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International audience; If weathered-derived solutes time series can characterize theexported weathering flux [kg/ha/yr] of a given catchment, thesetime series alone are uninformative to characterize theweathering reactivity of a given landscape, ie its weatheringdynamic, leading to a given water quality in streamwater.Similarly, weathering rates derived from batch reactors aredifficult to upscale to estimate the weathering reactivity at thefield scale (White and Brantley, 2003). For these reasons, wepropose here to develop a model-based coupling betweenintermediate age tracers (ages representative of circa 10 to 100years) with long and in stream dissolved silica time series (15 yrslong) to characterize the weathering enrichment rate (inmMol/yr) water in different Brittany catchments.We developed a quasi-2D hillslope model that represents thepartitioning between stratified groundwater flows and fast flowsarising close to the surface seasonally, when the aquiferintersects the land surface. Coupled to a Lagrangian particletracking technique, the model is able to represent thestreamwater transit time distributions dynamics at the catchmentscale. We then collected CFCs measurements in wells, springsand streams to characterize the mean transit time variability ofthe different water storages. We calibrate the different watercompartments (storage capacity and flux coming from thedifferent storages) with discharge time series and CFCsmeasurements.We found that the seasonal dynamic of the transit timedistributions is correlated with the DSi observations. By adding afirst order kinetic law to represent the apparent weatheringactivity, we were able to generate numerical DSi concentrationstime series and to reproduce the seasonal patterns observed. Wefound that the weathering rates in shallow Brittany aquifers arecirca 0.003 mMol/yr, consistent with previous weatheringestimated carried in crystalline aquifers

Germanium is a useful tracer of silicate weathering and secondary mineral formation in the Critical Zone because Ge/Si ratios are fractionated during incongruent weathering of silicates. We develop an estimate of the equilibrium fractionation coefficient between germanium and silicon for the precipitation of kaolinite using a solid-solution model. Thermodynamic properties and distribution coefficient were estimated using observations from natural systems, experimental data from analog phyllo-germanate minerals (Shtenberg et al., 2017) and a parametric method based on a sum of oxides approach with site-specific interaction parameters (Blanc et al., 2015). The estimated log D G e - S i ' for the incorporation of Ge into kaolinite at 25 °C and 0.1 MPa is equal to −3.4 ± 1.5. The estimated Δ G f ° for a fully Ge substituted kaolinite ( Ge 2 Al 2 O 5 ( OH ) 4 ) equals −3130 ± 15 (kJ/mol) and the estimated log K s p for Ge-kaolinite = 3.1 ± 1.5. We further develop a series of batch reaction models using a geochemical reactive transport code to test the estimated range of the Ge-Si equilibrium fractionation coefficient. In these series of models, we also investigate how precipitation dynamics can impact the Ge/Si ratios observed both in streams and soils. These models show that both precipitation kinetics and re-equilibration of the precipitated solid control the behavior of Ge/Si ratios at far-from-equilibrium timescales. While the actual length of these timescales remains to be determined by better constraints on kaolinite precipitation rates at environmental conditions; our models suggest that the lowest groundwater measured Ge/Si ratios should represent this equilibrium timescale.

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.

International audience; Weathering and erosion can affect the long-term ocean-atmosphere budget of carbon dioxide both through the consumption of carbonic acid during silicate weathering and through changes in the weathering and burial rates of organic carbon 1-4. Recent attention has focused on increased silicate weathering of tectoni-cally uplifted areas in the India-Asia collision zone as a possible cause for falling atmospheric CO 2 levels in the Cenozoic era 5-7. The chemistry of Neogene sediments from the main locus of sedimentary deposition for Himalayan detritus, the Bengal Fan, can be used to estimate the sinks of CO 2 from silicate weathering and from the weathering and burial of organic carbon resulting from Himalayan uplift. Here we show that Neogene CO 2 consumption from the net burial of organic carbon during Himala-yan sediment deposition was 2-3 times that resulting from the weathering of Himalayan silicates. Thus the dominant effect of Neogene Himalayan erosion on the carbon cycle is an increase in the amount of organic carbon in the sedimentary reservoir, not an increase in silicate weathering fluxes. Silicate weathering is typically incongruent, yielding both a solute and a secondary mineral phase, so direct evidence of chemical weathering can be found in the record of secondary minerals in sedimentary basins. The Bengal Fan and Ganges-Brahmaputra (GB) delta contain a huge volume of sediment derived from erosion of the India-Asia collision zone, with 6 X 10 6 km 3 deposited in the past 20 Myr (ref. 8). Isotopic data for Nd, Sr and O from Bengal Fan sediments show that the source for over 80% of the detritus since 20 Myr ago has been the high-grade metasedimentary rocks of the High Himalayan crystalline (HHC) sequence 9. Clastic and carbo-nate sediments of the Precambrian Lesser Himalaya (LH) and Palaeozoic-Mesozoic Tethyan Himalaya (TH) are the other important sources of sediment to the Bengal Fan during the Neogene.

We present a mineralogical and isotopic study of Early Miocene turbidites from the Bengal Fan, which are the oldest sediments recovered during ODP Leg 116. Clay fractions are dominated by illite and chlorite and have low δ18O (11–16‰). The clays are dominantly derived from a metamorphic precursor by physical erosion, and have undergone only minor low-T water-rock exchange and chemical weathering. Sr and Nd isotope analysis of silt and clay fractions and coarse muscovite, biotite and feldspar fractions are remarkably similar to those of the presently exposed High Himalaya Crystalline (HHC) sequence. The data show that a close analogue of the HHC was already subaerially exposed to active erosion during Early Miocene. RbSr data on the biotite separates has been used to evaluate the time elapsed between cooling through the closure temperature for Sr in biotite in the HHC (ca. 325°C) and final sedimentation in the Bengal Fan. Despite the large uncertainties in the initial 87 Sr 86 Sr values of sedimentary mixtures, our estimates of the elapsed time between cooling and deposition are short (4–14 Ma). Assuming a transport time scale (≤1 Ma), deduced exhumation rates are in the same range of modern values (1–3 mm/yr) since ca. 30 Ma. Given that (a) the rocks exposed at the surface, (b) the style of weathering, and (c) rates of exhumation were similar to the present, we infer that the factors controlling erosion of the Early Miocene Himalayan belt were also similar to the present. The data suggest that the Early Miocene Himalayan belt had significant relief and induced orographic effects on precipitation similar to the modern mountain range. It appears that the thrusting along the Main Central Thrust (MCT) or similar structures has uplifted metamorphic basement analogous to the present HHC at sufficiently high rate to maintain steep relief throughout Miocene time, despite high average erosion rates.