Continental sedimentary processes decouple Nd and Hf isotopes
Section snippets
INTRODUCTION
The present-day Nd and Hf isotopic compositions of most crustal and mantle-derived rocks correlate and form the “Terrestrial Array” (Vervoort et al., 1999, Vervoort et al., 2011) due to similar fractionation of Sm/Nd and Lu/Hf during magmatic processes. However, it is also recognized that some oceanic sediments, in particular oceanic ferromanganese crusts and terrigenous clays, deviate significantly from the Terrestrial Array towards higher εHf values relative to their εNd values (e.g. Albarède
THE GANGES FLUVIAL SYSTEM
The Ganges fluvial system is one of the main conveyors of material eroded from the Himalayan Range, delivering ca. 400 million tons of sediments per year (Lupker et al., 2011) to the ocean in the Bay of Bengal. Together with the Brahmaputra River, the Ganges ranks among the three largest fluvial systems in terms of annual sediment load delivered into the ocean (Milliman, 2001). The sediments accumulate in the Bengal fan, by far the largest fan in the world with its volume of 12.5 × 106 km3 and a
Sediment sampling
River sediments analyzed in this study were sampled over several years of field campaigns, in Nepal, India and Bangladesh. Some of these samples have already been analyzed for major elements (Lupker et al., 2012), Re–Os isotopes (Pierson-Wickmann et al., 2000), mineralogical proportions (Garzanti et al., 2007, Garzanti et al., 2010, Garzanti et al., 2011) and Corg isotopes (Galy et al., 2008).
To document the Nd–Hf isotopic variability of river sediments at a large scale, i.e. during their
Major and Trace Element Concentrations
Major element contents are presented in Supplementary Table B. For the purpose of this study, we focus on the SiO2 and Al2O3 contents of the studied bedloads, bank deposits and suspended loads, as shown in Fig. 2(a). Lupker et al., 2011, Lupker et al., 2012 describe the variations of other major elements. In general, SiO2 and Al2O3 contents are strongly correlated with sediment grain-size (Lupker et al., 2011). Bedloads and bank sediments are enriched in coarse-grained quartz and have high SiO2
Source Isotopic Variability
The Nd and Hf isotopic compositions of sediments along a river course are influenced by contributions of materials with different isotopic compositions that are supplied by tributaries. These source effects need to be evaluated before we can consider any other control on the geochemistry of river sediments.
REE barely fractionate during weathering and have been widely used for studying the provenance of detrital sediments. More specifically, ratios such as La/Yb, or Nd isotopic compositions of
SUMMARY AND CONCLUSIONS
This paper describes the first comprehensive study of Nd and Hf isotopic compositions in bedloads, suspended loads and bank sediments sampled along one of the largest rivers, the Ganges. Because source heterogeneity does not significantly affect the isotopic composition of the studied sediments, we could demonstrate that mineral sorting processes control the chemistry of Ganges River sediments and account for their entire Hf isotopic variability (>10 εHf). Bedload and bank sediments always have
Acknowledgments
We thank A. Galy, M. Lupker, V. Galy and M. Attal who collected most of the samples analyzed in this study, S. Bureau for her help in the clean lab, P. Telouk (ENS, Lyon) for assistance during MC-ICP-MS measurements at Lyon and N. Arndt (ISTerre, Grenoble) for discussions that improved the style and the content of the manuscript. We are also grateful to Julie Prytulak, two anonymous reviewers and the associate editor Steve Shirey for their very constructive comments on the manuscript. This
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