Which minerals control the Nd–Hf–Sr–Pb isotopic compositions of river sediments?
Introduction
Because they integrate large portions of continental crust exposed to weathering, river sediments from worldwide fluvial systems have been widely used to trace sediment provenance, estimate the average isotopic composition of the upper continental crust, and constrain its evolution through time (Goldstein et al., 1984, Goldstein and Jacobsen, 1988, Asmerom and Jacobsen, 1993, Allègre et al., 1996, Clift et al., 2002, Singh and France-Lanord, 2002, Millot et al., 2004, Kamber et al., 2005, Richards et al., 2005, Roddaz et al., 2005, Singh et al., 2008, Cina et al., 2009, Belousova et al., 2010, Hawkesworth et al., 2010, Wu et al., 2010, Dhuime et al., 2011, Padoan et al., 2011). Most studies assume that the isotopic compositions of river sediments reflect that of the drained continental area. However, depending on its size, shape, and density, detrital material transported by rivers is segregated within the water column by hydrodynamic processes: fast-settling coarse and heavy minerals concentrate in bedloads while fine, platy and light minerals are preferentially transported in suspension, and at different depths in the water column depending on their settling velocities (Singh and France-Lanord, 2002, Komar, 2007, Galy et al., 2008, Garzanti et al., 2008). Because chemical elements are carried in different amounts by the various minerals, it is now well described that mineral sorting processes lead to large chemical variability between bedload and suspended load sediments (Garzanti et al., 2010, Bouchez et al., 2011, Garzanti et al., 2011, Lupker et al., 2011, Lupker et al., 2012). In contrast, our knowledge of how such processes affect the Nd, Hf, Sr and Pb isotopic compositions of sediments is relatively limited. While several studies have pointed out Nd, Hf, Sr and Pb isotopic fractionations between fine and coarse-grained sediments due to mineral sorting processes (Patchett et al., 1984, McLennan et al., 1989, Revel et al., 1996, Eisenhauer et al., 1999, Singh and France-Lanord, 2002, Chauvel et al., 2008, Bayon et al., 2009, Carpentier et al., 2009, Chauvel et al., 2009, Vervoort et al., 2011, Garçon et al., 2013a, Garçon et al., 2013b), the influence of each mineral species on the isotopic composition of sediments has never been thoroughly quantified.
In this paper, we report trace element contents and Nd, Hf, Sr and Pb isotopic compositions for a large number of pure mineral fractions separated from a bedload and a bank sediment sampled at the outflow of the Ganga fluvial system in the Bangladesh delta. Using Monte Carlo simulations, we combine these data with mineral proportions reported by Garzanti et al., 2010, Garzanti et al., 2011 and Lupker et al. (2012) in river sediments sampled at the same location to (1) evaluate the individual contribution of each mineral species to the Nd, Hf, Sr and Pb isotopic budgets of bedloads and suspended loads and (2) determine if the known mineralogy of bedloads and suspended loads accounts for the observed difference in isotopic compositions. Finally, we discuss the implications of our results for sediment provenance studies based on isotope data and for large-scale isotopic partitioning between continental and oceanic sediments.
Section snippets
Studied area and samples
The studied mineral fractions were separated from two samples (bank sediment BGP 6 and bedload BR 717) collected at the outflow of the Ganga River, before the confluence with the Brahmaputra River (Fig. 1). The sampling site was selected because it is located downstream, far away from the mountain range. As a consequence, the sediments integrate all materials transported by the Ganga River and its tributaries. These rivers belong to one of the largest fluvial systems on Earth (Milliman and
Mineral separation
The starting material consists of several kilos of sediment collected during the monsoon season. With the exception of the clay fraction separated from bank sediment BGP 6, all mineral fractions were separated from bedload BR 717 dredged in the main channel of the Ganga River. This sample was first washed and dried, then divided into 3 grain-size fractions by sieving: 63–100 μm, 125–250 μm, and > 250 μm. For each fraction, we used successive centrifugations in heavy liquids (sodium metatungstate
Trace element concentrations
Trace element concentrations measured in the mineral fractions are reported in Table 1 and shown in Fig. 2 as spidergrams normalized to the average composition of upper continental crust (Rudnick and Gao, 2003). Except for mobile elements such as Cs, Rb, Ba, Sr, Li, Co, or Ni, heavy minerals (Fig. 2a) generally display higher trace element contents than light minerals (Fig. 2b). The trace element patterns of the various minerals are generally consistent with published values (e.g., Götze and
Discussion
River sediments are basically mixtures of mineral species. Some minerals, such as quartz, contain virtually no trace elements and their proportion in the sedimentary material influences only the silica content of the sediment but has no impact on most isotopic ratios. In contrast, other minerals very rich in certain trace elements or with very unusual isotopic compositions have the potential to significantly contribute or can even overwhelm the trace element and isotopic budget of river
Summary and conclusions
The Monte Carlo simulations performed in this study provide a framework to decipher the contribution of individual mineral species to the Nd, Hf, Sr and Pb isotopic budget of river sediments. They also provide clues to understand the isotopic differences observed between bedloads and suspended loads that are related to mineral sorting processes during the fluvial transport of sediments. The results of the Monte Carlo simulations are well constrained and could be used to understand the isotopic
Acknowledgments
We would like to thank A. Galy, V. Galy and M. Lupker for the sample collection, S. Andò for his help during mineral separation, S. Bureau and C. Poggi for their help in the clean lab, P. Telouk and P. Nonnotte for their assistance during isotopic measurements in Lyon and Brest, as well as N.T. Arndt and E. Lewin for the constructive discussions that helped to develop and interpret the results of the Monte Carlo simulations. We also greatly thank the editor Laurie Reisberg and the two
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