Dissolved rare earth elements in the Southern Ocean, southwest...

Geochemical Journal, Vol. 37, pp. 47 to 62, 2003

47

*Corresponding author (e-mail: [email protected]. jp)

Dissolved rare earth elements in the Southern Ocean, southwest ofAustralia: Unique patterns compared to the South Atlantic data

YOSHIYUKI NOZAKI* and DIA SOTTO ALIBO

Marine Inorganic Chemistry Division, The Ocean Research Institute, The University of Tokyo,Nakano-ku, Tokyo 164-8639, Japan

(Received April 26, 2002; Accepted July 18, 2002)

Dissolved yttrium and rare earth elements (YREEs) have been measured in the Antarctic CircumpolarCurrent Region, southwest of Australia. Their vertical features are fairly smooth, irrespective of the differ-ent water masses in the water column, suggesting that the biogeochemical processes including reversiblescavenging reactions and sinking particulate matter are important control on their distribution. Compari-son to the previous data at nearly the same latitude of ~40°S in the Southern Ocean of the Atlantic sector(German et al., 1995) indicates that our concentrations are significantly depleted in the lighter REEs. Thedifference in their mean concentrations systematically decreases with increasing atomic number, i.e., 67%for La, 40% for Nd, 15% for Sm and less than 5% for the REEs heavier than Tb and Y. Gadolinium is anexception to this trend with a deficit that is anomalously high at ~36% compared to those of its neighbors,Eu and Tb. Negative Gd-anomalies exceeding 30% are recorded for the subsurface waters, which contrastwith positive Gd anomalies of up to ~20% in the North Pacific Deep Water (Alibo and Nozaki, 1999). Therelative deficit of 62% for Ce(IV), perhaps fortuitously, fits well between those of La and Nd, despite itsvertical profile being different from those of strictly tri-valent YREE(III)s. These observations cannot beexplained simply by the REE fractionation during scavenging of particulate matter and/or regional varia-tion of scavenging intensity alone, and suggest that there must be a REE compositional difference in theexternal sources, e.g., from shallow water sediments and lithogenic materials sinking through the watercolumn.

The unique REE(III) patterns of the Circumpolar Deep Water (CDW) and the Antarctic Bottom Water(AABW) with a LREE depletion in the southeastern Indian Ocean appear to be originated from igneousrocks around the Indonesian Archipelago. This is in contrast to the REE(III) patterns of a LREE enrich-ment with a maximum at Nd-Sm, reflecting continental sedimentary rocks, in the CDW and AABW in theSouth Atlantic and the western Indian Ocean. The markedly different REE patterns in the eastern andwestern components of CDW and AABW indicate that they can serve as novel tracers in elucidating thedeep water circulation of the Indian Ocean.

(1996) and Nozaki (2001). Recently, we haveshown that the oceanic distributions of dissolvedYREEs are maintained by the kinetic balance be-tween supply from remineralization sources andremoval by particle scavenging (Nozaki and Alibo,2002). The vertical profiles of the light and mid-dle REEs (LREEs and MREEs) may be repre-sented by reversible scavenging processes like Thwhich appear to respond quickly to the variationof the vertical flux and its REE composition of

INTRODUCTION

Yttrium and the rare earth elements (YREEs)are an extremely coherent group in terms of chemi-cal behavior and have intensively been investi-gated in the field of marine geochemistry to elu-cidate particulate scavenging processes and char-acterize water masses. Progress in the YREEgeochemistry in the marine environment has beenthoroughly reviewed by Byrne and Sholkovitz

48 Y. Nozaki and D. S. Alibo

detrital particles, whereas Y and the heavy REEs(HREEs) are largely governed by the horizontalprocesses of ocean circulation with regenerationfrom particulate matter more like dissolved Si.These implications are based on the observationthat the vertical profiles of LREEs and someMREEs are variable from basin to basin, and arealso supported by the earlier studies in the land-locked basins like the Sulu Sea (Nozaki et al.,1999) and the South China Sea (Alibo and Nozaki,2000) where, regardless of absence of (or limited)horizontal advection, unique YREE patterns aredeveloped in the deep waters.

The Southern Ocean is an important pathwayfeeding the bottom and deep waters of the majoroceans, and is regarded as a key region in under-standing the role of biogeochemical cycling on thevariation of global climate. German et al. (1995)have reported the detailed profiles of 10polyisotopic REEs in the Atlantic sector of theSouthern Ocean. This is the only study reportingdissolved REEs concentrations in the AntarcticCircumpolar Region to date. Here, we have ob-tained a complete dataset on dissolved YREEs inthe water column in the South Australian Basin.By comparing these data, we show that the east-ern bottom-deep water component of theCircumpolar Deep Water (CDW) and the Antarc-tic Bottom Water (AABW) in the South Austral-ian Basin in the eastern Indian Ocean has REEpatterns markedly different from those of the west-ern component originating in the Weddell-EnderbyBasin which influences the South Atlantic and thewestern Indian Ocean (German et al., 1995;Bertram and Elderfield, 1993). Possiblegeochemical causes for this observation are dis-cussed in this paper.

METHODS

During the “Piscis Austrinus” Expedition us-ing R.V. Hakuho-Maru, one hydrographic stationwas occupied in the Antarctic Circumpolar Cur-rent regime (Fig. 1a), southwest of Australia onJanuary 14, 1996. Figure 1b shows the station lo-cation (PA-4; 40°00′ S, 110°00′ E; water depth

4637 m) together with other R.V. Hakuho-Marustations and those where the dissolved REE dataare available in the literature (German andElderfield, 1990; Bertram and Elderfield, 1993;German et al., 1995). We have already reportedthe REE data for the surface waters (Amakawa etal., 2000) and for oceanic profiles in the Sulu Sea(Nozaki et al., 1999), the South China Sea (Aliboand Nozaki, 2000), and the Bay of Bengal andAndaman Sea (Nozaki and Alibo, 2002). Meth-ods of sampling, shipboard filtration, and YREEanalysis on the shore-based laboratory have al-ready been described in those publications.

Briefly, water samples were collected by us-ing a SeaBird CTD and carousel 12L Niskin multi-bottle sampling system, filtered through 0.04 µmhollow fiber membrane immediately after sam-pling, and acidified to pH

Dissolved REE patterns in the Southern Ocean 49

Fig. 1. (a) Current system of the Southern Ocean, and the station locations of PA-4 and AJAX 47. (b) The stationlocations of the “Piscis Austrinus” Expedition of the R.V. Hakuho-Maru (solid circles), and others (open circles),including AJAX 47 (a solid square) in the South Atlantic, where dissolved REEs have been measured. The concen-trations of La in the surface waters are also given.


Tabl

e 1.

D

isso

lved

rar

e ea

rth

elem

ents

(in

pm

ol/k

g) i

n th

e So

uthe

rn O

cean

and

ave

rage

con

cent

rati

ons

in t

he N

orth

Pac

ific

Dee

p W

ater

(N

PD

W)

*Thi

s is

Nor

th P

acif

ic D

e ep

De e

p W

ate r

at

2500

± 1

00 m

; v a

lue s

are

fro

m A

libo

and

Noz

aki

(199

9).

**T

his

is t

he a

v era

ge o

f P

ost-

Arc

hean

Aus

tral

ian

Sedi

men

tary

Roc

k s;

v alu

e s a

re g

ive n

in

µmol

/kg

from

McL

enna

n (1

989)

.


Current (Fig. 1a) and similar biological produc-tivity in the Southern Ocean (Moore and Abbot,2000).

In Fig. 1b, literature values of dissolved La insurface waters for each station are also summa-rized. The concentrations are clearly lower in thesoutheastern Indian Ocean than in the South At-lantic, the western Indian Ocean including theArabian Sea, and the Bay of Bengal.

Table 1 also gives the average values of thePost-Archean Australian Sedimentary Rocks(PAAS; McLennan, 1989) and the North PacificDeep Water (‘NPDW’; Alibo and Nozaki, 1999).The shale values like PAAS are thought to be rep-resentative of the YREEs of the upper continentalcrust and have been used for normalization toeliminate the well-known distinctive even-oddvariation in natural abundance (the Oddo-Harkinseffect) and to visualize, to a first approximation,fractionation relative to the continental source.However, we use the values of ‘NPDW’ for nor-malization throughout this paper, because the‘NPDW’-normalization eliminates the commonfeatures appeared in the shale-normalization likea progressive heavier REE enrichment and a pro-nounced Ce anomaly, and can single outfractionation relative to a reference water mass,NPDW which is the end product in the route ofthe global ocean circulation. It should be recog-nized here that the values of ‘NPDW’ were ob-tained by the same analytical method and filtra-tion protocol, and also by the same analyst as thisstudy (Alibo and Nozaki, 1999). Therefore, theNPDW-normalized pattern of our REE data elimi-nates any possible artifact arising from the ana-lytical method or sample treatment and visualizesthe difference of YREE composition only in thewaters, although this does not apply to the databy German et al. (1995).

DISCUSSION

Comparison to previous Southern Ocean profiledata

German et al. (1995) have previously reportedthe detailed vertical profiles of dissolved REEs at

the “AJAX” station 47 (39°00.5′ S, 00°59.2′ E) inthe Cape Basin (see Fig. 1b). In Fig. 2, the verti-cal profiles of salinity, dissolved oxygen, and nu-trients at AJAX 47 (Reid and Nowlin, 1985) arecompared with those at our PA-4 station. The dis-tinct water masses can be identified at the AJAXstation; the Antarctic Intermediate Water (AAIW)with low salinity and high dissolved oxygencentered at ~650 m, the North Atlantic Deep Wa-ter (NADW) with high salinity, high dissolvedoxygen and low dissolved Si and nitrate around3000 m, and the Antarctic Bottom Water (AABW)with low potential temperature and high dissolvedSi near the bottom. Between AAIW and NADW,a maximum of dissolved Si and a minimum of dis-solved oxygen occur around 1600 m which is theupper Circumpolar Deep Water (UCDW). In con-trast, at PA-4, the vertical profiles (Fig. 2) aresomewhat simpler than those at AJAX 47 due toabsence of NADW. The salinity minimum iscentered at 1050 m and a broad salinity maximumaround 3000 m characterizes the lowerCircumpolar Deep Water (LCDW) by entrainmentof NADW. Cold and less saline Antarctic BottomWater (AABW) occupies the interval betweenCDW and the bottom.

Figure 3 compares the dissolved REE data atthe two sites. German et al. (1995) employed anisotope dilution thermal ionization massspectrometry (ID-TIMS) so that only 10polyisotopic elements were measured. They used0.4 µm Nuclepore membrane to filter water sam-ples. However, Alibo and Nozaki (1999) haveshown that the particulate and colloidal REEs thatcan be removed by 0.04 µm hollow fiber filtra-tion are generally less than 5% except for 35%for Ce. Thus, the difference in the filtration methodwould not cause any significant difference, andtherefore, we compare the data, directly. The REEprofiles show systematic increase with depth simi-lar to each other, although some elements like La,Nd, and Gd are significantly offset. The verticalprofiles of the REEs are much simpler than thoseof the hydrographic properties shown in Fig. 2,suggesting generally that the vertical geochemicalprocesses through interactions with sinking parti-


cles may be more important in determining thegross feature of oceanic profiles than the horizon-tal processes which carry the different watermasses. Nonetheless, that the HREEs and Y arestrongly correlated with dissolved Si (R2 = 0.882for Dy and R2 = 0.914 for Lu for combined data atthe two stations) also suggests that they are lessreactive to particles and behave more similarly todissolved Si than the LREEs (R2 (relative to Si) =

0.309–0.531) and MREEs (R2 (relative to Si) =0.533–0.806).

The most striking feature of Fig. 3 is that thevertical profiles of Eu, Dy, Er, Yb and Lu showalmost perfect match, whereas La, Ce, Nd, Sm andGd at PA-4 are significantly depleted relative tothose at AJAX 47 when compared for the samedepths. Calculated numerical average concentra-tions at the two locations and their difference are

Fig. 2. Comparison of the vertical profiles of salinity, dissolved oxygen and nutrients at PA-4 (open squares) andAJAX 47 (open circles). Note various water masses (see text for acronyms).


Fig

. 3.

C

ompa

riso

n of

ve r

tic a

l pr

ofil

e s o

f di

ssol

v ed

YR

EE

s at

AJ A

X 4

7 (o

pen

c irc

les)

and

PA

-4 (

soli

d c i

rcle

s).


given in Table 2. The relative depletion at PA-4systematically decreases from 67% for La to lessthan 5% for Dy to Lu with increasing atomicnumber, except for anomalously high value of 36%for Gd. Anomalous behavior of Gd (and La) inthe marine environment has been known for sometime, although its cause is not clear and is contro-versial (Masuda and Ikeuchi, 1979; DeBaar et al.,1985). More recently, Alibo and Nozaki (1999)have noted positive anomalies of dissolved La andGd in the shale normalized pattern for westernNorth Pacific waters. Lerche and Nozaki (1998)also noted a significant break at Gd in the corre-lation diagram of neighboring elements across thelanthanide series for sinking particles collected bysediment traps.

Interestingly, the difference of Ce, which prob-ably exists as Ce(IV) and shows its profile differ-ent from that of the other YREE(III)s, turns out tobe 62% (Table 2) which fits well in-between Laand Nd. This is probably fortuitous, since it is in-consistent with the higher particle reactivity of Cethan those of the YREE(III)s and Y.

Monoisotopic elements, and Ce and Gd anoma-lies

We report monoisotopic lanthanides and yt-trium for the first time in the Southern Ocean.They fit very well between those of neighboringelements (Fig. 3). Altogether, there is a tendency

that the vertical features systematically changefrom concave curves at the LREEs like Pr to con-vex curves at the HREEs like Tm, suggesting thattheir involvement in the biogeochemical cyclingvaries across the lanthanide series. The verticalprofile of Y resembles those of the HREEs, par-ticularly Ho, due to the similarity in ionic radii.The correlation coefficient (R2) between Y and Hois very high at 0.989 for the data. However, the Y/Ho molar ratio (Fig. 4) systematically decreasesfrom ~115 at the surface to ~95 near the bottomindicating that they are not homogenized by mix-ing within their mean oceanic residence times.These values are significantly higher than thecrustal values of 39–70 (average, ~55), suggest-ing that the fractionation takes place presumablyduring scavenging in the marine environment(Nozaki et al., 1997). The higher Y/Ho ratio inthe upper water column suggests that Ho is morerapidly taken up by phytoplankton or biogenicparticulate matter than Y does.

With the use of Pr and Tb values, we estimatedCe and Gd anomalies as shown in Fig. 5. AlthoughGd is significantly depleted as shown earlier, theGd anomaly defined as Gd/Gd* = 2[Gd]/([Eu]+[Tb]), where [ ] denotes shale normalizedvalues, may be enhanced if Tb is positively devi-ated. This appears to be the case (see Fig. 6b) andtherefore, it is better to refer as the Gd/Tb anomalyhere. The Ce anomaly is always negative (defined

Element AJAX 47 PA-4 Difference Difference(pmol/kg) (pmol/kg) (pmol/kg) (%)

La 23.47 7.71 15.76 67.1Ce 5.66 2.18 3.49 61.6Nd 15.66 9.35 6.31 40.3Sm 2.91 2.47 0.44 15.1Eu 0.77 0.72 0.04 5.6Gd 4.29 2.74 1.55 36.1Dy 5.15 5.08 0.08 1.5Er 4.85 4.71 0.15 3.0Yb 4.89 4.66 0.23 4.7Lu 0.792 0.789 0.003 0.4

Table 2. Comparison of the mean concentrations ofREEs in the Atlantic and Indian Ocean sectors of theSouthern Ocean

Fig. 4. The vertical profile of Y/Ho ratio at PA-4.


to its redox chemistry, its cause may not simplybe related to solution chemistry. Alibo and Nozaki(2002) have shown that the Gd/Tb molar ratios invarious water masses in the eastern Indian Oceanare clustered into two regimes of ~3.3 and ~5.9.The latter value is close to the value of ~6 in thewaters of the western South Pacific (Zhang andNozaki, 1996). This strongly suggests that the Gd/Tb anomaly in seawater is likely originated fromsource materials of different Gd/Tb ratios. Reli-able measurements of Tb in various rocks are rela-tively few to date, and the variability in the Gd/Tb ratios of the source materials needs to be in-vestigated in the future.

Unfortunately, Tb data are not available atAJAX 47, and therefore, the Gd/Tb anomaly can-not be estimated.

‘NPDW’-normalized REE(III) patternsThe available data show that the waters in the

Antarctic Circumpolar Current (ACC) region havemarkedly different YREE compositions depend-ing upon longitude. This may be best shown bynormalization of the YREE data to an appropriatereference water mass, i.e., the North Pacific DeepWater (Alibo and Nozaki, 1999). FollowingNozaki et al. (1999), the ‘NPDW’-normalized

Fig. 5. The vertical profiles of Ce anomaly (left) and Gd/Tb anomaly (right). The anomalies were estimated by theequations, Ce/Ce* = 2[Ce]/([La]+[Pr]) and Gd/Gd* = 2[Gd]/([Eu]+[Tb]), where [ ] denotes shale normalizedvalues. The values for AJAX 47 are from German et al. (1995).

as


Fig

. 6.

T

he ‘

NP

DW

’-no

rmal

ize d

RE

E(I

II)

patt

e rns

of

seaw

ate r

s at

AJ A

X s

tati

on 4

7 (a

) an

d PA

-4 (

b).


patterns of REE(III)s are compared in Fig. 6 forthe two stations AJAX 47 and PA-4.

Figure 6a shows that, at AJAX 47 in the CapeBasin, the Antarctic Bottom Water (AABW) hasthe LREE-enriched pattern with a broad maximumat Nd and Sm, which resembles the pattern of shale(Fig. 7). The gradual decrease of the Nd-Sm maxi-mum with increasing distance from bottom indi-cates that AABW mixes with the above North At-lantic Deep Water (NADW) with an almost flatpattern (i.e., the REE(III) composition is similarto that of ‘NPDW’). In contrast, AABW and CDWat PA-4 in the South Australia Basin exhibit aMREE enriched pattern with dual maxima at Euand Tb and a marked depression at Gd (Fig. 6b)that is clearly different from Fig. 6a. The patternwith a LREE depletion and MREE enrichmentsomewhat resembles those of igneous rocks likemid-oceanic ridge basalt (MORB; Fig. 7), al-though the Gd deficit is small in those rocks. Thecold bottom waters of the eastern basins of theIndian Ocean, such as the South Australia, Perth,and West Australia Basins, are fed ultimately fromthe Australian-Antarctic Basin that is separated inthe deeper depths from the Weddell-Enderby Ba-sin by the Kerguelen Plateu and Southern IndianRidge (see Fig. 1b; Mantyla and Reid, 1995). Thisunique pattern can also be traced equatorwardthrough the Perth Basin to at least 10°S in the WestAustralia Basin (PA-7 in Fig. 1b; Alibo andNozaki, 2002).

In the Upper Circumpolar Deep Water(UCDW) and the Antarctic Intermediate Water(AAIW), the REE data in the South Atlantic showa flat feature from La to Eu and then stepped to aslightly higher flat pattern from Gd to Lu (Fig.6a). In contrast, those in the southwest ofAustralia show progressive increase from La toEu, a marked deficit at Gd, and nearly flat patternfrom Tb to Lu (Fig. 6b). The surface waters atAJAX 47 show a somewhat flat top pattern fromNd to Er (Fig. 6a), whereas those at PA-4 showthe progressive increase from La to Ho, with onlyslight Gd depression, and then decline to Lu (Fig.6b). In summary, despite the fact that the south-eastern Indian Ocean comprises various water

Fig. 7. The ‘NPDW’-normalized REE(III) patterns ofthe Mid-Oceanic Ridge Basalt (MORB) and the Post-Archean Australian Sedimentary Rocks (PAAS) afterMcLennan (1989), and average tholeiitic basalt fromSunda arc, Indonesia after Whitford et al. (1979).

masses, an overall feature of the REE pattern isuniquely characterized by a systematic depletionin the LREEs from La to Sm and a marked deficitat Gd (except for surface waters) throughout thewater column. Thus, we will examine, below, thepossibility that such a unique REE pattern may bedeveloped by geochemical processes occurringwithin the ocean.

Variation of particle scavenging intensityEarlier studies on the REEs in suspended

particulate matter by Bertram and Elderfield(1993), Sholkovitz et al. (1994) and Alibo andNozaki (1999) have demonstrated that the parti-cle association occurs in the order of LREEs >MREEs > HREEs with an exception of anoma-lously high Ce due to its +4 oxidation state. Thus,the observed difference in the LREE profiles mayhave, at least partly, resulted from the REEfractionation during more intensified scavengingin the southeastern Indian Ocean than in the SouthAtlantic. However, we find no evidence that theparticulate flux is significantly different at PA-4than at AJAX 47 as shown in the map of chloro-phyll distributions in the Southern Ocean (Mooreand Abbot, 2000). The primary productivity of ~30mmol C m–2d–1 has been estimated by modelingthe vertical profiles of 228Ra and nitrate at PA-4


(Nozaki and Yamamoto, 2001). The relatively highproductivity and enhanced particle export are well-known and probably common to the whole Ant-arctic Circumpolar Current Region. The scaveng-ing intensity may be more sensitively reflected inthe distributions of particle-reactive Th isotopes.For example, 230Th with a half-life, 75200 y, whichis predominantly and uniformly produced by insitu decay of 234U in the water column, is expectedto be lower in the higher scavenging regime. AtPA-4, a 230Th profile increasing from 0.25 dpmm–3 at the surface to 1.3 dpm m–3 at 3022 m hasbeen obtained using large-volume (~250 L) watersamples (Yamada, Okubo and Nozaki, in prepa-ration). This profile is similar to that reported forthe Atlantic sector of the Southern Ocean (44°30′S, 10°27′ E) which shows an increase from 0.2dpm m–3 at the surface to 1.05 dpm m–3 at 4700 m(Rutgers van der Loeff and Berger, 1993). There-fore, a large difference in the particle flux is notexpected between the two locations. Intensifiedscavenging would also result in greater relativedepletion of Ce since it is more reactive to parti-cle surfaces and is more rapidly removed from thewater column than the other strictly tri-valentYREEs. Obviously, this is not the case (Table 2).Thus, there is no evidence that the particle scav-enging regime is more intense at PA-4 that atAJAX 47.

A transient model with the first-order irreversiblescavenging

Another explanation for the lower LREE andGd concentrations at PA-4 may be the smaller in-put of those elements from external sources in thesoutheastern Indian Ocean than in the South At-lantic, since the concentrations of YREEs inseawater can change by the imbalance betweensupply and removal. To examine this possibility,we use a simple time-dependent model of the ACCin which the particle scavenging is assumed tooccur irreversibly for the dissolved REEs. Sup-pose that during the eastward flow of theCircumpolar Current from the AJAX 47 station tothe PA-4 station, there is no supply from externalsources but significant removal by scavenging,

occurring with first order functionality for the dis-solved REEs. In such an extreme case, the con-centration of REEs would decrease from west toeast depending on the current speed (v, km/y) andscavenging rate constants of the REEs. The rela-tionship is given by the following equation.

ln[REE]AJAX47/[REE]PA4 = (H/v)/τREE

where H is the distance between AJAX 47 and PA-4 and τ is the first-order irreversible scavengingresidence time of the REE.

Whitworth and Nowlin (1987) estimatedgeostrophic eastward flow of roughly about 2cm s–1 between 40°S and 60°S at the Greenwichmeridian. Schmitz (1995) also has summarized thevolume transport of CDW to be ~53 Sv(Sverdrup = 106 m3s–1), corresponding to a meancurrent speed of ~1 cm s–1 for a meridional crosssection of 1200 miles and 4 km depth. These ve-locities set an upper limit of (H/v) = ~10 years forCDW to flow about 5050 km from the AJAX 47station to the PA-4 station. For La, the concentra-tion in the lower CDW decreases from 30 pmol/kg at AJAX 47 to ~10 pmol/kg at PA-4, and hence,the mean scavenging residence time (τ) for La isestimated to be 9 years. In reality, the CDW is amixture of various water masses, the North At-lantic Deep Water (NADW), Weddell Sea Water,Indian Ocean Deep Water, etc. Since the concen-trations of the LREEs and Gd at PA-4 are the low-est so far (Bertram and Elderfield, 1993; Germanet al., 1995), exchange of water with those differ-ent water masses along the eastward transport ofCDW would probably result in net increase forthose elements. Thus, the 9 year residence timefor La estimated above should be regarded as anupper limit. However, this value is comparable tothe residence time of fine particles and is unrea-sonably short for dissolved La of 270 years esti-mated earlier (Nozaki, 2001).

Similarly, using the equation, the mean resi-dence time of Ce is calculated to be 9.5 years, tak-ing the CDW concentrations of 5.7 pmol kg–1 atAJAX 47 and 2.2 pmol kg–1 at PA-4 (Fig. 3). ThisCe scavenging residence time which is compara-


ble with that of La is inconsistent with well-knownhigher particle association of Ce(IV) and occur-rence of negative Ce-anomaly in seawater. Clearly,the simple first-order irreversible scavengingmodel failed to explain the difference of LREEsin the CDW at AJAX 47 and PA-4. The dissolvedYREEs at PA-4, if they are, at least partly, origi-nated from the South Atlantic (AJAX 47), musthave quickly been replaced by the YREEs fromdifferent sources. This may be achieved by equi-librium exchange reactions between dissolved andrapidly sinking particulate phases, in which theREE patterns can vary locally depending onlithogenic sources. Nozaki and Alibo (2002) havedemonstrated that, in the Bay of Bengal, the dis-solved YREEs and their composition are stronglyinfluenced throughout the water column by thelarge input of terrigenous minerals from the Gan-ges and Brahmaputura Rivers. Adsorption anddesorption equilibrium between dissolved andparticulate phases can be reached within a fewmonths for Th (e.g., Nozaki et al., 1987). Suchrapid exchange has been demonstrated to occurfor Nd through its isotopic studies (Bertram andElderfield, 1993; Jeandel, 1993; Jeandel et al.,1995; Tachikawa et al., 1999).

YREE remineralization sources in the southeast-ern Indian Ocean

The above arguments suggest that some uniqueYREE source exists in the southeastern IndianOcean. Recently, Amakawa et al. (2000) andSholkovitz and Szymczak (2000) have shown thatriver input and remineralization from shallowwater sediments are predominant sources of dis-solved YREEs in the open ocean. Aeolian dustinput has been suggested to be important for theREEs in the surface waters of the North Pacific,but this conclusion was later retracted (Greaveset al., 1999, 2001). Nozaki and Alibo (2002) havefurther suggested that such dissolution also oc-curs from lithogenic materials sinking through thewater column. The subsequent reversible exchangemechanism between dissolved and particulatephases leads the LREEs to increase almost linearlywith depth as observed in various oceanic regions.

Since the surface waters may be more stronglyinfluenced by the external sources, we plotted thedissolved La concentrations in Fig. 1b. It showssignificantly low values ranging from 2.5 to 4.7pmol/kg in the southeast Indian Ocean as com-pared to 10.8 pmol/kg at AJAX 47 and 8.1–15.2pmol/kg in the western Indian Ocean. Apparently,such regional difference of La (and presumablyother REEs) in the surface waters is propagatinginto deeper waters via the biogeochemical proc-esses associated with sinking particles.

Amakawa et al.(2000) have previously re-ported the NPDW-normalized REE(III) patternsin the surface waters of the eastern Indian Oceanand its adjacent seas which varies depending onthe oceanic province. The REE(III) patterns in theoffshore region of the eastern Indian Ocean of 10–30°S best resemble those in the CDW and AABWat PA-4 (Fig. 6b). The different REE(III) patternsin the open ocean surface waters may not only bederived from different lithogenic sources but alsomay be modified by subsequent fractionation ofYREEs during scavenging by particulate matter.If fractionation of REEs occurs during repeatedirreversible scavenging by particulate matter, thenthe following relationship may hold,

[REE]measured = [REE]0(1 – Fp)n

where [REE]0 is the original concentration of dis-solved REEs imprinted by lithogenic sources, Fpis the particulate fraction of REEs, and n is thenumber of times of particle scavenging by whichthe dissolved REEs are removed from seawater.The particulate fraction (Fp) is dependent upon thereactivity of elements relative to marine particles.Based on the same filtration procedure as thisstudy, Alibo and Nozaki (1999) obtained the Fpvalues ranging from ~5% for the LREE(III)sto


number of n. Figure 8 shows that with an increaseof n, the depletion of LREEs is enhanced by scav-enging, whereas the HREEs become nearly flat.The pattern becomes more like what is observedin Fig. 6b except for a strong Gd deficit. Thus,combination of initial REE sources and subsequentREE fractionation by scavenging appears to de-termine the dissolved REE patterns in seawater.Nonetheless, the Gd deficit and Tb maximum (Fig.6b) are difficult to reproduce in Fig. 8. This sug-gests that a pattern of either low Gd and/or highTb is unlikely produced by particulate scaveng-ing and needs to be imprinted in the original REEsource.

Igneous rocks like oceanic basalt are signifi-cantly depleted in the LREEs than the HREEs ascompared to those of the continental crust andsedimentary rocks (Fig. 7; McLennan, 1989; Sanoet al., 1999). Indeed, tholeiitic series basalt fromthe Sunda arc, Indonesia (Whitford et al., 1979)shows the REE(III) pattern similar to MORB.Thus, the LREEs and Gd depletion in the dissolvedREEs in the eastern Indian Ocean may be due tothe influence of the Indonesian Archipelago fromwhich lithogenic materials may be transportedalong with the Indonesian Throughflow. Alterna-tively, erosion of volcanic materials in the bottomsediments may imprint such a unique REE pat-tern into the deep waters of the eastern IndianOcean as postulated recently by Lacan and Jeandel(2001). Mantyla and Reid (1995) suggest that thedeep and bottom water component in the easternIndian Ocean comes from the Ross Sea and Adeliecoast. Since we have no YREE data further southof the ACC, a possibility exists that the uniqueREE pattern may be developed in the Australian-Antarctic Basin and flows into the South AustraliaBasin. However, it is hard to identify igneous prov-inces in those regions or even Australian coastfrom which the dissolved YREE patterns depletedin the LREEs and Gd may be derived. Thus, thebest candidate of the YREE sources for the CDWand AABW southwest of Australia is the lithogenicparticles carried by the surface currents from theIndonesian Archipelago. The surface 228Ra con-centration of 24 dpm/m3 at PA-4, significantly

higher than


observation cannot be simply explained by thesystematic trend of the YREEs during scavengingby particulate matter, and must ultimately origi-nate from different external sources to the oceans.Apparently, the variation of the YREEs in the sur-face waters, which is strongly influenced by in-put from terrigeneous sources, is roughly propa-gated into the deeper waters through rapid equi-librium exchange reactions between dissolved andsinking particle phases. Thus, it seems likely thatthe REE(III) patterns depleted in the LREEs andGd in CDW and AABW southwest of Australiaare derived from the eastern Indian Ocean, wherethe YREE sources are strongly affected by theIndonesian Throughflow which carries the YREEsfrom igneous rocks of the Indonesian Archipelago.

The marked difference in the REE(III) patternsfound between the eastern and western deep-bot-tom water components indicates that the dissolvedYREEs serve as novel tracers in understanding thedeep circulation of the Indian Ocean. They behaveindependently with internal chemical consistencyand add a useful mean of characterizing the watermasses that complements conventionalhydrographic properties like salinity, temperatureand nutrients. This aspect is beyond the scope ofthis paper and will be described elsewhere (Aliboand Nozaki, 2002).

Acknowledgments—We thank Captain Y. Jinno, theofficers and crew of the R.V. Hakuho-Maru, and thescientific party of the “Piscis Austrinus” Expedition fortheir collaboration in the sampling. We are also grate-ful to Professor J. Reid for providing the hydrographicdata of the “AJAX” expedition, and Dr. C. German foruseful insights on the Indonesian Throughflow. Com-ments on the manuscript provided by Y. H. Li and ananonymous reviewer were useful. This work was par-tially supported by the Ministry of Education, Culture,Sports, Science and Technology (MEXT), Japan underthe Grant-in-Aid No. 13304045 to the University ofTokyo (Y. Nozaki, Principal Investigator).

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