Pb and Nd isotopes in NE Atlantic Fe–Mn crusts: Proxies for trace metal paleosources and paleocean...

PII S0016-7037(99)00068-X

Pb and Nd isotopes in NE Atlantic Fe–Mn crusts: Proxies for trace metal paleosources andpaleocean circulation

W. ABOUCHAMI,1,* S. J. G. GALER,1 and A. KOSCHINSKY2

1Max-Planck-Institut fu¨r Chemie, Postfach 3060, 55020 Mainz, Germany2FU Berlin, FR Rohstoff- und Umweltgeologie, Malteserstrasse 74-100, 12249 Berlin, Germany

(Received April22, 1998;accepted in revised form November11, 1998)

Abstract—We report high precision Pb isotopic data (2s # 100 ppm) together with Nd isotopes on depthprofiles from two Fe–Mn crusts from the eastern Atlantic basin. The profiles provide a 13 Ma record ofchanges in eastern North Atlantic Deep Water (ENADW), and over the past 8 Ma for Mediterranean OutflowWater (MOW). Pb isotope ratios in the two records display gradual and systematic changes, tracking eachother through time. The highly precise Pb isotopic data also resolve, for the first time, changes in all three Pbisotope ratios in a single Fe–Mn crust, and exhibit well-defined binary mixing lines in Pb isotope space.

The Pb isotopic record of the Tropic Seamount crust shows that eastern Atlantic Pb has been dominated bybinary mixing throughout the last 13 Ma. The Pb binary mixing lines further demonstrate that a change in Pbprovenance to the eastern Atlantic occurred at 8 and 4 Ma. This is shown by the distinctive Pb isotopiccompositions of the mixing components in the time intervals 4–0 Ma and 13–8 Ma.

Consideration of Pb and Nd isotope systematics show that from 4 Ma to the present, the two endmembercomponents correspond to NADW and Southern Component Water (SCW). The quasi-cyclic character of206Pb/204Pb variations, as well as age progressive changes in the Pb mixing proportions of NADW and SCWendmembers, appear to reflect changes in deep water circulation. In particular, strengthening of the Pb and Ndisotopic signal associated with NADW since 3–4 Ma in both the eastern and western Atlantic basins impliesthat NADW advection from the western into the eastern Atlantic has been in operation over the past 4 Ma.

During the period from 13 to 8 Ma, two entirely different sources of Pb and Nd existed in the easternAtlantic. The radiogenic Pb and unradiogenic Nd component may have originated in the Norwegian–Greenland seas and the low206Pb/204Pb—high«Nd endmember component in the Southern ocean or Tethys.This suggestion is supported by evidence from paleoceanographic studies indicating the importance ofGreenland–Scotland ridge activity in Miocene deep water production, and also the presence of a distinctivehigh d13C water mass in the Southern Ocean, derived either locally or flowing in via Tethys.

The 8 Ma record of the Lion Seamount crust (65GTV) demonstrates that Pb and Nd sources in MOWswitched from predominantly internal, Mediterranean (European) sources prior to 4 Ma, to mainly external(Saharan) sources after 4 Ma. The gradual increase of Pb isotope ratios seen following the end of theMessinian reflects enhanced input of Saharan dust into the water column in the eastern Atlantic. Thestrengthening of the “Saharan” isotopic signal from about 4 Ma also matches the documented increase inaridity and dustiness in the Saharan and sub-Saharan regions and coincides with the re-establishment of waterexchange between the Atlantic and Mediterranean after the Messinian. Although Pb and, to a lesser extent, Ndisotope ratios are distinct during the Messinian (6.5–5 Ma), there is no clear evidence for either a shutdownof MOW or a stronger North Atlantic signal during this period. The fact that a similar isotopic signal isobserved, with an even higher amplitude in the ENADW record, shows that this signal is a feature of the wholeeastern Atlantic.

From 3 to 4 Ma ago, a source of radiogenic Pb and unradiogenic Nd appears to have dominated not onlythe eastern Atlantic but the world oceans, since it is seen ubiquitously in other Fe–Mn crusts at this time. Thisisotopic signal must be conveyed around the globe via the ocean circulation or the atmosphere. While theclosure of the Panama gateway may have played an important role in these global changes in source(s) and/orfluxes of Pb and Nd to the oceans 3–4 Ma ago other events, in conjunction, such as the emergence of theGreenland–Scotland ridge, were probably also as influential.Copyright © 1999 Elsevier Science Ltd

1. INTRODUCTION

The global heat and salt budget of the oceans exerts the keycontrol on global ocean circulation patterns, with production offresh waters at high latitudes and evaporation at low latitudesgoverning the production and strength of North Atlantic DeepWater (NADW) through time (Broecker and Denton, 1989).The importance of the “great conveyor” (Broecker, 1991) in

driving climatic, oceanographic, and atmospheric changes hasbeen demonstrated by numerous paleoceanographic studies.During the Miocene, two major paleoceanographic events af-fected the heat and salt budget of the oceans causing majorchanges in both the climate and the thermohaline circulation:(1) the gradual closure of the Isthmus of Panama, which lead tothe cutoff of water exchange between the Atlantic and thePacific around 3–4 Ma ago, and (2) the Messinian salinity crisis(6.5–5 Ma), during which time the Mediterranean sea becameisolated from the world’s oceans, resulting in the deposition ofvast evaporite formations (Kennett, 1982).

*Address reprint requests to W. Abouchami, Max-Planck-Institut fu¨rChemie, Postfach 3060, 55020 Mainz, Germany; Fax (149) 6131 371051; E-mail: [email protected]

Pergamon

Geochimica et Cosmochimica Acta, Vol. 63, No. 10, pp. 1489–1505, 1999Copyright © 1999 Elsevier Science LtdPrinted in the USA. All rights reserved

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1489

It has been suggested that the closure of the Panama gate-way, about 3 Ma ago, provoked a strengthening of the thermo-haline circulation and increased production of deep water in theNorth Atlantic, although several periods of NADW productionhave also been documented earlier during the Miocene (Blancet al., 1980; Schnitker, 1980; Woodruff and Savin, 1989;Wright et al., 1992). On the other hand, opinions have divergedconcerning the importance the Messinian event had on theNorth Atlantic circulation (Blanc et al., 1980; Keigwin et al.,1987). In this connection, Johnson (1997) recently proposedthat dry climate in the Mediterranean region may have causedan increased outflow of highly saline Mediterranean waterwhich, in turn, could have lead to Pleistocene climate changesin the NW Atlantic.

The usefulness of the long-lived radiogenic isotopes of Pband Nd as tracers of ocean circulation is a consequence of theshort oceanic residence time of these elements (Craig et al.,1973; Elderfield and Greaves, 1982; Bertram and Elderfield,1993) relative to the turnover time of the oceans ('1500 yr)(Broecker and Peng, 1982). Although Pb and Nd are tracemetals whose concentrations are not conservative in seawater,their isotopic compositions still remain diagnostic of theirsources to seawater. To the extent that the isotopic compositionapproximates a conservative tracer—that is, over the time scaleof interest—Pb and Nd isotopes can fingerprint different watermasses. The observation that geographical Pb and Nd isotopicvariations in surface scrapings of hydrogenous Fe–Mn nodulesmirror the pattern of present-day deep water circulation hasshown the potential of these radiogenic isotopes for tracingwater mass movement and mixing (Abouchami and Goldstein,1995; Albarede and Goldstein, 1992; Albare`de et al., 1997;Aplin et al., 1986/87; Simonetti et al., 1995; Simonetti et al.,1996; Von Blanckenburg et al., 1996). More recently, paleosea-water Pb and Nd isotopic records have been obtained usingdated depth profiles from hydrogenous Fe–Mn crusts(Abouchami et al., 1997; Burton et al., 1997; Christensen et al.,1997; Ling et al., 1997). In these studies, long-term Pb and Ndisotopic changes were observed in Pacific and Atlantic Fe–Mncrusts and ascribed to changes in the deep water circulationfollowing the closure of the Panama gateway.

Mixing in the modern oceans can be traced using conserva-tive properties of water masses (salinity, temperature, PO4 . . . )whose covariations define linear mixing arrays (Broecker andPeng, 1982). Pb isotopes offer significant advantages for mon-itoring past deep water circulation compared to other isotopicsystems. Of the available isotopic tracers (Be, Sr, Nd, Hf, Os,and Pb), only Pb has three daughter isotopes that can definelinear mixing arrays when plotted in Pb–Pb isotope space.Although the relationships between different isotopic sys-tems—such as Pbvs. Nd—may provide some information onmixing processes (e.g., Abouchami et al., 1997), the differencesin the oceanic chemistry of the two elements (oceanic residencetime, conservative versus nonconservative species) make suchan approach less compelling for unravelling mixing than byusing Pb isotopes alone. However, resolution of Pb mixinglines does depend critically on the accuracy with which Pbisotope ratios can be measured. So far, because of limitations inanalytical precision only changes in206Pb/204Pb ratios havebeen resolvable within Fe–Mn crust records (Abouchami et al.,

1997; Burton et al., 1997; Christensen et al., 1997; Ling et al.,1997; O’Nions et al., 1998).

In this study, we report Pb and Nd isotopic compositions ondepth profiles from two Fe–Mn crusts from the eastern Atlanticbasin. The Pb isotopic data were obtained with an external 2sprecision of#100 ppm using the Pb triple spike method (Galer,1997, 1999; Galer and Abouchami, 1998). These data providerecords of long-term changes in eastern NADW (ENADW) andMediterranean Outflow Water (MOW). The ENADW recordhelps in reconstructing the evolution of NADW during itstransit from the western to the eastern basin. Further, thehistory of Nd and Pb in MOW is unknown at the present time,hampering an evaluation of how MOW and the Messiniansalinity crisis may have influenced the North Atlantic thermo-haline circulation (see Johnson, 1997).

The most striking feature of the two Pb isotopic records isthat they show gradual and contemporaneous changes overtime. The highly precise Pb isotope data also reveal changes inall three Pb isotope ratios outside analytical error and allow usto resolve binary mixing lines in Pb isotope space for the firsttime in a single Fe–Mn crust record. Our results demonstratethat Pb sources to the eastern Atlantic in the late Miocenediffered markedly from those in the Pliocene. This implies that:(1) ventilation of the eastern Atlantic prior to 8 Ma was quitedifferent from that of the past 4 Ma, and (2) NADW advectionfrom the western Atlantic basin into the eastern basin has beenin operation since about 3–4 Ma.

2. SAMPLING AND ANALYTICAL METHODS

Hydrogenous Fe–Mn crust samples were collected during Sonnecruise 83 (1992) in the NE Atlantic (Koschinsky et al., 1996). Crust121DK was dredged from Tropic Seamount which lies off West Africa(24°539N, 21°429W, 2000 m), while crust 65GTV comes from LionSeamount, west of the Straits of Gibraltar (35°209N, 15°209W, 1500 m)(Fig. 1). These crusts grew in the cores of NADW and MOW at ratesof 3 mm/Ma and 4.5 mm/Ma, respectively, as inferred from10Be data(Koschinsky et al., 1996). Both crusts are about 5 cm thick, corre-sponding to time records of ENADW and MOW back to 13 Ma(121DK) and 8 Ma (65GTV), respectively. Detailed description andelemental concentration profiles can be found in Koschinsky et al.(1996).

The crusts were sawn into slabs perpendicular to the growth surface.The samples in this study were drilled parallel to the laminations in theslab using a 1 mmdiameter drill bit. After total sample dissolution in6 N HCl, Pb was separated by anion exchange using HNO3–HBrmixtures (Lugmair and Galer, 1992). The rare earth elements (REE)were then collected as a group by cation exchange in HCl medium onmicrocolumns, and the Nd obtained from the REE fraction by cationexchange usinga-hydroxyisobutyric acid as eluent. Pb and Nd blanksaveraged 30 pg and 25 pg, respectively, over the course of this studyand are negligible.

Previous Pb isotopic studies of Fe–Mn crusts have shown that Pbisotopic variations can be relatively small (Abouchami et al., 1997;Burton et al., 1997; Christensen et al., 1997; Ling et al., 1997; O’Nionset al., 1998; Frank and O’Nions, 1998). These studies also did not havethe requisite precision to resolve any variations in207Pb/204Pb, whichapproach or are within the analytical uncertainty of measurement. Ingeneral,206Pb/204Pb ratios are reported with a “nominal” precision of;1‰, but this can be improved to;0.4‰ by a rigorous control of theamount of Pb loaded on the filament (Abouchami et al., 1997). In orderto be able to resolve small Pb isotopic variations, we have used a triplespike technique to correct for instrumental mass discrimination for eachsample (Galer, 1997, 1999; Galer and Abouchami, 1998). The ap-proach is similar to that of the double spike technique (e.g., Hamelin etal., 1985; Todt et al., 1996) but has a more favourable error propaga-tion. The triple spike composition was calibrated against the NBS-982

1490 W. Abouchami, S. J. G. Galer, and A. Koschinsky

standard (Todt et al., 1996). Repeat analyses of 10 ng NBS-981standards measured in static multicollection mode (corrected for cupbias) using the triple spike yielded206Pb/204Pb 5 16.94056 0.0015(690 ppm),207Pb/204Pb 5 15.49636 0.0016 (6103 ppm) and208Pb/204Pb5 36.72196 0.0044 (6120 ppm) (2sext., N 5 60). The correspond-ing external reproducibilities of duplicate triple spike analyses for thesamples measured in this study (Table 1) are690 ppm,6120 ppm, and6140 ppm, respectively (2sext., N 5 16), and aresimilar to those

obtained for the NBS-981 standard. Further details of the technique anddata reduction will be presented elsewhere. Some of the Nd isotopicanalyses were done in static multicollection mode, for which the LaJolla Nd standard yielded a value of 0.5118606 21 (2sext., N 5 8);the remainder were measured by dynamic multicollection (La Jolla Nd:0.5118326 13, 2sext., N 5 15). All 143Nd/144Nd ratios are normalizedto 146Nd/144Nd of 0.7219 and corrected to a nominal La Jolla143Nd/144Nd of 0.511860.

Fig. 1. Map of the central and north Atlantic region showing the location of NE Atlantic Fe–Mn crusts 121DK (TropicSeamount) and 65GTV (Lion Seamount). Crust 121DK lies at 2000 m depth in the core of North Atlantic Deep Water(NADW), while 65GTV (1500 m depth) sits in the core of Mediterranean Outflow Water (MOW). The location of NWAtlantic Fe–Mn crust Bm1969.05 studied by Burton et al. (1997) is also shown. Seafloor topography is illustrated by greystippled regions with breaks occurring at 0.2, 2.0, and 4.0 km depths. The pattern of deep water circulation is illustratedschematically, with arrows indicating the sense of flow of the principal water masses involved in the Atlantic deep watercirculation: Labrador Sea Water (LSW), ISOW (Iceland–Scotland Overflow Water), DSOW (Denmark Strait OverflowWater), NADW, SCW (Southern Component Water), and MOW. Present-day«Nd values from direct seawater measure-ments together with206Pb/204Pb ratios for these water masses inferred from Fe–Mn nodules are, respectively, NADW:213.5,$19.0; SCW:28.5,#18.85; LSW:218, n.a.; DSOW:28, n.a.; ISOW:27.7, n.a.; MOW:29.5, n.a. Mid-AtlanticRidge fracture zones important in water mass transport are Gibbs Fracture Zone (GFZ), Vema Fracture Zone (VFZ), andRomanche Fracture Zone (RFZ). At present, the eastern Atlantic basin is ventilated by waters flowing in from the westernbasin through the RFZ (Broecker et al., 1985).

1491Pb and Nd isotopes in NE Atlantic Fe–Mn crusts

3. RESULTS AND DISCUSSION

3.1. NE Atlantic Pb and Nd Isotopic Records and PbMixing Lines

The Pb and Nd isotopic variations in the depth profiles of theTropic Seamount (121DK) and Lion Seamount (65GTV)crusts—hereafter referred to as Tropic crust and Lion crust—are plotted in Fig. 2. The Pb isotopic record from Tropic crust,lying in ENADW off the west African coast, spans the last 13Ma and displays206Pb/204Pb, 207Pb/204Pb, and208Pb/204Pbratios of 18.83 to 18.98 (Fig. 2a), 15.69 to 15.73 (Fig. 2b), and

38.95 to 39.08 (Fig. 2c), respectively. The 8 Ma isotopic recordfrom Lion crust provides the first preanthropogenic Pb isotopiccharacterization of MOW (Figs. 2e, f, g). MOW proves to bequite distinctive and has lower Pb isotope ratios (206Pb/204Pb5 18.70–18.78;207Pb/204Pb 5 15.66–15.68, and208Pb/204Pb5 38.78–38.86) than ENADW. Although the two crusts lieonly about 1000 km apart from one another, their Pb isotopiccompositions are quite distinct, reflecting those of the watermasses at their corresponding depths.

The deeper parts of the Lion crust have been affected byphosphatization (Koschinsky et al., 1996) which potentially

Table 1. Pb and Nd isotopic compositions in the Tropic seamount crust 121DK and Lion seamount crust 65GTV.

SampleDepth range

(mm)Age(Ma) 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 143Nd/144Nd «Nd

121DK-0 surface 0 18.95256 28 15.69816 25 39.06186 67121DK-0a 18.94946 15 15.69526 14 39.05556 41121DK-1 2.0–3.0 0.83 18.87906 9 15.69196 9 38.99306 30 0.5120316 9b 211.84121DK-11 4.0–5.0 1.50 18.85286 9 15.69246 10 38.97436 32 0.5120496 5b 211.49121DK-2 6.0–7.0 2.17 18.84476 6 15.68946 7 38.95476 21 0.5120406 23b 211.67121DK-12 8.5–9.5 3.00 18.83206 9 15.69126 9 38.94556 28 0.5120536 12b 211.41121DK-12a 0.5120586 6b 211.31121DK-3 11.0–12.0 3.83 18.84136 14 15.69116 16 38.96196 52 0.5120606 8 211.28121DK-13 12.8–13.8 4.43 18.86886 10 15.69736 9 38.99956 28 0.5120736 12b 211.02121DK-13a 18.86926 10 15.69776 9 39.00086 28121DK-4 14.0–15.0 4.83 18.91216 26 15.71106 32 39.07086 107 0.5120636 17 211.22121DK-4a 18.91426 14 15.71356 17 39.07906 55121DK-5 17.0–18.0 5.83 18.91836 23 15.71276 29 39.07756 94 0.5120736 5 211.02121DK-5a 18.92026 22 15.71546 20 39.08746 52 211.02121DK-6 19.0–20.0 6.50 18.90436 15 15.71246 18 39.06106 59 0.5120626 12 211.24121DK-6a 18.90236 7 15.71076 7 39.05676 23 211.24121DK-7 23.0–24.0 7.83 18.87606 16 15.71056 19 39.06206 63 0.5120866 8 210.77121DK-7a 18.87506 7 15.70946 8 39.05956 25 210.77121DK-8 27.0–28.0 9.17 18.90756 16 15.70856 15 39.04266 42 0.5120676 13 211.14121DK-8a 18.90796 26 15.70906 29 39.04446 94121DK-9 31.0–32.0 10.50 18.94356 8 15.71616 41 39.04956 26 0.5120486 16b 211.51121DK-9a 18.94136 8 15.71406 9 39.04506 27121DK-10 35.0–36.0 11.83 18.96666 8 15.72146 8 39.05676 26121DK-10a 18.96706 10 15.72186 11 39.05816 37121DK-14 37.0–38.0 12.50 18.97976 8 15.72356 8 39.05726 25121DK-14a 18.98086 22 15.72506 27 39.06206 8965GTV-0 0.2–0.3 0.06 18.77456 16 15.67196 20 38.83576 64 0.5121146 19 210.2265GTV-1 2.0–3.0 0.56 18.78356 7 15.67876 9 38.85736 25 0.5120736 16 211.0265GTV-1a 18.78366 7 15.67886 8 38.85786 2565GTV-10 4.5–5.5 1.11 18.77316 9 15.67646 10 38.84176 32 0.5121226 24b 210.0765GTV-10a 18.77316 7 15.67656 8 38.84196 2465GTV-11 7.0–8.0 1.67 18.76536 7 15.67896 9 38.84166 28 0.5121336 10b 29.8565GTV-2 9.0–10.0 2.11 18.76516 9 15.67796 9 38.83936 24 0.5121126 13b 210.8165GTV-12 11.5–12.5 2.67 18.74816 8 15.67306 8 38.82406 27 0.5120956 13b 210.5965GTV-13 14.0–15.0 3.22 18.72796 8 15.66906 9 38.80876 27 0.5121396 14b 29.7365GTV-3 17.0–18.0 3.89 18.70756 8 15.66346 8 38.78926 23 0.5121516 17b

65GTV-4 21.0–22.0 4.78 18.70636 8 15.66126 8 38.78546 25 0.5121476 17b 29.5865GTV-5 24.0–25.0 5.44 18.71196 15 15.65876 18 38.78406 60 0.5121826 9 28.9065GTV-5a 18.71276 16 15.65976 19 38.78726 6365GTV-14 25.5–26.5 5.78 18.71566 9 15.66126 10 38.79086 29 0.5121566 5b 29.4065GTV-6 27.0–28.0 6.11 18.71286 9 15.65946 11 38.78276 35 0.5121606 6 29.3265GTV-6a 18.71486 10 15.66196 11 38.79096 3665GTV-6a 18.71436 8 15.66096 8 38.78586 2365GTV-6a 18.71636 8 15.66346 8 38.79406 2665GTV-7 30.0–31.0 6.78 18.70306 15 15.66376 16 38.79876 45 0.5121646 13b 29.2565GTV-7a 0.5121556 9b 29.4265GTV-8 33.0–34.0 7.44 18.69946 19 15.66416 22 38.79616 71 0.5121726 8b 29.0965GTV-9c 37.0–38.0 8.33 18.70006 8 15.66546 9 38.80106 27 0.5121586 9b 29.36Apatite 52–56 19.22126 51 15.69186 43 38.84966 114 0.5122056 27b 28.45

a Duplicate Pb or Nd isotopic analysis, same dissolution;b Nd dynamic multicollection analysis;c Lion seamount sample closest to the apatite layer.«Nd 5 (143Nd/144Ndsample/

143Nd/144NdCHUR 2 1) 3 104, 143Nd/144NdCHUR 5 0.512638. Propagated errors (2s) on Pb isotope ratios includeuncertainties in the triple spike composition and those for both mass spectrometer runs (see Galer, 1997, 1999; Hamelin et al., 1985).


might have modified its Pb and Nd isotopic composition. How-ever, this does not appear to have been the case for a numberof reasons. (1) Koschinsky et al. (1997) have shown that Pb andREE are not significantly depleted in Fe–Mn crusts during

phosphatization, in contrast to some other elements. Rather,they are probably incorporated locally into more stable phos-phatic phases, thus preserving their respective isotope ratios.(2) Pb and Nd isotopic measurements on apatite taken from the

Fig. 2. Pb and Nd isotopic variations plotted as a function of age in Tropic crust (121DK) and Lion crust (65GTV). Theages were derived from the10Be growth rates of 3 mm/Ma (121DK) and 4.5 mm/Ma (65GTV) reported by Koschinsky etal. (1996). Pb and Nd isotopic records for crust 121DK, representative of eastern NADW, are shown in (a)–(d), while thosefor crust 65GTV, reflecting MOW, correspond to (e)–(h). The error bars shown as insets in (f) and (h) correspond to theexternal 2s reproducibility based on standard analyses and sample duplicates; error bars on206Pb/204Pb ratios are the samesize as the symbols. The solid lines at 8 and 4 Ma are arbitrary divisions which mark times when breaks in the206Pb/204Pbrecord occur. The time period representing the Messinian salinity crisis (6.5–5 Ma) is shown as a stippled field. There isa gross similarity in the206Pb/204Pb ratio variation pattern in both crusts—for example, the increases from about 3 to 4 Maago and the maximum at 6 Ma are common. However, the amplitude of variation is about twice as large in 121DK comparedto that in 65GTV. Variations in the207Pb/204Pb ratio for the most part follow those of206Pb/204Pb, but a different patternis seen in the two crusts: in 121DK, the oldest section has higher207Pb/204Pb ratios than in the period 4–0 Ma, while 65GTVdisplays the opposite. In addition, during the Messinian207Pb/204Pb ratios increase in 121DK but decrease in 65GTV. In bothcrusts,208Pb/204Pb ratios increase from 3 to 4 Ma to the present, matching the206Pb/204Pb ratio variation.«Nd values do not showas much variation as the three Pb isotope ratios, but display a decrease from 3 to 4 Ma toward the present in both crusts.


lowermost part of the crust show that it has a radiogenicsignature (206Pb/204Pb 5 19.22; 207Pb/204Pb 5 15.69, and208Pb/204Pb 5 38.85;«Nd 5 28.5) (Table 1). Therefore, anycontamination from the apatite should shift the oxide-hosted Pbisotopic composition toward more radiogenic values and, fur-ther, there should be a positive correlation between phosphoruscontent and206Pb/204Pb ratio in the Mn oxide layers. However,the reverse is seen: high P contents (9%) in the bottom part ofthe crust are associated with low206Pb/204Pb and vice versa inthe upper 10 mm of the crust (P , 1%) (Koschinsky et al.,1996). (3) The maintenance of an isotopic contrast between theapatite and the closest Mn–Fe oxide layer (65GTV-9:206Pb/204Pb5 18.70;207Pb/204Pb5 15.67, and208Pb/204Pb538.80;«Nd 5 29.4) (Table 1) is strong evidence that phospha-tization did not significantly affect the Pb and Nd isotopiccompositions of the Fe–Mn oxides.

The high precision Pb isotopic data allow us to resolvehighly detailed variations in all three Pb isotope ratios in aFe–Mn crust for the first time. In Tropic crust,206Pb/204Pbratios show three maxima which occur at;13 Ma (18.98), 6Ma (18.92), and 0 Ma (18.95) (Fig. 2a). The maxima at 13 and6 Ma are followed by decreases in206Pb/204Pb, with minima at8 Ma (18.88) and 4 Ma (18.84). Thus, the pattern of variationin 206Pb/204Pb ratio has a quasi-cyclic character which wearbitrarily subdivide into three time slices: 13–8 Ma, 8–4 Maand 4–0 Ma (Fig. 2a). Within each “cycle”, the changes in206Pb/204Pb ratios occur gradually but quite systematically withtime—a pattern not documented from Fe–Mn crust recordsbefore. The206Pb/204Pb variations with time in Lion crustappear to mirror those of Tropic crust, but are less pro-nounced—at least for the record available back to 8 Ma(Fig. 2e).

The 207Pb/204Pb ratios display a slightly different pattern ofvariation with time than those of206Pb/204Pb. In the Tropiccrust, the period 13–5 Ma is characterized by high values(15.73–15.71) relative to the period 4–0 Ma (15.69–15.70)(Fig. 2b). The reverse pattern is observed in the Lion crust,where higher207Pb/204Pb ratios (15.68 vs 15.66) occur duringthe more recent period 4–0 Ma (Fig. 2f). It is important toemphasize, though, that the changes in207Pb/204Pb observedwithin each time interval are again gradual and progressive,like those of206Pb/204Pb. The208Pb/204Pb ratios remain rela-tively constant (;39.06) between 13 and 8 Ma in Tropic crust(Fig. 2c), while a slight decrease is observed from 8 Ma to 6 Ma(38.80–38.78) in Lion crust (Fig. 2g). From 4 Ma on, bothcrusts show a gradual increase in208Pb/204Pb ratios, mirroringthe rise seen in206Pb/204Pb ratios (Figs. 2c and g).

During the Messinian (6.5–5 Ma), Pb isotope ratios in bothcrusts are quite distinct from those at other times—in particular,206Pb/204Pb ratios are higher than those observed before theMessinian (Figs. 2a,e). By contrast,207Pb/204Pb and208Pb/204Pb increase in Tropic crust (Figs. 2b and c) but the oppositeis observed in Lion crust (Figs. 2f,g).

The Pb isotopic variations over time in both crusts displaywell-defined linear arrays in Pb–Pb isotope space (Fig. 3).Based on the206Pb/204Pb variations in the Tropic crust, threedifferent mixing lines can be distinguished with switches be-tween the mixing lines occurring at 8 and 4 Ma (Figs. 3a, b).Similarly, Pb isotope ratios in samples from the last 4 Ma inLion crust display a well-defined mixing line in both206Pb/

204Pb vs 207Pb/204Pb and206Pb/204Pb vs 208Pb/204Pb space(Figs. 3c, d).

The fact that linear arrays are found in207Pb/204Pb–206Pb/204Pb and208Pb/204Pb–206Pb/204Pb space indicates beyond anydoubt that these arrays are true mixing lines between two—andonly two—endmember components within each time slice.Perhaps more striking, though, is that there is a gradual changein Pb isotopic composition along the mixing lines with time.Thus, during each time slice, the endmember components hadfixed isotopic compositions and the Pb mixing proportions atthe locations of the crusts changed systematically and graduallywith time.

Nd isotopic compositions in Tropic crust («Nd 5 210.8 to211.8) are less radiogenic than those observed in Lion crust(«Nd 5 28.9 to 211) (Figs. 2d, h).«Nd values vary to someextent, but overall exhibit less variation than those of the threePb isotope ratios. The only trend seen is a general decrease in«Nd toward the present-day in both crusts, starting 3–4 Ma ago.

3.2. The Present-Day Atlantic: Ocean Circulation and Ndand Pb Sources

Three principal water masses contribute to the formation ofpresent-day NADW (Fig. 1): Denmark–Straits Overflow Water(DSOW) and Iceland–Scotland Overflow Water (ISOW) flowthrough the Gibbs Fracture Zone into the western basin and joinLabrador Sea Water (LSW), mixing to form NADW (Broeckerand Peng, 1982). The deep eastern Atlantic basin is ventilatedby waters from the western Atlantic flowing west-to-eastthrough low latitude fracture zones (Fig. 1). The RomancheFracture Zone (RFZ) is particularly important in this regard,where the inflowing NADW becomes mixed and diluted withSouthern Component Water (SCW) flowing northward into theAtlantic (Broecker et al., 1985). Intense vertical mixing occursat the RFZ, altering the characteristics of western NADWduring its transit into the eastern Atlantic basin (Polzin et al.,1996).

Among the three water masses contributing to NADW, LSWhas a distinct Nd isotopic composition with an«Nd value of218, which is acquired from ancient continental sources(Stordal and Wasserburg, 1986). LSW differs markedly fromthe more radiogenic DSW («Nd 5 28.6) and ISOW («Nd 5 27.7)(Piepgras and Wasserburg, 1987) and imparts a strongly unradio-genic Nd character to NADW («Nd 5 213.5). Ndisotopic mea-surements on seawater indicate that the present-day NADW«Nd of 213.5 in the western basin becomes more radiogenicfollowing its passage into the eastern basin («Nd ' 212) as aresult of the addition of Nd from SCW («Nd ' 28.5) (Jeandel,1993; Piepgras and Wasserburg, 1980; Piepgras and Wasser-burg, 1982; Piepgras and Wasserburg, 1987; Spivack and Was-serburg, 1988).

Pre-anthropogenic Pb isotope ratios for NADW and SCWhave been inferred from surface scrapings of hydrogenousFe–Mn nodules and crusts (Abouchami and Goldstein, 1995;Burton et al., 1997; Simonetti et al., 1995; 1996; Von Blanck-enburg et al., 1996). The assumption that such Fe–Mn depositsreflect seawater is justified on the basis of the consistencybetween their Nd isotope ratios with those of the water massesin which they sit (Aplin et al., 1986/87; Albare`de and Gold-stein, 1992; Albare`de et al., 1997). The most radiogenic Pb


isotopic signature in the oceans today is found in NADW(206Pb/204Pb $ 19) while SCW, by contrast, has a compara-tively low 206Pb/204Pb ratio (#18.85) (Abouchami and Gold-stein, 1995).

Another potential Pb and Nd source to the Atlantic is Sa-haran dust, particularly in view of the fact that the TropicSeamount crust is located directly on the “Saharan Air Layer”trajectory. At the present-day, the “Harmattan” winds transportSaharan aerosols into the eastern Atlantic during the summermonths—an influence that can be traced as far away as SouthAmerica (Prospero et al., 1981). The eolian dust flux to theeastern Atlantic is presently estimated to be 1000 mg/cm2/kyr(Duce et al., 1991) and has probably varied through time (Rea,1994). Saharan aerosols have unradiogenic Nd isotopic com-positions with«Nd values ranging from211 to215 (Goldsteinet al., 1984; Grousset et al., 1988, 1992; Henry et al., 1994;Rognon et al., 1996). In contrast, very few Pb isotopic data arepresently available on Saharan aerosols (Hamelin et al., 1989).For this reason, Pb isotopic compositions of deep sea sedimentsfrom the Atlantic (Sun, 1980) have been used as representativeof Saharan dusts, even though such sediments must containboth an authigenic (seawater) and an eolian (dust) component.Below, we discuss the nature of the mixing endmembers in-volved in the eastern Atlantic over the last 13 Ma in the light ofthese known Pb and Nd sources to the Atlantic, drawing in parton evidence from the Pb and Nd isotopic record of WNADW

derived from Fe–Mn crust Bm1969.05 from the NW Atlantic(Burton et al., 1997).

3.3. Tropic Seamount Crust: History of NADWAdvection into the NE Atlantic

Over the last 13 Ma, the Pb isotopic composition of theTropic Seamount crust has changed in two ways: both abruptlyand gradually. Such variations in all three Pb isotope ratiosreflect changes in the provenance of Pb supplied to the NEAtlantic through time. The abrupt changes can be seen in thejumps between binary mixing arrays at 8 Ma and 4 Ma;gradual, progressive changes, by contrast, are evident in thevarying proportions of the two endmembers contributing toform the linear binary mixing array within each time slice(13–8 Ma; 6.5–4 Ma and 4–0 Ma) (Figs. 3a,b and 4b,c). Theselinear arrays demonstrate that: (1) mixing took place between ahigh206Pb/204Pb and a low206Pb/204Pb component; (2) that themixing proportions of the two sources changed gradually andappreciably with time, and (3) the two components had differ-ent isotopic compositions at 13 and 4 Ma.

Figure 4 plots together the Pb and Nd isotopic data from theTropic crust, Lion crust, and NW Atlantic crust Bm1969.05(Burton et al., 1997), which are representative of ENADW,MOW, and WNADW, respectively. Also plotted are Pb and Ndisotopic compositions of Atlantic Fe–Mn nodules and crusts,

Fig. 3. Pb isotopic covariations in NE Atlantic Fe–Mn crusts. In (a) and (b),207Pb/204Pb vs206Pb/204Pb and208Pb/204Pbvs 206Pb/204Pb are plotted for Tropic crust 121DK. The samples have been subdivided based on the time slices defined inFig. 2 (0–4 Ma, 4–8 Ma, and 8–13 Ma). The corresponding ages (in Ma) are labelled next to each sample. The three linesdrawn were determined by linear regression through the data for each time slice. Since the data points fall within analyticalerror (see Fig. 2) of these lines within each time interval, it demonstrates that Pb in 121DK is dominated by binary mixing.Changes in the isotopic compositions of the binary mixing endmembers occurred at 8 and 4 Ma, implying that at least fourdifferent Pb sources have contributed to the eastern Atlantic over the past 13 Ma. Particularly striking is the age progressionalong the binary mixing lines, which reflects a gradual change in the relative proportions of the two Pb endmembers throughtime within each time slice. In (c) and (d), corresponding plots are shown for Lion seamount crust 65GTV. Pre-Messinianand Messinian period samples have distinct and more tightly grouped Pb isotopic compositions compared to those ofpost-Messinian samples. The latter also lie on a well-defined mixing line, and there is a gradual increase in Pb isotope ratiosalong the mixing line towards the present.


Atlantic sediments, and Saharan dusts. Although the Nd isoto-pic variations over the past 13 Ma are smaller than those seenin Pb isotopes (Fig. 2)—probably a result of the longer oceanicresidence time of Nd (;103 yr) compared to Pb (;102 yr)—Pband Nd isotopic relationships are useful for constraining theendmember components in the Tropic crust. From 13 to 8 Ma,206Pb/204Pb ratios decrease and«Nd values increase; by con-trast,206Pb/204Pb ratios increase and«Nd values decrease from

4 to 0 Ma, mirroring the trend defined by WNADW over thepast 13 Ma (Fig. 4a). Therefore, in order to explain the Pb andNd isotopic variations within each time slice, two componentsare required: a high206Pb/204Pb–low«Nd component and a low206Pb/204Pb–high«Nd component.

The relative206Pb/204Pb ratios in the ocean basins at thepresent-day are: Atlantic. Antarctic ' Indian . Pacific(Table 2). Let us assume this relative order to have been a

Fig. 4. (a)«Nd vs206Pb/204Pb, (b)207Pb/204Pb vs206Pb/204Pb, and (c)208Pb/204Pb vs206Pb/204Pb for Tropic crust (121DK)and Lion crust (65GTV). Also shown are data from NW Atlantic crust Bm1969.05 (Burton et al., 1997), Atlantic sediments(Sun, 1980), Atlantic Fe–Me nodules (Simonetti et al., 1995, 1996; Von Blanckenburg et al., 1996), Saharan dust (Hamelinet al., 1989) and sediment trap data (Hamelin et al., 1997). The arrows show the direction of the isotopic changes forwardswith time in the two crusts with corresponding age intervals indicated (see Fig. 3). In (a), boxes indicate the«Nd and206Pb/204Pb of WNADW, ENADW, MOW as inferred from crusts Bm1969.05 (Burton et al., 1997), 121DK and 65GTV(this study), respectively. The estimates of«Nd and206Pb/204Pb for SCW at 13 Ma are from Abouchami et al. (1997). Thedashed line in (b) and (c) represents the north (N) to south (S) trend defined by Atlantic Fe–Mn nodules (Simonetti et al.,1995, 1996; Von Blankenburg et al., 1996). The present-day isotope ratios of crust Bm1969.05 reported by VonBlankenburg et al. (1996) are shown as a crossed square.


long-standing feature of the oceans. In this case, consideringthe present-day eastern North Atlantic circulation, the twocomponents (high206Pb/204Pb and low 206Pb/204Pb) of thethree mixing lines (13–8 Ma, 6.5–4 Ma, and 4–0 Ma) seen inFig. 3 could correspond to Northern Component Water(NCW)—the equivalent of present-day NADW—and SCW,respectively. In NW Atlantic crust Bm1969.05, however, thePb isotopic covariations from the last 13 Ma define a singletrend and have, for a similar206Pb/204Pb ratio, lower207Pb/204Pb and208Pb/204Pb ratios than those found in Tropic Sea-mount crust—the latter exhibiting three distinct trends (Figs.4b,c). Table 2 reports the206Pb/204Pb ratios and«Nd values ofthe different endmembers examined below as potential compo-nents of the binary mixing lines during the different time slices.

3.3.1. The 4–0 Ma time slice: Pb mixing relationships andsources of Pb and Nd

Among the potential sources present in the Atlantic, onlyWNADW appears to have the appropriate isotopic features—namely 206Pb/204Pb ratios higher than 19 and low«Nd of213—to explain the high206Pb/204Pb–low «Nd component.This suggests that the latter may reflect WNADW (Fig. 4a). Butat first glance this suggestion appears to be ruled out by the low207Pb/204Pb ratio of WNADW inferred from NW Atlantic crustBm1969.05 (Fig. 4b). However, the Pb isotopic compositionfrom a surface scraping of this same crust reported by VonBlankenburg et al. (1996) (206Pb/204Pb5 19.236 0.02;207Pb/204Pb5 15.686 0.02;208Pb/204Pb5 39.456 0.07) is withinanalytical uncertainty of that implied by the Tropic crust datafor the “NADW” component in the 4–0 Ma period (Figs. 4b,c). Moreover, the present-day208Pb/204Pb ratios of NADWinferred from both the western and eastern Atlantic records are

in relatively good agreement, and consistent with the NADWcomposition deduced from regression of Atlantic nodule andcrust Pb isotope data (Fig. 4c). Taken together, these observa-tions suggest that the low207Pb/204Pb ratios in the NW Atlanticrecord by Burton et al. (1997) may, to some extent, reflectapplication of an inappropriate instrumental bias correction.

Two other lines of evidence support NADW as the high206Pb/204Pb–low «Nd component in the NE Atlantic over thepast 4 Ma: Firstly, both the western and eastern Atlantic recordsdisplay a secular decrease in«Nd values, trending towardspresent-day values of NADW in these basins (213 and212,respectively); secondly, there has been a simultaneous increasein 206Pb/204Pb ratios and decrease in«Nd values in both basinssince;3–4 Ma ago (Fig. 4a). Taken together, these observa-tions are fully consistent with the isotopic signal associatedwith NADW in the western basin being advected into theeastern Atlantic throughout the past 4 Ma.

Let us now consider the low206Pb/204Pb–high«Nd compo-nent in Figs. 3 and 4. Potential sources for this mixing com-ponent in the eastern Atlantic could be Saharan dusts, MOW, orSCW, all of which have relatively low present-day206Pb/204Pbratios (18.76, 18.78, and#18.85, respectively, see Table 2)and, with the exception of Saharan dust, more radiogenic Ndisotopic compositions than NADW. Of these possibilities, Sa-haran dust can immediately be excluded on the basis of itsunradiogenic Nd isotopic signature (Goldstein et al., 1984;Grousset et al., 1988, 1992; Henry et al., 1994; Rognon et al.,1996).

Pb–Nd isotopic covariations give some support to “MOW”being the low206Pb/204Pb–high«Nd component. This followssince MOW has a206Pb/204Pb ratio of 18.78 and an«Nd valueof about211, and lies at the unradiogenic extreme of the 4–0Ma trend defined by the Tropic crust data (Fig. 4a). However,the low Pb isotope ratios of MOW over the past 8 Ma lie

Table 2. Pb and Nd isotopic compositions of water masses inferred from Fe–Mn deposits

Water mass Age (Ma) «Nd206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

Present-dayWNADWa,b,c 0 213 19.0–19.2 15.66–15.70 39.0–39.20PDWd 0 24 18.70 15.62 38.70SCWc,e 0 26 to 28.5 18.70–18.90 15.63–15.65 38.70–38.90SCW* 0 28.5 18.85–18.95 15.64–15.66 38.85–38.95Saharan dustsf 0 211 to 215 18.76 15.69 n.a.MOWg 0 211 18.78 15.67 38.84ENADWf 0 212 18.95 15.698 39.06

PastWNADWh 13 210.9 18.84 15.63 38.84PDWd #10 24 18.68 15.62 38.67SCW* #10 27.4 18.75 15.63 38.76IOWi 13 27.8 18.75 15.73 39.14MOWg 8 29.0 18.70 15.66 38.80NCWg 13 #212 $18.95 #15.72 39.06“Tethys”g 13 $210 #18.87 #15.70 39.06

WNADW: Western North Atlantic Deep Water; PDW: Pacific Deep Water; SCW: Southern Component Water; IOW: Indian Ocean Water;ENADW: Eastern North Atlantic Deep Water; MOW: Mediterranean Outflow Water; NCW: Northern Component Water; “Tethys” is the southernsource at 13 Ma (see text).

* Isotopic compositions of SCW were calculated as 50:50 mixtures of Pb and Nd derived from NADW and PDW using equal Pb/Nd ratios in theendmembers (Abouchami et al., 1997).a Burton et al. (1997);b Von Blanckenburg et al. (1996);c Simonetti et al. (1995);d Abouchami et al. (1997);e Abouchami and Goldstein (1995);f Hamelin et al. (1989); Goldstein et al. (1984); Grousset et al. (1988); Henry et al. (1994); Rognon et al. (1996);g This study;h O’Nions et al. (1998);i Frank and O’Nions (1998).


systematically below the Tropic crust arrays in Pb isotopespace (Figs. 4b,c), indicating that MOW cannot be an endmem-ber component of the 4–0 Ma mixing line.

An alternative explanation is that the low206Pb/204Pb mixingendmember seen in the eastern Atlantic corresponds to SCW.Figure 4 shows that present-day SCW entering the SW Atlan-tic, as inferred from Fe–Mn nodule data, has generally low Pbisotope ratios (206Pb/204Pb ' 18.75; 207Pb/204Pb ' 15.64;208Pb/204Pb ' 38.80) and relatively high«Nd (about26) (Ta-ble 2). These features have been explained as reflecting those ofSCW during glacial periods (Abouchami and Goldstein, 1995),when NADW Pb and Nd contribution was reduced, or thepresence of a diagenetic Nd component in the nodules (Al-barede et al., 1997).

Testing SCW as this possible endmember is difficult since aPb and Nd isotopic series for SCW is currently not available.However,206Pb/204Pb and«Nd values for SCW;10 Ma agohave been inferred to be;18.70 and27.3 based on Pb and Ndisotopic covariations in Fe–Mn crust records from the Atlanticand Pacific, considering simple mixing between Atlantic andPacific waters (Abouchami et al., 1997). These isotopic featurescorrespond well to those of the low206Pb/204Pb–high «Nd

component. Estimates of the207Pb/204Pb and208Pb/204Pb ratiosof past SCW can similarly be obtained (Table 2). However,given the uncertainty on the present-day207Pb/204Pb ratio ofWNADW—15.62 of Burton et al. (1997) versus 15.68 of VonBlanckenburg et al. (1996)—such estimates are best consideredas only crude approximations. Taking this into account, we willuse a range from 15.66 to 15.70 for WNADW from surfacescrapings of nodules from the NW Atlantic (Simonetti et al.,1995; 1996; Von Blankenburg et al., 1996) and a value of 15.62for Pacific water as recorded by Fe–Mn crust Va13-2(Abouchami et al., 1997). If SCW is considered as a 50:50mixture of NADW and Pacific waters, its207Pb/204Pb ratio willlie between 15.64 and 15.66, a range consistent with that ofpresent-day SCW entering the SW Atlantic (15.63–15.65)(Table 2).

There is also a good agreement, given reasonable analyticaluncertainties, between the208Pb/204Pb ratio of the low206Pb/204Pb component of the mixing line and the present-day ratio ofSCW, as inferred from both Pb isotope data of surface scrap-ings from nodules and crusts (38.70–38.90), and the calculatedrange of 38.85–38.95 (Table 2). From these considerations,then, it appears that for the past 4 Ma SCW influence has beenpresent in the eastern Atlantic basin. Further, the Pb isotopiccomposition of SCW appears to have remained relatively con-stant during this period, as has already been inferred fromFe–Mn crust Va13-2 from the Central Pacific (Abouchami etal., 1997).

In summary, the Pb and Nd isotopic variations found inthe eastern Atlantic during the time interval 4 – 0 Ma appearto be best explained by mixing between WNADW (high206Pb/204Pb–low «Nd) and SCW (low 206Pb/204Pb– high«Nd). In turn, this would imply that the age progressivechanges in206Pb/204Pb ratio seen in Tropic crust (Fig. 2)record variations in the mixing proportions of Pb fromNADW and SCW entering the eastern Atlantic basin viaequatorial fracture zones (Fig. 1).

3.3.2. The 13–8 Ma time slice: Pb mixing relationships andsources of Pb and Nd

The high 206Pb/204Pb–low «Nd component in the easternAtlantic appears to have been WNADW during the period 4–0Ma. But this does not seem to be the case from 13 to 8 Ma. At13 Ma, WNADW is characterized by lower206Pb/204Pb (18.85)and higher«Nd (211) compared to its present-day equivalent(19.2 and 213, respectively) (Table 2). Such features arecompletely different from those of the high206Pb/204Pb–low«Nd component of the eastern Atlantic, which has much higherPb isotope ratios (206Pb/204Pb 5 18.96;207Pb/204Pb 5 15.73;208Pb/204Pb 5 39.1) although its«Nd of 211.5 is comparable(Table 2). Given that such a component is not present in thewestern Atlantic during the period 13–8 Ma, it raises thepossibility that the NE Atlantic may not have been ventilatedby waters derived from the western basin at all. In such a case,sources of Pb–whether northern or southern—would have beenquite different at this time in the eastern basin.

Other possible sources of radiogenic Pb and unradiogenic Ndto the eastern Atlantic between 13 and 8 Ma are MOW andSaharan dusts. SCW can be ruled out immediately based uponits estimated206Pb/204Pb of ;18.70 and«Nd of ;7.3 at;10Ma ago (Abouchami et al., 1997). Unfortunately, the record ofMOW from Lion crust does not go back beyond 8 Ma. How-ever, over the past 8 Ma, MOW always had systematicallylower Pb isotope ratios (18.70–18.78) and higher«Nd values(29 to 211) than those of ENADW. If the Mediterraneanoutflow into the Atlantic prior to 8 Ma also possessed suchisotopic characteristics, then MOW can also be excluded as thesource of radiogenic Pb and unradiogenic Nd to the easternAtlantic at that time.

Sahara dusts have an appropriate Nd isotopic signature toaccount for the high206Pb/204Pb–low«Nd component but theirPb isotopic composition is too unradiogenic. Nevertheless, theisotopic composition of the leachable Pb fraction of Saharandust—which presumably contributes to seawater—is unknownat present due to the effect of anthropogenic contamination(Hamelin et al., 1997). If the radiogenic Pb in Saharan dustparticles is preferentially leached, as has been shown to occurduring weathering (Erel et al., 1994; Harlavan et al., 1998),then Saharan inputs may be a potential candidate for the high206Pb/204Pb component at 13 Ma. On the other hand, the sourceareas of Saharan dusts may have changed through time as aresult of shifts in wind belts and may thus have been isotopi-cally heterogeneous. Therefore, it is difficult at present toassess properly the Saharan dust contribution to the Pb budgetof the eastern Atlantic.

The last possibility is that the eastern Atlantic was ventilateddirectly from northern water sources. There is no evidence forpresent-day penetration of ISOW or DSOW—the other twonorthern contributors to NADW—directly into the eastern At-lantic basin (Fig. 1). This is simply due to the low densities ofthese waters relative to those flowing through equatorial frac-tures zones (Broecker et al., 1985). But was this always thecase? Recent Nd and Sr isotopic studies of the clay fractionfrom North Atlantic sediment cores show that there was achange in the clay provenance from Icelandic sources (high«Nd

and low 87Sr/86Sr) in interglacial periods, to old Europeansources (low«Nd and high87Sr/86Sr) during glacial periods


(Bout-Roumazeilles et al., 1998). The Pb isotopic compositionsof these two sources are also expected to differ—Icelandicvolcanic sources being less radiogenic in207Pb/204Pb (#15.55;see compilation in Mertz and Haase, 1997) than old crustalterranes from Europe (see Gwiazda et al., 1996 for summary).Therefore, we speculate that the source of radiogenic Pb andunradiogenic Nd present in the eastern Atlantic during theperiod 13–8 Ma may have been located in the Norwegian andGreenland Seas, with old terranes from the Scandinavian shieldacting as sources of Pb and Nd to ENADW. If correct, thiswould imply that deep water flow into the eastern Atlantic priorto 8 Ma may have occurred directly from the north rather thanvia the western basin and equatorial fracture zones as at thepresent-day.

In summary, during the period 13–8 Ma an entirely differentradiogenic Pb and unradiogenic Nd source was present in theeastern Atlantic and available data do not show its presence inthe western Atlantic. At the moment, the provenance of thissource remains unclear, but possible candidates include a north-ern water source—located in the Norwegian–Greenland Seas—MOW, and Saharan dusts.

Having discussed the nature of the high206Pb/204Pb–low«Nd

source, we now turn our attention to the low206Pb/204Pb–high«Nd component (Figs. 3 and 4). None of the sources known tocontribute to the eastern Atlantic appear to possess the appro-priate isotopic features of this component. For example, Sa-haran dust cannot be responsible for the increase in«Nd valuesobserved from 13 to 8 Ma, even though it could explain thedecrease seen in206Pb/204Pb (Fig. 4a). SCW is also an unlikelycandidate for the low206Pb/204Pb–high«Nd component giventhe high 208Pb/204Pb ratio of 39.1 of this component. Thisfollows since SCW is a mixture of Pacific and Atlantic waterswhich both had low208Pb/204Pb ratio prior to 8 Ma, as recordedby crusts from the NW Atlantic and Pacific (Abouchami et al.,1997; Burton et al., 1997; Christensen et al., 1997; Ling et al.,1997) (Table 2). Furthermore, WNADW at 13 Ma had too low207Pb/204Pb and208Pb/204Pb ratios to account for those in thismixing endmember. It can be concluded that the low206Pb/204Pb–high«Nd component reflects an entirely different sourcefrom the ones discussed above. This implies that not one butboth mixing endmembers during the time interval 13–8 Ma hada completely different origin from those in the more recentperiod 4–0 Ma.

One source which has not been considered up until now ispaleo-Tethys water. Abouchami and Goldstein (1995) noticedthat the Northern Indian ocean is characterized by higher207Pb/204Pb and208Pb/204Pb ratios than the Southern Indian ocean fora given206Pb/204Pb ratio. The recently published Pb and Ndisotopic record of an Indian ocean crust SS63 (Frank andO’Nions, 1998; O’Nions et al., 1998) indicates that at about 13Ma, Indian ocean water had relatively low206Pb/204Pb (18.75),high 207Pb/204Pb (15.73), and high208Pb/204Pb (39.14) ratios,while «Nd values (27.8) were similar to those measured inpresent-day Mn nodules from the Indian ocean (Albare`de et al.,1997). Such isotopic characteristics are entirely consistent withthose of the low206Pb/204Pb–high«Nd component in the timeslice 13–8 Ma (Figs. 3 and 4). If paleo-Tethys water is indeedthis mixing component, some important implications follow forthe paleocean circulation in the Atlantic prior to 8 Ma (seebelow).

In conclusion, eastern Atlantic Pb has, without question,been dominated by only two components during the time slices13–8 Ma and 4–0 Ma. Over the last 4 Ma, Pb and Nd isotopicvariations in the Tropic crust reflect mixing of variable propor-tions of Pb and Nd derived from two endmembers—NADWand SCW. The gradual increase in Pb and Nd contribution fromNADW and corresponding decrease from SCW in the easternAtlantic basin reflect a strengthening of the isotopic signalassociated with WNADW from about 4 Ma toward the present-day, in agreement with the western Atlantic record of crustBm1969.05. This further implies that advection of NADWfrom the western into the eastern Atlantic basin has been inoperation throughout the last 4 Ma. The Pb isotopic data alsoreveal that the provenance of Pb and Nd were entirely differentprior to 8 Ma, possibly suggesting a completely different pat-tern of abyssal circulation at that time.

3.4. Global Correlation of Pb Isotopes with PaleoceanCirculation

We have shown above that the provenance of Pb and Nd inthe eastern Atlantic changed quite abruptly at about 4 Ma, andthat the Pb isotopic compositions of both mixing endmembersshifted synchronously. Such changes provide a record of mod-ifications to Pb and Nd inputs and/or their fluxes to seawater,which in turn are ultimately controlled by climate, ocean, andatmosphere circulation patterns, and paleogeography.

During the Miocene, several climatic and tectonic eventscontributed to changes in surface and deep water circulation(see Kennett, 1982). Of particular importance to changes in thethermohaline circulation in the Atlantic region were the fol-lowing events: (1) the emergence of the Greenland–ScotlandRidge 12–15 Ma ago (Wright and Miller, 1995) which coin-cided with the closure of Tethyan Passages at about 12 Ma(Woodruff et al., 1981); (2) The uplift of Central America,starting around 8 Ma (Collins et al., 1996), leading to gradualrestriction of water exchange between the Pacific and Atlantic,ending 3–4 Ma ago (Keigwin, 1982); (3) the Messinian salinitycrisis (6.5–5 Ma); (4) the onset of Labrador Sea Water (LSW)production (;3–4 Ma) and increased aridity in the Saharanregion at about the same time; and finally, (5) the initiation ofNorthern Hemisphere glaciations during the Pliocene (2.6 Ma).Although Atlantic deep water production has varied since theMiocene, paleoceanographic data and General CirculationModels (GCM) results indicate that the “conveyor” may havebeen in operation since the early Miocene, albeit perhaps in-termittently (Blanc et al., 1980; Maier-Reimer et al., 1990;Mikolajewicz and Crowley, 1997; Murdock et al., 1997; Woo-druff and Savin, 1989; Woodruff et al., 1981; Wright andMiller, 1995; Wright et al., 1992).

Changes in the strength of LSW inputs at around 3–4 Mawould dramatically affect both the Nd and Pb isotopic compo-sition of NADW because LSW has the most distinctive Nd, andpresumably also Pb, isotopic composition compared to othersources of Pb and Nd (i.e., from DSOW and ISOW) contrib-uting to NADW. In the NW Atlantic, the shift toward radio-genic Pb and unradiogenic Nd observed at around 3–4 Ma inFe–Mn crust Bm1969.05 has been ascribed to an increasedcontribution from LSW (Burton et al., 1997). Such an expla-nation is quite consistent with the Pb and Nd isotopic changes


seen in the eastern Atlantic record since about 4 Ma, furthercorroborating our conclusion that the NADW isotopic signalhas been advected into the eastern Atlantic since that time.

More important, though, is that the Pb binary mixing arraysin Tropic crust reveal age progressive changes in the mixingproportions of Pb contributed from NADW and SCW, with anincreased contribution from NADW from 4 Ma toward thepresent-day. These results are in complete agreement withpaleoceanographic data and recent GCMs which have stressedthe role of the closure of the Isthmus of Panama (8–3 Ma) incontrolling the strength of the thermohaline circulation, wherefinal constriction at about 3 Ma leads to the “turn-on” ofNADW (Maier-Reimer et al., 1990; Mikolajewicz and Crow-ley, 1997; Murdock et al., 1997).

During the Messinian period (6.5–5 Ma), there is a shifttoward higher Pb isotope ratios (Fig. 2). Although the shiftcould conceivably be related to decreased Mediterranean Out-flow, the shift is in the opposite sense to that predicted for areduced outflow—namely diminished deep water production inthe North Atlantic (Reid, 1979; Johnson, 1997) and thussmaller contributions from radiogenic northern sources. A morepromising explanation for the Pb isotopic shift is tectonicmovements in and around the fracture zones of the Mid-Atlantic Ridge. Such movements could cause a deepening orshallowing of sills, modifying both the flow structure andrelative proportion of water masses flowing through from thewestern into the eastern basin. Indeed, the presence of 56 1Ma-old limestones from the RFZ area indicates that parts of theRFZ were at or above sea level at this time (Bonatti et al.,1996). If the RFZ sill was shallower, the flow of cold southernwaters (found below 4 km today) eastwards would diminishand, consequently, SCW influence in the eastern basin mayhave been less or ceased altogether. In such a case, a greaterinfluence from northern sources would be expected, whichseems to be consistent with the higher Pb isotope ratios ob-served (Fig. 2).

Prior to 8 Ma, the eastern Atlantic appears to have beendominated by entirely different Pb and Nd sources than those inthe period 4 Ma to the present. We suggested these sources maycorrespond to a northern source—most likely located in theNorwegian–Greenland seas—and a source which may haveoriginated in Tethys. GCMs have placed special emphasis onthe role of the closure of the Panama Gateway in Atlantic oceancirculation changes. But an alternative point of view has beenput forward by Wright and Miller (1995), who suggested thatNCW (NADW today) production may have been regulated bythe tectonic history of the Greenland–Scotland ridge (Vogt,1972). Thus, their reconstruction of NCW production is corre-lated with Icelandic magmatic activity, such that episodes ofhigh NCW flux are in phase with periods of low magmaticproductivity and vice versa. In addition, Miller et al. (1987)have related the increased ventilation in the eastern basins ofthe North Atlantic at about 15 Ma to a bottom water sourcefrom the Norwegian–Greenland Sea. From these studies, itfollows that if production of NCW was high during periods oflow Icelandic magmatic activity, Pb and Nd inputs from vol-canic sources would have been smaller, and the Pb and Ndisotopic characteristics of NCW should be dominated by inputsfrom old crustal terranes from the Scandinavian shield. Thisfavors our suggestion that a Norwegian–Greenland source may

indeed correspond to the high206Pb/204Pb–low«Nd componentin the eastern Atlantic prior to 8 Ma. Therefore, based upon thePb isotopic record from the eastern Atlantic basin and thesepaleoceanographic studies (Miller et al., 1987; Wright andMiller, 1995), it appears that the principal Pb and Nd sources aswell as production of deep water in the North Atlantic werelocated in the Greenland–Norwegian seas during the period13–8 Ma, with the Labrador sea contribution gaining impor-tance from around 3 Ma onwards—a suggestion also consistentwith recent GCM predictions (Mikolajewicz and Crowley,1997).

As to the origin of the low206Pb/204Pb–high«Nd componentin the eastern Atlantic during the period 13–8 Ma, a Tethyiansource is implicated by the similarities in its Pb and Nd isotopiccomposition to that of Indian ocean water. This possibility isfurther supported by evidence for a highd13C water massduring the Miocene in the Atlantic. According to Woodruff andSavin (1989), this “warm saline deep water” originated in theeastern Tethys and flowed into the Southern Ocean where itwas redistributed into the Pacific and Atlantic. Wright et al.(1992) proposed that the source of this highd13C water wasmore probably located in the Southern Ocean and remainedactive until the late Miocene. We cannot be conclusive aboutthe exact origin of the low206Pb/204Pb–high«Nd component—namely whether it originated in the Northern Indian Ocean or inthe Southern Ocean, for which no isotopic record is yet avail-able. Nevertheless, it is highly significant that the Pb and, to alesser extent Nd, isotopic record of Tropic crust reveals distinctsource provenances for these elements in the eastern Atlanticprior to and after 8 Ma. The fact that these changes areassociated with tectonic events and correlate with deep watercirculation changes in the Atlantic demonstrates the over-whelming sensitivity of Pb isotopes for global change prove-nance studies.

3.5. The Record of Mediterranean Outflow Water:Lion Seamount Crust 65GTV

The Mediterranean sea has a negative water balance: lesssaline waters from the North Atlantic flow in at the surface andare replaced by an outflow of highly saline and dense Medi-terranean water at depth. Mediterranean Outflow Water(MOW) enters the North Atlantic via the Straits of Gibraltar ata rate of 1.6 Sv and then flows northward into the Norwegiansea where it contributes to the formation of NADW (Reid,1979) (Fig. 1).

A major paleoenvironmental change happened during theMessinian (6.5–5 Ma) when the Mediterranean basin becameisolated from the Atlantic, leading to dessication and depositionof large amounts (;106 km3) of evaporites (Hsu¨ et al., 1977).The Messinian event was significant enough to cause a drop inthe total salt budget of the world oceans and may have affectedthe thermohaline circulation, although opinion is divided onthis question (Blanc et al., 1980; Keigwin et al., 1987). Reid(1979) has stressed the importance of highly saline Mediterra-nean waters in initiating deep water production in the Norwe-gian–Greenland seas and in the Labrador sea—the main com-ponents contributing to NADW. Recently, Johnson (1997)suggested that MOW influences Northern latitude climate bydriving warm surface waters into the Labrador sea. In his


model, the ensuing upwelling of cold waters in the Norwegian–Greenland seas at the Scotland–Faeroe sill might possiblytrigger the growth of ice sheets in Canada. These studieshighlight the need for a better understanding of the history ofthe Mediterranean outflow.

The Pb and Nd isotopic record from Lion seamount crust isused, firstly, to assess the provenance of Pb and Nd in MOWthrough time, and secondly, to evaluate the effects of theMessinian salinity crisis on Pb and Nd sources during this timeperiod.

3.5.1. Sources of Pb and Nd to MOW

The Mediterranean outflow is a mixture of Atlantic SurfaceWater and waters originating from within the Mediterraneanbasin (Wust, 1961). Present-day MOW has a more radiogenic«Nd (29.4) than that of NADW (213.5 6 0.5), to which itcontributes 10%–30% Nd (Spivack and Wasserburg, 1988).Isotopic data on particulate and dissolved Nd in the westernMediterranean sea show that atmospheric inputs dominate theNd budget, with 90% derived from the Sahara («Nd 5 2136 1) and the remainder coming from Europe («Nd 5 211.2)(Henry et al., 1994).

The Pb and Nd isotopic profile of Lion crust (Fig. 2) providesthe first isotopic characterisation of Pb in the Mediterraneanoutflow and the first time record of its Pb and Nd isotopiccomposition. From 4 Ma onwards (after the Messinian crisis),206Pb/204Pb ratios gradually increase from 18.71 at 4 Ma to18.78 today, indicating that a change in the source of Pboccurred around 4 Ma ago. Nd isotopic compositions in Lioncrust are less variable than those of Pb, with«Nd values varyingbetween29.7 and211 from 4 to 0 Ma, while slightly moreradiogenic Nd is found prior to 4 Ma («Nd 5 29 to 29.6)(Fig. 2h).

Potential sources of Pb and Nd in the Mediterranean regioninclude volcanic and crustal sources, both of which have highlyvariable isotopic compositions; in addition, a Saharan dustcontribution is highly likely given the location of the Lion crustwithin the trade winds belt (Fig. 1). Furthermore, the significantenrichment in Fe, Pb, Al, and Si observed in eastern Atlanticcrusts has been related to the influence of terrigeneous materialderived from the nearby African continent (Koschinsky et al.,1995). Of these possibilities, a significant contribution fromvolcanic sources seems to be precluded, since207Pb/204Pbratios in Lion crust are higher than those of recent volcanicrocks from the Mediterranean region (see Esperanca and Crisci,1995 and references therein). Trace metal measurements inaerosols (Dulac et al., 1987) also support this view.

Several studies have shown that the Saharan dust plume is amajor source of trace metals to the Mediterranean and that itcan, for example, make a significant contribution to the Ndisotopic budget of MOW (Boyle et al., 1985; Frost et al., 1986;Greaves et al., 1991; Henry et al., 1994; Measures and Edmond,1988; Piepgras and Wasserburg, 1983; Spivack and Wasser-burg, 1988; Van Green et al., 1988). In Fig. 4, the Saharanaerosol Pb isotope data of Hamelin et al. (1989) lie near to thepresent-day Pb isotopic composition of Lion crust. This sug-gests that Saharan dust may constitute the radiogenic endmem-ber of the 4–0 Ma binary mixing line (Fig. 4b). The gradualincrease in Pb isotopic ratios in Lion crust along the binary

mixing line since 4 Ma (Fig. 3d) can then be explained in termsof an increasing Saharan dust contribution towards the present.Such a strengthening of the Saharan dust plume from about 4Ma onwards is in complete agreement with the increase inNorth Africa aridity observed at this time in eolian recordsfrom the equatorial Atlantic (see summary in Rea, 1994).

The Pb isotopic compositions observed in MOW prior to 4Ma are lower than those afterwards, and show the influence ofa Pb source quite distinct from either Saharan dust or volcanicsources. Since Nd isotopic data for Atlantic surface sedimentscontaining Saharan components are currently lacking, we esti-mate the Pb and Nd isotopic compositions of Saharan dust viathose of their potential source terrains (Fig. 5)—circum-Med-iterranean granitoids (Juteau et al., 1986). While samples olderthan 4 Ma have similar Pb and Nd isotopic compositions tothose of Alpine granitoids, most samples younger than 4 Maoverlap the restricted field defined by granitoids from NWAfrica.

During the period 8–4 Ma,«Nd values are more radiogenicin Lion crust than during the last 4 Ma and are consistent withatmospheric and riverine inputs of Nd derived mostly fromwithin the Mediterranean basin. Nd isotopic compositions ineastern Mediterranean waters («Nd 5 27.7) (Henry et al.,1994) and rivers (Nile:«Nd 5 23.3; Seyhan:«Nd 5 26.2;Tarsus:«Nd 5 26.3) are more radiogenic than those in riversfrom the western Mediterranean basin (Po:«Nd 5 210.8;Rhone: «Nd 5 29.7) (Fig. 5) (Frost et al., 1986) and supportthis suggestion.

Therefore, the Pb and Nd isotopic records of Lion crust takentogether appear to indicate a switch from European (eolianand/or riverine) to predominantly Saharan inputs—mainly orig-inating from NW Africa—to MOW at about 4 Ma. The “euro-pean” endmember component still remained after 4 Ma, but hada comparatively lesser influence on MOW. Overall, these prov-enance changes seen in Lion crust seem to be consistent withthe 8 Ma record of Saharan climate reported by Tiedemann etal. (1989).

The change of Pb and Nd source in Lion crust occurs, withinthe resolution of our sampling, some time between 5 and 4 Ma,and matches the timing of initiation of water exchange betweenthe Mediterranean and the Atlantic at the end of the Messiniansalinity crisis. This, combined with the development of aridconditions in Africa around 4 Ma ago, would have allowed thetransport of the Saharan dust signal into the Mediterranean viathe inflow of Atlantic surface waters. The trace metal enrich-ment of deep Mediterranean waters today has been explained inthis manner (Boyle et al., 1985; Measures and Edmond, 1988;Van Green et al., 1988).

The significance atmospheric inputs may have on the REEand, especially, Pb budgets of the oceans still remains a matterof debate (Albare`de and Goldstein, 1992; Bertram and Elder-field, 1993; Elderfield and Greaves, 1982; Frost et al., 1986;Jeandel et al., 1995; Jones et al., 1994; Shimizu et al., 1994;Von Blanckenburg et al., 1996). Abouchami et al. (1997) haveargued that the decrease in Pb isotope ratio since the lastinterglacial seen in Central Pacific Fe–Mn crust Va13-2 reflectsincreased eolian inputs from Central America carried by thetrade winds. The evidence presented here that the Saharan dustplume has influenced the Pb isotopic composition of MOWalso demonstrates that atmospheric inputs may be an important


component in the mass balance of Pb in seawater, and espe-cially in regions of high eolian fluxes.

3.5.2. Effects of the Messinian salinity crisis

By comparing the Pb and Nd isotopic records from theAtlantic and the Mediterranean outflow, some insight can begained into the effects the Messinian salinity crisis had onNorth Atlantic deep water circulation. This exercise is possiblebecause the Pb and Nd isotopic signatures of NADW andMOW are distinct from one another. While it is clear thatMOW had a distinct isotopic composition during the Messinian(Fig. 2)—reflecting mainly internal European sources of Pb andNd to the Mediterranean basin—there is no strong evidence fora shutdown of MOW at that time. This follows, since if therewere no outflow, the Pb and Nd isotopic composition of Lioncrust, located just west of the Straits of Gibraltar, should reflecteither surface or deep Atlantic waters.

To elaborate on this point, present-day Atlantic surface wa-ters contributing to MOW have more radiogenic Nd («Nd 529.5 to 211.4) than deep waters («Nd 5 213.5) (Jeandel,1993; Piepgras and Wasserburg, 1980; Piepgras and Wasser-burg, 1987; Spivack and Wasserburg, 1988). The hydrocastprofile taken at station TTO-TAS80 (27°509N, 30°329W) nearLion crust shows that present-day NADW at this location hasan «Nd of 212.6 (Spivack and Wasserburg, 1988). If, duringthe Messinian, the Nd isotopic composition of Atlantic surfacewaters was more radiogenic than NADW (i.e.,.212.6), itcould account for the Nd isotopic signature of Lion crust at thattime.

The second possibility would be that the Lion crust wasbathed in NADW rather than Atlantic surface waters. But the

«Nd of ENADW during the Messinian is known to be211 fromTropic crust, and is clearly inconsistent with the value of about29 contemporaneously observed in Lion crust. The Pb isotopicrecord of Lion crust cannot be entirely reconciled with thisexplanation either. On the one hand, there are similar patternsof 206Pb/204Pb variation in both crusts between 6 and 5 Ma,with the amplitude of variation in Lion crust being about halfthat of Tropic crust. On the other hand, the207Pb/204Pb ratiosin Lion crust decrease during the period 6.5–5 Ma while thoseof ENADW increase, firmly ruling out any strong influencefrom ENADW during the Messinian in Lion crust.

Overall, the Pb and Nd isotopic record from the Lion Sea-mount crust provides little indication that the Mediterraneanoutflow stopped completely for long periods of time during theMessinian. The fact that the predominant contribution of Pband Nd to MOW was from internal sources during this time,does suggest that any Atlantic water inflow into the Mediter-ranean then was restricted and/or intermittent. Such a view issupported by87Sr/86Sr ratios in Messinian-age ostracods fromthe eastern Mediterranean which show some freshwater influ-ence with occasional values typical of seawater, suggestive oftransient flow of marine waters into the Mediterranean (Mc-Culloch and De Deckler, 1989).

Whether MOW has a strong influence on the PleistoceneNorth Atlantic climate and circulation, as suggested by Johnson(1997), or did so in Messinian times, is hard to evaluate.Nevertheless, as a result of Nd contributed from MOW,present-day NADW becomes one«Nd unit more radiogenic(Spivack and Wasserburg, 1988), and from 4 Ma to the present«Nd changes in the NW Atlantic did not exceed 1-epsilon unit(212 to213) (Burton et al., 1997)—a variation similar to thatobserved in Lion crust (29 to 210). This suggests that Nd

Fig. 5. Evaluation of potential Pb and Nd sources to Lion crust 65GTV using a plot of«Nd vs 206Pb/204Pb. Literature datasources: Circum–Mediterranean granitoids (Juteau et al., 1986); ranges of206Pb/204Pb in sediment trap and Cape Verdeaerosol (Hamelin et al., 1997; Hamelin et al., 1989) and«Nd in Saharan dusts (Goldstein et al., 1984; Grousset et al., 1988;Grousset et al., 1992);«Nd in rivers (Frost et al., 1986; Goldstein et al., 1984);«Nd in MOW and Eastern Mediterraneanwaters (E. Med.) (Spivack and Wasserburg, 1988). The arrow shows the trend of increasing206Pb/204Pb and decreasing«Nd

in the crust from 4.8 Ma to the present. This trend implies a switch from internal, Mediterranean sources prior to and duringthe Messinian, to predominantly external, NW African sources afterwards, probably in the form of eolian dust. An eoliandust source appears to have become increasingly important since;4 Ma ago.


contribution from MOW to NADW, at least, did not changedrastically over the past 8 Ma.

4. CONCLUSIONS

Pb and Nd isotopic depth profiles are presented for twoFe–Mn crusts from the NE Atlantic: crust 121DK from Tropicseamount and 65GTV from Lion seamount provide a timerecord of eastern NADW and MOW back to 13 Ma and 8 Ma,respectively. Pb isotopic data were obtained at high precision(2sext # 100 ppm) with the use of a Pb triple spike.

In both crusts, Pb isotope ratios exhibit systematic, gradual,and synchronous variations through time. Over the past 13 Ma,eastern Atlantic Pb—as inferred from Tropic crust—appears tohave been dominated by binary mixing between Pb from south-ern and northern sources, with abrupt shifts in the compositionsof these two endmembers occurring at about 4 Ma and 8 Ma.From 4 to 0 Ma, the two endmembers correspond to NADWand SCW, while from 13 to 8 Ma, mixing occurred betweentwo distinct sources—a northern source located in the Green-land–Norwegian sea and a southern or Tethyan source. Fur-thermore, age progressive changes in the relative proportions ofthe endmembers appear to be correlated with known pale-oceanographic circulation changes. For example, Pb and Ndisotopic variations during the time slice 4–0 Ma are consistentwith increased inputs of LSW into the western Atlantic basinand advection of the western NADW isotopic signal into theeastern basin. During the time period 13–8 Ma, by contrast, Pbisotopic compositions in the two basins were different, suggest-ing that little or no advection from the western into the easternAtlantic basin took place. Thus, ventilation and deep watercirculation in the eastern basin seem to have been quite differ-ent in these two time periods.

The Pb isotopic record of Lion Seamount crust indicates thatsources of Pb to MOW switched from predominantly internalMediterranean sources to external Saharan sources at about 4Ma. Before and during the Messinian (6.5–5 Ma), Pb and Nd inMOW were largely controlled by riverine, and possibly eolian,inputs derived from within the Mediterranean basin. Followingthe re-establishment of fully open marine conditions at the endof the salinity crisis, there appeared a “new” Pb and Nd sourcewhich grew in importance towards the present. We attributethis influence to Saharan dust input, probably derived from NWAfrica and added to the Mediterranean basin via Atlantic sur-face water inflow. The strengthening of this “Saharan” isotopicsignal since 4 Ma is entirely consistent with increased ariditydocumented in the Saharan and sub-Saharan regions of Africastarting at about this time.

From 3 to 4 Ma onwards, a gradual change toward moreradiogenic Pb and unradiogenic Nd is observed in Fe–Mncrusts from the NE Atlantic (this study), NW Atlantic (Burtonet al., 1997), the central Pacific (Abouchami et al., 1997;Christensen et al., 1997; Ling et al., 1997), the Arctic ocean(Winter et al., 1997) and the Indian ocean (Frank and O’Nions,1998). The fact that the Pb and Nd isotopic response was globalsuggests that changes in the sources of these elements and/orfluxes must somehow reflect changes in the ocean and atmo-sphere circulation patterns and climate worldwide. The closureof the Isthmus of Panama seems to have played a decisive rolein governing the thermohaline circulation, but it may not have

been the sole cause of the global changes in sources to theoceans over the last 13 Ma—though probably the most impor-tant one 3–4 Ma ago.

Acknowledgments—We wish to particularly thank Steve Goldstein forlong-standing discussions on paleoceanographic applications of radio-genic isotopes. Francis Grousset, Bruno Hamelin, and R. Tiedemannare thanked for providing useful pointers to “Saharan” dust. A. Simo-netti is acknowledged for discussion on Atlantic ocean lead and A.Michard for her interest in this work. Al Hofmann provided usefulcomments and is especially thanked for encouragement. A. Bollho¨fer,S. Bederke-Raczek, H. Buckardt, and Th. Kenntner helped at severalstages of this study. We are grateful to B. Hamelin and two anonymousreviewers for their comments and suggestions which lead to significantimprovements to the manuscript. Figure 1 was prepared using the GMTprogram of P. Wessel and W. H. F. Smith. The samples were collectedon Cruise SO 83 of the RV Sonne and was funded by the GermanMinistry for Research and Technology (Project No. 03R424A6). Dur-ing this study, W. A. was supported by DFG Grant No. II C6-Ho1026/10-2.

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