isotope fractionation in the Amazon River basin controlled by the weathering regimes, Geochimica et Cosmochimica Acta (2015), doi: http://dx. AbstractWe report Li isotope composition (δ 7 Li) of river-borne dissolved and solid material in the largest River system on Earth, the Amazon River basin, to characterize Li isotope fractionation at a continental scale. The δ 7 Li in the dissolved load (+1.2 to +32 ) is fractionated toward heavy values compared to the inferred bedrock (-1 to 5 ) and the suspended sediments (-6.8 to -0.5 ) as a result of the preferential incorporation of 6 Li into secondary minerals during weathering. Despite having very contrasted weathering and erosion regimes, both Andean headwaters and lowland rivers share similar ranges of dissolved δ 7 Li (+1.2 to +18 ). Correlations between dissolved δ 7 Li and Li/Na and Li/Mg ratios suggest that the proportion of Li incorporated in secondary minerals during weathering act as the main control on the δ 7 Li diss across the entire Amazon basin. A "batch" steady-state fractionation model for Andean and lowland rivers satisfactorily reproduces these variations, with a fractionation factor between weathering products and dissolved load (α sec−dis ) of 0.983. Two types of supply-limited weathering regimes can be identified for the lowlands : "clearwaters" with dominant incorporation of Li in secondary minerals, and "black waters" (e.g. Rio Negro) where dissolution of secondary minerals enhanced by organic matter produces low δ 7 Li. Apart from the black waters, the δ 7 Li of Andean and lowland rivers is negatively correlated to the denudation rates with the lowest δ 7 Li corresponding to the rivers having the highest denudation rates. In contrast, the main tributaries draining both the Andes and the lowlands have higher δ 7 Li compared to other rivers. We propose that part of the dissolved Li derived from weathering in the Andes is re-incorporated in sediments during transfer of water and sediments in floodplains and that this results in an increase of the dissolved δ 7 Li along the course of these rivers. Unlike other rivers, the dissolved δ 7 Li in the main tributaries is best described by a Rayleigh fractionation model with a fractionation factor α sec−dis of 0.991. Altogether, the control imposed by residence time in the weathering zone and floodplain processes results in (i) a non-linear correlation between dissolved δ 7 Li and the weathering intensity (defined as W/D) and (ii) a positive relationship between the dissolved Li flux and the denudation rate. These results have important implications for the understanding of past ocean δ 7 Li and its use as a paleo weathering proxy.

[1] Residual solid products of erosion display a wide range of size, density, shape, mineralogy, and chemical composition and are hydrodynamically sorted in large river channels during their transport. We characterize the chemical and isotopic variability of river sediments of the Amazon Basin, collected at different water depths, as a function of grain size. Absolute chemical concentrations and Sr and Nd isotopic ratios greatly varies along channel depth. The Al/Si ratio, tightly linked to grain size distribution, systematically decreases with depth, mostly reflecting dilution by quartz minerals. A double-normalization diagram is proposed to correct from dilution effects. Elements define fan-shaped patterns and can be classified in three different groups with respect to hydrodynamic sorting during transport in the Amazon: (1) "poorly sorted" insoluble elements like Al, Fe, Th, and REEs, (2) "well-sorted" insoluble elements like Zr and Ti, mainly carried by heavy minerals, and (3) alkali (Na to Cs) and alkali-earth elements (Mg to Ba), for which a large variety of patterns is observed, related, for alkali, to their variable affinity for phyllosilicates. Sr isotopes show that the Amazon River at the mouth is stratified, the Madeira-and Solimões-derived sediments being preferentially transported near the channel surface and at depth, respectively. The comparison between the Solimões and Madeira rivers shows how the interplay between grain sorting, weathering, and crustal composition controls the composition of the suspended river sediments.

constrained. Here, we quantify POC source in the Mackenzie River, the main sediment 24 supplier to the Arctic Ocean 11,12 and assess its flux and fate. We combine measurements 25 1 Hilton, R. G., et al., Revised version for Nature, 12 th May 2015, doi:10. (Fig. 1). The δ 13 C org values and Al/OC total ratios support this inference (Extended Data Fig. 2). 92Using an end member mixing analysis 10,13 we quantify POC petro content of sediments matter turnover in terrestrial ecosystems is more rapid (Fig. 2c) with water discharge (Fig. 2b) could be important in setting the variability of POC biosphere age 123 carried by the river (Fig. 2c)

Samples of river bank sediments and rocks were collected from across the entire study 3 site in order to constrain the elemental and isotopic composition of different lithologic end-4 members. Sub-samples of the river bank sediments were separated using a riffle splitter and 5 powdered in a ball mill. Rock samples were disaggregated using an agate mortar and pestle 6 before being ground in a ball mill. Only measurements of Na, Ca, Mg, and Sr concentrations are discussed in this paper. The vessel. During the reaction, the digestion vessels were heated from below with a hotplate. 25The liberated H 2 S gas was passed through a condenser and bubbled through a solution of 26

[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

The two long-term sources of atmospheric carbon are CO 2 degassing from metamorphic and volcanic activity, and oxidation of organic carbon (OC) contained in sedimentary rocks, or petrogenic organic carbon (OC petro). The latter flux is still poorly constrained. In this study, we report Particulate Organic Carbon (POC) content and 14 C-activity measurements in Amazon River sediments, which allow for estimates of the OC petro content of these sediments. A large

Publisher's copyright statement: NOTICE: this is the author's version of a work that was accepted for publication in Earth and Planetary Science Letters. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reected in this document. Changes may have been made to this work since it was submitted for publication. A denitive version was subsequently published in Earth and Planetary Science Letters, 401, 1 September 2014, 10.1016/j.epsl.2014.061. Additional information:Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The erosion of major mountain ranges is thought to be largely cannibalistic, recycling 50 sediments that were deposited in the ocean or on the continents prior to mountain uplift. 51Despite this recognition, it has not yet been possible to quantify the amount of recycled 52 material that is presently transported by rivers to the ocean. Here, we have analyzed the 53 Li content and isotope composition (δ 7 Li) of suspended sediments sampled along river 54 depth profiles and bed sands in three of the largest Earth's river systems (Amazon, 55Mackenzie and Ganga-Brahmaputra rivers). The δ 7 Li values of river-sediments 56 transported by these rivers range from +5.3 to -3.6‰ and decrease with sediment grain 57 size. We interpret these variations as reflecting a mixture of unweathered rock fragments 58(preferentially transported at depth in the coarse fraction) and present-day weathering 59 products (preferentially transported at the surface in the finest fraction control the evolution of climate through the consumption of atmospheric carbon dioxide 83 (Berner et al., 1983;Gaillardet et al., 1999b; Raymo et al., 1988; Walker et al., 1981), 84 3 shape the Earth's surface through chemical denudation and soil production (Heismath et 85 al., 1997) and modify the composition of the continental crust (Lee et al., 2008; Liu and 86 Rudnick, 2011; Rudnick, 1995). Geochemical mass budgets of river-borne material 87 (dissolved and particulate phases) show that rivers transport both "present-day 88 weathering products" (new solid materials formed during the residence of sediment in 89 river basins by present-day water-rock interactions) and carry "inherited weathering 90 products" derived from older sedimentary rocks, which have been subject to previous 91 weathering episodes (Bouchez et al., 2011a;Gaillardet et al., 1999a). Moreover, on the 92 basis of the Nd isotope record of shales, Veizer and Jansen (1979, 1985) sugg...

: Earth surface erosion and weathering from the 10Be (meteoric)/9Be ratio. v351-352, pp 295-305 2012, doi j/epsl.2012.07.022 AbstractThe isotope ratio of the meteoric cosmogenic nuclide 10 Be to the mineral-derived stable isotope 9 Be discloses both the Earth surfaces' denudation rate and its weathering intensity. We develop a set of steady state mass balance equations that describes this system from a soil column over the hillslope scale to an entire river basin. The prerequisites making this new approach possible are: 1) the 9 Be concentration of parent rock (typically 2.5 ± 0.5 ppm in granitic and clastic sedimentary lithologies) is known; 2) both Be isotopes equilibrate between the fluids decomposing rock and reactive solids formed during weathering; and 3) a critical spatial scale is exceeded at which the fluxes of both isotopes into and out of the weathering zone are at steady state over the time scale of weathering (typically ~10 kyr). For these cases the isotope ratios can be determined in bulk sediment or soil, on leachates from the reactive (adsorbed and pedogenic mineral-bound) phase in sediment or soil, and even on the dissolved phase in river water. The 10 Be/ 9Be ratio offers substantial advantages over the single-isotope system of meteoric 10 Be. The latter system allows to directly determine erosion rates only in the case that 10 Be is fully retentive in the weathering zone and that riverine sorting has not introduced grain size-dependent 10 Be concentration gradients in sediments. We show the feasibility of the 10 Be/ 9 Be tracer approach at the river scale for sediment and water samples in the Amazon basin, where independent estimates of denudation rates from in situ-produced 10 Be exist. We furthermore calculate meaningful denudation rates from a set of published 10 Be/ 9 Be ratios measured in the dissolved load of globally distributed rivers. We conclude that this isotope ratio can be used to reconstruct global paleodenudation from sedimentary records.