Hf and Nd isotopes in marine sediments: Constraints on global silicate weathering

Type Publication
Date 2009
Language English
Copyright 2009 Elsevier B.V. All rights reserved.
Author(s) Bayon Germain1, Burton K. W.2, 3, Soulet Guillaume1, 4, Vigier N.5, Dennielou BernardORCID1, Etoubleau Joel1, Ponzevera Emmanuel1, German C. R.6, Nesbitt R. W.7
Affiliation(s) 1 : Inst Francais Rech Exploitat Mer, Dept Geosci Marines, F-29280 Plouzane, France.
2 : Open Univ, Dept Earth Sci, Milton Keynes MK7 6AA, Bucks, England.
3 : Univ Oxford, Dept Earth Sci, Oxford OX1 3PR, England.
4 : Univ Paul Cezanne Aix Marseille 3, CEREGE, F-13545 Aix En Provence 4, France.
5 : CRPG, F-54501 Vandoeuvre Les Nancy, France.
6 : Woods Hole Oceanog Inst, Woods Hole, MA 02543 USA.
7 : Natl Oceanog Ctr, Southampton SO14 3ZH, Hants, England.
Source Earth and Planetary Science Letters (0012-821X) (Elsevier), 2009 , Vol. 277 , N. 3-4 , P. 318-326
DOI 10.1016/j.epsl.2008.10.028
WOS© Times Cited 62
Keyword(s) Congo fan, Seawater array, Continental erosion, Silicate weathering, Hafnium isotopes, Neodymium isotopes
Abstract The combined use of Lu-Hf and Sm-Nd isotope systems potentially offers a unique perspective for investigating continental erosion, but little is known about whether, and to what extent, the Hf-Nd isotope composition of sediments is related to silicate weathering intensity. In this study, Hf and Nd elemental and isotope data are reported for marine muds, leached Fe-oxide fractions and zircon-rich turbidite sands collected off the Congo River mouth, and from other parts of the SE Atlantic Ocean. All studied samples from the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable Nd isotopic composition (epsilon(Nd)similar to - 16), indicating that Fe-hydroxides leached from these sediments correspond to continental oxides precipitated within the Congo basin. In marked contrast, Hf isotope compositions for the same samples exhibit significant variations. Leached Fe-hydroxide fractions are characterized by epsilon(Hf) values (from -1.1 to +13) far more radiogenic than associated sediments (from -7.1 to -12.0) and turbidite sands (from -27.2 to -31.6). epsilon(Hf) values for Congo fan sediments correlate very well with Al/K (i.e. a well-known index for the intensity of chemical weathering in Central Africa). Taken together, these results indicate that (1) silicate weathering on continents leads to erosion products having very distinctive Hf isotope signatures, and (2) a direct relationship exists between epsilon(Hf) of secondary clay minerals and chemical weathering intensity. These results combined with data from the literature have global implications for understanding the Hf-Nd isotope variability in marine precipitates and sediments. Leached Fe-hydroxides from Congo fan sediments plot remarkably well on an extension of the 'seawater array' (i.e. the correlation defined by deep-sea Fe-Mn precipitates), providing additional support to the suggestion that the ocean Hf budget is dominated by continental inputs. Fine-grained sediments define a diffuse trend, between that for igneous rocks and the,seawater array', which we refer to as the 'zircon-free sediment array' (epsilon(Hf)=0-91 epsilon(Nd)+3.10). Finally, we show that the Hf-Nd arrays for seawater, unweathered igneous rocks, zircon-free and zircon-bearing sediments (epsilon(Hf)= 1.80 epsilon(Nd)+2.35) can all be reconciled, using Monte Carlo simulations, with a simple weathering model of the continental crust. (C) 2008 Elsevier B.V. All rights reserved.
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Bayon Germain, Burton K. W., Soulet Guillaume, Vigier N., Dennielou Bernard, Etoubleau Joel, Ponzevera Emmanuel, German C. R., Nesbitt R. W. (2009). Hf and Nd isotopes in marine sediments: Constraints on global silicate weathering. Earth and Planetary Science Letters, 277(3-4), 318-326. Publisher's official version : http://doi.org/10.1016/j.epsl.2008.10.028 , Open Access version : http://archimer.ifremer.fr/doc/00000/6451/