11(<100 yr) to climate change and may act as a key component in the atmospheric CO 2 level short-term regulation (Beaulieu et al., 2012). However, the relative role of climatic parameters on chemical weathering is still debated. Model simulations and geochemical studies of river-born material highlight either a temperature dependence (Walker et al., 1981), where warming promotes chemical weathering, or a predominance of other parameters like mechanical erosion (Raymo and Ruddiman, 1992;Gaillardet et al., 1999;Riebe et al., 2001;Donnadieu et al., 2004) and vegetation (Bayon et al., 2012). To address this question, another possible approach consists in investigating past continental environments through the study of marine or floodplain deposits. However, because in large river systems sediment transport and storage operates over >10 4 yr timescales (Granet et al., 2010), a significant time lag may exist between the time when sediments acquired their geochemical characteristics, reflecting palaeoenvironmental conditions, and their final deposition.Here we use the lithium (Li) isotopic composition of clays from sedimentary records in Himalayan basins to determine how chemical weathering intensity has varied over the past 40 ka and particularly since the Last Glacial Maximum (locally older than 24 ka; Owen et al., 2002). In order to minimise the time lag between source and deposit locations, we have focused on alluvial deposits located in the headwater areas of the Ganges and Yamuna Rivers.While little isotope fractionation occurs during mineral dissolution clay formation induces strong fractionations at low temperature, whereby 6 Li is preferentially incorporated into clays compared to 7 Li, resulting in a strong enrichment of 7 Li in waters (Burton and Vigier, 2011). Thus, it has been shown that the d 7 Li composition of natural waters can be used as a proxy for chemical weathering rates at the catchment scale, since the heaviest d 7 Li compositions in water are associated with the areas characterised by low catchment-wide silicate weathering rates (Fig. 1). Several studies have also shown that the d 7 Li composition of solid weathering products (i.e. soils, river sediments) is sensitive to chemical weathering conditions, where lower d7 Li values reflect more intensive leaching (Burton and Vigier, 2011).To evaluate the reliability of the records studied, we investigated three different regions of the Himalaya and its piedmont in India: the upper Yamuna River basin, the Alaknanda River basin and the Donga Fan (Fig. S-1). Depositional ages for the alluvial deposits reported here were previously constrained by optically-stimulated luminescence (OSL) dating and range from 9 to 41 ka (Singh et al., 2001;Ray and Srivastava, 2010). Both bulk sediments and clay-sized fractions were analysed for Li isotopes. In bulk sediments, mineralogical abundances and Sr isotopes were also measured (see Supplementary Information). et al., 1981;Berner et al., 1983), accounting for nearly half the consumption of atmospheric carbon diox...
