A new approach to the Nd residence time in the ocean: the role of atmospheric inputs

ELSEVIER

Earth and Planetary Science Letters 170 (1999) 433–446 www.elsevier.com/locate/epsl

A new approach to the Nd residence time in the ocean: the role of atmospheric inputs K. Tachikawa Ł , C. Jeandel, M. Roy-Barman Legos (CNES=CNRS=UPS), Observatoire Midi-Pyre´ne´es, 14 av. E. Belin, 31400 Toulouse, France Received 20 April 1998; accepted 2 May 1999

Abstract Concentrations of rare earth elements (REE) and Nd isotopic ratios were analyzed for seawater, filtered suspension and sediment trap samples collected in the tropical Atlantic Ocean (EUMELI program, EUtrophic, MEsotrophic and oLIgotrophic sites, 20ºN, 18º–21ºW). This is the first REE=Nd dataset on solution and different-sized particles collected at the same site. We present direct evidence of the Nd isotopic exchange between particulate lithogenic fraction and seawater without significant mass transfer. This exchange is probably one of the main factors that simultaneously constrains the Nd concentration and isotopic ratio budget. We propose a new approach to estimate the residence time of Nd in the ocean (−Nd ) based on isotopic exchange: 200 yr < −Nd < 1000 yr. The exchange requires a partial dissolution of lithogenic Nd. We estimate that the fraction of soluble Nd proportion in atmospheric dust is of the order of 20% based on the isotopic ratios. We suggest that the partial dissolution of atmospheric fallout is probably one of the main REE sources of the ocean.  1999 Elsevier Science B.V. All rights reserved. Keywords: rare earths; Neodymium isotope ratios; marine environment; particles; processes; geochemistry; residence time

1. Introduction Scavenging of elements in solution by marine particles fractionates dissolved rare earth elements (REE) in the ocean: light REE (LREE) are preferentially removed by marine particles whereas the heavy REE (HREE) remain in solution. This fractionation makes the REE a potential tracer of the scavenging process [1–3]. The REE in marine particles consist of two fractions (lithogenic and authiŁ Corresponding

author. Present address: Department of Earth Sciences University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. Fax: C44 1223 333 450; E-mail: [email protected]

genic) of various origins. In this paper, the lithogenic fraction indicates the particulate continental material, such as aluminosilicates, and the authigenic (non-lithogenic) fraction indicates the part formed in the water column such as biogenic components, oxyhydroxide precipitates and elements scavenged from seawater. Additional information concerning the origin of the REE can be obtained with the Nd isotopic ratio (143 Nd=144 Nd) that distinguishes the lithogenic=authigenic and authigenic=authigenic fractions of various origins. Thus, the REE concentrations coupled with the Nd isotopic ratios are powerful tracers to investigate the scavenging process and to predict the fate of elements brought from the continent.

0012-821X/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 1 2 7 - 2

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K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446

The purpose of this paper is to investigate whether the reversible scavenging observed for Th [4–6] occurs for the REE. When dissolution (or desorption) of particulate Nd is balanced with scavenged Nd, this process may modify the Nd isotopic ratio without any significant mass transfer. We define this process as ‘dissolved=particulate exchange’. The Nd exchange is suggested by the oceanic budget of this element but direct evidence has not yet been reported [7,8]. We will present direct evidence of the Nd exchange and propose a new approach to the Nd residence time in the ocean. Results on the filtered suspensions [3] and trapped materials ([2] and this study) will be combined with new results of dissolved REE to describe the Nd budget.

2. Samples and analytical procedures All the samples were collected during the EUMELI program (JGOFS-France, Fig. 1). The three studied sites (EUMELI sites) are in the tropical NE Atlantic: the Eutrophic site (E-site, 20ºN, 18ºW), the Mesotrophic site (M-site, 18ºN, 21ºW) and the Oligotrophic site (O-site, 21ºN, 31ºW). Both aeolian dust flux and primary=exported production decrease from

the African coast (E-site) to the open ocean (O-site) [9,10]. The hydrology of the sites is described in [11]. Non-filtered seawater samples (5–10 l) were collected during the EUMELI 3 and EUMELI 4 cruises (September 1991 and June 1992) at the three sites. They were acidified with distilled 6 N HCl (pH < 2) on board. Suspended matter (>0.65 µm) and large sinking particles were collected using an insitu filtration system and time series sediment-traps, respectively [2,3,12]. The dissolved REE were preconcentrated with either coprecipitation with hydrous ferric oxides or complexation with HDEHP and H2 MEHP on a C18 cartridge [13]. REE recovery (¾100% for all samples) is monitored by isotopic dilution of Nd (LREE) and Yb (HREE; [12]). Blank contributions of these procedures are 1–3% for light REE (LREE) and <1% for heavy REE (HREE). The filtered suspensions were completely dissolved [14], whereas the trapped materials were leached with 0.6 N HCl [15] or 25% acetic acid [14]. An aerosol collected at Niamey (14ºN, 2ºE) was processed with the same chemical treatments as the trapped materials. REE, Mn, Al and Th concentrations of all the samples were measured by ICP–MS (ELAN 5000). The precision of the particulate samples was de-

Fig. 1. Map showing the three EUMELI sites: Eutrophic site (E-site, 20ºN, 18ºW), Mesotrophic site (M-site, 18ºN, 21ºW) and Oligotrophic site (O-site, 21ºN, 31ºW). The three EUMELI sites are characterized by a decreasing gradient of the mass flux from the African coast to the open ocean (see text).

K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446

scribed in [2,3,12]. The precision of seawater analysis is better than 5% for the LREE and 10– 15% for the HREE. We do not present Eu data of some samples because of large Ba oxide interferences. 143 Nd=144 Nd ratios were measured by TIMS (Finnigan MAT 261), using normalization to 146 Nd=144 Nd D 0.7219. Our results will be presented using "Nd notation defined by "Nd.0/ D [(143 Nd=144 Nd)sample =0.512638 1] ð 104 [16].

3. Results 3.1. REE concentrations Dissolved REE concentrations are presented in Table 1. REE=Al values of the filtered suspensions, the Niamey aerosol and mean REE=Al values of all trapped materials and their standard deviations are presented in [3] and Table 2, respectively. Detailed results of trapped materials are available on request from the first author. Fig. 2 displays a comparison of shale-normalized REE patterns for seawater, filtered suspensions and mean trapped material collected at 250 m, 1000 m and 2500 m at the M-site. The REE patterns of the seawater samples are characterized by HREE enrichment relative to LREE and a negative Ce anomaly. Shale-normalized La=Yb ratios of seawater vary from 0.42 to 0.50, whereas the

Fig. 2. Shale-normalized REE patterns of seawater samples (this study), filtered suspensions [3] and trapped materials (this study) at 250 m, 1000 m and 2500 m at the M-site. Mean REE concentrations are used to construct the REE patterns of the trapped materials. To make the comparison between the three reservoirs easier, the following presentations were used. (1) REE=Al ratios are used instead of REE concentrations for the filtered suspensions and trapped materials. Note that the REE concentrations in the suspensions (pmol=kg) correspond to REE amounts existing in volume unit of filtered seawater, whereas those in the trapped material (g=g) correspond to REE amounts existing in weight unit of the trapped particles. Thereby, REE concentration of the average shale is also divided by Al concentration (10%) to establish the REE pattern of the suspensions and trapped materials. (2) The REE concentrations of the seawater samples were multiplied by 5 ð 106 , to be plotted in the same figures as the particulate samples. To eliminate the artificial ‘zig-zag’ pattern on the HREE-side due to shale normalization, monoisotopic REE data are not presented on the REE pattern [2].

435

Ce anomaly ranges between 0.13 and 0.56 (Fig. 3). This typical seawater REE pattern is observed for all seawater samples, except for a surface sample (50 m) collected at the E-site. This sample shows a flat REE pattern as African dusts do (this study; [17,18]). Since the dust flux is very high at the E-site, this REE pattern is probably related to a partial dissolution of lithogenic particles. By contrast with the seawater samples, the filtered suspensions and trapped material show REE patterns

436

Table 1 REE concentrations (pmol=kg) and Nd isotopic ratios of seawater samples Sample a

" Nd(0) š 2Ž

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

143 Nd=144 Nd

E-site E4 E 50 E4 E 100 E4 E 200 E4 E 500 E4 E 1100

50 100 200 500 1100

84.0 17.3 23.2 24.8 25.1

146.6 13.7 13.8 10.1 24.2

18.7 3.54 5.26 5.08 4.55

73.2 18.7 21.9 23.2 24.6

11.5 3.91 4.88 4.36 4.11

nd nd nd nd nd

9.24 4.56 5.20 5.27 4.79

1.27 0.79 0.91 0.74 0.78

8.22 5.35 6.31 5.54 5.16

1.73 1.53 1.34 1.39 1.46

5.07 4.64 4.98 5.08 4.43

0.70 0.67 0.69 0.65 0.66

4.21 4.23 4.71 3.60 4.99

0.65 0.67 0.71 0.70 0.76

nd 0:512020 š 10 0:512011 š 10 0:511974 š 11 0:511971 š 11

nd 12:1 š 0:2 12:2 š 0:2 12:9 š 0:2 13:0 š 0:2

M-site E3 M 50 C 60 E3 M 140 E3 M 250 E3 M 500 E3 M 1000 E3 M 2500 E4 M 500 E4 M 1000

55 140 250 500 1000 2500 500 1000

24.3 24.6 35.8 27.9 23.6 40.7 nd nd

18.7 16.2 39.7 20.6 7.6 8.5 nd nd

5.17 5.50 8.44 5.80 4.39 5.65 nd nd

24.6 26.5 30.9 33.7 29.2 22.7 nd nd

5.11 4.38 4.88 4.77 3.98 5.06 nd nd

nd nd nd nd nd nd nd nd

4.13 4.07 4.95 4.35 4.44 5.05 nd nd

0.77 0.72 1.03 0.74 0.75 0.93 nd nd

5.64 4.69 5.56 5.88 5.36 6.56 nd nd

1.19 1.12 1.47 1.38 1.48 1.87 nd nd

4.18 3.88 5.50 4.76 4.95 5.64 nd nd

0.58 0.51 0.84 0.66 0.75 0.93 nd nd

3.94 3.43 4.51 4.35 4.74 5.62 nd nd

0.59 0.52 0.78 0.67 0.86 0.99 nd nd

0:512012 š 10 0:512046 š 11 0:512013 š 11 0:512085 š 15 0:512053 š 16 0:512044 š 10 0:51204 š 11 0:512051 š 10

12:2 š 0:2 11:6 š 0:2 12:2 š 0:2 10:8 š 0:3 11:4 š 0:3 11:6 š 0:2 11:8 š 0:2 11:5 š 0:2

O-site E3 O 50=1 b E3 O 50=2 b E3 O 100 E3 O 175 E3 O 250 E3 O 500 E3 O 2500 E4 O 1000 E4 O 2500

50 50 100 175 250 500 2500 1000 2500

nd 24.3 19.1 18.0 19.6 20.3 25.0 21.5 nd

28.3 28.1 17.4 15.4 10.1 4.7 18.1 8.4 nd

6.05 6.23 4.40 3.81 4.17 4.02 4.92 3.90 nd

27.3 27.7 20.8 17.7 18.6 18.2 19.5 16.5 nd

5.78 5.10 3.73 3.53 3.49 3.83 3.37 3.26 nd

1.42 nd nd nd nd nd nd nd nd

6.15 5.89 4.69 4.29 4.78 3.95 3.44 4.11 nd

1.03 1.03 0.74 0.72 0.74 0.78 0.66 0.71 nd

6.89 7.14 5.45 5.62 5.68 5.56 5.20 5.08 nd

1.58 nd 1.32 1.44 1.28 1.40 1.26 1.45 nd

4.74 4.69 4.21 4.28 4.09 5.39 4.63 4.41 nd

0.63 0.67 0.59 0.62 0.63 0.71 0.66 0.63 nd

3.44 3.76 3.60 3.71 3.43 4.50 4.15 4.82 nd

0.59 0.62 0.59 0.59 0.63 0.72 0.70 0.77 nd

0:512025 š 12 0:511999 š 20 0:512054 š 11 0:512084 š 10 0:512099 š 11 0:512100 š 10 nd nd 0:512029 š 10

12:0 š 0:2 12:5 š 0:4 11:4 š 0:2 10:8 š 0:2 10:5 š 0:2 10:5 š 0:2 nd nd 11:9 š 0:2

295.2

592.3

71.7

263.4

49.9

10.6

40.7

7.7

33.8

8.15

22.12

3.73

20.23

3.49

Shale c (µmol=kg)

nd D not determined. a Sample names ‘E3’ and ‘E4’ correspond to the samples collected during EUMELI 3 and EUMELI 4 cruises, respectively. b Duplicates. Only sample E3 O 50=2 was preconcentrated using the method of Shabani et al. [13]. c Average shale compositions reported by De Baar et al. [34], Haskin and Haskin [35] and Piper [36].

š 2Ž

K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446

Depth (m)

K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446 Table 2 Mean REE=Al values and their standard deviations (10 of Niamey aerosol and lithogenic reference Trapped material Mesotrophic site 250 m (n D 14) 1000 m (n D 12) 2500 m (n D 22) Oligotrophic site 250 m (n D 2) 1000 m (n D 8) 2500 m (n D 18) Niamey aerosol (n D 1) Lithogenic reference a

5

437

g=g) of trapped material from the EUMELI sites together with REE=Al ratios

La

Ce

Pr

Nd

Sm

Eu

Gd

av std av std av std

56.0 7.55 58.8 8.91 53.6 8.01

117 17.2 130 19.7 113 15.0

13.6 2.45 14.1 2.12 12.4 1.82

51.2 10.1 52.9 8.7 46.5 6.99

10.1 2.25 10.2 1.69 8.94 1.49

nd nd nd nd nd nd

8.62 2.45 8.76 1.91 7.21 1.40

av std av std av std

68.3 5.96 62.9 3.40 61.1 2.96

138 12.4 130 7.64 131 8.54

16.6 2.06 15.0 0.854 14.3 0.728

62.8 8.76 55.7 2.68 53.6 2.68

12.0 1.75 10.6 0.501 10.2 0.567

nd nd nd nd nd nd

10.5 2.68 8.26 1.05 8.34 0.590

70.2 38.9

138 78.5

17.4 9.0

61.7 33.2

11.1 6.2

2.11 nd

9.82 4.6

Dy

Er

Yb

Lu

7.65 1.90 7.83 1.44 6.32 1.09

4.39 1.11 4.49 0.845 3.65 0.623

4.37 1.13 4.32 0.826 3.46 0.651

0.657 0.187 0.667 0.144 0.529 0.110

8.99 1.84 7.80 0.376 7.29 0.539

4.90 0.867 4.31 0.190 4.08 0.297

4.46 0.612 3.91 0.072 3.50 0.351

0.624 0.112 0.565 0.022 0.503 0.042

5.86 2.3

6.99 2.1

0.857 nd

10.6 4.3

nd D not determined. a The lithogenic reference (component ‘lith 2’) values correspond to the lowest REE=Al ratios of all analyzed bulk samples. Except for Er and Yb, samples selected as references are trapped materials: ‘II2M22’ for La, Ce, Pr and Nd and ‘IV4M2’ for Sm, Gd and Dy [2,12]. Sample taken as reference for Er and Yb is filtered suspension at 500 m at the M-site [3].

characterized by LREE enrichment relative to HREE and a positive Ce anomaly (Fig. 2). Even though these REE fractionations are small for the trapped material, the same pattern is systematically repeated for most of the samples. The REE fractionations are

more emphasized for the filtered suspensions than for the trapped material. In addition, depth profiles of the Ce anomaly observed on the filtered suspensions display a ‘mirror image’ of those of the seawater samples (Fig. 3). The Ce anomaly evolves with

Fig. 3. Depth profiles of Ce anomaly observed in the seawater samples and filtered suspensions at the three EUMELI sites. Cerium anomaly is defined by Ce anomaly D 2(Ce=Ceshale )=(La=Lashale C Pr=Prshale ). A Ce anomaly of less than 1 corresponds to a Ce depletion, whereas a value greater than 1 corresponds to a Ce enrichment relative to its neighboring REE.

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K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446

Fig. 4. Depth profiles of " Nd(0) of the seawater samples (open symbols: this study) and the filtered suspensions (closed symbols: [3]) at the E-site, M-site and O-site. The solid line in the E-site figure recalls the " Nd(0) depth profile of the M-site seawater. The " Nd(0) of the E-site seawater are systematically 1¾2 "-unit more negative than the M-site values, although the water masses are identical.

depth, negatively in the seawaters (0.13–0.91) and positively in the filtered suspensions (0.79–1.59). 3.2. Nd isotopic ratios 143

Nd=144 Nd values for seawater samples are presented in Table 1. New data on Nd isotopic ratios of the trapped materials and the Niamey aerosol are available in EPSL on line (EPSL Online Background Dataset 1 ). Fig. 4 presents the comparison of "Nd.0/ between the seawater samples and filtered suspensions. In the surface layer, the "Nd.0/ of the seawater samples ( 13 to 12) are similar to those of the filtered suspensions at the three sites. Below 250–500 m at the M-site and O-site, the seawater values become about 1 "-unit less negative than the suspension values, although the vertical variation is similar (Fig. 4). At the E-site, the similarity of seawater and suspension "Nd.0/ are maintained at all depths. We note that the E-site seawater "Nd.0/ values ( 13.0 to 12.1) are 1–2 "-units more negative than those at the M-site ( 12.2 to 10.8, solid line in Fig. 4). This discrepancy between the two sites is not related to different water masses because temperature and salinity identify the same 1 http://www.elsevier.nl/locate/epsl, mirror site: http://www.elsevier.com/locate/epsl

water masses at both sites [11]. This point will be discussed later. Fig. 5 presents "Nd.0/ distributions of all the trapped samples ([2] and this study) subjected to mild chemical leachings (0.6 N HCl or 25% acetic acid): the labile (acid soluble) and refractory (acid non-soluble) fractions. The labile "Nd.0/ (ca. 12) are systematically closer to the seawater values at all depths at both sites. The labile "Nd.0/ do not vary significantly with depth (Fig. 5). The refractory "Nd.0/ (ca. 14 at the M-site and ca. 13 at the O-site) overlap the data on lithogenic material estimated from weakly leached sediments and aerosols of this region . 14:4 š 0:2 at the M-site and 13:1 š 0:5 at the O-site [19]. Our leaching result on the Niamey aerosol implies that the dust contains an easily soluble fraction which is isotopically different from the remaining refractory residue (see EPSL Online Background Dataset 1 ).

4. Discussion In the following, we will estimate the proportion of authigenic Nd in the filtered suspensions with (1) the bulk Nd concentration and (2) the Nd isotopic ratio. The Nd exchange does not modify the particulate Nd concentration but may modify the isotopic

K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446

439

Fig. 5. Distributions of labile and refractory " Nd(0) in the trapped materials collected at 250 m, 1000 m and 2500 m ([2] and this study). The labile (soluble) and refractory (non-soluble) fractions are estimated by leaching experiments using 0.6 N HCl or 25% acetic acid. Figures in the histograms present mean " Nd(0) and their standard deviations (1¦ ) in each fraction. Ranges of the seawater values and lithogenic material are estimated from this study and [19] (seawater: see Table 1; dust: 14:4 š 0:2 and 13:1 š 0:5 at the M-site and O-site, respectively).

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K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446

ratio. Therefore, one can quantify the exchanged Nd proportion as the discrepancy of the two estimates, if this process occurs. Using the estimated authigenic REE proportions, we will assess the REE partition coefficients .K d / to predict REE carriers of the EUMELI suspensions. Finally, we will propose a new approach to the oceanic Nd residence time using the suspension and trapped material results. To quantify the proportion of authigenic REE .X auth /, we apply a binary mixing model in which we assume that marine particles are constituted of homogeneous authigenic and lithogenic fractions. However, the Niamey aerosol contains easily soluble lithogenic REE with a Nd isotopic ratio distinct from the more refractory part. We assume that these weakly bonded REE (component ‘lith 1’) dissolve at the ocean=atmosphere interface. Therefore, the REE concentrations and Nd isotopic ratios in the bulk particles are determined by the mixing of an authigenic (component ‘auth’) and a refractory lithogenic fraction (component ‘lith 2’). 4.1. Estimation of authigenic REE proportion 4.1.1. Estimate based on the bulk concentration Since Al is essentially contained in the lithogenic fraction, we can correct the lithogenic REE contributions to the bulk particulate REE using REE=Al ratios. The authigenic REE concentration (REE)auth is calculated by subtracting the lithogenic REE concentration (REE)lith 2 from the bulk REE concentration (REE)bulk : .REE/auth X auth D .REE/bulk D

.Al/bulk Ð .REE=Al/lith 2 .REE/bulk     Al REE Ð REE bulk Al lith 2

.REE/bulk

D1

(1)

The (REE=Al)lith 2 is defined as the lowest REE=Al ratio of all our bulk samples (Table 2). Since African dust contains the soluble lithogenic component 1, it is not appropriate as component 2 (note that lithogenic reference values are much lower than the aerosol values in Table 2). Our choice is based on the hypothesis that samples with the lowest REE=Al ratio are the least ‘contaminated’ by the authigenic

REE. The estimated authigenic Nd proportion in the trapped material with this method is between the one estimated from acetic acid leaching and from HCl leaching (EPSL Online Background Dataset 2 , and [12]). The HCl leaching was too strong, yielding the partial dissolution of the lithogenic fraction whereas acetic acid leaching is milder than that with HCl [2]. These observations suggest that our lithogenic reference is reasonably constrained. We calculate in situ partition coefficient .K d / of REE between the filtered suspensions and seawater using the X auth (Eq. 1): [REE]ads .g=g/ Kd D [REE]diss .g=ml/ [REE]bulk .g=ml/ Ð X auth =Cp .g=ml/ (2) [REE]diss .g=ml/ The [REE]diss is the dissolved REE concentration given in Table 1. The authigenic REE concentration [REE]ads is a product of the bulk REE concentration in the suspensions [REE]bulk (g=ml) [3], the X auth (%) and the concentration of the suspended matter Cp (200–300 µg=l in the surface and 20–30 µg=l in deepwater at the EUMELI sites, Table 3). The Cp estimate is based on nephelometry profiles [20] and weights of filtered suspensions [21] at the EUMELI sites. The resulting K d (log unit) ranges between 3.7 and 6.5 at the three sites. The main uncertainty of K d arises from the particle concentration .Cp / and authigenic REE proportions .X auth /. In the worst case the Cp varies by a factor of 1.5 and the X auth varies š10%, resulting in a variation of K d 0.2 log unit. The range of K d estimated here is in reasonable agreement with values (5–7 log unit) in the western Indian Ocean [7]. We compare our in situ K d with theoretical values based on thermodynamic constants. A linear free energy relationship is expected between the surface adsorption constants and the first hydrolysis constants of O-donor surfaces. Therefore, the K d is expressed as a function of the first hydrolysis and complexation constants considering soluble ligands [22]. The soluble REE are assumed to be complexes of the first and second carbonate, sulfate and chloride and free ion. La-normalized K d values are used to simplify D

2

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K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446

441

Table 3 Partition coefficients (Kd ) of REE between the filtered suspensions and seawater at the EUMELI sites (log unit) together with in situ [CO23 ] concentrations and thermodynamic constants for a value determination log.K d /

Site Depth Particle concentration a (m) (µg=l)

In situ [CO23 ]T b (µmol=kg)

E

50 100 500 1100

250 200 50 50

195 177 105 97

4.0 5.2 5.3 4.5

4.4 5.9 6.6 5.8

4.2 5.3 5.4 4.6

4.3 5.2 5.4 4.8

4.4 5.2 5.6 5.2

4.5 5.1 5.4 4.9

4.2 4.8 5.1 3.7

4.2 4.5 5.0 4.2

3.7 4.5 4.5 4.3

M

50 100 2500

200 60 30

200 145 130

5.2 5.3 5.0

5.5 6.1 6.3

5.1 5.4 5.2

5.0 5.2 5.2

5.0 5.2 5.2

4.9 5.2 5.1

4.7 5.0 4.7

4.6 4.9 4.5

4.4 4.9 4.5

O

50 100 1000

80 50 20

262 254 97

5.3 5.4 5.3

5.4 5.3 6.5

5.2 5.2 5.4

5.3 5.3 5.3

5.1 5.2 5.1

4.8 4.9 5.0

4.9 5.0 4.8

4.6 4.6 4.6

Thermodynamic constants (at 25ºC) c 0 log.þ1MOH /

8.66 not used

8.27

8.16

7.96

7.93

7.81

7.73

7.60

SW / log.þ1MCO3

4.57 not used

5.01

5.12

5.33

5.27

5.45

5.56

5.73

SW log.þ2MCO3 / 0:7 log.þ1MSO4 / 0:7 / log.þ1MCl

7.92 not used

8.82

9.00

9.28

9.26

9.57

9.80

10.12

1.95 not used

1.95

1.97

2.00

1.99

1.95

1.92

1.91

0.41 not used

0.44

0.48

0.52

0.57

0.59

0.62

0.63

La

Ce

Pr

Nd

Sm

nd nd nd

Gd

Dy

Er

Yb

nd D not determined. a The particle concentration is estimated from nephelometry profiles [20] and filtered suspension weight [21]. b [CO2 ] D [CO2 ] C [NaCO ] C [CaCO0 ] C [MgCO0 ]. In situ concentrations are calculated using salinity, temperature, pH, 3 T 3 3 3 3 alkalinity, phosphate and silica concentrations (see text). c Compilation of Byrne and Sholkovitz [23]: the first hydrolysis constants (zero ionic strength), the first and second carbonate complexation constant in seawater, sulfate complexation constants (0.7 mol=kg ionic strength) and chloride complexation constants (0.7 mol=kg ionic strength).

the equation (see [22] for a detailed derivation of the equation): # " 0 /a .þ1MOH (3) log.K d /n-La D log XX 1C þi j Ð [L i ] j n-La where XX i

i

þi j Ð [L i ] j D

j

X SW .þ1MCO C [CO23 ] 3

aD

j

C

SW þ2MCO [CO23 3

2

] C

0:7 þ1MSO [SO24 4

]C

0:7 þ1MCl [Cl

Thermodynamic constants come from the compilation of [23] (Table 3). The constant a characterizes the O-donor surfaces. Amorphous iron hydroxides are characterized by a D 1:17 [24], carboxylic groups of organic matter have a value of a D 1:61 [25] and for Mn oxides, a D 5. Replacing log.K d /n-La by the in-situ log.K d /n-La , we estimate a for the suspended matter at the EUMELI sites:

]/

0 where: þ1MOH is the first hydrolysis constants (zero SW SW ionic strength); þ1MCO , þ2MCO are complexation 3 3 C constants for MCO3 and M(CO3 )2 in seawater; 0:7 is complexation constant for MSOC þ1MSO 4 (0.7 4 0:7 mol=kg ionic strength); þ1MCl is complexation constant for MCl2C (0.7 mol=kg ionic strength).

  XX j .log.K d /n-La /in situ C log 1 C þi j Ð [L i ] i 0 log.þ1MOH /n-La

j

n-La

(4) Carbonate complexes are the most abundant dissolved REE species in seawater. In this calculation, we use in-situ [CO23 ] concentration (free plus ion-paired: Table 3) estimated at each depth using

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K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446

program CO2SYS version 01.04 (by E. Lewis), using temperature, salinity, alkalinity, pH, dissolved silica and phosphate concentrations (Web site, http:==www.obs-vlfr.fr=jgofs=html=bdjgofs.html). Concentrations of free sulfate and chloride ions are 0.0095 mol=kg and 0.56 mol=kg, respectively [23]. Temperature dependencies of the constants do not influence a, within our temperature range (6.2 to 24.4ºC). We obtain a D 1:36 š 0:37 (average value for the three sites and the standard deviation) for EUMELI suspended matter. Cerium is not included for this calculation because of its redox chemistry. Uncertainties on this K d estimate due to the particle concentration and the authigenic REE proportion are minimized because La-normalized values are used. This value of a agrees very well with am-Fe(OH)3 value .a D 1:17/ and is comparable with the value for organic matter .a D 1:61/ whereas it is significantly lower than the value for Mn oxides .a D 5/. This result suggests that in spite of the formation of the Ce anomaly, the Mn oxides are not the only REE carrier at the EUMELI sites. 4.1.2. Estimate based on Nd isotopic ratio The bulk "Nd.0/ of the marine particles are expressed by the mixing of the components ‘auth’ and ‘lith 2’ [2]: ."Nd /bulk D X auth Ð ."Nd /auth C .1 X auth D

."Nd /bulk ."Nd /auth

."Nd /lith 2 ."Nd /lith 2

X auth / Ð ."Nd /lith 2 (5a) (5b)

where ."Nd /bulk , ."Nd /lith2 and ."Nd /auth are the "Nd.0/ of the bulk, the component ‘lith 2’ and the component ‘auth’, respectively. The ."Nd /bulk is directly measured. For ."Nd /lith2 , we take the values of weakly leached sediments and aerosols . 14:4 š 0:2 at the M-site and 13:1š0:5 at the O-site [19]. We assume that the authigenic fraction of the suspended matter is in isotopic equilibrium with the surrounding seawater [8,26] and that ."Nd /auth corresponds to the seawater value (Table 1). In the case of the trapped material, the assumption of authigenic Nd in isotopic equilibrium with seawater may not be relevant because rapidly sinking particles may integrate the authigenic Nd from different depths. This problem has been treated elsewhere [2].

Fig. 6. Depth profiles of authigenic Nd proportion in the M-site filtered suspensions. The proportion estimated from the concentrations (Eq. 1) is systematically lower than that estimated from the isotopic ratios (Eq. 5b).

4.2. Neodymium exchange between seawater and filtered suspensions Fig. 6 presents the depth profiles of the authigenic Nd proportion calculated for the M-site filtered suspensions using the concentrations on one hand (Eq. 1) and the isotopic ratios on the other hand (Eq. 5b). The two profiles have similar shapes but the estimates based on the "Nd.0/ (70% at the surface and 40% in the deep water) are systematically higher than those based on the concentrations (50% at the surface and 20–30% below 1000 m: Fig. 6). We do not believe that this discrepancy results from our lithogenic references (Nd=Al and Nd isotopic ratio): to obtain consistent authigenic proportions of the two methods, it would be necessary to decrease either (Nd=Al)lith 2 by a factor of 2 or ."Nd /lith2 by 1 "-unit, that is ."Nd /lith D 15:4 š 0:2/ which are unfeasibly low values of Nd=Al and "Nd.0/ in this region (Table 2, [18,19]). If the particulate matter loses some lithogenic Nd to the solution and gains the same amount of Nd from the solution, the net Nd gain is zero. When the authigenic and lithogenic "Nd.0/ are different, the exchange modifies the "Nd.0/ of the marine particles but not their Nd=Al ratio. While the concentration estimate accounts only for excess Nd relative to the

K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446

lithogenic reference, isotopes keep a record of the whole exchange process. The authigenic Nd proportion estimated from the concentration corresponds to the adsorbed Nd, whereas the authigenic Nd proportion estimated from the isotopic ratios corresponds to the sum of adsorbed and exchanged Nd. Thus, the difference between the two estimates (20–30%) corresponds to the exchanged Nd content. At the E-site, the seawater "Nd.0/ is very similar to the suspension values and it is significantly more negative than at the other sites (Fig. 4). The more negative "Nd.0/ at the E-site is a good example of the Nd exchange. At this site, the surface seawater REE pattern shows a dust-like shape, suggesting that significant partial dissolution of lithogenic REE occurs (Table 1). In addition, this area is located on the continental slope where the sediment contains a large amount of organic matter [10]. Thus, high dust flux and reducing sedimentary conditions could induce a release of lithogenic Nd [27]. Since the dissolved Nd concentration is not higher at the E-site (except for 50 m samples) than at the other sites, scavenging of Nd from seawater may occur simultaneously with lithogenic Nd release. 4.3. Residence time of Nd in the ocean The Nd exchange observed at the EUMELI sites seems to be a key to solving the ‘Nd paradox’ (the discrepancy of Nd residence times based on the concentrations and isotopic ratios). The dissolved Nd depth profile shows a nutrient-like shape. During deepwater circulation, the dissolved Nd concentration increases from the Atlantic to the Pacific Ocean due to remineralization of particulate Nd, as observed with dissolved silica concentration. This accumulation effect suggests that the residence time of dissolved Nd is relatively long (¾104 yr [7]). However, the "Nd.0/ of seawater varies with oceanic basins, suggesting a shorter Nd residence time (a few hundred years or less [28]) than the global circulation time (¾1500 yr [29]). Dissolved=partic ulate Nd exchange was suggested as the process required to balance "Nd.0/ in deepwater [7]. Arguments of these authors are that: (1) laterally transported dissolved Nd flux is not sufficient to obtain distinct "Nd.0/ in the deep Atlantic ( 12.1), Indian ( 8.3) and Pacific ( 3.5); (2) for a

443

consistent "Nd.0/ , it is necessary to transport surface Nd to the deep water; (3) this influx is canceled by particulate Nd formation in deep water to maintain mass balance. Consequently, a seawater=authigenic Nd exchange is proposed. This exchange is an essential process for the deepwater Nd budget but it does not constrain oceanic Nd residence time because it concerns only intra-oceanic Nd fluxes and describes neitherthe Nd flux into the ocean nor out of the ocean. We focus on lithogenic=authigenic Nd exchange which allows an estimate of real scavenged Nd flux. Scavenged Nd flux is quantified using the data of trapped material and filtered suspensions. The combination of authigenic=authigenic and lithogenic=authigenic Nd exchange must be considered to establish the Nd budget in the whole ocean. The Nd residence time .−Nd / in the ocean is quantified from the flux into the ocean (river runoff and atmospheric fallout) or the flux out of the ocean: −Nd D Q=F

(6)

where Q is total dissolved Nd content in the ocean .Q D 5:6 ð 1012 g [8] and F is the in-flux or out-flux of Nd. We note that refractory lithogenic Nd just crossing the water column should not be taken into account in the Nd flux to seawater. Using a river runoff (after Nd removal in estuaries) of 5:0 ð 108 g=yr [30] and soluble Nd flux from dust of 5:4 ð 108 g=yr (atmospheric fallout of 9:1 ð 1014 g=yr [31], Nd concentration in dust of 30 µg=g [19] and partial dissolution of Nd in dust of 2% [17]), we obtain a global Nd residence time of ¾5000 yr. This value (4 to 5 times more than the oceanic mixing rate) is incompatible with the heterogeneous distribution of the Nd isotopic signal in the ocean. Below, we compare this residence time with those estimated from the authigenic Nd flux based on "Nd.0/ at the EUMELI sites. 4.3.1. Nd residence time based on the trapped material Settling large particles is considered to be the main flux of particulate material to the seafloor. The authigenic Nd flux out of the ocean is given by the following equation: Fout D Fm ð [Nd] ð X auth

(7a)

where Fm is mass flux estimated from trapped ma-

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Table 4 Global Nd out-flux based on the trapped material from EUMELI sites

2

1) b

Mass flux (g m yr Nd concentration in large particles (µg=g) b Adsorbed and exchanged Nd proportion (%) c Nd out-flux (g m 2 yr 1 ) d Surface area (%)

Marginal region a

Pelagic region a

65 10 40 2.6 ð 10 10

13 12 45 7.0 ð 10 90

4

Global Nd out-flux (g=yr) e

5

3.2 ð 1010

a Marginal

and pelagic regions correspond to the M-site and O-site, respectively. Mass fluxes and Nd concentrations in large particles at each region are based on the mean values of trapped material collected at 2500 m at the M-site and O-site [2,12]. c Adsorbed and exchanged Nd proportions are estimated using Eq. 5b (see text), assuming that (i) lithogenic " Nd(0) values are 14:4 š 0:2 at the M-site and 13:1 š 0:5 at the O-site [19] and (ii) authigenic " Nd(0) values are 12:4 š 0:4 at the M-site and 12:0 š 0:4 at the O-site [2,12]. d (Nd outfluxes) D (mass fluxes) ð (Nd concentrations in large particles) ð (adsorbed and exchanged Nd proportion). e Global Nd out-flux is estimated by a surface weighted mean of the two region fluxes. b

terial, [Nd] is Nd concentration in the trapped material, X auth is authigenic Nd proportion estimated with "Nd.0/ (Table 4). Since our deepest traps were at 2500 m, the Fout may be overestimated due to the authigenic Nd remineralization between 2500 m and the seafloor; consequently, this estimate gives a lower limit of −Nd . To obtain a global Nd out-flux, we calculated a weighted average of authigenic Nd fluxes at the continental margin (10% of the ocean surface represented by the M-site) and open ocean (90% of the ocean surface represented by the O-site: Table 4). We obtain a global Fout of 3:2 ð 1010 g=yr that yields a −Nd of ¾200 yr. 4.3.2. Nd residence time based on the filtered suspensions Most of the trace metal removal from seawater is due to the uptake on small particles. Therefore, the −Nd can be estimated from suspension data if their average settling speed is known [4]. Fout is given by the following equation: Fout D [Nd] ð X auth ð S

(7b)

where [Nd] is the Nd concentration in the filtered suspensions, X auth is authigenic Nd proportion estimated with "Nd.0/ (Table 5), S is settling speed of the filtered suspensions. Using a 230 Th inventory obtained at a station close to EUMELI sites (28ºN, 22ºW; [32]), we estimate the

settling speed of the suspensions at 800 m=yr (Table 5). The Nd concentration in suspensions at the bottom of the ocean is estimated at 0.25 pmol=kg from M-site and O-site results (Table 5). The authigenic Nd proportion in the suspensions is assumed to be 50% (this study), giving Fout D 5:1 ð 109 g=yr.

Table 5 Global Nd out-flux based on the filtered suspensions from EUMELI sites Nd concentration (pmol=kg) a Adsorbed and exchanged Nd proportion (%) b Settling speed of suspended matter (m=yr) c

0.25 50 800

Global Nd out-flux (g=yr) d

5.1 ð 109

a

Nd concentration in suspended matter is based on the M-site and O-site results. b Adsorbed and exchanged (authigenic) Nd proportion is estimated using Eq. 5b (see text). c Settling speed of suspended matter is estimated from 230 Th inventory obtained at a close station (28ºN, 22ºW: Colley and Thomson [32]) to EUMELI sites. At the steady state, 230 Th influx (in situ production) is balanced with the out-flux (particle flux) P D [Th]p ð S where P is production rate (P D 0.0263 ð Z) (in dpm m 2 yr 1 ), Z is depth (in m). We use a mean depth of the ocean Z D 3800 m), [Th]p is particulate 230 Th concentration at 3800 m of 0.13 dpm=m3 [32] and S is settling speed of suspended matter (m=yr). Using the above equation, we obtain S D 800 m=yr. d Global Nd out-flux is the product of the Nd concentration, authigenic Nd proportion, settling speed and ocean surface area (361 ð 1012 m2 ).

K. Tachikawa et al. / Earth and Planetary Science Letters 170 (1999) 433–446

Using Eq. 3, we obtain a −Nd of ¾1000 yr based on the suspended matter. These two estimates allow us to constrain −Nd in the ocean with respect to isotopic variation in the marine particles: 200 yr < −Nd < 1000 yr. This short residence time suggests that more than 20% of dust-hosted Nd dissolves if the Nd in-flux is the sum of river runoff and the partial dissolution of dust. Our calculation suggests that the contribution of atmospheric fallout to Nd flux into the ocean is probably underestimated and=or that another Nd source contributes to the oceanic Nd budget. Another possible source is remobilization of Nd from riverborn particles [33]. The contribution of these sources to the global ocean will be presented using a tenbox model (Tachikawa et al., in prep.). The shorter residence time calculated based on exchange induces isotopic heterogeneity without concentration change. We believe this to be the key to resolving the ‘Nd paradox’.

5. Conclusions We report the first data set of REE patterns and Nd isotopic ratios of seawater, filtered suspensions and trapped material collected at the same sites. Our results confirm that the LREE are preferentially removed by the suspended matter and vertically transported by the large sinking particles. The Nd exchange between the lithogenic and authigenic fractions is one of the important factors that constrains the global budget of both Nd concentrations and isotopic ratios. We propose a new approach to estimating oceanic Nd residence time .−Nd / and we obtain a −Nd of between 200 yr and 1000 yr based on the Nd isotopic ratios in the marine particles. Our results suggest that more than 20% of partial dissolution from atmospheric fallout and=or other additional sources are required for the oceanic Nd budget. The higher dust dissolution and=or the other additional sources may contribute to the budget of other REE and of other reactive metals brought from the continent, although the contribution is not apparent because these elements do not have specific isotopic signatures.

445

Acknowledgements We thank N. Leblond, P. Brunet, B. Reynier and M. Valladon for their help during the analytical work. F. Grousset and C. Brunet kindly provided the aerosol sample and CO2SYS, respectively. G. Berger is thanked for the calculation of the thermodynamic constants. We are grateful to M. Kastner, H. Elderfield, M. Bau and an anonymous reviewer for their instructive comments. Thanks to D. Lyness who improved the English text. This work was supported by the EUMELI (JGOFS-France) program and by the Institut National des Sciences de l’Univers. [MK]

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