Rare earth element transport in the western North Atlantic inferred from Nd isotopic observations

Geochrmrca d Cosmochrnrca Ana Vol. 51, pp. 1257-127 Q Pergamon Journals Ltd. 1987. Printed m U.S.A.

I

0016.7037/87/$3.00

+ .OO

Rare earth element transport in the western North Atlantic inferred from Nd isotopic observations* D. J. PIEPGRAS’ and G. J. WASSERBURG The Lunatic Asylum of the Charles Arms Laboratory, Division of Geological and Planetary Sciences. California Institute of Technology, Pasadena, CA 91125, U.S.A. (Received May 5, 1986; accepted in revisedform February 13, 1987)

Abstract-The isotopic composition of Nd in the water column from several western North Atlantic sites and formational areas for North Atlantic Deep Water shows extensive vertical structure at all locations. In regions where a thermocline is well-developed, large isotopic shifts (2 to 3 c units) are observed across the base of the thermocline. Regions without a thermocline are characterized by much more gradual shifts in isotopic composition with depth. In general, the data reveal an excellent correlation between the Nd isotopic distribution in the western North Atlantic water column and the distribution of water masses identified from temperature and salinity characteristics. NADW, as identified from T-S properties, is also characterized by a well-defined isotopic composition having tNd(0) = - 13.5 f 0.5. This signature is associated with waters identified as NADW from high latitudes near formational areas in the Labrador Sea down to the equatorial region. The isotopic signature of NADW would appear to be formed by a blend of more negative waters originating in the Labrador Sea (~~~(0)< - 18) and more positive waters originating in the overflows from the Norwegian and Greenland Seas (~~~(0)= -8 to -10) and is consistent with classical theories on the formation of NADW. The isotopic signature of NADW is propagated southward to the equator where it is gradually being thinned out by mixing from above and below with more radiogenic Nd associated with northward-spreading Antarctic Intermediate and Bottom Waters. The preservation of the isotopic signature of NADW over these large distances indicate that the REE undergo extensive lateral transport. The isotopic composition of Nd is largely conservative over the time scales of mixing within the Atlantic in spite of the intrinsic nonconservative behavior of neodymium. Nd concentration gradients generally show surface waters to be depleted in Nd relative to deep waters, which must require vertical transport processes. However, isotopic differences in the water column preclude the local downward transport of REE from the surface into underlying deep waters as a simple explanation of the concentration gradient. The apparent decoupling of REE in NADW from overlying (local) surface waters and the increasing concentration with depth provide a conflict with simple vertical transport mechanisms that is not yet resolved. INTRODUUION THE “‘Nd/‘44Nd ratio varies with geologic time due to the radioactive decay of 14’Sm (tllz = 1.06 X 10” years). In the marine environment, the isotopic composition of Nd is fixed by the source(s) of REE and is not affected by isotopic fractionation. This permits the distinction between sources which cannot be determined from concentration measurements. The major objectives of this work are to identify the broader characteristics of the Nd isotopic distribution in the Atlantic as they relate to the sources and transport of the REE in this ocean basin. Therefore, we want to establish the relationship between the Nd isotopic compositions of these waters with the origin and circulation of water masses identified on the basis of temperature and salinity observations. To meet these objectives, samples were collected in the western North Atlantic between 7”N and 54”N to provide broad meridional coverage. The samples include waters representative of major water masses in the North Atlantic including southward-flowing North Atlantic Deep Water (NADW) and northward-flowing Antarctic Bottom Water (AABW). In addition, surface samples were collected on a tran-

* Division Contribution No. 4 118 (49 1). ’ Present address: Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA 02 138, U.S.A.

sect across the Atlantic at 36”N to the mouth of the Mediterranean. Previous studies of Nd in the western North Atlantic near 30”N (OCE 63) have shown the water column in this region to be stratified with respect to Nd isotopic composition (PIEPGRAS and WASSERBURG, 1980, 1983). The measured ‘43Nd/‘44Nd ratio (represented here as q+,(O)) was found to decrease with depth through the thermocline from a surface maximum of -9.5 to a minimum of CNd(O)= -13Satthe hd(O) = base of the thermocline (- 1 km). Below 1 km, the waters are entirely associated with NADW and tNd(0) remains fairly uniform to the ocean bottom (- 5 km). This suggests that NADW has a characteristic Nd isotopic composition. The isotopic differences in the water column indicate that the source of REE in the present surface waters is different from that supplying the underlying deep waters. The concentration of Nd at this station (OCE 63) was found to increase regularly with depth and appears to be a regular feature of Nd and the other rare earths in the oceans (PIEPGRAS and WASSERBURG, 1982, 1983; ELDERFIELD and GREAVES, 1982; KLINKHAMMER et al., 1983; DE BAARet al., 1983, 1985). If the deep water enrichment was due to the downward transport of Nd from the overlying surface waters (without other inputs), we would expect the deep water isotopic signature to reflect the surface source. The clear 1257

1258

D. 1. Piepgras and G. J. Wasserhurg

difference in Q*(O) between surface and deep water indicates that the regular increase in concentration of Nd with depth cannot simply be accounted for by resolution of Nd scavenged from the present surface waters into the deep waters. The high Nd concentrations must be supplied either by dissolved Nd transported in bottom currents, by injection of Nd remobilized in underlying deep-sea sediments or by transport from surface layers with a different t&O) than presently observed. The source of relatively nonradiogenic Nd in the deep waters associated with NADW (C&O) = - 13.5) in the western North Atlantic has not been unequivocally identified, but requires a substantial contribution from old, continentally derived Nd. Studies of Nd in the “dissolved” load of the Mississippi and Amazon Rivers (STORDAL and WASSERBURG, 1986) and in the suspended and “dissolved” loads of numerous other rivers (GOLDSTEIN et ul.. 1984; GOLDSTEIN and JACOBSEN, 1986) indicate that most major drainages are characterized by tNd(0) from -8 to - 1 1. These values are too radiogenic to provide the source of all the Nd in the deep waters but may provide the source in some surface waters. Data for some bulk sediments in the Atlantic (GOLDSTEIN and O'NIONS, 198 1) indicate that they have Nd which is also too radiogenic to be the source of the deep waters. We suggested earlier (PIEPGRAS and WASSERBURG, 1983) that the Nd with low values of eNd(0) which contribute to NADW may be derived from runoff from ancient Precambrian terranes into the Arctic seas as North Atlantic Deep Water is formed by the sinking of cold surface waters in this region. A recent study of Baffin Bay shows the Nd in these waters to be dominated by Archean sources with tNd(0) as low as -25 (STORDAL and WASSERBURC;. 1986). These workers have shown that the Nd flux from Baffin Bay could contribute -30% of the nonradiogenic component of Nd to NADW. The other sources contributing Nd are not fully identified and it must be demonstrated whether NADW can be associated with a particular isotopic composition. The location and mechanisms by which various water masses are mixed to form NADW is a matter of active study (see SWIFT, 1984). If NADW has a clearly identifiable Nd isotopic signature, it should be possible to identify interactions of NADW with the northward transport of REE in Antarctic Bottom Water (AABW) and Intermediate Water (AAIW). These Antarctic water masses have C&O) = -9 in their purest form (PIEPGRAS and WASSERBURG, 1982) which is distinguishable from the values we associate with NADW. Insofar as e&O) can be shown to be strongly coupled with the major Atlantic water masses, we would have to infer that both the water masses and Nd have comparable lifetimes for mixing and Nd precipitation. In this paper, we present new data to address the above problems. In conjunction with the study of Baffin Bay (STORDAL and WASSERBURC;, 1986). we present strong evidence for a northerly source for the Nd iso-

topic signature of NADW. We will show thar there is a close correlation between tNd(0) and T-S properties in the water column and that this is consistent with the REE transport being dominated by the known circulation of major Atlantic water masses. The problems associated with the vertical transport processes of the REE will be discussed in the context ofthe Nd isotopic results.

Figure i summarizes the major water masses ot the western basin of the Atlantic and the general flow pattern. In addition to the warm water sphere, three major water masses dominate in this basin: northward-spreading Antarctic Intermediate Water (AAIW); Antarctic Bottom Water (AABW): and southward-spreading NADW. AABW is formed primarily ln the Weddell Sea and is a mixture of Shelf Water and Antarctic Circumpolar Water (PICKARD, 1979). This mass is fairly homogeneous, characterized in its purest form by restricted values 34.67L of temperature and salinity near @= 0°C and S (BROECKERand TAKAHASHI,1980). AABW spreads northward in the western basin beneath the southward-spreading NADW with which it mixes. Although highly diluted. AABW can he traced from its high silica content as far north as 45”N (BROECKER,1979). Antarctic Intermediate Water also forms with relatively restricted temperature and salinit! values tn regions just south of the Antarctic polar front. 1his water mass forms at the sea surface and is characterized by low salinity. In its purest form, Antarctic lntermediatc Water has temperatures near 0 = 1 to 2°C and S * 33.8%~(DILIKI(‘H PI ul.. 1980). This water sinks to a depth of -900 meters and spreads northward, mixing with overlying surface waters and underlying NADW and is recognized by the salinity minimum at this depth as far north as 25”N. In contrast to the restricted compositions ot AAIW and AABW. NADW is characterized by a fairly wide range 01 temperature and salinity exhibiting considerable structure with depth. The origin of this structure in NADW is not clearl) understood. The temperatures and salinities of NADW plot along an approximately linear segment on a H-S diagram ranging from 0 = 6°C and S =: 35.1%0 near the top of this water mass down to values near 2.O”C and 34.895r near the bottom. In subsequent discussion we will refer to water masses which have this full range of 0-S characteristics as being matrrrc’ NADW. This occurs principally in mid-latitudes where there is a well-developed thermocline. During its southward flow mature NADW will gradually mix with less saline Antarctic water masses which are flowing northward. We will define this as modified NADW which can be recognized by its departure from the B-S charactiristics of mature NADW as defined above. At higher latitudes of the North Atlantic, the formational areas of NADW are approached. In these regions. major contributing sources come together and many of the T-S characteristics of mature NADW are mixed in. We will refer to this newly formed water mass as profo NADW. The structure and wide range in 8-S values associated with NADW indicate that it is composed of a mixture of several components. The major components include the overflows from the Norwegian and Greenland Seas over the IcelandScotland Ridge and the Denmark Strait, the Labrador Sea Water, and the Mediterranean outflow. These components oi NADW are each characterized by narrowly defined ranges 01 temperature and salinity. According to SWIFT(19845, the Iceland-Scotland overflow (0 = -0.5”C. S s 34.91%0)originates in the Norwegian Sea and enters the eastern basin where ir mixes with the Northeast Atlantic Water (0 8°C‘. S u 35.25%). This mixture enters the western basin through the Gibbs Fracture Zone near 53”N having 8 .= 1.8" to 3.O”C and S = 34.96 to 34.987~. The contribution from the Denmark Strait overflow originates in the Greenland-Iceland Seas

REE transport

5000 m W-WARM WATER SPHERE NADW‘NORTH

ATLANTIC DEEP WATER

in the Atlantic

Ocean

AIW= ARCTIC INTERMEDIATE WATER ABW=

ARCTIC BOTTOM WATER

1259

AAIW:

ANTARCTIC INTERMEDIATE WATER AABW=ANTARCTlC

BOTTOM

WATER

FIG. 1. Cross section of the western Atlantic Ocean showing the principal water masses and their general direction of flow (after DIETRICH et al., 1980). Approximate sampling locations for this study are indicated by vertical dashed lines.

and has temperatures near 0°C and salinities near 34.9%0 (SWIFT, 1984). This overflow is identified as Arctic Bottom Water (ABW) in Fig. 1. These sources from the Denmark Strait and Iceland-Scotland overflows mix to provide the relatively well-oxygenated lower NADW (DIETRICH et al., 1980). Another major component is the Labrador Sea Water. CLARKE and GASCARD (1983)identified Labrador Sea Water as having 0 = 2.9”C and S = 34.84%0. In its purest form, the Labrador Sea Water is the major source of Arctic Intermediate Water (AIW) shown in Fig. 1. After sinking and mixing with other NADW sources, the low salinity Labrador Sea Water component accounts for the salinity minimum associated with middle NADW. Further south, the Mediterranean outflow (0 k 2O”C, S Z 38%0) sinks to a depth of - 1000 meters in the eastern basin and spreads southwest across the Atlantic and accounts for the relatively high temperatures (0 = 4 to 6°C) and the salinity maximum (S = 35.0 to 35.1%0) associated with upper NADW at latitudes below 35”N. BROECKER and TAKAHASHI (1980) adopted a stricter definition of NADW to include only those waters with properties derived from the northern sources. They define the lower boundary to be the 2°C discontinuity. Below this temperature, they consider the waters to include admixtures of AABW with overlying NADW. Their upper boundary lies near the 4°C isotherm at the dissolved silica minimum at a depth of - 1.8 km at 42”N. Above this depth is a dissolved silica maximum associated with Antarctic sources. The salinity minimum associated with Antarctic Intermediate Water is not propagated above about 25”N, so we will use the broader classification of mature NADW defined above. This includes the higher temperatures and salinities imparted by the Mediterranean outflow at the upper boundary. SAMPLING The sampling sites are shown in Fig. 2 and listed in Table 1 along with the associated water masses and cruises. Temperature, salinity and nutrients were measured at each site to properly identify water masses. Samples were collected of representative waters from the major sources which are believed to mix to form NADW. These include samples from the Faeroe Channel and Denmark Strait overflows (and their presumed sources in the Norwegian and Greenland Seas) collected during the Transient Tracers in the Oceans North Atlantic Study (TTO/NAS). Labrador Sea Water was collected during the CSS Hudson 83-036 cruise in the southeastern Labrador Sea and includes samples representative of newly formed NADW. To establish the distribution of Nd isotopes, profiles of the water column were collected at several more southerly locations. The water column was sampled near 36”N during cruise 109 of the RV Atlantis II. At this latitude, NADW is fully matured and occupies the entire water column below the

thermocline (see Fig. 1). Another mid-latitude profile (OCE 63; see PIEPGRAS and WASSERBURG, 1980) near 28”N is also reviewed here. Finally, a profile was collected during the TTO Tropical Atlantic Study (TTO/TAS) near 7”N in a region where NADW is observed to lie at depths between intermediate and deep water masses of Antarctic origin. Surface samples were collected at most of the above locations. In addition, surface samples were collected during cruise 109 of the RV Atlantis II from an east-west transect along 36”N to identify variations in sources of REE in the sea surface and their relationship to North Atlantic surface circulation, METHODS

AND

ANALYSIS

Unfiltered samples of seawater were collected either in tefIon-coated GO-FL0 or Niskin bottles with teflon-coated springs. In most cases, sampling was performed during a hydrocast immediately following a CTD cast. The T-S data from the CTD cast were used to select sampling depths for the hydrocast and the bottles were positioned accordingly on a stainless steel hydrowire. During the TTO/TAS expedition, GO-FL0 bottles were mounted on a 24-position rosette containing a CTD. Surface samples were collected from the bow of the ship with a plastic bucket immediately upon arriving on station and before the ship came to rest in order to minimize contamination. After collection, samples were transferred to lo-liter polyethylene containers for storage and acidified with ultrapure 10N HCl to pH - 2. Samples were drawn for determination of salinity, oxygen and nutrients. The rare earths were extracted using two different methods. In most cases, after return to the laboratory, the acidified samples were spiked with 14’Sm and 15’Nd tracers and 75 mg Fe and allowed to equilibrate. The REE were then extracted by co-precipitation with Fe (OH), with yields >95% (see PtEPGRaS and WASSERBURG, 1980). The second method was a shipboard extraction procedure used on the TTO/TAS expedition. In this procedure, the REE were extracted from seawater by ionexchange techniques onto a chelating resin using a procedure modified after that originally developed by DE BAAR (1983). Biorad Chelex 100 (100-200 mesh) resin was packed to a depth of 7 cm (Hz0 adjusted) in individual 1.5 cm plastic columns. Four liter subsamples of seawater at ambient pH (unacidified) were spiked on board ship immediately after collection with preweighed Sm and Nd tracers. After allowing about twenty-four hours for equilibration, the samples were drawn through the Chelex columns using a peristaltic pump downstream of the columns. The ends of the columns were then sealed to prevent drying. The REE were later eluted from the resin using 4N HNO,. Yields for this procedure were >90%. Comparisons of the two procedures were made for several of the TTO/TAS samples. After extraction of the REE, Sm and Nd were separated (PIEPGRAS and WASSERBURG,

D. J. Piepgras and G. .I. Wasserburg

1260

FIG. 2. Locations of sampling sites for this study. Cruise designations and corresponding station numbers are indicated. The solid line indicates the stations used to create the cross section in Fig 12.

1980). Total procedural blanks for the chemical separation of Nd were typically 150 picograms. The isotopic composition and concentration of Nd was measured with the Lunatic I mass spectrometer on samples of 10 to 20 nanograms Nd. Neodymium was analyzed as NdO+ at typical ion beam currents of 4 to 8 X 1O-l* amp (see PIEPGRASand WASSERBURC 1980, 1985). RESULTS Salinity, dissolved oxygen and nutrient data for most samples are listed in Table 2. Results of Nd isotopic and concentration measurements are given in Table 3. In addition, 8-S curves are shown for all stations for which vertical profiles have been measured for Nd. These are discussed for each station along with the Nd results. We first present the results on the two stations chosen to sample “mature” NADW. After establishing the isotopic characteristics of this water mass, we will examine its modification during southward flow and then present the data on stations representing water masses that can contribute to NADW. North Atlantic Deep Wafer (NAD W) A-II 109-I Station 30. The 8-S curve is shown in Fig. 3. The points labeled A, B, C, etc. on this and all subsequent 0S diagrams correspond to positions where a large change in slope of 8 vs. S occur and represent the approximate compositions of the various end member components which were mixed to produce these curves. The segment of the B-S curve between points A and B correspond to the top 5 meters of the water column. The region between points B (0 = 23°C. S ;5 36.2461)and C (0 = 18°C S x 36.5%, depth c 200 m) represents the seasonal thermocline. The fairly linear segment between points C and D (0 = 7”C, S c 35.12%0, depth = 950 m) corresponds to waters within the main thermocline. Below

point D, 0-S remains fairly linear to the ocean bottom at point E(8 z=z1.84”C S Y 34.89%0, depth = 4850 m). and has flS properties representative ofNADW as defined earlier. Some structure in the deep waters clearly exists as indicated by the kinks in the O-Scurve in this region. The slight change in slope near point E toward lower temperature and salinity corresponds to the two-degree discontinuity (BROECKERand ‘TAKAHASHI, 1980) and indicates that a very dilute component of AABW may be present. Nutrients are depleted in the surface waters and mcreasc to a maximum near 800 m for PO:- and NO; and near I 100 m for Si02. Below these maxima, PO:- and NO; remain fairly constant with depth to the bottom, while Si02 continues to increase substantially to a bottom water value of 34.6 pmol/ kg. This high SiOl concentration is also consistent with a very dilute component of AABW in the bottom waters. Dissolved oxygen exhibits a minimum (166 Fmol/kg) at depths corresponding to the PO!- and NOT maxima and has fairly uniform high values (-265 pmol/kg) in the deep waters. These results are fairly typical of nutrient and oxygen profiles in the western North Atlantic and will not be described for remaining stations. Results of Nd concentrations (CM) are shown in Fig. 4. There is a small maximum of C,, at the surface. but C,, is generally lowest in the near-surface waters and increases with depth. There is a relatively large increase in C,, between 800 and 1100 m corresponding to the interval across the base of the thermocline at point D on the 0-S curve (Fig. 3). C,, gradually increases with depth between 1100 and 3000 m. Below 3000 m, C,, changes more rapidly and there is a very large increase at the bottom. The general trend of increasing C,, with depth is similar to that reported elsewhere (PIEffiR4S and WASSEFCBURG,1982, 1983; ELDERFIELDand GREAVES, 1982; DE BAAR et al., 1983; KLINKHAMMER et al.. 1983; Dr: BAAR et al., 1985). Figure 5 shows c&O) as a function of depth. e,,(O) is most radiogenic in the near-surface waters of the seasonal ther-

REE transport in the Atlantic Ocean Teble

1.

Cruise

Sampling

and Station

locations

and water

Location

masses represented. Water fiaeeee Sampled

Labrador

Current

Hudson 83-036 LC

54’29’41” 56’19’24”

“udeon 83-036 Station 9

54014’51” N 52’07’24” W

Surface wete?z, Labrador Sea Water. North Atlantic Deep Water

Hudson 83-036 Station 11

52’05’50” 47~Ol’lS”

Surface wlte=, Labrador Sea Water North Atlantic Deep Water

TTOlNAS

142

61’21’ 8’01’

144

67’40’ N 3017’ w

Surface water, Norwegian Sea Deep Water

76’53’ N 1002’ E

Greenland

149 167

64’03’ 33’20’

Lsbrador Sea Weter, Denmark Strait Overflow

109-l station

30

36’15’38” 61”58’27”

AII 109-l stetion

36O13’00” N 5Z007’12” W

Surface

veter

39

109-I station

36’15’49” 19’57’27”

N w

Surface

“ate=

79

AII 109-L station 95

36’17’41” 10’02’27”

N W

Surface

“ate=

OCS 63 station

29°53’oo” 76’14’12”

N

nlermocline

1

OCE 63 station

27’57’14” N 7OO23’25” W

North Atlantic

2

OCE 63 station

27’01’42” N 74920’00” W

Surface

3

OCE 63 station

27°06’30” 74’21’00”

N W

North Atlantic

4

TTOITAS station

1°02’4S” 49’41’42”

N w

Am.¶zonRiver

44

TI‘O/TAS Stetion

63

7044’ N 40°42’ W

Stetian TTO/NAS

station TTOINAS TTOINAS Station AI1

AI1

N W

N W

Faeroe

N w

N W N W

Channel Overflow

Sea Deep Water

Surface “*teT, nlermocline weter, North Atlantic Deep water

“eter

W Deep Water

“ete= Deep Wafer

Surface water, TlIermocline w*te=, Antarctic Intel-mediate water, North Atlantic Deep water. Antarctic Sottom weter‘

mocline, having uniform ~~~(0)= -9.5. Below this, in the main thermocline, there is a small shift of - 1 e unit toward less radiogenic Nd which remains rather uniform in this region with ++,(O) = -10.5. Below 1 km, t&O) = - 13.5 and the isotopic composition remains uniform within analytical error to the ocean bottom. The deep water samples below 1000 m lie within the region identified as NADW (see segment DE in Fig. 3) and have tNd(0) within the range previously inferred for this water mass. Two important observations can be made from this isotopic distribution. First, there is clear evidence of stratification of C&O) above and below the thermocline. Second, there is a sharp jump in concentration at the same level (see inset, Fig. 4). The concentration changes cannot be due to mixing of component C with D, as tNd(0) and C,, should then change smoothly with depth rather than exhibiting large shifts. There is a good correlation between the isotopic shifts and the characteristics inferred from 0-S. ~~~(0)is quite uniform within regions of linear 8-S, suggesting a close association between isotopic compositions and water masses. OCE 63. The Nd data from several closely spaced stations in the western Sargasso Sea (PIEPGRAS and WASSERBURG, 1980) are reviewed here. We show a O-Sdiagram in Fig. 6 for Crawford 17 Station 3 16 (FUGLISTER, 1960) which is somewhat east of the general sampling site but should be representative. Using the convention in Fig. 3, point C (0 = 19°C. S = 36.58%0,depth = 100 m) represents the base of the seasonal thermocline. Temperature and salinity values between points C and D (19= 6”C, S z 36.05%0,depth = 1000 m) correspond to the main thermocline. Below the thermocline at point D’ (0 = 4.6”C, S = 35.07%0, depth o 1400 m) lies the salinity maximum imparted by the Mediterranean outflow and associated with upper NADW. The linear segment of 0-S between

1261

points D’ and E ((I = 2°C S z 34.91%0, depth = 4100 m) corresponds to typical values of NADW. At point E, there is a clear shift in slope of 0-S toward lower temperatures and salinities. This corresponds to the two-degree discontinuity of BROECKER and TAKAHASHI (1980) and depths below this to E’ (19zz 1.7”C, s = 34.87%0,depth = 5 100 m) at the ocean

D. J. Piepgras and G. J. Wasserburg

1262 Table

3.

Results

of

Nd isotwic

Depth

Table

and

meadu~ements.

concentration CNd

lwq,p CNd(0)b (pmol/kg) ‘bSpJd, --__-

(m) Hudson

83-036,

100

Labrador

12.03

83-036,

5

Station

24.96

0.511111

20.04

83-036,

5

Station

21.07

AI1

“.511040

19.2”

.I 5.4

0.511”52 tie

13.87

OCS 63,

t0.3

Station

22””

340”

-15.8

“CE

15.5 f0.4

63,

“.5,,11”

Scation

sl.4

63,

Station

TTOITAS,

Station

Ib.7,

0.511142

3000

17.33

“.5111bi

125

0.511160 f?-!

TTOINAS, 65

Station

0.511455 t29

22.05

“.5llO68

20”

15.25

13.511204

390

15.46

i,.51127?

59”

14.77

“.5ii24”

790

15.88

iJ.5ii24”

9R”

16.22

O.iii2Oi

,990

17.33

C1.Sirib4

(Fe-pprj

17.54

11.51114’1

291”

18.37

“.511203

369”

25.72

0.51121”

(Fe-ppr)

27.18

0.511193 i?O

t27

to.4

(Fe-ppt)

3750

0.511321

*I8

16.29

TTO/NAS, 2800

Station

--13.3

*20

l“.4 -13.4

t2R

l.5

+zlA

--7.7 t” .6

iF2R

*1+

-10.3

t25

f0.5

0.511359 i24

-9.5

t19

fO.5

f20

149

16.78

0.511299

+,a

-.10 . 7

t?h

9.4

TTO/NAS, Station 167 840 2310

AII

109-l. 5

200

16.5”

“.511124

fo.4 -8.6 f0.5

4280

26.48

20.59

Lt2” 0.511408 ~24

11.511231 t22

481”

30.09

li.511243

14.42

0.511361 t23

-9.5

(Fe-ppt)

31.68

11.511252

13.59

0.511365

-9.4

station

14.63

0.51?3”3

14.63

0.511318 f27

‘%d/144Nd

t.27

t0.5 -14.0 ro.4

1800

18.44

0.511157 i22

-13.5 f0.4

3000

18.86

0.511180 f13

-13.0 fO.3

4000

26.34

0.511161 t23

--I).4 to.4

4850

62.46

0.511153 fib

.-I 3.6 fO.3

-10.9

39

l27

44,

Amazon

-9.9 fO.5

Riverd S. .

“.511394 t”

f26

a Isotopic

0.511128 f21

0.511342

373.66

t0.5

IS.02

station

0

-10.3

1100

7.90

Station

-1” .6 f0.4

0.511288

109-I.

TTOITAS,

*0.4

15.25

5

*22

M.4

8””

AII

I

tlh

f2i 600

-14.

30

t20 400

I

t2

t0.5

144

14.28

extraction)

-13.R

142

2L.35

750

(Chelex

-14.1

t0.4

*20

63

“.>:I136

-13.4

0.5111bS

-1,. +,I.

18.23

0

250”

4 0.5ii146

f-O.4

*2”

-o., f” .,
n.511351 *47

131

-14.4

0.511124

I >.I ‘

il!

3

_-

50

4 1””

t19

*u..

!!.111161

22.1,

“CE

TTOINAS , Station

-;;,

f0.5

-14.7

19.41

to.’

f??

0.5ilO96

3850

-,(i.‘t

“.‘ii,,74

HI.5

18.“9

le.23

*it

2

17.82

1000

3500

-ii.,*

0.511287

__

,000

-15.1 io.4

17.6S

0.5112b4

t30

0.511073 221

2000

95

tO.6

IS.16

18.09

Station 12.48

300

-14.4

800

1500

Li’.

OCE b3, station 1

Kl.3

-11.9

t26 500

109-1,

:

-tel.’

f22

“.51”933

21.7”

“.51131”

1,

i26 125

‘Ndf iI

*20

-1j.R

0.51?14i t17

Hudson

i*YNd

___-___

t3,3

f29 2550

“%d, 79

9.29

0

0.511059

18.23

5

9

116 1200

%d

(pmOl/kg) - Cm) AII 109-1, station

Current 0.510510 izn

Hudson

s

3 (continued)

Depth

romposltions -

are

normalized

/)

t,

1.138305.

Reported errore 20 of the mean. concentrations “ere determined on samples spiked with 15%d typical errors (ore less than “.5%. ENI

POT TTOITAS

from

z

[

b ’

the

lrsNd)sample 1]

.511847 Str,tir,n 63,

samples

All semples separates. were also separates discussion,

( lb3Nd/

by

two

RSE were different

.

methods.

from ehelex gome ena1yses

performed on Fe end are Indicated. see text.

precipitation For further

results ere was filtered filter.

I//.

extracted

were analyzed POT comppariaon,

d The Amazon River liter sample which 0.45 “rn Nlxleopore

il~r iln.i

for a onethrough a

REE transport in the Atlantic Ocean

1263 CNd(x10-12#$

(TTCVTASsh 63) A~--__~~

I (A-II 109-l Stn 30) A

S0l.PPO )

.E,

0'

34.5

35.0

35.5

Salinity

36.0

b&J

36.5

0 A-I] 109-TSin30 oOCE 63

37.c

a) Potential temperature vs. salinity for A-II 109-l Station 30 (solid curve). On this and subsequent B-Sdiagrams, the position of major changes in the slope of 0 vs. S are indicated by the letters and their approximate depth in the water column is shown. In general, these correspond to boundaries between different water masses. The segment of the curve between points C and D represents the main thermocline waters. The region between points D and E is representative of NADW. b) Potential temperature vs. salinity at TTO/TAS Station 63 (dashed curve). The core of AAIW is clearly shown by the salinity minimum at D”. The core of NADW is indicated by point D. The two-degree discontinuity is located near point E. Below this, there is a significant contribution to 0 and S from AABW. Inset shows expanded scale of 8-S for the region D” to E’ for TTO/TAS Station 63. Distinct curvature of 0 vs. S can be seen in the region D to E’ due to mixing of AABW with NADW.

I

IO

1

I

20

30

I

I

I

40

50

60

-E

CNd (pmolhg)

FIG. 4. a) Concentration of Nd as a function of depth for A-II 109-l Station 30 (solid circles). The inset shows an expanded concentration scale for CNdin the upper 3 km of the water column. A sharp increase in concentration is observed between 800 and 1100 meters, which is coincident with the base of the thermocline. The depths labeled at the right hand side of the figure correspond to the depths labeled on the 0-S diagram in Fig. 3 for A-II 109-l Station 30. b) C,, vs. depth for OCE 63 (open circles).

015

43

bottom are interpreted to contain admixtures of AABW and C NADW. These data indicate that this more southerly station differs from A-II 109-l Station 30 by the presence of a recognizable Mediterranean outflow component in the upper D lNADW and a dilute but more recognizable AABW component. I C,, has been measured for only three depths (Fig. 4) and I exhibits an increase with depth. The profile of++,(O) is shown 2t in Fig. 5. One sample from the seasonal thermocline has t&O) = -9.6, similar to corresponding samples from the 36”N station. Also, one sample from the main thermocline has ~~~(0) = - 10.9. Aithough this result is within analytical error of the 3mixed layer sample, the value lies in the direction associated with the small range of thermocline values of -10.5 from 36”N (Fig. 5). The deep waters at and below the base of the I thermocline (point D) are distinct from the above lying waters 4with a clear shift of -2 t units. All samples within the region II-A-II 109-tSin 30 DDE are identified as NADW and have rather uniform ~~~(0) o---OCE 63 = - 13.4. These values are entirely within the range of values dwe associated with NADW from the 36”N station. In general, -E 5this isotopic profile is congruent with that from 36”N, maintaining a good correlation to 0-S features consistent with the FIG. 5. a) ~~~(0)as a function of depth for A-II 109-1 Station gross similarities of the hydrography of these two sites. 30 (solid squares). Note the large shift across the base of the The Nd isotopic signature of NAD W. At mid-latitudes (28’ thermocline (point D) and the very uniform eNd(0)values asto 36”N) in the western North Atlantic, NADW has acquired sociated with NADW. Error bars are the 20 errors on the all of its temperature and salinity characteristics, indicating isotopic measurements (Table 3). b) tNd(0) as a function of that it is fully matured in this region. It occupies essentially depth for OCE 63 (open circles). Once again, note the very the entire water column below the base of the thermocline uniform values for NADW below point D and the large isotopic difference from overlying thermocline and surface waters. (- 1000 m) and has only a very minor Antarctic component

T-i

4

I

/

I

I

I

I

D. J. Piepgras and G. J. Wasserburg

1264 SALINITY

(“/cd

FIG. 6. Temperature VS.salinity for Crawford Station 3 17 in the Sargasso Sea (data are from FUGLISTER, 1960) and is taken to be representative of T-S at the OCE 63 sampling sites (Fig. 2). Thermocline waters are in the region CD. The salinity maximum at D’ corresponds to a Mediterranean component and marks the upper boundary of NADW which lies between DE. A shift in T-S below E indicates the presence of dilute AABW.

in the bottom waters below 4000 m. The results presented above for two stations (Fig. 5) indicate that NADW in its mature form has a uniform and characteristic isotopic composition of end(O)= - 13.5 -+ OS. The 8-S characteristics in this region (Figs. 3 and 6) show NADW to be dominated by an effective, two-component mixture and that the water mass called NADW is an inhomogeneous blend of these components. As Q+,(O)is uniform, this indicates that the sources of REE contributing to these two effective components of NADW are the same within I c unit. C,, exhibits a regular increase with depth within NADW (Fig. 4), but is not correlated linearly with 0 and thus does not appear compatible with a simple. two-component mixing model. This indicates that vertical REE transport involving particles or sols must be a fundamental issue of the overall REE transport problem. There is a rather abrupt change in Z&O) and C,, at the region of the overlying thermocline. With regard to the isotopic composition of Nd associated with mature NADW. the results above show it to be much less radiogenic than overlying waters in this region.

continuity and marks the lower boundary of NADW. Below this are cooler temperatures and lower salinities which extend to the sea bottom at point E’(0 = I .?“C, S Q 34.80%0,depth r 48 10 m) and correspond to a diluted component of AABW. From silica budgets (BROECKER,1979) it can be estimated that about 40% of the bottom water can be attributed to i\ABW, the rest being NADW. The distinct curvature of the 8-S diagram below point D indicates the presence of at least one other end member and may reflect extensive mixing or interaction of AABW with NADW above the two-degree discontinuity as well. It is clear from these 8-S data that mature NADW has been modified due to mixing with Antarctic water masses. The results of C,, for sample splits prepared by Chelex extraction methods are shown in Fig. 7. There is a clear max. imum in the surface of CNd- 18.2 pmol/kg. Below this. C’Iuli is fairly uniform to about 1000 meters, at - 15-i 6 pmol/kg. While there is some structure in CN, in this depth interval. there is no obvious correlation to corresponding U-Sproperties. Below 1000 meters, C,, increases regularly to a bottom water maximum of 30.1 pmol/kg. Repeat analyses on unfiltered and acidified subsamples which were extracted using Fe coprecipitation methods yield higher concentrations to varying degrees. Deep water values agree to within 6% of Chelex numbers, but the surface sample shows a 2 I % higher Nd concentration by the Fe precipitation method. ‘This may be due to leaching of REE from suspended particulate matter during storage of the acidified sample. For the Chelex method, in which samples were not acidified, particulate matter was probably removed onto the resin without equilibrating with the spikes. The excess concentrations by the Fe method agree with previous estimates of particulate contributions to the total REE budget of 520% (PIEPGRASand WASSERBUR(;. 1982) and are generally consistent with vertical particle distributions in the Atlantic which show mid-depth minima and surface and bottom maxima (BREWERet al.. 1976). The high surface water concentrations at this site (both by Chelex and Fe precipitation methods) may reflect an extreme surface par-

CN,,(xto-‘*g/g)

c-3

-----

4

-4-i

!,B

TTO/TAS Stn.63

7’44’N,

40°4Z’W

.

I/’

chelex

Modified NAD W

As mature NADW spreads southward, it begins to encounter and interact with water masses of Antarctic origin. It is shown from T-S observations (cf: Fig. 1) that, as NADW approaches equatorial regions, it is substantially eroded from above and below by northward-spreading Antarctic Intermediate and Bottom Waters. TTO/TAS Station 63. The 0-S curve (see Fig. 3) exhibits clear differences in the water column structure from that at mid-latitudes. The upper 120 meters (points A to C) is similar in structure to the upper 200 meters at 36”N (Fig. 3) with point C (0 = 22.5”C. S = 36.74%0, depth 2 120 m) corresponding to the base of the seasonal thermocline. Below this depth to the salinity minimum at point D” (0 = 5.4”C. S = 34.61%0, depth = 800 m) are waters of the main thermocline. The salinity minimum corresponds to the core of northward-spreading AAIW (CJ:Fig. 1). Below the core of AAIW, the salinity increases to an intermediate depth maximum at about point D (0 m 4.3”C, S = 34.995, depth o 1500 m) which corresponds to the upper boundary of NADW. Point E (0 FT2.O”C, S = 34.92%0,depth = 3 100 m) corresponds to the approximate depth of the two-degree dis-

1. i5

Lo

20

x) 25 &&llOl~kg)

L

3:

FIG. 7. CNdvs. depth for TTO/TAS Station 63. Solid circles correspond to CNddetermined for Chelex separates. while open circles represent Cud in samples separated by Fe precipitation (see text for further details). Note the relatively large maximum at the surface which may reflect an atmospheric input.

REE transport

in the Atlantic

title maximum due to transport of Saharan dust in the Tradewinds. This would also account for the high REE surface concentrations in the Northeast Atlantic as suggested by ELDERFIELD and GREAVES (1982). ~~~(0) as a function of depth is shown in Fig. 8. In contrast to mid-latitude sites presented above, this surface water has a very low e&O) of - 13.9 and requires a different source of REE. Though isotopically similar to NADW values, it is unlikely that these signatures are related. It is more likely that this nonradiogenic surface water signal reflects either an atmospheric component or REE advected from more easterly masses in the North Equatorial Current. t&O) increases to a maximum of -11.2 near 400 meters. Within the analytical uncertainty, this maximum extends into the salinity minimum of AAIW which is labeled D” on the right side of Fig. 8. We note that the shift towards more radiogenic Nd in the region of the salinity minimum is in the direction inferred for Antarctic sources of Nd (C&O) = -9; PIEPGRAS and WASSERBURG, 1982) and is consistent with the O-S properties, Below the salinity minimum, c&O) decreases to a mid-depth minimum of - 13.3 near 2000 meters. This is at depths near the core of NADW (point D) and is indistinguishable from isotopic values associated with this water mass at mid-latitudes (Fig. 5). Below 2000 meters, ~~~(0) increases again to a bottom water maximum of - 11.8. This shift also is in the direction inferred for Antarctic sources of Nd and is comparable in magnitude to mixtures of AABW and NADW determined from hydrographic parameters (see below). The shape of the deep water isotopic profile suggests mixing between AABW and NADW and is consistent with the curvature of the 8-S diagram in this depth interval. The isotopic distribution at this site is clearly different from those at the sites presented above, but again demonstrates a close correlation between isotopic properties and 0-S properties. There is again a clear contrast between ~~~(0) above and below the thermocline. It follows that the deep water values are most reasonably attributed to the blending of water masses resulting from the lateral transport of Nd associated with southward-spreading NADW with northward-spreading AAIW and AABW. With the exception of the surface sample, cNd obtained by the Fe precipitation and Chelex methods is the same within error. The surface sample by the Fe precipitation method is 1.3 e units lower than obtained on the Chelex sample. In principle, this difference could be accounted for by a higher particulate contribution having more negative tNd(0) and would

744’N. 40.42’W .

Lhh

m Fe ppl 5

E’

1

FIG. 8. C&O) vs. depth for TTO/TAS Station 63. Isotopic shifts in the direction of Antarctic sources of REE (e,,(O) = -9) are observed with AAIW and bottom waters associated with dilute AABW. The isotopic signature of mature NADW is clearly recognizable in the core of NADW near point D. Note that surface waters are much less radiogenic than at mid-latitude sites shown in Fig. 5.

Ocean

I265

F 34 80

34.90 Sai.l%.l

3500

D

/

A______-__.--------~’ _ ---1

Hudson 83-036 Hudson 83-036 33.5

/I’

Sin II Sin 9 34.0

34.5

3513

Salinity (%J

FIG. 9. a) Potential temperature vs. salinity for Hudson 83036 Station 1 I (solid curve). Proto NADW is recognized in the region below point D while Labrador Sea water masses lie at depths above point D. b) 0 vs. S for Hudson 83-036 Station 9 (dashed curve). Once again, proto NADW occupies the region between DE while Labrador Sea waters lie at depths above point D. The inset shows expanded 8-S scales for depths below point G for both stations.

be consistent ples.

with the concentration

differences

in these sam-

Proto NAD W At the higher latitudes of the North Atlantic and southeastern Labrador Sea, a substantial portion of NADW has already been formed. These waters are cooler and lack the higher salinity imparted by the Mediterranean component which characterizes mature NADW. Two sites were studied to determine cNd of this proto NADW. Hudson 83-036. Station 1 I is located over the Labrador Basin (Fig. 2). The 8-S diagram (Fig. 9) shows that temperature decreases and salinity increases from surface values at point A (6 = 8.2”C, S z 34.28%0) through a shallow thermocline to a depth of 300 meters at point F (0 z 4.4”C, S = 34.80%0). This depth corresponds to the approximate top of the Labrador Sea Water. The region between F and G (0 = 3.3”C, S of Labrador = 34.90%0, depth = 1500 m) is representative Sea Water. Below point G, both 0 and S rise slightly to the temperature maximum at point D (19x 3.4”C. S = 34.93%0, depth = 1800 m). This corresponds to the top of NADW as defined by CLARKE and GASCARD (1983). This water mass extends to the bottom at point E (0 = 1.4”C, S = 34.92%0, depth = 3900 m), This water is much colder and has slightly lower and more constant salinity than mature NADW at midlatitudes but is still considered to represent NADW. The colder temperature of the bottom water and curvature of the upper portion of this region of the B-S curve (above 3000 m) indicates that NADW at this site is being mixed from its individual components and must still be quite immature. Three water components are indicated from Fig. 9. Extrapolating from the linear segment of O-S above point E to the intersection with the segment along G-D shows one component to have the composition 0 = 3.6”C, S = 35.0%0 which is similar to waters flowing through the Gibbs Fracture Zone (SWIFT, 1984). The cold bottom water at point E suggests an original end member of Denmark Strait overflow water, while point G reflects the third component which is Labrador Sea Water. This scenario for NADW at this site is entirely consistent with the general oceanography of the region outlined earlier. Station 9 is located over the continental slope off Labrador (Fig. 2). The 0-S diagram is also shown in Fig. 9. The major water mass features

1266

D. J. Piepgras and G. J. Wasserburg

of Station I 1 are also found here. However, this site is characterized by a much colder (0 o 1.9”Q relatively fresher (S = 33.4%0), surface layer. Below the surface, both B and S increase to a maximum at point F (B = 3.8”C. S = 34.86%0, depth = 300 m). Between points F and G (0 = 2.56”C, S = 34.88’&, depth = 1200 m), the water column has nearly uniform salinity and only a small decrease in 0. This region corresponds to Labrador Sea Water. Below point G, 0 and S rise slightly to a 9 maximum at point D (0 =, 2.8”C, S = 34.92%0,depth m 1400 m). This again corresponds to the top of NADW. Below point D, a single water mass occupies the entire water column to the bottom at point E. One other sample was collected from the Labrador Current on Hamilton Bank (LC, Fig. 2) at a depth of 100 meters. The salinity was 32.689%0 (Table 2) and is the lowest salinity analyzed from this region. Figure 10 shows CNdfor Stations 9 and I 1. In general, C’Nd at both stations are highest in the surface waters, decrease to mid-depth minima, and increase again to the bottom. The profile for Station 11 shows the most detail, exhibiting a subsurface maximum of 2 1.7 pmol/kg at 125 meters, below which the concentration drops steadily to a value of 18.2 pmol/kg at 800 m. Between 800 and 1500 meters, the concentration remains constant. Below 1500 meters, the Nd concentration decreases again to a mid-depth minimum of 16.7 pmol/kg at 2500 meters. The Nd concentration then increases again to the bottom, reaching a deep water maximum of 19.4 pmol/ kg. The profiles differ remarkably from elsewhere in the world ocean where near-surface waters have lower CNd relative to intermediate and bottom waters. There is no apparent depthrelated correlation between C,, and hydrographic properties; however. the surface concentrations appear to be inversely correlated with salinity. The Labrador Current sample has the lowest salinity (S = 32.689%.~)and the highest CN,, = 32.0 pmol/kg, followed by Station 9 which has intermediate salinity (S = 33.385%0)and C,, = 25.0 pmol/kg. Station 11 has the highest surface salinity (S = 34.285k) and the lowest CNd = 2 1.1 pmol/kg. It is reasonable to infer that the large fresh water flux necessary to produce low salinity surface waters may carry with it a substantial REE flux. Extremely high Nd concentrations (>2000 pmol/kg) have been measured for some Greenland rivers (GOLDSTEIN and JACOBSEN, 1986) which supports this conclusion.

CN,,(xl(r’2g/g)

FIG. 11. t,+,(O)VS.depth for Hudson 83-036 StatIons 9 and 1 1. Note that deep waters associated with proto NADW have Q.&O)similar to mature NADW further south. At depths associated with Labrador Sea Water, c&O) is more negative than NADW with the lowest values at the surface.

Figure 11 illustrates e&O) at Stations 9 and 1 1 and shows regular increases in ~~~(0)as a function of depth. For Station 9, e,(O) increases from a surface minimum of -15.4 to a bottom water maximum of - 13.8. At Station 11, the surface waters have ~~~(0)= - 17.9 and then rises sharply to -- 15.8 at 125 meters. Below 125 meters, e&O) increases regular11 with depth to a maximum value of -- 13.4 at 3000 meters and then remains constant to the bottom. In the deepest waters below 2500 meters associated from physical oceanographic considerations with NADW, +,(O) is within the rangeof values we attribute to mature NADW. There appears to be a general correlation with B-S properties and tNd(0) at these sites. The linear segment of 9-S below 3000 meters at Station 11 has uniform t&O). Above 3000 meters, where mixing from above is obvious in the 8-S curve, we begin to see a shift in C&O) toward less radiogenic Nd. Between the surface and 3000 meters, the isotopic distribution has the appearance of a simple mixing curve. There are no large isotopic shifts (except at the surface of Station 11) which coincide with points labeled on the 8-S diagrams in Fig. 9. The gradual shift in t&O) between surface and deep waters suggests some vertical mixing of Nd which may be related to the lack of a well-developed thermocline and pycnocline in the region. As with other sites in this study, it is once again clear from the isotopic differences that the present surface waters cannot be the only source of Nd to the deep waters. The sample from Hamilton Bank of the Labrador Current has Tad = -26.1. This requires a very ancient source terrane (>2.6 AE) which is consistent with fresh water runoff from the Canadian Shield. The low surface water isotopic compositions at Stations 9 and 11 also require substantial contributions from old source terranes. However, there is no correlation observed between surface CNdand e&O) as there is for CINdand salinity. Other components

HUDSON83-036 0 stn. 9 (54’N,52.W) l Sin.It (52.N.47*W) I

1

/

15

20 ‘&(pmOl/kg)

25

FIG. 10. CN, vs. depth for Hudson 83-036 Stations 9 and 11. Both stations show high surface concentrations, mid-depth minima and increases again in the deep waters.

of NADH’

The above data show that proto NADW has acquired the isotopic characteristics of mature NADW in the deep waters of the Labrador Basin below 2500 meters. The less radiogenic Nd associated with the upper waters of the Labrador Sea which comprise one source of NADW require other sources which have more radiogenic Nd to mix with the Labrador Sea component to make the e&O) = - 13.5 associated with mature NADW. We analyzed samples whieh represent the source waters for contributions to NADW from the Norwegian and Greenland Seas and their overflows. TTO/NAS. Eight samples of different water masses were analyzed from six stations (Table 1, Fig. 2). 0-S data are not

REE transport in the Atlantic Ocean reported here, but salinity, oxygen and nutrient data are listed in Table 2. Waters derived from this region range in C,, from 14.3 to 21.4 pmol/kg and ~~~(0)ranges from -7.7 to -10.7 (Table 3). One sample from Station 142 represents Faeroe Channel overflow water at the sill depth and is the most radiogenic, having e,,(O) = -7.7. Two samples from the Norwegian Sea (Station 144) are less radiogenic, but have similar isotopic compositions of -10.3 (65 m) and -9.5 (3750 m). One sample from Greenland Sea Deep Water (Station 149) has c&O) = - 10.7 and is similar to Norwegian Sea surface water. The bottom water sample from Station 167 in the Irminger Sea represents Denmark Strait overflow water and has an isotopic composition of t&O) = -8.6. An intermediate depth sample from Station 167 (840 m) has its origins in the Labrador Sea (T. TAKAHASHI, pers. commun.). This last sample has e,,(O) = - 14.1 which is clearly consistent with a source in the Labrador Sea. All other samples have tNd > -13.5. Mixtures of these overflow waters with the more negative q+,(O)values associated with Labrador Sea and Baffin Bay (STORDALand WASSERBURG, 1986) sources could, in principle, yield the isotopic signature of NADW. Surface samples in the North Atlantic To evaluate the surface waters as potential sources of deep water isotopic signatures, we collected samples along an eastwest transect at 36”N in the North Atlantic Gyre during cruise A-II, 109-l (see Fig. 2). The results (Table 3) show a regular decrease in C&O) from -9.5 in the western basin to - I 1.4in the eastern basin. These values are all too radiogenic to provide the source of the nonradiogenic Nd component in NADW. They are close to values associated with waters in the Norwegian and Greenland Seas and may be contributing to these basins. The heterogeneity of these surface waters must be dependent on injection from distinct sources, but the tNdvalues are not attributable to any specific source. CNdin these surface waters range from 7.9 to 14.4 pmol/kg. The lowest concentrations are found in the central portions of the gyre, while the highest are nearest the continental margins. The high concentrations near the margins indicate runoff may be a primary source of REE in the surface waters. The low concentrations in the interior imply that advection of REE to the central gyre is slow relative to removal rates. DISCUSSION We have attempted to identify the broader characteristics of the Nd isotopic distribution in the western North Atlantic. A regular pattern emerges which correlates well with the distribution of major water masses as identified from their T-S characteristics. The data show the following features: 1) There is extensive vertical structure in the isotopic distribution at all sites studied. Regions where a thermocline is well-developed are characterized by abrupt Nd isotopic shifts across the base of the thermocline, whereas regions without a thermocline are characterized by gradual shifts. 2) Mature NADW has a well-defined isotopic composition of ~~~(0) = - 13.5 f 0.5. Two mid-latitude locations (see Fig. 5) show ~~(0) to be uniform in NADW within this range through a 4 km section of water column below the base of the thermocline to the ocean bottom which show the regular T-S characteristics (Figs. 3 and 6) of NADW. There is no evidence at either of these sites for substantial contributions of Nd from any other sources. 3) A more southerly vertical profile (TTO/TAS Station 63) was studied to monitor the isotopic composition of NADW during southward

I267

flow. The B-S data for this section (Fig. 3) show that NADW has been modified during southward flow by interaction with northward-flowing AABW at depth and AAIW at shallower levels (< 1500 m) resulting in mixing of NADW from above and below. The isotopic data show that the core of NADW has t,.+JO) = - 13.3 which is identical to mature NADW. Waters mixing in from the top and bottom of NADW have tNd(0) = - 11.2 and -11.8, respectively (see Fig. 8) and are in the direction of Antarctic sources (~~~(0) = -9, REPGRAS and WASSERBURG, 1982). For bottom water, we can calculate a mass balance between NADW and AABW from silica budgets. If we assume simple twocomponent mixing between a northern (NADW) source of silica (Si x 12 pmol/kg) and a southern (AABW) source (Si = 125 pmol/kg) as suggested by BROECKER (1979), we find that the bottom water sample at TTO/TAS Station 63 is comprised of -60% NADW and 40% AABW. From the mixing proportions inferred for NADW and AABW (or AAIW), we calculate that tNd(0) for the mixture should be -11.7 which is in agreement with the measured value of - 11.8. Further, it is evident from the 0-S properties in the depth interval from -2000 m to the bottom that the waters result from mixing between NADW and AABW. In this same zone, ~~~(0) is found to change regularly. It therefore appears that interaction between NADW and northward-spreading AAIW and AABW as determined from t&O) observations closely parallels effects of this interaction as determined from 8-S measurements. The data unambiguously show that the tNd(0) of mature NADW is well-identified and would appear to be conserved during southward flow. The source of REE in NAD W It follows from previous inferences (PIEPGRAS and 1986) and from data presented here that the sources of REE must lie upstream of mature NADW. The ++, of mature NADW is much less radiogenic than in present surface waters at mid-latitudes and the Nd carried in the dissolved and detrital loads of major rivers. Data for the dissolved loads of the Mississippi River (~~~(0) =-loto-12;STORDALand WASSERBURG, 1983) and the Amazon River (t&O) = -8.9; see Table 3) are similar to mid-latitude surface waters and clearly cannot account for the deep water isotopic signatures. River-suspended loads have average -11 (GOLDSTEIN et al., 1984) and are also +Jd(O) = too radiogenic. In addition, one mid-latitude, western basin, deep-sea sediment analyzed by GOLDSTEIN and O’NIONS (198 1) had t&O) = - 11.3, similar to average river fluxes and much more radiogenic than NADW. Mixtures which comprise NADW require components with more negative values of cNd(O)than the -9 to - 1 I of surface waters and major fluvial sources at mid to low latitudes. The sources of REE in NADW must contain a component derived from terranes which are older than 1.3 AE since, for a source age T, tNd(O) = QJT z 25.1 AE-’ x (-0.4) x T(AE) (DE PAOLO and WASSERBURG, 1976). WASSERBURG,

1268

D. J. Piepgras and G. J. Wasserburg

The formation of NADW is known to take place in high latitude seas where winter cooling results in the sinking of surface waters to form deep water masses. Subsequent mixing of these cold water masses provide the major sources for NADW as described earlier. Some of these high latitude seas are adjacent to ancient Precambrian terranes (T 3 2.5 AE) where runoff could supply a substantial source of REE with very negative ~~~(0).We have shown that in the southeastern Labrador Sea, newly formed proto NADW attains the isotopic characteristics of mature NADW in the deepest waters. The isotopic signature of NADW results from mixtures of components more radiogenic than NADW (t&O) > -13.5) with less radiogenic components (Q&O) < - 13.5). Our investigation of the major high latitude sources of NADW reveals that the Labrador Sea may provide the major source of nonradiogenic Nd to NADW. Nd being transported in the southwardflowing Labrador Current has Tad = -26. I, implying a -2.6 AE source terrane consistent with the age of surrounding continental land masses. In addition, both surface and deep water flows from Baffin Bay are contributing very negative Nd (tag < - 18) to the Labrador Sea (STORDALand WASSERBURG,1986). These workers have found isotopic compositions as low as -25 in Baffin Bay and overflows through the Davis Strait with t&O) = - 18 to -19. If the Nd balance in NADW is governed by inputs of tNd(0) x -9 and - 18. they estimate that the flux from Baffin Bay through the Davis Strait could contribute -30% of the total Nd in the less radiogenic component in NADW. Convective mixing during cold winter months is known to occur in the western Labrador Sea to depths of 2000 meters or more (CLARKE and GASCARDE, 1983) and could transport the low t&O) surface REE into the deep water. The very negative ~~~(0)values associated with Labrador Sea Water and Baffin Bay outflow require that other components of NADW must have more positive values to make up a final value of --13.5. We have shown from limited sampling that contributions from

the Greenland and Norwegian Seas through the Denmark Strait and Faeroe Channel overflows are indeed more radiogenic than mature NADW having ++,(O) u -8 to -9. To determine the relative contributions from each of these sources to the total REE budget it Will be necessary to know f& and eNdfor at least one of the major contributors. However, given the generally higher Nd concentrations in our Labrador Sea samples relative to sources in the Norwegian and Greenland Seas, it is reasonable to assume that the Labrador Sea. which contributes -30% of the water to NADW (SWIR, 1984), could dominate the REE budget of NADW. Where and how these three major sources are mixing cannot be determined from our data, but ir must occur somewhere within this northerly region. The Mediterranean Sea, which contributes to the overall higher salinities and temperatures of mature NADW, does not appear to be a major contributor to the REE and Nd isotopic composition in the western basin (PIEPGRASand WASSERBURG,1983). Table 4 summarizes our estimates of t&O) and c‘,,, in the major water masses of the western basin of the North Atlantic. In addition, tNd(O)associated with the major known components which contribute directly to the make-up of NADW are listed above. For cornparison, we also show published t&O) values in the “dissolved” river and suspended loads. It should be noted that none of the rivers studied represent drainage into the high latitude seas where we have shown the source of nonradiogenic Nd to be imparted to NADW. The meridional distribution 0f’t~~(0) in thtz western North Atlantic Figure 12 summarizes the Nd isotopic data in a cross section through the western North Atlantic from the Labrador Sea to the equator. Extensive meridional transport of Nd associated with NADW is demonstrated by the propagation of tNd(O) = -- ix.5 waters from high latitudes in the Labrador Sea down to the equator. Very negative Nd is seen to penetrate from surface waters into the deep waters in the Labrador

Table 4. Nd isotopic compositionof major water massee in the western North Atlantic, components in Nm, and in fluvial sources of RFX. ENd(O)

CNd (pl%=l/&)

NADW

-13.5

20-30

AABW'

-9

30

AAIW

-9

15

Mid-latitude Surface Water

-9.5

15

Labrador Sea Surface Water

-17

21-25

-

Baffin Bay2 Average

-20

35

L&mark strait overflow

-8.6

20

Paeroe channel overflow

-7.7

21

MediterraneanOverflow3

-6

17

-10 to -12

25

-9

350

Hfssissippi River2 Amazon River2 Average River4 Suspended Load

----_

-11.5

1. PiepRres and Uaseerburg (1982). 2.

Stordal and Wasserburg (lYfJ6).

3.

Goldstein et al. (1984).

Piepwas and Wasserburg (1983). 4.

REE transport in the Atlantic Ocean

E&l INTHE

WESTERN

1269

NORTH ATLANTIC

FIG. 12. Meridional cross section of c&O) in the western basin from the Labrador Sea to the equator. Position of t&O) contours between stations are estimated from known 0-S relationships. Note the extent of very uniform &O) between - 13 and - 14 associated with NADW.

Sea where it mixes with less radiogenic sources of Nd which we infer to originate in the Norwegian and Greenland Sea overflows. At this point, the major isotopic characteristics of NADW have been acquired in the deepest waters. This mass now flows southward and under the warm, near-surface waters of the middle latitudes where a thermocline is well-developed. At this point, the upper waters are characterized by more radiogenic Nd at the surface (~~~(0) N -9.5) and a very strong isotopic gradient across the base of the thermocline which separates the more negative NADW from the more positive surface waters. The presence of this strong isotopic gradient indicates that NADW is decoupled from overlying thermocline and surface waters with respect to sources and transport of REE and precludes any significant vertical mixing across this boundary on a short time scale. The isotopic distribution shown suggests some mixing between these more radiogenic surface waters with the less radiogenic waters derived in the Labrador Sea along the northern edge of the warm water sphere where the thermocline breaks down upon crossing the Polar Front. Upon reaching low latitudes, NADW encounters northwardspreading AAIW and AABW. The effect is clearly seen in the isotopic distribution as a pinching out of the less radiogenic NADW isotopic signature by the more positive signatures associated with the intermediate and bottom waters of Antarctic origin. The nonradiogenic surface water in the equatorial region probably reflects REE which are either transported from easterly sources by the North Equatorial Current or sources derived from atmospheric fluxes. It is different from surface sources at mid-latitudes and cannot be receiving significant fluxes of REE from the Amazon. Comparison of the isotopic distribution shown in Fig. 12 with the water mass distributions shown in Fig. 1 demonstrates the excellent correspondence between tNd(0) and water masses and supports our conclusion that the REE are transported laterally in association with the llow of water masses. No evidence was found for significant bottom fluxes from the isotopic data. Consequently, the isotopic composition of Nd would appear to be conservative in ocean waters on the time

scale of Atlantic Ocean mixing in spite of the nonconservative elemental behavior of Nd and suggests that the residence time of Nd must also be of the same order as mixing times in the Atlantic. Insofar as Nd from Atlantic sources can be observed in the bottom waters of the Central Pacific (PIEPGRAS and WASSERBURG, 1982) related to AABW flow, we estimate that the residence time of Nd must be greater than 1000 years.

The role

ofvertical transport

We have shown from the Nd isotopic distribution that the REE must undergo extensive horizontal transport in the oceans. However, the ubiquitous presence of concentration gradients in the oceans must require vertical transport processes as well to account for the REE distributions. Most treatments of oceanic trace element distributions have been in the framework of one-dimensional advection-diffusion models (cJ CRAIG, 1974) ignoring horizontal processes entirely. Extensive work on this problem with particular concern for the transport of a very reactive element dominated by particles such as 230Th was correctly done by BHAT et al. ( 1969). For some time, it was common practice to treat the vertical transport process with a generic source for elements which were not in fact produced by radioactive decay in order to describe the effects of particulates (cJ: CRAIG, 1969, 1974; CHAN et al., 1976). We have previously considered this approach to the REE, properly taking into account the sequestering and resolution of these elements on particles with a separate constitutive equation for the particle distribution as a function of depth (PIEPGRAS and WASSERBURG, 1982). We look at several mechanisms which, in principle, could lead to a concentration gradient for Nd such as typically observed. The first mechanism is a one-dimensional model of scavenging by settling particles. Removal of near-surface REE onto settling particles could result in the general depletion of REE in surface waters relative to underlying deep waters. Resolution (or particle disaggregation) in the deep waters could

I270

1). J. Piepgras and G. J. Wasserburg

account for higher deep water concentrations. However, the surface source of Nd should then be reflected in the deep water isotopic composition. Insofar as large isotopic differences exist in the water column between surface and deep waters at many locations, the surface waters must be decoupled from deep waters. This would seem to preclude the possibility of signiticant amounts of REE scavenged from local surface sources being redissolved into the deep waters and governing the composition. If significant amounts of REE are being scavenged from surface waters, then the data would indicate that REE removal must be associated with rapidly sinking particles with little or no resolution into the deep waters. To explain both the isotopic data and concentration data by Nd exchange on settling particulates which originate in the near-surface regions will require that the surface waters are highly variable in space and time and that their average value be tNd = -13.5. Another possibility is that, away from the formation site of NADW. the surface layers are virtually decoupled from NADW (i.e., relatively low Nd transport by particles from this region), and that particulate transport is dominant below the thermocline by particles that originate in a region where CNdiS UnifOrmly ~-13.5. This would then alter the concentration without conflicting with the isotopic data. This implies a particle source (possibly ~01s) of a rather large size which is regenerated presumably by chemical processes. If the time scale for vertical transport is less than twenty years, this requires particle sizes of over 2 pm. Another mechanism, generally considered to operate in addition to scavenging, is upward advection and diffusion of REE released from sediments. This process could account for the high concentrations associated with deep waters, provided the sediment sources are compatible isotopically with overlying deep waters. If benthic fluxes were the primary source of REE in NADW, transport to the base of the thermocline (and removal by surface waters) would be necessary to account for the uniform t&O) of NADW. This view is in conflict with the data reported here which shows that the isotopic signature of NADW is associated with the water mass itself and is not plausibly due to injection from the bottom during southward flow. In addition, the time scales for deep water renewal in the North Atlantic may limit the extent of upward transport from a bottom source. For diffusion, the transport distance X is approximated by the expression X2 = Dt, where D is the diffusion coefficient and t is time. In the oceans, D = 1 cm’/s (cf CRAIG, 1974). For advection, X = 01 where v is the upward advective velocity which is estimated to be 10-j cm/s. Using these values for D and v, we find that the advective transport equals the diffusive transport when X = 1000 meters, and that the time scale for transport across this distance is 300 years. For heights less than 1000 meters above the bottom, diffusion will dominate the transport. For transport to shallower depths, advection will dominate. Thus, the time required to transport Nd to the base of

the thermocline (-4000 meters above the bottom) is - 1200 years. This is considerably longer than the estimated residence time of NADW of 250 years (WORTHINGTON. 1976). Further consideration shows that a model of upward transport requires that the upper boundary of NADW would have to release Nd into the thermocline and be transported away. l+om all of the above arguments, we conclude that injectron from bottom sources is not a reasonable explanation. In all of these cases, a major concern is the time scale for developing the concentration gradient in the deep water. Horizontal time scales for advcction in NADW are known to be very fast. Atmospheric chlorofluoromethanes originating in the Labrador Sea have reached the equator at depths corresponding to NADW on a time scale of only 23 years (WEISS CI LI/ 1985). This corresponds to horizontal velocities of up to - i cm/s which would appear to be much faster than the velocities for vertical advective or diffusive processes. This suggests that larger scale instabilities which are rapid (such as those which cause the sinking of Arctic waters) are responsible for the vertical transport of chlorofluoromethanes into NADW. Insofar as concentration gradients in Nd are observed in the deep waters of the Labrador Sea where NADW is very young, the data suggest that the gradients may be produced very early on in the formation of the water mass as a result of mixing processes at the sources or by dissolving particulates of large size (22 microns). Other processes may act to maintain these gradients dounstream from where they were developed. .4cknowledgements-We wish to thank J. Sarmiento and I. Takahashi for allowing us to participate in the TTO/TAS expedition. We also wish to thank the PACODF crew for making the hydrographic measurements on the samples collected durine. this cruise. T. Takahashi ako kindlv nrovided us with the T’TO/NAS samples. C. Wunsch provided us with ship time and on-board hydrographic measurements for samples collected from cruise A-II 109-l. J. Lazier provided the ship time for the Labrador Sea work. Nutrient analyses on these samples were kindly measured by P. Jones at the Bedford Institute of Oceanography. Finally, we wish to thank the crews of the RV Atlantis II, RV Knorr and the CSS fir&on for their support during the collection of these samples. We are grateful to M. P. Bacon for his review, the associate editor for his comments and H. De Baar for his criticisms. Their comments helped to improve this report. This work was suonorted in part by-grants from the Nat&al Science Foundat& (NSF OCE 8308884 and NSF OCE 8320516) and the National Aeronautics and Space Administration (NAG 9-43). Division Contribution No. 4 118 (49 1). Ed&vial handling: H. Elderheld REFERENCES BHATS. G., KRISHNASWAMY S., LAL D., RAMAand MOORS W. S. (1969) 234Th/238Uratios in the ocean, Earth Plane/ Sci. Lett. 5, 483-49 1. BREWERP. G., SPENCERD. W.. BISCAVEP. E., HAWLEYA.. SACHSP. L., SMITHC. L., KADARS. and FREDERICKSJ. (1976) The distribution of particulate matter in the Atlantic Ocean. Earth Planet. Sci. Lett. 32, 393-402. BROECKER W. S. (1979)A revised estimate for the radiocarbon

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