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Hafnium and Neodymium Isotopes in Surface Waters of the Eastern Atlantic Ocean: Implications for Sources and Inputs of Trace Metals to the Ocean

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Hafnium and Neodymium Isotopes in Surface Waters of the Eastern Atlantic Ocean: Implications for Sources and Inputs of Trace Metals to the Ocean Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 540–557 www.elsevier.com/locate/gca Hafnium and neodymium isotopes in surface waters of the eastern Atlantic Ocean: Implications for sources and inputs of trace metals to the ocean J. Rickli a,*, M. Frank b, A.R. Baker c, S. Aciego a, G. de Souza a, R.B. Georg d, A.N. Halliday d a ETH Zurich, Institute for Isotope Geochemistry and Mineral Resources, Clausiusstrasse 25, CH-8092 Zurich, Switzerland b IFM-GEOMAR, Leibniz Institute of Marine Sciences, 24148 Kiel, Germany c School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK d Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK Received 14 January 2009; accepted in revised form 30 September 2009; available online 7 October 2009 Abstract We present hafnium (Hf) and neodymium (Nd) isotopic compositions and concentrations in surface waters of the eastern Atlantic Ocean between the coast of Spain and South-Africa. These data are complemented by Hf and Nd isotopic and con- centration data, as well as rare earth element (REE) concentrations, in Saharan dust. Hafnium concentrations range between a maximum of 0.52 pmol/kg in the area of the Canary Islands and a minimum value of 0.08 pmol/kg in the southern Angola Basin. Neodymium concentrations also show a local maximum in the area of the Canary Islands (26 pmol/kg) but are even higher between ~20°N and ~4°N reaching maximum concentrations of 35 pmol/kg. These elevated concentrations provide evidence of inputs from weathering of the Canary Islands and from the partial dissolution of dust from the Sahara/Sahel region. The inputs from ocean island weathering are also reflected in radio- genic Hf and Nd isotopes. The Hf isotopic compositions of dust samples themselves are highly variable, ranging between e = 20 and 0.6. The Hf À À combined Hf and Nd isotopic compositions of dust plot close to the “terrestrial array” during periods of appreciable dust load in the atmosphere. During low atmospheric dust loading combined Hf and Nd isotopic compositions similar to seawater are observed. Most of the variability can be explained in terms of variable degrees of zircon loss from the dust samples, which in turn is linked to sorting during atmospheric transport to the eastern Atlantic Ocean and possibly presorting by sedimentary redistribution on the continent. In addition, increasing relative proportions of radiogenic clay minerals with decreasing grain size may contribute to the radiogenic Hf isotopic compositions observed. While the Nd isotopic composition in the surface ocean reflects the Nd isotopic composition of the Saharan dust adjacent to the Sahara/Sahel region, the release of Hf from that dust appears to be incongruent and results in surface ocean Hf isotopic compositions which are ~10 eHf more radiogenic than the bulk dust. Radiogenic Hf appears to be released from clays and possibly from trace apatite. Rare earth element patterns of dust samples indicate the presence of apatite but provide no evi- dence for ferromanganese grain coatings, suggesting that such coatings are insignificant in the release of Hf and Nd from Sah- aran dust to the surface ocean. The Nd isotopic composition of the surface waters becomes less radiogenic south of the equator, most likely reflecting the release of Nd from Congo river sediments. The release of Hf from Saharan dust and the Congo river sediments, however, does not produce distinct Hf isotopic signatures in the surface ocean, implying that the mobile fraction of Hf integrated over large continental areas is isotopically uniform. The Hf isotopic uniformity in the surface ocean means that the limited variability in * Corresponding author. E-mail address: [email protected] (J. Rickli). 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.10.006 Hafnium and neodymium isotopes in Atlantic surface waters 541 deep water isotopic compositions is consistent with a short deep water residence time and reflects homogenous continental inputs rather than efficient deep water homogenization. Ó 2009 Elsevier Ltd. All rights reserved. 1. INTRODUCTION mermann et al., 2009a,b) and ferromanganese crusts (FeMn) (Godfrey et al., 1997; Lee et al., 1999; Piotrowski The radiogenic hafnium (Hf) and neodymium (Nd) iso- et al., 2000), which have recorded past seawater isotopic topic composition of past seawater as recorded in authi- compositions also reveals a linear relationship of Hf and genic marine sediments provides insights into paleo- Nd isotopes, which is referred to as the “seawater array” oceanographic conditions and continental weathering on (Albare`de et al., 1998; David et al., 2001). This array is, Cenozoic (e.g. Frank, 2002), as well as on Quaternary (Pio- however, shifted towards more radiogenic values for a gi- trowski et al., 2005; Foster et al., 2007; Gutjahr et al., 2008) ven Nd isotopic composition when compared to terrestrial time scales. Due to the relatively short deep water residence rocks. The offset between the two arrays most likely reflects times of these elements, which corresponds to 500–2000 yr incongruent weathering of the continental crust with re- for Nd (Jeandel et al., 1995; Tachikawa et al., 1999; Siddall spect to Hf (e.g. Bayon et al., 2006; van de Flierdt et al., et al., 2008) and a few hundred years for Hf (Rickli et al., 2007). A significant contribution to the incongruent weath- 2009; Zimmermann et al., 2009a) the isotopic variability ering of Hf probably arises from the “zircon effect”, e.g. the of the elemental inputs to the oceans is preserved. The retention of unradiogenic Hf in the weathering resistant resulting variations between different water masses have mineral zircon (van de Flierdt et al., 2007). River data from proven useful as fingerprints of global circulation (e.g. the Vosges mountains, however, indicate that the “zircon Goldstein and Hemming, 2003). free crust” is also weathered incongruently as a function In order to better understand the behavior of these iso- of differences in the Hf isotopic composition of other min- topic tracers and interpret records of past seawater isotopic erals (Bayon et al., 2006). compositions, improved constraints on the sources of Hf An alternative explanation for the radiogenic Hf isoto- and Nd to the ocean are required. The variability of both pic composition of seawater involves hydrothermal contri- isotopic systems over time can reflect reorganization of oce- butions of Hf from oceanic venting systems (White et al., anic circulation (e.g. Piotrowski et al., 2005) but can also re- 1986; Godfrey et al., 1997; Bau and Koschinsky, 2006). late to changes in the contributions of external sources, for Although the presence of Hf-complexing fluorides in instance reflecting differing weathering regimes (Piotrowski hydrothermal systems suggests that fluids might carry sig- et al., 2000; van de Flierdt et al., 2002). nificant amounts of radiogenic Hf into the deep ocean, the The external sources of Nd to the ocean have been studied relationship between Hf and Nd observed in seawater pro- extensively (e.g. Elderfield et al., 1990; Greaves et al., 1999; vides evidence against this source. Since there is no net Haley et al., 2004; Lacan and Jeandel, 2005; Johannesson hydrothermal contribution of Nd to seawater (Michard and Burdige, 2007). Although, these studies clearly docu- et al., 1983; German et al., 1990; Halliday et al., 1992), ment that rivers and groundwater, dust, and fluxes from mar- locally variable Hf contributions from hydrothermal sys- ine sediments including exchange processes between tems would result in a decoupling of Hf and Nd isotopes dissolved Nd and sediments add Nd to the ocean, the relative and as a consequence no relationship of Hf and Nd iso- importance of these sources is currently not well defined. topes in seawater should be observed (van de Flierdt In contrast, there is little direct information on the et al., 2007). sources of Hf to the ocean. It is thought that Hf is delivered We present a continuous set of dissolved (<0.45 lm) to the ocean in large part by rivers. Removal of Hf in estu- Hf and Nd isotopic compositions and concentrations from aries significantly reduces this flux but the efficiency of re- eastern Atlantic surface waters sampled along a transect moval of ~50% (Godfrey et al., 2008) is similar to that of from the Bay of Biscay to Cape Town (Fig. 1A, RV Nd (45–80%, Elderfield et al., 1990). A comparison between Polarstern expedition ANTXXIII/1). The data set is used the Hf and Nd isotopic compositions in seawater indicates to identify Hf and Nd inputs to the surface ocean from some fundamental differences in the sources and processes different potential sources, such as the Madeira and Can- delivering Nd and Hf to the ocean. Terrestrial rocks display ary Islands, Saharan and Namibian dust, as well as large a close relationship between Hf and Nd isotopic composi- African rivers. The study is complemented by the first tion, which is referred to as the “terrestrial array” (Vervoort combined Hf and Nd isotopic compositions of aerosols et al., 1999). This coupling reflects the analogous behavior from the northeastern Atlantic Ocean between 30°N and of the Sm/Nd and Lu/Hf isotope systems in igneous pro- the equator. Hafnium and neodymium isotopic composi- cesses. Crustal rocks have low Sm/Nd and Lu/Hf elemental tions are expressed as eHf and eNd units, which are the ratios, resulting in the development of relatively unradio- deviations of the 176Hf/177Hf and 143Nd/144Nd ratios in genic isotopic compositions over time, contrasting with samples from those of the Chondritic Uniform Reservoir young mantle derived rocks which have more radiogenic (CHUR) of 0.282769 (Nowell et al., 1998) and 0.512638 isotopic compositions.
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