Increasing chemical weathering in the Himalayan system since the Last Glacial Maximum

Abstract

1 Continental chemical weathering is central in Earth's surface biogeochemical 2 cycles as it redistributes elements across reservoirs such as the crust and the oceans. 3 However the evolution of weathering through time and its response to external forcing 4 such as changes in climate remain poorly constrained. In this work, a composite 5 sediment record from the Bay of Bengal is used to document the evolution of chemical 6 weathering in the Himalayan system (Himalayan range and Indo-Gangetic floodplain), 7 the world largest sediment conveyor to the oceans, since the Last Glacial Maximum 8 (LGM). The degree of weathering of the sediments is documented using mobile to 9 immobile ratios such as K/Si and H2O + /Si as well as detrital calcite abundance. Robust 10 weathering proxies are derived by correcting the chemical composition of sediment for 11 sorting effects that occur during transport and deposition. The Bay of Bengal record is 12 also further compared to the chemical composition of modern river sediments from the 13 Ganga & Brahmaputra basin. Weathering proxies all indicate that the sediments 14 exported by the Ganga & Brahmaputra Rivers became increasingly weathered over the 15 past ~21 kyr, whereas, Sr, Nd and major elements suggest a constant sediment 16 provenance in the system over the last 21 kyr. These changes in the degree of weathering 17 of the sediments show that the weathering flux exported by the system to the Indian 18 Ocean during the LGM was significantly lower than at present and demonstrate that 19 chemical weathering in continental scale basins such as the Ganga & Brahmaputra 20 responds to Late Quaternary climate changes. 21 137 Cs, 210 Pb and 228 Ra gamma spectrometry (Michels et al., 1998). Scarce hemipelagic layers 65 were excluded from the study in order to focus on turbiditic sediments representing the Ganga 66 & Brahmaputra detrital signal (Pierson-Wickmann et al., 2001; Weber et al., 1997). We 67 interpret our data by constructing a composite Log from the shelf and levees core following 68 the approach detailed in Galy et al. (2008). 69 River suspended river sediments were filtered through 0.22 µm PES membrane filters 70 within 24 h of sampling and dried in the lab. Major element concentration was determined by 71 Lupker, M., France-Lanord, C., LavÈ, J., Bouchez, J., Galy, V., MÈtivier, F., 577 Gaillardet, J., Lartiges, B., Mugnier, J.-L., 2011. A Rouse-based method to integrate the 578 chemical composition of river sediments: Application to the Ganga basin. A., Wiedicke, M., 1998. The 586 submarine delta of the Ganges-Brahmaputra: cyclone-dominated sedimentation 587 patterns. Marine Geology 149, 133-154. 588 Pierson-Wickmann, A.-C., Reisberg, L., France-Lanord, C., Kudrass, H., 2001. Os-Sr-589 Nd results from sediments in the Bay of Bengal: Implications for sediment transport and 590 the marine Os record. Paleoceanography 16, 435-444. 591

Impact of sediment–seawater cation exchange on Himalayan chemical weathering fluxes

“…It should nevertheless be kept in mind that our estimates are formulated based on the Himalayan system at present. On longer timescales, the variability in both sediment (Goodbred and Kuehl, 2000) and weathering fluxes (Lupker et al, 2013) means that the relative importance of cation exchange fluxes in the global weathering budget has likely varied and hence should be treated carefully. Finally, it is worth mentioning that these estimates of weathering impact on the carbon cycle do not take into account the role of chemical weathering through sulfuric acid (Galy and France-Lanord, 1999;Turchyn et al, 2013) that is known to also contribute to the weathering budget of Himalayan erosion and does counteract long-term carbon sequestration (Calmels et al, 2007).…”

Clay mineralogy, strontium and neodymium isotope ratios in the sediments of two High Arctic catchments (Svalbard)

“…Additionally, εNd variations in ocean sediments are often interpreted in terms of changing source regions with the isotopic composition of individual source regions remaining constant though time. However, recent studies have hinted at seasonal variations in εNd in river sediments (Viers et al, 2008;Garçon et al, 2013;Lupker et al, 2013), raising the possibility that the εNd value of sediment exported from individual source regions may not remain constant over time. Thus, although εNd is a reliable tracer of source, one region may contain multiple end-members, whose relative contributions vary over time.…”

“…The radiogenic isotope tracers 143 Nd / 144 Nd (expressed as εNd) and 87 Sr / 86 Sr are often used together to understand where and how sediment is generated and weathered, enabling source regions to be characterised (Goldstein and Jacobsen, 1988;Cameron and Hattori, 1997;Tricca et al, 1999;Peucker-Ehrenbrink et al, 2010;Lupker et al, 2013;Clinger et al, 2016). εNd is particularly suited to being a source tracer because, unlike Sr and Rb which are fluid mobile, Sm and Nd are immobile and behave very similarly during chemical weathering such that the Sm / Nd ratio does not fractionate during weathering (McCulloch and Wasserburg, 1978) and therefore variations in εNd are predominantly controlled by age (Goldstein and Jacobsen, 1988) rather than the weathering of specific minerals which can affect the Rb-Sr system (e.g.…”

U-Th and <sup>10</sup>Be constraints on sediment recycling in proglacial settings, Lago Buenos Aires, Patagonia

“…But its role in controlling CO 2 concentrations and climate variations over glacial-interglacial cycles is contentious (Foster & Vance, 2006;Vance et al, 2009;Lupker et al, 2013;VonBlanckenburg et al, 2015;Cogez et al, 2015). Understanding whether weathering varied over these cycles requires robust determinations of sediment transfer times.…”