The isotopic composition of neodymium in the North Pacific

Geochimico n Cosmochimica Ada Vol. 52, pp. 1373-1381 Copyright 0 1988 Pergamon Press pk. Printed in U.S.A.

The isotopic com~sition

00167037/88/$3.00

of n~ymium

t .OO

in the North Pacific*

IXNALD J. PIEFGRAS~ and STEINB. JACOBSEN Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02 138, U.S.A.

(Received August 12, 1987; accepted in revisedform March 22, 1988)

Abstract-We have determined the isotopic composition of neodymium in two water column profiles in the western North Pacific and in bottom waters from three other locations in the North Pacific. The vertical profiles reveal that the water column is stratified with respect to isotopic commotion. Mid-depths associated with Pacific Deep Water are uniform having e&O) = -3. Shallow waters are variable, but deep waters are less radiogenic having ~~(0) = -4.4 to -5 in the western basin. The more negative bottom waters in the western North Pacific provide evidence for the advective transport of rare earth elements from Antarctic sources to this region. Bottom waters in the eastern North Pacific are more radiogenic (c&O) = -3.3 to -3.8) indicating very little transport of REE from Antarctic sources to this basin. Overall, the isotopic composition of Nd in Pacific bottom waters appears to be conservative when compared to the silica distribution in spite of evidence for Nd removal. The findings are generally compatible with other parameters such as temperature, salinity and nutrient distributions. The presence of Nd isotopic signatures associatedwith Antarctic sources in these waters provides evidence for a small component of rare earth elements of North Atlantic origin in the bottom waters of the North Pacific. This is the 6rst tracer evidence to corroborate the suggestion of REIDand LYNN (197 I) that some of the tem~~ture and salinity properties of the deep waters of the North Pacific originated in the Norwegian and Greenland S&s. INTRODUCTION

On a global scale, the major ocean basins are each characterized by distinct isotopic compositions. The average values of eNd(0)in each ocean basin are as follows: Pacific, t&O) m -3; Indian, e&O) = -8; Atlantic, t&O) = - 12 (PIEPGRAS et al., 1979; GOLDSTEINand O’NIONS, 198 1). These differences reflect the relative importance of continents ver,ws oceanic crustal sources of REE in the different ocean basins. The negative values of q&O) observed for all ocean basins indicate that the REE supply to the oceans is dominated by continental sources. For the Atlantic, PIEPGRA~et al. (I 979) estimated that at least 90% of the REE must be from continental sources. This is supported by recent estimates of the weighted average of t&O) = - 10.8 to - 12.6 in rivers draining into the Atlantic (GOLDSTEIN and JACOBSEN, 1987). The more radiogenic Pacific values, however, require a substantial contribution (up to 50%) of a depleted mantle derived component of REE. This would be consistent with the much greater level of volcanic activity in and around the Pacific basin. PIEPGRASand WASSERBURG(1985) measured Sm and Nd concentrations in mid-ocean ridge hydrothermal waters in the Pacific and concluded that hydrothermal fluxes of REE could not account for the more radiogenic Nd isotopic signature of Pacific waters. Instead, they suggested that weathering of exposed volcanic terranes in the Pacific could be a probable source of the radiogenic Nd. This conclusion is sup ported by the river flux data of GOLDSTEINand JACOBSEN (1987). Their average river flux to the Pacific is t&O) = -2.9 to -3.7 in agreement with the average Pacific Ocean value and reflects large inputs of radiogenic Nd from weathering of volcanic terranes. On a finer scale, the Nd isotopic dist~bution within ocean basins exhibits considerable structure (PiEFGRAS and WAS SERBURG, 1980, 1982, 1983, 1987; STORDALand WASSERBURG, 1986). This structure is closely related to both sources of REE and their transport in association with the circulation of water masses. This was clearly demonstrated recently by PIEPGRASand WASSERBURG(1987) for the western North Atlantic. They were able to show that North Atlantic Deep Water (NADW) has a characteristic Nd isotopic signature

NATURALLYOCCURRINGVARIATIONS in the t43Nd/‘44Nd ratio in crustal sources of rare earth elements (REE) to the oceans and the short residence times of REE in the oceans relative to the time it takes to homogenize the isotopic cornposition of Nd in the oceans make it an ideal tracer for studying the sources and transport of REE in the oceans. The abundance of ‘43Nd changes through time due to the radioactive decay of ‘47Sm(half life = 1.06 - 10” years). The *43Nd/ 144Ndratios [represented by eNa(O)as defined in Table 21 of erustal rocks will, therefore, reflect their age and Stn/Nd ratio. The evolution of the ls3Nd/‘@Nd ratio in the bulk earth is believed to follow a simple growth curve corresponding to the SmfNd ratio of chondritic meteorites (DEPAOLO and WAS&BURG, 1976). Terrestrial differentiation processes, however, have segregated material into light REE depleted oceanic mantle and light REE enriched continental crust reservoirs with distinctive ages and Sm/Nd ratios. Oceanic crustal rocks (mid-ocean basal@ ocean islands, and island arcs) derived from the depleted mantle and/or recycled oceanic crust (Sm/Nd > chondritic) are characterized by positive values of q.@). In contrast, the light REE enriched continental crust (Sm/Nd c chondritic) is characterized by negative values of e&O). Thus, sources of REE to the oceans derived from weathering of oceanic and continental crustal rocks can be readily distinguished by their isotopic composition. Incomplete mixing of these isotopically diverse sources of REE results in large variations of ~~~(0)in the oceans which reflect both the sources and transport behavior of the REE in the oceans. This is demonstrated by both the broader and finer scale features of the Nd isotopic distribution in the oceans. -_* Presented at the Conference on “Isotope Tracers in Geochemistry and Geophysics”in honor of ProfessorGerald J. Wasserburg’sSixtieth Birthday, March 23-25, 1987, Pasadena, California. t Present addresss: St. Croix Valley Natural Gas Co., River Falls, WI 54022, U.S.A. 1373

D. J. Piepgras and S. B. Jacobsen

1374

(q&O)= - 13.5f 0.5) that is compatible with the isotopic compositions of the respective sources (Norwegian, Greenland, and Labrador Seas) which mix to produce this water mass. The isotopic signature associated with NADW could be traced during the southward flow of this water mass from high latitude to equatorial waters (the southern most region studied) where in~m~on with Antarctic Intermediate (AAIW) and Bottom Water (AABW) began to mix out the NADW signature from above and below with more radiogenie waters (Antarctic waters have ~~~(0) N -9; PIEFGFUS and WASSERBURG,1982). The correlation of the Nd isotopic distribution in the water column to temperature and salinity properties which independently identify the water masses was excellent and indicated that the isotopic composition of Nd is conservative over the transit time of a water mass. Only a few data points are available for the isotopic composition of Nd in the Pacific waters (PIEPGRASet al., 1979; PIEPGRAS and WASSERBURG,1982). A water column profile based on 3 sample depths in the central South Pacific (PIEPGRAS and WASSERBLJRG, 1982) exhibited extensive vertical stratification ranging from t&O) ~250 in the surface waters to &d(O) = -8 in the bottom waters. The bottom water signature was inferred to represent nearly pure AABW and was consistent with T-Sdatafor the sample. The large difference between surface anddeep water isotopic signatures suggested that transport of REE in association with the spreading of water masses was also true for South Pacific waters. In this study, we present new data for the isotopic composition of Nd in the North Pacific water column. The major goals of this study were to determine the sources of REE in the North Pacific and to evaluate their transport behavior in relation to the known circulation of water masses in this basin. Deep circuIation in the North Pacific is very sluggish due to the lack of a northern source of deep water such as is produced in the North Atlantic. Evidence for this can be found in the chemistry of deep Pacific waters such as depletion in oxygen and enrichment in nutrients and other chemical elements relative to the well-ventilated Atlantic. One possible consequence of slow lateral circulation in the deep waters is that vertical mixing could dominate the distribution of nonconservative species such as the REE. If true, the Nd isotopic distribution in the water column would be expected to be uniform. The data we present here for two water column profiles and three other bottom water sites will demonstrate that there is horizontal transport of REX in the North Pacific. There is clear vertical Nd isotopic structure in the water coiumn, which, in part, can be related to REE transport in northward spreading AABW and can be traced to 47”N in the western North Pacific. PACIFIC

WATER

MASSES

Figure I summarizes the major water masses of the Pacific Ocean. Four water masses dominate the thermohaline circulation of the Pacific. These include two intermediate water masses, deep water, and bottom water. One of the intermediate water masses is the Antarctic Intermediate Water (AAIW). This water mass is generated in the Southern Ckx?anin regions just poleward of the oceanic polar front, a 7.oneof convemcz of wind-generated surface currents that encircles Antarctica. In this region, an excess of precipitation over evaporation produces low salinity waters (S x 33.8%). This water sinks upon cooling to a depth of about 900 meters and flows northward into all ofthe major oceans. In the Pacific it is recognized by an intermediate depth salinity minimum that can be traced northward to the equator.

*

WATER W=WAR&I #NER SPHERE AiW=PREIK I~~R~~AT~ WATER AAlW: ANTkKCII‘~“~~~~~~A~~ AAlW 1 liRiARcT!CBOTTOM WliiER

POW: PdCifiCOEEPXNER

FIG. 1. Cross section of the Pacific Ocean at 160”W showing the major water masses and their direction of flow (after DIETRICHet al., 1980).Note that a flow direction for PDW has not been inferred.

Similarly, Arctic Intermediate Water (AIW) forms near the polar front in the North Pacific and spreads southward at a similar depth and with similar salinity as AAIW. According to DIETRICHet al. (1980), neither AAIW nor AIW crosses the equator in significant volumes. Below the intermediate waters is Pacific Deep Water (PDW). This water mass has fairly uniform temperature and salinity (0 = 1.1 to 2.2’C, S = 34.65-34.75s) atdeptibelow 2000 m (RCKARD,1979). The origin and circulation of PDW is unclear. Unlike the Atlantic, there is no northern source of deep water formation in the Pacific. It is generally assumed, therefore, that PDW derives its properties from mixtures of overlying intermediate waters and underlying bottom waters. This scenario is supported by temperature and salinity observations reported by REIDand LYNN(1971) that show the deep water salinity maximum (imparted fmm NADW) in the South Pacific gradually being mixed out to the nor& by mixing in lower salinity bottom and intermediate waters. From the equator northward, the tr-S relationship in the deep water is linear. The last major water mass in the Pacific is the Antarctic Bottom Water (IBM). This water mass forms with fairly homogeneous ~rn~ratu~ and salinitv near 8 = 0°C and S = 34.67960(BROECKER and.TA~HASHI, 198Oj. AA3W flows into the Pacific through the Macquarie Ridge south of New Zealand and spreads northward along the bottom at a depth below -4 km (WARREN, 1973). CRAIGd at. (1972) showed that in the western South Pacific, the northward spreading AABW was separated f%omoverlying PDW by a “benthic front” characterized by abrupt changes in temperature, salinity and nutrients. AABW fiows into the Central Basin through a passage northeast of Samoa (REID and LONSDALE,1974). From there, a portion of the flow enters the Northwest Pacific Basin east of Wake Island and the rest into the Northeast Pacific through passages in the Line Islands Ridge (EDMOND etal., 197 I). According to DIETRICH et al. (1980), AABW can be traced as far as 50’N. In the near surface waters at low to mid latitudes is the warm water sphere. These waters exhibit a wide range in temperature and salinity and are characterized by a well developed thermocline. Its properties are a&ted primarily by processes at the sea surface such as seasonal temperature variation and precipitation or evaporation. The circulation of the warm water sphere is laqely wind driven and separates the Pacific into two major circulation gyres, the clockwise rotating North Pacific gyre and counterclockwise South Pacific gyre. In addition, there is a subpolar gyre in the North Pacific poleward of the polar front (near 45”N) which has counterclockwise rotation. More detailed discussions of Pacific water masses and circulation can be found in texts by DIETRICH er al. ( 1980) and PACKARD(1979). SAMPLES

AND METHODS

!%mplesof the North Pacific water column were collected for Nd isotopic analysis from five locations during the T-Pacific Sections cruises (R/V Thomas Thompson) during the summer of 1985. The locations of these sites are shown in Fii. 2 and are given in Table I. The two sites from 47”N (TPS47 39-l and 80-l) are located in the subpolar gyre. Two sites from 24”N (TPS24 76- 1 and 27 1- 1) are located in the North Pacificgyre while the third site from 24”N (TPS24 35 I - 1) is located in the PhiIlipine Sea. These samples provide a range of North Pacific water types including PDW, AIW. surface waters and bottom waters. Complete water column profiles for 6&O) have

Pacific seawater Nd

1375

was transferred to clean 10 liter polyethylene jerrycans and acidified with -50 ml high purity 6 N HCI. The samples were not filtered. Spiking, chemical separation of the REE, and mass spectrometry procedures followed those of PIEPGRASand WASSERSURG (1980, 1985), except that mass spectrometry was done on a VG Isomass 543. Replicate analyses of some samples (see Table 2) demonstrate

our ability to reproduce measurements of the 143Nd/‘44Ndratio in small samples (10 to 20 ng). Many of our samples were too small for repeat analysis, however. To ensure the overall reproducibility of our results, periodic analysis of 10 to 18 ng of the Caltech Nd@ normal

076-I

(WA~ERBURG et al., 198 I) subjected to the seawater chemistry pro-

cedure was made. The average of eight analyses was c&O) = - 14.4 + 0.5 in excellent agreement with values obtained at Caltech on the Lunatic I mass spectrometer (WASSERRLJRG et al, 1981; D. PIE&GRAS, unpublished data). Thus, data reported here can be directly compared to previous measurements of the isotopic composition of Nd in seawater made at Caltech and reported in the literature.

Sm and Nd blanks were closeiy monitored throu~out this work. New columns and resins were used and the initial blanks were highest, but they decreasedwith use. At the start of this work, total chemistry blanks were - 100 pg for Nd and -20 pg for Sm. After about three months, total blank levels dropped to 25 to 45 pg Nd and 2 to 4 pg Sm and have remained constant near this level. In all cases, blanks were less than 1% of sample size and are considered negligible. RG. 2. Locations of sites sampled for this study. Stations 39-1 and 80-l are to TPS47 sites, while stations 76-1, 171-1, and 351-l are TPS24 sites. MC 80 st. 3 1 is the site of a South Pacific profile reported by PIEPGRAS and WASSERBURG (1982) and reviewed here.

been determined for two locations only, TPS47 39- 1and TPS24 27 l1. Only bottom waters were analyzed from the remaining sites. !!iample sizes ranged from 6 to 9 liters. Samples were collected in

10 liter Niskin type samplers mounted on a 36-p&e rosette quipped with a CTD for in situ measurement of conductivity, temperature and depth. After drawing samples for the determination of salinity, oxygen and nutrients, the remaining water from each Niskin bottle

RESULTS Table 1 lists the results for hydrographic measurements of potential temperature (e), salinity, dissolved oxygen, and nutrients (PO:-, NO;, and SiOz). Results of Nd isotopic measurements and Sm and Nd concentrations are given in Table 2. We have constructed 8-S curves for those stations where water column profiles of Sm and Nd have been determined. These are shown in Fig. 3. Each 8-S curve was constructed

TABLE

TABLE 1. Sample locations and ky~ographic B

Depth

TPS “3’ 39-l

!;,;l,Xi.O’

Salinity

2.799

33:703

364 g: 1249 1795 2692 3592 4481 6408

3.334 2.717 3.645 2.233 1.791 1.389 1.201 1.097 1.063

34.061 34.356 34.250 34.489 34.583 34.651 34.674 34.683 34.688

8&l

(r;l;9.6’

PO:

N, ;;1;7;8.2’ E)

195

Tl”;

02

N, ;Y;5.6’

293 106 :: 26 34 13 143 157 161 W) 148

1.18 2.94 3.30 3.23 3.21 3.16 3.03 2.77 2.61 2.51 2.49

2.53

Depth

data%. NGg

12.4 39.9 45.6 44.6 44.2 44.1 42.1 38.8 37.3 36.6 36.3

37.7

SiO2

17.8 90.6 118.6 139.9 149.6 171.2 178.2 166.2 159.0 155.1 153.4

171.6

TPS 47 39-l 19: 364 600 800 1249 1795 2692 3592

TPS 47 8&l 5174

2. Sm-Nd

isotopic and concentration

Nb

Sma

15.9 22.2 22.9 24.4

2.88 4.09 4.12 4.52 4.70 5.07 5.54 6.44 7.26 8.14 8.60

0.1141 0.1162 0.1132 0.1167 0.1170 0.1171 0.1174 0.1118 0.1196 0.1196 0.1220

0.511840151 0.511731*13 0.511739*18 0.511715+18 0.511673+21 0.511694*25 0.511693*27 0.511692*10 0.5116?3*~ 0.51164&17 0.511667+20 0.511635+18

-0.1 -2.3 -2.1 -2.6 -3.4 -3.0 -3.0 -3.0 -3.4 -3.0 -4.7 -4.1

:!I; 29.8 34.2 38.7 42.9 44.4 44.4

761

f~;$.O’

TPS y 271-;&17.2

N, gtY86.9’

W)157

2.56

37.8

159.3

0.1 3.2 9.7 29.5 43.8 43.5 46.8 39.0 37.0 36.3

1.5 2.9 10.6 54.9 121.5 132.9 153.6 154.9 147.0 142.5

36.9

146.5

0.02 33.4 31.6

0.3 112 166

NS;M;628.2’ E)

184 381 640 1046 1194

17.465 14.117 6.834 3.640 3.144

34.378 34.461

211 212 200 136 47 57

;z 4195 3073

1.748 1.326 1.107 1.046

34.617 34.663 34.683 34.689

105 137 161 169

if; 0.71 2.03 2.9% 2.91 2.71 2.57 2.44 2.40

1(yo’433.0’ NJ&l3.6’

E) 158

2.51

S, 15;oi9’ 165 195

W)t 0.13 2.41 2.24

TKJ24 35-l

Marj~haa 2800 4500

34.797 Z!::

80 Station 31 (3y40’ 1 34;36

f Depth given in meters, potential temperature (0) in ‘C, in per mil and nutrient data in ~01 k t Hydrographic data from K. BRULAb (pers. comm.).

salinity

$$

data.

ENd(o)C

1.0 I 0.3 l 0.4 * 0.4 + 0.4 * 0.5 * 0.5 * 0.4 * 0.4 l 0.3 * 0.4 * 0.4

l

62.8

12.6

0.1265

0.511680+16

-3.3

l

0.3

TPS 24 Xi-1 51.7 4621

10.2

0.1237

0.511653*1?

-3.8

l

0.3

-3.2 -4.4 -3.5 -3.7 -2.0 -3.0 -3.8 -3.4 -4.5 -4.8 -5.0 -5.0

TPS 24 271-l Tk”24

(pmol/kg)

I@SRl m

0.4 0.4 0.4 0.3 0.3 0.4 0.3 0.4 0.3 0.6 0.3 0.5

1s:

6.75 5.41

1.43 1.14

z:

z:, 1046 1104 2000 2999 4195

15.1 7.91 20.0 20.9 28.2 34.0 37.0

2.85 1.65 3.65 3.80 5.13 6.36 6.84

0:1315 0.1180 0.1155 0.1147 0.1150 0.1150 0.1167

567; ‘I

37.0 35.0 35.0

6:s -

o-L179 -

0.511685+26 0.511623*18 0.511670*21 0.511656+14 0.511701*16 0.511603*20 0.511651*16 0.511672*19 0.511617*17 0.511Mfli33 0.511~0~14 0.511590*28

6.30

0.1160

0.511616r13

-4.5

0.511861+30 0.511617*36 0,511435*20

+0.3 * 0.8 -4.5 * 0.7 -8.1 l 0.6

TPS 24 361-l 34.5 5926 Mar. C&m. 28:: 4500

80 Station 3ld 17.8 2.85 25.5

-

-

l

I * * * + f * * * * *

* 0.3

a Uncertainti~ in Sm aad Nd ~U~a~aLi~s i 0.5%. b Ncmnatid to l~zNd/‘~Nd = 1.138305. c Parts in 104 deviation from a chondritic uniform reservoir CHUR) having “aNd/‘uNd = 0.511847 (JACOBSEN and WASSERBUR 6 , 1984). d Data from PIEPGRAS and WASSERBURG (1982).

D. J. Pitpgras

1376

0.0 1 “““““‘I 33.0 32.5

”

”

33.5

34.0

Salinity

(X0)

”

I

”

34.5

“1 35.0

RG. 3. Potential temperature verslls salinity diagrams for TPS47 39- 1 in the western subpolar gyre and TPS24 27 I- I in the western North Pacific g,yre. The circles correspond to the 8-S characteristics of the samples analyzed for Nd isotopic composition. The inset diagram shows an expanded scale for Band S in the deep waters below -1500 m.

from a cubic spline fit of discrete salinity and CTD temperatures determined for all 36 rosette samples collected at each site. The 0-S values for those samples analyzed for Sm and Nd and listed in Table 1 are labeled on the diagrams. The 8S curves and other hydrographic results are described below along with the Sm and Nd results for each station. Results for Mar. Chem. 80 in the South Pacific (PIEPGRAS and WASSERXJRG, 1982) are also fisted in the data tables and were reviewed in the intr~u~on. TFS47 39-l. This station is located in the subpoiar gyre over the Northwest pacific Basin. The B-S characteristics are shown in Fig. 3 and are typical of subpolar waters. Surface salinity is very low (S = 32.777%) reflecting an excess of precipitation over evaporation. Salinity increases rapidly in the upper 1 km to S = 34.4% and then continues to increase gradually to a bottom water maximum of S = 34.688%. There is shallow seasonal thermocline with Bdecreasing from a surface value of 10.214OC to a minimum of 1525°C at 76 m. Below this, 6 rises slightly to a maximum of 3.335”C at 366 m before deereasing to a bottom water value of 1.066”C. The inset in Fig. 3 shows B-S in expanded scale for depths below - 1500 m. From this it can be seen that B-S is linear for depths beiow -2500 m. The low bottom water temperature suggests the presence of dilute AABW as one component in the 8-S mixture. Dissolved oxygen is highest in near surface waters (9 =s 300 pmollkg) and decreases to a minimum (0, = 20 pmol/kg) at -500 m. The oxygen remains below 100 rmol/ kg to a depth of -2 100 m and then increases gradually to a bottom water maximum of - 161 rmol/kg. PO:- and NO; are lowest at the surface, and increase to a maximum (PO:- = 3.28 pmol/kg, NO: TZ:46.1 tumol/kg) near the oxygen minimum (-500 m). Below this depth, both PQZ- and NO; decrease to the bottom where POj- s;c 2.5 rmol/kg and NO; =5 36.7 pmol/kg. SiO, is similar to PO:- and NO? ex-

and S. B. Jacobsen

cept that the SiOr maximum (SiOr = 178.2 pmof/kg) occurs well below the Or minimum at a depth of - 1800 m. Below this Si02 decreases to a bottom water value of 153.4 pmol/ kg. According to EDMOND ef al. (1979), the deep water silica minimum is an advective feature resulting from low silica AABW (Si = 100 to 120 pmol/kg) flowing northward under high silica PDW (Si = 180 pmoi/kg). Dissolved O2 and nutrient profiles at other locations are similar in their general features to those described above and will not be presented for other stations. The resuhs of Nd isotopic measurements are shown in Fig. 4 with solid squares. +&Of is highest in the surface water having t&O) = 0. There is a sharp decrease to c&O) = -2.3 at 200 m. Below this there is only a small decrease to c&O) = -3 which remains constant within error at depths between 800 and 2700 m. At depths below 2700 m, t&O) decreases gradually to a bottom water value of -4.4 (the average of two analyses reported in Table 2). The isotopic data indicate that depths associated with Pacific Deep Water have fairly uniform e&O) = -3, identical to the estimated value for average Pacific waters given earlier. The shift toward less radiogenic values in the bottom waters is in the dire&an inferred for Antarctic sources of Nd and is broadly consistent with the hydrographic data discussed above. The radiogenic surface waters could indicate that the REE receive contributions from weathering of nearby volcanic terrains such as the Kuril and Aleutian Islands. Nd and Sm concentration profiles are shown in Fig. 5 (black squares). Both Sm and Nd are lowest in the surface waters (CNcI= 15.9 pmol/kg, Cs,,, = 2.88 pmol/kg). Between

* TPS47

39-l

TPS47 0 TPS24 0 TPS24 0 TPS24

60-l 76-1 271- 1 351-l

l

I

FIG. 4. e&O) as a function of depth for five North Pacik sites. The symbols correspond to those used to show station locations in Fig. 2. Vertical pro&a have been de@&ned for two sites only flB47 391-l and TPS24 271-f). The r~~~~ results &own are bottom water samples. Error bars are 2 c of the mean.

Pacific seawater Nd

Sm bmob’kd

Nd (pmob’kd 0

20

40

600

4

6

12

1

-2

E

5 x3

H p4 5

6 Concentration of a) Nd and b) Sm versusdepth for North Pacific samples. Symbols are as in Fig. 4. FIG. 5.

200 m and 4500 m their concentrations exhibit a linear increase with depth. The highest con~~tr~tions are in the bottom waters (C,, = 44.4 pmol/kg, Csm = 8.60 pmol/kg) but are slightly lower than the trend indicated by the interval from 200-4500 m, suggesting a slight depiction in the bottom waters. TFS24 271-f. This station is located in the North Pacific gyre. The 8-S diagram for this station is shown in Fig. 3 and illustrates the major di%zences in the water column structure of the North Pacific gyre from the subpolar gyre discussed above. Salinity increases fram a sur&acevalue of S = 34.926% to a maximum of 34.974~ at -50 m. Below this, salinity decreases to a minimum of 34.~~7~ at -640 m. Salinity then increases to a bottom water vafue of 34.6~9~. The middepth salinity minimum corresponds to the core of southward spreading Arctic Intermediate Water that originates in the subpolar gyre. Potential tempemture decreases from a su&ace high (@ = 24.791”C) to a bottom water minimum (6 = 1.046Y). There is a well developed thermocline that extends from the surface to the depth of the salinity minimum. Bottom water temperatures indicate a slightly greater AABW component than at TPS47 39-1 which is also indicated by the nutrient data. An expanded scale 0-S diagram is also shown in Fig. 3 for the deep waters. From this it appears that 19-s is linear for depths below - 1200 m. Compared to the subpolar gyre station, this station is c~~cte~ed by higher salinity for a given temperature at depths above -2500 m. Below this depth, the 8-S curves are almost identical except for the slightly cooler temperatures in the bottom waters at this more southerly location. This suggests that the deep water mass features are nearly identical at these two sites. The Nd isotopic profile is shown in Fig. 4 and is indicated by the open squares. e&O) varies through the thermocline ranging from -3.2 to -4.4. ct&O) = -3.7 at the base of the the~~line in the core of AIW. Below the ~e~ocline to a depth of -3000 m, ~~(0) varies only slightly about an average value of = -3.4. In the deep waters below 3000 m, ~(0~ exhibits a shift toward less radiogenic values reaching a minimum of -5-O in the bottom water, once again in the direction inferred for AABW sources of REE. Fig. 4 shows that the

1377

isotopic profile for TPS24 27 l- 1 at depths below - 800 m is nearly identical to that for TPS47 39-t in the subpolar gyre. The concentrations ofSm and Nd are shown in Fig. 5 and are indicated by the open squares. Once again, Sm and Nd are depleted in surface waters relative to deep waters. However, both Sm and Nd reach m~imum values in the deep waters near 4200 m. Sm and Nd both show slightly lower values in the bottom waters relative to 4200 m for TPS24 27 l-f. This is true for other REE as well (P~EPCZA~and JACOBSEN,19X7), This depletion in bottom water REE content here and that suggested for TPS47 39- 1 is similar to the silica feature (Table l), but the deep water maximum for the REE occurs at much greater depth than for Si. In general, Sm and Nd concentrations of TPS24 27 l- I are lower than for TPS47 39-l at comparable depths. Other samples. In addition to the two profiles above, we have determined Nd isotopic compositions in bottom waters from three other locations in the North Pacific. TPS47 80- 1 is in the subpolar gyre in the eastern North Pacific. q+,(O) = -3.3 in this sample, indicating no significant Antarctic component. This is consistent with its warmer tem~ratu~ (6 = 1.115“C) and high silica content ( 17 1.6 pmol/kg). Nd and Sm concentrations are the highest values found in this study (CNd = 62.8 pmolfkg, Csm = 12.6 pmol/kg). TPS24 76-l is located in the eastern basin of the North Pacific gyre. This bottom water has e&O) = -3.8. The lower temperature (@= I. 106OC) and silica content (Csi = 159.3 ~rno~g~ indicate that a small component of AABW may be present. The isotopic result is consistent with this but the analysis is within error of TPS47 80-l. Sm and Nd concentrations are lower than at TPS47 80- 1 but much higher than other locations (C& = 5 1.7 pmot/kg, Cs, = 10.2 pmol/kg). The fast station sampled is TPS24 35 1-t in the Philippine Sea. The bottom water here has q+,(O) = -4.5 s~e~ing that a dilute component of AABW is present. Tire temperature, however, is much higher than elsewhere (0 = I .204Y) and the salinity slightly lower (S = 34.679%) indicating perhaps another source for the bottom water here. Sm and Nd concentrations are nearly identical to TPS24 27 I - 1, having C&d = 34.5 pmol/kg and C’s, = 6.39 pmol/kg. DMXJSSION The isotopic composition of Nd in the western North Pacific exhibits significant vertical structure that requires horizontal transport of REE in the water column from various sources. The distribution is broadly compatible with water mass distribution and circulation in this region. At depths between 1000 m and 3600 m associated with PDW, ~~~(0) is very uniform, averaging -3.2. At depths below 3600 m, tNd(0)gradually decreases (Fig. 4) to a bottom water value of -5.0 at TPS24 271-l in the North Pacific gyre, and -4.4 at TPS47 39-l in the subpolar gyre. Insofar as surface and middepth waters are more radiogenie, simple downward transport of REE from high levels in the water column on settling particles cannot account for the lower bottom water values. Three possible sources could provide the less radiogenic Nd observed in these bottom waters; pore water fluxes, resuspension of bottom water sediment, or the influx of less radiogenic Antarctic Bottom Water. There are no pore water

D. J. Piepgras and S. 3. Jacobsen

1378

data for REE in Pacific sediments, but data for Atlantic deepsea sediments (PIEPGRAS et al, 1986) indicate that pore water fluxes from oxic marine sediments are not a source of REE to the overlying water column. Resuspension of bottom water sediment seems unlikely as well. Although some isotopic data for Pacific sediments indicate that they could provide a source of less radiogenic Nd (GOLDSTEINand O’NIONS, 198 l), resuspension of bottom water sediment would be expected to add REE to the bottom waters resulting in increased concentration gradients in the deep waters. However, our water column profiles show evidence of a small REE depletion in the bottom waters relative to mid-depth trends (Fig. 5). We therefore conclude that the influx of northward spreading AABW (E&O) = -8 to -9) provides the most probable source of less radiogenic Nd in the bottom waters of the western North Pacific. This inte~re~tion is compatible with temperature, salinity and nutrient data which also show evidence of an Antarctic component as described in the results section. Evidence for an Antarctic Bottom Water source of REE in the Pacific can also be found in Mn nodule data. The Mnnodule data summarized by STAUDIGELel al. ( 1985) in their Fig. 2 are in the range -4 to -5 for the North Pacific and are similar to the bottom water values reported here for the same area. They felt the Mn-nodule data made sense in the framework of global ocean circulation, but they could not tie their results to actual water column data. Similar conclusions were reached by APLIN et al. (I 986) who felt the flow of AABW could be determined from Nd isotopic ~s~butions in Mn-nodules. The agreement between bottom water and Mn-nodule isotopic data indicates they have similar sources of REE and supports the conclusions reached by these authors. Bottom waters from other locations in the North Pacific indicate that AABW penetration is more dominant in the western basin. Two eastern basin bottom waters have Q.&O) = -3.8 and -3.3 (TPS24 76-i and TPS47 80- 1, respectively). These values approach PDW values. This is consistent with hydrographic data that show these bottom waters to be warmer and higher in nutrients (Table 1) than western basin sites. The observations are consistent with the known spreading of AABW into the North Pacific as outlined earlier. The isotopic composition of Nd in the bottom water of the Philippine Sea (TPS24 3.51-1, t&O) = -4.5) suggests that some AABW flows into this region as well. Although this is not easily explained in relation to the warmer temperature in this basin (1.204”Q there is evidence from silica distributions as discussed below that support this conclusion. To further evaluate the distribution of bottom water isotopic results and their relationship to AABW sources, we consider the case of two component conservative mixing between a southern (AABW) and northern (PDW?) end-member. For conservative mixing of these two components, 1 = AABW and 2 = PDW, we have in the mixture (tw): CNdM

=

CNd,

CSiM

=

CSilXl

xl

+

+

cNd7,(

CSi*(l

1 -

-

XI)

X0

(1)

(2)

where the Cij’S refer to the concentration of species i (here Si or Nd) in j (= I, 2 or &f) and X, r&rs to the mass fraction of component j in the mixture. A plot of Csi versus C,, must

yield a straight line, if conservative mixing of the two components occurred. The equation for conservative Nd isotopic mixing, of the two ~m~nents is given by: tNdM

=

%dl

CNdlxl

+

tNdZCNd2t

CNd,&

+

cNd2(1

1 -

xl)

(3)

xl)

where the refer to the E&O) values in j. A plot of tNd versus CNdwill be non-linear for conservative two-component mixing. For the AABW end-member, we use C,, = 28 pmol/ kg and +&Of = -8.6 (PIEPG~ZASand WASSERBURG,1982). These are values for Antarctic waters as they leave the Pacific sector. It is unlikely that the isotopic composition of this endmember could be more radiogenic than measured in the Drake Passage. However, the Nd concentrations could differ since the extreme variations of Nd measured in the water column profiles make it difi%ult to get a precise estimate of end-member concentrations. For the northern end-member, we USe CNd = 68 plllOl/kg and t&O) = -3.0. The Nd COD centration estimate for the northern end-member is justified from the bottom water result for TPS47 80- 1 which has CN~ = 62.8 pmol/kg. The end-member isotopic com~sition [c&O} = -31 is consistent with the fairly uniform PDW values observed in this data set and previous data (PIEPGRASet al., 1979). The resulting mixing curve for q&O) versus CNd is shown in Fig. 6a. Also shown are bottom water data for this study and one sample from the South Pacific (P~EPGRASand WASSERBURG,1982). In general, the data do not show a good fit 6Ndj’s

b -s

-7

-6

-4

%&jf”) FIG. 6. a) CM versus eNdfor bottom water samples from the Pacific. The solid curve with open circles corresponds to a conservative mixing curve between northern and southern components as defined in the text. Open circles denote each 20s change in amount of northern or southern component. Bottom water samples are. shown in solid squares with error bars. Note the poor fit to the conservative mixing curve. b) Cs versus eNa(O)for Pa&c bottom waters. The conservative mixing curve was calculated as in Fig. 6a. The data show an excellent fit to the conservative mixing curve.

Pacific seawater Nd

to the calculated mixing curve. Most samples fall well below the curve indicating that they are too low in Nd concentration, too high in isotopic com~sition, or both. If two eminent mixing is assumed to be valid, then two processes, either separately or in conjunction, could account for the observed deviation from the mixing curve. One process is removal of Nd in bottom waters by scavenging. Evidence for Nd removal will be given below. A second process is vertical mixing between bottom waters and PDW into the northward spreading AABW. Both processes could produce the observed distribution in Fig. 6a. The lack of a good fit may also indicate that two component mixing is not a valid assumption. The inferred non-conservative mixing of Nd in Fig. 6a depends critically on the choice of q&O) for the AABW end member. However, for the remaining live points to be close to a conservative mixing line it is necessary that e&O) > -6 for C,, - 25 pmol/kg. In light of the data of PIEPCRASand WASSERBURG(1982) for the AABW we consider this unlikely. Since Nd is not considered to be conservative, it may be useful to compare the isotopic composition of bottom waters to a conservative species. We have chosen silica because there is a large difference in con~ntration between end-members, which is not the case for salinity and temperature. Silica in bottom waters is generally considered to be conservative. It bar. been suggested as an indicator for determining the amount of southern (AABW) component in Atlantic bottom waters (BROECKER,1979). CRAIG et al. ( 1972) observed a “benthic front” in the southwest Pacific and suggested that silica is conservative along this front which separates northward spreading AABW from overlying PDW. Conservative behavior of silica in Pacific bottom waters has been suggested by EDMONDet al. (1979) as well. Because conservative mixing for chemical species is linear, we can su~titute C’s for C,, in equation (3) and construct a mixing curve for Csi versus +&O). This curve is shown in Fig. 6b. We have used C’si = 100 pmol/kg for the AABW end-member and Csi = 180 pmol/kg in the northern endmember. The AABW value we have chosen for Csi may be too low. For example, CRAIG et al. ( 1972) show bottom silica values of 120 pmol/kg in the southwest Pacific. However, if silica is conservative in bottom waters, a lower value is necessary to account for the silica concentration in the South Pacific bottom water sample Mar. Chem. 80, Station 3 1 which has Csi = 106 pmol/kg (Table 1). Lower silica values (95 to 103 pmol/kg) have been reported for the Southeast Pacific and Wendell-Enderby Basin (EDMONDet al., 1979), but it is not clear that they contribute directly to western Pacific waters. In order to be consistent with our data set, we will assume the lower silica in AABW to be correct. When plotted on a Csi versus CN~(O) diagram, the data show an excellent fit to the conservative mixing curve (Fig. 6b). All samples are within analytical error of the curve. This result is sawing, since Fig. 6a clearly indicates that isotopic mixing is not conservative with respect to Nd concentration. The good fit to the silica mixing curve suggests that Nd is proportionately removed from the respective end-members during transport such that the apparent conservative mixing of t&O) is preserved with respect to silica. Removal of Nd relative to Si in the bottom waters is indicated for all sites studied. Fig. 7 shows C,, versus Csi for bottom waters. In all

1379

25 I 90

I, 110

I 130

150

170

190

Csibmob’kgI FIG. 7. CNd versus Cs in Pacific bottom waters. The conservative mixing line is shown connecting end-members used to calculate the mixing curves in Fig. 6a. Note, all of the data lie below the mixing line suggesting removal of Nd relative to Si in the bottom waters.

cases, the data points lie below the combative mixing line shown, indicating the preferential removal of Nd. This evidence for Nd removal and the fit of the isotopic data to mixing proportions based on silica concentrations suggests that near bottom scavenging of REE is the principal reason for the poor fit of C,, and +&O) to the conservative mixing curve in Fig. 6a. Western North Pacific upper waters show two distinct trends. At 47”N (TPS47 39-1, Fig. 4), t&O) decreases from a surface maximum (t&O) = -0.1) through the upper waters until the uniform PDW values (z&O) = -3) are reached at 800 m depth. In contrast, surface waters at 24”N (TPS24 271-l) are more negative (q.,,,(O)= -3.2) and the water column is relatively uniform to a depth of 3 km. Insofar as the subpolar gyre provides the source ofArctic Intermediate Water in the North Pacific, we would expect more radiogenic values to be associated with AIW at the more southerly location in the North Pacific gyre. The lack of an ~~~(0) maximum in AIW at 24*N suggests that vertical mixing in the upper waters has mixed out the northern source. The isotopic differences in the surface waters clearly reflect source differences. The radiogenic surface water in the subpolar gyre must receive injections of REE from the weathering of island arc terranes such as the nearby Kurils and Aleutian Islands. Surface waters at one South Pacific site also have c&O) near 0 indicating a source of radiogenic Nd (Table 2). As in the subpolar gym, there is an abundance of volcanic islands that could provide the source of radiogenic Nd to these surface waters. At 24”N, surface waters are fed by the North Equatorial Current which flows from the eastern Pacific. The relative lack of volcanic island terranes in the path of this water could explain why lower surface water values are observed at Station 27 l- 1. In the western North Atlantic, large isotopic shifts were observed across the base of the thermocline (PIEPGRASand WASSERBURG,1987). The absence of a similar feature in the Pacific reflects fundamental differences in the deep circulation of the Pacific. The isotopic shift in the western North Atlantic probably exists due to the strong northward flow of the Gulf

I380

D.

J. Piepgras and S. B. Jacobsen

Stream over the isotopically distinct waters associated with southward 5owing NADW. In the North Pacific, there is no deep water formation due to the lack of saline waters at high latitudes where winter convection could produce them. Consequently, deep water flow in the Pacific is sluggish compared to the Atlantic and vertical mixing becomes more pronounced. In this respect, the North Pacific, in general, is similar to the eastern North Atlantic, where weaker eastern boundary flow’results in a more uniformly mixed water column with respect to en,,(O)(see PIEPGRASand WASSERBURG, 1983). Other REE, the heavy REE in particular also exhibit bottom water depletions relative to overlying PDW (PIEPGRAS and JACOBSEN,1987). We believe that, as in the case of silica, this feature is best explained by bottom transport of AABW low in REE under PDW higher in REE content. More detailed comparisons of the REE data to Si and other nutrients will be made elsewhere (PIEPGRASand JACO~SSEN, in prep.). REE concentration data provide further evidence of advecl_tivetransport of the REE in the bottom waters of the North Pacific. The Sm and Nd con~ntra~ons shown in Fig. 5 i&&rate this. Both elements indicate depletions in the bottom waters relative to overlying deep waters. This feature is very similar to Si protiles in the Pacific (EDMONDet al., 1979) and have been interpreted as resulting from AABW having lower Csi spreading northward under PDW having higher Csi. Though there is evidence of Nd and Sm removal as illustrated in Fig. 7, the overall similarity to Si profiles suggests a similar process affecting the REE distributions. CONCLUSIONS The Nd isotopic data presented here have demonstrated that the BEE undergo extensive lateral transport in the bottom waters of the Pacific. BEE associated with AABW spreads northward in the Pacific primarily within western boundary currents. They can be detected by their Nd isotopic signature at latitudes up to 47”N. Vertical mixing is probably the major process that acts to dilute the AABW isotopic signature with the more radiogenic PDW. However, vertical mixing rates are not suffioient to mask the effects of horizontal transport in the bottom waters associated with western boundary flow. This finding has interesting implications for the global circulation of deep waters. REID and LYNN ( 197 1) suggested that temperature and salinity properties in the bottom waters ofthe North Pacific had characteristics that originated in the ovetfiows from the Norwegian and Greenland Seas via mixing through the Antarctic circumpolar current. Their study was based on the examination of temperature and salinity along a density stratum that outcrops at the sea surface in the Norwegian and Greenland Seas and extends below 4000 m in the North Pa&c. Their conclusions were based on the assumption that major 5ow and mixing will take place along such surfaces of constant density. Since the deep water formed in the Norwegian and Greenland Seas is characterized by high salinity, this source of deep water can be traced througbout much of the world ocean on the chosen isopycnal surface as a salinity maximum. The Nd isotopic signatures of North Pacific bottom waters also suggest characteristics that originated in the high latitude seas of the North Atlantic. PIEPCRAS

and WASSERBURC~ (1982) showed that t&O) in Antarctic circumpolar waters in the Drake Passage was dominated by Atlantic sources of Nd (50 to 75%) with the remaining Nd coming from Indian and Pacific sources. The primary source of Atlantic waters in Antarctic circumpolar waters is southward flowing NADW. Recently. PIEPGRASand WASSERB~JRG ( 1987) showed that the isotopic signature of NADW originates from mixtures of Labrador Sea water (q&O) z;- .-.18 to - 20) and Norwegian and Greenland seawater (tag = --8). Insofar as a major component of the REE in AABW can be traced back to the Norwegian, Greenland and Labrador Seas. Nd isotopic signatures in the North Pacific associated with AABW provide direct evidence for the North Atlantic origins of REE properties in North Pacific bottom waters. This observation confirms REID and LYNN’S ( 197 I ) suggestion that the Norwegian and Greenland Seas contribute properties to bottom waters in the Pacific and also identifies the importance of the Labrador Sea contribution. The isotopic data here are consistent with Pacilic Deep Water attaining its properties as a result of mixing between bottom waters and overlying waters. REII) ( I98 I) argued that there did not need to be any water entering the North Pacific at mid-depth, rather the temperature and salinity properties could be derived from vertical mixing of deeper and shallower waters. In all of the areas studied in the Pacific. surface waters are more radiogenic than bottom waters. The intermediate values associated with PDW (tXd(0) 2 --3) are consistent with the vertical mixing hypothesis. This is supported by the overall trend of PDW values. PIEPGRASef al. ( 1979) reported Nd isotopic values near ~~(0) = --3.5 for depths associated with PDW in the eastern North Pacific. The more radiogenic values they reported relative to our western basin PDW data are consistent with our observations which indicate less pcnetration of AABW into the eastern basin. In the South Pacific (Table 2) PDW has t&O) = ---4.5 (PIEPCRASand WASSERBURG, 1982). The lower value is consistent with the much greater AABW component in the bottom water at this site. Acknow/edgemen(s-We wish to thank J. Edmond and C. Measures for their assistancein collecting samples from TPS24 sites. L. Talley graciously collected samples for this work from TPS47 sites. We also thank the PACODF personnelfrom Scripps Institute ofOceanography for providing the hydrographic analyses of the samples. We thank H. Staudigel and an anonymous reviewer for helpful comments on the manuscript. This work was supported by National Science Foundation grants OCE-85-00674 and EAR-84-51 172. Edmrial

handling: D. J. DePaolo

REFERENCES APLIN A., MICHARD A. and ALSAR~DE F. (1986) “‘Nd/‘4JNd tn Pacific ferromanganese encrustations and nodules. Earl/r P/arm. sci. Lm. 81.7-14. BROECKERW. S. (1979) A revised estimate for the radiocarbon age of North Atlantic Deep Water. J. Geophys. Rex 84. 32 18-3226. BROECKER W. S. and TAKAHASHI T. (1980) Hydrography of the Central Atlantic---III, The North Atlantic deep-water complex Deep-Sea Res. 27A, 59 I-6 13. CRAIG H., CHUNG Y. and F~ADERO M. (1972) A benthic front in the South Pacific. Earth Planet. So. Left. 16. 50-65. DEPAOLQ D. J. and WASSFXBURGG. J. ( 1976) Nd isotopic variations and petrogenetic models. G’eophw. Res. LetI. 3, 249-252.

Pacific seawater Nd DETRICH

G., KALLE K., KRAuss W. and SIELDERG. (1980) Generaf Oceanography: An Introduction. J. Wiley & Sons, 626~. EDMONDJ. M., CHUNGT. and SCLATERJ. G. (197 1) Pacific bottom water: Penetration east around Hawaii. J. Geophys. Res. 76,80898097. EDMONDJ. M., JACOBSS. S., GORDONA. L., MANTYLAA. W. and WEISS R. F. (1979)Water column anomalies in dissolved silica over opaline pela@c sediments and the origin ofthe deep sea silica maximum. J. Geophys. Res. 84,7809-7826. GOLDSTEINS. J, and JACOESENS. B. (1987) The Nd and Sr isotopic systematics of river water dissolved material: Implications for the sources of Nd and Sr in seawater. Chem. Geoi. Isotope Geoscience Secfion 66-245-272. GOLDSTEINS. L. and O’NIONS R. K. (1981) Nd and Sr isotopic relatio~hips in pelagic clays and fe~ornang~~ deposits. Nature 2X$324-327. JACOBSENS. B. and WASSERBURGG. J. (1984) Sm-Nd isotopic evolution of chondrites and achondrites, II. Earth Planet. Sci. Leti. 67, 137-150. PICKARDG. L. ( 1979) Descriptive Physical Oceanography. Pergamon Press, 233~. PIEPGRASD. J. and JACOBSENS. B. (1987) Rare earth elements and Nd isotopes in the N. Pacific (abstr.). Eos 68,470. PIEPGRASD. J. and WASSERBURG G. J. (1980) Neodymium isotopic variations in seawater. Earth Planet. Sci. Lett. 50, 128-l 38. PIEPGRASD. J. and WASSERBURGG. J. (1982) The isotopic composition of neodymium in waters flowing through the Drake Passage. Science 217,207-214. PIEPGRASD. J. and WASSERBURG G. J. (1983) Influence ofthe Mediterranean outflow on the isotopic commotion of n~yminm in waters of the North Atlantic. f. Geophys. Res. 88, 5997-6006. PIEPGRASD. J. and WASSERBURG G. J. (1985) Strontium and neo-

1381

dymium isotopes in hot springs on the East PacificRise and Guaymas Basin. Earth Planet. Sci. Left. 72, 341-356. PIEERAS D. J. and WASSERBLJRG G. J. (1987) Rare earth element transport in the Western North Atlantic inferred from Nd isotopic observations. Geochim. Cosmochim. Acta 51, 1257- 127 1. PIEFGRASD. J., WASSERBURGG. J. and DASCH E. J. (1979) The isotopic composition of Nd in different ocean masses. Earth Planet. Sci. Lett. 45, 223-236. F’IEFGRASD. J., JACOBSEN S. B. and SHOLKOVITZE. R. (1986) REE in pore waters of western N. Atlantic sediment (abstr.). Eos 67, 1008. REID J. L. (I 98 1) On the deep circulation of the Pacific Ocean, In Proceedings of a Workshop on Physical Oceanography Related fo the Seabed Disposal o~~igh-Ievel Nu~Iear Waste. (ed. M. G. MAR. IETTA), #~nd8~ 1776, pp. 144-184. Sandia National Lab. Publ. REID J. L. and LONSDALEP. F. ( 1974) On the flow of water through the Samoan Passage. J. Phys. Oceanogr. 4, 58-73. REIDJ. L. and LYNNR. J. (197 1) On the influence ofthe NorweluanGreenland and Weddell Seas upon the bottom waters of the Indian and Pacific Oceans. Deep-Sea Res. 18, 1063-1088. STAUDIGELH., DOYLE P. and ZINDLERA. (1985) Sr and Nd isotope systematics in fish teeth. Earth Planet. Sci. Lett. 76, 45-56. STORDALM. C. and WASSERBURG G. J. (1986) Neodymium isotopic study of Baffin Bay water: Sources of REE from very old tenanes. Earth Planet. Sci. Lett. 77, 259-272. WARREN B. A. (1973) Transpacific hydrographic sections of Lats 43% and 28“s: The SCORPIO expedition, II, Deep water. Deep Sea Res. 20,209-238. W~~ERBURG G. J., JACOBSENS. B., DEPAOLOD. J., MCCULLOCH M. T. and WEN T. (198 1) Precise dete~ination of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochim. Cosmochim. Acta 45,23 1l-2323.