Ocean particle chemistry: The fractionation of rare earth elements between suspended particles and seawater

Geochimicaet CosmochimicaActa, Vol. 58,No. 6, pp. 1567-1579.1994

1994Elsevier Science Ltd Printed in theUSA. All rights reserved 0016-7037194 $6.00+ .OO

Pergamon

Ocean particle chemistry: The fractionation of rare earth elements between suspended particles and seawater EDWARD R. SHOLKOVITZ,’

WILLIAM

M. LANDING,’

and BRENT L. LEWIS’*

‘Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole. MA 02543, USA

‘Department of Oceanography, Florida State University, Tallahassee. FL 32306. USA (Rweivcd .-lugx~t 5, 1993; accepted in revisedJiwn Dcc~ernlwr6. 1993)

Abstract-Sargasso Sea suspended particles were sequentially digested with three chemical treatments (acetic acid, mild HCI/HN03, and HF/HN03/HCl in a bomb). The latter two treatments dissolve detrital minerals, while the acetic acid removes surface coatings (organic matter and Mn oxides). The rare earth element (REE) composition of the surface coatings. in marked contrast to the crust-like REE composition of the two detrital phases, is extensively fractionated with respect to both filtered seawater and the crust. Surface coatings are responsible for the removal and fractionation of REEs from seawater and, as such. play a key role in the marine geochemical cycles of trace elements. Relative to seawater, the surface coatings are systematically enriched tenfold across the trivalent REEs from Lu to La and develop large positive Ce-anomalies. The Ce-anomalies of the coatings switch from being negative (seawater-like) in the upper 100 m to being strongly positive at greater depths. The ingrowth of Ce and LREEs on particle surfaces reflects the in situ oxidation of dissolved Ce( III) to particulate Ce( IV). and the preferential removal of LREE( 111)s over HREE( 111)s. REEs(III) fractionation of this type is consistent with particle/solution models. Both processes appear to be related to the in situ formation of Mn oxide particles from the oxidation of dissolved Mn( II) in the upper 200 m of the water column. Preferential removal of LREEs in the upper waters is countered by their preferential release at depth due to remineralization of surface coatings on particles. A new method is explored for estimating the residence time of suspended particles by combining Ce concentration data of dissolved and surface-bound phases with the Ce( III) oxidation rate measurements of MOFFETT ( 1990). A Ce-based residence time of thirteen days is similar in magnitude to the value calculated from U-234Th disequilibria in the Sargasso Sea. AGAWA, 1978: HONEYMAN and SANTSCHI, 1989: SANTSCHI

INTRODU(JTION

and HONEYMAN, 199 1). In particular, the disequilibrium of 2’4Th from its parent, dissolved 23BU. has been applied as a time-dependent probe of partitioning and fractionation processes (e.g., TSUNOGAI and MINAGAWA, 1978: BACON and ANDERSON, 1982; BRULAND and COLE, 1986: MOORE and MILLWARD, 1988; MORAN and BUESSELER, 1992 ). The rare earth elements (REEs) have chemical properties which make them an excellent natural probe of particle/solution interactions and redox reactions at surfaces. The REEs consist of fourteen elements which form a series from the lightest REE (La) to the heaviest REE (Lu 1. With the exception of multiple oxidation states for Ce, the other REEs have a trivalent oxidation state in most natural waters; Eu( II) in hydrothermal waters is another exception ( MICHARD et al., 1983). As a result of the f-electron shell being progressively filled, there is a gradual decrease in ionic radius (lanthanide contraction) and small but systematic changes in the chemical properties across the REE series. With respect to seawater, where REE-carbonate ion complexes are the dominant dissolved species. the most important property is the systematic increase in carbonate complexation from the light to heavy REE(III)s (CANTRELL and BYRNE 1987; BYRNE and KIM, 1990, LEE and BYRNE. 1992. 1993: MILLERO. 1992). Increased complexation from La to Lu leads to a decrease in the proportions of free REE( III) ions across the series. This change in speciation in turn results in fractionation, whereby the light REEs ( LREEs) are preferentially adsorbed to surfaces as the heavy REEs (HREEs) are preferentially retained in

THE PARTITIONINGOF trace elements

between particles and seawater is controlled by the interplay of surface and solution chemistry. Surface binding and solution complexation are fundamental processes underlying the cycling of trace elements in the oceans ( SCHINDLER, 1975: BALIS~RIERIet al., 1981: HUNTER, 1983; NYFFELER et al.. 1984: HONEYMAN and SANTSCHI. 1988). The extent to which trace elements are bound to different types of inorganic and biogenic particles (colloidal, suspended, and large, fast-sinking types) will affect their removal rates and the composition of seawater. Removal by particles also affects other major oceanic processes, such as remineralization in the water column, diagenesis in the sediments. and preservation of the sedimentary record. One outstanding question with respect to particle/solution interactions is the extent to which trace elements fractionate during the partitioning process( es). Fractionation is defined here as the relative change in solution or particle composition for a group of trace elements due to geochemical reactions. Fractionation could occur during the adsorption of dissolved species onto particles and during particle-particle interactions such as the transformation of colloidal, to suspended, to large sinking particles. Particle-particle transformations have been the focus of considerable research (e.g.. TSLJNOGAI and MIN-

* Prrsenntuddress: College of Marine Studies. University of Delaware, Lewes, DE 19958, USA. 1567

E. R. Sholkovitz, W. M. Landing, and B. L. Lewis

1568

solution ( KOEPPENKASTROPet al., 1991; KOEPPENKASTROP and DE CARLO, 1992,1993;SHOLKOVITZ,1992). Cerium is the only REE with redox transformations at ambient oceanic conditions (GOLDBERG et al., 1963; DE BAARet al., 1985: MOF’FETT,1990). The preferential removal of dissolved Ce( III) occurs as a consequence of redox reactions. MOFFETT ( 1990) has experimentally shown that biologically mediated oxidation of dissolved Ce( III) to a more insoluble form of Ce( IV) occurs in the upper water column of the Sargasso Sea. The abiotic oxidation of Ce( III) on the surfaces of Mn oxides has also been reported from laboratory experiments using synthetic minerals ( KOEPPENKASTROP and DE CARLO ( 1992) The chemical properties of REEs have been used to model the particle/solution interactions oftrace elements in seawater ( ELDERFIELD,1988; BYRNEand KIM, 1990; ELDERFIELDet al., 1990; DE h4R et al., 199 1; EREL and MORGAN, 199 1; LEE and BYRNE, 1992, 1993; EREL and STOPLER, 1993; KOEPPENKASTROPand DE CARLO, 1993). These models employed complexation constants for REE( III) solution interactions and estimated binding constants for surface interactions, and concluded that fractionation occurs because of differences in the relative affinity of REE(III)s, for surface adsorption to particles and for complexation with ligands in seawater. Figure 1 serves as a conceptual picture of these competitive processes and as a framework for the discussion and interpretation of our data. As emphasized in Fig. 1, coatings of organic matter and oxides of Mn and Fe play a major role in the binding of trace elements to particle surfaces (e.g., BALISTIERIet al., 1981; HUNTER, 1983).

This paper builds upon an earlier study of the upper ocean chemistry of dissolved REEs in the Sargasso Sea ( SHOLKOVITZ and SCHNEIDER,199 1) and reports the REE composition of three phases resulting from sequential chemical digestion of suspended particles. This paper will demonstrate that coatings on suspended particles carry the chemical imprint of the in situ removal of REEs from seawater and that extensive REE fractionation between particle coatings and seawater develop in the upper water column. Samples and Sample Treatments

The samples come from an April 1989 water column profile in the Sargasso Sea (3 I “46’N and 64” 12’W), approximately 100 km southwest of Bermuda. Details are given in LANDINGand LEWIS ( 199I ) and SHOLKOVITZ and SCHNEIDER ( 1991). Briefly, 28 L of seawater were collected and filtered through 0.4 lrn ( 142 mm) Nuclepore filters. Two liters of filtered water samples for REE analysis were acidified with purified HCl to pH 2. The filters were returned to Florida State University for chemical digestions and analysis of Fe, Mn, and Al. Aliquots of the digestions were sent to Woods Hole for REE measurements: these aliquots represented particles collected from 5 to 20 L of seawater. LANDING and LEWIS ( 1991) describe the digestion protocols, the analytical methods. and the vertical distributions of Al. Fe, and Mn in the three digestions. Three digestions were carried out sequentially on each filter. Filter blanks were also carried through the digestions. The first digestion consisted of 25% acetic acid at room temperature for 4 h. This digestion is used to release adsorbed cations, carbonates, and reactive Mn oxyhydroxides. It also will release Fe carried as amorphous oxides. The second digestion (herein called strong acid) consisted of a 2M HCl/ 1M HNO, mixture at room temperature for 4 h. This digestion will release strongly bound cations not released by the acetic acid treatment and will attack certain types of alumi-

CoatifJgs (organic or oxide)

Ce(lll) -Ce(lV) Detrital materials

Oceanic Particles

nCO:-

r\

_ La .._. Ce .__.Nd .._. Eu ._.. Er . . . . Lu _ nCO*-

I

I1

Seawater

3 Complexes

1

+

Oceanic Removal via Particle Settling

v FIG. 1.Schematic model of REE fractionation between particles and seawater. Main features include ( 1) the systematic variation in the relative affinity of REE( III) for complexation to solution carbonates and binding to particles, (2) the enhanced formation of particulate Ce due to the oxidation of Ce( III) to Ce( IV), and (3) presence of surface active coatings on detrital particles. These features lead to fractionation of REE between seawater and particles and to fractionation via the settling of large sinking particles.

REE fractionation by particles in seawater

1569

TABLE I RAREEARTHELEMENT and MANGANESEDATA: SARGASSOSEA PARTICLES and FILTERED SEAWATER SAMPLE

La

Ce

Nd

Sm

Eu

Cd

Dy

Er

Yb

Lu

Jkptflype

CeAIlOlll.

Mn

1~/AC 15JSW

15960 15660 15880 3510 920

5330 5840 4730 4040 551 0.47

0.02 1.94

3O/Ac 3o/sw

15540 15040 15640 3490 920

5120 6020 4710 4070 558 0.46

0.02 2.06

45/Ac 45/sw

15530 14000 16010

3500

920

5100

nd

nd

nd

nd

0.43

1.78

6olAC 6O/sw

194 15686

160 13536

138 15525

22 3464

4.0 906

21 5068

14 5860

13 4770

4.0 4150

0.10 571

0.44 0.41

0.06 1.93

105/Ac 105/sw

393 14876

337 264 12107 15482

43 3463

9.0 936

37 5266

21 5890

10 4770

5.0 4120

0.30 558

0.47 0.38

0.11 1.33

15OlAc 15o/sw

368

909

46

9.0

nd

27

11

6.0

0.50

1.60

0.48 0.71

2OO/Ac 2OO/sw

319 15382

968 218 10815 15918

45 3479

9.0 880

38 5274

27 5860

13 4750

7.0 4080

1.00 557

1.64 0.33

0.49 0.65

255/Ac 255lSTRONG 255lSW

375 61 147 15480

1083 233 244 69 242 99 11110 16780

45 19 15 4180

11.2 4.7 3.9 890

44 19 12 5830

30 17 9 5470

14 8.8 5.6 4560

8.1 7.6 5.2 3970

0.57 0.97 0.51 548

1.60 1.83 0.90 0.33

0.32 0.02 0.004 0.37

34OlAc 34OMCl

343 117

1123 395

53 28

12.0 6.3

53 26

37 22

17 11.1

10.1 10.1

1.01 1.21

1.70 1.65

0.23 0.03

34O/Bomb 34OlSW

203 396 139 22 4.7 15280 9570 16160 3950 870

255/Bomb

0.03

231

267 111

16 13 7.6 7.3 0.96 1.05 5570 5590 4582 3980 526 0.29

0.004 0.27

75OlAc 352 75O/STBONG 142

1183 308 609 183

62 42

12.5 9.7

59 39

42 33

20 11.8 1.22 1.68 17.2 15.6 2.03 1.86

0.23 0.03

750/T%mb 75OlSW

294 20450

578 5130

203 16030

32 3270

6.5 880

23 7280

18 5150

10.8 4450

10.5 4200

1.27 517

1.06 0.18

0.01 0.29

lOGQ/Ac lOOO/STRONG

395 178

1216 585

339 195

65 44

15.0 9.6

61 45

45 35

23 17.8

11.6 16.0

nd 2.01

1.55 1.52

0.24 0.03

1OOO/Bomb 1Ooo/sw

348 620 229 37 7.5 24400 5940 21200 5250 880

29 23 13.6 12.7 nd 0.97 6730 5590 4940 4590 685 0.12

0.01 0.38

15OO/Ac

437

1306 400

80

17.6

74

54

25

16.5 1.75 1.48

0.23

15OO/STRONG

166

500

36

7.8

32

26

13.1

nd

15OO/Bomb 15oO/sw

315 564 219 34 6.9 26QOO 6820 21400 5410 890

24 21 12.1 11.8 1.63 0.96 6850 5710 5080 4760 695 0.13

0.01 0.42

2OOO/Ac 2CWSTBONG 2OWBomb 2cWsw

336 164 380 23300

995 462 755 6350

321 158 280 19400

64 32 46 5100

13.8 nd nd 820

60 48 nd 6480

42 22 27 5400

21 11.4 15.9 4930

13.6 9.6 16.0 4560

1.44 1.18 1.98 665

0.17 0.02 0.01 0.42

Blank/At Blank/Strong Blank/Bomb

32 hd 33

35 33 26

23 16 16

hd bd 2

bd M hd

bd hd M

0.6 0.4 0.7

0.3 0.4 0.7

bd bd td

hd td hd

181

nd

1.40

1.44 1.36 1.05 0.14

0.02

0.002 0.002 0.003

ALLREE CONCENTRATIONS

IN FMOLKG WATER Mm CONCENTRATIONS IN NMOLACG OF WATER AC = ACETIC ACID ~~3F.370~: &R~NG = STRONG ACID DIGESTION BOMB = BOMB DIGESTION:

Blank= FILTER BLANK REECONCENTFUTIONS

SW = FILTERED SEAWATER bd = BELOW DETECI-ION nd = NO DATA

nosilicates (i.e., clays) and crystalline Fe oxides. The third digestion (previously called bomb) was carried out in a sealed Teflon bomb at 100°C using a mixture of HCI, HN09, and HF. The bomb digestion was a two part procedure. After a first treatment with a HCI/HNOj mixture, HF was added for a second treatment. The bomb digestion will dissolve aluminosilicates and many accessory minerals not released by the strong acid. It will not release REEs from highly refmctory minerals. zircon being an important example (SHOLKOVITZ, 1990).

As with all chemical digestions, the results are method dependent. SHOLKOWTZ ( 1989) argued that the REE composition resulting from chemical digestions of sediments may be subject to major artifacts such as the readsorption of REEs released from a mild type of treatment. The strong acid and bomb digestions pose little problem in this regard. The main concern lies in the acetic acid digestion. The use of large volume to weight ratios of acetic acid to suspended partitles (20 ml to about 3 mg) will minimize readsorption problems ( SHOLKOVITZ.1989).

1570

E. R. Shoikovitz, W. M. Landing, and B. L. Lewis Ce-anomaiy

Using one-liter volumes of filtered seawater, the co~cen~tions of La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb, and Lu were measured by isotope dilution thermal ionization mass spectrometry (SHOLKOVITZ and SCHNEIDER, 1991; PIEPGRASand JACOBSEN,1992) to precisions ofO.l-2% ( SHOLKOVITZ, 1993; SHOLKOVITZet al., 1992). The three different types ofdigestionsof a blank filter gave small REE concentrations (less than 1 fmol/kg for Dy and Er; Table 1); the concentrations of Eu, Gd, Yb. and Lu were below detection limits. Blank concentrations of La and Nd for all three digestions, relative to their concentrations in the 750 m particulate sample, by way of example, ranged between 7 and I 1%.For Nd, this value was 3-5%; for the bomb digest of Sm it was 6% while Dy and Er ranged between 1 and 6%. The mass spectrometer counting statistics for the particle samples yielded errors in isotopic ratios of between 0.1 and 1%for all REEs; most errors on most samples fell nearer to 0.1% To assess the worst case precision for the particle REE data, we applied a 1% error to the three types of digests. With the exception of Lu (2-4%), the resulting errors on the concentrations of all the samples ranged between 1 and 2%.

0.50

0.25

Filtered Water 500 t

750 -

E

1000 1

22500ro

RESULTS

REE Concentration (pmol/kg water)

Types of Data In order to discuss the different facets of the marine chemistry of the REEs, the resuIts will be described and illustrated in three different ways. First, vertical profiles ofconcentrations and dist~butions between the three types of particle digestions will be presented. Second, shale-normalized abundances provide insights into the fractionation of the trivalent REEs and the development of Ce-anomalies. Normalization is a common geochemical practice as it removes the natural variations in the absolute concentrations of REEs and allows a comparison to the REE composition of the upper crust, for which shale is a proxy (e.g., HASKINS et al., 1966; HENDERSON, 1984a,b: TAYLOR and MCLENNAN, 1985). Third, normalization of particle compositions to the REE compositions

FIG. 3. Vertical profiles of dissolved (co.22 Frn) La, Ce, and Nd from the Sargasso Sea. The profile of the calculated Ce anomaly is also shown.

of fiitered seawater provides a quantitative measure of fractionation associated with the pa~itioning process in the oceans. This treatment assumes some type of equilibrium process between REEs in seawater and on the surfaces of particles. Table 1 contains the REE and Mn data for the three particle

digestions and filtered seawater; seawater data covering the upper 750 m has been reported in SHOLKOVITZ and SCHNH-

Nitrate (~mol/l) Total Suspended Carbon &g/l)

12

s 0 Q

Total Particulate Al (nmol/kg water,l 3 4 5 6 7

8

1250 1500

Dissolved Mn -A- Pa~iculate Mn

-b-

2250

t 2500 f 0.0 2*501/

0

50

-O- Total ParticulateAl

I

I

0.5

1.0

I

I

1.5

2.0

I 2.5

Mn Concentration (nmol/kg water) 100

150

200

250

300

350

400

Dissolved Oxygen (bmol/l) Chlorophyll-a (n&) FIG. 2. Vertical profiles of nitrate, dissolved oxygen and chlorophyllu (data only to 500 m), and total suspended carbon (data onIy to 300 m) from the April 1989 station in the Sargasso Sea. Data from G. Cutter (pers. commun.).

FIG. 4. Vertical profiles in the Sargasso Sea of dissolved Mn and total particulate Mn and Al. A high dissolved value (2.7 rmol/kg) at 440 m was measured but not plotted. For almost all the samples, greater than 90% of the particulate Mn concentration is carried in the phase released by the acetic acid digestion; in contrast, the major portion of particulate Al is carried in the strong acid and bomb digestions (LANDING and LEWIS, 1991).

REE fractionation

by particles

in seawater

1571

rise in magnitude to 650 m, below which depth they remain fairly constant. Large Mn gradients exist in the upper 300 m. Dissolved Mn has a large near-surface maximum of 20 nmol/kg and then decreases markedly to reach a minimum value of 0.3 nmol/kg at 300 m. Total particulate Mn. in contrast, has a subsurface maximum between 150 and 200 m. This in situ biologically mediated oxidation of dissolved Mn( II) to particulate Mn (IV) is a well documented feature of the Sargasso Sea (SUNDA et al., 1983; SUNDA and HUNTSMAN, 1988). As the acetic acid digest carries the major portion (>85%) of the total particulate Mn (LANDING and LEWIS, 199 1; Table 2), this phase must be Mn oxyhydroxide formed within the water column. Particulate Al (Fig. 4) and particulate Fe (LANDING and LEWIS, 199 1) have very different profiles than does particulate Mn. Unlike Mn, the major fraction of Al (>90%) and Fe (>80%) are carried in the detrital phases (e.g.. strong acid plus bomb digestions). Hence, the percentages of Al and Fe in the acetic acid digestion remain low and uniform with the exception of samples at 90-125 m. This zone of in situ oxidation of Mn (II) also has slightly higher acetic acid-soluble Al and Fe fractions. This feature might reflect uptake from seawater. The 440 m sample stands out as having unusually high concentrations of both dissolved Mn and particulate Al

DER ( 199 1) .The three digestions

were not carried out on all of the samples (255,340,750, 1000, 1500, and 2000 m samples only). Only the acetic acid digestion was analyzed for the shallower samples (60, 105, 150, and 200 m) Note that the REE and Mn concentration data are reported on a per-kg-of-seawater scale as accurate weights of particles could not be measured. Hence, our particle/seawater ratios are not true distribution coefficients ( KD) which use particle REEs concentrations based on the weight of the particles, In lieu of particle weight data, total carbon concentrations (Fig. 3 ) and total particulate Al (Fig. 4) can serve as indicators of the vertical variation in particle concentration (Fig. 2). Since surface coating are the active phase with respect to the marine chemistry of REEs, a KD based on total particle weight is irrelevant with respect to interpreting oceanic processes.

Non-REE Data Temperature data reveal a 40-50-m mixed layer. Profiles of dissolved oxygen, nitrate, and chlorophyll-a show the following features: sharp maximum in chlorophyll-a at 90 m. small oxygen minimum at 750 m, and nitrate depletion in the upper 75 m (Fig. 2; G. Cutter, pers. commun.). Nitrate concentrations, below detection in the upper 100 m, gradually

TABLE 2 SARGASSOSEA SUSPENDED PARTICLES PERCENTAGE DISTRIBUTION OF REEs and Mn AMONGST THREE DIGESTIONS

Sample 255

La

Ce

Nd

Sm

Eu

Gd

Dy

Er

Yb

Lu

Mn

m

%AceticAcid

64

69

58

51

56.6

59

%Strong Acid %Bomb %P

10 25 4

16 15 12

17 25 2

24 19 2

23.1 19.7 2.2

25 16 1

54 30 16 1

50 31 19 0.6

58 5 37 0.4

28 47 25 0.4

93 6 1 48

340m ?&Acetic Acid IStrong Acid %BClmb %P

52 18 31 4

59 21 21 17

52 21 27 3

51 27 21 3

52.2 21.4 20.4 2.6

56 27 17 2

51 31 18 1

48 31 21 1

37 37 27 1

32 38 30 1

88 10 1 49

Acid %Strong Acid %Bomb BP

45 18 37 4

50 26 2.4 32

44 26 29 4

46 31 24 4

43.6 33.8 22.6 3.2

49 32 19 2

45 35 19 2

42 36 23 1

31 41 28 1

27 45 28 1

87 11 2 48

1000 m %Acetic Acid %Strong Acid IBOMB %P

43 19 38 4

50 24 26 29

44 26 30 3

45 30 25 14

46.7 29.9 23 0.5

45 33 21 2

44 34 22 2

42 33 25 1

29 40 32 1

nd nd nd nd

86 10 4 42

1500 m %Acetic Acid %Strong Acid %Bomb %P

48 18 34 3

55 21 24 26

50 23 21 4

53 24 23 3

55 24 21 4

57 25 18 2

53 26 21 2

50 26 24

nd nd nd nd

nd nd nd nd

89 7 5 39

38 19 43 4

45 21 34 26

42 21 37 4

45 23 32 3

nd nd nd nd

nd nd nd nd

46 24 30 2

44 24 33 1

35 25 41

31 26 43 1

85 11 5 32 -

750

m

%Acetic

1

2000m %Acetic Acid %Suong Acid %Bomb %P

% P = (TOTAL PAFtTtCLES)(TGTAL

1

PARTICLES + FILTERED SEAWATER) WHERE TGTAL PARTICLES = SUM OF THREE DIGESTIONS nd= NO DATA

x 100

1572

E. R. Sholkovitz, W. M. Landing, and B. L. Lewis Ce anomaly

(Fig. 4). This feature may reflect offshore transport. However, no corresponding increases are observed in particulate Mn or in dissolved and particulate REEs (Table 1). Rare Earth Element Concentrations and Distributions The water column profiles of dissolved La, Ce, and Nd concentrations and Ce anomalies are presented in Fig. 3. The Ce anomaly is defined as being equal to (3 X Ce/Ce,,,,,)/( 2 X La/Lashale + Nd/Nd$,,,,,). Negative Ce anomalies and positive Ce anomalies are defined as values of less and greater than I, respectively. The main feature in Fig. 3 is the contrast between Ce and its trivalent neighbors La and Nd over the upper 550 m. There is extensive depletion of dissolved Ce ( 15.7 pmol/kg at 15 m-6.3 pmol/kg at 550 m), while La and Nd concentrations show little variation over this depth range. Cerium reaches a minimum at 750 m and then increases slightly at greater depths. There is also extensive downward depletion of dissolved Mn over the upper 250 m (Fig. 4). The profile of dissolved Mn differs from that of Ce in that Mn reaches a minimum concentration at a much shallower depth (250 m vs. 750 m). Below 750 m there is a small rise in the Ce concentration. The removal of Ce from the dissolved phase leads to the development of more negative Ce anomalies with increasing depth. Surface waters have anomalies of 0.45 while the 750 m sample has a value of 0.18. The trivalent REE concentrations increase between 500 and 1000 m and then level off. Remineralization of REE(lII)s in the deep water has been observed in other studies ( ELDERFIELD and GREAVES, 1982: DE BAAR et al., 1985: ELDERFIELD. 1988: BERTRAM and ELDERFIELD, 1993). There is fractionation amongst the dissolved REE( 111)s over the 2000 m sampling interval in the form of the HREEs (Dy, Er, Yb, and Lu), showing smaller increases below 500 m than do the LREEs (La and Nd; Table 1). For example, Yb increases by about 15%whereas Nd increases by 37%. Fractionation, a key feature of this study, will be discussed later in more detail. The total (sum of three digestions) particle concentrations of the REE(III)s comprise between 0.4 and 4% of the REE( III ) inventory (this being defined as the sum of the dissolved and the total particle concentrations). There is a systematic decrease across the series with 4% of La and 0.4% of Lu present as particles. There are no significant changes in these percentages over the 2000 m sampling interval. In contrast, Ce is preferentially enriched in the suspended particles, and the Ce particle concentration increases over the upper 550 m of the water column. The percent total particulate Ce in the Ce pool is as follows: 12% at 255 m, 17% at 340 m, 32% at 750 m. 29% at 1000 m, 26% at 1500 m, and 26% at 2000 m. Figure 5 shows the vertical distributions of the acetic acid digestions for particulate La. Ce, and Nd concentrations and the resulting Ce anomalies. Lanthanum and neodymium concentrations double between 60 and 105 m and then increase more gradually over the remaining part of the water column. The Ce concentration increases markedly (about sevenfold) between 60 m and 225 m. and then remains fairly uniform (the high value at 550 m being an exception). The ingrowth of Ce onto particles mirrors the depletion of dis-

1250

22501 0

1, 200

1 400

1 600

I 800

I I I I I 1000 1200 1400

1 1600

REE Concentration (fmol/kg water)

FIG. 5. Vertical profiles of La, Ce, and Nd concentrations from the acetic acid digestion of suspended particles from the Sargasso Sea. The Ce anomaly profile is also shown.

solved Ce (Fig. 3). The acetic acid digests of the 60 and 105 m particles have anomalies of 0.44 and 0.47 (like seawater) while samples at 150 m and deeper have values of 1.4- 1.6.Hence, a major feature in our particle-coating dataset is the shift from negative to positive Ce anomalies between 10.5 and 150 m. These data confirm prior experimental studies ( MOFFETT, 1990) that there is in situ oxidation of dissolved Ce(III) to particulate Ce( IV) in the upper water column of the Sargasso Sea. The production of particles with positive Ce anomalies corresponds with the production of a subsurface maximum in particulate Mn oxyhydroxide (Fig. 4). This suggests that the oxidation of Ce( III) is directly coupled to the redox chemistry of Mn. The distribution of the REEs between the three chemical digestions are given in Table 2. For brevity, consider the shallowest and deepest samples. At 255 m about 50-70% of the REEs are contained in the acetic acid digest: the strong acid digest carries lo-30%, while the bomb digest carries 15-25%. At 2000 m the acetic acid digest contains a smaller percent of the REEs ( 30-50%). while the strong acid and bomb digest account for lo-40% and 20-40%, respectively. Fractionation of REEs Shale-normalized REE patterns for the 255 m and the 1000 m samples illustrate the main features of all the samples (Figs. 6 and 7 ). These figures compare filtered seawater with those of the three particle digestions (acetic acid, strong acid, and bomb, and their sum totals). We first consider the 255 m sample. The filtered seawater has a shale pattern typical of seawater, and a negative Ce anomaly within an overall HREE-enriched composition. With respect to the particles, the outstanding feature is that both the strong acid and bomb digests have flat patterns indicative of crust-like REE compositions. This is consistent with the major fraction of Al being in these two digestions (LANDING and LEWIS, 1991).

REE fractionation by particles in seawater I”““““““’

255 m -3 + + -+ -O-

0.01

’ ’ La Ce

’

’ Nd

’

’ ’ Sm Eu

’

Gd

’

’ Dy

’

Aceticacid Strongacid Seawater Bomb Total particle

’ Er

’

’

Yb

y Lu

FIG. 6. Shale-normalized REE ratios of samples from 255 m in the Sargasso Sea. Samples include seawater (dissolved fraction), the three digestions of the suspended particles (acetic acid, strong acid, and bomb), and the total particulate composition (sum of three digestions).

In marked contrast, the acetic acid digest is LREE-enriched and exhibits a large positive Ce anomaly. The acetic acid digest has a pattern which is the mirror image to that of seawater; the largest decrease in shale-normalized ratios occurs at the heavy end of the series (from Gd to Lu). In detail, the bomb digest shows a subtle but systematic decrease from La to Lu. and the HCI /HNOX digest has a small positive Ce anomaly. As the acetic acid digest contains the largest proportion of the particulate REEs, the shale pattern of the total particle composition is also LREE- and Ce-enriched. Only at the HREE end of the series (Yb and Lu) do the three fractions contribute equal proportions. For the shale-normalized patterns of the 1000 m sample digests (Fig. 7), the features described above for the 255 m sample hold. The only substantial difference is that the acetic acid digestion is less dominant in terms of the total particulate REEs. A closer look at the shale patterns of the acetic acid digests for suspended particles in the upper water column (60 through 550 m) reveal other important insights into the development of REE fractionation (Fig. 8). The striking feature is the fact that both the 60 m and 105 m samples have negative Ce anomalies like their seawater counterparts. The acetic acid

La Ce

’

Nd

’

’ ’ ’ Sm ELI Gd

’

’

Dy

’

’ Er

’

’

’

Yb Lu

1573

digest then switches to a positive Ce anomaly between 105 m and 150 m, our next deepest sample. Below this depth, the anomaly increases to reach a maxima ( 1.4-1.6) at 550 m (Fig. 5 ). The extent of fractionation for the REE (Ill) also increases between 60 m and 105 m. The 60 m digest has an almost flat REE (Ill) pattern with small depletions at Yb and Lu only. By 105 m, the REE( Ill) patterns are strongly HREEdepleted. The shale-normalized data in Figs. 6, 7, and 8 lead to several important geochemical observations. Superimposed on detrital phases with a REE composition representative of the upper crust are surface phases or coatings with highly fractionated compositions and positive Ce anomalies. The acidacetic data confirm that the oxidative removal of Ce (Ill) to particulate Ce (presumably Ce( IV)) and the preferential uptake of LREE (III) and MREE( Ill) relative to the HREE( Ill) are driven by processes operating in the upper few hundred meters of the Sargasso Sea. These processes are controlled by reactions with surface coatings. Moreover, there is largescale REE fractionation between suspended particles and seawater and between the composition of surface coatings and the upper crust. REE uptake and fractionation coincides with the in situ formation of Mn oxide-rich particles. The subsurface maximum in total particulate Mn between 125 and 200 m is predominantly composed of Mn oxides, easily released by the acetic acid digestion. Iron and aluminum, in contrast, are mainly carried by the strong acid and bomb digests; the only increase in the acetic acid Fe concentrations is a small one between 60 and 90 m (LANDING and LEWIS, 199 1) . This suggests that Mn oxyhydroxides play a key role in the fractionation of REEs in the upper oceans. However, our digestion protocol can not rule out the possibility that organic coatings may be equally important as well. The REE normalization of acetic acid digests to filtered seawaters is illustrated in Fig. 9 for the nine samples between 60 m and 2000 m. The main features are ( 1) a systematic and tenfold decrease in the ratios from La to Lu across the REE( Ill) series, and (2) large peaks in Ce below 105 m. The large size of the Ce peaks results from the combination of Ce removal from the dissolved phase and Ce uptake by the particles. The seawater-normalized plots of all samples fall closely together even though the absolute concentrations show large variations within the 2000 m profile. The extent of fractionation can be assessed by comparing the acetic acid digest/ seawater ratios for La and Yb, LREE( Ill) and HREE(llI),

1

FIG. 7. Shale-normalized REE ratios of samples from 1000 m in the Sargasso Sea. Samples include seawater (dissolved fraction), the three digestions of the suspended particles (acetic acid, strong acid. and bomb). and the total particulate composition (sum of three digestions).

FIG. 8. Shale-normalized REE ratios of the acetic acid digestion of suspended particles from the Sargasso Sea. Only samples in the upper 550 m are shown here.

1574

E. R. Sholkovitz.

(a)

W. M. Landing,

1

a-

60m

-o+

105m 200m

+

255m

f

340m

O.ll La

Ce

Nd

Sm

Eu Gd

Dy

Er

Yb

Lu

(b) ‘oo’ol-----7 s 0

-b -c4 -0+

lO.O-

k

340m 750m 1OOOm 1500m 2000m

and B. L. Lewis

ation by comparing the lightest REE( III), La, with the second heaviest, Yb. The La/Nd ratio provides a good contrast to La/Yb, as the La and Nd should exhibit less fractionation due to their nearness within the REE( III) series. With respect to particles, the La/Yb ratio increases by a factor of 1.6 between 60 and 105 m, reflecting the preferential removal of La in the upper oceans. Between 105 m and 550 m this ratio reverses direction and decreases sharply (by three and a half-fold), indicating the preferential release of LREE during remineralization. Between 550 and 2000 m, the La/Yb ratio does not change appreciably. The La/Yb ratio of the filtered seawater, while more subtle in its vertical variation, does show systematic changes. These changes are in opposite direction to those of the acetic acid digestions. The La/Yb seawater ratio decreases in the upper 100 m (4.03.6) and then increases to a value of 5.1 at 2000 m. While the La/Yb ratios of seawater and suspended particles are

(a)

Seawater: LaNb Ratio 0 0;

2

4

6

a

10

1

I

12

14

I

I

16

-e 250 -

E p 9 a$

500 -

sg Orn m&q l.OOp

750 -

8 Ti

~iooo2 z a” 1250 -

“‘“i La Ce

Nd

Sm Eu

Gd

Dy

Er

Yb

Lu

FIG. 9. (a) The REE composition of acetic acid digestions normalized to the REE composition of filtered seawater for samples in the upper 340 m of the Sargasso Sea. y-axis is unitless and is shown as log scale X 100: the y-axis is percent REEs contained in the form of acetic acid releasable particles. (b) The REE composition of acetic acid digestions normalized to the REE composition of seawater for samples between 340 m and 2000 m in the Sargasso Sea.

respectively. The means and standard deviations of the ratios for the nine samples are 1.85 (0.39) for La and 0.27 (0.05) for Yb. Hence, there is a sevenfold increase in the partitioning of La relative to Y b when acetic acid digests are normalized against their respective seawater concentrations. Fractionation accompanying the removal and remineralization in the water column is a critical part of the overall cycling of REEs. The observation of increasing LREE(III)/ HREE(II1) ratios of seawater with increasing water depth has been interpreted as being controlled by a biogeochemical cycle ( ELDERFIELD and GREAVES, 1982; DE BAAR et al., 1985; ELDERFIELD, 1988; BERTRAM and ELDERFIELD, 1993). Having the composition of surface coatings provides more insights into particle/solution interactions. Fractionation in the Sargasso Sea can be best addressed by comparing the vertical profiles of the La/Nd and La/Yb ratios of filtered seawater and acetic acid digests of suspended particles (Fig. I0a.b). The La/Yb ratio allows a quantification of fraction-

10

20

30

40

50

Particles: L&b

A+

I

I

0.8

60

70

60

Ratio

Filtered seawater Particles: acetic acid digest

I

I

I

I

1 .o

1.2

1.4

1.6

I 1.6

LalNd Ratio

(a) Vertical profiles of the La/k% ratios of filtered seawater and the acetic acid digestions of suspended particles from the Sargasso Sea. (b) Vertical profiles of the La/Nd ratios of filtered seawater and FIG. IO.

the acetic acid digestions Sea.

of suspended

particles

from the Sargasso

REE fractionation by particles in seawater converging at depth due to fractionation during remineralization, the particles remain significantly more LREE-enriched throughout the 2000 m section. Both the particle and seawater phases show vertical La/Nd variations reflecting fractionation (Fig. lob). As predicted, the variation in the La/Nd ratios are smaller than their La/Yb counterparts. La/ Nd variations are small, but measurable and systematic. Between the surface and 250 m, the preferential removal of La relative to Nd in the acetic acid digestion (La/Nd increases from 1.4 to 1.6) is mirrored by a small decrease in the La/ Nd ratio ( 1.00-0.92) in the filtered seawater. Between 250 m and 550 m, the seawater ratio reverses directions indicating the preferential release of La off particles to the seawater. Below 1000 m, the La/Nd ratio of the two phases converge and cross. This convergence and crossover of La/Nd ratios. from starting ratios of 1.0 and 1.6 in the surface waters, exemplifies the active nature of fractionation resulting from surface/solution interactions. This crossover is not observed for La/Yb ratio as these two REE( 111)sexhibit a greater extent of fractionation between seawater and surface coatings. As shown in the above paragraph, biogeochemical reactions within the water column yield systematic fractionation between the surface coatings of suspended particles and filtered seawater. The preferential removal of LREE( III )S onto surface coatings in the upper few hundred meters is countered by the preferential release of LREE( 111)s at depth. Given that suspended particles only comprise a small fraction (0.44%)) of the total REE(III)s in seawater. the REE inventory in the filtered seawater cannot be balanced by a one-time release of REEs from suspended particles. The variations in the concentration and fractionation of the REEs in filtered seawater must reflect a recycling process in which particle/ solution interactions of REEs are complexly related to the upper ocean processes such as the upwelling of nutrients and the formation and remineralization of biogenic particles. Particle Residence Time as Deduced from Cerium Compositions Here we explore an idea that the redox chemistry of Ce can be used to estimate the residence times of suspended particles in the upper ocean. The calculation requires the concentrations of dissolved and surface-adsorbed Ce and rate measurements of the oxidation of dissolved Ce( III) to particulate Ce( IV). The Ce concentration of the acetic acid digest represents Ce which has been removed from the water onto the surface coatings of suspended particles. The model is a steady-state one which assumes that the removal of dissolved Ce( III) is balanced by an export of particulate Ce( IV), which in turn is driven by the oxidation of Ce( III). The production of particulate Ce(IV) from dissolved Ce( III) is taken to be a biologically mediated process ( MOFFETT, 1990). We use the MOFFE~T ( 1990) Ce radiotracer-based oxidation rates which come from a May 1989 study of the Sargasso Sea. The rates are corrected for the nonoxidative uptake of Ce( 111) onto particles. Our water column samples were collected one month earlier from a nearby location. The ratio of dissolved to particulate Ce represents the number of cycles that are required to remove dissolved Ce through particle scavenging and settling. While steady-state conditions are unrealistic for

1575

the upper ocean, this is the first data set with sufficient Ce data to attempt an estimate of a Ce-based residence time. Consider the upper 150 m, the depth region for which MORAN and BUESSELER ( 1992) have calculated 234Th-based particle residence times. The rate of Ce(II1) oxidation increased with depth and varied from 0 at the surface to 0.8% per day at 200 m ( MOFFETT. 1990). Using a rate of 0.3% per day as an average for the upper 150 m and a value of 26 for the ratio of dissolved Ce to adsorbed particulate Ce (Table 1 ). a particle residence time of 13 days results. Given the averaged values used in the calculation and the limited amount of Ce data and rate measurements, our particle residence times have uncertainties at best ofa factor of4. MORAN and BUESSELER ( 1992) estimated particle residence times for a nearby Sargasso Sea location using U-‘j4Th disequilibria; their results are based on samples collected from the upper 150 m in April 1991. Their 214Th-based residence times are estimated to be 6 1 days for dissolved Th. 6 days for colloids, 18 days for small suspended particles, and 0.3 days for large particles. Their small particles are defined in the same manner as our suspended particles (>0.22 pm). In spite of the large uncertainties in our Ce-based model, our particle residence time ( 13 days) agrees with the short ( 18 days) values derived from their ‘34Th-based models. The addition of Ce data for sediment traps would help constrain a Ce-based residence time. DISCUSSION There have been only a few studies of the REE composition of particles. Sediment trap data have been reported by MURPHY and DYMOND (1984), MASUZAWA and KOYAMA ( 1989), and FOWLER et al. ( 1992 ) Suspended particle compositions have been reported for anoxic basins by DE BAAR et al. ( 1988) and GERMAN and ELDERFIELD ( 1989, 1990). and for a hydrothermal plume by GERMAN et al. ( 1990): all these can be considered special cases. The most similar set of suspended particle measurements ( to our study ) is that of BERTRAM and ELDERFIELD ( 1993). who present seawater and particle data from two profiles from the Indian Ocean, one from the Somali Basin, and one from the Madagascar Basin. Their samples were filtered through 0.4 pm filters. and the resulting particles were digested with 6 M HCl at room temperature. While it is not possible to make absolute comparisons between the data sets as the digestion protocols are different. their 6 M HCl digestions will have released REEs bound to oxide phases and other surface phases and attacked certain types of detrital minerals. We will compare the sum of our acetic acid and strong acid digests with their 6 M HCl digestion, the best match of the two digestion protocols. Figure 1 1 (a.b,c) shows our data and the data Of BERTRAM and ELDERFIELD ( 1993). both normalized to the REE composition of the corresponding filtered seawater samples. This figure reveals a quantitatively consistent picture of REE fractionation for the Sargasso Sea and the Indian Ocean. All three data sets show fractionation across the series and the preferential uptake of Ce onto suspended particles. The partitioning of REE( 111)s can be assessed by comparing the particle/seawater ratios of Nd and Yb. The Nd ratios (mean and standard deviation) are 1.76(0.23). X62(0.78). and 2.48 (0.42 ) for the Madagascar Basin (five samples between

E.

1576

R. Sholkovitz. W. M. Landing, and B. L. Lewis

*

La Ce 100.0,

,

,

Sm Eu Gd

Nd (

,

(

1

,

I

Dy ,

,

La Ge ,

,

Sm Eu Gd

Nd

,

,

,

,

,

/

Nd

Sm Eu Gd

,

,

,

300m -Q- 5OOm h 1180m -CL 20OOm 4 2515m

Oy ,

/

t

,

,

,

-U -O++

Oy

i

Yb Lu

Er

+

Somali Basin

La Ce

(

,

z?

O.‘O~

Yb Lu

Er

Madagascar Basin

*

D

100.0

340m 750m 10OOm WJOm 2000m

-3 4 -0*

10.00

,

/

,

125m 365m 785m 1300m 1805m

Er

-

Yb Lu

FIG. Il. (a) Sargasso Sea: The REE composition of suspended particles (sum of acetic acid and strong acid digestions) normalized to the REE composition of filtered seawater. Note log scale. (b) Madagascar Basin ( BERTRAM and ELDERFIELD, 1993): The REE composition of suspended particles (6 N HCl digestion) normalized to the REE com~sition of filtered seawater. Note log scale. Only one sample had La data. (c) Somali Basin ( BERTRAM and ELDERFIELD, 1993): The REE composition of suspended particles (6 N HCI diges-

tion) normalized to the REE composition of filtered seawater. Note log scale. 300 and 25 15 m), the Somali Basin (six samples between 125 and 2300 m) and the Sargasso Sea (six samples between 255 and 2000 m), respectively. Ytterbium has the following ratios for the same three basins: 0.40(0.1 l), 0.80(0.31). and 0.53 (0.10). These partitioning values will depend on the total particle concentrations ofeach sample (neither study has these data). The extent of fractionation across the REE( 1iI)s can be quantified by comparing the p~cle/~awater pa~tioning

of Nd to that of Yb; this ratio of a ratio is independent of the particle load. Neodymium was selected as the Madagascar data do not include many La values. The NdjYb ratios yield means and standard deviations of 4.63( 1.18), 3.57( 1.12), and 4.57(0.24) for the Madagascar Basin, the Somali Basin, and the Sargasso Sea. respectively. Hence, relative to seawater concentrations, Nd is removed onto particles by a factor of 4-5 times that of Yb. Given the use of different digestions (and their inherent limi~tions~ and the different oceanographic regimes of the three sites, there is remarkable similarity in the partitioning and fractionation of REE( III)s and the development of Ce anomalies. In addition, experimental studies of REE uptake onto various oxide surfaces in a seawater media yield Nd/ Yb ratios of 3.4-4.8 (KOEPPENKASTROP and DE CARLO, 1992). Field and laboratory results confirm that solution/ surface interactions control the removal and fractionation of REEs. The relative affinity of REEs for complexation and adsorption is a fundamental feature of REE chemistry and responsible for their marine chemistry. Sediment trap data also provide a means of quantifying fractionation between solution and particle phases and for determining in situ processes such as remineralization, uptake, and flux. The most comprehensive REE study is that of FOWLER et al. ( 1992), who deployed traps at depths of 200,500, 1000, and 2000 m in the northwest Mediterranean Sea. They report REE data for material collected for six I3day deployments over a total period often weeks. Their samples were digested in strong acid ( HNOX/HF) using microwave heating. Figure 12 compares the shale-normalized patterns of the following samples: 200 and 2000 m sediment trap samples from 18 June to 1 July 1990 period in the Mediterranean Sea and our 60 and 2000 m suspended particle samples from the Sargasso Sea. Only acetic acid digest data are available for the 60 m sample; this is the most diagenetically important phase. Our 2000 m pattern is calculated using the total REE particle concentrations, the most similar match to the FOWLER et al. ( 1992 ) type of digestion. As shown in Fig. 12, their 200 m trap had a crust-like composition, while their 2000 m trap was both elevated in REE concentrations and highly fractionated. This fmctionation takes the form of a positive Ce anomaly ( 1.18) and a large enrichment in LREEs. Many of FOWLER et al, ( 1992) deep traps from other time periods have similar fractionated compositions. Figure 12 also shows that our 2000 m suspended particles have the same fractionated compositions as their 2000 m traps. In fact, particles from all depths below 150 m in the Sargasso Sea exhibit the same type of fractionation (Figs. 6, 7). Like FOWLER et al. ( 1992) 200 m sample, our shallowest sample (60 m) also shows little fractionation. Hence, this comparison demonstrates that there is a general geochemical process operating in the oceans, whereby both sediment-trapped particles and suspended particies are similarly fractionated with respect to REE( III)s, and enriched in Ce. Their time-series data allow FOWLER et al. ( 1992) to make other important interpretations. They noted that the extent of fractionation in sediment-trapped material is much less well developed in the deep water samples after a phytoplank-

ton bloom. The bloom yields a large increase in total sediment trap flux throughout the water column. After a high flux pe-

REE fractionation by particles in seawater

X-0-

10

60m Sargasso Sea (acetic acid) 2000m Sargasso Sea (total particle)

111

Ol”“““““IVl La Ce

Nd

Sm Eu Gd

1

Dy

Er

Yb

Lu

0

FIG. 12. A comparison of shale-normalized REE ratios of suspended particles from the Sargasso Sea and sediment trap particles from the Mediterranean Sea (FOWLERet al.. 1993). Sargasso samples include 60 m (acetic acid digestion) and 2000 m (total of three digestions). Mediterranean Sea samples include 200 m and 2000 m (microwave digestions using a mixture of HNO, and HF). Note different scales on the y-axes: Sargasso Sea concentrations based on weight of water while trap data based on weight of particles.

riod, deep samples show Ce anomalies that are negative and REE(III) patterns that are flat (features like their 200 m sample in Fig. 12). They suggested that the extent of fractionation is inversely related to the residence time and size of particles in the water column. That is, short residence times of fast-sinking particles lead to little fractionation between particles and seawater, whereas longer residence times of small, slowly sinking particles lead to extensive fractionation of REE (111)s and the biologically mediated oxidation of Ce( III) on surfaces. Hence, they proposed that the extent of fractionation is a kinetically controlled interaction between solution and particle phases. This conclusion was also reached by KOEPPENKASTROP et al. ( 1991) and KOEPPENKASTROP and DE CARLO ( 1993 ), based on laboratory uptake experiments using inorganic oxides of Mn and REE radiotracers in a seawater media. If the fractionation of REE(III)s between particles and seawater is a kinetically controlled process and depends on the size and residence time of particles, then the REEs should be useful probes for the study of particle-particle interactions, such as the transformation from colloidal to small-suspended to large-sinking particles (e.g., HONEYMAN and SANTSCHI, 1989; MORAN and BUESSELER, 1992). Measurements of REEs would complement those of Th isotopes and allow different insights into the geochemistry of particle/seawater reactions. For example, ocean colloids would be predicted to be highly fractionated with respect to bigger particles and seawater, a phenomenon recently reported for river water ( SHOLKOVITZ. 1992 ). MASUZAMA and KOYAMA (1989) attributed the positive Ce anomalies of their sediment trap samples (Japan Sea) to a biologically mediated oxidation process associated with the presence of Mn oxide particles. Manganese oxide-enhanced oxidation of Ce( III) is consistent with our observations. Their

1.577

shale-normalized REE patterns do not show any consistent form with which to draw conclusions about REE( 111)s fractionation. Only their 2750 m sample is slightly LREE-enriched; the other four samples have flat or HREE-enriched patterns. Sediment trap particles from the eastern equatorial Pacific Ocean (MURPHY and DYMOND, 1984) are strikingly different in that their shale-normalized patterns are like those of seawater, HREE-enriched, and negative Ce anomalies. The reason for this difference may lie in the more detrital nature of these samples, or different degrees of uptake and remineralization within the water column. The PALMER and ELDERFIELD ( 1985) study of the REE composition of foraminifera tests from core tops has shown that the major proportion of the REE inventory is contained within FejMn oxide coatings. The coating/shale patterns are similar to those of seawater. Hence, the REE signature of foraminifera coatings is opposite to that of suspended particles reported in this paper. Because the vertical profiles of dissolved REEs are most similar to the profiles of Si (as compared to nitrate and phosphate), REE-Si relationships have been used to suggest that there is a biogeochemical coupling between the two cycles (DE BAAR et al.. 1985; ELDERFIELD, 1988). BERTRAM and ELDERFIELD ( 1993 ) noted that the profiles of the HREEs are more similar to Si than are those of the LREEs and the MREEs. It is difficult to reconcile a geochemical coupling between Si and REEs. Our data point to the uptake and release of REEs from surface coatings (soft parts) while Si is released from dissolved siliceous plankton (hard parts) in deep waters ( BROECKER and PENG, 1982 ). As our data do not cover the depths below 2 km, one could argue that the cycling of REEs switches from a soft part mode to a hard part mode as coatings are remineralized at depth. Coatings, however, are ubiquitous features of all oceanic particles. Even if there is a switch from a soft to a hard part mode, adsorption of REEs onto siliceous surfaces from seawater should still favor the LREEs over the HREEs. Clearly the data of the type presented here open up intriguing questions about the marine chemistry of particles. Our results are consistent with a recent argument by BERTRAM and ELDERFIELD ( 1993) that the vertical variation in the neodymium isotopic composition of seawater is being driven by the reversible exchange of Nd between particles and seawater. SUMMARY

1) The extensive fractionation

of the REE (III ) between Sargasso Sea water and suspended particles can be qualitatively explained by a solution/surface competition model (Fig. 1). The progressive decrease from La to Lu in the extent to which REEs(II1) are removed by suspended particles results from an increase in the solution complexation constants. 2) The chemical digestion data demonstrate that coatings or adsorbed phases (released by acetic acid digestion) on suspended particles are responsible for the removal and fractionation of REEs( III) from seawater. In contrast, the detrital phases have crust-like REE compositions. of Mn appear to be a key surface site 3) The oxyhydroxides

1578

4)

5)

6)

7)

8)

E. R. Sholkovitz, W. M. Landing. and B. L. Lewis

for the oxidation ofdissolved Ce( III) to particulate Ce( IV) and the fractionation of REE( 111)s during sorption. These processes become important between a water depth of 100-200 m, in concert with the in situ oxidation of dissolved Mn( 11) to particulate Mn( IV) oxides. The extent of fractionation between seawater and suspended particles is similar for samples collected in the Sargasso Sea (this study) and the Indian Ocean ( BERTRAM and ELDERFIELD, 1993 ). This suggests that the complexation and binding constants controlling the partitioning may be applicable on an ocean-wide basis. The preferential removal of LREE( 111)s onto particles in the upper zoom is followed by their preferential release at depth. The REEs associated with surface coatings are labile. Hence, the REEs are involved in an active geochemical cycle during removal and remineralization. With samples only to 2000 m, we cannot address deep ocean processes. The combination of Ce data for seawater and particle coatings and measured rates of Ce( III) oxidation ( MOFFETT. 1990) lead to particle residence times in the Sargasso Sea which are similar to those based on Th isotopes. Studies are needed to test the validity of Ce-based residence times. The data Of BERTRAM and ELDERFIELD ( 1993) and those in this paper provide better estimates of in situ binding constants, which in turn should help refine speciation and residence time models ofthe type developed by CANTRELL and BYRNE ( 1987), EREL and STOPLER ( 1993), and LEE and BRYNE (1993). The results of this study have important implications with respect to the geochemical connection between continental and oceanic particles and the sedimentary record. In particular, we cite the recent study by MURRAY and LEINEN ( 1993) who show that surface sediments across the equatorial region of the Pacific Ocean vary greatly in their absolute and relative abundances of REE(III)s and their Ce anomalies. A large proportion (>80%) of the REE inventory of their most biogenic sediments is in excess of the REE inventory supported by detrital minerals. Hence, the uptake of REEs from seawater and/or porewater is a major process controlling the REE composition and fractionation of deep ocean surface sediments. Comprehensive data sets on the REE composition of seawater. aeolian particles. suspended particles, sinking particles, and sediments are now needed to make the geochemical connection between the different sources and phases.

AcX-noM,ledgmenls-Special tribute goes to David Schneider who carried out the mass spectrometric determinations in the Isotope Geochemical Facility at WHO1 and to Gregory A. Cutter (Old Dominion University) who organized and was chief scientist on the Sulfide Experiment Cruise which was supported by NSF grant OCE88-0037 1 to GAC. Thanks to Julie Palmieri for assisting in the REE analyses. Reviews by Drs. De Carlo. Elderfield, and German are appreciated. This study was supported by NSF grants OCE-8711032 and OCE-9101466 to ERS and by grant OCE-86 13638 to WML. This paper has WHO1 contribution number 8464.

Editorial handling: L. S. Balistrieri

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