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Anomalies in rare earth distributions in seawater: Gd and Tb
0016-7037/85/$3 00 + .Xl
Cecchrm!ca PI Cosmochrnuca Acfa Vol. 49. pp. 1961-1969 0 Pcrgamon Pres Ltd. 1985. Printal in U.S.A.
Anomalies in rare earth distributions in seawater: Gd and Tb* HEIN J. W. DE BAAR’, PETER G. BREWER
and MICHAEL P. BACON
Woods Hole Oceanographic Institution, Woods Hole, MA 02543 (Received August 21, 1984; accepted in revised form June 2 I, 1985)
Abstract-We have measured profiles of the rare earth elements (REE) in Atlantic and Pacific Ocean waters. The data, normalized versus shales, exhibit a pronounced anomaly in Gd relative to its neighbors Eu and Tb in the REE series such that the Gd concentrations are high by 30-50%. Closer inspection reveals that the anomaly is made up of both elevated Gd and depressed T%concentrations, likely associated with solution chemistry shifts in the transition from an exactly half filled 4felectron shell. Anomalies in Gd and ‘I% solution complexation are also indicated by the Turner-Whitfield-Dickson speciation model. The overall trend of heavy REE(III) enrichment in seawater and the Gd/Tb anomaly described here tend to support scavenging as an important removal mechanism for the REE from seawater. INTRODUCTION
Gd is one of the more difficult REE for determination by neutron activation analysis. For Gd determination the samples WE HAVE PREVIOUSLY described the distribution of the were counted after a six-week cooling period during which interfering short-lived isotopes (e.g. ‘%m) decayed away. rare earth elements (REE) in waters of the Atlantic and Pacific Oceans (DE BAARet al., 1983, 1985). These Concentration values based on the 97 keV and 103 keV peaks of long-lived isotope ‘53Gdusually agreed within ten percent. data, combined with recent contributions Of PIEPGRAS For the combined Pacific and Atlantic data sets both estimates and WASSERBURG (1982, 1983), ELDERFIELD and yielded a mean ratio
GREAVES(1982) and KLINICHAMMER et al. (1983) now permit a much more complete description of the abundances and controls of the REE in seawater than has hitherto been possible. In examining our results we have noted that the behavior of Gd appears always to be anomalous when compared to its neighbors Eu and Tb in the REE series. The anomaly is such that the concentration of dissolved Cd appears to be too high by 30-5090, so that its oceanic distribution would tend to place it between the heavier REE Dy and Ho, rather than in its true place in the series according to atomic number. This behavior appeared to be so odd that we chose to delay reporting our results for Gd in seawater in our first paper on the subject (DE BAAR et al.. 1983). However, a careful examination of this first Atlantic data set, as well as the later data from the Pacific Ocean (DE BAAR et al., 1985). shows the effect to be real, and we present here a discussion of the anomaly and its likely causes. The feature is of considerable interest, because, with the REE exhibiting such systematic trends in their geochemical properties. such anomalies potentially provide a very specific probe into the fundamental processes affecting their oceanic distributions. METHODS The REE were quantitatively extracted from seawater by CHELEX 100 ion exchange chromatography. After subsequent purification by cation and anion exchange, the samples were analyzed with neutron activation followed by high resolution gamma spectrometry. A full account of analytical methods has been reported elsewhere (DE BAAR, I983,1984).
’ Present address: Department of Earth Sciences, University of Cambridge, Cambridge CB3 OEZ, UK. * W.H.O.I. Contribution No. 575 I.
Gd (97 keV)/Gd (103 keV) = 1.08 f 0.14
(1)
which does not differ significantly from one. The 14% standard deviation is less than the observed sysremafic 30-50% positive offset that we discuss here. All reported Gd concentrations represent a simple arithmetic mean of both values, although the 97 keV peak exhibits somewhat better counting statistics. Various types of systematic erron have been considered. For instance, interference by an unnoticed peak of another radionuclide might have occurred. It is unlikely, however, that both peaks of ‘53Gd (97 and 103 keV) would be on top of two hidden peaks of similar size. The sample spectra were very clean and appeared devoid of any peaks of elements other than the REE. Moreover, we found no possible candidates for hidden peaks in the 97-103 keV range after a careful search of the literature and gamma-ray energy tables for minor gamma peaks of long-lived radionuclides. Systematic errors in peak integration were avoided by comparing the net peak areas calculated by two different spectral analysis programs. Finally, concentrations based on the very weak signal of shortlived ‘59Gd served as an independent check (within about -+20%) of the ls3Gdderived values. Systematic errors in all three peaks of two different Gd isotopes are deemed unlikely. The determination of Tb is also based on a 48-hour count for each sample, after at least six weeks’ cooling in order to obtain a very low and stable background devoid of any interferences. Concentrations calculated from three different, very strong peaks (98, 299 and 879 keV) were always in excellent agreement. Reported values are based on the 299 keV peak which has slightly better counting statistics than the other peaks. Standards prepared from pure metals (Ames Laboratory) were used for the Pacific Ocean and Cariaco Trench data sets. For the Atlantic data an older standard mixture, made up from REE oxides, was used (F. FREY, pers. commun.). The new standards do potentially have a higher accuracy. Additional seawater data taken from ELDERFIELDand GREAVEZS (1982), which we will use in our discussion, is again based on oxide standards. More recently pure metal standards have also been used in their laboratory, and no significant differences between the two types of standards have been found (H. ELDERFIELD,pers. commun.). Recent intercalibration of pure metal standard mixtures of Dr. Eldertield and ourselves yielded
1961
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H. .I. u’. de Baar. P. G. Brewer and M. P. Bacon
good agreement within the error of the IDMS analytical technique (Table I). NORMALIZATION-DEFINITION OF AN ANOMALY
The vertical distributions of Gd and neighboring elements Sm, Eu and Tb are very similar both in the northwest Atlantic Ocean (Fig. 1) and the eastern equatorial Pacific Ocean (DE BAAR et al., 1984). Yet after normalization versus shales a distinct positive Gd anomaly is readily apparent (Figs. 2 and 3). Tabular listings of the data are to be found for the Atlantic in DE BAAR et al. ( 1983) and for the Pacific in DE BAAR et al. ( 1984). As mentioned earlier, the Gd data were not previously reported for the Atlantic. and Table 2 here remedies this omission.
Table
1.
Element
&xvimetric pIm1.g-’
La CC
Pr Nd Sm Eu Gd : EO
&r rm rb LU
Intercalibration* ILMS pma1.g-’
515.02
516.2
1122.4 262.51 827.5 100.4 15.64 256.07 105.h 19.06
1123.3 -027.b 100.0 15.85 251.8 106.5 --
63.Ll 98.2 32.36 90.18 16.05
-90.2 -89.5 16.57
* Calibration with Isotope Dilution Mass Spectrometry at Cambridge (U.K.) of Woods Hole seawater-type REEstandard mixture,
made up gravi-
wtrically from pure REE metals (DE BMR. 1983). An amount of standard mixture corresponding to about 5 kg seawater was mixed with isotopically
enriched Cambridge spikes. The latter had been calibrated versus a similar standard at Leeds (U.K.), which in itself is in ercellent agreement with a Mains standard (ELDERFIELD and GRWVES, pers. which co-. ). Elements run well with IDMS(Nd, Sm, Cc) agree very well. and there is no reason for the graviwtric concentrations of the other elements to be any less This includes accurate. the gravirtric values for Pr. Tb, Eo and I’m, which cannot be determined by IDMS.
The magnitude of an anomaly depends on the normalization scheme, which has to be selected carefully. The cosmic abundance patterns of the REE are related to various stages of nucleosynthesis (HENDERSON. 1982). As a result their abundances are dominated both by a systematic decrease in concentration with increasing atomic number and by the well-known predominance of elements with even atomic numbers. Smaller fractionations in the ocean due to marine chemical processes are only revealed after these dominant effects are removed by normalization. Two normalization schemes were given serious consideration: 1)against a weighted mean of abundances in chondritic meteorites (EVENSEN ef al.. 1978). representative of the relative REE distribution in the bulk earth; and 2) against an arithmetic mean of abundances in three composites made up from North American. European and Russian Platform shales (HASKIN and HASKIN. 1966). We have chosen to normalize our data wr.sus the shale reference. This is the most appropriate for our work, since shales represent not only average crustal abundance, but also the abundances in terrestrial and marine sediments, and are expected to resemble the aeolian or riverine input of terrestrial material into the ocean basin. Moreover the Eu signals of seawater (ELI/ Sm = 0.22-0.26) and shales (Eu/Sm = 0.22) are very similar(DE BAAR et al., 1985).in contrast to chondrites (Eu/Sm = 0.37). In other words there appears to be no Eu anomaly in typical seawater/shale ratios. This is important for defining the Gd anomaly relative to its neighbors Eu and Tb in the series. Given the gradual enrichment of heavy REE (HREE) in seawater, higher Gd/Sm and Gd/Eu ratios than in shales are expected and were indeed found (Fig. 4). However, the very same HREE enrichment would lead to lower rather than the observed higher Gd/Tb ratios. The ratio Gd/Ho. Ho being the next measured element, is indeed lower in seawater than in shales. There appears to be a crossover somewhere between Tb and Ho where the Gd/REE ratio shifts from too high values back to the expected lower ratios than in shales. In other words, Gd seems to fit better among the heavier elements Dy and Ho than between Eu and Tb. where it formally belongs according to atomic number. This unusual behavior of Gd can be better resolved by definition of a Gd anomaly in a manner similar to our earlier discussion of the pronounced C’e anomaly (DE BAAR er al., 1983, 1985): (Gd/Gd*),,, = 2(Gd/Gd,,,)/(Eu/Eu,,,
+ Tb/Tb,,&
(2)
where Gd* represents the expected behavior of Gd as interpolated between neighboring elements Eu and Tb. When measured Gd values are plotted vcvxu Gd*. almost all data points fall above the Gd/Gd* -- 1i I line for shales (Fig. 5). ELDERFIELDand C~KEAVES (1982) did not measure monoisotopic Tb, and there appears to be no distinct Gd anomaly in their REF
1963
REE in seawater 2 4 6 8
12 16
4 ,8 1; I$
04 08 12 16
j-i7-IJ-yJ~
FIG. I.The
vertical distributions of Sm, Eu, Gd and Tb in the western North Atlantic Ocean (34”N.
58’W). Data from Table 2 and DE BAAR etal. (1983).
patterns versus shales. However, from their data we calculated Gd* by using the next element Dy instead. In this way the linear interpolation stretches a longer span of the curved REE distribution pattern, and the resulting values of Gd* are slightly less reliable. Nevertheless most of their data also fall above the l/ 1 line. No clear trends appear in the vertical profiles of the above defined Gd anomalies (Fig. 6). The signal is rather noisy in the Atlantic Ocean, although there may be a maximum at mid-depth. At the Pacific site the Cd anomaly is extremely uniform with depth. The shale normalized REE data shown in Figs. 2 and 3 are plainly not represented by simple, smooth
curves. Thus the Gd anomaly we describe must be tested to see if it represents a statistically significant signal above the noise in the data. We can in fact test numerous combinations of our REE data to see if significant departures from the mean represented by near neighbor data exist. For argument, then, we define anomalies in Sm, Tb, Tm and Yb analogous to that for Gd, i.e. Sm* = l/4(Pr/PrM, Tb* =
+ 3Eu/EutiJ + HO/HO&~,)
(3)
‘12
(Eu/Euti,
Tm* =
%
(Ho/HoadC + 2Yb/YbMC)
(5)
Yb* =
%
(Tm/Tmati,
(6)
+ Lu/Lu,,).
(4)
We did not report data on Nd in Atlantic Ocean waters, since at that early stage of our work reliable data could not be obtained. In our Pacific data set, however, Nd results are reported, and there we define 03
005 005
02
f&fF
Sm* = % (Nd/Ndhtie + 2Eu/Eutie). a2
a%
a2
9’,
a2
\43
0.2
0.05 n_
!!a P a3
p
/-To2
(7)
In Table 3 we show the mean and standard deviation ofthe ratio REE/REE* in each water column. Students’ t test is used to test the hypothesis that these ratios are different from I .O. In the Atlantic Ocean data set it is clear that anomalies in both Gd and Tb exist and that the visual effect in Fig. 2 of elevated Gd concentrations is made up of both elevated Gd and reduced Tb concentrations. In the Pacific Ocean data anomalies appear for each element; however, the Gd/Tb effect is by far the largest. Given the systematic trends in the data shown in Figs. 2 and 3, it is not surprising that the secant approach used here consistently reveals small departures from a linear interpolation. However, it is the clearly predominant Gd/Tb shift that is of interest here. REE FRACl-IONATIONS IN SEAWATER
Mechanisms conlrolling REE in seawater
0081 I_0
, cc pr
, Nd
sn
/
1
I
Eu
Gd
Tb
1
1
Ho
,
I
I
Tm
Yb
I ,008 Lu
FIG.2. REE distribution patterns normalized versus shales in the western north Atlantic Ocean.
In order for an REE anomaly to exist some mechanism for fractionation relative to its neighbors must operate. Two simultaneous mechanisms are believed to control the distributions of REE(II1) in seawater (DE BAAR et al., 1985): 1) association with skeletal material, possibly calcareous; and 2) adsorptive scavenging by
1964
H. .I W. de Baar. P. G. Brewer and M. P. Bacon
10 08 06
00
00
oo+ 006 006
0004
FIG. 3. REE distribution patterns normalized versus shales in the eastern equatorial Pacific Ocean
particles. The corresponding fundamental properties for fractionation of the REE(II1) cations are: 1) different ionic radii; and 2) different relative affinity of chemical bonding with either the adsorbing surface or the stabilizing complexers in seawater solution. Both concepts depend largely on the electron shell filling of the REE(II1) cations. settling
ionic radii
The well-known lanthanide contraction leads to a gradual decrease in ionic radius from 1.03 A for I_a(III) to 0.8 A for Lu (III) in six-fold coordination (SHANNON, 1976). There appears to be a small but distinct discon-
tinuity in the contraction between Gd and Tb. WC have hypothesized (DE BAAR d ai.. 1985) that REE” may substitute for Ca*+ in carbonate skeletal material and that involvement of the REE in the uptake/dissolution cycle may be partly responsible for enhanced REE concentrations in deep water relative to surface water and in the deep Pacific relative to the deep Atlantic. This hypothesis is not incompatible with the water column observations, but the REE concentrations in foraminifera (PALMER. 1983)appear to be too low to have an impact on the oceanic REE distributions (DE BAAR ef al., 1985). Also. REE fractionations resulting from this cycle were considered to be small. This also seems true for the observed Gd/Tb anomalies.
1965
REE in seawater Table 2. Gd concentrations in the north-west Atlantic Ocean (33058'~. 5E"05'W)*
DEPTH
(In)
mol.kg-')
10
4.9
48 95 143 493 643 793 992 1184 1377 1729 2490 2870 3253 4309 4378 4426
4.6 4.9 5.2 4.7 4.5 4.2 4.4 4.9 4.0 4.5 6.3 6.1 7.1 11.1 10.6 12.7
tween the strongly (Th, Pa, Fe) and less strongly (Ni.
Cd) hydrolyzed elements. Several classes of adsorptive sites (clay minerals, Fe/Mn oxide coatings or various organic functional groups) on the particles are potentially involved. BALISTRIERIet al. ( 198 1) further explored the SCHINDLER (1975) model in an analysis of metal scavenging residence times derived from sediment trap experiments. They attempted to constrain the options with respect to plausible surface sites but were forced to use a small group of trace elements with rather different chemical properties. In this respect the group of REE with very
10
. 7
I
I .*m* Irn
*Data for other REE are to be found in DE BAAR et al. (19831. Sampling depths have been revised slightly.
For instance, a strong preferential uptake of Gd in skeletal material would lead to much higher Gd anomalies in deep waters compared to surface waters. In fact the Gd anomaly appears to be very constant with depth, except for a slight enhancement in the core of the North Atlantic Deep Water (Fig. 6). While some biogenic Gd fractionation may occur, it appears to be too small to account wholly for the very uniform Gd/Tb anomaly in seawater.
48 f
1
l
1 ‘.
NW.Atlantx l . Cariaco Rephcates * Sargasso Sea .
Pacific II,,,\I,llljll,
Particle
scavenging
Many trace elements in seawater are adsorbed by the surfaces of fine suspended particles and are then removed from the water column. This continuous process of scavenging (GOLDBERG, 1954; CRAIG, 1974; TUREIUAN, 1977) has been subjected to considerable study by marine chemists. The exact chemical nature of the scavenging mechanism probably varies from element to element. SCHINDLER( 1975) first proposed a formal model for the adsorptive removal of the trace metals from the oceans in which scavenging was seen as competition for the free metal between solution ligands and particle surfaces. The particle surface/metal ion interaction was represented as an equilibrium with surface -OH groups, in accordance with the general observation that oceanic residence times correlate well with the tendency for hydrolysis. Such general trends between fundamental chemical properties and scavenging residence times of the elements are a matter of considerable debate (LI, 198 I ; WHITFIELDand TURNER, 1982, 1983); however, the hydrolysis constants for the REE would clearly place these elements in a residence time hierarchy be-
2-I
. a*
of/,,
,, ,
(((,,
,
.A
-
.
,,
,
,,
8
‘4
l
'76
Gd/pmol kg-y FIG. 4. Scatter plots of Sm. Eu, Tb and Ho versus Gd. Preliminary Cariaco Trench data taken from DE BAAR(1983).
H. J. W. de Baar. P. G. Brewer and M. P. Bacon
The equilibrium model of BALISTRIERIP! LI/.i I% I ) clearly implies reversibility. That reversible adsorption does indeed occur was shown by BACONand ANDEKSON ( 1982). By analogy with their reversible model the scavenging removal of REE from seawater is envisioned A’ . NW ATLANTIC OCEAN as a two step process: 1) equilibration of REE(lII) bex E ATLANTIC OCEAN A URIACO TRENCH tween inorganic complexes in solution and s&ace sites n PACIFIC OCEAN on small suspended particles; and 21 periodic rcmovai of small suspended particles (BACONN ui.” 1985). the latter being swept out either by settling biogcnic debris from surface waters or by zooplankters at all depths FIG. 5. Measured Gd versus the expected value Gd* which package fine suspended particles into large rap= % (Eu/Eb + TbjTb-) normalized versus shales. For idly settling fecal pellets. the eastern North Atlantic Ocean (ELDERF~ELD and GREAVES. Chemical fractionations within the REE series arc 1982) Gd* = % (2 Eu/Eb f Dy/Dy,). For the same data sets, a very similar trend was found upon normalization versus thought to occur exclusively during the equilibration chondrites instead (DE BAAR 1983). Preliminary Cariaco step. In order for the process to create the Gd/Tb Trench data taken from DEBAAR( 1983). anomaly that we observe, we must invoke either a smaller binding constant of the Gd3+ ion with the surface sites on particles or preferential complexing of the similar, gradually varying chemical properties is Gd3+ ion in solution (the opposite for Tb). Preferential promising, since one can assume that they will all complexing of Gd3+ in solution is supported by shifts probably interact with the same type surface site. The in the fundamental properties at this place in the REE possibility of their simultaneous involvement in a bio- series and by a solution speciation model. as discussed geochemical cycle, however. would blur the picture. below. With respect to the tirst argument, there are neither constraints for the possible surface sites nor firm inGd - Anomaly : Gd/Gd * formation on their binding constants for REE(III) adro sorption. It is interesting to note, however. that two model organic compounds, EDTA (WHEELWRIGHTcl al., 1953) and iminodiacetic acid (THOMPSONef ai.. .. I967), have higher formation constants for the heavier . . REE. A significant fractionation due to these differ. . ences of REE binding constants of such functional groups on particles would lead to a depletion, rather I l than the observed enrichment, of heavy REE in seawater. If these model compounds are representative . indeed for the open ocean, then differences in REE . binding constants of surface sites appear to be too small . to exert a control on seawater fractionation 10
14
14
. L-n
!
:
Fundamental
NWATLANTIC
E ATLANTIC CARIACO
E.EPACIFIC
FIG. 6. Vertical profiles of Gd/Gd* in the western North Atlantic Ocean, the eastern Atlantic Ocean (ELDERF~ELD and GREAVES,1982), the Cariaco Trench (preliminary data from DE BAAR, 1983) and the Pacific Ocean (DE BAAR n al.. 1985).
properk~
Gd (Z = 64) is the eighth element in the series of 15 REE (57La to “Lu). This location halfway along the series leads to an exactly half-filled inner 4felectron shell for the Gd(III) cation. There appears to be a special stability associated with the exactly half-filled 4J‘shell The sum of the first three ionization potentials exhibits
1967
REE in seawater
a distinct minimum at Gd (Fig. 7). The relatively larger decrease in ionic radii between Gd and Tb has been mentioned above and is indicative of similar discontinuities in, for instance, hydration and complexation constants. Similar, but considerably weaker, energetic advantages have been invoked for exactly 114filled (between Nd and Pm) and ‘/4filled (between Ho and Er) 4fshells of the REE(II1) ions. These combined discontinuities between the third and the fourth, exactly at the seventh (Gd), and between the tenth and eleventh 4felectron filling are known as tetrad effects (NUGENT, 1970; SIEIURSIU, 197 1; DZURINSKII, 1980). The stronger effect exactly at Gd would be better detectable than the weaker effects between Ho and Er and between Nd and Pm. Moreover, Pm does not exist in nature, and Er was not measured in our samples. Solution chemistry
In attempting to translate the above unique properties of Gd into predictions for aqueous solution, the effects of hydration ought to be taken into account. Unfortunately a quantitative treatment of the hydration concept in seawater is hampered by significant experimental and theoretical problems (WHITFIELD, 1975; WHITFIELD and TURNER, 1983). With respect to the REE(III), the Gd(II1) ion has an exactly spherically symmetrical ligand field which in itself might lead to a unique hydration pattern. There are, however, many other factors and concepts involved. As a result there appears to be no simple explanation of tetrad effects observed even in simple aqueous solutions (WILLIAMS, 1982). Prediction of tetrad effects for hydration in complex aqueous solutions (e.g., seawater) is even more difficult. There is some indication, however. that the lighter members La(III) to Nd(III) have a ninefold coordination, whereas the series Gd(II1) to
Lu(II1) is generally eightfold coordinated. In other words, there would again be a break at about Gd. However, all the above evidence is based on measurements in concentrated solutions, and different REE(II1) hydration numbers may dominate more dilute media such as seawater. The speciation of metal ions by formation of complexes in seawater has been assessed by many workers. Here we will use the most recent and careful compilation by TURNER et al. (198 1). Their calculated speciation for the REE(II1) is shown in Fig. 8. Of particular interest is the importance of the REE-carbonate complexes. These complexes dominate the speciation, yet the formation constants have not been measured directly. TURNER ef al. (198 1) estimated the constants through reliance on the correlation between measured oxalate and carbonate complexes: thus there is some uncertainty here (R. H. BYRNE, pet-s. commun.). Clearly a more detailed study of REE(III)-complexes in seawater is needed. Yet for now the best available model (TURNER et al., 1981) leads to some interesting predictions. In their results a very distinct Gd/Tb anomaly appears for almost every dissolved species, including the free ion itself (Fig. 8). There is also a striking resemblance between the total amount of REE(II1) complexes and the shale normalized pattern observed in seawater (Fig. 8). In other words, not only do the heavier REE indeed appear to be more stabilized due to their generally stronger complexation in seawater (GOLDBERG et al., 1963), but a minor anomaly in speciation is also reflected in our measurements for the elements Gd and Tb. MASUDAand IKEUCHI( 1979) expected to find such a Cd anomaly (or actually a full-fledged tetrad effect) in seawater, but they had to rely on interpolations between nine measured REE in only one sample. Our data confirm their expectations. DISCUSSION
Number 0
of Eleclrons
2
4
in Inner
6
8
4F
10
Shell 12
(4
3511,,,,,,,,,,,,1 Lo
Ce
Pr
Nd
Pm
91
Eu
Cd
Tb
Dy
Ho
Er
Tm
Yb
IN
FIG. 7. The sum of the first three ionization potentials of the REE. Note the marked decrease at half-filled (Cd) and completely filled (Lu) 4j’electronshells. A minor discontinuity may occur between Ho and Er, but absence of Pm data precludes observation of a similar anomaly between Nd and Pm. From FAKTOR and HANKS (1969).
It is interesting to consider whether the REE patterns observed in seawater, including the Gd/Tb anomalies, should be reflected in authigenic or biogenic phases leaving the ocean. Various marine deposits, in particular ferromanganese nodules. have often been noted to exhibit a REE pattern with positive Ce anomaly and heavy REE depletion, i.e. the inverse of the typical seawater pattern (GLASBY, 1973; PIPER, 1974; ADDY, 1979; RANKIN and GLASBY, 1979; ELDERFIELDet al.. 198 1). It is sometimes implied that such deposits rep resent the complementary sink presumably requited for the existence of the seawater pattern. The argument might go as follows: Assuming a hat shale pattern for the input, the REE are subsequently distributed between the solid and liquid phases. with the distribution coefficient roughly proportional to the percentage free ion in seawater (Fig. 8). After the solid phase (i.e., fine suspended particles) is removed, the remaining REE in the parcel of water would have the observed pattern with a positive Gd anomaly. Would the adsorbed au-
1968
H. J. W. de Baar. P. G. Brewer and M. P. Bacon
L
w4-?‘--T~-&~LOcaR Nd
c
RrcentogeFree
l
1250m Depth, Fucef!c / OCP?Oll
L-T--
Ions
oas
-
be in the pool of dissolved species (Fig. 8). also with a seawater pattern. The minor fraction adsorbed on particles would by and large exhibit a flat shale pattern. As a result. we believe that the REE composition o! marine authigenic deposits should. ONuverugc. match the shale pattern of continental inputs. The patterns of REE observed in seawater demonstrate that the chemical processes operating in the oceanic water column are capable of separating elements with very similar chemical properties. This is also illustrated by the fractionation that occurs between the actinides Th and Pa. which are believed to be partitioned differentially between a vertical flux governed by particulate transport and a horizontal flux driven by uptake at ocean boundaries (ANDERSON et ui 1983a,b). Recently very similar vertical distributions were also reported for the elements Ni and Pd (LEE;. 1983) and for Cu and Ag (MARTIN of 01.. 1983) at VERTEX II, the site of our Pacific REE study (DE BAAR et al., 1985). Elemental ratios were found to be about 4 times (for Ni/Pd) and about IO times (for Agi Cu) greater than crustal abundance ratios based on TAYLOR (1964). There is some uncertainty in the assessment of crustal abundances, but the apparent oceanic fractionation of these group 8 and I b elements may well be affected by chemical processes similar to those governing the fractionation of the actinide or REE series.
t ‘i-7’ Tm m L”
FIG. 8. Above: Percentages of REE(III) species predicted by the model of TURNER ef al. ( 198 1)for seawater of pH 8.2, 25°C and I atm pressure. Significant variations with temperature and pressure are expected to occur because of the variation of carbonate concentration with temperature and pressure (BROECKERand PENG, 1982). Below: The mirror image (log-scale) of percentage free REE(III) ion compared with the shale-normalized distribution pattern for deep Pacific water. The light REE(III), with a higher percentage free ion, are more depleted in seawater. The Gd anomaly in seawater is compatible with the anomaly in the percentage free ion.
thigenic phase (to be distinguished from biogenic and detrital components) of settling large particles or marine sediments then have the complementary (i.e., inverted) pattern, with I ) a gradual depletion of the heavy REE and 2) a Gd depletion? The above is a static or batch type description of what in fact is a continuous, dynamic process, and the actual situation indeed appears to be rather different. The continuous supply by rivers and aerosols of reactive REE with a flat shale pattern would tend to dilute the HREE enrichment and Gd anomaly of the seawater pattern. In order to counteract this trend strictly by
scavenging removal, then the authigenic fraction of settling particles must essentially exhibit a flat shale pattern without a Gd anomaly. This is also expected from the Turner-Whitfield-Dickson model, provided the total inventory (dissolved species + fine suspended particles) of a parcel of water has a seawater pattern. At low particle concentrations most of the REE would
CONCLUSIONS
I) Seawater, normalized wrsus either shales 01 chondrites, consistently exhibits a positive Gd anomaly and a negative Tb anomaly. II) The well-known anomalous physical and chemical properties associated with the shift from an exactly half-filled 4feIectron shell could very well lead to the observed Gd/Tb fractionation in seawater. Ill) The observed anomaly is compatible with the predicted speciation of REE(III) in seawater, in combination with scavenging removal as the dominant control of REE distributions in the oceans. .4cknowledgemenls-The authors are greatly indebted to Rebecca Belastock, Alan Fleer, Fred Frey, Hugh Livingston, Pieter Nella, Peter Sachs and Deborah Shafer for their help and advice during various stages of the project. Bob Byrne kindly reviewed an earlier draft of the manuscript. We are grateful to Harry Elderheld for many stimulating and informative discussions. This research was supported by US Department of Energy Contract DE-ASO2-76EV03566 and Office of Naval Research Contract NOOOl4-82-C-00 19 NR 083-004. Editorial
handling: S.
E. Calvert REFERENCES
ADDYS. K. ( 1979) Rare earth element patterns in manganese nodules from northwest Atlantic. Grtnhim. 17osmcxhim. Acta 43, 1105-l
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1969
REE in seawater ANDERSON R. F., BACONM. P. and BREWERP. G. (1983a) Removal of 2?h and “‘Pa from the open ocean. Eurfh Planet. Sci. Lett. 62, l-23. ANDERSON R.F.,BAcoN M.P.and BREWER P.G.(I 983b) Removal of “rh and I’Pa at ocean margins. Earth Planet. Sci. Lett. 64.73-90.
BACONM. P. and ANDERSON R. F. (1982) ~st~bution of thorium isotopes between dissolved and particulate forms in the deep sea. J. Geophys. Res. 87,2045-2056. BACON M. P.,HUH C.-A., FLEER A. P. and DEUSERW. G. (1985) Seasonaiity in the flux of natural radionuclides and plutonium in the deep Sargasso Sea. Deep-Sea Res. 32, 273-286. BALISTRIERIL., BREWERP. G. and MURRAYJ. W. (1981) Scavenging residence times and surface chemistry. DeepSea Res. 28, 101. BROECKERW. S. and PENG T. H. (1982) Tracers in the Sea Eldigio Press of Columbia University, Palisades, NY. 690 p. CRAIG H. ( 1974) A scavenging model for trace elements in the deepsea. Earth Planet. Sci. Leti. 23, 149-159. DE BAAR H. J. W. (1983) The marine aeochemistrv of the rare earth elements. Ph:D. Thesis, WHOI/MIT, 2%pp. DE BAAR H. J. W. ( 1984) Neutron activation analysis of rare earth elements in seawater. Proc. Intl. Symposium on the Use and Development ofLow and Medium Flux Research Reactors. Special Volume Atomkernenergie-Kemtechnik, Supplement 44,702-709. DE BAAR H. J. W., BACONM. P. and BREWERP. G. (1983) Rare-earth dist~butions with a positive Ce anomaly in the western North Atlantic Ocean. Nature 301, 324-327. DE BAARH. J. W., BACONM. P.. BREWERP. G. and BRULAND K. W. ( 1985) Rare earth elements in the Atlantic and Pacific Oceans. Geochim. Cosmochrm. .4cta 49, 1943-1959. DZURINS~U~B. F. (1980) Tetrad effects. Russian J. fnorg. Chem. (in English) 41, 25. ELDERRELDH. and GREAVESM. J. (1982) The rare earth elements in sea water. Nature 2%, 214-219. ELDERHELDH., HAWKESWORTH C. J., GREAVE~M. J. and CALVERTS. E. (198 la) Rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments. Geochim. Cosmochim. Acta 45,513-528. ELDERRELDH., HAWKESWORTH C. J.. GREAVESM. J. and CALVERTS. E. ( 198 1b) Rare earth zonation in Pacific ferroman~n~ nodules. Ge~him. Cosm~him. Acta 45, 1231-1234.
EVENSENN. M., HAMILTONP. J. and D’NIONS R. K. (1978) Rare-earth abundances in chondritic meteorites. Geochim. Cosmachim. Acta 42,
1199-i 2 12.
FAKTOR M. M. and HANKS R. (1969) Calculation of third ionisation potentials of the Ianthanides. J. inorg. Nucl. Chem. 31, 1649-1659.
GLASBYG. P. f 1973) Mechanisms of enrichment of the rarer elements in marine manganese nodules. Mar. Chem. 1, 105-125. GOLDBERGE. D. (1954) Marine geochemistry 1. Chemical scavengers of the sea. J. Geol. 62, 249-265. GOLDBERG E. D., KOIDE M., SCHMITT R. A. and SMITH R. H. (2963) Rare-earth distributions in the marine environment. 3. Geophys. Rex f&4209-42 17. HASKIN M. A. and HASWN L. A. (1966) Rare earths in European shales: a redetermination. Science 154, 507-509. HENDERSONP. ( 1982) Inorganic Geochemistry. Pergamon Press, Oxford. 353 p. KLINKHAMMERG.. ELDERRELDH. and HUDSONA. t 1983) Rare earth elements in seawater near hydrothemai &nts: Nature305, 185-188. LEE D. S. ( 1983) Palladium and nickel in north-east Pacific waters. Nuture 305,4?-48. LI Y. H. (1981) Ultimate removal mechanisms of elements from the ocean. Geachim. Cosmochim. Acta 45,1659- 1664.
MARTIN J.H.,KNAUER G. A. and GORDON R. M. (1983) Silver distributions and fluxes in north-east Pacific waters. Nature 305, 306-309. MASLJDAA. and IKEUCHIY. (1979) Lanthanide tetrad effect observed in marine environment. Geochem. J. 13, 19-22. NUGEN~ L. J. ( 1970) Theory of the tetrad effect in the lanthanid~lI1) and a~inid~III) series. 3. Inorg. Nuci. C&em. 32.3485-349 I. PALMERM. R. (I 983) Rare earth elements and Nd and Sr isotopes in the Atlantic Ocean. Ph.D. Thesis, Univ. of Leeds, 284 pp. PIEPGRASD. J. and WASSERBURG G. J. (1982) Isotopic composition of neodymium in waters from the Drake Passage. Science 217,207-214. PIEPGRASD. J. and WASSERBURGG. J. (1983) Influence of the Mediterranean outflow on the isotopic composition of neodymium in waters of the North Atlantic. J. Geophys. Res. 88, 5997-6006. PIPER D. Z. (1974a) Rare earth elements in ferromanganese nodules and other marine phases. Geochim. Cosmochim. Acta 38, 1007-1022.
PIPER D. Z. ( 1974b) Rare earth elements in the sedimentary cycle: a summary. Chem. Geoi. 14,285-304. RANKIN P. C. and GLASBYG. P. (1979) Regional distribution of rare earth and minor elements in manganese nodules and associated sediments in the southwest Pacific and other localities. In Marine Science Vol. 9, Marine Geo1og.v and Oceanography aj”the Pacific Manganese Nodule Province (eds. J. L. BISCHOFFand D. Z. PIPER)pp. 68 i-697. Plenum.
SCHINDLERP. W. (1975) Removal of trace metals from the oceans: a zero order model. Thalassia .Jugosiavica 11( I/2), 101-111. SHANNONR. D. (1976) Revised effective ionic radii and systematic studies of interatomic distances in halides and chaicogenides. Acta Crvst. A32, 75 l-767. SIEKIR~KIS. (197 I) The shape of the Ianthanide contraction as refiected in the changes of the unit ceil volumes, ianthanide radius and the free energy of complex formation. J. Inorg. Nucl. Chem. 33,377-386.
TAYLORS. R. ( 1964) Abundance of chemical elements in the continental crust: a new table. Geachim. Cosmochim. Acta 28, 1273-1285.
THOMPSON L.C., SHAFERB. L., EDGARJ. A. and MANNILA K. D. ( 1967) Complexes of the rare earths, n-substituted imin~iacetic acids. In ~nthanide/Aczin~de Chemistry(eds. P. R. FIELDSand T. MOELLER)Adv. Chem. Series, 71, 169-179. TUREIUANK. K. (1977) The fate of metals in the oceans. Geochim. Cosmochim. Acta 41, 1 i39- 1144. TURNER D. R., WHITRELD M. and DICKSONA. G. (1981) The equilibrium speciation of dissolved components in freshwater and seawater at 25°C and I atm pressure. Gecchim. Cosmochim. Acta 45.855-88
I.
WHEELWRIGHT E. J., SPEDDINGF. H. and %HWARZENBACH G. (1953) The stability of the rare earth complexes with ethyienediaminetetraacetic acid. J. Amer. Chem. Sot. 75, 4196-4201.
WHITF~ELDM. (1975) Sea Water as an Electrolyte Solution. In Chemical Oceanugraph~~ feds. J. P. RILEYand G. SWRROW),Vol. I. pp. 44-172. Academic Press. London. WHITRELD M. and TURNER D. R. ( 1982) Ultimate removal mechanisms of elements from the ocean-a comment. Geochim. Casmochim. Acta 46, 1989-1992.
WHITFIELDM. and TURNER D. R. (1983) Chemical periodicity and the speciation and cycling of the elements. In Trace Meials in Seu Wafer (eds. C. S. WONG, E. BOYLE, K. W. BRULAND,J. D. BURTONand E. D. GOLDBERG), pn 7 19-750. Plenum Press. New York. W~LEAMS R. J. P. ( 1982) The chemistry of Ianthanide ions in solution and in biological systems. Structure and Bonding (Springer, Berlin) SO, 80-l 19.
Cecchrm!ca PI Cosmochrnuca Acfa Vol. 49. pp. 1961-1969 0 Pcrgamon Pres Ltd. 1985. Printal in U.S.A.
Anomalies in rare earth distributions in seawater: Gd and Tb* HEIN J. W. DE BAAR’, PETER G. BREWER
and MICHAEL P. BACON
Woods Hole Oceanographic Institution, Woods Hole, MA 02543 (Received August 21, 1984; accepted in revised form June 2 I, 1985)
Abstract-We have measured profiles of the rare earth elements (REE) in Atlantic and Pacific Ocean waters. The data, normalized versus shales, exhibit a pronounced anomaly in Gd relative to its neighbors Eu and Tb in the REE series such that the Gd concentrations are high by 30-50%. Closer inspection reveals that the anomaly is made up of both elevated Gd and depressed T%concentrations, likely associated with solution chemistry shifts in the transition from an exactly half filled 4felectron shell. Anomalies in Gd and ‘I% solution complexation are also indicated by the Turner-Whitfield-Dickson speciation model. The overall trend of heavy REE(III) enrichment in seawater and the Gd/Tb anomaly described here tend to support scavenging as an important removal mechanism for the REE from seawater. INTRODUCTION
Gd is one of the more difficult REE for determination by neutron activation analysis. For Gd determination the samples WE HAVE PREVIOUSLY described the distribution of the were counted after a six-week cooling period during which interfering short-lived isotopes (e.g. ‘%m) decayed away. rare earth elements (REE) in waters of the Atlantic and Pacific Oceans (DE BAARet al., 1983, 1985). These Concentration values based on the 97 keV and 103 keV peaks of long-lived isotope ‘53Gdusually agreed within ten percent. data, combined with recent contributions Of PIEPGRAS For the combined Pacific and Atlantic data sets both estimates and WASSERBURG (1982, 1983), ELDERFIELD and yielded a mean ratio
GREAVES(1982) and KLINICHAMMER et al. (1983) now permit a much more complete description of the abundances and controls of the REE in seawater than has hitherto been possible. In examining our results we have noted that the behavior of Gd appears always to be anomalous when compared to its neighbors Eu and Tb in the REE series. The anomaly is such that the concentration of dissolved Cd appears to be too high by 30-5090, so that its oceanic distribution would tend to place it between the heavier REE Dy and Ho, rather than in its true place in the series according to atomic number. This behavior appeared to be so odd that we chose to delay reporting our results for Gd in seawater in our first paper on the subject (DE BAAR et al.. 1983). However, a careful examination of this first Atlantic data set, as well as the later data from the Pacific Ocean (DE BAAR et al., 1985). shows the effect to be real, and we present here a discussion of the anomaly and its likely causes. The feature is of considerable interest, because, with the REE exhibiting such systematic trends in their geochemical properties. such anomalies potentially provide a very specific probe into the fundamental processes affecting their oceanic distributions. METHODS The REE were quantitatively extracted from seawater by CHELEX 100 ion exchange chromatography. After subsequent purification by cation and anion exchange, the samples were analyzed with neutron activation followed by high resolution gamma spectrometry. A full account of analytical methods has been reported elsewhere (DE BAAR, I983,1984).
’ Present address: Department of Earth Sciences, University of Cambridge, Cambridge CB3 OEZ, UK. * W.H.O.I. Contribution No. 575 I.
Gd (97 keV)/Gd (103 keV) = 1.08 f 0.14
(1)
which does not differ significantly from one. The 14% standard deviation is less than the observed sysremafic 30-50% positive offset that we discuss here. All reported Gd concentrations represent a simple arithmetic mean of both values, although the 97 keV peak exhibits somewhat better counting statistics. Various types of systematic erron have been considered. For instance, interference by an unnoticed peak of another radionuclide might have occurred. It is unlikely, however, that both peaks of ‘53Gd (97 and 103 keV) would be on top of two hidden peaks of similar size. The sample spectra were very clean and appeared devoid of any peaks of elements other than the REE. Moreover, we found no possible candidates for hidden peaks in the 97-103 keV range after a careful search of the literature and gamma-ray energy tables for minor gamma peaks of long-lived radionuclides. Systematic errors in peak integration were avoided by comparing the net peak areas calculated by two different spectral analysis programs. Finally, concentrations based on the very weak signal of shortlived ‘59Gd served as an independent check (within about -+20%) of the ls3Gdderived values. Systematic errors in all three peaks of two different Gd isotopes are deemed unlikely. The determination of Tb is also based on a 48-hour count for each sample, after at least six weeks’ cooling in order to obtain a very low and stable background devoid of any interferences. Concentrations calculated from three different, very strong peaks (98, 299 and 879 keV) were always in excellent agreement. Reported values are based on the 299 keV peak which has slightly better counting statistics than the other peaks. Standards prepared from pure metals (Ames Laboratory) were used for the Pacific Ocean and Cariaco Trench data sets. For the Atlantic data an older standard mixture, made up from REE oxides, was used (F. FREY, pers. commun.). The new standards do potentially have a higher accuracy. Additional seawater data taken from ELDERFIELDand GREAVEZS (1982), which we will use in our discussion, is again based on oxide standards. More recently pure metal standards have also been used in their laboratory, and no significant differences between the two types of standards have been found (H. ELDERFIELD,pers. commun.). Recent intercalibration of pure metal standard mixtures of Dr. Eldertield and ourselves yielded
1961
1962
H. .I. u’. de Baar. P. G. Brewer and M. P. Bacon
good agreement within the error of the IDMS analytical technique (Table I). NORMALIZATION-DEFINITION OF AN ANOMALY
The vertical distributions of Gd and neighboring elements Sm, Eu and Tb are very similar both in the northwest Atlantic Ocean (Fig. 1) and the eastern equatorial Pacific Ocean (DE BAAR et al., 1984). Yet after normalization versus shales a distinct positive Gd anomaly is readily apparent (Figs. 2 and 3). Tabular listings of the data are to be found for the Atlantic in DE BAAR et al. ( 1983) and for the Pacific in DE BAAR et al. ( 1984). As mentioned earlier, the Gd data were not previously reported for the Atlantic. and Table 2 here remedies this omission.
Table
1.
Element
&xvimetric pIm1.g-’
La CC
Pr Nd Sm Eu Gd : EO
&r rm rb LU
Intercalibration* ILMS pma1.g-’
515.02
516.2
1122.4 262.51 827.5 100.4 15.64 256.07 105.h 19.06
1123.3 -027.b 100.0 15.85 251.8 106.5 --
63.Ll 98.2 32.36 90.18 16.05
-90.2 -89.5 16.57
* Calibration with Isotope Dilution Mass Spectrometry at Cambridge (U.K.) of Woods Hole seawater-type REEstandard mixture,
made up gravi-
wtrically from pure REE metals (DE BMR. 1983). An amount of standard mixture corresponding to about 5 kg seawater was mixed with isotopically
enriched Cambridge spikes. The latter had been calibrated versus a similar standard at Leeds (U.K.), which in itself is in ercellent agreement with a Mains standard (ELDERFIELD and GRWVES, pers. which co-. ). Elements run well with IDMS(Nd, Sm, Cc) agree very well. and there is no reason for the graviwtric concentrations of the other elements to be any less This includes accurate. the gravirtric values for Pr. Tb, Eo and I’m, which cannot be determined by IDMS.
The magnitude of an anomaly depends on the normalization scheme, which has to be selected carefully. The cosmic abundance patterns of the REE are related to various stages of nucleosynthesis (HENDERSON. 1982). As a result their abundances are dominated both by a systematic decrease in concentration with increasing atomic number and by the well-known predominance of elements with even atomic numbers. Smaller fractionations in the ocean due to marine chemical processes are only revealed after these dominant effects are removed by normalization. Two normalization schemes were given serious consideration: 1)against a weighted mean of abundances in chondritic meteorites (EVENSEN ef al.. 1978). representative of the relative REE distribution in the bulk earth; and 2) against an arithmetic mean of abundances in three composites made up from North American. European and Russian Platform shales (HASKIN and HASKIN. 1966). We have chosen to normalize our data wr.sus the shale reference. This is the most appropriate for our work, since shales represent not only average crustal abundance, but also the abundances in terrestrial and marine sediments, and are expected to resemble the aeolian or riverine input of terrestrial material into the ocean basin. Moreover the Eu signals of seawater (ELI/ Sm = 0.22-0.26) and shales (Eu/Sm = 0.22) are very similar(DE BAAR et al., 1985).in contrast to chondrites (Eu/Sm = 0.37). In other words there appears to be no Eu anomaly in typical seawater/shale ratios. This is important for defining the Gd anomaly relative to its neighbors Eu and Tb in the series. Given the gradual enrichment of heavy REE (HREE) in seawater, higher Gd/Sm and Gd/Eu ratios than in shales are expected and were indeed found (Fig. 4). However, the very same HREE enrichment would lead to lower rather than the observed higher Gd/Tb ratios. The ratio Gd/Ho. Ho being the next measured element, is indeed lower in seawater than in shales. There appears to be a crossover somewhere between Tb and Ho where the Gd/REE ratio shifts from too high values back to the expected lower ratios than in shales. In other words, Gd seems to fit better among the heavier elements Dy and Ho than between Eu and Tb. where it formally belongs according to atomic number. This unusual behavior of Gd can be better resolved by definition of a Gd anomaly in a manner similar to our earlier discussion of the pronounced C’e anomaly (DE BAAR er al., 1983, 1985): (Gd/Gd*),,, = 2(Gd/Gd,,,)/(Eu/Eu,,,
+ Tb/Tb,,&
(2)
where Gd* represents the expected behavior of Gd as interpolated between neighboring elements Eu and Tb. When measured Gd values are plotted vcvxu Gd*. almost all data points fall above the Gd/Gd* -- 1i I line for shales (Fig. 5). ELDERFIELDand C~KEAVES (1982) did not measure monoisotopic Tb, and there appears to be no distinct Gd anomaly in their REF
1963
REE in seawater 2 4 6 8
12 16
4 ,8 1; I$
04 08 12 16
j-i7-IJ-yJ~
FIG. I.The
vertical distributions of Sm, Eu, Gd and Tb in the western North Atlantic Ocean (34”N.
58’W). Data from Table 2 and DE BAAR etal. (1983).
patterns versus shales. However, from their data we calculated Gd* by using the next element Dy instead. In this way the linear interpolation stretches a longer span of the curved REE distribution pattern, and the resulting values of Gd* are slightly less reliable. Nevertheless most of their data also fall above the l/ 1 line. No clear trends appear in the vertical profiles of the above defined Gd anomalies (Fig. 6). The signal is rather noisy in the Atlantic Ocean, although there may be a maximum at mid-depth. At the Pacific site the Cd anomaly is extremely uniform with depth. The shale normalized REE data shown in Figs. 2 and 3 are plainly not represented by simple, smooth
curves. Thus the Gd anomaly we describe must be tested to see if it represents a statistically significant signal above the noise in the data. We can in fact test numerous combinations of our REE data to see if significant departures from the mean represented by near neighbor data exist. For argument, then, we define anomalies in Sm, Tb, Tm and Yb analogous to that for Gd, i.e. Sm* = l/4(Pr/PrM, Tb* =
+ 3Eu/EutiJ + HO/HO&~,)
(3)
‘12
(Eu/Euti,
Tm* =
%
(Ho/HoadC + 2Yb/YbMC)
(5)
Yb* =
%
(Tm/Tmati,
(6)
+ Lu/Lu,,).
(4)
We did not report data on Nd in Atlantic Ocean waters, since at that early stage of our work reliable data could not be obtained. In our Pacific data set, however, Nd results are reported, and there we define 03
005 005
02
f&fF
Sm* = % (Nd/Ndhtie + 2Eu/Eutie). a2
a%
a2
9’,
a2
\43
0.2
0.05 n_
!!a P a3
p
/-To2
(7)
In Table 3 we show the mean and standard deviation ofthe ratio REE/REE* in each water column. Students’ t test is used to test the hypothesis that these ratios are different from I .O. In the Atlantic Ocean data set it is clear that anomalies in both Gd and Tb exist and that the visual effect in Fig. 2 of elevated Gd concentrations is made up of both elevated Gd and reduced Tb concentrations. In the Pacific Ocean data anomalies appear for each element; however, the Gd/Tb effect is by far the largest. Given the systematic trends in the data shown in Figs. 2 and 3, it is not surprising that the secant approach used here consistently reveals small departures from a linear interpolation. However, it is the clearly predominant Gd/Tb shift that is of interest here. REE FRACl-IONATIONS IN SEAWATER
Mechanisms conlrolling REE in seawater
0081 I_0
, cc pr
, Nd
sn
/
1
I
Eu
Gd
Tb
1
1
Ho
,
I
I
Tm
Yb
I ,008 Lu
FIG.2. REE distribution patterns normalized versus shales in the western north Atlantic Ocean.
In order for an REE anomaly to exist some mechanism for fractionation relative to its neighbors must operate. Two simultaneous mechanisms are believed to control the distributions of REE(II1) in seawater (DE BAAR et al., 1985): 1) association with skeletal material, possibly calcareous; and 2) adsorptive scavenging by
1964
H. .I W. de Baar. P. G. Brewer and M. P. Bacon
10 08 06
00
00
oo+ 006 006
0004
FIG. 3. REE distribution patterns normalized versus shales in the eastern equatorial Pacific Ocean
particles. The corresponding fundamental properties for fractionation of the REE(II1) cations are: 1) different ionic radii; and 2) different relative affinity of chemical bonding with either the adsorbing surface or the stabilizing complexers in seawater solution. Both concepts depend largely on the electron shell filling of the REE(II1) cations. settling
ionic radii
The well-known lanthanide contraction leads to a gradual decrease in ionic radius from 1.03 A for I_a(III) to 0.8 A for Lu (III) in six-fold coordination (SHANNON, 1976). There appears to be a small but distinct discon-
tinuity in the contraction between Gd and Tb. WC have hypothesized (DE BAAR d ai.. 1985) that REE” may substitute for Ca*+ in carbonate skeletal material and that involvement of the REE in the uptake/dissolution cycle may be partly responsible for enhanced REE concentrations in deep water relative to surface water and in the deep Pacific relative to the deep Atlantic. This hypothesis is not incompatible with the water column observations, but the REE concentrations in foraminifera (PALMER. 1983)appear to be too low to have an impact on the oceanic REE distributions (DE BAAR ef al., 1985). Also. REE fractionations resulting from this cycle were considered to be small. This also seems true for the observed Gd/Tb anomalies.
1965
REE in seawater Table 2. Gd concentrations in the north-west Atlantic Ocean (33058'~. 5E"05'W)*
DEPTH
(In)
mol.kg-')
10
4.9
48 95 143 493 643 793 992 1184 1377 1729 2490 2870 3253 4309 4378 4426
4.6 4.9 5.2 4.7 4.5 4.2 4.4 4.9 4.0 4.5 6.3 6.1 7.1 11.1 10.6 12.7
tween the strongly (Th, Pa, Fe) and less strongly (Ni.
Cd) hydrolyzed elements. Several classes of adsorptive sites (clay minerals, Fe/Mn oxide coatings or various organic functional groups) on the particles are potentially involved. BALISTRIERIet al. ( 198 1) further explored the SCHINDLER (1975) model in an analysis of metal scavenging residence times derived from sediment trap experiments. They attempted to constrain the options with respect to plausible surface sites but were forced to use a small group of trace elements with rather different chemical properties. In this respect the group of REE with very
10
. 7
I
I .*m* Irn
*Data for other REE are to be found in DE BAAR et al. (19831. Sampling depths have been revised slightly.
For instance, a strong preferential uptake of Gd in skeletal material would lead to much higher Gd anomalies in deep waters compared to surface waters. In fact the Gd anomaly appears to be very constant with depth, except for a slight enhancement in the core of the North Atlantic Deep Water (Fig. 6). While some biogenic Gd fractionation may occur, it appears to be too small to account wholly for the very uniform Gd/Tb anomaly in seawater.
48 f
1
l
1 ‘.
NW.Atlantx l . Cariaco Rephcates * Sargasso Sea .
Pacific II,,,\I,llljll,
Particle
scavenging
Many trace elements in seawater are adsorbed by the surfaces of fine suspended particles and are then removed from the water column. This continuous process of scavenging (GOLDBERG, 1954; CRAIG, 1974; TUREIUAN, 1977) has been subjected to considerable study by marine chemists. The exact chemical nature of the scavenging mechanism probably varies from element to element. SCHINDLER( 1975) first proposed a formal model for the adsorptive removal of the trace metals from the oceans in which scavenging was seen as competition for the free metal between solution ligands and particle surfaces. The particle surface/metal ion interaction was represented as an equilibrium with surface -OH groups, in accordance with the general observation that oceanic residence times correlate well with the tendency for hydrolysis. Such general trends between fundamental chemical properties and scavenging residence times of the elements are a matter of considerable debate (LI, 198 I ; WHITFIELDand TURNER, 1982, 1983); however, the hydrolysis constants for the REE would clearly place these elements in a residence time hierarchy be-
2-I
. a*
of/,,
,, ,
(((,,
,
.A
-
.
,,
,
,,
8
‘4
l
'76
Gd/pmol kg-y FIG. 4. Scatter plots of Sm. Eu, Tb and Ho versus Gd. Preliminary Cariaco Trench data taken from DE BAAR(1983).
H. J. W. de Baar. P. G. Brewer and M. P. Bacon
The equilibrium model of BALISTRIERIP! LI/.i I% I ) clearly implies reversibility. That reversible adsorption does indeed occur was shown by BACONand ANDEKSON ( 1982). By analogy with their reversible model the scavenging removal of REE from seawater is envisioned A’ . NW ATLANTIC OCEAN as a two step process: 1) equilibration of REE(lII) bex E ATLANTIC OCEAN A URIACO TRENCH tween inorganic complexes in solution and s&ace sites n PACIFIC OCEAN on small suspended particles; and 21 periodic rcmovai of small suspended particles (BACONN ui.” 1985). the latter being swept out either by settling biogcnic debris from surface waters or by zooplankters at all depths FIG. 5. Measured Gd versus the expected value Gd* which package fine suspended particles into large rap= % (Eu/Eb + TbjTb-) normalized versus shales. For idly settling fecal pellets. the eastern North Atlantic Ocean (ELDERF~ELD and GREAVES. Chemical fractionations within the REE series arc 1982) Gd* = % (2 Eu/Eb f Dy/Dy,). For the same data sets, a very similar trend was found upon normalization versus thought to occur exclusively during the equilibration chondrites instead (DE BAAR 1983). Preliminary Cariaco step. In order for the process to create the Gd/Tb Trench data taken from DEBAAR( 1983). anomaly that we observe, we must invoke either a smaller binding constant of the Gd3+ ion with the surface sites on particles or preferential complexing of the similar, gradually varying chemical properties is Gd3+ ion in solution (the opposite for Tb). Preferential promising, since one can assume that they will all complexing of Gd3+ in solution is supported by shifts probably interact with the same type surface site. The in the fundamental properties at this place in the REE possibility of their simultaneous involvement in a bio- series and by a solution speciation model. as discussed geochemical cycle, however. would blur the picture. below. With respect to the tirst argument, there are neither constraints for the possible surface sites nor firm inGd - Anomaly : Gd/Gd * formation on their binding constants for REE(III) adro sorption. It is interesting to note, however. that two model organic compounds, EDTA (WHEELWRIGHTcl al., 1953) and iminodiacetic acid (THOMPSONef ai.. .. I967), have higher formation constants for the heavier . . REE. A significant fractionation due to these differ. . ences of REE binding constants of such functional groups on particles would lead to a depletion, rather I l than the observed enrichment, of heavy REE in seawater. If these model compounds are representative . indeed for the open ocean, then differences in REE . binding constants of surface sites appear to be too small . to exert a control on seawater fractionation 10
14
14
. L-n
!
:
Fundamental
NWATLANTIC
E ATLANTIC CARIACO
E.EPACIFIC
FIG. 6. Vertical profiles of Gd/Gd* in the western North Atlantic Ocean, the eastern Atlantic Ocean (ELDERF~ELD and GREAVES,1982), the Cariaco Trench (preliminary data from DE BAAR, 1983) and the Pacific Ocean (DE BAAR n al.. 1985).
properk~
Gd (Z = 64) is the eighth element in the series of 15 REE (57La to “Lu). This location halfway along the series leads to an exactly half-filled inner 4felectron shell for the Gd(III) cation. There appears to be a special stability associated with the exactly half-filled 4J‘shell The sum of the first three ionization potentials exhibits
1967
REE in seawater
a distinct minimum at Gd (Fig. 7). The relatively larger decrease in ionic radii between Gd and Tb has been mentioned above and is indicative of similar discontinuities in, for instance, hydration and complexation constants. Similar, but considerably weaker, energetic advantages have been invoked for exactly 114filled (between Nd and Pm) and ‘/4filled (between Ho and Er) 4fshells of the REE(II1) ions. These combined discontinuities between the third and the fourth, exactly at the seventh (Gd), and between the tenth and eleventh 4felectron filling are known as tetrad effects (NUGENT, 1970; SIEIURSIU, 197 1; DZURINSKII, 1980). The stronger effect exactly at Gd would be better detectable than the weaker effects between Ho and Er and between Nd and Pm. Moreover, Pm does not exist in nature, and Er was not measured in our samples. Solution chemistry
In attempting to translate the above unique properties of Gd into predictions for aqueous solution, the effects of hydration ought to be taken into account. Unfortunately a quantitative treatment of the hydration concept in seawater is hampered by significant experimental and theoretical problems (WHITFIELD, 1975; WHITFIELD and TURNER, 1983). With respect to the REE(III), the Gd(II1) ion has an exactly spherically symmetrical ligand field which in itself might lead to a unique hydration pattern. There are, however, many other factors and concepts involved. As a result there appears to be no simple explanation of tetrad effects observed even in simple aqueous solutions (WILLIAMS, 1982). Prediction of tetrad effects for hydration in complex aqueous solutions (e.g., seawater) is even more difficult. There is some indication, however. that the lighter members La(III) to Nd(III) have a ninefold coordination, whereas the series Gd(II1) to
Lu(II1) is generally eightfold coordinated. In other words, there would again be a break at about Gd. However, all the above evidence is based on measurements in concentrated solutions, and different REE(II1) hydration numbers may dominate more dilute media such as seawater. The speciation of metal ions by formation of complexes in seawater has been assessed by many workers. Here we will use the most recent and careful compilation by TURNER et al. (198 1). Their calculated speciation for the REE(II1) is shown in Fig. 8. Of particular interest is the importance of the REE-carbonate complexes. These complexes dominate the speciation, yet the formation constants have not been measured directly. TURNER ef al. (198 1) estimated the constants through reliance on the correlation between measured oxalate and carbonate complexes: thus there is some uncertainty here (R. H. BYRNE, pet-s. commun.). Clearly a more detailed study of REE(III)-complexes in seawater is needed. Yet for now the best available model (TURNER et al., 1981) leads to some interesting predictions. In their results a very distinct Gd/Tb anomaly appears for almost every dissolved species, including the free ion itself (Fig. 8). There is also a striking resemblance between the total amount of REE(II1) complexes and the shale normalized pattern observed in seawater (Fig. 8). In other words, not only do the heavier REE indeed appear to be more stabilized due to their generally stronger complexation in seawater (GOLDBERG et al., 1963), but a minor anomaly in speciation is also reflected in our measurements for the elements Gd and Tb. MASUDAand IKEUCHI( 1979) expected to find such a Cd anomaly (or actually a full-fledged tetrad effect) in seawater, but they had to rely on interpolations between nine measured REE in only one sample. Our data confirm their expectations. DISCUSSION
Number 0
of Eleclrons
2
4
in Inner
6
8
4F
10
Shell 12
(4
3511,,,,,,,,,,,,1 Lo
Ce
Pr
Nd
Pm
91
Eu
Cd
Tb
Dy
Ho
Er
Tm
Yb
IN
FIG. 7. The sum of the first three ionization potentials of the REE. Note the marked decrease at half-filled (Cd) and completely filled (Lu) 4j’electronshells. A minor discontinuity may occur between Ho and Er, but absence of Pm data precludes observation of a similar anomaly between Nd and Pm. From FAKTOR and HANKS (1969).
It is interesting to consider whether the REE patterns observed in seawater, including the Gd/Tb anomalies, should be reflected in authigenic or biogenic phases leaving the ocean. Various marine deposits, in particular ferromanganese nodules. have often been noted to exhibit a REE pattern with positive Ce anomaly and heavy REE depletion, i.e. the inverse of the typical seawater pattern (GLASBY, 1973; PIPER, 1974; ADDY, 1979; RANKIN and GLASBY, 1979; ELDERFIELDet al.. 198 1). It is sometimes implied that such deposits rep resent the complementary sink presumably requited for the existence of the seawater pattern. The argument might go as follows: Assuming a hat shale pattern for the input, the REE are subsequently distributed between the solid and liquid phases. with the distribution coefficient roughly proportional to the percentage free ion in seawater (Fig. 8). After the solid phase (i.e., fine suspended particles) is removed, the remaining REE in the parcel of water would have the observed pattern with a positive Gd anomaly. Would the adsorbed au-
1968
H. J. W. de Baar. P. G. Brewer and M. P. Bacon
L
w4-?‘--T~-&~LOcaR Nd
c
RrcentogeFree
l
1250m Depth, Fucef!c / OCP?Oll
L-T--
Ions
oas
-
be in the pool of dissolved species (Fig. 8). also with a seawater pattern. The minor fraction adsorbed on particles would by and large exhibit a flat shale pattern. As a result. we believe that the REE composition o! marine authigenic deposits should. ONuverugc. match the shale pattern of continental inputs. The patterns of REE observed in seawater demonstrate that the chemical processes operating in the oceanic water column are capable of separating elements with very similar chemical properties. This is also illustrated by the fractionation that occurs between the actinides Th and Pa. which are believed to be partitioned differentially between a vertical flux governed by particulate transport and a horizontal flux driven by uptake at ocean boundaries (ANDERSON et ui 1983a,b). Recently very similar vertical distributions were also reported for the elements Ni and Pd (LEE;. 1983) and for Cu and Ag (MARTIN of 01.. 1983) at VERTEX II, the site of our Pacific REE study (DE BAAR et al., 1985). Elemental ratios were found to be about 4 times (for Ni/Pd) and about IO times (for Agi Cu) greater than crustal abundance ratios based on TAYLOR (1964). There is some uncertainty in the assessment of crustal abundances, but the apparent oceanic fractionation of these group 8 and I b elements may well be affected by chemical processes similar to those governing the fractionation of the actinide or REE series.
t ‘i-7’ Tm m L”
FIG. 8. Above: Percentages of REE(III) species predicted by the model of TURNER ef al. ( 198 1)for seawater of pH 8.2, 25°C and I atm pressure. Significant variations with temperature and pressure are expected to occur because of the variation of carbonate concentration with temperature and pressure (BROECKERand PENG, 1982). Below: The mirror image (log-scale) of percentage free REE(III) ion compared with the shale-normalized distribution pattern for deep Pacific water. The light REE(III), with a higher percentage free ion, are more depleted in seawater. The Gd anomaly in seawater is compatible with the anomaly in the percentage free ion.
thigenic phase (to be distinguished from biogenic and detrital components) of settling large particles or marine sediments then have the complementary (i.e., inverted) pattern, with I ) a gradual depletion of the heavy REE and 2) a Gd depletion? The above is a static or batch type description of what in fact is a continuous, dynamic process, and the actual situation indeed appears to be rather different. The continuous supply by rivers and aerosols of reactive REE with a flat shale pattern would tend to dilute the HREE enrichment and Gd anomaly of the seawater pattern. In order to counteract this trend strictly by
scavenging removal, then the authigenic fraction of settling particles must essentially exhibit a flat shale pattern without a Gd anomaly. This is also expected from the Turner-Whitfield-Dickson model, provided the total inventory (dissolved species + fine suspended particles) of a parcel of water has a seawater pattern. At low particle concentrations most of the REE would
CONCLUSIONS
I) Seawater, normalized wrsus either shales 01 chondrites, consistently exhibits a positive Gd anomaly and a negative Tb anomaly. II) The well-known anomalous physical and chemical properties associated with the shift from an exactly half-filled 4feIectron shell could very well lead to the observed Gd/Tb fractionation in seawater. Ill) The observed anomaly is compatible with the predicted speciation of REE(III) in seawater, in combination with scavenging removal as the dominant control of REE distributions in the oceans. .4cknowledgemenls-The authors are greatly indebted to Rebecca Belastock, Alan Fleer, Fred Frey, Hugh Livingston, Pieter Nella, Peter Sachs and Deborah Shafer for their help and advice during various stages of the project. Bob Byrne kindly reviewed an earlier draft of the manuscript. We are grateful to Harry Elderheld for many stimulating and informative discussions. This research was supported by US Department of Energy Contract DE-ASO2-76EV03566 and Office of Naval Research Contract NOOOl4-82-C-00 19 NR 083-004. Editorial
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