Deep-sea coral aragonite as a recorder for the neodymium isotopic composition of...

Deep-sea coral aragonite as a recorder for the neodymiumisotopic composition of seawater

Tina van de Flierdt a,b,!, Laura F. Robinson c, Jess F. Adkins d

aDepartment of Earth Science and Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AZ, UKbLamont-Doherty Earth Observatory and Department of Earth and Environmental Sciences, Columbia University, 61 Route 9W,

Palisades, NY 10964, USAcDepartment of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

dCalifornia Institute of Technology, MS100-23, 1200 E. California Boulevard, Pasadena, CA 91125, USA

Received 15 March 2010; accepted in revised form 27 July 2010; available online 6 August 2010

Abstract

Deep-sea corals have been shown to be useful archives of rapid changes in ocean chemistry during the last glacial cycle.Their aragonitic skeleton can be absolutely dated by U–Th data, freeing radiocarbon to be used as a water-mass proxy. Forcertain species of deep-sea corals, the growth rate allows time resolution that is comparable to ice cores. An additional proxyis needed to exploit this opportunity and turn radiocarbon data into rates of ocean overturning in the past.

Neodymium isotopes in seawater can serve as a quasi-conservative water-mass tracer and initial results indicate that deep-sea corals may be reliable archives of seawater Nd isotopes. Here we present a systematic study exploring Nd isotopes as awater-mass proxy in deep-sea coral aragonite. We investigated five di!erent genera of modern deep-sea corals (Caryophyllia,Desmophyllum, Enallopsamia, Flabellum, Lophelia), from global locations covering a large potential range of Nd isotopic com-positions. Comparison with ambient seawater measurements yields excellent agreement and suggests that deep-sea corals arereliable archives for seawater Nd isotopes.

A parallel study of Nd concentrations in these corals yields distribution coe"cients for Nd between seawater and coralaragonite of 1–10, omitting one particular genus (Enallopsamia). The corals and seawater did however not come from exactlythe same location, and further investigations are needed to reach robust conclusions on the incorporation of Nd into deep-seacoral aragonite.

Lastly, we studied the viability of extracting the Nd isotope signal from fossil deep-sea corals by carrying out stepwisecleaning experiments. Our results show that physical removal of the ferromanganese coating and chemical pre-cleaning havethe highest impact on Nd concentrations, but that oxidative/reductive cleaning is also needed to acquire a seawater Nd iso-tope signal.! 2010 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

Relating changes in ocean chemistry to oceanographicchanges, and ultimately to climate change, is one of the fun-damental goals in earth system science research. Progress inthe field requires identification of reliable archives of seawa-ter chemistry, and successful development and applicationof oceanographic proxies. An archive that has attractedgrowing attention over the past 15 years has been the ara-gonitic skeleton of deep-sea corals.

0016-7037/$ - see front matter ! 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.gca.2010.08.001

! Corresponding author at: Department of Earth Science andEngineering, Imperial College London, South Kensington Cam-pus, Exhibition Road, London SW7 2AZ, UK. Tel.: +44 20 75941290; fax: +44 20 7594 7444.

E-mail address: [email protected] (T. van deFlierdt).

www.elsevier.com/locate/gca

Available online at www.sciencedirect.com

Geochimica et Cosmochimica Acta 74 (2010) 6014–6032

http://dx.doi.org/016/j.gca.2010.08.001

http://dx.doi.org/10.1016/j.gca.2010.08.001

mailto:[email protected]

In contrast to tropical shallow-water corals, deep-seacorals do not rely on symbiotic algae and are able to growin the absence of sunlight. They are often referred to ascold-water corals, benthic corals, azooxanthellate corals,or hermatypic corals (Cairns, 2007). Deep-water coralsare in the order cnidaria, and include stony corals (Sclerac-tinia), soft corals (Octocorallia), black corals (Antipatha-ria), and hydrocorals (Stylasteridae) (Roberts et al., 2006).Some species build massive colonies, providing a construc-tional framework and habitat similar to shallow-water cor-al reefs (e.g., Lophelia pertusa, Solenosmilia variabilis;Freiwald et al., 2004). Some species are solitary, e.g., Des-mophyllum dianthus. Overall there are 615 known azooxan-thellate scleractinian species that can live below 50 m waterdepth; approximately three quarters of them are solitaryspecies, the rest are colonial (Cairns, 2007). Their distribu-tion is global, from the Norwegian Sea (70"N) to the RossSea (78"S), in water depths down to 6328 m, and at watertemperatures of !1 "C to over 20 "C (Keller, 1976; Stanleyand Cairns, 1988).

Research e!orts on deep-sea corals are increasing, andone important avenue is the geochemical record of deep-ocean changes archived in their skeletons. Compared to tra-ditional sedimentary archives, the advantages of deep-seacorals as climate archives can be summarized in three mainpoints. First, scleractinian corals contain high concentra-tions of uranium allowing for accurate dating by U-seriestechniques (Cheng et al., 2000; Mortlock et al., 2005). Sec-ondly, growth rates of 0.5–2 mm/year in the solitary coralD. dianthus allow for records of sub-decadal resolution overthe duration of a century or more (Adkins et al., 2002,2004; Risk et al., 2002). Growth rates in colonial corals of5 mm/year for the vertical extension in Enallopsamiarostrata and 4–26 mm/year for L. pertusa can produce evenhigher resolution records (Wilson, 1979; Bell and Smith,1999; Adkins et al., 2002, 2004; Orejas et al., 2008; see alsoRoark et al. (2009) for growth rates of other deep-sea cor-als). Finally, deep-sea corals can be abundant in areaswhere common paleoceanographic archives such as foram-inifera are scarce (i.e., high current locations with low sed-imentation rates or low pH locations).

Common geochemical proxies including carbon iso-topes, oxygen isotopes, and Mg/Ca ratios appear to bestrongly a!ected by physiological processes during biomin-eralization in the skeletons of deep-sea corals, (e.g., Smithet al., 2000; Adkins et al., 2003; Shirai et al., 2005; Cohenet al., 2006; Sinclair et al., 2006; Blamart et al., 2007; Gag-non et al., 2007; Montagna et al., 2007). These vital e!ectsseverely limit our ability to use proxies to reconstruct keyparameters of the ocean in the past, such as water temper-ature, pH, or nutrient content. On the other hand, increas-ing knowledge of the characteristics and mechanisms ofvital e!ects combined with detailed studies of the structureof the coralline aragonite allow us to look beyond vital ef-fects. For example, Sr/Ca ratios measured exclusively inaragonite from the center of calcification centers of D. dian-thus appear to be correlated to temperature, (Gagnon et al.,2007) as are the stable isotopes of Sr in L. pertusa (Rugge-berg et al., 2008). Cohen et al. (2006) furthermore foundthat Sr/Ca ratios in Lophelia skeletons reflect seasonal tem-

perature, as well as seasonal changes in the mass fraction ofaragonite precipitated from the calcifying fluid.

Thus far the most successful tracer used in scleractiniandeep-sea corals is radiocarbon. The radiocarbon content ofdissolved inorganic carbon (DIC) in seawater is an importanttool for constraining the rate of deep-water circulation. In amodern calibration study Adkins et al. (2002) showed thatseveral species of deep-sea corals incorporate the same D14C(14C/12C ratio is expressed as D14C, the ratio of a sample tothe preindustrial prenuclear atmospheric standard in unitsof &) value as seawater DIC. Coupled with U–Th dates,14C analyses in deep-sea corals can provide a direct measure-ment of past seawater D14C. Such paired measurements inscleractinian deep-sea corals have been extremely valuablein placing constraints on ocean chemistry during times ofrapid climate change (e.g., Adkins et al., 1998; Manginiet al., 1998; Goldstein et al., 2001; Schroder-Ritzrau et al.,2003; Frank et al., 2004; Robinson et al., 2005; Eltgrothet al., 2006; Cao et al., 2007; Robinson and van de Flierdt,2009). Theoretically radiocarbon measurements from deep-sea corals canbeusedasadynamic tracerofoceancirculation.However, calculating ocean ventilation rates is not possiblewithout quantification of water-mass endmember mixing.

Neodymium isotope ratios have the potential to providean independent water-mass mixing proxy. In the modernocean di!erent water masses are characterized by distinctNd isotopic compositions, whose values are ultimately de-rived from the continents and delivered to the oceanthrough weathering, erosion, and particle–seawater interac-tion (Piepgras et al., 1979; Goldstein and O’Nions, 1981;Frank, 2002; Goldstein and Hemming, 2003; Lacan andJeandel, 2005). 143Nd is produced by radioactive decay of147Sm, and the 143Nd/144Nd in seawater is primarily a func-tion of the age of the continental sources. In the globaloceans today there are two main endmembers, which havedistinctive dissolved Nd isotopic values. North AtlanticDeep Water (NADW) has low values reflecting the old con-tinental crust surrounding the North Atlantic (eNd =!13.5 ± 0.5; Piepgras and Wasserburg, 1987; eNd is thedeviation of a measured 143Nd/144Nd ratio from the “bulkEarth” value of 0.512638; Jacobsen and Wasserburg, 1980).Deep water in the North Pacific has higher values reflectingcontribution from young volcanic arcs (eNd = !3 to !6; forrecent summaries see van de Flierdt et al. (2004) and Amak-awa et al. (2009)). Intermediate Nd isotopic compositionsare found in Circum-Antarctic and Indian Ocean deepwaters (eNd " !7 to !9; Piepgras and Wasserburg, 1982;Bertram and Elderfield, 1993; Jeandel, 1993). The isotopicdi!erences between the ocean basins highlights that the res-idence time of Nd in the ocean is shorter than the globalturnover time of deep waters (sNd " 400–1000 years; Tac-hikawa et al., 2003), making it a valuable tracer for globalwater-mass distributions.

In order to use Nd isotopes to trace water-mass mixingon time-scales relevant to rapid climate change, we requirearchives with precise age control, high temporal resolution,and preservation of the seawater Nd signal. Commonly usedarchives are ferromanganese crusts, fossil fish teeth, plank-tonic and benthic foraminifera, and the ferromanganeseoxide fraction leached from marine sediments (for recent

Neodymium isotopes in deep-sea corals 6015

summaries, see Frank (2002), Goldstein and Hemming(2003), Martin and Scher (2004), Vance et al. (2004),Klevenz et al. (2008)). Ferromanganese crusts and fossil fishteeth record millions of years of seawater history, but fail toprovide sub-millennial time resolution. Analyses of leach-ates of marine sediments have provided high-resolution re-cords of Nd isotopes during the last glacial cycle (e.g.,Rutberg et al., 2000; Piotrowski et al., 2005; Gutjahret al., 2008; Pahnke et al., 2008). However, several studieshave demonstrated that the sedimentary signal does notalways record the Nd isotopic composition of the bottom-most seawater (for a recent discussion, see Gutjahr et al.(2008)). Calcitic shells of planktonic and benthic foraminif-era on the other hand appear to record seawater eNd (Vanceand Burton, 1999; Burton and Vance, 2000; Vance et al.,2004; Stoll et al., 2007; Klevenz et al., 2008), but the Ndconcentrations are generally too high for incorporation ofseawater Nd within the calcite structure only (Vance et al.,2004; Haley et al., 2005; Martinez-Boti et al., 2009).

Here we provide a systematic study of the neodymiumisotopic composition of modern (i.e., <500 year old) and fos-sil deep-sea scleractinian corals. Two approaches are used toinvestigate whether the aragonitic skeletons of deep-sea cor-als record the Nd isotopic composition of seawater. First, wecarefully test various cleaning steps and their e"ciency inrecovering seawater Nd isotopic composition. Second, wecompare the Nd isotopic composition of modern deep-seacorals with that of close-by seawater. In the following we willpresent results, demonstrating that five di!erent species ofsolitary and colonial deep-sea corals are reliable archives ofseawater eNd, allowing for well-dated and potentially high-resolution records of past seawater Nd isotopes.

2. SAMPLES AND METHODS

All carbonate samples recovered from the seafloor arelikely to be contaminated to some degree by detrital sedi-

ment, organic matter or ferromanganese coatings. All thesecontaminant phases have di!erent geochemical composi-tions than the aragonitic skeleton of corals, and it is there-fore vital to ensure that all samples are thoroughly cleanedbefore analysis. An important part of this study consist oftesting the e"ciency of cleaning protocols for modern andfossil corals based on the methods developed by Shen andBoyle (1988) and Cheng et al. (2000). The full cleaning pro-cedure applied in this study is outlined in Table 1 includingphysical and chemical cleaning steps.

2.1. Coral cleaning experiments

Samples for cleaning experiments were selected toachieve three objectives: (i) determine extent of cleaning re-quired for modern corals, (ii) determine extent of cleaningrequired for fossil corals, and (iii) monitor the full cleaningprocedure step-by-step for both modern and fossil corals.

For the first objective we used two modern D. dianthussamples, one from the NW Atlantic (Smithsonian ID62309/Nd07-08; note that Nd08 was also included in themodern calibration), and one from the western slope o!Chile (Alb#9/Nd09-13) (Table 2). The latter sample wascoated with dried yellow organic tissue from the coralpolyp.

For the second objectivewe chose twoD. dianthus individ-uals from the NW Atlantic with ages between 30 and 41 kyr(ALV-3892-1421-001-030 and ALV-3892-1315-001-010).

For the third objective we used three D. dianthus, onewith an age of "200 ka from the NW Atlantic covered inthick ferromanganese coating (ALV-3889-1326-002-B7),and two D. dianthus from o! Chile (Alb#1/Nd14-19: whitearagonite with no apparent coating; Alb#7/Nd21-27: cov-ered with brown to black ferromanganese coating)(Fig. 1). While they are from two di!erent oceanographicsettings (NW Atlantic and Chilean shelf), all three coralsare of the species D. dianthus, to exclude potential biases

Table 1Cleaning procedure for deep-sea coral aragonite.

Action Comment

Step 1 Scraped coating from thecoral surface

Organic or ferromanganese coating is scraped from the coral surface to be analyzed separately

Step 2 Uncleaned coral No cleaning is performed prior to dissolutionStep 3 Physical cleaning Macroscopically visible coatings are removed using a dremel tool as far as possible. Evidence of

reworking by endolithic organisms is drilled outStep 4 Pre-cleaning Physically cleaned sample is exposed to 1:1 mixture of 30% hydrogen peroxide and 1 N sodium

hydroxide for 20 min while being ultrasonicated. The treatment is repeated until no further FeMn oxidecoatings and organics are visible. To remove any remaining organic stain on the aragonitic skeleton,samples are dipped in a 1:1 mixture of 30% peroxide and 1% perchloric acid

Step 5 Oxidative cleaning Physically and chemically pre-cleaned pieces of coral are transferred to acid-cleaned vials, and washed inmethanol (20 min in an ultrasonicator) and dipped in 0.2% nitric acid. The oxidizing step consists of a 1:1mixture of 30% hydrogen peroxide and 1 N sodium hydroxide applied for 20 min in a heatedultrasonicator

Step 6 Reductive cleaning Steps 3–5 are followed by applying a mixture of citric acid, ammonium hydroxide, and hydrazine inorder to remove trace metals associated with iron and manganese oxides. The step is followed by a repeattreatment with the oxidizing solution (step 5)

Step 7 Final step Pieces of coral that went through steps 3–6 are ultrasonicated in EDTA solution to remove re-adsorbedtrace metals. A final leach is performed in dilute nitric acid (0.2% for 1 min)

Note: For steps 3–7 we analyzed the coral aragonite after the cleaning step. All leaching solutions were discarded. Samples were dried andcrushed in an agate mortar and pestle to facilitate dissolution.

6016 T. van de Flierdt et al. /Geochimica et Cosmochimica Acta 74 (2010) 6014–6032

from analysis of di!erent species. To carry out step-by-stepcleaning experiments on these three corals, a large ("7 g)sub-sample from each corals was crushed into chunks,and divided into seven sub-samples of "1 g. All sub-sam-ples were exposed to di!erent degrees of physical and chem-ical cleaning, followed by U–Th dating and Nd isotopeanalyses as detailed in Tables 3, 4. Aliquots for Nd concen-trations were removed prior to dissolution of samples.

2.2. Modern calibration

For the modern calibration a range of di!erent speciesof modern (i.e., mostly <500 year old) deep-sea corals fromthe global ocean, spanning water depths from 46 to 2180 m,were investigated (Fig. 2 and Table 2). Fifteen out of the 16modern coral samples were loans from the Smithsonianinvertebrate collection, and were selected and sampled withthe assistance of curator S. Cairns. One further samplecomes from a DSV Alvin cruise to the New England sea-mount in May–June 2003 (see also van de Flierdt et al.,2006). Where possible samples were chosen from locationsin close proximity to existing seawater eNd profiles (Fig. 2).However, due to the scarcity of seawater data and of coralspecimens it was di"cult to achieve a perfect match in allcases (see Section 4). Smithsonian ID 47443 (west of Chile)is not near any seawater data, but was chosen because ofthe large number of samples available. This sample, as wellas Smithsonian ID 11964 (North Atlantic), has only beenused to examine U, Th, and Nd concentrations, and notNd isotopes (Tables 3 and 4).

For each coral we cut about 1 g of aragonite using a dre-mel tool. In the case of the solitary species D. dianthus, F.alabastrum, and F. apertum each sample was taken fromadjacent septae. In the case of the colonial species L. per-tusa, L. prolifera, E. rostrata, and Enallopsamia profundaeach sample was as close as possible to a single polyp(Fig. 1). All sub-samples were exposed to physical andchemical pre-cleaning, followed by U–Th dating and Ndisotope analyses. Aliquots for Nd concentrations were re-moved prior to dissolution of the cleaned samples. Smithso-nian ID corals 62308 and 45669 were part of an earlierexperiment than the rest of the samples, and underwentthe full chemical cleaning procedure prior to analyses forNd isotopes, but were not analyzed for U–Th contents.

2.3. Column chemistry and mass spectrometry

2.3.1. U–Th datingAfter exposing each weighed aliquot of carbonate to dif-

ferent steps of physical and chemical cleaning (Table 3),samples were dissolved in 8 N nitric acid. For samples fromsteps 1 and 2 HCl and heat was applied to ensure completedigestion (exception: FeMn crust of ALV-3892-1315-001-010, which was analyzed after a 6 M HCl leach for15 min). Each sample was spiked with a mixed 229Th–236Uspike, treated with a Fe co-precipitation step and loadedon anion-exchange columns (Edwards et al., 1986). The ini-tial 8 N nitric acid rinses were collected for Nd isotopechemistry. Purified U and Th aliquots were collectedseparately and were measured by Neptune multi-collector

Table 2Sample information on corals used for modern calibration and cleaning experiments.

Sample ID Lab ID Species Water depth location

Modern calibration62309 Nd08 D. dianthus 613–430 40N 22.760 67W 39.34 NW Atlantic62308 LT17 D. dianthus 1850–1950 38N 45.80 72W 41.60 NW AtlanticALV-3891-1459-003-011 UAM06/LT12 D. dianthus 1176 38N 56.920 61W 1.620 New England Seamounts78459 MA6 D. cristagalli 2110–2180 38N 45.80 72W 39.10 NW Atlantic15584 MA11 F. alabastrum 1330 36N 420 74W 300 NW Atlantic80750 MA1 E. profunda 45.7 32N 120 79W 150 O! Florida80797 MA8 E. profunda 455 31N 49.60 78W 45.80 O! Florida62577 MA10 E. rostrata 1098 28N 060 77W 080 O! Florida1081198 MA12 L. pertusa 370–387 32S 53.50 50W 25.30 South Atlantic45669 LT15 D. dianthus 494–384 56S 060 66W 190 Drake Passage93951 MA9 E. rostrata 1200 18N 38.70 158W 16.80 Hawaii92795 MA3 C. ambrosia 1097 33N 24.250 135E 30.50 NW Pacific

47443 MA4 F. apertum 1500–1666 53S 130 75W 410 Chilean slope11964 MA7 F. alabastrum 862 42N 55.50 50W 510 North Atlantic

80404 MA5 D. dianthus 380–400 35N 26.50 4W 14.20 Alboran Sea261291 MA2 L. prolifera 802 30N 510 79W 240 O! Florida

Cleaning experiments62309 Nd07-08 D. dianthus 613–430 40N 22.760 67W 39.34 NW AtlanticALV-3892-1315-001-010 UAB01/Nd04-06 D. dianthus 1713 38N 12.500 60W 32.000 New England SeamountsALV-3892-1421-001-030 LT30/Nd01-03 D. dianthus 1657 38N 12.500 60W 32.000 New England SeamountsAlb#09 Nd09-13 D. dianthus "800 m 48S 09 74W 36 Chilean slopeAlb#07 Nd21-27 D. dianthus "800 m 48S 09 74W 36 Chilean slopeAlb#01 Nd14-19 D. dianthus "800 m 48S 09 74W 36 Chilean slopeALV-3889-1326-002-B7 UAL20/Nd28-34 D. dianthus 1723 33N 51.200 62W 39.500 New England Seamounts


inductively coupled plasma mass spectrometry (MC-ICP-MS) with bracketing standards of CRM-145 for U, and anin-house Th standard calibrated against CRM-145 andHU-1 (for details, see Robinson et al. (2005)).

2.3.2. Nd concentrations and isotopesAliquots for Nd concentration analyses were taken prior

to dissolution of coral aragonite and were spiked with a150Nd spike. Purification of rare earth elements was carriedout using Eichrom TRU-spec# resin. Analyses of 146Ndand 150Nd were made on an Element 2 single collectorICPMS and 147Sm was monitored to assess contributionof Sm to 150Nd.

Neodymium isotopes were extracted from the wash solu-tion of the U-series anion-exchange chemistry. Rare earth

elements were separated from the remaining major andtrace elements on Eichrom TRU-spec# resin. Neodymiumwas subsequently separated from the other REE using a-hydroxyisobutyric acid (HIBA) and cation resin. Neodym-ium isotopes were measured at Lamont-Doherty EarthObservatory on a VG (Micromass) Sector 54 thermal ioni-zation mass spectrometer by dynamic multi-collection. Pro-cedural blanks were less than 10 pg (n = 3). Samples weremeasured as NdO+ loaded as Si gel sandwiches, where eachsamples was baked at 1050–1100 "C for 4 h prior to slowheating. A 146Nd/144Nd ratio of 0.7219 was used to correctfor mass fractionation. Typical 144Nd beams for coral runswere between 100 and 200 mV, maintained for up to 300 ra-tios. One third of all samples yielded larger beams (up to450 mV). Replications of 5–35 ng La Jolla Nd standards

Fig. 1. Photographs of some of the corals analyzed. (a–f, left) Corals used for modern calibration experiments (a = Smithsonian ID 62309, D.dianthus; b = Smithsonian ID 1081198, Lophelia pertusa; c = Smithsonian ID 80404, D. dianthus; d = Smithsonian ID 15584, Flabellumalabastrum; e = Smithsonian ID 93951, Enallopsamia rostrata; f = Smithsonian ID 92795, Caryophyllia ambrosia). (g–k, right) Corals used forcleaning experiments (g = ALV-3889-1326-002-B7; h = Alb#7; i = Alb#1; j = Alb#9; k = ALV-3892-1315-001-010). White and black barsare 1 cm each. For more sample information, see Table 2.


Table 3U-series data summary for all corals.

Sample ID Lab ID Cleaning 238U(ppm)

232Th(ppb)

d234U(&)

(230Th/238U)act 232Th/230Th(atom)

Ageraw(years)

Agecorr(years)

d234Uinitial (&)

Modern calibration62309 Nd08 Fully

cleaned2.995 ± 0.001 0.21 ± 0.02 147.8 ± 0.7 0.00077 ± 0.00002 5512.9 ± 466.5 73 ± 2 39 ± 37 147.8 ± 0.7

62308 LT17 Fullycleaned

ALV-3891-1459-003-011

UAM06/LT12

Fullycleaned

See van de Flierdt et al. (2006)

78459 MA6 Pre-cleaned 3.210 ± 0.005 0.44 ± 0.00 148.1 ± 0.9 0.00175 ± 0.00003 4765.4 ± 100.7 166 ± 3 99 ± 68 148.2 ± 0.915584 MA11 Pre-cleaned 4.764 ± 0.007 10.67 ± 0.05 147.7 ± 0.6 0.00377 ± 0.00003 36011.0 ± 195.6 359 ± 3 <080750 MA1 Pre-cleaned 4.914 ± 0.020 0.40 ± 0.00 147.7 ± 0.9 0.00301 ± 0.00003 1635.3 ± 23.2 286 ± 3 247 ± 41 147.8 ± 0.980797 MA8 Pre-cleaned 5.344 ± 0.008 0.27 ± 0.00 147.3 ± 0.5 0.00105 ± 0.00001 2882.1 ± 64.0 100 ± 1 75 ± 25 147.4 ± 0.562577 MA10 Pre-cleaned 4.965 ± 0.004 1.48 ± 0.01 149.5 ± 0.6 0.00549 ± 0.00006 3287.7 ± 35.4 523 ± 6 377 ± 144 149.7 ± 0.61081198 MA12 Pre-cleaned 3.687 ± 0.004 0.32 ± 0.01 147.5 ± 0.6 0.00057 ± 0.00002 9135.2 ± 380.2 54 ± 2 12 ± 43 147.5 ± 0.745669 LT15 Fully

cleaned93951 MA9 Pre-cleaned 5.170 ± 0.007 0.05 ± 0.01 146.7 ± 0.6 0.00239 ± 0.00003 265.1 ± 30.3 228 ± 3 223 ± 8 146.8 ± 0.692795 MA3 Pre-cleaned 4.479 ± 0.014 1.05 ± 0.01 146.5 ± 1.0 0.00189 ± 0.00003 7490.7 ± 101.5 180 ± 3 66 ± 112 146.5 ± 1.1

47443 MA4 Pre-cleaned 3.835 ± 0.010 0.12 ± 0.00 146.5 ± 0.9 0.00123 ± 0.00002 1505.8 ± 61.3 117 ± 2 102 ± 17 146.6 ± 0.911964 MA7 Pre-cleaned 5.009 ± 0.010 2.40 ± 0.01 147.9 ± 0.5 0.00241 ± 0.00002 12056.4 ± 113.8 229 ± 2 <0

80404* MA5 Pre-cleaned 3.434 ± 0.010 2.15 ± 0.01 149.0 ± 0.9 0.06193 ± 0.00034 612.3 ± 1.4 6043 ± 39 5737 ± 330 151.4 ± 1.1261291* MA2 Pre-cleaned 4.510 ± 0.011 1.52 ± 0.01 144.5 ± 0.9 0.09688 ± 0.00052 210.6 ± 0.9 9641 ± 61 9476 ± 218 148.4 ± 1.0

Cleaning experiments62309 Nd07 Pre-cleaned 3.301 ± 0.001 0.62 ± 0.02 148.1 ± 0.6 0.00080 ± 0.00002 14374.5 ± 516.2 76 ± 2 <0

Nd08 Fullycleaned

See values under modern calibration

ALV-3892-1315-001-010

Nd04 FeMn crust 9.640 ± 0.071 154037.92 ± 683.59 146.5 ± 2.1 43.24654 ± 0.19547 22377.5 ± 13.3

Nd05 Pre-cleaned 4.543 ± 0.001 133.4 ± 0.7 0.27515 ± 0.00192 !55.2 ± !0.7 30,196 ± 262Nd06 Fully

cleaned4.200 ± 0.001 1.07 ± 0.06 135.4 ± 0.7 0.27445 ± 0.00292 56.0 ± 3.1 30,042 ± 386 29,918 ± 510 147.4 ± 0.9

ALV-3892-1421-001-030

Nd01 FeMn crust 1.631 ± 0.048 5626.81 ± 226.54 93.3 ± 22.1 12.56192 ± 0.41395 16633.2 ± 840.5

Nd02 Pre-cleaned 3.107 ± 0.001 127.3 ± 0.7Nd03 Fully

cleaned2.652 ± 0.001 0.21 ± 0.02 130.5 ± 0.6 0.35734 ± 0.00368 13.5 ± 1.3 41,161 ± 537 41,122 ± 577 146.5 ± 0.9

Alb#09 Nd09 FeMn crust 0.678 ± 0.008 54.05 ± 0.32 120.9 ± 6.2Nd10 Pre-cleaned 3.596 ± 0.001 146.8 ± 0.6Nd11 Fully

cleaned3.580 ± 0.001 1.23 ± 0.02 143.2 ± 0.7 0.00173 ± 0.00003 12060.9 ± 243.5 165 ± 3 <0

(continued on next page)

Neodym

iumiso

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indeep

-seacorals

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Table 3 (continued)

Sample ID Lab ID Cleaning 238U(ppm)

232Th(ppb)

d234U(&)

(230Th/238U)act 232Th/230Th(atom)

Ageraw(years)

Agecorr(years)

d234Uinitial (&)

Duplicate Nd12 Pre-cleaned 3.646 ± 0.001 0.38 ± 0.02 138.5 ± 0.9 0.00175 ± 0.00006 3566.1 ± 210.9 168 ± 6 117 ± 56 138.6 ± 0.9Nd13 Fully

cleaned3.638 ± 0.001 0.60 ± 0.02 150.3 ± 0.6 0.00154 ± 0.00002 6418.9 ± 222.6 147 ± 2 67 ± 80 150.3 ± 0.6

Alb#07 Nd21 FeMn crust 6.797 ± 0.184 1013.57 ± 7.17 63.4 ± 6.7 0.09726 ± 0.00138 92866.2 ± 1317.7Nd22 No cleaning 4.934 ± 0.002 17.37 ± 0.07 143.1 ± 0.8 0.00523 ± 0.00003 40803.8 ± 209.2 500 ± 4 <0Nd23 Phys.

cleaned4.512 ± 0.001 0.82 ± 0.02 145.5 ± 0.8 0.00520 ± 0.00004 2110.6 ± 44.2 497 ± 4 408 ± 89 145.6 ± 0.9

Nd24 Pre-cleaned 4.388 ± 0.001 0.28 ± 0.02 145.6 ± 0.9 0.00531 ± 0.00005 729.9 ± 51.1 508 ± 5 476 ± 37 145.8 ± 0.9Nd25 Oxidat.

cleaned3.984 ± 0.001 0.32 ± 0.02 149.2 ± 0.7 0.00530 ± 0.00004 921.0 ± 50.8 505 ± 5 465 ± 44 149.4 ± 0.7

Nd26 Reduct.cleaned

4.102 ± 0.001 0.23 ± 0.02 147.5 ± 0.7 0.00537 ± 0.00004 637.9 ± 45.8 512 ± 5 485 ± 33 147.7 ± 0.7

Nd27 Fullycleaned

4.166 ± 0.001 0.27 ± 0.02 147.3 ± 0.7 0.00537 ± 0.00005 725.6 ± 46.8 513 ± 5 481 ± 37 147.5 ± 0.8

Alb#01 Nd14 FeMn crust 5.795 ± 0.053 52.44 ± 1.83 115.9 ± 2.9 0.00609 ± 0.00025 89910.2 ± 4700.7Nd15 No cleaning 5.469 ± 0.002 6.32 ± 0.03 145.1 ± 0.7 0.00231 ± 0.00002 30329.8 ± 262.4 220 ± 2 <0Nd16 Phys.

cleaned4.928 ± 0.001 0.44 ± 0.01 143.1 ± 0.5 0.00233 ± 0.00008 2339.8 ± 98.8 222 ± 7 178 ± 50 143.2 ± 0.5

Nd17 Pre-cleaned 4.382 ± 0.002 0.14 ± 0.02 146.9 ± 0.7 0.00202 ± 0.00003 938.4 ± 118.9 192 ± 3 177 ± 19 147.0 ± 0.7Nd18 Oxidat.

cleaned4.936 ± 0.002 0.25 ± 0.02 146.2 ± 0.7 0.00235 ± 0.00003 1310.5 ± 87.3 224 ± 3 199 ± 28 146.3 ± 0.8

Nd19 Reduct.cleaned

4.367 ± 0.001 0.16 ± 0.02 146.8 ± 0.7 0.00238 ± 0.00003 925.9 ± 116.4 227 ± 3 209 ± 22 146.9 ± 0.7

ALV-3889-1326-002-B7

Nd28 FeMn crust 12.602 ± 0.033 117319.57 ± 806.51 140.1 ± 2.5 61.85267 ± 0.27727 9115.3 ± 49.6

Nd29 No cleaning 3.440 ± 0.003 650.70 ± 3.75 89.0 ± 2.1 1.43837 ± 0.00870 7965.4 ± 15.5Nd30 Phys.

cleaned3.019 ± 0.001 3.75 ± 0.03 108.2 ± 0.7 0.92618 ± 0.00404 81.1 ± 0.4 185,955 ± 2386 185,364 ± 2950 182.8 ± 2.7

Nd31 Pre-cleaned 2.834 ± 0.001 1.04 ± 0.02 94.2 ± 0.8 0.92814 ± 0.00410 24.0 ± 0.5 194,523 ± 2720 194,345 ± 2892 163.2 ± 2.8Nd32 Oxidat.

cleaned2.871 ± 0.001 2.39 ± 0.02 104.1 ± 0.7 0.93406 ± 0.00408 53.9 ± 0.5 192,237 ± 2575 191,840 ± 2954 179.1 ± 2.8

Nd33 Reduct.cleaned

2.658 ± 0.001 0.59 ± 0.03 101.0 ± 0.7 0.92578 ± 0.00416 14.6 ± 0.6 189,518 ± 2541 189,411 ± 2647 172.5 ± 2.5

Nd34 Fullycleaned

2.878 ± 0.001 0.94 ± 0.03 95.6 ± 0.8 0.92737 ± 0.00427 21.3 ± 0.6 193,314 ± 2752 193,156 ± 2905 165.1 ± 2.7

Calculated Uranium series ages are reported in years before date of analysis and are corrected for 230Th initial. For further details on the dating method, see Robinson et al. (2005). All errors are 2rstandard errors. Samples with a # are significantly older than the rest of the modern calibration samples.

6020T.van

deFlierd

tet

al./Geochim

icaet

Cosm

ochim

icaActa

74(2010)

6014–6032

over the course of the sample analyses (June 2005–Novem-ber 2007), run at 144Nd beam sizes similar to the coral runs,gave an average of 0.511847 ± 0.000026 (2r external stan-

dard deviation; n = 69). This long term average howeverintegrates over quite a few maintenance sessions. Hencewe used the external error from each analytical session to

Table 4Neodymium isotope and concentration data summary for all corals.

Sample ID Lab ID Cleaning Nd (ppb) 143Nd/144Nd ± 1r SE eNd ± 2r SE

Modern calibration62309 Nd08 Fully cleaned 18.1 0.511861 ± 0.000008 !15.16 ± 0.3062308 LT17 Fully cleaned 0.511906 ± 0.000010 !14.29 ± 0.40ALV-3891-1459-003-011 UAM06/LT12 Fully cleaned 0.511879 ± 0.000023 !14.80 ± 0.8678459 MA6 Pre-cleaned 3.3 0.511940 ± 0.000014 !13.61 ± 0.5415584 MA11 Pre-cleaned 17.7 0.511912 ± 0.000005 !14.17 ± 0.20Re-run 0.512199 ± 0.000006 !8.57 ± 0.2280750 MA1 Pre-cleaned 16.8 0.511957 ± 0.000006 !13.28 ± 0.2480797 MA8 Pre-cleaned 27.0 0.512062 ± 0.000006 !11.23 ± 0.2262577 MA10 Pre-cleaned 24.0 0.511926 ± 0.000006 !13.89 ± 0.221081198 MA12 Pre-cleaned 4.4 0.512149 ± 0.000021 !9.54 ± 0.8245669 LT15 Fully cleaned 0.512165 ± 0.000024 !9.22 ± 0.9493951 MA9 Pre-cleaned 51.3 0.512518 ± 0.000005 !2.34 ± 0.2092795 MA3 Pre-cleaned 0.512462 ± 0.000037 !3.43 ± 1.46

47443 MA4 Pre-cleaned 8.711964 MA7 Pre-cleaned 38.0

80404* MA5 Pre-cleaned 29.0 0.512184 ± 0.000006 !8.85 ± 0.24261291* MA2 Pre-cleaned 20.2 0.512087 ± 0.000006 !10.75 ± 0.22

Cleaning experiments62309 Nd07 Pre-cleaned 36.6 0.511885 ± 0.000006 !14.69 ± 0.22

Nd08 Fully cleaned 24.9 See values aboveALV-3892-1315-001-010 Nd04 FeMn crust 0.511955 ± 0.000009 !13.32 ± 0.34

0.511960 ± 0.000004 !13.23 ± 0.16Nd05 Pre-cleaned 67.7 0.511990 ± 0.000010 !12.63 ± 0.38Nd06 Fully cleaned 54.2 0.511982 ± 0.000007 !12.80 ± 0.26

ALV-3892-1421-001-030 Nd01 FeMn crust 0.511965 ± 0.000005 !13.13 ± 0.20Nd02 Pre-cleaned 11.8 0.512034 ± 0.000010 !11.78 ± 0.38Nd03 Fully cleaned 0.512009 ± 0.000010 !12.27 ± 0.38

Alb#09 Nd09 FeMn crust 0.512372 ± 0.000033 !5.19 ± 1.30Nd10 Pre-cleaned 22.2 0.512367 ± 0.000015 !5.29 ± 0.58Nd11 Fully cleaned 28.0 0.512419 ± 0.000008 !4.26 ± 0.30

Duplicate Nd12 Pre-cleaned 11.4 0.512408 ± 0.000014 !4.48 ± 0.56Nd13 Fully cleaned 16.3 0.512370 ± 0.000013 !5.22 ± 0.52

Alb#07 Nd21 FeMn crust 0.512308 ± 0.000009 !6.44 ± 0.36Nd22 No cleaning 61.7Nd23 Phys. cleaned 18.8Nd24 Pre-cleaned 9.6 0.512343 ± 0.000017 !5.76 ± 0.68Nd25 Oxidat. cleaned 17.2 0.512416 ± 0.000014 !4.33 ± 0.54Nd26 Reduct. cleaned 7.2 0.512412 ± 0.000017 !4.41 ± 0.66Nd27 Fully cleaned 9.8 0.512424 ± 0.000012 !4.17 ± 0.48

Alb#01 MA16 FeMn crust 0.512444 ± 0.000015 !3.79 ± 0.58Nd15 No cleaning 75.7 0.512483 ± 0.000005 !3.02 ± 0.20Nd16 Phys. cleaned 12.4 0.512465 ± 0.000010 !3.38 ± 0.38Nd17 Pre-cleaned 7.6 0.512468 ± 0.000014 !3.31 ± 0.54Nd18 Oxidat. cleaned 9.6 0.512444 ± 0.000011 !3.79 ± 0.42Nd19 Reduct. cleaned 7.7 0.512421 ± 0.000032 !4.23 ± 1.24

ALV-3889-1326-002-B7 Nd28 FeMn crust 0.511956 ± 0.000005 !13.30 ± 0.18Nd29 No cleaning 1587.0 0.511942 ± 0.000006 !13.58 ± 0.22Nd30 Phys. cleaned 235.0 0.511995 ± 0.000006 !12.54 ± 0.22Nd31 Pre-cleaned 54.2 0.511957 ± 0.000007 !13.29 ± 0.28Nd32 Oxidat. cleaned 0.511985 ± 0.000007 !12.74 ± 0.28Nd33 Reduct. cleaned 0.511987 ± 0.000009 !12.70 ± 0.36Nd34 Fully cleaned 0.511988 ± 0.000007 !12.69 ± 0.28

143Nd/144Nd ratios are corrected relative to a La Jolla of 0.511858. Measured La Jolla values over the course of the four measurement sessionsare displayed in Electronic Annex and yielded external reproducibilities between 30 and 55 ppm on 143Nd/144Nd. In all figures the largest erroris plotted for eNd (typically the internal error). eNd calculated with CHUR of 0.512638.


plot on our data (see Electronic Annex). Where internal 2rstandard errors of individual runs were larger than theexternal error, the internal error is reported. All reported143Nd/144Nd ratios were normalized to a nominal valuefor the La Jolla Nd standard of 0.511858. eNd was calcu-lated using a CHUR of 0.512638 (Jacobsen and Wasser-burg, 1980). The Nd in our first chemistry batch stuck tothe Tru Spec resin, most likely due to some organic remainsin the samples after U–Th anion-exchange chemistry. Theresulting low yields caused unusually large error bars onthe 143Nd/144Nd measurements (Table 4). We consequentlychanged our procedure to add a aqua regia step prior toREE chemistry, which resolved the issue and gave im-proved Nd yields.

3. RESULTS

3.1. Cleaning experiments

3.1.1. Modern corals—pre-cleaning experiments for Ndisotopes

Both experiments carried out (Fig. 3) show that the Ndisotopic composition of pre-cleaned and fully cleaned mod-ern deep-sea corals is indistinguishable. These experimentswere carried out prior to making the decision on how muchcleaning the modern samples were exposed to, and demon-strated that pre-cleaning is su"cient (see Tables 3 and 4).Neodymium concentrations on the same samples did notshow any consistent trends (Table 4). In sample 62309 theNd concentration decreased from 36.6 to 24.9 ppb, whilefor sample Alb#9 there was an increase in Nd concentra-tions from the pre-cleaned (22.2 and 11.4 ppb) to the fullycleaned sub-samples (28.0 and 16.3 ppb). The Nd concen-trations of pre-cleaned modern corals (3.3–51.3 ppb;n = 14; Table 4) showed a similar range of Nd concentra-

tions than fully cleaned modern corals (16–25 ppb; n = 4;Table 4).

3.1.2. U, Th, and Nd concentrations in ferromanganesecoating

Ferromanganese coatings were found to contain 232Thconcentrations of up to 154 ppm, with 232Th/230Th ratiosdependent on the oceanic location of the coral. These con-centrations are six orders of magnitude higher than ob-served in cleaned aragonite. 238U concentrations in thecoatings range from 5.8 to 12.6 ppm, never more than a fac-tor of "2–4.5 di!erent to cleaned aragonite, consistent withthe results of Cheng et al. (2000). The dried organic matter

Fig. 2. Global map showing the locations of modern deep-sea corals used in this study as red dots. The labels denote the regional names usedthroughout the text. Yellow circles broadly outline the two areas from which we used D. dianthus samples for cleaning experiments (Chileanslope and NW Atlantic). Black dots denote seawater locations from the literature used for comparison (see also Table 5). (For interpretationof the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Experiments on two modern D. dianthus samples from theNW Atlantic Ocean (left) and the Chilean slope (right) to comparethe Nd isotopic composition of pre-cleaned aragonite with fullycleaned aragonite from the same coral. The experiment has beenduplicated on the Chilean slope corals. Error bars are 2r standarderrors reported in Table 4.


on Alb#9 had a lower 238U concentrations than the fullycleaned coral aragonite (Alb#9–Nd09; Table 3 andFig. 1), though Cheng et al. (2000) occasionally found highU in this fraction (see sample 85080-D). Neodymium con-centrations in coatings are presumably very enriched com-pared to the values reported for cleaned corallinearagonite. If similar to ferromanganese crusts and nodules,Nd concentrations will range from 100 to 300 ppm (Heinet al., 2000), which is a factor of "1000 to 10,000 higherthan found in modern corals. No Nd concentration datawere collected on the coatings.

3.1.3. U, Th, and Nd concentrations in step-by-step cleaningexperiments

Results for the step-by-step cleaning experiments areshown in Fig. 4 and are reported in Tables 3 and 4. The firstobservation is that 232Th and Nd concentrations decreaseduring the cleaning procedure while 238U concentrations de-crease for some samples, but increase for others. Second, thelargest changes in 232Th, 238U, and Nd concentrations arecaused by the physical cleaning step. Uncleaned corals showconcentration ranges of 6–651 ppb for 232Th, 62–1587 ppbfor Nd, and 3.3–5.5 ppb for 238U. By the end of step 3, (phys-ical cleaning) concentrations of 232Th decrease to 0.4–3.8 ppb, concentrations of Nd decrease to 12.4–235 ppb,and concentrations of 238U decrease to 3.0–4.9 ppb. Thirdly,the concentrations for all elements do not change signifi-cantly after the pre-cleaning (step 4). Furthermore, the finalrange of concentrations in fully cleaned fossil corals showsa factor of 6 variability for 232Th (0.2–1.2 ppb), a factor of5.5 variability for Nd (9.8–54.2), and factor of 1.6 variabilityfor 238U (2.7–4.2 ppb). For all three elements, the final cleancoral aragonite concentrations are in the range observed formodern samples, with no correlation to the age of the coral(Tables 3 and 4). Sample ALV-3889-1326-002-B7 has anage of"200 ka.Despite well developed ferromanganese coat-ings, the two Chilean slope samples are less than 500 yearsold. The d234Uinitial values for the fully cleaned corals are allwithin the seawater range, except for the oldest sample,ALV-3889-1326-002-B7 (d234Uinitial = 165.1&; d234U =([(234U/238U)meas/(

234U/238U)eq] ! 1) $ 103; d234Uinitial =d234Uekt). All d234U ratios for ALV-3889-1326-002-B7 arehigh, indicative of alteration to the U-series system, mostlikely due to alpha-recoil processes (Robinson et al., 2006).All reported ages—other than for ALV-3889-1326-002-B7—are considered to be accurate.

Fig. 4. Stepwise cleaning experiments for nine di!erent deep-seacorals (D. dianthus) from two di!erent regions (Chilean slope andNW Atlantic Ocean). Results are shown for Nd, 238U, and 232Thconcentrations. For more explanation, see text.

Fig. 5. Stepwise cleaning experiments for six di!erent deep-seacorals (D. dianthus) from two di!erent regions (Chilean slope andNW Atlantic Ocean). Results are shown for Nd isotopes on twodi!erent scales (right scale for NW Atlantic corals, left scale forChilean corals). For more explanation, see text.


3.1.4. Neodymium isotopes in step-by-step cleaningexperiments

Neodymium isotopic compositions for the individualcleaning steps are reported in Table 4, with results forcleaning sequences on three D. dianthus from the Chileanslope (Alb#1, Alb#7, and Alb#9) and four D. dianthusfrom Alvin dives at the New England seamounts (NWAtlantic) shown in Fig. 5. The isotopic ratios of all coralsreach that of fully cleaned coral aragonite after the oxida-tive leaching (step 5). In two of the three samples for whichthe full cleaning sequence was analyzed there is no large dif-ference between the isotopic composition of the coatingsand the aragonitic skeleton.

3.2. Modern calibration

3.2.1. U, Th, and Nd concentrations232Th and 238U concentrations are typical for modern

deep-sea corals with 0.1–2.4 ppb and 3.0–5.3 ppb, respec-tively (outlier in 232Th: sample 15584, 10.7 ppb; Table 3).No correlation exists between U and Th concentrationsand coral genus (Fig. 6). Neodymium concentrations range

from 9 to 38 ppb in Flabellum (alabastrum and apertum),from 17 to 51 ppb in Enallopsamia (rostrata and profunda),from 3 to 29 ppb in D. dianthus, and from 4 to 20 ppb in L.(prolifera and L. pertusa). The total range of Nd concentra-tions in deep-sea corals is 3.3 to 51.3 ppb (factor 15.5). Thisis smaller than that observed for 232Th (factor 24) but largerthan for 238U (factor 1.7) on the same sample set (Tables 3and 4). There is no correlation between 232Th and Nd con-centrations, and a weak correlation between 238U and Ndconcentrations (R2 = 0.53). As for U and Th, there is nocorrelation of Nd concentrations with coral genus(Fig. 6). U–Th ages show that all except two of our moderncalibration samples have ages close to the modern day (i.e.,<500 years old), and none had ages older than the Holo-cene (Table 3). All modern calibration samples yieldd234Uinitial values within error of modern seawater valuesindicating closed-system behavior (Edwards et al., 1986).

3.2.2. Nd isotopesFourteen modern deep-sea corals were analyzed for their

Nd isotopic composition encompassing five di!erent generaof colonial and solitary deep-sea corals (D. dianthus, F. ala-bastrum/apertum, C. ambrosia, E. rostrata/profunda, and L.pertusa/prolifera). The data span most of the natural rangeof Nd isotopes observed in present day seawater reachingfrom eNd = !15.2 in the North Atlantic to !2.3 in the cen-tral North Pacific. Results for the sample ALV-3891-1459-003-011 have been published previously (van de Flierdtet al., 2006).

4. DISCUSSION

4.1. Modern corals reflect the Nd isotopic composition ofambient seawater

For practical reasons, the proximity between locationsof deep-sea coral recovery and published Nd isotope seawa-ter profiles served as the main criterion for initial sampleselection at the Smithsonian. In order to compare Nd isoto-pic compositions of modern deep-sea corals with ambientseawater values, it is however important to consider localand regional hydrography, and to identify matching watermasses. This need is highlighted in Fig. 7, which comparesNd isotope results for Smithsonian ID 62309, a D. dianthusfrom intermediate water depth (430–613 m) in the NWAtlantic Ocean, with the closest-by seawater location (Sta-tion AII 109-1; Piepgras and Wasserburg, 1987). The valuesat the same water depth deviate by 4.9 epsilon units. The500 m salinity contour map reveals that the coral was grow-ing in Labrador Sea water, which occupies the uppermost1000 m of the water column in the NW Atlantic (Talleyand McCartney, 1982), but is not present at Station AII109-1. Comparison of the coral results with a seawater pro-file from the Labrador Sea (Hudson 93-036 Station 11; Pie-pgras and Wasserburg, 1987) yields excellent agreement(Fig. 7).

To test the match in hydrography between coral andseawater locations, we here take the simplified approachto compare salinity values for the respective locations (Ta-ble 5; salinity data are taken from the WOA05 database).

Fig. 6. Neodymium, U, and Th concentrations in four di!erentgenera of modern deep-sea corals (Flabellum, Enallopsamia, Des-mophyllum, Lophelia).


Of the 14 locations from where corals were selected for ourmodern calibration 11 show salinity values that agree with-in 0.1 psu of the nearest seawater site (Fig. 8). The threeoutliers are from the South Atlantic (Smithsonian ID1081198, L. pertusa) and o! Florida (Smithsonian ID80750, E. profunda; Smithsonian ID 80797, E. profunda).For the latter case the corals were recovered from less salinewaters than observed at the nearest seawater station. Thisobservation is important, as freshwater inputs from thecontinent are one of the major modulators of seawaterNd isotopes.

When comparing the Nd isotopic compositions for thesame corals and nearby seawater stations, 12 out of 14deep-sea corals are identical within error (Fig. 8 and Ta-ble 5). Seawater and coral values are positively correlatedand fall on a 1:1 line (r2 = 0.99; Fig. 8). The two outliersare from o! Florida (Smithsonian ID 80750, E. profunda,o! Florida; Smithsonian ID 261291, L. prolifera, o! Flor-ida). These results are further illustrated in Fig. 9 and high-light that while the overall comparison from the combinedNd isotope and salinity data looks very promising, twolocations emerge as problematic: the South Atlantic andthe region o! Florida.

In the following we will discuss the data for these tworegions in more detail, and show how study of hydrogra-phy, continental inputs, and coral age can be used to fur-ther substantiate the value of our modern calibration ofdeep-sea corals as Nd isotope seawater archives.

4.1.1. Depth profile o! Florida and sample o! SouthAmerica—inputs of Nd from the continents

Two E. profunda samples (45.7 m and 455 m waterdepths, o! Florida) and one L. pertusa sample (370–387 m)

show small ("0.5 psu), but nevertheless significant o!setsin terms of salinity at the coral collection site, and salinityat the closest-by seawater collection station and water depth.All three samples are derived from locations that are closer tothe continents than the seawater stations (Figs. 2 and 10).One goal of our modern calibration was to find sites wherewe could access corals from a wide depth range in a smallgeographic area. One such area is the continental slope o!the coast of Florida. Unfortunately, the nearest seawaterNd isotope data come from a composite station in the Sar-gasso Sea (Piepgras andWasserburg, 1980). The coral-basedNd isotope depth profile shows clear deviations from theopen ocean seawater profile, with mismatches of !3.7 and+2.3 epsilon units at the subsurface and at "802 m waterdepths, respectively (Figs. 9 and 10). The two samples thatshow deviations in salinity (lower values), however, are thesample from the subsurface and from "455 m.

The continents are the ultimate source of Nd to theocean: local Nd inputs at any ocean margin therefore havethe potential to alter the seawater isotopic composition. Acompilation of coastal geology and Nd isotopes suggeststhat the North American continent in this area has an epsi-lon Nd value of about !14 (Jeandel et al., 2007), similar tothe value seen in the 45 m water depth E. profunda. The sitewhere the coral was collected is, furthermore, close to theoutflow of the Savannah River which drains the Appala-chian Mountains. Zircon crystals collected from the Savan-nah River show Grenvillian, Neoproterozoic, and Paleozoicages which should yield relatively negative Nd isotopiccompositions (Eriksson et al., 2003). Fresher surface watersalinity values at the coral site compared to the open oceansite support our interpretation that river waters are influ-encing the coral site, causing the negative eNd deviation

Fig. 7. (a) Salinity contour map of the NW Atlantic Ocean at 500 m water depth. Plotted with OceanDataView using the WOA2005 dataset.White square marks the location of the seawater location most proximal to all corals analyzed from the NW Atlantic (Piepgras andWasserburg, 1987). Black circle marks location of Smithsonian ID 62309, a D. dianthus sample from intermediate water depth. Black square:seawater station in the Labrador Sea (Piepgras and Wasserburg, 1987). (b) Seawater profiles for both seawater locations shown in (a) as wellas the data point for the D. dianthus coral, which matches the Labrador Seawater Nd isotopic composition.


(Fig. 10). This interpretation however remains to be con-firmed by future seawater collection and analyses on theNorth American shelf. Although the L. pertusa in the SouthAtlantic shows a similar deviation in terms of salinity val-ues (Fig. 7), there is no corresponding Nd isotope e!ect.One possible interpretation is that continental inputs tothe area have a similar Nd isotopic composition as ambientseawater (eNd " 10; Pahnke et al., 2008). However, as the

salinity o!set is in the opposite direction as expected forfreshwater inputs (i.e., the coral location more proximalto the continent shows a higher salinity than the seawaterlocation), it is unlikely that we are looking at significantNd input from the continent. This conclusion is furthersupported by the fact that the Nd isotopic compositionsof seawater and coral site are similar, and that the Nd con-centration in the coral is very low (4.5 ppb; Table 4).

Table 5Neodymium isotope and concentration data summary for all modern corals and close-by seawater data.

Coralsample ID

Nd(ng/g)

Nd/Ca(nmol/mol)

eNd ± 2r SE/SD

Seawater stationname

Waterdepth(m)

Nd(ng/kg)

Nd/Ca(mol/mol)

eNd ± 2rSE/SD

Reference Kd

62309 18.1 12.56 !15.16 ± 0.30 Hudson 93-036 Station 11 500 3.13 2.11 !15.5 ± 0.4 1 5.9562308 !14.29 ± 0.55 AII 109-1 Station 30 1800 2.60 1.75 !13.5 ± 0.4 1ALV-3891-1459-003-001

!14.8 ± 0.43 AII 109-1 Station 30 1100 2.20 1.48 !14.0 ± 0.4 1

78459 3.3 2.29 !13.61 ± 0.54 AII 109-1 Station 30 1800 2.60 1.75 !13.5 ± 0.4 1 1.3115584 17.7 12.28 !14.17 ± 0.40 AII 109-1 Station 30 1100 2.20 1.48 !14.0 ± 0.4 1 8.2880750 16.8 11.66 !13.28 ± 0.36 OCE 63 Station 3 50 !9.6 ± 0.9 380797 27.0 18.74 !11.23 ± 0.40 OCE 63 Station 1 300 2.00 1.35 !10.9 ± 0.6 3 13.8962577 24.0 16.65 !13.89 ± 0.40 OCE 63 Station 2 1000 !13.1 ± 0.4 31081198 4.4 3.05 !9.54 ± 0.82 SAVE 302 470 1.50 1.01 !10.0 ± 0.4 4 3.0245669 !9.22 ± 0.94 327 650 1.31 0.88 !9.2 ± 0.8 593951 51.3 35.60 !2.34 ± 0.40 MW98-13-107 (1.14) 1140 2.41 1.63 !2.1 ± 0.23 6 21.9092795 !3.43 ± 1.46 LM-6/11 1009 3.03 2.04 !3.8 ± 0.3 747433 8.7 6.0411964 38.0 26.3780404* 29.0 20.12 !8.85 ± 0.36 Station C 400 3.48 2.35 !8.9 ± 0.3 2 8.57261291* 20.2 14.02 !10.75 ± 0.22 OCE 63 Station 2 1000 !13.1 ± 0.4 3

References: 1, Piepgras and Wasserburg (1987); 2, Tachikawa et al. (2004); 3, Piepgras and Wasserburg (1980); 4, Jeandel (1993); 5, Piepgrasand Wasserburg (1982); 6, Vance et al. (2004); 7, Amakawa et al. (2004).

Fig. 8. Modern calibration for Nd isotopes in aragonitic deep-sea coral skeletons. (a) Deep-sea coral Nd isotopes versus seawater Ndisotopes. Seawater values are taken from the literature from stations closest to the place of coral recovery. References are given in Fig. 9.Di!erent symbols and colors highlight di!erent coral genera. Gray symbols are corals discussed explicitly in the text. Gray circle: coral o!Florida with a non-modern age (i.e., early Holocene). Gray square: coral o! Florida from the subsurface, influenced by continental run-o!.All other samples plot on the 1:1 line. (b) Salinity comparison for the same locations for corals and seawater plotted in (a). All but threesamples plot on a 1:1 line. Symbols as in (a). For further explanations, see text. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)


4.1.2. Holocene seawater history o! FloridaFinally, we discuss the one coral that does not stand out

in the salinity comparison, but does in the Nd isotope com-parison (Figs. 8 and 9; L. prolifera, Smithsonian ID261291,o! Florida, 802 m water depth; "2.3 epsilon units higherthan Sargasso Sea water). The U–Th data for this coralshows that it has an early Holocene age of "9500 years (Ta-ble 3). Since there is the possibility of a change in strengthof the intermediate water-mass flow o! Florida during theHolocene (for example, see Lynch-Stieglitz et al. (2009)),we exclude this coral from our calibration. In principle,more radiogenic Nd isotopic compositions in intermediatedepth seawater o! Florida could originate from AntarcticIntermediate Water (e.g., Jeandel, 1993; Pahnke et al.,2008) or the Gulf of Mexico through the Florida Straits(Caribbean seawater probably has an epsilon Nd close to!10.7; Frank et al., 2006).

In summary: Of the 14 corals selected for our moderncalibration, all except two were close to modern in age.Nine of the remaining twelve samples had local salinity val-ues within 0.1 psu of seawater sites representing the watermasses bathing the coral sites. Neodymium isotopic compo-sitions of these nine corals are positively correlated with

seawater values on a 1:1 line (r2 = 0.99). This calibrationprovides a very robust demonstration that coralline arago-nite incorporates seawater neodymium isotopes.

4.2. Incorporation of Nd into Aragonite Lattice

Neodymium isotopes in modern deep-sea corals reflectseawater Nd isotopes demonstrating that the incorporationof Nd into coralline aragonite happens without fractionat-ing the 143Nd/144Nd ratio either through physical or biolog-ical processes. This result is not surprising, given thatroutine analysis of 143Nd/144Nd ratios involves mass biascorrection using a natural 146Nd/144Nd ratio. Hence anypotential stable isotope e!ect is removed from the final143Nd/144Nd ratio. This should however not underminethe significance of Nd isotopes as a new tracer in deep-seacoral research, a research field that has been complicatedby ‘vital e!ects’ on traditional paleoceanographic proxies.A similar observation has already been made for Nd iso-topes in planktonic foraminifera (e.g., Vance et al., 2004)and recently also for benthic foraminifera (Klevenz et al.,2008). The mechanism by which the Nd signal observedin calcitic foraminifera is transported to the sediment re-

Fig. 9. Comparison of Nd isotope results for deep-sea corals with most proximal seawater Nd isotope profiles. Corals are shown as coloredsymbols, and seawater profiles are shown as black diamonds connected by a line. The plot on the lower right hand side shows the corals fromo! Florida which are discussed in more detail in the text. Otherwise, the coral aragonite Nd isotopic compositions show excellent agreementwith seawater Nd isotope compositions. Seawater data are taken from Piepgras and Wasserburg (1980, 1982, 1987), Jeandel (1993),Tachikawa et al. (2004), Amakawa et al. (2004), and Vance et al. (2004). (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)


mains the subject of continuing debate (Pomies et al., 2002;Vance et al., 2004; Martinez-Boti et al., 2009). Consistentobservations however suggest concentrations of 0.1–2.5 lmol/mol Nd in cleaned planktonic foraminifera (seeVance et al. (2004) for a recent summary), a factor 100–500 higher than expected if assuming direct incorporationfrom seawater into the calcite lattice (Shaw and Wasser-burg, 1985). This observation, in conjunction with othermetal/Ca ratios in foraminifera, led Haley et al. (2005)and Vance et al. (2004) to propose a role for organic chem-istry during foraminiferal calcite precipitation and/or redoxconditions in the water column.

A number of studies have investigated the incorporationof Nd into shallow-water corals. Shaw and Wasserburg(1985) were the first to report much lower Nd concentra-tions in cleaned primary biogenic carbonates (low-Mg cal-cite and aragonite). They reported data on two aragoniticshallow-water corals (Acropora sp., Siderastrea radians)yielding 2.8 and 67 nmol/kg Nd. Subsequent work on shal-low-water corals of di!erent species (Goniastrea pectinata,Oulastrea crispate Porites lutea cf., Stylophora pistillata,Diploria labyrinthiformis, Diploria strigosa, Porites aster-oids, Montastrea annularis) revealed a wider concentrationrange of 2.8–385 nmol/kg (Shaw and Wasserburg, 1985;Sholkovitz and Shen, 1995; Akagi et al., 2004; Wyndhamet al., 2004). When compared to nearby seawater theseNd concentrations translate to partition coe"cients (Kd)of "1–3 (Kd = (Nd/Ca)coral/(Nd/Ca)seawater). Sholkovitzand Shen (1995) suggested that incorporation of Nd intothe coral lattice is aided by the similar radii of Nd3+ andCa2+ with the complication of maintaining charge balanceovercome by the occurrence of Nd in seawater as carbonatecomplex.

Results presented in Tables 4 and 5 contain the first Ndconcentration data for deep-sea scleractinian corals. TheNd concentration range from 23 to 355 nmol/kg in our sol-

itary and colonial corals is lower than for most calciticforaminifera ("0.1–2.5 lmol/kg with the exception of astudy by Pomies et al. (2002) and some samples in Marti-nez-Boti et al. (2009)), and similar to the published concen-tration range of shallow-water corals. No clear relationshipcan be found when comparing the new concentration datafor scleractinian corals to nearby seawater station Nd con-centrations (Table 5). The range of calculated distributioncoe"cients for the deep-sea corals is larger than for shal-low-water corals, at 1.3–21.9 (Table 5). The highest Kds inour sample set are associated with Enallopsamia. This colo-nial coral has a large surface area, particularly around pol-yps, and can be hard to clean, due to its delicate structure(Fig. 1). It is possible that more Nd is adsorbed on surfacesduring formation, or that the cleaning procedure did not re-move all surface Nd. Kds for the other genera are well below10, suggesting that most of the Nd analyzed from the cor-alline aragonite is derived from direct incorporation fromseawater. No correlation of distribution coe"cients withwater depth is to be observed. The distribution coe"cientscalculated here may be subject to a number of biases.Firstly, the cleaning procedure may not remove all surfaceneodymium and/or organic matter trapped inside the skel-eton. Secondly, the seawater stations used for comparisonmay not be close enough to provide a proper comparison(Fig. 2). This spatial separation may be especially impor-tant close to the continents as shown by Akagi et al.(2004). However, experimental data on rare earth elementpartitioning into aragonite also show distribution coe"-cients that are greater than 1, with a range of 4–4.5 (Tera-kado and Masuda, 1988), implying preferential uptake ofNd relative to Ca in the aragonitic skeleton. Further studiesare needed, particularly on combined seawater and deep-sea coral samples from the exact same locations, to under-stand and quantify Nd incorporation in the aragoniticskeletons.

Fig. 10. Regional map for the area o! Florida showing as red circles the deep-sea coral locations and as black squares locations for seawatermeasurements. The Nd isotope profile defined by the corals deviates from the seawater profile defined by direct water analyses. Also shown isthe salinity data for all locations highlighting that the water becomes fresher closer to the continent. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)


4.3. Preservation of the seawater signature over time

In order to use deep-sea corals as archive of past seawa-ter Nd isotopic compositions, it is necessary to demonstratethat the original isotopic signature is not compromised bydiagenesis or contamination.

The biggest concern is whether contaminant phases canbe removed, most importantly detrital grains and ferroman-ganese oxides. Both of these phases contain orders of mag-nitudes more Nd than seawater or coralline aragonite(upper continental crust: 26 ppm, Taylor and McLennan,1985; ferromanganese crusts and nodules: "200 ppm, Heinet al., 2000; typical seawater: 0.001–0.004 ppb). Shaw andWasserburg (1985), for example, reported results for twofossil corals to have much higher (103–104 times) Nd con-centrations than modern corals, and suggested that thisenrichment in Nd was derived from Fe–Mn hydroxides ascareful cleaning was not carried out prior to analyses.

Our sequential cleaning steps show that indeed there is amarked change in Nd concentrations from uncleaned tophysically cleaned corals (Fig. 4 and Table 4). The oldestcoral in our series of experiments (ALV-3889-1326-002-B7, "220 kyr) is surrounded by a thick black ferromanga-nese oxide coating. An uncleaned piece of this coral yieldsNd concentrations two orders of magnitude above the levelof modern corals (Figs. 1 and 4). Significant Nd concentra-tion changes in the order of 69% and 84% can also be ob-served for the two Holocene corals from the Chilean shelf(Alb#7, Alb#1). Physical cleaning is the most importantstep in the cleaning procedure. The next step, the pre-clean-ing, apparently removes most remainders of the ferroman-ganese coating as Nd concentrations after physical cleaningand pre-cleaning are very similar to modern corals for mostfossil corals (7.6–67.7 ppb; Fig. 4 and Table 4).

In some cases Nd isotopes show a behavior decoupledfrom Nd concentrations (Figs. 4 and 5). While the concen-trations are mainly governed by contaminating phases, theNd isotopic composition in such phases, especially the ferro-manganese oxides can be very similar to the composition ofthe readily cleaned aragonite. This is the case if the fossilcoral grew in a water mass that has had the same Nd isoto-pic composition as the modern ambient water mass, as is thecase for the heavily coated NW Atlantic coral ALV-3889-1326-002-B7 ("220 kyr old). This coral shows the sameNd isotopic composition throughout the cleaning processdespite a factor of 30 of change in Nd concentration. Thecoral was collected from the New England seamounts, andits ferromanganese coating probably integrates over the past220 kyr of seawater history. An unchanged Nd isotopiccomposition over the same period has also been reportedby Foster et al. (2007) using laser ablation Nd isotope datafrom a ferromanganese crust from the New England sea-mounts. Supporting this conclusion, deglacial corals fromthe New England seamounts also exhibit no change (vande Flierdt et al., 2006). However, and as pointed out before,these result should be treated with caution as the U–Th dataindicate an open-system behavior for this particular coral,and di!usion of Nd cannot be excluded. In general we rec-ommend to not analyze fossil corals for their Nd isotopiccomposition, if they do not yield a precise U–Th age.

In contrast to ALV-3889-1326-002-B7, some other coralsshow a distinct di!erence between the FeMe crusts, un-cleaned coral aragonite, and clean coral aragonite, demon-strating that diagenetic overprinting and/or di!usion isnot an issue for Nd isotopes in corals that are identifiedby U–Th data as closed systems. Coral Alb#7 from the Chil-ean shelf exhibits a shift in the Nd isotopic compositionfrom !6.4 ± 0.4 in the coating to !4.2 ± 0.5 in the cleanedaragonite. The pre-cleaned aliquot has an intermediate Ndisotopic composition (eNd = !5.8 ± 0.7). Overall, the threeChilean corals used for the cleaning experiments (Alb#1,7, 9) indicate that the coral has to pass at least the oxidativecleaning step to remove contaminant phases (Fig. 5). Afterthis cleaning step, the Nd isotopes of all three Chilean coralsare indistinguishable from one another. Coral ALV-3892-1421-001-030, recovered from the New England seamounts,shows a shift in Nd isotopes from !13.1 ± 0.2 in the ferro-manganese coating to !12.3 ± 0.4 in the cleaned aragonite.Even though the average Nd isotopic composition may haveremained unchanged over the past 220,000 years (Fosteret al., 2007), this coral hints to fluctuations on short time-scales in the NWAtlantic during the last glacial period. Pre-viously published results from a set of two corals (one mod-ern and one "17 kyr old) from the Drake Passage alsosupport our conclusion that fossil deep-sea corals, if cleanedcarefully, can record eNd values of past seawater (Robinsonand van de Flierdt, 2009). At this location the modern coralaragonite matches the seawater Nd isotopic composition(eNd = !9.2 ± 0.9), while the fossil coral does not(eNd = !6.4 ± 0.4). The ferromanganese coating scrapedo! the fossil coral recorded an epsilon Nd of !7.0 ± 0.4,identical to results from four ferromanganese crusts in thearea (Albarede et al., 1997), and intermediate between themodern and deglacial seawater values.

5. CONCLUSIONS

We have presented a comprehensive test of the idea thatdeep-sea corals can be used as an archive for Nd isotopes inthe past ocean. Analyses of five di!erent genera of modernscleractinia (Dianthus, Caryophyllia, Flabellum, Lophelia,Enallopsamia) provide robust evidence that these genera re-flect seawater Nd isotope compositions. Neodymium con-centration data indicate a distribution coe"cient below 10for all genera except Enallopsamia. Cleaning experimentson fossil corals confirm that contaminant Nd can be re-moved successfully, and that the cleaned coralline aragonitehas similar Nd concentrations to modern corals. This tar-geted study demonstrates that deep-sea corals can providewell-dated, and potentially high-resolution records of sea-water Nd isotopic compositions in the past, allowing theirapplication to a broad range of questions in paleoclimateresearch.

ACKNOWLEDGMENTS

This study was supported by NSF Grant OCE-0623107 toT.v.d.F. and L.F.R., NSF Grant ANT-063678 to L.F.R., a MarieCurie International Reintegration grant (IRG 230828) and NERCGrant NE/F016751/1 to T.v.d.F., and NSF Grant OCE-0929272to J.F.A. We thank the Lamont geochemistry group for their help


in keeping the labs and the mass specs running smoothly, particu-larly Jenna Cole for initial help in getting familiar with the TIMS,and Sidney Hemming for invaluable discussions while developingthe coral measurements on the TIMS. Alex Gagnon is thankedfor help with coral sampling. We are very grateful to StephenCairns from the Smithsonian Museum in Washington (DC) forlending us the majority of the coral specimen used for this studyand for fruitful discussion. Constructive reviews by M. Gutjahrand two anonymous referees, as well as the editorial handling ofM. Bar-Matthews are gratefully acknowledged.

APPENDIX A. SUPPLEMENTARY DATA

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.gca.2010.08.001.

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Associate editor: Miryam Bar-Matthews


http://dx.doi.org/10.1029/2006PA001294

http://dx.doi.org/10.1029/1999JC000285

http://dx.doi.org/10.1029/1999JC000285

http://dx.doi.org/10.1029/2003PA000957

Deep-sea coral aragonite as a recorder for the neodymium isotopic composition of...

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