Global neodymium–hafnium isotope systematics — revisited

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Abstract

Global Nd–Hf isotope systematics can be mainly described with two linear arrays, the global silicate Earth array (“the terrestrial array”) and the global ferromanganese crust and nodule array (”the seawater array”). The offset between these two arrays provides evidence for the sources and mechanisms by which these elements are added to ocean water. However, the reason for this offset is under debate, with the two preferred hypotheses being (i) incongruent release of Hf during continental weathering and (ii) hydrothermal contribution of Hf to the seawater budget.

Here we present new Nd and Hf isotope data on glacio-marine core-top sediments from around the perimeter of the Antarctic continent. The results range from εHf =  30.0 to εHf = + 3.9 and εNd =  21.3 to εNd = + 0.9, reflecting the large range of basement ages and lithologies around the Antarctic continent. In Nd–Hf isotope space, they confirm the systematic correlations found in rocks from other parts around the world and provide valuable insights into the previously underrepresented group of sediments with very old provenance.

In this paper we revisit the cause for the offset of the seawater array from the terrestrial array using simple mass balance considerations. We use these calculations to test to what degree the seawater array could be a product of preferential weathering of “non-zircon portions” of the upper continental crust, implying retention of zircons in the solid residue of weathering. Lutetium–Hf and Sm–Nd evolution and mixing calculations show that the global seawater array can be generated with continental sources only. On the other hand, a predominantly hydrothermal origin of Hf in the ocean is not possible because the seawater Hf isotopic composition is significantly less radiogenic than hydrothermal sources, and requires a minimum fraction of 50% continental Hf. While hydrothermal sources may contribute some Hf to seawater, continental contributions are required to balance the budget.

Introduction

Combined Nd and Hf isotope analyses have become widely used in studies of solid Earth geochemistry, but their application to the low-temperature environment is still limited (for a recent review see van de Flierdt et al., 2004a). Both elements show variable compositions in seawater and their residence times in the ocean are thought to be similar (500–2000 yr; Jeandel, 1993, McKelvey, 1994, Jeandel et al., 1995, Godfrey et al., 1996, Godfrey et al., 1997, McKelvey and Orians, 1998, Lee et al., 1999, Tachikawa et al., 1999, David et al., 2001, Tachikawa et al., 2003). A number of studies have recently addressed the sources of Nd and Hf in the ocean and internal cycling processes (e.g., Lacan and Jeandel, 2005, Bau and Koschinsky, 2006, Bayon et al., 2006, Jacobson and Holmden, 2006). In the case of Nd, these studies have been accompanied by a growing number of applications of Nd isotopes as a tracer for past water mass mixing and ocean circulation given that various archives in the ocean have been shown to record the Nd isotopic composition of ambient seawater (e.g., Rutberg et al., 2000, Frank, 2002, Goldstein and Hemming, 2003, Bayon et al., 2004, Martin and Scher, 2004, Piotrowski et al., 2005, Vance et al., 2004).

Due to the very low concentrations of Hf in seawater (Godfrey et al., 1996, McKelvey and Orians, 1998) and associated analytical difficulties in measuring Hf isotope compositions, our overall understanding about fractionation processes and internal cycling of Hf in the ocean is not as well developed as it is for Nd. However, recent studies by Zimmermann et al., 2004, Zimmermann et al., 2005 and Bayon et al. (2006) have taken advantage of the advancement of MC-ICP-MS (multiple collector inductively coupled plasma mass spectrometry) techniques to measure the Hf isotope composition of seawater and river waters. The results from these studies indicate that ferromanganese crusts and nodules reliably reflect the Hf isotopic composition of seawater. This is an important confirmation because global Nd–Hf isotope systematics for most mantle and crustal rocks show a tight positive correlation called the “terrestrial array” (Vervoort et al., 1999), whereas ferromanganese crusts and nodules form a “seawater array”, oblique to the terrestrial array with higher Hf isotope ratios for a given Nd isotope composition (Albarède et al., 1998).

Based on ferromanganese crust studies, it has been suggested that the Hf isotopic composition of seawater can serve as a tracer for enhanced physical weathering resulting from glaciation because of the significant fractionation of Lu and Hf within the sedimentary system (Patchett et al., 1984, Piotrowski et al., 2000, van de Flierdt et al., 2002, van de Flierdt et al., 2004a). A recent study by Bau and Koschinsky (2006) questions this interpretation on the basis of an “elemental perspective” on Nd–Hf systematics in the ocean and proposes the contrasting view that seawater Hf is dominated by hydrothermal inputs due to efficient scavenging of dissolved riverine Hf in the estuaries.

Here we revisit the discussion on the origin of elevated Hf isotopes in seawater (e.g., Patchett et al., 1984, White et al., 1986, Godfrey et al., 1997, Albarède et al., 1998, Piotrowski et al., 2000, van de Flierdt et al., 2002, Bau and Koschinsky, 2006) by taking an “isotopic perspective” on global Nd–Hf systematics. We highlight the problem by presenting new data for detrital Nd–Hf isotope compositions from Antarctica and compare them to existing Southern Ocean Nd–Hf isotope data from ferromanganese nodules. The circum-Antarctic Nd–Hf trends faithfully follow the global systematics. As developed below, the global isotope mass balance implies that while hydrothermal contributions of Hf to the ocean are possible, continental contributions are absolutely required to explain the observed variation and distribution of Hf isotope ratios in global seawater.

Section snippets

Antarctic and Southern Ocean Nd–Hf isotope systematics

Constraining southern high latitude Nd–Hf systematics is important for several reasons. Firstly, the Southern Ocean plays a crucial role in the present-day global ocean current system (as part of the “global ocean conveyor belt”; Broecker, 1991). The connections among all three major ocean basins provided by the ACC not only permit efficient global water-mass exchange, but also control the transport of heat and other properties that influence climate (e.g., Schmitz, 1995, Rintoul et al., 2001).

Circum-Antarctic data in the context of global Nd–Hf isotope systematics

Neodymium and Hf isotopes in nearly all terrestrial bulk rock samples form a linear positive correlation, the terrestrial array (Vervoort et al., 1999, and references therein; Fig. 2). All data analyzed so far to characterize seawater, which are ferromanganese crust and nodule proxy data, plot off this trend and form the seawater array (Albarède et al., 1998). Authigenic and detrital Nd and Hf data from the circum-Antarctic realm (Roy et al., under revision, van de Flierdt et al., 2006; this

Conclusions

We presented new Hf and Nd isotope data for bulk sediments from proximal locations around the Antarctic continent. Together with previously published ferromanganese nodule data from the Antarctic Circumpolar Current, the data mimic the global Nd–Hf isotope systematics (e.g., the seawater array and the terrestrial array). In the light of the ongoing discussions on the global relationship between the terrestrial Nd–Hf isotope array and the seawater Nd–Hf isotope array, we revisited the two main

Acknowledgements

This study was supported by NSF grants OPP 00-88054 and ANT 05-48918, the Comer Science and Education Foundation, and a Lamont Postdoctoral Fellowship. We thank Marty Q. Fleisher, N. Gary Hemming, Allison M. Franzese, and Jennifer M. Cole for their help in keeping the labs and the mass specs running smoothly, and E. Troy Rasbury and Gilbert N. Hanson for letting us use their laboratory set up at Stony Brook for flux fusion. The manuscript benefited from discussions with B. Haley and M. Gutjahr,

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