Elsevier

Geochimica et Cosmochimica Acta

Volume 121, 15 November 2013, Pages 177-195
Geochimica et Cosmochimica Acta

Continental sedimentary processes decouple Nd and Hf isotopes

https://doi.org/10.1016/j.gca.2013.07.027Get rights and content

Abstract

The neodymium and hafnium isotopic compositions of most crustal and mantle rocks correlate to form the “Terrestrial Array”. However, it is now well established that whereas coarse detrital sediments follow this trend, fine-grained oceanic sediments have high Hf ratios relative to their Nd isotopic ratios. It remains uncertain whether this “decoupling” of the two isotopic systems only occurs in the oceanic environment or if it is induced by sedimentary processes in continental settings. In this study, the hafnium and neodymium isotopic compositions of sediments in large rivers is expressly used to constrain the behavior of the two isotopic systems during erosion and sediment transport from continent to ocean.

We report major and trace element concentrations together with Nd and Hf isotopic compositions of bedloads, suspended loads and river banks from the Ganges River and its tributaries draining the Himalayan Range i.e. the Karnali, the Narayani, the Kosi and the Marsyandi Rivers. The sample set includes sediments sampled within the Himalayan Range in Nepal, at the Himalayan mountain front, and also downstream on the floodplain and at the outflow of the Ganges in Bangladesh. Results show that hydrodynamic sorting of minerals explains the entire Hf isotopic range, i.e. more than 10 εHf units, observed in the river sediments but does not affect the Nd isotopic composition. Bedloads and bank sediments have systematically lower εHf values than suspended loads sampled at the same location. Coarse-grained sediments lie below or on the Terrestrial Array in an εHf vs. εNd diagram. In contrast, fine-grained sediments, including most of the suspended loads, deviate from the Terrestrial Array toward higher εHf relative to their εNd, as is the case for oceanic terrigenous clays. The observed Nd–Hf decoupling is explained by mineralogical sorting processes that enrich bottom sediments in coarse and dense minerals, including unradiogenic zircons, while surface sediments are enriched in fine material with radiogenic Hf signatures. The data also show that Nd–Hf isotopic decoupling increases with sediment transport in the floodplain to reach its maximum at the river mouth. This implies that the Nd–Hf isotopic decoupling observed in worldwide oceanic clays and river sediments is likely to have the same origin. Finally, we estimated the Nd–Hf isotopic composition of the present-day mantle if oceanic sediments had never been subducted and conclude that the addition of oceanic sediments with their anomalous Nd–Hf isotopic compositions has slowly shifted the composition of the Earth’s mantle towards more radiogenic Hf values through time.

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

References (86)

  • K. David et al.

    The Hf isotope composition of global seawater and the evolution of Hf isotopes in the deep Pacific Ocean from Fe–Mn crusts

    Chem. Geol.

    (2001)
  • B. Dupré et al.

    Major and trace elements of river-borne material: the Congo Basin

    Geochim. Cosmochim. Acta

    (1996)
  • J. Gaillardet et al.

    A global geochemical mass budget applied to the Congo Basin rivers: erosion rates and continental crust composition

    Geochim. Cosmochim. Acta

    (1995)
  • A. Galy et al.

    Weathering processes in the Ganges–Brahmaputra basin and the riverine alkalinity budget

    Chem. Geol.

    (1999)
  • V. Galy et al.

    Loading and fate of particulate organic carbon from the Himalaya to the Ganga–Brahmaputra delta

    Geochim. Cosmochim. Acta

    (2008)
  • M. Garçon et al.

    Beach placer, a proxy for the average Nd and Hf isotopic composition of a continental area

    Chem. Geol.

    (2011)
  • E. Garzanti et al.

    Quantifying sand provenance and erosion (Marsyandi River, Nepal Himalaya)

    Earth Planet. Sci. Lett.

    (2007)
  • E. Garzanti et al.

    Mineralogical and chemical variability of fluvial sediments 1. Bedload sand (Ganga–Brahmaputra, Bangladesh)

    Earth Planet. Sci. Lett.

    (2010)
  • E. Garzanti et al.

    Mineralogical and chemical variability of fluvial sediments 2. Suspended-load silt (Ganga–Brahmaputra, Bangladesh)

    Earth Planet. Sci. Lett.

    (2011)
  • L. Godfrey et al.

    The Hf isotopic composition of ferromanganese nodules and crusts and hydrothermal manganese deposits: Implications for seawater Hf

    Earth Planet. Sci. Lett.

    (1997)
  • S. Goldstein et al.

    Nd and Sr isotopic systematics of river water suspended material: implications for crustal evolution

    Earth Planet. Sci. Lett.

    (1988)
  • J. Götze et al.

    Distribution of REE and trace elements in size and mineral fractions of high-purity quartz sands

    Chem. Geol.

    (1994)
  • W.W. Hay

    Detrital sediment fluxes from continents to oceans

    Chem. Geol.

    (1998)
  • M. Lupker et al.

    Predominant floodplain over mountain weathering of Himalayan sediments (Ganga basin)

    Geochim. Cosmochim. Acta

    (2012)
  • S.M. McLennan et al.

    Geochemical and Nd-Sr isotopic composition of deep-sea turbidites: crustal evolution and plate tectonic associations

    Geochim. Cosmochim. Acta

    (1990)
  • J.D. Milliman

    River inputs

  • P.J. Patchett et al.

    Hafnium/rare earth element fractionation in the sedimentary system and crustal recycling into the Earth’s mantle

    Earth Planet. Sci. Lett.

    (1984)
  • T. Pettke et al.

    Radiogenic Hf isotopic compositions of continental eolian dust from Asia, its variability and its implications for seawater Hf

    Earth Planet. Sci. Lett.

    (2002)
  • A.-C. Pierson-Wickmann et al.

    The Os isotopic composition of Himalayan river bedloads and bedrocks: importance of black shales

    Earth Planet. Sci. Lett.

    (2000)
  • J. Prytulak et al.

    Astoria Fan sediments, DSDP site 174, Cascadia Basin: Hf–Nd–Pb constraints on provenance and outburst flooding

    Chem. Geol.

    (2006)
  • A. Richards et al.

    Himalayan architecture constrained by isotopic tracers from clastic sediments

    Earth Planet. Sci. Lett.

    (2005)
  • J. Rickli et al.

    The hafnium–neodymium isotopic composition of Atlantic seawater

    Earth Planet. Sci. Lett.

    (2009)
  • J. Rickli et al.

    Hafnium and neodymium isotopes in surface waters of the eastern Atlantic Ocean: implications for sources and inputs of trace metals to the ocean

    Geochim. Cosmochim. Acta

    (2010)
  • J. Rickli et al.

    Controls on the incongruent release of hafnium during weathering of metamorphic and sedimentary catchments

    Geochim. Cosmochim. Acta

    (2013)
  • D.M. Robinson et al.

    The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes

    Earth Planet. Sci. Lett.

    (2001)
  • P. Singh

    Major, trace and REE geochemistry of the Ganga River sediments: influence of provenance and sedimentary processes

    Chem. Geol.

    (2009)
  • P. Singh

    Geochemistry and provenance of stream sediments of the Ganga River and its major tributaries in the Himalayan region, India

    Chem. Geol.

    (2010)
  • S.K. Singh et al.

    Tracing the distribution of erosion in the Brahmaputra watershed from isotopic compositions of stream sediments

    Earth Planet. Sci. Lett.

    (2002)
  • M. Singh et al.

    Sediment characteristics and transportation dynamics of the Ganga River

    Geomorphology

    (2007)
  • T. Stichel et al.

    The hafnium and neodymium isotope composition of seawater in the Atlantic sector of the Southern Ocean

    Earth Planet. Sci. Lett.

    (2012)
  • J. Stummeyer et al.

    The composition of suspended matter from Ganges–Brahmaputra sediment dispersal system during low sediment transport season

    Chem. Geol.

    (2002)
  • J.K. Tripathi et al.

    Geochemistry of sediments of the Ganges alluvial plains: evidence of large-scale sediment recycling

    Quat. Int.

    (2007)
  • T. van de Flierdt et al.

    Global neodymium–hafnium isotope systematics—revisited

    Earth Planet. Sci. Lett.

    (2007)
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