combined effects of lichen, rainfall, and temperature on silicate weathering[END_REF][START_REF] Clergue | Influence of atmospheric deposits and secondary minerals on Li isotopes budget in a highly weathered catchment, Guadeloupe (Lesser Antilles)[END_REF][START_REF] Dessert | Erosion of Deccan Traps determined by river geochemistry; impact on the global climate and the (super 87) Sr/ (super 86) Sr ratio of seawater[END_REF][START_REF] Gaillardet | Global silicate weathering and CO consumption rates deduced2 from the chemistry of large rivers[END_REF][START_REF] Li | Temperature dependence of basalt weathering[END_REF]P Louvat and Allègre, 1998;[START_REF] West | Tectonic and climatic controls on silicate weathering[END_REF]. Over geological timescale (>10 5 years), silicate weathering, with organic matter burial, are major sinks of atmospheric carbon. Weathering of silicate rocks consumes atmospheric CO2 and transforms it to dissolved HCO3 - [START_REF] Gaillardet | Global silicate weathering and CO consumption rates deduced2 from the chemistry of large rivers[END_REF], which further precipitates with calcium carbonate in the ocean. As CO2 is a greenhouse gas, silicate rock weathering can be a critical feedback control on the climate [START_REF] Brady | Direct effects of CO2 and temperature on silicate weathering: Possible implications for climate control[END_REF][START_REF] Walker | Weathering rates in 106 gm km -2 yr -1 of dissolved Si02 . Parentheses signify[END_REF].

However, at present, the relative importance of the various continental areas and various types of lithologies having played a key role in the carbon cycle is still debated, in particular over the Cenozoic. The emplacement of the Himalayas (and its uplift) may have been critical but this is not consensual thus far. Other mountainous regions may have been of importance, in particular those rich in basaltic lithologies, and are little considered yet [START_REF] Li | Evolution of carbon cycle over the past 100 million years[END_REF]. Indeed, all silicate rocks do not present the same sensitivity to weathering. Among them, basaltic rocks can be weathered more easily than any other igneous ones (e.g. [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF]. Some studies [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF][START_REF] Gaillardet | Global silicate weathering and CO consumption rates deduced2 from the chemistry of large rivers[END_REF][START_REF] Louvat | Present denudation rates on the island of réunion determined by river geochemistry: Basalt weathering and mass budget between chemical and mechanical erosions[END_REF]P Louvat and Allègre, 1998) indicate that basaltic rocks are weathered 8 time faster than granitic rocks. [START_REF] Millot | The global control of silicate weathering rates and the coupling with physical erosion. new insights from rivers Can[END_REF] show that the silicate weathering rate is 10 to 100 times larger for basaltic regions than for granitic areas. Basalt weathering has thus been estimated as an important sink of CO2 at the global scale [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF] and it is calculated to be responsible of 30% of the present-day CO2 consumption [START_REF] Gaillardet | Global silicate weathering and CO consumption rates deduced2 from the chemistry of large rivers[END_REF]. The Deccan Traps were investigated in great details (e.g. [START_REF] Das | Chemical weathering in the Krishna Basin and Western Ghats of the Deccan Traps, India: Rates of basalt weathering and their controls[END_REF][START_REF] Dessert | Erosion of Deccan Traps determined by river geochemistry; impact on the global climate and the (super 87) Sr/ (super 86) Sr ratio of seawater[END_REF][START_REF] Kisakürek | Behaviour of Li isotopes during continental weathering: The Bidar laterite profile, India[END_REF][START_REF] Vigier | Mobility of U-series nuclides during basalt weathering: An example from the Deccan Traps (India)[END_REF], as well as the Siberian Traps (e.g. [START_REF] Pokrovsky | Basalt weathering in Central Siberia under permafrost conditions[END_REF][START_REF] Zakharova | Chemical weathering of silicate rocks in Aldan Shield and Baikal Uplift: Insights from long-term seasonal measurements of solute fluxes in rivers[END_REF]. However, some large basaltic provinces have not been studied yet. For example, weathering of the Ethiopian Traps (upper Blue Nile Basin) is not well constrained, despite the importance of this region in terms of climate, size, tectonics and age (30 Ma). A recent carbon-climate modeling study highlights that the Ethiopian Traps were the world-wide most important hotspot of silicate weathering during the Oligocene [START_REF] Lefebvre | Was the Antarctic glaciation delayed by a high degassing rate during the early Cenozoic? Earth Planet[END_REF]. The invoked reason is that, during this period, the Ethiopian Traps were located within the equatorial latitudes, affected by high precipitation rates and high temperatures, two parameters known to strongly increase basaltic weathering rates [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF][START_REF] Goddéris | Rates of consumption of atmospheric CO2 through the weathering of loess during the next 100 yr of climate change[END_REF]. At present the Ethiopian Traps are influenced by monsoonal climate, with most precipitation occurring during the summer period. The monsoon intensity in North-East Africa fluctuated significantly in the past, with, for instance, particularly high peaks during high insolation periods such as the African Humid period (14.5 to 8 ka, [START_REF] Lamb | 150,000-year palaeoclimate record from northern Ethiopia supports early, multiple dispersals of modern humans from Africa[END_REF][START_REF] Shanahan | The time-transgressive termination of the African humid period[END_REF]). Numerous studies show that monsoon variations controlled physical erosion and the Blue Nile sediment flux during the Quaternary period [START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Revel | 21,000 Years of Ethiopian African monsoon variability recorded in sediments of the western Nile deep-sea fan[END_REF][START_REF] Revel | 100,000 Years of African monsoon variability recorded in sediments of the Nile margin[END_REF]. As shown by lithium isotope data for the Nile Basin [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF] and for the Himalayas [START_REF] Dosseto | Rapid response of silicate weathering rates to climate change in the Himalaya[END_REF], or hafnium isotopes for the Congo Basin [START_REF] Bayon | Use in Iron Age Central Africa[END_REF], continental silicate weathering can respond rapidly to monsoon fluctuations. Thus, the impact of the Ethiopian Traps on the atmosphericocean carbon system deserves to be assessed both at present and in the past.

In fact, from sedimentary records located in the Nile deep sea fan, the importance of physical erosion in this region, and the reactivity of sediment transport to hydro-climate change have extensively been highlighted. Neodymium isotopes measured in the silt fractions [START_REF] Blanchet | Asynchronous changes in vegetation, runoff and Erosion in the Nile River watershed during the Holocene[END_REF], 2013[START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF], 2010) of Quaternary sediments have evidenced covariations of terrigenous flux from the Blue Nile River with monsoon intensity. More recently, [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF] have explored the use of Li isotope composition of the clay fractions to determine paleo-variations of silicate weathering in this region. Indeed, lithium isotope composition of river waters and sediments can be used to characterize the weathering intensity [START_REF] Bouchez | Weathering Intensity in Lowland River Basins: From the Andes to the Amazon Mouth[END_REF], 2013[START_REF] Dellinger | Tracing weathering regimes using the lithium isotope composition of detrital sediments[END_REF], 2015, 2014;[START_REF] Pogge Von Strandmann | Lithium isotope behaviour during weathering in the Ganges Alluvial Plain[END_REF]. Lithium is powerful to trace silicate weathering because it is principally derived from the chemical erosion of silicate minerals [START_REF] Kisakurek | Li and δ7Li in Himalayan rivers: Proxies for silicate weathering? Earth Planet[END_REF][START_REF] Millot | Behaviour of lithium and its isotopes during weathering in the Mackenzie Basin, Canada[END_REF]. During chemical erosion in soils or in weathering profiles, no significant fractionation during rock dissolution can be highlighted [START_REF] Pistiner | Lithium-isotope fractionation during continental weathering processes[END_REF][START_REF] Ryu | Variation of lithium isotope geochemistry during basalt weathering and secondary mineral transformations in Hawaii[END_REF][START_REF] Verney-Carron | Experimental determination of the role of diffusion on Li isotope fractionation during basaltic glass weathering[END_REF][START_REF] Wimpenny | The behaviour of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt[END_REF]. In contrast, large isotope fractionations occur during clay neoformation, in favor of the light 6 Li (into octahedral sites).

At a watershed scale, simple modeling taking into account most recent knowledge on the magnitude of Li isotope fractionation indicate that high clay δ 7 Li may correspond to low ratio of dissolution/clay neoformation rate [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Bouchez | Modeling novel stable isotope ratios in the weathering zone[END_REF], or low weathering intensity [START_REF] Dellinger | Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes[END_REF].

A large amount of Li studies is focused on the river dissolved phases [START_REF] Millot | Behaviour of lithium and its isotopes during weathering in the Mackenzie Basin, Canada[END_REF][START_REF] Pogge Von Strandmann | Lithium isotope behaviour during weathering in the Ganges Alluvial Plain[END_REF][START_REF] Vigier | The relationship between riverine lithium isotope composition and silicate weathering rates in Iceland[END_REF]. However, recently, the potential of river suspended particles [START_REF] Dellinger | Tracing weathering regimes using the lithium isotope composition of detrital sediments[END_REF][START_REF] Dellinger | Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes[END_REF][START_REF] Dellinger | Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion[END_REF][START_REF] Millot | The global control of silicate weathering rates and the coupling with physical erosion. new insights from rivers Can[END_REF], and of sediments deposited in terraces, lake or delta [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Dosseto | Rapid response of silicate weathering rates to climate change in the Himalaya[END_REF] to trace past variations of silicate weathering has also been demonstrated [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Dosseto | Rapid response of silicate weathering rates to climate change in the Himalaya[END_REF][START_REF] Lechler | Lithium-isotope evidence for enhanced silicate weathering during OAE 1a (Early Aptian Selli event)[END_REF]. By studying the Nile Delta during the last 32kyr, [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF] show that clays deposited by the Nile River were mostly smectite formed in the Ethiopian Traps. Their δ 7 Li values co-vary with climate proxies, and in particular with precipitation variations. The controls of lithium isotope composition for clays formed in the Ethiopian Traps are therefore important to investigate further, and should help to refine our understanding of the relationships between climate and basalt weathering.

In this study, we target to provide chemical erosion rates and related CO2 consumption rate in the region of the Blue Nile River, the major river draining the Ethiopian Traps basins. For this, we use the first large dataset of chemical compositions of water and particles from most rivers draining the Ethiopian Traps. The objectives are (1) to study weathering rates, their controls, and their relationships with clay Li isotope composition in this region, and (2) to give an estimation of CO2 consumption rate during an intensive runoff period, well-known and well evidenced in Nile delta sedimentary cores, the African Humid Period (14.5-8kyr).

Study area

The Ethiopian Continental Flood Basalt (CFB) province is located in the North and West Ethiopian highlands (Fig 1 & Fig S1). It covers a surface of about 800,000 km² and has an estimated volume ranging between 1 and 1.5 million km 3 [START_REF] Rochette | Magnetostratigraphy and timing of the Oligocene Ethiopian traps[END_REF]. They erupted 30 Ma ago over a short time period, 1-2 Ma [START_REF] Hofmann | Timing of the Ethiopian flood basalt event and implications for plume birth and global change[END_REF]. The lave pile thickness varies from 500 to 2000 m with maximum thickness of 3000 m in the northern part of the plateau [START_REF] Mohr | The Ethiopian flood basalt province[END_REF]. This CFB province is mainly composed by transitional basalts of low-Ti and high-Ti affinities [START_REF] Pik | Isotopic and trace element signatures of Ethiopian flood basalts: Evidence for plume-lithosphere interactions[END_REF][START_REF] Pik | The northwestern Ethiopian Plateau flood basalts: classification and spatial distribution of magma types[END_REF], and is covered by a rhyolite terminal formation on the western part of the plateau, along the main escarpment [START_REF] Ayalew | Source, genesis, and timing of giant ignimbrite deposits associated with Ethiopian continental flood basalts[END_REF].

The Ethiopian Traps exhibits a mean altitude of ~ 3000 m with flat under lying remnants of the original Oligo-Miocene surface. The volcanoes emplaced on the plateau present a higher elevation up to 4600 meters. The Ethiopian Highlands have been uplifted since at least 25 Ma [START_REF] Pik | Stability of the Upper Nile drainage network (Ethiopia) deduced from (U-Th)/He thermochronometry: Implications for uplift and erosion of the Afar plume dome[END_REF] with concomitant development of the two main basins of the Nile and Tekeze rivers. The plateau presents important border escarpments with steep slopes, especially along the roughly N-S marginal graben on its Eastern side (Fig. 1). These highlands have been dissected by rapid development and propagation of the Nile and Tekeze basins towards its most and early uplifted Eastern edge [START_REF] Pik | Stability of the Upper Nile drainage network (Ethiopia) deduced from (U-Th)/He thermochronometry: Implications for uplift and erosion of the Afar plume dome[END_REF], which acted as the main tectonic drainage divide since Oligo-Miocene times. This tectonic divides is separating the two large westward basins (Nile and Tekeze) from the short and parallel eastward basins developed along the marginal graben after initiation of extension 25 Ma ago [START_REF] Wolfenden | Evolution of the northern Main Ethiopian rift: Birth of a triple junction[END_REF]. This tropical region is submitted to the African monsoon, with a mean annual temperature around 21°C. Thus, 90% of the annual precipitations occur during the summer month (Korecha and Barnston, 2007) (Fig 1). Precipitations display an East-West gradient in Ethiopia: rainfall increase from 200 mm/year on the Indian Ocean to 1800 mm/year in the Highlands. At present, the Blue Nile River (major affluent of the Nile River) and its tributaries drain the Ethiopian Traps. Thus, this high topography region is estimated to contribute to 60 % of the total water discharge of the main Nile and to 60 % of its terrigenous sediments [START_REF] Fielding | A detrital record of the Nile River and its catchment[END_REF][START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF][START_REF] Nyssen | Rainfall erosivity and variability in the Northern Ethiopian Highlands[END_REF]. In the past, during the last 30 kyr, the positioning and the intensity of the monsoon rain belt has oscillated [START_REF] Shanahan | The time-transgressive termination of the African humid period[END_REF], in relation with the precession parameter [START_REF] Tuenter | The response of the African summer monsoon to remote and local forcing due to precession and obliquity[END_REF] which lead to stronger precipitation every 21 kyrs. These strong runoff variations are recorded by Ethiopian lake sediments [START_REF] Amaral | Palynological evidence for gradual vegetation and climate changes during the African Humid Period termination at 13°N from a Mega-Lake Chad sedimentary sequence[END_REF] and by the sediments transported by the Nile River and deposited in its delta [START_REF] Blanchet | Asynchronous changes in vegetation, runoff and Erosion in the Nile River watershed during the Holocene[END_REF][START_REF] Castañeda | Hydroclimate variability in the Nile River Basin during the past 28,000 years[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Revel | 100,000 Years of African monsoon variability recorded in sediments of the Nile margin[END_REF].

Sampling and methods

Sampling

All samples come from the northern Ethiopian plateau (Fig 1), which encompasses the large catchment of the Blue Nile (64 000 km 2 ), several smaller sub-basins (1000 to 6000 km 2 ), as well as small catchments emplaced directly on the top of the plateau (5 to 100 km 2 ). Two sampling campaigns have been carried out in June 2004 (ETH04) and in March 2005 (ETH05). At each sample site, sand, mud and river waters were collected. Waters were filtered on the field with 0.2µm filters and immediately acidified with purified nitric acid.

Sands and muds were sampled on the river banks after mixing 5 different aliquots of about 100g each. The difference between mud and sand is the visible granulometry: sand samples correspond to coarse sized mineral (> 2mm grain) rich-particles and the mud samples to the finest grain deposits. At the laboratory, clay-size fractions were extracted from the mud samples before chemical and isotope analyses (see section 3.3.2).

Runoff Modeling

The hydrology of the Blue Nile upper basins is strongly controlled by seasonal precipitations, with strong rains and flooding observed during the summer monsoon, from June to September (e.g. Conway, 2009). These conditions are not appropriate for providing a reliable estimate of annual discharge and runoff for each studied zone, using classical methods. In order to provide these estimations for each sampled sub-basin, necessary for weathering rate calculations, we used a numerical model based on the IDRISI © GIS software. This model was run using the plateau topography and available public rainfall data. In order to avoid unconstrained biases linked to evapotranspiration, the output has been convincingly calibrated with current discharge data available for the upper Blue Nile basin [START_REF] Conway | A water balance model of the Upper Blue Nile in Ethiopia[END_REF]. These calibrated discharge and runoff data for each sub-basin are presented in Table 1.

Analytical protocol

Chemical analyses

Major and trace element concentrations were analyzed in waters and sediments using ICP-AES and ICP-MS at the SARM National Facilities in Nancy (France) (http://helium.crpg.cnrs-nancy.fr/SARM/index.html). Anions have been measured using ionic chromatography Dionex ICS3000-S with AG11-HC/AS11-HC column. Corresponding uncertainties and detection limits are described in (http://helium.crpg.cnrsnancy.fr/SARM/pages/roches.html).

Lithium isotope analyses of the clay fractions

Clays were extracted from the mud samples, by decantation of the sediments in 50ml of ultrapure water mixed with 20µl of sodium hexametaphosphate (100mg/l). This separation allows to extract particles with a diameter and a density lower than 2 µm and 1.8 respectively. Clay fractions were finely crushed into an agate mortar and 10 mg of the fine powder was mineralized using a concentrated HF/HNO3/HCl solution in a clean laboratory at LOV, using ultrapure acids. The solution was evaporated and the residue dissolved in 1.00N HCl. The total dissolution of samples was systematically verified by the absence of residue after 20 min of centrifuging at 4000rpm.

For major and trace element analyses, an aliquot was diluted 300 times and analyzed by ICP-OES. For lithium isotope analyses, an aliquot containing ≈60ng Li was introduced on a cationic resin column and eluted with 1.00N HCl, following the method published in [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Vigier | Quantifying Li isotope fractionation during smectite formation and implications for the Li cycle[END_REF]. This separation was performed twice and allowed Li recovery and purification. For all samples, the total cationic concentration of the sample was constrained to be less than 10% of the total cationic capacity of the column resin. All seawater and rock reference materials were submitted to the exact same protocol.

Lithium isotope measurements were performed using a Neptune Plus (Thermofisher) multicollector inductively coupled plasma spectrometer (MS-ICP-MS) at the Ecole Nationale Supérieur de Lyon (CNRS National Facilities). Sample was introduced through an Aridus II desolvating system before ionization to increase sensitivity (1V 7 Li / ppb Li). We used the jet and X cones, as described in [START_REF] Balter | Natural variations of lithium isotopes in a mammalian model[END_REF]. To correct for instrumental mass bias, we used the standard bracketing technique, using the LSVEC standard. The total blank procedure is systematically lower than 10pg Li. The accuracy of isotopic analysis was verified by measuring Li7-N reference solution [START_REF] Carignan | Three secondary reference materials for lithium isotope measurements: Li7-N, Li6-N and LiCl-N solutions[END_REF] and various reference materials (BE-N basalt, GH granite and seawater). Without chemical purification, an average δ 7 Li value of 30.2±0.4‰ (2SD, n=32) is obtained for the pure Li7-N standard, in agreement with published and theoretical value. After chemical purification, mean δ 7 Li values are 30.3±0.4‰ (2SD, n=22), 5.12±0.9‰ (2SD, n=5), 1.64 ±0.6‰ (2SD, n=3) and 31.1±0.6‰ (2SD, n=6), for Li7-N, BE-N basalt, GH granite and seawater respectively. All values for reference materials compare well with published values [START_REF] Carignan | Three secondary reference materials for lithium isotope measurements: Li7-N, Li6-N and LiCl-N solutions[END_REF][START_REF] Millot | Accurate and high-precision measurement of lithium isotopes in two reference materials by MC-ICP-MS[END_REF].

Results

Runoff and discharge

As described in section 3.2, we have estimated runoff values using a specific numerical model including published data for rainfall and discharge, and regional topography. The runoff values determined for 19 sub-basins range from 190 to 273 mm/yr with a mean value of 242 mm/yr. The corresponding calculated discharges range from 11x10 5 m 3 /yr (for the Ulet Wenz River) to 1.8x10 10 m 3 /yr (for the Blue Nile river), with a mean value of 19x10 8 m 3 /yr (Table 1).

For the Blue Nile river, the calculated discharge is close to the annual discharge measured at the Roseires Dam Station (http://www.grdc.sr.unh.edu/html/Polygons/P1663800.html) :

4.7x10 10 m 3 /yr. The estimated average runoff value of 242 mm/yr for the Ethiopian Traps is in line with the value published in ( [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF], 129 mm/year). These two comparisons indicate that our model estimates representative values for the runoff and river discharge.

The average runoff for this region is two times lower than the estimated value for the Deccan Traps (463 mm/year). It is also lower than the one determined for the Siberian Traps (324 mm/yr). Compared to small tropical basaltic areas such as Hawaii (1612 mm/yr) and the Reunion Island (2433 mm/yr), the Ethiopian runoff is also low. This is partly due to their topography, their small surface area compared to the Ethiopian Traps (800,000 km² vs 16760 km² in Hawaii and 2512 km² in the Reunion Island), and the occurrence of frequent hurricanes and tropical storms. In Iceland, high runoff (1883-2432 mm/yr) values are mostly due to ice melting, which introduces water in the riverine system during the summer season [START_REF] Gislason | Direct evidence of the feedback between climate and weathering[END_REF].

Major and trace elements

River waters

In waters, the dominant cations are, in increasing order of concentrations: Si, Mg, Na, and Ca, with mean values of 447 (± 219), 1768 (± 502), 712 (± 332) and 883 (± 417) µmol/l respectively. These concentrations are corrected from the atmospheric input using Cl concentration and oceanic X/Cl molar ratios, following the method detailed in [START_REF] Berner | The Global Water Cycle[END_REF]. After corrections the concentration of Mg*, Na*, and Ca* are 880 (± 415), 68 (± 26) and 574 (± 285) µmol/l respectively.

On average, molar Ca*/Na* and Mg*/Na* ratio are 2.5 (± 0.9) and 0.4 (± 0.6), respectively (Table 2). For comparison, average Ca/Na and Mg/Nain Ethiopian basalts are 3.9 (± 1.3) and 3.2 (± 2.7) respectively, and in the Ethiopian rhyolite are 0.07 (± 0.06) and 0.03 (± 0.04).

When compared with other regions of similar lithologies, river water chemical composition displays roughly a single trend between basalts and rhyolites (Fig. 2). From this trend, it can be estimated that the dissolved contribution coming from rhyolite weathering is low (<15%) compared to basaltic rocks.

The dominant anions are, in increasing order of concentrations: HCO3 -SO4 2-, Cl -, NO3 -, and F, with mean values of 3170 (406 to 6039) 202 (15 to 880), 156 (15 to 348), 71 (4 to 292) and other rivers draining basaltic regions (e.g. [START_REF] Dessert | Erosion of Deccan Traps determined by river geochemistry; impact on the global climate and the (super 87) Sr/ (super 86) Sr ratio of seawater[END_REF].

River bed particles

Sands are enriched in SiO2 compared to mud samples (Table 3), (51.9 ± 10.6 % on average vs 43.2 ± 2.6 % for muds) and Na2O (2.2 ± 0.8 % vs 1.1 ± 0.5 %), while depleted in Al2O3 (11.7 ± 3.4 % vs 13.0 ± 3.5 % in mud), CaO (7.0 ± 3.4 % vs 4.7 ± 2.1 % in mud) and Fe2O3 (11.4 ± 3.4 % vs 13.5 ± 1.5%). TiO2 and MgO (5.7 ± 4.0 % vs 5.8 ± 3.1%) concentrations are similar between sands and muds: (≈3.1 ± 1.4 wt % and ≈5.8 ± 3.1 wt % respectively). Si/Al ratio is lower by ≈30% in muds compared to sands, which is consistent with occurrence of aluminosilicate minerals (clays/smectite) in the collected muds. The chemical composition of <2µm clay fractions extracted from mud samples, contains, on average, 24.2% (± 4.7 %) Al2O3, 22.2% (± 1.8 %) Fe2O3, and 15.1% (± 6.9 %) MgO. We note that the mud samples collected in river banks are not representative of the clay fraction separated at the laboratory and rather reflects a mixing between clays and primary phases. Thus, clay fractions (<2µm) were preferentially used for assessing weathering processes in this region. Some trace elements measured in the solid phases can be used to study chemical erosion degree or intensity [START_REF] Louvat | Present denudation rates on the island of réunion determined by river geochemistry: Basalt weathering and mass budget between chemical and mechanical erosions[END_REF]P Louvat and Allègre, 1998) 2 and3), when compared to bedrocks, Ethiopian river waters display elevated U/Th and Sr/Th ratios (respectively >2 and > 8405) while the solid phases display low U/Th and Sr/Th values (respectively <0.3 and <87.8). Sr and U are two mobile elements during water-rock interactions and are usually depleted in the solid phase, in contrast with Th, which is an immobile element that can easily adsorb, except in organic-rich environments [START_REF] Dupré | Major and trace elements associated with colloids in organic-rich river waters: Ultrafiltration of natural and spiked solutions[END_REF]. Figure 3 also indicates that the river particles are mostly derived from basaltic rocks, as previously described, and that the influence from rhyolite weathering is minimal, except for one sample (Nazero Wenz), which is located in a rhyolite rich area (Figure S1).

Lithium isotope compositions of clay size (<2µm) fractions

Ethiopian bedrock samples exhibit a mean δ 7 Li value of 5±0.5‰ (n=3) [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF] (Fig. 4), which compares well with the range published for intraplate basalts. Compared to these rocks, or to tholeiitic basalts (3.1 to 5.2 ‰, [START_REF] Elliott | Lithium isotope evidence for subductionenriched mantle in the source of mid-ocean-ridge basalts[END_REF]), and to OIB basalts (3.1 -5.2 ‰, [START_REF] Genske | Lithium and boron isotope systematics in lavas from the Azores islands reveal crustal assimilation[END_REF]) clays deposited by Ethiopian rivers are significantly enriched in 6 Li: clay δ 7 Li values range from 0.3‰ (± 0.5‰) to 4‰ (± 0.5‰) , with a mean value of 2.0‰ (± 1.2‰). This is consistent with preferential enrichment in light lithium isotope during smectite neoformation, as seen experimentally [START_REF] Vigier | Quantifying Li isotope fractionation during smectite formation and implications for the Li cycle[END_REF] and in basaltic soils [START_REF] Huh | Behavior of lithium and its isotopes during weathering of Hawaiian basalt[END_REF][START_REF] Kisakürek | Behaviour of Li isotopes during continental weathering: The Bidar laterite profile, India[END_REF][START_REF] Pistiner | Lithium-isotope fractionation during continental weathering processes[END_REF][START_REF] Ryu | Variation of lithium isotope geochemistry during basalt weathering and secondary mineral transformations in Hawaii[END_REF].

In the literature, a large range of δ 7 Li values is observed in suspended particles (SPM) of rivers draining basaltic regions e.g. in Iceland (with values from -1.3 to 7.5‰, [START_REF] Von Strandmann | Riverine behaviour of uranium and lithium isotopes in an actively glaciated basaltic terrain[END_REF]) and in the Azores archipelago (3.5 to 9.5‰, [START_REF] Von Strandmann | Assessing the role of climate on uranium and lithium isotope behaviour in rivers draining a basaltic terrain[END_REF]). Also in soils, the range of published δ 7 Li for bulk soils samples is large (-15 to 10‰, [START_REF] Clergue | Influence of atmospheric deposits and secondary minerals on Li isotopes budget in a highly weathered catchment, Guadeloupe (Lesser Antilles)[END_REF][START_REF] Pistiner | Lithium-isotope fractionation during continental weathering processes[END_REF][START_REF] Ryu | Variation of lithium isotope geochemistry during basalt weathering and secondary mineral transformations in Hawaii[END_REF][START_REF] Schmitt | Processes controlling the stable isotope compositions of Li, B, Mg and Ca in plants, soils and waters: A review[END_REF][START_REF] Von Strandmann | Lithium, magnesium and silicon isotope behaviour accompanying weathering in a basaltic soil and pore water profile in Iceland[END_REF]). These observed variations are possibly due to mixing between clay and primary minerals, because these samples are not necessarily representative of clay pure endmember [START_REF] Dellinger | Lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion[END_REF]. Here, clays were extracted from river sediments in order to minimize the influence of primary minerals.

Discussion

Assessing weathering processes at the basin scale

As described in section 4.2, chemical and isotopic compositions of river waters and particles can give indications on weathering rates and/or intensity at the watershed scale. Some elements are particularly mobile during basalt alteration (e.g. Sr, K, Na), and preferentially lost from weathering profiles after a short period of time [START_REF] Chadwick | Changing sources of nutrients during four million years of ecosystem development Terrestrial biogeochemists traditionally distinguish atmospherically derived from rock-derived elements[END_REF][START_REF] Ryu | Variation of lithium isotope geochemistry during basalt weathering and secondary mineral transformations in Hawaii[END_REF]. Thus, as described in section 4.2, Sr and U are depleted in sands and mud samples, while enriched into the dissolved phases (Fig. 2) of the corresponding rivers. Also, Na, Ca and Mg concentrations are typical of waters draining basaltic rocks (Fig. 3). However, using elementary ratios, or elementary concentrations, it is often difficult to distinguish the role of lithology from the one of dissolution or leaching process, because of the chemical diversity of the drained bedrocks. As described in previous works [START_REF] Pistiner | Lithium-isotope fractionation during continental weathering processes[END_REF][START_REF] Vigier | Quantifying Li isotope fractionation during smectite formation and implications for the Li cycle[END_REF][START_REF] Wimpenny | The behaviour of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt[END_REF], lithium isotopes provide a complementary mean to examine chemical erosion processes [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Dellinger | Riverine Li isotope fractionation in the Amazon River basin controlled by the weathering regimes[END_REF]. [START_REF] Bastian | Thèse de doctorat Bastian Luc Impact des variations de la mousson Africaine sur l' érosion chimique des silicates dans le bassin versant du Nil depuis[END_REF], have analyzed 55 clay fractions extracted from Nile deep sea fan core sediments. Their δ 7 Li values vary between 4 and -1.2 ‰ during the last 32,000 years. In their study, clay δ 7 Li were related to the ratio between dissolved Li flux and the flux of Li incorporated in the secondary phases (F /F ). In rivers currently draining the Ethiopian Traps, clay fractions display variable δ 7 Li values, from 0.3‰ to 4.0 ‰, a range that is similar to the 0-32 kyr clays deposited in the Nile delta. Clay δ 7 Li and Mg/Ti ratios display a positive correlation (shown in fig. 4), which suggests that both proxies are impacted by similar processes during basalt weathering. This is not surprising since Li + essentially substitutes to Mg 2+ during smectite formation [START_REF] Decarreau | Partitioning of lithium between smectite and solution: An experimental approach[END_REF][START_REF] Vigier | Quantifying Li isotope fractionation during smectite formation and implications for the Li cycle[END_REF], and smectite is the most abundant phase carried by rivers draining the Ethiopian Traps [START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF].

Using a simple modeling, based on mass balance, clay δ 7 Li values have been shown to be essentially explained by the ratio of leaching versus clay formation rates, and thus reflect different weathering intensities [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Bouchez | Modeling novel stable isotope ratios in the weathering zone[END_REF][START_REF] Vigier | The relationship between riverine lithium isotope composition and silicate weathering rates in Iceland[END_REF][START_REF] Vigier | A new approach for modeling Cenozoic oceanic lithium isotope paleo-variations: The key role of climate[END_REF][START_REF] Von Strandmann | Lithium, magnesium and silicon isotope behaviour accompanying weathering in a basaltic soil and pore water profile in Iceland[END_REF]. The behavior of Li isotopes at the scale of a watershed or of a continent has been described with the following equations, considering a simple mass balance between bedrock, rivers and soils at the watershed scale. It is defined such that all the lithium released by rock dissolution is either incorporated into secondary phases formed in weathering profiles or released into superficial waters (and in fine in rivers).

F δ F δ F δ equation 1
where F , F , and F are the Li flux related to dissolution, secondary phase precipitation and soil water outflow respectively. δ , δ , δ are the Li isotopic signature (δ 7 Li value) of the dissolved phase, the secondary phase, the river water and the bedrock, respectively. Since basalt dissolution is isotopically congruent, δ = δ !"# [START_REF] Verney-Carron | Experimental determination of the role of diffusion on Li isotope fractionation during basaltic glass weathering[END_REF][START_REF] Wimpenny | The behaviour of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt[END_REF]. The isotope fractionation between dissolved phase and the secondary phases is noted Δ %&'( . After simplification, the isotope composition of the secondary phases is given by:

δ Δ %&'( F F ) δ *+(,-./ 0 Δ %&'( Equation 2
This equation states that if F 3 is close to F 3 , then δ δ *+(,-./ . If F 3 is low compared to F 3 , then δ Δ %&'( .

The positive correlation between clay δ 7 Li and Mg/Ti cannot be explained by a binary mixing between bedrocks and Li-rich clays, as illustrated in Figure 4. Thus, a similar mass balance between rocks and weathering products, as for δ 7 Li, was applied to clay Mg/Ti. For solving it, we assumed that no Ti is released into water during dissolution (due to the high immobility of this element during water/rock interactions) (Ti 0 . Thus Mg/Ti ratio of secondary phases can be described as:

Mg 89 : ;< 1 F => F => ) . 89 " @ A B CD E!"# ) Equation 3
The relation between the Li and the Mg flux becomes:

F F ) . G F => F => ) Equation 4
with β reflecting the Li/Mg ratio of the clay fraction (normalized to the source rock one). Following our modeling, the various δ 7 Li and Mg/Ti values are best explained by variations of the weathering intensity, leading also to a large range of clay Li/Mg (β parameter). For a large part, the β values are greater than 1 (Fig. 4). This indicates that Li is more retained in the weathering profiles than Mg (compared to source rocks). This is consistent with observations made in various Hawaiian weathering profiles where kaolinite prevail [START_REF] Ryu | Variation of lithium isotope geochemistry during basalt weathering and secondary mineral transformations in Hawaii[END_REF].

As shown in

Thus, the trend between clay δ 7 Li and clay Mg/Ti is likely due to variable weathering conditions. However, no clear relationship with the runoff, the topography or vegetation can be highlighted for this group of rivers. Since Mg is involved in the vegetation cycle, and Li is not, perhaps a loss of Mg in relation with recent plant and root activity can also explain part of this variability. More work would certainly need to be undertaken in the most vegetated areas of this region since no convincing correlation between δ 7 Li or Mg/Ti and physical parameters such as the slope, runoff, vegetation and land use can be highlighted here. This suggests that, at present, basalt weathering and soil production rate in the Blue Nile Basin are variable in space, and that this variation is likely to be a complex function of several parameters.

Weathering rates and controls in the Upper Blue Nile Basin

As discussed above, the relatively large range of clay Mg/Ti and δ 7 Li reflects a significant variation of weathering conditions in the Ethiopian Traps region while the lithology is dominantly basaltic. It is possible to use the chemical composition of river waters in order to quantify present-day weathering rates (Table 2) at the scale of each corresponding watershed (e.g. [START_REF] Gaillardet | Global silicate weathering and CO consumption rates deduced2 from the chemistry of large rivers[END_REF][START_REF] Stallard | Geochemistry of the Amazone[END_REF]. The Total Dissolved Solid (TDS) is defined by the sum of major mobile cations (Ca, Mg, K and Na) concentrations in the dissolved phases of rivers. These concentrations are corrected from the atmospheric input using Cl concentration and oceanic X/Cl molar ratios, following the method detailed in [START_REF] Das | Chemical weathering in the Krishna Basin and Western Ghats of the Deccan Traps, India: Rates of basalt weathering and their controls[END_REF]. TDS* values (* indicate corrected from atmospheric inputs) estimated from concentrations measurement in the dissolved phase of the Blue Nile Basin rivers range from 9.5 to 111.0 mg/l, with a mean value of 67 mg/l. In the diagram TDS* versus temperature, the Blue Nile tributaries plot along the same trend defined by all other basaltic regions studied thus far (Fig. 5a). This first indicates the good representative nature of our sampling and methods. The average TDS* for the Ethiopian Traps is as high as the one determined for the Deccan Traps (54 mg/l) and for Java Island (43 mg/l) [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF][START_REF] Li | Temperature dependence of basalt weathering[END_REF], which are also characterized by elevated annual temperature (27°C and 24.8 °C respectively).

These three regions display river TDS* values significantly greater than in Hawaii (7.4 mg/l) and in Iceland (8.8-10.3 mg/l), displaying both lower surface temperatures on average (16 and 2 °C respectively). This trend confirms that, for basaltic basins, temperature is a key parameter which controls river TDS* [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF][START_REF] Li | Temperature dependence of basalt weathering[END_REF].

Using the TDS* values, the estimated annual discharge, and the watershed area, it is possible to determine silicate weathering rates. These rates in the Ethiopian Traps basins range from 2.6 to 24.4 tons/km 2 /year, with a mean value of 16.1 tons/km 2 /year (Table 2). For the Blue Nile River, sampled at the output of the Ethiopian Traps (see map Fig. 1), the silicate weathering rate is 11.3 tons/km 2 /year. The mean value estimated for chemical erosion rate in the Ethiopian Traps, based on 17 data (Table 2), is consistent with the positive correlation previously observed with the runoff (see Fig. 5b, [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF]). In contrast to TDS*, weathering rates in Ethiopia are low compared to other regions of similar lithology. For instance, the weathering rate in the Deccan Traps is 25.1 tons/km 2 /year, 48.4 tons/km 2 /year for La Reunion, and 152 tons/km 2 /year for Java [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF]. Lower runoff in the Ethiopian Traps (242 mm/years) compared to these regions is certainly a key factor. However, other parameters may also play a role, in particular temperatures, the age of the basaltic rocks [START_REF] Li | Temperature dependence of basalt weathering[END_REF], the soil age [START_REF] Ryu | Variation of lithium isotope geochemistry during basalt weathering and secondary mineral transformations in Hawaii[END_REF][START_REF] Vigier | Mobility of U-series nuclides during basalt weathering: An example from the Deccan Traps (India)[END_REF], and formation of glassy material in volcanically active area (e.g. Java Island and Iceland) [START_REF] Gislason | Direct evidence of the feedback between climate and weathering[END_REF][START_REF] Gislason | Mechanism, rates, and consequences of basaltic glass dissolution: II. An experimental study of the dissolution rates of basaltic glass as a function of pH and temperature[END_REF][START_REF] Vigier | The relationship between riverine lithium isotope composition and silicate weathering rates in Iceland[END_REF].

Overall, both river particles and water composition can provide valuable information on chemical erosion rate in the various basins. However here there is no correlation between river particles and water chemical compositions. This is likely to be due to a recent decoupling between soil (or weathering profile) formation and river aqueous phase. Indeed, large soil reservoir where clays have formed can encompass different weathering conditions, issued from past climate variations, and may therefore not be representative of the current state. Recent observations in the Nile deltaic sediments show that both chemical and physical erosion responded rapidly to hydro-climate change in the last 30kyr [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF].

This argues for a residence time of clay-rich sediments that is not above a few thousand years at most, in coherence with elevated topography of the Traps where clays mainly come from.

The observed decoupling between clays and waters must then have occurred just recently, perhaps due to intensification of anthropic activities in this region [START_REF] Lanckriet | The Late-Holocene geomorphic history of the Ethiopian Highlands: Supportive evidence from May Tsimble[END_REF]

CO2 consumption and SO4 2-concentration due to silicate weathering

From river water chemical compositions, it is possible to estimate the CO2 consumption rates related to silicate weathering. In the literature, several methods have been used, using corrections from source rocks in mixed lithology basins, calcium concentration in silicate rivers [START_REF] Goddéris | Rates of consumption of atmospheric CO2 through the weathering of loess during the next 100 yr of climate change[END_REF], or HCO3 -concentrations in carbonate-free watersheds (e.g. [START_REF] Das | Chemical weathering in the Krishna Basin and Western Ghats of the Deccan Traps, India: Rates of basalt weathering and their controls[END_REF]. Since the Ethiopian Traps are essentially constituted of silicate rocks, with little vegetated areas, major cations concentrations in rivers have only been corrected for atmospheric inputs, as described in section 5.1. HCO3 -concentrations are determined using the following electronic mass balance:

[HCO M N ] 2[Ca PQ ] [K Q ] 2[Mg PQ ] 0 2[SO T NP ] 0 [F N ] 0 [NO M N ] VWXYZ9[\ 5
where all concentrations are corrected for atmospheric inputs (see section 4.2.1). The average pH of the Ethiopian rivers (7.5-8.2, (MERID, 2005)) makes it to possible to assume that HCO3 -is the dominant anion compared to CO3 2-. Thus, in order to determine the CO2 consumption related to silicate weathering, we consider that 1 mole of HCO3 -exported by rivers corresponds to 1 mole of consumed atmospheric CO2. CO2 consumption rates are then calculated using the river discharge and the basin surface area (for specific rates). Some recent studies [START_REF] Calmels | Sustained sulfide oxidation by physical erosion processes in the Mackenzie River basin: Climatic perspectives[END_REF][START_REF] Li | Sulfuric acid as an agent of carbonate weathering constrained by δ13CDIC: Examples from Southwest China[END_REF][START_REF] Torres | The acid and alkalinity budgets of weathering in the Andes-Amazon system: Insights into the erosional control of global biogeochemical cycles[END_REF] show that sulfate mineral dissolution produces sulfuric acid that can enhance bedrock and mineral dissolution, in particular in sedimentary or carbonate rich context. Any silicate weathering due to sulfuric acid may lead to an overestimation of the CO2 consumption rate if this effect is not considered into the calculations. In the rivers studied here, dissolved sulfate concentrations do not correlate to dissolved nitrate but rather with elements that are mobile during silicate dissolution (for example Ca and Mg). This suggests that sulfates are not significantly related to fertilizer addition but may mostly derive from weathering and oxidizing processes. This is not surprising since this region is not intensively cultivated, and is also supported by small amounts of nitrate in waters (see section 4.2.1). The use of Equation ( 5) to calculate HCO3 - allows us to avoid a potential bias due to sulfuric acid in the determination of CO2 consumption rate.

Table 2 show that specific CO2 consumption rate due to basalt dissolution in Ethiopian Traps vary from 0.11x10 6 mol/km²/yr (for the Ulet-Wenz basin) to 1.33 x10 6 mol/km²/yr (for the Ketchiin Abeba Wenz), and is 0.76x10 6 mol/km²/yr on average. The CO2 consumption rate cover a large range, possible due to local -small scale -effects. Thus, for the comparison with literature data, we use the two values estimated for the Blue Nile River, which integrates most waters draining the Ethiopian Traps. The Blue Nile River was sampled at two different seasons (ETH04-2; Juin-04 and ETH05-52; Mars -05). The two samples lead to similar CO2 consumption rate, of 0.49 and 0.52 mol/km²/yr respectively, but are low compared to other basaltic regions. For comparison, this flux is estimated to be 1.26 10 6 mol/km²/yr for the Deccan Traps and 6.41 10 6 mol/km²/yr for Java Island [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF]. The CO2 consumption rate due to silicate weathering in the Blue Nile basin is slightly higher, than the value published by [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF] for the Ethiopian Traps, i.e. 0.15x10 6 mol/km²/yr.

When using the [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF] theoretical equation fCO2 = Rf ₓ323.44 exp(0.0642 T)

where fCO2, Rf and T are respectively the specific atmospheric CO2 consumption (mol/km²/year), the runoff (mm/year) and the temperature (°C), the corresponding consumption rate is also slightly lower (0.16x10 6 mol/km²/year) that our calculated value.

In order to determine the relative role of the Blue Nile Basin in the present-day carbon cycle, one needs to estimate its total CO2 consumption flux (in mol/year). The average CO2 consumption rate is calculated to be 0.65x10 12 mol/year. This is significantly higher than in Iceland (0.070 x10 12 mol/year), the Reunion Island (0.006 x10 12 mol/year) or the Azores (0.0004 x10 12 mol/year) (see Fig. 6). However, this value remains lower than for the Deccan (0.392 x10 12 mol/year) or than SE Asia (1.033 x10 12 mol/year). The upper Blue Nile represent a CO2 consumption close to the global average ( [START_REF] Gaillardet | Global silicate weathering and CO consumption rates deduced2 from the chemistry of large rivers[END_REF], see Fig. 6), and only 7.5 % of the total CO2 consumed by silicate weathering on Earth. It corresponds to 7.8.10 3 Gt of carbon consumed by year, which is the equivalent of 0.4% of the total human carbon emission per year. Our results demonstrate that, at present, the Ethiopian Traps weathering has a negligible impact on the carbon cycle, and thus on global climate.

In the past, this could however be different. First, as described in the Introduction, the modeling study from [START_REF] Lefebvre | Was the Antarctic glaciation delayed by a high degassing rate during the early Cenozoic? Earth Planet[END_REF], show that, during the Oligocene, the Ethiopian Traps was likely the most important region on Earth in terms of silicate weathering. More recently, since 100kyr, the North East African region is known to have been submitted to extreme variations of precipitation and runoff. The monsoon rain belt oscillated together with the precession cycle of 21,000 years. This induced succession of "humid" and "arid" periods that affected sediment flux and physical erosion in this region. This is evidenced in lakes where arid periods are characterized by hiatus or paleo-soils in the sedimentary records and water level variations (for lake Turkana, lake Abhé [START_REF] Forman | Variations in water level for Lake Turkana in the past 8500 years near Mt. Porr, Kenya and the transition from the African Humid Period to Holocene aridity[END_REF][START_REF] Garcin | Late Pleistocene-Holocene rise and collapse of Lake Suguta, northern Kenya Rift[END_REF][START_REF] Hoelzmann | Hydrological response of an east-Saharan palaeolake (NW Sudan) to early-Holocene climate[END_REF]). This is also observed in the marine cores of the Nile deep-sea-fan, with much higher sedimentation rates during humid periods [START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Revel | 21,000 Years of Ethiopian African monsoon variability recorded in sediments of the western Nile deep-sea fan[END_REF][START_REF] Revel | 100,000 Years of African monsoon variability recorded in sediments of the Nile margin[END_REF].

Based on both observations and modeling, it is possible to provide first extrapolation of our results to some critical period of the Quaternary period. Indeed, recent studies illustrate that during this time, silicate weathering rates were able to respond rapidly to climate change [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Dosseto | Rapid response of silicate weathering rates to climate change in the Himalaya[END_REF][START_REF] Rothacker | Impact of climate change and human activity on soil landscapes over the past 12,300 years[END_REF]. This region was submitted to large changes of the African Monsoon intensity [START_REF] Shanahan | The time-transgressive termination of the African humid period[END_REF] while runoff likely plays a key role on basalt weathering and related CO2 consumption ( [START_REF] Goddéris | Rates of consumption of atmospheric CO2 through the weathering of loess during the next 100 yr of climate change[END_REF], see also Fig. 5b and8). Since 6 kyr, the climate in the Ethiopian Traps is characterized by relatively low monsoonal precipitation and low runoff. However, between 15 and 8 kyr, during the African Humid Period (AHP), this region was submitted to much higher precipitation rates [START_REF] Blanchet | High-and low-latitude forcing of the Nile River regime during the Holocene inferred from laminated sediments of the Nile deep-sea fan[END_REF][START_REF] Revel | 21,000 Years of Ethiopian African monsoon variability recorded in sediments of the western Nile deep-sea fan[END_REF]. Climate modeling studies, using LMDZ and ORCHIDEE models, determined an increase of precipitation in the Ethiopian Traps of 1.4 to 2.5 times larger during the AHP ( [START_REF] Marzin | The role of the ocean feedback on Asian and African monsoon variations at 6 kyr and 9.5 kyr BP[END_REF]Vadsaria and Ramstein, Pers. Comm.). This corresponds to a runoff ranging between 340 and 600 mm/years. By applying this factor, CO2 consumption rate during African Humid Period may thus have increased up to values of 0.91-1.5x10 12 mol/yr (brown bar in Fig. 6), which corresponds to the most elevated current rates. These estimations are in line with the study of [START_REF] Lefebvre | Was the Antarctic glaciation delayed by a high degassing rate during the early Cenozoic? Earth Planet[END_REF], which showed that the importance of this region in the carbon cycle during the Oligocene period. However, it is important to note that during the Oligocene, the global temperature was also higher, while for the AHP, the runoff increase in North Africa is likely not related to a significant increase of temperature [START_REF] De Boer | Rapid succession of plant associations on the small ocean island of Mauritius at the onset of the Holocene[END_REF].

These results also suggest a potential increase of global CO2 consumption during more humid periods due to increase of silicate weathering. Indeed, other large basaltic regions (e.g.

Deccan, Parana) are also located in the tropical zone influenced by monsoon. This is likely a key for understanding past rapid climate change and the influence of tropical basins on the carbon cycle.

Conclusion

This study examines the chemical composition of river water and sediments in the Ethiopian In the recent or the remote past, such as during the African Humid Period (from 14.500 to 8.000 years BP) or during the Oligocene (30 Ma) when this region was under equatorial latitudes, the runoff was increased to much higher values that modelers have estimated. Based on these results and weathering models, we find comparable results for CO2 consumption rate for both periods. The determined CO2 consumption rate for the African Humid Period range between 0.91 and 1.5.10 12 mol/yr. This indicates that during humid periods the Ethiopian traps could have played a significate role on carbon cycle and on climate.

Overall, this study brings out the key role of a region that is understudied thus far in terms of weathering and climate paleo-variations. More generally, the influence on global climate of tropical basins influenced by monsoon would deserve to be assessed more precisely, in particular those of basaltic lithology. for Gondar, Bahar Dar, Kombolcha and Addis Ababa, between 1991and 1997for Fiche and between 1994and 1997 for Lalibellathe. [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF].

Bedrock data come from [START_REF] Pik | Isotopic and trace element signatures of Ethiopian flood basalts: Evidence for plume-lithosphere interactions[END_REF]. [START_REF] Benedetti | Water-rock interactions in tropical catchments: field rates of weathering and biomass impact[END_REF][START_REF] Dessert | Erosion of Deccan Traps determined by river geochemistry; impact on the global climate and the (super 87) Sr/ (super 86) Sr ratio of seawater[END_REF][START_REF] Gaillardet | Global silicate weathering and CO consumption rates deduced2 from the chemistry of large rivers[END_REF][START_REF] Gislason | Chemical weathering of basalt as deduced from the composition of precipitation, rivers and rocks in SW Iceland[END_REF][START_REF] Leemans | The IIASA Database for Mean Monthly Values of Temperature, Precipitation, and Cloudiness on the Global Terrestrial Grid[END_REF][START_REF] Louvat | Etude géochimique de l'érosion fluviale d'iles volcaniques à l'aide des bilans d'éléments majeurs et traces[END_REF]Pascale Louvat and Allègre, 1998;[START_REF] Louvat | Present denudation rates on the island of réunion determined by river geochemistry: Basalt weathering and mass budget between chemical and mechanical erosions[END_REF][START_REF] Meybeck | Composition chimique des ruisseaux non pollués de France[END_REF][START_REF] Négrel | Erosion sources determined by inversion of major and trace element ratios and strontium isotopic ratios in river water: the Congo Basin case[END_REF][START_REF] Négrel | Natural and anthropogenic budgets of a small watershed in the Massif Central (France): chemical and strontium isotopic characterization of water and sediments[END_REF][START_REF] Riotte | U colloidal transport and origin of the 234U/238U fractionation in surface waters: new insights from Mount Cameroon[END_REF] recent Nile delta fan sediments, [Li] = 100 ppm [START_REF] Bastian | Thèse de doctorat Bastian Luc Impact des variations de la mousson Africaine sur l' érosion chimique des silicates dans le bassin versant du Nil depuis[END_REF][START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF].

River numbers are the same as in figure 1. 3), and the black circles come from the compilation of [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF] [START_REF] Benedetti | Water-rock interactions in tropical catchments: field rates of weathering and biomass impact[END_REF][START_REF] Dessert | Erosion of Deccan Traps determined by river geochemistry; impact on the global climate and the (super 87) Sr/ (super 86) Sr ratio of seawater[END_REF][START_REF] Gaillardet | Global silicate weathering and CO consumption rates deduced2 from the chemistry of large rivers[END_REF][START_REF] Gislason | Chemical weathering of basalt as deduced from the composition of precipitation, rivers and rocks in SW Iceland[END_REF][START_REF] Leemans | The IIASA Database for Mean Monthly Values of Temperature, Precipitation, and Cloudiness on the Global Terrestrial Grid[END_REF][START_REF] Louvat | Etude géochimique de l'érosion fluviale d'iles volcaniques à l'aide des bilans d'éléments majeurs et traces[END_REF]Pascale Louvat and Allègre, 1998;[START_REF] Louvat | Present denudation rates on the island of réunion determined by river geochemistry: Basalt weathering and mass budget between chemical and mechanical erosions[END_REF][START_REF] Meybeck | Composition chimique des ruisseaux non pollués de France[END_REF][START_REF] Négrel | Erosion sources determined by inversion of major and trace element ratios and strontium isotopic ratios in river water: the Congo Basin case[END_REF][START_REF] Négrel | Natural and anthropogenic budgets of a small watershed in the Massif Central (France): chemical and strontium isotopic characterization of water and sediments[END_REF][START_REF] Riotte | U colloidal transport and origin of the 234U/238U fractionation in surface waters: new insights from Mount Cameroon[END_REF] and corresponds to most other basaltic areas studied thus far. (B) Plot of chemical erosion rate as a function of runoff (average based on 17 data, see Table 3). The blue diamond reflects the Ethiopian Traps (this study) and the other symbols come from [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF] and display other basaltic basins with active zones (in black) and inactive zone (in white). The dashed lines correspond to the TDS* concentrations. for the African Humid Period (AHP, see text for more details, and the grey one is for the Oligocene and was published by [START_REF] Lefebvre | Was the Antarctic glaciation delayed by a high degassing rate during the early Cenozoic? Earth Planet[END_REF].

  . In the diagram Sr/Th versus U/Th (Fig 3, Table

Figure 4 ,

 4 some basins are characterized by low clay δ 7 Li and low Mg/Ti values, (ie Jara Wenz (ETH05-11), Borkena Wenz (ETH05-13), Robi Wenz (ETH05-4), Meri Wenz (ETH05-28), Tikur Wenz (ETH05-19)). For these small basins, the weathering model indicates that soil formation rate (Fsp) is low compared to the leaching rate (Flea). Other areas display high clay δ 7 Li values associated to high Mg/Ti ratios (Ketching Abeba Wenz (ETH05-25), Gereb Ara Wenz (ETH05-40), Tekeze (ETH0535), Hormat Wenz (ETH05-47), Ala-Wiha (ETH05-20), Nazero Wenz (ETH05-6)).

Fig 1 :

 1 Fig 1: A. Map showing samples localization and the studied area. Sample name on the map are: 1: Jara Wenz (ETH05-11); 2:Borkena Wenz (ETH05-13) ; 3: Robi Wenz (ETH05-4) ; 4: Meri Wenz (ETH05-28) ; 5: Tikur Wenz (ETH05-19) ; 6: Ketchin Abeba Wenz (ETH05-25) ; 7: Gereb Ara Wenz (ETH05-40) ; 8: Tekeze (ETH05-35) ; 9: Hormat Wenz (ETH05-47) ; 10: Ala-Wiha (ETH05-20) ; 11: Nazero Wenz (ETH05-6) .B. Measured rainfall (mm/week) temporal variations in various places of the Ethiopian Traps. Reported values corresponds to the average of rainfall between1961 and 1997 for Gondar, Bahar Dar, Kombolcha and Addis Ababa, between 1991 and 1997 for Fiche and between 1994 and 1997 for Lalibellathe.

Fig 2 :

 2 Fig 2: Mg/Na molar ratio as a function of Ca/Na molar ratio, for river dissolved phases. Both ratios are corrected from atmospheric inputs as described in section 4.2.1. The blue circles are for samples analyzed in this study. Orange circles are for other basaltic regions reported in[START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF].

Fig 3 :

 3 Fig 3: Sr/Th as a function of U/Th (wt ratios) in the dissolved phases and solid phases of the sampled rivers. Bedrock values come from[START_REF] Pik | Isotopic and trace element signatures of Ethiopian flood basalts: Evidence for plume-lithosphere interactions[END_REF]

Fig 4 :

 4 Fig 4: Clay δ 7 Li as a function of clay Mg/Ti for the main Blue Nile Basin rivers (blue diamonds). Green lines correspond to the result of the model described in section 5.1. for different β values. The dotted line corresponds to the theoretical mixing curve between bedrock and isotopically fractionated clay endmembers. Mixing model conditions: Bedrock δ7Li= 5‰, [Li] = 10 ppm (this study); clays δ 7 Li= -2.3‰ (clay δ 7 Li measured in the most

Fig 5 :

 5 Fig 5: (A) TDS* (Total Dissolved Solids corrected for atmospheric inputs) as a function of temperature. The blue diamond corresponds to the Ethiopian Traps (average based on 17 data, see Table3), and the black circles come from the compilation of[START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF] (Benedetti et

Fig 6 :

 6 Fig 6: Total CO2 consumption in mol/year[START_REF] Gaillardet | Global silicate weathering and CO consumption rates deduced2 from the chemistry of large rivers[END_REF] and annual CO2 consumption in GtC/yr due to silicate weathering for the largest rivers system. The blue bar corresponds to the current CO2 consumption rate estimated in Ethiopia based on our measurements in river waters. The brown bar indicate the CO2 consumption rate estimated

  This is bet explained by the low runoff values observed in Ethiopia at present (242 mm/yr), despite regular occurrence of the monsoon. This demonstrates that, at present, the Ethiopian Traps weathering has a negligible impact on the carbon cycle, and thus on climate.

	Traps in order to characterize silicate weathering and corresponding CO2 consumption in this
	region. Lithium isotopic composition of the clay fractions was also examining and present a
	positive correlation with clay Mg/Ti ratios, in relation with variable weathering conditions in
	this region.
	The average weathering rate (16.1 tons/km 2 /yr) and CO2 consumption rate (0.65.10 12 mol/yr)
	determined for the Ethiopian Traps are lower than for other basaltic regions (e.g. Deccan or
	SE Asia).

Table 1 :

 1 Characteristics of the studied watersheds.

Table 2 :

 2 Chemical composition of river waters, Total Dissolved Solid (TDS), silicate weathering rate and related CO2 consumption rate for the major basins of the Ethiopian

	Traps.
	Table 3: Chemical composition of sand, mud and clay samples (clay fractions have been
	physically separated following the procedure described in part 3.3.2)

Supplementary informations

Fig S1: Ethiopian geological map. The black circles indicate our sampling sites.

Table 1 :

 1 Characteristics of the studied watersheds.

	Sample number	Sample name	Si (µmol/l)	Mg (µmol/l)	Ca (µmol/l)	Na (µmol/l)	K (µmol/l)	Al (µmol/l)	Sr (µg/l)	Th (µg/l)	U (µg/l)	F µmol/l -	Cl µmol/l -	NO 3 -µmol/l	SO 4 2-µmol/l	Mg* (µmol/l)	Ca* (µmol/l)	Na* (µmol/l)	K* (µmol/l)	F µmol/l -*	NO 3 -* µmol/l	SO 4 2-* µmol/l	HCO 3 -* µmol/l	TDS* (µmol/l)	TDS* (mg/l)	Total chemical weathering tons/km 2 /year	CO2 mol/km2/year 6 10
	ETH04-2	Nil (Juin-04)		304	530	494	70	3405	135	0.005	0.1	8.5	76	43	146	296	529	427	68	8.5	43	146	1808	1320	40.9	11.2	0.49
	ETH 05 -52	Nil (Mars-05)		341	584	414	61	398	174	< L.D.	0.2	8.9	70	4	171	334	582	352	60	8.9	4	171	1897	1328	41.9	11.4	0.52
	ETH04-9	Jemma Juin-04)		323	688	650	141	3946	162	0.011	0.6	12.2	116	100	252	311	686	548	139	12.2	100	252	2075	1683	53.1	12.4	0.48
	ETH05-51	Jemma (Mars-05)		684	1173	959	98	93	371	< L.D.	0.5	17.7	127	14	269	671	1171	847	96	17.7	14	269	4070	2785	86.5	20.1	0.95
	ETH 05 -50	Wench't Wenz		585	1035	1219	92	264	257	< L.D.	0.6	14.2	165	8	402	568	1032	1073	89	14.2	8	402	3551	2762	83.3	21.0	0.90
	ETH 05 -13	Borkena Wenz		499	1334	734	59	26	265	< L.D.	0.3	8.5	345	292	125	464	1327	429	52	8.5	292	125	3546	2273	76.4	20.5	0.95
	ETH 05 -19	Tikur Wiha (Woldia)		789	1110	688	70	198	237	< L.D.	0.9	9.7	138	115	98	775	1107	566	68	9.7	115	98	4089	2516	78.9	17.9	0.93
	ETH04-4	Wegel Tena		775	1141	901	65	3625	236	0.006	0.8	9.1	273	77	272	748	1135	659	60	9.1	77	272	3880	2602	81.2	19.0	0.91
	ETH04-5	Bashillo		663	1022	940	60	4823	227	0.007	0.7	10.1	164	16	182	647	1019	795	57	10.1	16	182	3809	2518	77.1	20.5	1.02
	ETH 05 -25	Ketchin Abeba Wenz		1565	1357	1055	71	62	488	< L.D.	0.9	8.8	348	132	212	1530	1350	748	65	8.8	132	212	6039	3692	111.0	24.4	1.33
	ETH 05 -28	Meri Wenz		1365	1386	869	69	458	672	< L.D.	0.5	9.2	184	11	880	1346	1382	706	65	9.2	11	880	4466	3500	106.9	21.2	0.89
	ETH 05 -35	Tekeze amont		1768	1170	761	48	101	360	< L.D.	1.2	7.3	298	129	205	1739	1164	497	43	7.3	129	205	5826	3442	102.0	22.4	1.28
	ETH04-10	plateau-N-Addis		169	304	273	56	443	115	< L.D.	0.0	7.7	52	154	22	164	303	228	55	7.7	154	22	1014	750	23.5	6.2	0.27
	ETH04-8	Plateau E-Addis		88	160	147	51	3359	53	0.006	0.0	6.4	54	21	15	83	159	99	50	6.4	21	15	582	391	12.6	3.4	0.16
	ETH 05 -2	Ulet Wenz		59	133	102	20	2225	44	0.002	0.0	5.1	15	26	27	57	133	89	20	5.1	26	27	406	299	9.5	2.6	0.11
	ETH 05 -4	Robi Wenz		250	721	734	73	307	114	< L.D.	0.4	12.2	56	59	70	244	720	684	72	12.2	59	70	2478	1720	53.3	13.4	0.62
	ETH 05 -6	Nazero Wenz		581	1161	1155	100	68	174	0.002	0.5	21.3	168	8	90	565	1158	1007	97	21.3	8	90	4354	2826	87.1	22.5	1.13

Table 2 :

 2 Chemical composition of river waters, Total Dissolved Solid (TDS), silicate weathering rate and CO 2 consumption rate for the major basins of the Ethiopian Traps

Acknowledgments

This work was supported by BQR Geoazur, CNRS INSU-SYSTER grants and ANR INTOCC. We thank P. Telouk (LGLTPE) for his assistance with the ENS-Lyon Neptune plus. We thank also P.H Blard and B. Marty (CRPG) for their help on the field and fruitful discussions.