Rapid response of silicate weathering rates to climate change in the Himalaya

Chemical weathering of continental rocks plays a central role in regulating the carbon cycle and the Earth's climate (Walker et al., 1981; Berner et al., 1983), accounting for nearly half the consumption of atmospheric carbon dioxide globally (Beaulieu et al., 2012). However, the role of climate variability on chemical weathering is still strongly debated. Here we focus on the Himalayan range and use the lithium isotopic composition of clays in fluvial terraces to show a tight coupling between climate change and chemical weathering over the past 40 ka. Between 25 and 10 ka ago, weathering rates decrease despite temperature increase and monsoon intensification. This suggests that at this timescale, temperature plays a secondary role compared to runoff and physical erosion, which inhibit chemical weathering by accelerating sediment transport and act as fundamental controls in determining the feedback between chemical weathering and atmospheric carbon dioxide. Disciplines Medicine and Health Sciences | Social and Behavioral Sciences Publication Details Dosseto, A., Vigier, N., Joannes-Boyau, R., Moffat, I., Singh, T. & Srivastava, P. (2015). Rapid response of silicate weathering rates to climate change in the Himalaya. Geochemical Perspectives Letters, 1 (1), 10-19. Authors Anthony Dosseto, Nathalie Vigier, Renaud C. Joannes-Boyau, Ian Moffat, T Singh, and Pradeep Srivastava This journal article is available at Research Online: http://ro.uow.edu.au/smhpapers/2952 Letter Geochemical Perspectives Letters Geochem. Persp. Let. (2015) 1, 10-19 | doi: 10.7185/geochemlet.1502 Geochem. Persp. Let. (2015) 1, 10-19 | doi: 10.7185/geochemlet.1502 10 11 (<100 yr) to climate change and may act as a key component in the atmospheric CO2 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 >104 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 6Li is preferentially incorporated into clays compared to 7Li, resulting in a strong enrichment of 7Li in waters (Burton and Vigier, 2011). Thus, it has been shown that the d7Li composition of natural waters can be used as a proxy for chemical weathering rates at the catchment scale, since the heaviest d7Li 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 d7Li composition of solid weathering products (i.e. soils, river sediments) is sensitive to chemical weathering conditions, where lower d7Li 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). The d7Li compositions of bulk sediments vary between -1.54 and +1.98 ‰, while clay-sized fractions show a much broader range. Most samples show low d7Li values best explained by significant 6Li enrichment during clay formation (Burton and Vigier, 2011). The d7Li values in bulk samples and clay-sized fractions evolve consistently as a function of time: both decrease between 35 and 25 ka, © 2015 European Association of Geochemistry Rapid response of silicate weathering rates to climate change in the Himalaya A. Dosseto1,*, N. Vigier2,3, R. Joannes-Boyau4, I. Moffat5,6, T. Singh7, P. Srivastava8 Abstract doi: 10.7185/geochemlet.1502doi: 10.7185/geochemlet.1502 Chemical weathering of continental rocks plays a central role in regulating the carbon cycle and the Earth’s climate (Walker et al., 1981; Berner et al., 1983), accounting for nearly half the consumption of atmospheric carbon dioxide globally (Beaulieu et al., 2012). However, the role of climate variability on chemical weathering is still strongly debated. Here we focus on the Himalayan range and use the lithium isotopic composition of clays in fluvial terraces to show a tight coupling between climate change and chemical weathering over the past 40 ka. Between 25 and 10 ka ago, weathering rates decrease despite temperature increase and monsoon intensification. This suggests that at this timescale, temperature plays a secondary role compared to runoff and physical erosion, which inhibit chemical weathering by accelerating sediment transport and act as fundamental controls in determining the feedback between chemical weathering and atmospheric carbon dioxide. Received 9 December 2014 | Accepted 23 January 2015 | Published 20 February 2015