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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. The Hf–Nd isotopic composition (Jacobsen and Wasserburg, 1980), respectively, in parts of seawater (Godfrey et al., 2009; Rickli et al., 2009; Zim- per 10,000. 542 J. Rickli et al. / Geochimica et Cosmochimica Acta 74 (2010) 540–557

A 45°W 30° 15° 0° 15°E Temperature (°C) C 50°N 18 22 26 30 sw 1,2 40° sn 1 AC sn 4,5,6 30° f 6,8,9 20° CC f 12,14 NEC f 16,18,2 10° f 23 ECC ITCZ GC f 27 0° f 33, sw 4 B BRC f 33,34 18° 14° 10° SEC Congo f 38 10°

32° d sn 5 f 44,47,50 20° sn 6 Selvagen Is. Benguela Current CanaryBRC Is. BC 30° S 28° 34 35 36 37 f 6 Salinity

Fig. 1. (A) Cruise track illustrating the sampling locations and schematic representation of the surface ocean currents. Abbreviations are as follows: Azores Current, AC; Canary Current, CC; North Equatorial Current, NEC; Equatorial Counter Current, ECC; South Equatorial Current, SEC; Guiana Current, GC; Brazil Current, BRC; Benguela Current, BC (for references see Section 3.1). Fish (f), snorkel (sn) and the regular seawater inlet (sw) describe the used sampling equipment, for which details are given in Section 2.1. A more detailed map of the area of the Canary Islands and temperature (dashed line) and salinity (solid line) in the surface ocean along the cruise track are given in (B) and (C).

2. SAMPLING AND METHODS procedure (Dodson et al., 1936) applied to commercially available ferric chloride hexahydrate, which resulted in 2.1. Seawater solution blanks of 1.5 pg Hf and 0.8 pg Nd per 100 mg of Fe. For each sample a 2 l aliquot was filtered and acidified A detailed description of the sampling procedures and for the determination of Sm, Nd and Hf concentration by methods can be found in Rickli et al. (2009). Surface seawa- isotope dilution. ter samples between 100 and 150 l in volume were collected Hafnium and neodymium were separated from the other with three different sampling systems while the ship was elements in the clean lab facilities at ETH Zurich following underway. The primary method used was a stainless steel previously established ion chromatographic methods fish with a Teflon inlet connected to PVC tubing towed (Patchett and Tatsumoto, 1980; Pin and Zalduegui, 1997; alongside the ship collecting water at 2–5 m depth (Croot Mu¨nker et al., 2001). In the first step Hf and Nd were sepa- and Laan, 2002). The second method used a snorkel, rated from the primary matrix elements and each other by mounted in the moon pool of the ship, collecting water cation exchange chromatography (42 ml resin bed, through an all-Teflon tubing system from approximately AG50W-X8), based on the work of Patchett and Tatsumoto 11 m below the hull. A few additional samples were ob- (1980). Hafnium was subsequently separated from the tained through the regular seawater inlet system of the ship, remaining heavy rare earth elements (HREEs) and the other which consists of polypropylene (Table 1). Three shallow high field strength elements (HFSEs) using Eichrom Ln spec samples from within the topmost 200 m of the water col- resin (Mu¨nker et al., 2001). The Nd fraction was loaded on a umn were collected using 12-l Niskin bottles attached to a second cation exchange column (1.4 ml resin bed, AG50W- CTD-rosette (~90 l). All samples were filtered through X8) to remove Ba (eluted in 2 M nitric acid) from the REEs 0.45 lm nitrocellulose membranes, and Hf and Nd were (subsequently eluted in 6 M hydrochloric acid), preceding subsequently pre-concentrated by co-precipitation with fer- the final separation of Nd from Sm following Pin and Zal- ric hydroxide on board the ship. The added amount of duegui (1997). cleaned Fe corresponded to ~5 mg/l. The ferric chloride Procedural blanks, including shipboard contributions solution used had been cleaned through a back extraction were <41 pg Hf and <150 pg Nd. The shipboard blank Table 1 Hafnium and neodymium isotopic compositions and elemental concentrations of Hf, Sm and Nd in the eastern Atlantic surface ocean. The reproducibilityofSmandNdconcentrationsestimated from repeated processing of ~1.5 l sample aliquots is better than 1% (1 SD). For Hf concentrations it corresponds to 5% at 0.4 pmol/kg and 14% at 0.1 pmol/kg (1 SD).

Sample Date Start Start End End Distance Salinity Temperature eHf Internal External Hf eNd ±0.3 Nd Sm latitude longitude latitude longitude (km) (start) (°C) (start) error 2 SEM error 2 SD (pmol/kg) (pmol/kg) (pmol/kg) sw 1 22/10/2005 44.59N 7.72W 44.54N 7.82W 9.5 – – 0.0 ±0.6 ±0.6 0.29 10.9 10.5 2.0 À sw 2 22/10/2005 44.00N 9.01W 44.00N 9.01W 0.1 – – 1.4 ±0.5 ±0.6 0.34 11.3 11.9 2.4 À waters surface Atlantic in isotopes neodymium and Hafnium snorkel 1 25/10/2005 40.31N 10.32W 40.22N 10.37W 10.5 36.10 19.25 0.2 ±0.5 ±0.6 0.30 10.8 13.3 2.7 À À snorkel 4 27/10/2005 34.22N 14.10W 34.11N 14.17W 13.7 36.65 22.00 0.0 ±0.5 ±0.6 0.32 10.5 13.3 2.7 À snorkel 5 27/10/2005 32.41N 15.20W 32.29N 15.28W 15.3 36.83 22.90 2.6 ±0.6 ±0.6 – 8.5 – – À snorkel 6 28/10/2005 30.34N 16.45W 30.20N 16.53W 16.7 36.85 23.01 10.5 ±0.8 ±0.6 0.44 8.4 25.6 5.2 À fish 6 29/10/2005 26.18N 17.13W 25.99N 17.34W 29.8 36.96 24.39 3.8 ±0.5 ±0.6 – 8.9 – – À fish 8 29/10/2005 24.55N 18.95W 24.40N 19.08W 21.5 36.95 24.68 3.2 ±0.4 ±0.6 0.52 8.4 23.9 4.9 À fish 9 30/10/2005 22.87N 20.23W 22.66N 20.38W 27.4 36.66 24.64 2.3 ±0.6 ±0.6 0.33 11.4 18.7 3.9 À fish 12 31/10/2005 19.16N 20.91W 18.90N 20.91W 28.1 36.52 25.95 1.7 ±0.7 ±0.8 0.13 12.6 24.7 5.0 À fish 14 31/10/2005 17.47N 20.94W 17.24N 20.95W 25.6 36.50 26.98 0.0 ±0.5 ±0.8 0.30 12.1 28.8 5.8 À fish 16 1/11/2005 14.66N 20.98W 14.39N 20.92W 30.5 35.71 28.37 0.1 ±0.5 ±0.6 0.38 12.1 34.5 7.2 À fish 18 1/11/2005 12.58N 20.54W 12.37N 20.50W 24.1 35.71 28.85 0.9 ±1.1 ±0.8 0.38 11.5 33.7 7.0 À fish 20 2/11/2005 10.60N 20.14W 10.37N 20.08W 26.6 35.62 29.75 1.2 ±0.5 ±0.6 0.37 11.5 30.8 6.5 À fish 23 3/11/2005 7.61N 17.99W 7.43N 17.84W 26.5 34.67 29.14 0.8 ±0.5 ±0.6 0.36 11.2 29.4 6.3 À fish 27 4/11/2005 4.32N 15.24W 4.12N 15.07W 29.3 34.35 28.45 0.1 ±0.5 ±0.6 0.38 11.5 29.7 6.3 À fish 30 5/11/2005 0.81N 12.44W 0.45N 12.15W 51.4 36.06 26.68 1.2 ±0.9 ±0.8 0.22 12.5 18.9 3.8 À sw 4 5/11/2005 0.69S 11.27W 0.69S 11.27W 0.0 35.99 26.64 1.9 ±0.9 ±0.6 0.22 11.9 19.2 3.8 À fish 33 6/11/2005 3.17S 9.34W 3.33S 9.21W 22.9 36.31 25.81 0.7 ±0.8 ±0.8 0.18 12.3 14.9 2.9 À fish 34 7/11/2005 4.79S 8.08W 4.95S 7.95W 23.0 36.05 24.52 1.0 ±0.9 ±0.8 0.17 13.8 13.1 2.5 À fish 38 9/11/2005 11.53S 2.78W 11.55S 2.77W 2.8 36.03 23.42 0.9 ±0.6 ±0.8 0.24 14.6 17.8 3.3 À fish 44 11/11/2005 17.23S 1.79E 17.40S 1.93E 24.1 35.94 19.16 0.1 ±0.8 ±0.8 0.11 16.5 11.3 1.9 À À fish 47 12/11/2005 20.55S 4.54E 20.72S 4.67E 23.0 35.78 18.96 1.0 ±1.0 ±1.5 0.08 15.4 10.3 1.9 À fish 50 13/11/2005 23.95S 7.38E 24.13S 7.54E 26.1 35.41 18.04 2.4 ±1.5 ±2.2 0.10 14.4 10.9 2.0 À Station 69/14 2/11/2005 10.62N 20.12W 40 m depth 35.59 28.48 0.8 ±0.6 ±0.6 0.55 11.9 30.5 6.3 À À 100 m depth 35.59 15.84 1.9 ±0.5 ±0.6 0.49 12.9 17.1 3.4 À 200 m depth 35.32 13.11 1.7 ±0.4 ±0.6 0.48 12.4 18.1 3.5 À 543 544 J. Rickli et al. / Geochimica et Cosmochimica Acta 74 (2010) 540–557 was estimated from the individual blanks of the involved chemicals aboard the ship, assuming no significant contri- bution from the atmosphere (dust) and the laboratory on the ship because the exposure time was short. Blank contri- External error 2 SD butions correspond to less than 1.3% for most of the Hf seawater samples, excluding the three southernmost sam- ples for which blanks are <2.9%. Neodymium blanks are less than 0.2% of the smallest sample analyzed. No correc- tions for blank were applied for Hf and Nd isotopic compo- Internal error 2 SEM sitions. For isotope dilution measurements of Sm, Nd and 9.30.6 ±0.6 ±1.3 ±0.6 ±1.5 3.3 ±1.8 ±2.2 9.4 ±0.3 ±0.6 7.77.4 ±0.5 ±0.5 ±0.7 ±0.7 8.4 ±0.5 ±0.7 150 149 178 8.9 ±0.6 ±0.7

Hf, a mixed Nd/ Sm and Hf spike was added to 10.6 ±0.320.0 ±0.9 ±0.7 ±1.5 À À À À À À À À Hf e the 2 l sample aliquots and left for five days to equilibrate. À À Subsequently, Hf and Nd were co-precipitated with ferric hydroxides and separated from the main matrix elements by cation exchange chromatography. 9.8 10.4 12.9 13.2 À TIMS, ±0.45 (2 SD) À À 2.2. Dust À

During cruise ANT XXIII/1, aerosol samples were col- lected for trace metal analysis using a high volume 3 1 9.8 12.6 12.4 13.0 13.2 13.3 12.5 (1 m minÀ ) collector equipped with single Whatman 41 12.5 À Nd e MC-ICP-MS, ±0.3 (2 SD) À À À À À À cellulose filters (Table 2). Whatman 41 filters have no rigor- À ously defined porosity, but collection efficiency has been

shown to be very high (>99.9%) for particles as small as m l

0.3 lm(Pszenny et al., 1993). The filters were acid-washed ) 3 (consecutive washes in 0.5 M HCl and 0.1 M HCl) before g/m l Bulk Atmospheric dust load ( use in order to reduce trace metal blanks. Collecting filters Dust >1 were generally changed once each day. Average air volume sampled corresponded to ~1400 m3. Wind direction was continuously monitored and the sampler was switched off when there was a risk of contamination from the ship’s ex- Distance (km) haust. The filters were stored in separate, sealed, plastic bags, frozen immediately and returned to the laboratory at the University of East Anglia for later analysis. Aerosol )yieldingconsistentresultsforHfandNdisotopes.ReplicatemeasurementsofNdisotopiccompositions 2.2 samples were also obtained from two other cruises in the End longitude (W) northeast tropical Atlantic, employing very similar sam- pling procedures: IRONAGES III in September/October

2002 (Baker et al., 2006a) and M55 in October/November End latitude (N) 2002 (Baker et al., 2006b). During M55, aerosol samples were size segregated using a cascade impactor (Baker et al., 2006b). Only the upper stages (particle diameter

>1 lm) were sub-sampled for this study, as this fraction Start longitude (W) contained the majority of the aerosol mineral dust. Aerosol dust particles were removed from the filters in a single-step procedure. The filters were placed in pre-cleaned 50 ml cen- trifuge tubes and covered in ultrapure water (MilliQ-ele- Start latitude (N) ment). Samples were agitated in an ultrasonic bath for 30 min resulting in desorption of most of the dust particles. The dust and the water were then transferred into Teflon vials and evaporated to dryness. The residue was subse- quently treated with 4 ml of aqua regia and a few drops of perchloric acid at 120 °C, in order to oxidize residual cel- lulose fibers from the filter as well as organic compounds Sample Date TM 20 2,3/11/02 8.4 24.6 10.6 24.8 248 73.76 ±1.15 TM 20TM 24 Replicate 6,7/11/02 10.6 17.1 11.0 18.9 210 69.11 ±1.08 within the dust. In following steps, the dust samples were TM 25 7,8/11/02 11.0 19.0 11.0 20.4 151 55.81 ±0.88 dissolved at 140 °C on a hot plate in a mixture of nitric, hydrofluoric and perchloric acid (7 M 1 ml, 28 M 1 ml, 7M 50ll), evaporated to dryness, and re-dissolved in a mixture of hydrochloric and hydrofluoric acid (6 M 2 ml, 28 M 2 ml) over several days. Dissolution after these steps Table 2 Hafnium and neodymium isotopic compositionsprocessed of as dust two samples aliquots and(IRONAGES through atmospheric III dust the TM loading two 04, for di ff erent M55 samples separation TM collectedCruise schemes 24) during (Section were ANT performed XXIII/1, IRONAGES on III the and same M55. processed Sample Nd M55 fraction, TM yielding 20 consistent was results between MC-ICP-MS and TIMS measurements. ANT XXIII/1ANT XXIII/1ANT TM XXIII/1 07ANT TM XXIII/1 08 31/10-1/11/05ANT TM XXIII/1 09 1,2/11/05 18.4 TM 10 2,3/11/05 TM 11 3,4/11/05 20.9 13.9 4,5/11/05 10.5 20.8 7.0 13.9 20.1 3.7 17.5 20.8 10.6 14.7 7.0 20.1 497 3.7 17.5 0.3 14.7 370 5.26 12.0 477 ±0.28 480 6.26 483 ±0.34 1.36 ±0.09 1.21 0.68 ±0.09 ±0.06 IRONAGES III TM 04 6,7/10/02 32.0 21.5 30.0 20.0 257 94.2 ±0.7 M55 M55 M55 was complete. M55 Hafnium and neodymium isotopes in Atlantic surface waters 545

Two different procedures were used for the separation of samples (Table 1 and 2). The average JMC 475 values of the Hf and Nd from dust for isotopic determinations. Small different sessions were within 93 ppm of the accepted liter- samples <5 mg (after dissolution and associated Si loss) ature value of 0.282160 (Nowell et al., 1998), to which all including ANT XXIII/1 TM 10, 11 and M55 TM 25 (Table measurements were normalized. 2) were processed using a low blank procedure developed Neodymium isotopic compositions measured by MC- for small rock samples in order to minimize the risk of sig- ICP-MS were corrected for mass bias to 146Nd/144Nd of nificant blank contributions (Aciego et al., 2009). Larger 0.7219 applying an exponential mass fractionation law. samples ranging between 12 to 45 mg in mass including External reproducibility was estimated by repeated measure- ANT XXIII/1 TM 07–09, IRONAGES III TM 04 and ments of JNdi-1 over the course of a measuring session M55 TM 24 were chromatographically processed in a (n = 14–16) and was always better than 0.33 eNd (2 SD). slightly modified scheme of the procedure described for sea- The average JNdi-1 values of the different sessions were with- water above. Neodymium was collected together with the in 74 ppm of the accepted literature value of 0.512115 (Tana- other REEs as a separate fraction of the Hf separation fol- ka et al., 2000), to which all measurements were normalized. lowing Mu¨nker et al. (2001) and was subsequently sepa- The Nd isotopic compositions of small dust samples rated from Sm (Pin and Zalduegui, 1997). One sample (<20 ng Nd), as well as a two replicate measurements of (M55 TM 20) was processed as two aliquots through both 10 ng Nd aliquots of large dust samples, were carried out separation procedures yielding consistent Hf and Nd isoto- on a Thermo Finnigan Triton TIMS at ETH Zurich. Sam- pic compositions (Table 2). A blank for the isotopic deter- ples were loaded onto double outgassed Re filaments in minations of the dust samples was obtained by processing a ~1 ll of 0.5 M hydrochloric/0.5 M nitric and trace amounts blank filter piece of similar size as those processed with the of phosphoric acid. Mass fractionation correction was per- samples and corresponded to 12 pg of Hf and 56 pg of Nd, formed in the same way as for the MC-ICP-MS measure- respectively, representing <1% for Hf and <0.4% for Nd of ments and Nd was measured as a metal. The external the smallest sample analyzed. reproducibility deduced from replicate measurement of For the three dust samples collected during M55 REE 10 ng JNdi-1 (n = 10, average = 0.512101) corresponds to patterns and Hf concentrations were determined on sepa- 0.45 eNd (2 SD). Samples were all normalized to the ac- rate sample aliquots of 1–2 mg. Rare earth element patterns cepted JNdi-1 value of 0.512115 (Tanaka et al., 2000). were measured on a Nu AttoM (Nu Instruments) single col- The replicate determinations of the Nd isotopic composi- lector high-resolution ICP-MS without any further chemi- tion of the samples IRONAGES III TM 04 and M55 TM cal processing (see Section 2.3). The Hf concentrations 24 by MC-ICP-MS and TIMS yielded consistent results were measured by isotope dilution and involved a separa- within analytical uncertainty (Table 2). tion of Hf from the main matrix elements by cation ex- The REE composition of dust samples was analyzed change chromatography and subsequent measurement on using a Nu AttoM ICP-MS at the Department of Earth Sci- a MC-ICP-MS. Rare earth element and Hf concentration ences, University of Oxford. For the REE analysis the elec- blanks were monitored by processing a representatively tro-static scan mode was used. Each run consisted of 25 sized blank filter piece and were negligible (<0.1% of the cycles, with a dwell time of 300 and 1 ls delay between peak sample sizes analyzed). jumps. Typical in-run precision (1 SD) was better than 1% for an acquisition time of less than 1 min. Samples were 2.3. Mass spectrometry spiked with 1 ppb of In (as an internal standard) and were introduced in 2% HNO3 + 0.05% HF into a peltier-cooled The Hf and Nd isotopic compositions of dust and sea- (7 °C) cyclonic spray chamber using a concentric PFA l- water, with the exception of a few small Nd dust samples flow nebulizer (Elemental Scientific Inc.) with an uptake 1 (<20 ng Nd), as well as seawater Nd and Hf concentrations rate of 100 ll minÀ . A 4-point calibration using REE multi were determined on a Nu Plasma MC-ICP-MS at ETH element standard solutions (1.0, 0.75, and 0.1 ppb concen- Zurich. Measured Hf isotopic compositions were corrected trations and a running acid blank solution) resulted in an for mass bias to a 179Hf/177Hf of 0.7325 applying an expo- r2 better than 0.9998 for each element. The In-spike was nential mass fractionation law. The measured 176Hf inten- used for drift correction. Between 1 and 2 mg of the dust sity was corrected for interferences by monitoring 172Yb samples were digested and taken up in 6 ml 2% 175 and Lu assuming identical mass fractionation for Yb, HNO3 + 0.05% HF. Aliquots for La and Ce measurements Lu and Hf. Interference corrections were small and apply- were diluted 10 or 20 times, due to the higher abundances of ing element-specific fractionation corrections as proposed these elements. Aliquots for Pr-Lu were diluted 3–6 times, by Chu et al. (2002) does not alter the results significantly depending on the sample amount. External reproducibility (less than 0.02 eHf). The external reproducibility of the of the measurements was estimated from analyzes of differ- 176Hf/177Hf determinations was estimated from repeated ent USGS rock standards (AGV-2, BCR-2, BHVO-2) and measurements of JMC 475 over the course of a measuring corresponded to 4% for La–Gd and 7% for Er–Lu. session (n = 9–20). In order to derive a representative esti- mate for the differently sized Hf samples, JMC 475 solu- 2.4. Air mass back trajectories tions were run at variable concentrations and over variable integration times. For most samples the estimated Air mass back trajectories for arrival heights of 500, reproducibility corresponds to between 0.6 and 1 eHf (2 1000 and 3000 m above the ship’s position were obtained SD), with the exception of a few Hf-poor seawater and dust from the NOAA Air Resources Laboratory (Hysplit model, 546 J. Rickli et al. / Geochimica et Cosmochimica Acta 74 (2010) 540–557

FNL data set, Draxler and Rolph, 1997) and were used to- dust is transported across the subtropical eastern Atlantic gether with the isotopic data to constrain the source area of Ocean at low altitudes (<2000 m) by the northeastern trade the collected dust. Atmospheric dust loadings were deter- winds and at higher altitudes by the Saharan Air Layers mined from Al concentration measurements in the dust (SAL). The high altitude transport of dust by the SAL dur- by assuming that all aerosol Al was derived from mineral ing boreal summer dominates the overall export of dust to dust and that the abundance of Al in mineral dust is the subtropical North Atlantic (e.g. Prospero, 1990). 7 wt.% (Baker and Jickells, 2006). For the IRONAGES Northeastern trade winds are, however, significant dust car- III and M55 cruises total aerosol Al was determined after riers as well, in particular over distances of up to 1500 km complete digestion with nitric and hydrofluoric acids, as de- offthe northwestern African coast (Chiapello et al., 1997). scribed elsewhere (Baker et al., 2006a,b). For ANT XXIII/1 The global sources of aerosols can be identified using the total aerosol Al was determined by neutron activation anal- Total Ozone Mapping Spectrometer (TOMS) and the de- ysis of portions of the aerosol filters. rived aerosol index (Herman et al., 1997). The hotspots of dust generation in northern Africa identified from several- 3. ENVIRONMENTAL SETTING year averages include the Bode´le Depression (Chad), the Taoudeni Basin (Mauritania, Mali) and to a lesser degree 3.1. Surface ocean hydrography the region to the north of the Tibesti massif (Chad, Lybia) (e.g. Middleton and Goudie, 2001; Moreno et al., 2006). The seawater samples were mainly taken from the eastern Arid areas of southern Africa include the Namib and the boundary currents of the subtropical gyres of both hemi- Kalahari Desert. Clay mineral characteristics in the surface spheres, and from the equatorial current system (Fig. 1A). sediments of the Southeastern Atlantic Ocean indicate that these areas are also significant sources of terrigenous mate- Between ~36°N (snorkel 4) and ~10°N (fish 18) the currents represent the easternmost prolongation of the eastward flow- rial to the adjacent ocean basins (Petschick et al., 1996). As ing Azores Current (AC), followed by the Canary Current in the northern hemisphere, dust transport in the southern hemisphere occurs from the Ross latitudes towards the (CC), commencing at ~30°N(Stramma, 1984). The Canary Current flows southwestward along the west coast of Africa ITCZ within the trade winds. towards the equator and closes the northern subtropical gyre by feeding into the North Equatorial Current (NEC, Fedo- 4. RESULTS seev, 1970). Close to the African coast between ~10°N and ~3°N (fish 20, 23, 27) surface water flows eastward as part 4.1. Hafnium and neodymium in the surface ocean of the Equatorial Counter Current (ECC, Richardson and Reverdin, 1987). The South Equatorial Current (SEC), a re- Hafnium and neodymium concentrations show similar gion of uniform westward flow, extends from ~3°Nto~25°S patterns along the transect of this study (Fig. 2). Hafnium (fish 30 to fish 50) (Peterson and Stramma, 1991). The south- concentrations range between a maximum of 0.52 pmol/ ern branch of the SEC south of 15°S is fed by the Benguela kg in the area of the Canary Islands and a minimum value Current (BC), which carries relatively cold and fresh waters of 0.08 pmol/kg in the southern Angola Basin. As with Hf, to lower latitudes. The properties of the BC reflect contribu- Nd concentrations also show a local maximum in the area tions from Subantarctic surface waters and entrainment of of the Canary Islands (26 pmol/kg) but are even higher be- upwelling waters along the African coast (e.g. Peterson and tween ~20°N and ~4°N, reaching maximum concentrations Stramma, 1991). of 35 pmol/kg. The most prominent features for both ele- Salinity and temperature measured at the bow of the ments are thus the elevated concentrations around the Can- ship during the cruise are displayed in Fig. 1B. The low ary Islands and in the area to the west and the south of the salinity between ~10°N and the equator marks the meridio- Sahara/Sahel region, as well as relatively low concentra- nal band of high precipitation associated with the inter- tions in the southern Angola and northern Cape Basin (fish tropical convergence zone (ITCZ). Equatorial upwelling is 44, 47, 50). In addition, a local and smaller maximum in Nd poorly visible in the salinity and temperature data and is and Hf concentrations occurs at 12°S. only reflected by a slight decrease in salinity around the Hafnium shows relatively uniform isotopic composi- equator. A further area of lowered salinity between ~3°S tions in the range of e = 0.2 to +2.4 in most surface Hf À and ~13°S reflects fresh water discharge of the Congo river. waters, with the exception of the samples taken in the vicin- The salinity decrease south of 15°S is due to the advection ity of the Canary Islands (Fig. 2, Table 1). North and south of less saline waters of the BC. of the Canaries a pronounced excursion to more radiogenic isotopic compositions is observed reaching a maximum of 3.2. Dust source areas and wind systems eHf = +10.5 (snorkel 6) to the north of the Islands. In par- allel with Hf isotopic variations, Nd isotopes are also The Sahara and Sahel regions, extending in latitude shifted to more radiogenic compositions of e = 8.9 to Nd À from approximately 25°Nto14°N, represent the vast areas 8.4. Neodymium isotopes display uniform values of À of arid to semiarid climate in northern Africa. As such, they e ~ 11 close to Spain/Portugal and e ~ 12 in the Nd À Nd À constitute the largest global source of dust to the ocean, area of highest Nd concentrations offthe northwest African estimated at 400–700 million tons per year, which repre- coast. In the southern hemisphere, from 5°S southwards, sents almost half the global annual flux (Middleton and Nd isotopic compositions become increasingly unradiogen- Goudie, 2001 and references therein). Saharan and Sahelian ic and reach a minimum e and of 16.5 in the southern Nd À Hafnium and neodymium isotopes in Atlantic surface waters 547

A B C D E

+10.5 Latitude 20°S 10° 0° 10° 20° 30° 40°N

0.1 0.3 0.5 10 20 30 -1 1 3 -16 -12 -8 0.17 .19 .21

Hf, pmol/kg Nd, pmol/kg εHf εNd Sm/Nd

Fig. 2. Hafnium and neodymium concentrations (A and B), Hf and Nd isotopic compositions (C and D), and Sm/Nd ratios (E) for surface seawater of the eastern Atlantic Ocean. The error bars of the Hf isotopic compositions (C) correspond to the average external reproducibility of the reported samples (0.7 eHf, 2 SD). For Nd isotopic compositions, Hf, Sm and Nd concentrations, and Sm/Nd ratios the errors are smaller than the symbol size (see Table 1).

Angola Basin. For the two southernmost samples (fish 44, the trade winds (trajectories at 500 and 1000 m altitude), 47, 50) this trend is reversed and the isotopic compositions shifting to more easterly directions at higher altitudes become more radiogenic again. The pattern observed for (3000 m, Fig. 4A). Apart from sample ANT XXIII/1 TM Sm/Nd ratios is similar to the pattern observed in eNd. 09, which shows a similar back-trajectory to TM 07 and However, Sm/Nd does not show a prominent shift to higher 08, the samples collected north of 10°N including those ratios close to the Canary Islands but rather remains consis- from M55 and IRONAGES III show similar isotopic char- tently at high ratios around 0.2 in the Northern Hemi- acteristics. Their Nd isotopic compositions fall within a sphere, until a marked drop to lower values occurs south narrow range between e = 13.3 and 12.4, and their Nd À À of 4°N, reaching a minimum ratio of 0.17 at 17°S. Hf isotopic compositions are also relatively uniform, vary- Combined Hf and Nd isotopic compositions of the seawa- ing between e = 10.6 and 7.5. Combined Hf and Nd Hf À À ter samples illustrate that Atlantic surface ocean waters, sim- isotopic compositions of these northern samples plot close ilar to other seawater (Godfrey et al., 2009; Rickli et al., 2009; to or slightly above the “terrestrial array” (Fig. 3). Atmo- 3 Zimmermann et al., 2009a,b), are shifted towards radiogenic spheric dust loadings exceeded 5 lgmÀ during the collec- Hf isotopic compositions for a given Nd isotopic composi- tion of TM 07 and 08 on cruise ANTXXIII/1, but were tion relative to the “terrestrial array” (Fig. 3). Most Atlantic 10 to 20 times higher during M55 and IRONAGES III surface water samples cluster in an isotopically uniform area, (Table 2). Rare earth element concentrations in the mea- which is represented by the samples taken offthe coast of sured dust samples (M55 TM 20, 24 and 25) show slight Spain/Portugal and northwestern Africa. Isotopically dis- enrichments by a factor of up to 1.8 relative to Post-Arche- tinct samples stem from the vicinity of the Canary Islands an Australian Shale (PAAS, Taylor and McLennan, 1985 (more radiogenic Hf and Nd) and the region of unradiogenic Fig. 5, Table 3). The patterns relative to PAAS are rela- Nd isotopic compositions in the southern hemisphere. With tively flat: a slight relative enrichment is observed for Sm, the exception of the most extreme Hf isotope data point from Eu, Gd, while Er, Yb, and Lu are slightly depleted relative near the Canaries, the isotopic compositions of all samples to the other REEs. Hafnium displays concentrations corre- plot on or near the “seawater array” derived from FeMn sponding to 62–68% of PAAS Hf abundances. crusts and nodules (Albare`de et al., 1998; David et al., Air mass back trajectories for the three southerly sam- 2001; van de Flierdt et al., 2006). ples collected during ANT XXIII/1 (ANTXXIII/1 TM 09-11) reflect the shift from northeastern trade to southeast- 4.2. Air mass back trajectories, atmospheric dust loadings, ern trade winds as the ITCZ is crossed (Fig. 4A). Although REE patterns and the Hf and Nd isotopic compositions of a northeasterly origin for TM 09 is indicated by the back- dust trajectory for this sample, its Nd isotopic composition is identical to those found further south with southeastern Air mass back trajectories for the dust samples collected air mass arrivals from the open ocean at 500 at 1000 m alti- north of 10°N indicate arrivals from the northeast within tude. The Hf isotopic compositions of the southerly 548 J. Rickli et al. / Geochimica et Cosmochimica Acta 74 (2010) 540–557

Since weathering and sedimentary sorting has little effect 30 Atlantic surface seawater, this study Atlantic mixed layer to deep waters on Nd isotopic compositions (Goldstein et al., 1984; Frank, Arctic seawater 2002 and references therein) we use these to identify varia- 20 Pacifc seawater FeMn crusts and nodules tions in dust sources. Neodymium isotopic signatures in fine-grained sediments (<30 lm) in northern Africa show 10 Seawater array a range of compositions increasing gradually from e ~ 18 in northern Mauretania to e ~ 12 in north- Nd À Nd À

Hf 0 ern Guinea (Fig. 4B; Grousset et al., 1998). The Nd isotopic ε variability in the dust source areas is, however, not reflected Terrestrial array -10 in clearly distinguishable dust signatures over the eastern Dust, this study: Atlantic Ocean. The only clear distinction is observed be- ANT XXIII/1 tween an area of unradiogenic Nd isotopic compositions -20 M55, > 1µm IRONAGES III in dust (e = 14.5 to 11.2) stretching in NE–SW direc- Nd À À tion offthe coast of Africa and more radiogenic Nd isotopic Crustal rocks -30 Mantle rocks compositions close to the African coast south of ~10°N (e = 10.4 to 9.1, Goldstein et al., 1984; Grousset Nd À À -20 -15 -10 -5 0 5 10 15 et al., 1988, 1998; this study). In agreement with earlier ε data, the dust samples reported here allow a distinction be- Nd tween unradiogenic northerly dust (ANT XXIII/1 TM 07, Fig. 3. Hafnium–neodymium isotope systematics of surface sea- 08; M55) influenced by the unradiogenic Nd sources of water and dust obtained in this study together with compiled Mauretania and Mali and southerly dust with dominating literature data. Mantle and crustal rocks define a linear trend, contributions from Guinea and Senegal (ANT XXIII/1 which is referred to as the “terrestrial array” (Vervoort et al., 1999; TM 09-11) (Grousset et al., 1998). This transition in dust van de Flierdt et al., 2006). Ferromanganese crusts and nodules origin is consistent with the observed air mass back trajec- define a linear relationship referred to as the “seawater array” tories (Fig. 4A). As noted by Grousset et al. (1998), the Nd (Albare`de et al., 1998; David et al., 2001; van de Flierdt et al., isotopic composition of dust is never as unradiogenic as its 2006). Atlantic surface waters (this study), Atlantic mixed layer to source area, arguing for mixing with an atmospheric deep waters (Godfrey et al., 2009; Rickli et al., 2009), Pacific background. (Zimmermann et al., 2009a), and Arctic seawater (Zimmermann et al., 2009b) plot on or close to the “seawater array” defined by Hafnium isotopes, unlike Nd isotopes, are prone to sed- FeMn crusts and nodules. They are also characterized by more imentary sorting given that unradiogenic Hf contained in radiogenic Hf isotopic compositions for a given Nd isotopic zircons will be concentrated in coarse-grained sediments composition than observed for rocks. African dust shows variable (Patchett et al., 1984). The predominately physical weather- Hf–Nd isotopic characteristics, mostly controlled by the variability ing processes in northern Africa appear to induce variabil- of the Hf isotopic composition. ity in the Hf concentrations, creating depletion in depot centers of fine-grained sediments, such as the Bode´le samples are remarkably variable, with eHf values between depression (Moreno et al., 2006), possibly implying a signif- 0.6 and 20. Two of them show a combined Hf and icant fractionation of Hf isotopic compositions. Addition- À À Nd isotopic composition similar to seawater, whereas the ally, chemical weathering may also affect the bulk Hf most unradiogenic sample in Hf (eHf = 20) lies on the isotopic compositions of rocks due to the preferential À unradiogenic side of the “terrestrial array”. Atmospheric weathering of high Lu/Hf phases (Bayon et al., 2006) but dust loadings during the collection of these southern sam- this process remains to be documented by soil data. Chem- ples were significantly lower than further north and did ical weathering and sorting on the continent thus poten- 3 not exceed 1.4 lgmÀ (Table 2). tially affect the Hf isotopic compositions of the soil material to be transported to the ocean by wind. However, 5. DISCUSSION it is not possible to distinguish the effect of sorting on Hf isotopic compositions during redistribution on the conti- 5.1. Hafnium and neodymium isotopic compositions of dust nent from that occurring during the eventual transport to the eastern Atlantic, and we therefore look at sorting in The Hf and Nd isotopic composition of the sampled general. dust can, in principle, be affected by three main factors: The low abundance of Hf in the dust samples (<70% of (i) the source area of the dust which determines the bulk PAAS concentrations) likely reflects the depletion of zir- Hf and Nd isotopic composition of the rocks reflecting their cons in dust compared to PAAS (Table 3, Fig. 5). It is “crustal age” (more precisely their crustal residence time, noted that the reported concentrations and isotopic compo- Goldstein et al., 1984), (ii) weathering and sedimentary sitions of dust from M55 were measured on the size fraction sorting on the continents, which potentially redistributes >1 lm, which implies that REE patterns, Hf concentrations the bulk Hf and Nd isotopes into isotopically distinct reser- and Hf isotopes reported here may be slightly different from voirs and (iii) sorting during generation of dust at the site of bulk dust measurements. The depletion in zircons is ex- deflation and during eolian transport to the ocean. These pected due to the high density of zircon, and it is supported three factors will be discussed below to explain the observed by slight relative depletions of the heavy REEs Er, Yb and Hf and Nd isotopic compositions of the dust. Lu in the dust samples (Gromet and Silver, 1983). Its high Hafnium and neodymium isotopes in Atlantic surface waters 549

AB 30°W 20° 10° 0° 10°E 30°W 20° 10° 0° 10°E 40°N ANT XXIII/1

IRONAGES III -8.5 -144 -9.4 / -13 -1616 30° -17 Algeria -13.4 /-12.1 -1717

Mauri- -16 20° -10.6 / -12.6 -14.6 tania Mali -9.3 / -12.4 -13.6 -144 Niger -11.2 Senegal -8.4 / -12.9 -14.2 -8.9 / -12.5 -12.7/-11.2 Guinea 10° -7.5 / -13.3 -0.6 / -9.8 -12.7 -20 / -9.8 -13.9 -3.3 / -10.4 -14.5 0° -12.8 -9.1 Increasing TOMS aerosol index 10°S

Fig. 4. (A) Hafnium–neodymium isotopic compositions (eHf/eNd) of dust samples collected during ANT XXIII/1, M55 and IRONAGES III. Air mass back trajectories (120 h) are provided for the samples collected during ANTXXIII/1 (Dashed lines 500 m, dotted lines 1000 m, solid lines 3000 m). Grey contours mark averaged atmospheric aerosol concentrations derived from TOMS over the western part of the Sahara/ Sahel between 1992 and 2005 (modified from Moreno et al., 2006). (B) Contour lines (dotted) of Nd isotopic compositions in fine sediments (<30 lm) of northern Africa (Grousset et al., 1998) and previously reported Nd isotopic compositions of dust collected over the northeastern Atlantic Ocean (Goldstein et al., 1984; Grousset et al., 1988, 1998).

Table 3 iability in the samples taken during M55 can be explained Rare earth element and Hf concentrations in ppm for dust samples by variable loss of zircons during continental redistribution collected during M55, performed on the fraction >1 lm. Sampling processes and final transport to the eastern Atlantic Ocean. locations are given in Table 2. As with the samples collected during M55, the Hf isoto- M55 TM 20 M55 TM 24 M55 TM 25 pic compositions in most other samples are consistent with La 33 53 47 variable degrees of zircon loss. This is clearly the case for Ce 84 111 97 the northerly samples collected during ANT XXIII/1 (TM Pr 9.9 12.8 11.2 07, 08), which are less radiogenic than the samples collected Nd 37 48 43 during M55, implying a slightly larger fraction of the zir- Sm 6.9 9.1 8.0 cons still present. Two of the southerly dust samples (TM Eu 1.4 2.0 1.7 09, 11) show combined Hf and Nd isotopic compositions Gd 5.9 8.0 7.2 which are similar to seawater. This is consistent with a near Dy 5.0 6.8 6.1 complete loss of zircons and would result in Hf isotopic ER 2.7 3.8 3.5 composition as radiogenic as e = +2.6 (Fig. 6). However, Yb 2.4 3.5 3.2 Hf the highly unradiogenic southerly dust sample (TM 10, Lu 0.34 0.46 0.41 e = 20) remains hard to explain, given that it is even less Hf 3.1 3.4 3.1 Hf À radiogenic than zircons of a corresponding crustal age. Variable loss of zircons seems to control the variations density, however, does not lead to a complete absence of in Hf isotopic compositions of dust from similar sources. zircon in dust (Sholkovitz et al., 1993; Pettke et al., 2002). The proportion of lost zircons, in turn, is related to atmo- The expected Hf isotopic composition with decreasing Hf spheric dust loadings. Faster settling velocities of large dust concentration resulting from zircon loss can be estimated particles lead to a decrease in grain size as dust loadings de- employing mass balance calculations and depleted mantle crease (Baker and Jickells, 2006), which is accompanied by evolutions for Hf and Nd isotopes, as outlined by van de a more complete loss of zircons. In support of this interpre- Flierdt et al. (2007) (Fig. 6). Lowering Hf concentrations tation, the most radiogenic Hf isotopic compositions were 3 from upper continental crustal values of ~6 ppm (Rudnick associated with low dust loadings of <1.4 lgmÀ during and Gao, 2003) to 3–3.4 ppm through such loss is accompa- ANT XXIII/1 (TM 09, 11). Apart from the “zircon effect”, nied by a change in Hf isotopic compositions from a second mineralogical control may lead to radiogenic Hf e = 15.1 to 9.2 to 7.9, which is consistent with our isotopic compositions of dust. As grain sizes decrease, the Hf À À À observations (Table 2). Thus the observed Hf isotopic var- relative proportion of clays in dust increases (Glaccum 550 J. Rickli et al. / Geochimica et Cosmochimica Acta 74 (2010) 540–557 and Prospero, 1980). Bayon et al. (2009) report a fair cor- relation between Al/K and eHf in fine-grained detrital sedi- Zircon free crust ments from the Congo Basin, probably implying that secondary clay minerals incorporate radiogenic Hf isotopic compositions from ambient solutions during formation. An increase in the relative proportion of clays in dust thus pos- sibly also contributes to the observed radiogenic Hf isotopic 1450 Ma, εNd = -9.9 Hf compositions of TM 09 and 11. ε It should be noted that the model assumptions by van de Flierdt et al. (2007) are not fully constrained to date and Bulk crust await better quantification. Based on the assumption that the “seawater array” is caused solely by the “zircon effect” an estimate of the fraction of the crustal Hf in zircon of 65– 1650 Ma, εNd = -12.7

70% is derived. However, there is evidence that incongruent -15 -10 -5 0 5 weathering of the “zircon free crust” also contributes to the offset between the “terrestrial array” and the “seawater 23456 array” (Bayon et al., 2006, 2009). We conclude that our Hf (ppm) observations of the Hf isotopic composition of dust are well Fig. 6. Hafnium isotopic compositions as a function of Hf explained by the model of van de Flierdt et al. (2007), concentrations in dust. The relationship assumes that the initial which, however, is not based on independent constraints concentration in the dust corresponds to typical upper continental on the partitioning of Hf between the various minerals of crustal values of ~6 ppm (Rudnick and Gao, 2003) and decreases the crust. only due to the loss of zircons along the transport path. Bulk crust and zircon Hf isotopic composition were calculated from crustal 5.2. Hafnium and neodymium composition of the surface residence times of 1450 Ma for the southerly and 1650 Ma for the ocean northerly dust samples, as derived from the corresponding Nd isotopic compositions of e = 9.9 and 12.7 and assuming a Nd À À fraction of 66% of crustal Hf in zircons (see van de Flierdt et al., 5.2.1. The area of the Canary Islands 2007). The samples collected during M55 (diamonds) are consistent The isotopic and elemental distribution of Hf and Nd with the model suggesting that variable degrees of zircon loss can between ~36°N and ~10°N reflects the southward advection explain most of the observed Hf isotopic variability in dust. of the two elements within the Azores and the Canary Cur- rents, the contribution of Hf and Nd from external sources, as well as the scavenging of the elements from the surface In spite of the general current direction, it is possible that ocean. The variability in isotopic compositions and the pro- the radiogenic Hf isotopic composition observed at ~30°N nounced concentration maxima strongly indicate that there was associated with a northward-travelling eddy transport- are two major sources for both elements, i.e. Saharan dust ing Nd and Hf from the Canary Islands. Although this can and highly radiogenic contributions from oceanic islands. not be excluded, salinity and temperature along the transect The most radiogenic Hf isotopic composition of do not give a clear indication of such an eddy at the time of = +10.5 (snorkel 6) was observed north of the Canary eHf sampling (Fig. 1B). The most radiogenic Hf isotopic com- Islands at 30 N. Given the southwestward surface flow ~ ° position is observed in the vicinity of the small Selvagen Is- in the area (Zhou et al., 2000), this radiogenic Hf isotopic lands located at 30.2°N and 15.8°W. From the above signature could in principle be advected from the north considerations these islands are the most likely source of and reflect release of Hf from Madeira. However, the loca- the highly radiogenic surface ocean Hf isotopic composi- tion of Madeira ( 33 N, 17 W) suggests that the sample ~ ° ~ ° tions observed. from 32 N (snorkel 5) should then be even more radio- ~ ° The fact that the surface ocean Hf isotopic composition genic in Hf than the one at 30 N, which is not the case. ~ ° is more strongly affected by the release of trace metals from the Selvagen Islands than that of Nd indicates that Hf is re- leased very efficiently. This suggests that the Hf/Nd ratio is 2.0 less fractionated than during weathering of differentiated rocks, which is consistent with a lower abundance of zir- 1.5 cons (see van de Flierdt et al., 2007). The highly radiogenic Hf isotopic composition indicates that the local Hf budget 1.0 is completely dominated by Hf release from this source.

Dust / PAAS M55-20 Although this conclusion is supported by isotopic evidence, 0.5 M55-24 the increase in Hf concentration in the area of the Selvagen M55-25 0.0 Islands from north to south (0.3–0.44 pmol/kg) is less pro- nounced than that of Nd (13.3–25.6 pmol/kg). This implies La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu Hf that Hf is scavenged more efficiently from the surface ocean Fig. 5. Rare earth element patterns and Hf concentrations for dust than Nd, indicating a shorter surface ocean residence time. samples collected during M55. The concentrations are normalized As noted above, the Hf isotopic maximum at ~30°N most to PAAS abundances (Taylor and McLennan, 1985). likely reflects the release of Hf from the relatively small Hafnium and neodymium isotopes in Atlantic surface waters 551

Selvagen Islands. It may well be that similarly radiogenic A Hf isotopic compositions occur immediately south of the South North Canary Islands. Dust input flux Flux from 5.2.2. The Sahara/Sahel region and the release of Hf and Nd Oceanic islands, J(x) from dust Partial South of the Canary Islands at 24°N the concentrations dissolution of Hf and Nd decrease. This decrease is accompanied by a change in the isotopic compositions of both elements, Surface ocean Advection, v which therefore can not only reflect the scavenging of ad- mixed layer Scavenging, k vected elements from the north but must involve addition of unradiogenic Hf and Nd or dilution with less radiogenic, B Dust input flux Hf- and Nd-poor surface waters. Due to the relatively sim- ple hydrographic conditions and isotopic constraints on the external inputs, a simple model is developed here to esti- Partial mate the scavenging rate of Nd and the external fluxes from dissolution the oceanic islands and the Sahara between 36°N and 10°N. In steady state, assuming constant north to south advection Surface ocean (v), a constant scavenging rate (k) and external fluxes (J(x)) mixed layer Scavenging, k that only vary with latitude, the system will be approxi- Fig. 7. Model approaches to estimate the surface ocean residence mately governed by the following equation: time of Nd (A) and the partial dissolution of Hf and Nd from dust (B). (A) The surface ocean residence time of Nd (s =1/k, where k @C @C 0 v k C J x corresponds to the scavenging rate) is estimated using the north to @t ¼ ¼À  @x À  þ ð Þ south variability in Nd concentrations and isotopic compositions The equation is valid for each isotope, and the input between 35° and 10°N in relation to the external inputs in a fluxes can be weighted accordingly by variable abundances dynamic model of the surface ocean. Model parameters are given in of 143Nd in the inputs. The model does not include a term Table 4. (B) The release of Hf and Nd from Saharan dust is estimated based on the observed surface ocean concentration for eddy diffusion, as the system is clearly dominated by adjacent to the Sahara/Sahel, using a static model and parameters advection as evidenced by a Pe´clet number of ~270. The (residence time/dust flux) given in Table 5. assumption of steady state is currently not supported by data: It may be that the concentrations and isotopic compo- Table 4 sitions of Nd in the surface ocean undergo seasonal vari- Model parameters for an estimate of the residence time of Nd in the ability depending on the residence time of Nd in the surface ocean to the south of the Canary Islands. The model and surface ocean and on the temporal variability of Nd release the results are illustrated in Figs. 7A and 8. The resulting surface ocean residence times correspond to ~1/10 to ~1/7 of a year and from dust and oceanic islands. For instance, a short resi- 2 1 dence time of only a few months would be expected to re- imply large Saharan dust fluxes between 210 and 320 g mÀ yrÀ . sult in temporal variations in the Nd concentrations of Current velocitya 10–15 cm/s the surface ocean, reflecting variations in dust deposition Mixed layer depth 50 m rates. On the other hand, a residence time of several years Nd isotopic composition of inputs would even out temporal variability in the inputs in terms Canary Islands e = 8b (+5.3c) Nd À of fluxes and isotopic compositions. Given that we cur- Saharan dustd e = 12.7 Nd À rently do not have constraints on the short-term variability Dust Nd concentratione 40 ppm of Nd isotopes and concentrations in the surface ocean, we Nd solubilityf 20% make the simplifying assumption of steady state. An illus- tration of the model and a description of the parameters Estimated Scavenging residence time 1/7–1/10 yr are given in Fig. 7A and Table 4, respectively. Assuming typical current speeds of 10–15 cm/s for the Canary Current Estimated maximum Nd fluxes (Zhou et al., 2000), the scavenging rate would need to be on Canary Islands 3.5–5.1 107 pmol/m2 yr 1  7 2 the order 7–10 yrÀ Sahara 1.2–2.2 10 pmol/m yr in order to explain the swift change in  2 concentrations and isotopic compositions of Nd south of Corresponding Saharan 210–320 g/m yr dust fluxes (assuming a the Canaries at 24°N(Fig. 8). The corresponding residence solubility of 20%) time of Nd in the surface ocean of 1/10 to 1/7 of a year a would demand high Saharan dust fluxes of 210– Zhou et al. (2000). 2 1 b Nd isotopic composition observed in the surface ocean in this 320 g mÀ yrÀ assuming a large dissolving Nd proportion of 20% (Tachikawa et al., 1999) in order to be consistent study. c Average Nd isotopic composition for Canary Islands rocks (cf. with the high concentrations of 32 pmol/kg in the surface ~ GEOROC). ocean adjacent to the Sahara/Sahel region (Table 4). This d Average Nd isotopic composition from this study. deposition rate is unrealistically high, as observations in e Typical concentrations of Nd in dust (Goldstein et al.,1984; surface sediment, sediment traps and direct collection of Grousset et al., 1998, this study). dust indicate that deposition rates are rather on the order f Tachikawa et al. (1999). 552 J. Rickli et al. / Geochimica et Cosmochimica Acta 74 (2010) 540–557

Table 5 Estimates for the solubility of Nd and Hf from dust assuming 50 steady state concentrations adjacent to the Sahara/Sahel region. The model is illustrated in Fig. 7B. 30 Dust fluxa 30 g/m2 yr Nd (pmol/kg) Mixed layer depth 50 m 10 Dust Nd concentrationb 40 ppm -6 Dust Hf concentrationc 3.2 ppm -8 Assumed residence time Nd Ndd 1–7 yr ε -10 Hf 1–3 yr

-12 Nd (10 Observed concentration between 17° and 11°N Nd 32 pmol/kg Hf 0.36 pmol/kg yr)) 7 2 6 pmol / (m Solubility estimates

Nd 2.7–19.2% 4 Hf 1.1–3.3% 400 2 a Average estimate from different literature sources (Mahowald 2 200 yr)) et al., 1999; Bory et al., 2002; Torres-Padro´n et al., 2002). b Dust flux (g / (m 0 0 Typical concentrations of Nd in dust (Goldstein et al.,1984; 10 15 20 25 30 35 Grousset et al., 1998, this study). c Latitude (°N) This study. d Elderfield and Greaves (1982). Fig. 8. Simulation of the Nd concentration and isotopic distribu- tion to the south of the Canary Islands using variable Nd input fluxes and scavenging residence times. Model parameters are given however, argues for a shorter residence time of Nd closer in Table 4. For current velocities of 10 cm/s (solid line) and 15 cm/s to 1 yr. (dashed line) scavenging residence times of ~1/10 to ~1/7 of a year In agreement with earlier studies (e.g. Greaves et al., appear plausible. More radiogenic Nd isotopic compositions next 1994), our results suggest that dust is a significant source to the Canary Islands (dotted line), which were not observed during 1 for the marine REE budget. Although an immediate effect this study, would demand faster scavenging rates of ~14 yrÀ . of partial dust dissolution is observed in the surface ocean in this study and also in surface waters of the Pacific ocean 2 1 of 10–50 g mÀ yrÀ at 20°W between 30° and 10°N(Maho- (Greaves et al., 1999), deep ocean Nd records indicate little wald et al., 1999; Bory et al., 2002; Torres-Padro´n et al., sensitivity of the North Pacific deep water isotopic compo- 2002). sitions to variations in dust fluxes over time (van de Flierdt Although the quick transition observed for Nd isotopes et al., 2004). In fact, van de Flierdt et al. (2004) exclude a between 24° and 22°N suggests a highly reactive behavior Nd solubility of more than 3.4% for the dust deposited over of Nd in the surface ocean, the observed pattern most likely the North Pacific because a larger solubility should be re- also reflects dilution of the Nd isotope signal originating flected in a more pronounced variability in the Nd isotopic from the oceanic islands with less radiogenic surface waters. composition of Pacific deep waters over the last 5 million As a consequence the short residence time of 1/10 to 1/7 of years. These observations possibly indicate that the solubil- a year, suggested by the model, is probably too low. Using ity varies between the dust sources of the Pacific and the an oceanic residence time for Nd in the surface ocean of 1– Atlantic realm. 7 yr as proposed by Elderfield and Greaves (1982),an Estimating the release of Hf from dust particles by estimate of the solubility of Nd from dust can be made un- assuming a shorter surface ocean residence time than that der the assumption of steady state concentrations adjacent of Nd on the order of 1–3 years, indicates that Hf release to the Sahara/Sahel and typical estimates of atmospheric corresponds to less than 3.3% (Table 5). 2 1 fluxes of ~30 g mÀ yrÀ (Table 5, Fig. 7B, Mahowald The above considerations provide estimates of the solu- et al., 1999; Bory et al., 2002; Torres-Padro´n et al., 2002). bility of Nd and Hf from dust, based on the observed sur- For a residence time of 7 years, Nd release from dust would face ocean concentrations and assuming steady state correspond to 2.7% of the bulk Nd in agreement with lab- (Fig. 7B). In addition, the REE patterns and the Hf isoto- oratory observations (Greaves et al., 1994; Arraes-Mescoff pic composition bear information on the mineral phases et al., 2001), whereas for a short residence time of 1 yr a lar- that release these elements. Hafnium isotopic compositions ger solubility of 19.2% similar to the proposed value of Tac- in the surface ocean adjacent to the arid regions of north- hikawa et al. (1999) is implied. The missing quantification western Africa range between eHf = +0 and +1.2, in of the dilution of the radiogenic signal by less radiogenic marked contrast to the Hf isotopic compositions of the surface waters hampers a clear preference of the combina- collected dust. It is thus clear that the Hf released from tion short residence time/high dissolution or long residence dust, which is reflected in elevated Hf concentrations, time/low dissolution. The fast change in Nd isotopic com- can not be identical to the observed bulk Saharan dust positions and decrease in concentration south of 24°N, Hf isotopic compositions of e = 10.5 and 7.5. It is Hf À À Hafnium and neodymium isotopes in Atlantic surface waters 553 noted here that, depending on the atmospheric dust load, The Nd isotopic composition of surface waters south of the Hf isotopic composition of Saharan dust may become the equatorial region starting at 5°S are distinctly less radio- similar to seawater, reflecting a more complete loss of zir- genic than any observations north of this latitude (Fig. 2). cons (see Section 5.1). In order to reconcile the observed The least radiogenic signature is as low as e = 16.5 Nd À surface ocean Hf isotopic compositions with the Hf isoto- and occurs in the central Angola basin (fish 38, Table 1). pic compositions of the dust, it is concluded that Hf is The source of this unradiogenic Nd is most likely Congo mainly released from secondary weathering phases such river sediments. A number of arguments support this idea. as clays and FeMn coatings, which are likely to contain Neodymium isotopic compositions of dust from the Namib radiogenic Hf as a result of fractionation during weather- and Kalahari deserts are mostly in the range of e = 12 Nd À ing, or from primary minerals with high Lu/Hf such as to 3(Grousset et al., 1992). Although a comprehensive À apatite. The formation of FeMn coatings is generally data set does not exist, the available data clearly indicate accompanied by a prominent enrichment of Ce relative that this source is too radiogenic to explain the negative to the neighboring REEs (Thiagarajan and Lee, 2004), surface ocean isotopic compositions observed. Dust export which is not observed for the REE patterns reported here to the surface ocean from these areas is also relatively small (Fig. 5). There is thus no evidence that release of Hf and compared to the Saharan/Sahelian area, and deposition 2 1 Nd from FeMn coatings is significant, although the absence rates over the ocean only locally exceed 5 g mÀ yrÀ (e.g. of coatings in the samples from M55 does not preclude that Mahowald et al., 2005). Highly negative Nd isotopic com- such dust coatings may be observed at other sampling occa- positions between e = 24.6 and 18.8 are, however, ob- Nd À À sions. On the other hand, the slight MREE enrichment in the served in Congo river-borne FeMn coatings and deep sea samples of M55 supports the presence of apatite (cf. Ohr turbiditic sands in the Angola basin (McLennan et al., et al., 1994), indicating that apatite may contribute to the re- 1990; Bayon et al., 2004). The most likely source of unrad- lease of Hf and Nd. Overall, it is likely that clays dominate iogenic Nd to the surface ocean in the Angola Basin is thus the release of Hf and Nd, given their large abundance in Afri- the reduction of river-borne FeMn coatings. Due to the can dust ranging between 62% and 82% (Glaccum and Pros- high productivity in the surface ocean (van Bennekom pero, 1980; Avila et al., 1997). and Berger, 1984) and the relatively low oxygen concentra- tions below 100 lmol/kg between 100 and 700 m water 5.2.3. The equatorial region and the southern hemisphere depth on the continental shelf and slope of the Angola Ba- The constancy in Nd concentrations and isotopic com- sin (e.g. WOCE line A8), the coatings are probably reduced positions between 19°N and 4°N suggest that Saharan dust under anoxic conditions close to the sediment seawater also constitutes the main Nd source south of 10°N. Advec- interface and sustain a flux of unradiogenic Nd into the tion of Nd from the west within the Equatorial Counter water column (Haley and Klinkhammer, 2003). While the Current does not affect the surface ocean Nd budget signif- release of trace elements from river-borne coatings from icantly. Although the Nd isotopic composition of the dust the Congo has a strong effect on Nd isotopes, no concurrent in this study indicates a transition to more radiogenic Nd shift is observed for Hf isotopes. This is probably not due to isotopic compositions of e ~ 10 in aerosols between a mass balance effect since Hf/Nd ratios in surface seawater Nd À 10°N and 2°N(Fig. 4), surface ocean Nd isotopic composi- of the Angola basin (0.008–0.013) are similar to elemental tions remain relatively uniform. Neodymium fluxes origi- ratios observed in the dissolved load of the Congo river nating from more radiogenic southern African source (0.015, Dupre´ et al., 1996) and by inference probably also areas are therefore less important for the surface ocean in river-borne FeMn coatings. Coatings in marine sedi- Nd budget in this region. This conclusion is consistent with ments display Hf/Nd ratios (0.005–0.019, Gutjahr, 2006) their location at the transition to the inner tropics, which which are similar to seawater ratios, indicating that their are less significant dust sources due to higher humidity, formation, at least in the marine environment, does not im- and is also supported by surface sediment signatures part an elemental fractionation. As Congo river-borne (Grousset et al., 1998). The annual migration of the ITCZ, coatings have Hf isotopic compositions ranging between reaching ~5°N latitudes during boreal winter, results in the 1.1 and +1.3 (Bayon et al., 2009), which is similar to sur- À deposition of Saharan dust south of 10°N, thus explaining face ocean waters of the Basin, also Hf, and not only Nd, the Nd isotopic compositions observed in the surface ocean could be derived from these coatings. at these latitudes. South of 4°N the influence of Saharan The reversed trend towards more radiogenic Nd isotopic dust dissolution declines and Hf and Nd concentrations compositions in the southernmost Angola Basin and the decrease to lower values of 0.2 and 20 pmol/kg, Northern Cape Basin, which is also observed for Hf iso- respectively. topes, coincides with the freshening of the surface ocean At 12°S a concentration peak of Hf and Nd is observed. and indicates the advection of more radiogenic isotopic These elevated concentrations occurred within the band of compositions within the Benguela Current. The low ele- reduced salinity in the surface ocean between 3° and 13°S mental concentrations at these latitudes suggest that Sub- reflecting the discharge of the Congo. The elevated concen- antarctic surface waters, which contribute to the Benguela trations possibly result from the desorption of Hf and Nd Current, are poor in Nd and Hf. This is consistent with ear- from particles delivered by the Congo. Extensive release lier observations on Nd close to the surface in the Southern of REE in the high salinity region of estuaries, after the Ocean (Jeandel, 1993) and is also expected from the low removal through coagulation at low salinities, has been atmospheric fluxes to the Southern Ocean reflecting its observed for the Amazon (Sholkovitz and Szymczak, 2000). remoteness from continental inputs (Wagener et al., 2008). 554 J. Rickli et al. / Geochimica et Cosmochimica Acta 74 (2010) 540–557

6. CONCLUSIONS continental inputs, although hydrothermal fluxes can not be excluded at present. Hafnium and neodymium isotopic compositions and concentrations in the surface ocean of the eastern Atlantic ACKNOWLEDGEMENTS basin indicate three different input sources. Both elements receive radiogenic inputs from the weathering of the Can- B. Zimmermann and M. Gutjahr are acknowledged for the ini- ary and Selvagen Islands. Further south, elevated Hf and tial laboratory support, F. Oberli for support with MC-ICP-MS. We thank chief scientist M. Rutgers van der Loeffand the crew Nd concentrations reflect Hf and Nd inputs from the par- of ANT XXIII/1 of RV Polarstern, as well as the involved cruise tial dissolution of Saharan dust. Unradiogenic Nd is re- participants for their support to collect the large surface water sam- leased from Congo river sediments, most likely by ples. We thank C. Jeandel and an anonymous reviewer for their reduction of river-borne FeMn coatings. 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