Nd isotopes in deep-sea corals in the North-eastern Atlantic

Nd isotopes in deep-sea corals in the North-eastern Atlantic

Quaternary Science Reviews 29 (2010) 2499e2508 Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.c...

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Quaternary Science Reviews 29 (2010) 2499e2508

Contents lists available at ScienceDirect

Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev

Nd isotopes in deep-sea corals in the North-eastern Atlantic Kevin Copard a, b, *, Christophe Colin a, Eric Douville b, Andre Freiwald e, Gudmundur Gudmundsson c, Ben De Mol d, Norbert Frank b a

Laboratoire des Interactions et Dynamique des Environnements de Surface (IDES), UMR 8148, CNRS-Université de Paris-Sud, Bâtiment 504, 91405 Orsay Cedex, France Laboratoire des Sciences du Climat et de l’Environnement (LSCE), Laboratoire mixte CNRS-CEA, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France c Icelandic Institute and Museum of Natural History Po Box 5320, Hlemmur 3, 112 Reykjavik, Iceland d GRC Geociències Marines, Parc Científic de Barcelona, Campus Diagonal - Universitat de Barcelona, Adolf Florensa 8, 08028 Barcelona, Spain e Senckenberg Meeresforschung, Südstrand 40, 26382 Wilhelmshaven, Germany b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 November 2009 Received in revised form 17 May 2010 Accepted 18 May 2010

Neodymium (Nd) concentrations and isotopic signatures of living and fossil deep-sea coral species Lophelia pertusa, Desmophyllum dianthus and Madrepora oculata from the northeast Atlantic Ocean have been investigated in order to test the ability of deep-sea corals to reconstruct the seawater Nd isotopic signature and past changes of ocean circulation in the eastern North Atlantic. Small quantities of Nddless than 45 ng/gdare incorporated into the aragonite skeleton of living deep-sea corals that dwell at upper intermediate depths throughout the Northeast Atlantic. Rigorous cleaning techniques are needed in order to avoid Nd contamination from manganese-oxide and iron hydroxide coatings. Moreover, Nd isotopic compositions have been measured using thermal ionization mass spectrometry (TIMS) by Nd-oxide method. Our data indicate that the isotopic signatures of modern corals are similar to those of adjacent water masses, implying that deep-sea corals can serve as an archive of the seawater Nd isotopic compositions in the past. The first results from few fully-cleaned fossils corals collected within the Porcupine Seabight and the southwest Rockall Bank reveal significantly higher eNd for corals dated between 150  40 and 3060  90 yrs than those of the living corals located in similar areas. This suggests rapid hydrological variations along the eastern margin of the North Atlantic Ocean at intermediate water depth with higher contribution of the Mediterranean Overflow Waters (MOW) or other temperate Atlantic mid-depth water masses (ENACW or NAC) in the past. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Seawater displays distinct Neodymium (Nd) isotopic values derived ultimately from continental weathering, erosion and particle-seawater interactions (Piepgras et al., 1979; Goldstein and O’Nions, 1981; Frank, 2002; Goldstein and Hemming, 2003; Lacan and Jeandel, 2005b). The Nd isotopic composition (143Nd/144Nd) is expressed as eNd, which is the deviation of the 143Nd/144Nd ratio from its average value in the ‘‘bulk Earth’’ 143Nd/144Nd ratio of 0.512638 in parts per 104 (Jacobsen and Wasserburg, 1980), expressed as:

eNd ¼ ([(143Nd/144Nd)sample/0.512638]1)  10000. * Corresponding author. Laboratoire des Interactions et Dynamique des Environnements de Surface (IDES), UMR 8148, CNRS-Université de Paris-Sud, Bâtiment 504, 91405 Orsay Cedex, France. Tel.: þ33 1 69 82 35 37. E-mail addresses: [email protected] (K. Copard), [email protected] (A. Freiwald), [email protected] (G. Gudmundsson), [email protected] (B. De Mol). 0277-3791/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2010.05.025

Unlike nutrient proxies, such as d13C or Cd/Ca, Nd isotopes are not known to be affected by biological processes, and thus they can serve as a quasi-conservative water mass tracers (Frank, 2002; Goldstein and Hemming, 2003; Martin and Scher, 2004; Vance et al., 2004). The Nd isotopic composition of seawater is today recognized as powerful tracers of water mass provenance and mixing (Lacan and Jeandel, 2004a,b,c, 2005b). Moreover, it was shown that sedimentary archives such as foraminifera, the authigenic Fe-Mn oxyhydroxide fraction of sediments, and ferromanganese crusts, allow tracing past ocean circulation changes on various times scales (Vance and Burton, 1999; Piotrowski et al., 2004, 2005; Foster et al., 2007). Recently, Van de Flierdt et al. (2006) produced first seawater Nd isotope reconstructions using U-series-dated solitary deep-sea coral species Desmophyllum dianthus from the New England Seamounts in the north-western Atlantic Ocean. This study revealed minor variations of the intermediate water isotopic compositions throughout the last deglaciation in agreement with the expected Nd isotope composition of Glacial North Atlantic Deep Waters (GNADW). Foster et al. (2007), using eNd in ferromanganese

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crusts over glacial/interglacial periods, also concluded that there was no significant change in the eNd for the northern deep water masses such as GNADW and Glacial Labrador Sea Water (GLSW). These results imply a long-term stability of the continental Nd sources added to the seawater in the north Atlantic. Other records obtained, for example, for an authigenic Fe-Mn oxyhydroxide fraction of the sediments collected in the South Atlantic have demonstrated large variations of Nd isotopic signatures (3 eNd) throughout the past climate cycle, which have been linked to changes in ocean circulation, i.e., variable export of North Atlantic Deep Water (NADW) into the southern Ocean (Rutberg et al., 2000; Piotrowski et al., 2004; Piotrowski et al., 2005). Within the eastern North Atlantic Ocean, deep-sea corals are widespread along the margins from the Strait of Gibraltar to the Norwegian shelf. These corals dwell at upper intermediate depths and are thus influenced mainly by upper intermediate water recirculation throughout the north-eastern Atlantic basin (Bower et al., 2002; Lavender et al., 2005). Consequently, by using framework building corals, such as Lophelia pertusa and Madrepora oculata, that have contributed to coral constructions dating back to 2.6 Ma (Kano et al., 2007) Nd-isotopic signatures might possibly be retrieved to record the northward advection and mid-depth recirculation of surface and intermediate waters in these areas. To test the ability of this new archive to trace seawater Nd isotopic signatures, we have investigated living and fossil deep-sea corals, such as L. pertusa, M. oculata, D.dianthus, from the northeast Atlantic. A first step in our study was to establish chemical protocols to extract the Nd incorporated into the corals’ aragonitic skeleton from seawater in order to determine the isotopic composition of the coral and thus seawater. The Nd concentration of corals can be easily altered by non-carbonate contamination; therefore rigorous cleaning procedures are required. Here, we demonstrate that, after mechanical and chemical cleaning, the isotopic composition of the sampled living corals matches the one of seawater. Additionally, we briefly present and discuss Nd isotopic

composition of fossil deep-sea corals collected within Rockall Trough and Porcupine Seabight at water depths influenced by intermediated water re-circulation. 2. Samples and hydrological setting Fifteen living corals of species L. pertusa, M. oculata and D. dianthus and 11 fossil coral samples from the eastern North Atlantic margin and the Nordic Seas have been analysed (Table 1). Deep-sea corals selected for this study have been collected at water depths comprised between 101 and 1325 m. Twelve deep-sea corals have been sampled in the gravity cores MD01-2454G, MD01-2455G and MD01-2463G. These cores were collected during the R/V Marion Dufresne expedition ‘‘MD123’’ in September 2001. The core MD012454G was retrieved from the eastern slope of the Rockall Bank (55 31017Ne15 390 08W, 747 m water depth) on an unnamed mound colonised by deep-sea corals (Fig. 1). Core MD01-2455G was retrieved at the same location at 635 m water depth. Gravity core MD01-2463G (888 m) was recovered from top of Thérèse Mound located in the Belgica Mound Province of Porcupine Seabight. Several other deep-sea corals have been collected by ROV (B08-04, CARACOLE PL 127-05 and KGS 16/132-10), by box core (Alk samples and VH samples) or by sledge. For the eastern North Atlantic, the present hydrodynamic regime within the depth range of the studied corals is stipulated through strong bottom water currents with peak velocities of up to 100 cm/s (White, 2006). These high currents are driven by the northward moving of the Eastern North Atlantic Central Waters (ENACW) (Rockall Bank) and the Mediterranean Overflow Waters (MOW). Actually, the northward flow of the MOW in the North Atlantic is limited to the Porcupine Seabight (Holliday et al., 2000; New and Smythe-Wright, 2001). However, Lozier and Stewart (2008) have recently shown that the northward penetration of the MOW along the eastern boundary of the North Atlantic is characterised by a great temporal variability in relation to the North

Table 1 Localisation, species and water depth (m) of the deep-sea coral samples investigated in this study. Ages, Nd concentration, Nd/Ca ratio, Mn/Ca ratio, and eNd value of the living and fossil deep-sea corals are also presented. In cases were statistical uncertainty is better the 0.3 eNd reproducibility (2s) the global accuracy (i.e. 0.3) is quoted. In cases statistical uncertainty exceeds the reproducibility and thus 2s statistical uncertainties are quoted. All samples have been cleaned with protocol described in Table 2. Sample name

Specie

Latitude

Longitude

Water depth (m)

age (yr)

Nd (ppb)

Nd/Ca (nmol/mol)

Mn/Ca (mmol/mol)

eNd

SEGE 303 VH 97-315 B08-04 CARACOLE PL 127-05 CARACOLE KGS 16/132-10 MD01-2454G top Pos 241 364 DS Dos G Pos 241 362 DS 9H Pos 292 526-1 Alk 232 BG 1163 Alk 231 BG1158 Bioice 2472 Bioice 2424H VH-95-163 JR-111 MD01-2455G 0e5 cm A MD01-2455G 0e5 cm B MD01-2455G 7e8 cm MD0- 2455G 10 cm MD01-2455G 20 cm MD01-2455G 21e27 cm MD01-2455G 25 cm MD01-2455G 31e32 cm MD01-2455G 45e48 cm MD01-2463G top MD01-2463G 10e11 cm

M. oculata L. pertusa M. oculata L. pertusa L. pertusa L. pertusa D. dianthus D. dianthus L. pertusa L. pertusa L. pertusa L. pertusa L. pertusa L. pertusa L. pertusa L. pertusa L. pertusa L. pertusa L. pertusa L. pertusa L. pertusa L. pertusa M. oculata M. oculata L. pertusa L. pertusa

35 57.530 N 42 46.590 N 46 54.630 N 52 18’N 55 32’N 55 31.17’N 57 25.36’N 59 20.93’N 59 11.06N 59 05.64N 59 05.73N 63 070 N 63 100 N 64 04.95N 70 15.81N 55 33’N 55 33’N 55 33’N 55 33’N 55 33’N 55 33’N 55 33’N 55 33’N 55 33’N 51 26’N 51 26’N

5 46.33’W 11 46.880 W 5 19.620 W 13 02’W 15 40’W 15 39.08’W 11 43.11’W 10 33.71’W 17 12.70W 10 47.87E 10 47.70E 21 380 W 20 090 W 08 01.63E 22 27.71E 15 40’W 15 40’W 15 40’W 15 40’W 15 40’W 15 40’W 15 40’W 15 400 W 15 40’W 11 460 W 11 460 W

101 823 693 600 850 747 1003e1214 1117e1325 516 88 120 666 495e575 289 254 635 635 635 635 635 635 635 635 635 888 888

alive alive alive alive alive alive alive alive alive alive alive 50 <40 alive alive 150  40 240  120

NA 38.0 11.4 16.0 19.2 11.3 23.7 30.2 38.5 6.1 8.1 42.7 19.5 12.6 18.6 56.2 35.0 51.5 65.6 309.8 112.2 241.8 234.8 222.8 55.2 131.9

NA 26.7 9.0 11.1 13.8 8.8 17.0 26.6 26.1 4.6 4.2 32.5 14.7 9.1 13.1 48.3 30.5 45.0 56.1 264.0 98.2 214.0 201.7 191.3 39.4 104.3

NA 0.05 0.14 0.06 0.93 0.12 0.07 0.19 0.28 1.94 1.33 9.77 0.19 0.64 2.37 7.83 6.17 6.03 4.41 6.72 3.56 9.35 6.01 6.01 0.84 0.21

9.8 12.2 11.5 13.3 13.3 13.8 14.0 13.3 13.1 13.5 13.0 10.2 11.0 14.1 13.3 12.2 12.4 12.9 11.9

190  190 850  200

3060  90 261  33 851  99

                  

0.3 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.4 0.3 0.3 0.3 0.3 0.3

12.4  0.3

12.3  0.3 12.7  0.3

K. Copard et al. / Quaternary Science Reviews 29 (2010) 2499e2508

100 mg of sample was necessary for Nd, Mn and Ca concentration measurements.

0.51190 0.51189

Nd/

144

Nd

0.51188

143

2501

+2

3.2. Ca, Mn and Nd concentrations analyses

0.51187 mean value

0.51186 0.51185 0.51184

-2

0.51183 0.51182 0

20

40

60 80 100 Analysis number

120

140

Fig. 1. Long-term reproducibility of 300-ng loads of the Nd La Jolla standard, running as NdOþ. Mean value of 143Nd/144Nd is 0.511859  0.000018 (2s).

Atlantic Oscillation (NAO) index and the subpolar gyre expansion, implying the possibility of short-lived penetration of the MOW into the Rockall Trough. Below these water masses, the Labrador Sea Water (LSW) flows eastward at water depths ranging from 1600 to 1900 m and re-circulates along the topographic basin boundaries (Ellett and Martin, 1973; Holliday et al., 2000; New and SmytheWright, 2001). In addition, the local topography results in internal basin scales waves that favour nutrient and food storage in mound regions (De Mol et al., 2002; Kenyon et al., 2003). Strong wind forcing through the westerlies result in a mixed layer confined to less than 600 m (Holliday et al., 2000). Thus, corals develop in near thermo-cline waters of temperatures generally ranging from 4 to 12 C, which originate either in the temperate Atlantic area (MOW and ENACW) or the Labrador Sea. In particular, water masses from the Labrador Sea are entrained underneath the North Atlantic Current (NAC) and are a dilute component of the water masses at upper intermediate depths similar to MOW from the temperate east Atlantic (Lozier, 1999; Bower et al., 2002; Lavender et al., 2005; Lozier and Stewart, 2008). 3. Methods 3.1. Cleaning procedure of deep-sea corals Coral polyps were sliced in half and rinsed in MilliQ water to remove sediments from the external and internal surface of the deep-sea coral fragments. Then, the inner and outermost surface of the skeleton were polished using a diamond-bladed saw in order to retrieve an opaque and translucent pure aragonite skeleton. At this step, we avoid specimens that reveal skeleton alteration through boring organisms. This mechanical cleaning was followed by two weak attacks with dilute ultraclean 0.5N hydrochloric acid in an ultrasonic bath for 10 min to remove further potential residual Fe-Mn coatings. Samples were then rinsed several times with MilliQ water. In our samples, the Fe-Mn staining was minor or almost absent, indicating the quick burial of the samples by sediment. Living corals do not have any staining due to the activity of the mucus that keeps the skeleton free of chemical and biological alternations. Cleaned polyp fragments were dried and crushed in an agate mortar in order to obtain a homogenous powder representative of the average bulk composition of the coral polyps. Two aliquots of each sample are taken for trace element analyses and for combined Nd isotopic measurements and U-series dating of the fossil specimens. Samples were then dissolved in dilute ultraclean hydrochloric acid. On average about 600 mg of coral was used for U-series dating and for the Nd-isotopic measurements. An additional

Ca, Mn and Nd concentrations of the deep-sea corals were analysed using a quadruple ICP-MS XseriesII CCT (Thermo Fisher Scientific) at the LSCE by the measurement of their isotopes 46Ca, 55 Mn and 146Nd and by using appropriate carbonate standards (JCp1 coral, JCt-1 clam, Arag. AK, BAM 3). Samples and standard solutions have been systematically adjusted at 100 ppm Ca through dilution, without further chemistry. This is because (1) dominant Ca signals need to be avoided as high concentrations can increase salt deposition on cones affecting ICP-MS stability and (2) adjusted Ca concentration levels introduced into ICP-QMS at 100 mL/min provide a way to control matrix effects due to the presence of Ca. Instrumental calibration based on the standard addition method was achieved using a mono-elementary standard solution for each element (Harding et al., 2006) and routinely measuring carbonate standards (JCp-1 and JCt-1 (Okai et al., 2004)). To compensate the signal derivation of a few percentage values during a day, a standard (JCp-1) was run every three samples, and additionally three internal standards (9Be, 115In and 185Re) were added to each sample. The concentrations of Nd in blanks are negligible as compared to the lowest concentration of 1.8 ppt  19% measured in the corals. Internal reproducibility for Nd on the JCp-1 standard (100 ppm Ca, 7 ppt Nd, n ¼ 22) was five percent (2s) for Nd and Mn and one percent for Ca. Detection limits are: 0.6 ppm for Ca, 15 ppt for Mn and 0.6 ppt for Nd, and lowest measured values are two to three times higher than the detection limits. 3.3.

230

Th/U dating and

143

Nd/144Nd analyses

For the U-series dating, samples are dissolved in ultraclean 3N HNO3 and are spiked with a mixed 229Th, 233U, 236U triple spike. Ion exchange resin UTEVA is used to purify U and Th from all other trace and minor elements following the procedure described in detail by Frank et al. (2004) and Douville et al. (2010). REE and all minor and trace elements are eluted using 2.5 ml of 3N HNO3, while U and Th are adsorbed on the UTEVA resin (Douville et al., 2010). U and Th are eluted with 2.5 ml of 3N HCl and 3 ml of 1N HCl, respectively. U and Th were then loaded onto single degassed Re filaments in a sandwich of graphite. The U and Th isotope analyses were performed on a TIMS (Finnigan MAT 262) using a method described in detail by Frank et al. (2004). An Nd-oxide technique for TIMS was used to determine the 143 Nd/144Nd ratios of corals, which is mandatory because Nd is incorporated into the coralline aragonite in small traces. NdOþ mass analyses by the Nd-oxide technique include Nd16Oþ, Nd17Oþ and Nd18Oþ. Potential isobaric interferences from Sm, Ce and Pr imply an efficient chromatographic extraction before the Nd isotopic analyses. For Nd purification, we used a TRU-Spec and a LnSpec resin following the method described in detail by Pin and Santos Zalduegui (1997). In this method, samples are loaded using 2 ml of 1N HNO3 on preconditioned TRU-Spec columns (83 mg portion of TRU-spec). The unwanted cations were eluted using 5 portions of 0.5 ml portion of 1N HNO3. TRU-Spec columns were then placed over Ln-Spec columns. The LREEs were then eluted from the upper (TRU-Spec) column using seven portions of 0.1 ml of 0.05N HNO3. After decoupling from the TRU-Spec columns, La, Ce and Pr were rinsed from the Ln-Spec columns with 3.25 ml of 0.25N HCl. Nd was then eluted with an additional 2.5 ml of 0.25N HCl. The Ln-Spec resin provides efficient separation of Nd from Sm, Ce and La. The separation of Nd from Pr is less efficient, producing

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significant isobaric interference with NdOþ for masses of 158 (141Pr17O) and 159 (151Pr18O) during the acquisition of the 143 Nd/144Nd ratio. Thus, mass 157, corresponding to 141Pr16O, is also measured in order to correct these interferences. The samples were loaded using 0.5N HNO3 in the smallest possible increments onto single Re degassed filaments (0.7 mm  0.04 mm, Re Alloys commercial grade ribbon). A current of 0.1 A was continuously applied during the sample loading. Once the sample had dried on the filament, 2 mL of silica gel (0.4 g SiO2 with 5.6 g H3PO4 in 44.8 g H2O) was loaded on top of the sample. The filament current was then increased until the filament glowed dark red. Then, the current was quickly reduced to zero. 143 Nd/144Nd isotopic ratios were analysed on a 6 Faraday Collector Finnigan MAT 262 thermal ionization mass spectrometer (LSCE, Gif/Yvette) using dynamic multiple collection of metal-oxide masses of 157, 158, 159, 160, 161, 162 and 163 measured in two steps (157e159 and 158e163). Samples were run for 200 scans with the 142 NdOþ intensity being higher than 0.5, up to 1.5 V. Ce and Sm were continuously monitored during all of the NdOþ analyses and were not found to be present. PrOþ isobaric interferences were measured and corrected line by line during offline analysis. Overall Pr to Nd ratios (141Pr/143Nd) varied between 0.1 and 0.6 and corrected 142Nd/144Nd ratios are found to be indistinguishable from those of Pr free samples and standards. However, in a few case 142 Nd/144Nd was slightly elevated compared to reference values, which was due to a small amount of Ce isobaric interferences, which however did not disturb measured 143Nd/144Nd ratios. To further test our interference corrections, standards of Nd have been spiked with Pr. Results have shown, in agreement with other previous studies (e.g. Harvey and Baxter, 2008), that interference corrections are efficient. The Nd isotope ratios were corrected for mass fractionation relative to 146Nd/144Nd ¼ 0.7219 using a power law. The oxygen isotope composition that was used for the NdOþ analyses was 18O/16O ¼ 0.002085 and 17O/16O ¼ 0.000391. These ratios have been analysed several times during the course of our analysis using PrOþ measurements or using 150Nd17Oþ and 150 Nd18Oþ, and they have been found to be constant. The measured Nd blank was well below significant levels. Replicate analyses (n ¼ 124) of the La Jolla standard yield a 143Nd/144Nd range between 0.511827 and 0.511884 with a mean of 0.511858  0.000010 (Fig. 1).

This mean value compares well to its certified value of 0.511850  0.000013 and we therefore consider that there is a negligible machine bias of 0.00001 that was taken into consideration for the sample data’s re-treatment following a bracketing technique. The major variance observed in the replicate analyses of the standards can be explained by the quality of the filament deposition and the filament geometry. Several analyses with the same filament yield far more reproducible values as compared to multi-filament analyses strongly supporting this idea. In contrast, the observed variability cannot be explained by potentially variable oxygen isotope composition. For example one would expect that isotopic ratios measured several times on a single filament should be affected as the oxygen reservoir is continuously reduced, which is not observed here. Replicate analyses have also been made for several samples of deep-sea corals (Colin et al., 2010). One of them is presented in this study; sample MD01-2455G 0e5 cm B presented in Table 1 and sample MD01-2455 0e5 cm b presented in Table 2 are replicates and show similar concentrations of Nd and Mn as well as similar Nd isotopic composition.

4. Results 4.1. Chronological framework The 230Th/U data and ages as well as one radiocarbon age of the fossil deep-sea corals are reported in Table 3. Seven coral ages obtained for the upper 50 cm of core MD01-2455G (SW Rockall Bank) and MD01-2463G (Porcupine Seabight) range between 150  40 and 3060  90 yr. Two additional deep-sea corals were collected from similar depth ranges in core MD01-2455G, but due to strong residual 232Th contaminations U-series dating was not feasible. However, both samples are most likely characterised by an age corresponding to the late Holocene, i. e. less than 3000 yr. For the 15 living specimens used in this study, we attributed an age close to the date of collection. L. pertusa corals grow with individual rates of several mm per year and, consequently, coral polyps less than 1.5 cm in size should correspond to less than a few years of growth. For specimens of the solitary corals D. dianthus, Adkins et al. (2004) determined far slower growth rates, of less than

Table 2 Results of the Nd concentration, Nd/Ca ratio, Mn/Ca ratio and eNd value for two samples from the core MD01-2455G (55 330 N e 15 400 W, 635 m water depth) obtained after different cleaning procedures. ‘‘Bulk sample’’ corresponds to deep-sea corals samples that were not cleaned. ‘‘Mechanical cleaning’’ corresponds to the coral aragonite after polishing of the inner and outermost surface coral skeleton using a diamond-bladed saw. ‘‘Dilute HCl attacks cleaning’’ corresponds to the cleaning of the coral skeleton by two weak attacks with dilute ultraclean HCl in an ultrasonic bath. ‘‘Mechanical residue’’ (corresponding to the leachate fraction from polishing) and ‘‘chemical residue’’ correspond to analyses done on the residual fractions obtained from the two steps of the cleaning procedure. The sample MD01-2455G 0e5 cm b corresponds to a replicate of sample MD01-2455G 0e5 cm B presented in the Table 1. Mass percentage (not determined for oxidative and reductive cleaning) corresponds to the ratio between coral fragment after cleaning (or residue obtained during cleaning) and un-cleaned bulk sample. The 0.3 eNd error corresponds to reproducibility (2s). Sample name/cleaning procedure MD01-2455G 0e5 cm b (L. pertusa,/age U/Th ¼ 240  120 yrs) Bulk sample - not cleaned Mechanical cleaning Mechanical cleaning and dilute HCl attacks cleaning Oxidative cleaning Reductive cleaning Mechanical residue (Fe-Mn coating) Chemical residue MD01-2455G 21e27 cm (L. pertusa/age U/Th ¼ 850  200 yrs) Bulk sample - not cleaned Mechanical cleaning Mechanical cleaning and dilute HCl attacks cleaning Oxidative cleaning Reductive cleaning Mechanical residue (Fe-Mn coating) Chemical residue

Mass percentage

Nd (ppb)

100 61.3 52.0 ND ND 18.4 9.3

308.7 154.2 29.6 51.2 25.1 1109.3 245.0

100 64.9 50.8 ND ND 21.5 14.1

402.9 419.2 144.2 99.4 44.4 1617.7 618.8

Mn/Ca (mmol/mol)

eNd

236.8 114.1 22.4 38.4 19.0 771.3 197.3

92.1 12.4 1.1 0.4 0.1 163.6 28.8

12.5 13.1 12.3 NA NA 11.9 NA

278.7 303.4 99.8 73.3 33.4 1032.2 492.1

11.7 7.7 3.4 1.1 1.0 49.9 14.8

12.6 12.6 13.1 NA NA 12.5 NA

Nd/Ca (nmol/mol)

 0.3  0.3  0.3

 0.3

 0.3  0.3  0.3

 0.3

K. Copard et al. / Quaternary Science Reviews 29 (2010) 2499e2508

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Table 3 U-series measurements and ages and radiocarbon age for fossil corals of core MD01-2455G and MD01-2463G. Further U-series ages on both sediment cores are published by Frank et al. (2009) and Douville et al. (2010). 232Th values given in bold characters indicate strong residual detritus or Fe-Mn contamination of samples. Those samples were thus rejected for further data treatment and age determination. Age* reflect ages calculated based on a simple correction model considering precipitation of initial 230Th from seawater taking seawater (232Th/230Th) activity ratios ranging from 14 to 6 into account (Frank et al., 2004).

MD01-2455G MD01-2455G MD01-2455G MD01-2455G

Gif-1286 Gif-1287

MD01-2455G 21e27 cm MD01 2455G 25 cmA

4.248  0.016 4.767  0.006

Gif-1290b Gif-0012 SAC-40152

MD01 2455G 45e48 cm MD01-2463G top MD01-2463G 10 cm

5.017  0.008 15.204  0.059 3.909  0.012 0.800  0.010 AMS-14C ARTEMIS/LSCE

0e5 cm A 0e5 cm B 10 cm 20 cm

U (ppm)

4.043 3.610 4.196 4.520

   

0.0019 0.0046 0.0062 0.019

Th (ppb)

0.953 2.671 8.534 21.56

   

0.015 0.032 0.014 0.23

5.523  0.014 42.08  0.27

d234UM (&)

(230Th/238U)

(230Th/232Th)

1.6 4.2 5.4 8.9

0.00229  0.00012 0.00494  0.00024 0.00781  0.00014

29.7  1.6 20.4  1.0 11.7  0.2

140.4  7.5 145.0  2.6

0.01299  0.00034

30.5  0.8

145.3 150.2 146.9 151.0

   

1 mm per year, implying that the collected aragonite samples have formed over several years to several decades. 4.2. Nd concentration The Nd concentrations in modern framework buildings and solitary deep-sea corals were analysed here for the first time on modern (living) deep-sea reef frameworks and solitary corals and they ranged from 6 to 43 ppb, while those of fossil deep-sea corals ranged from 35 to 310 ppb (Table 1). The Nd/Ca ratio of the modern corals is in the order of 4.2e32.5 nmol mol1, while those of the fossil corals range from 30 to 264 nmol mol1. Mn/Ca ratios for the modern corals range from 0.05 to 9.8 mmol mol1 with most values close to or lower than 1 mmol mol1. In contrast to the Nd/Ca ratios of fossil corals, the Mn/Ca range is comprised between 0.2 and 9.3 mmol mol1, thus in a similar order of magnitude, as compared to modern corals. For all the modern cleaned corals, the Nd/Ca ratio was uncorrelated with the Mn/Ca ratio (R2 ¼ 0.11 in the best case, Fig. 2a). In addition, note that the coral Nd concentration and the Mn/Ca ratio do not yet seem to be species-dependent. On the contrary, Nd concentrations for cleaned fossil corals demonstrate higher values than modern ones, suggesting the presence of a residual contamination in the aragonite skeleton and/or reprecipitation of Nd during the cleaning process that could not be removed even applying very rigorous cleaning procedures. Here, we briefly evaluated the cleaning procedure using two fossil coral fragments for which Nd concentrations were measured during the different steps of the mechanical and chemical cleaning procedure (Table 2). For both fossil corals, the cleaning procedure leads to a strong decrease of the Mn and Nd concentrations of the coral fragments (Fig. 2b). Nd concentrations decrease by a factor of two through mechanical cleaning and by a factor of ten using acid leaches. The Mn concentration decreases by a factor three upon mechanical cleaning and by a factor of 100 upon chemical cleaning. Through the here applied cleaning procedure a weight loss of approximately 50% occurs, which highlights that the removed parts of the coral are strongly enriched in Nd and Mn as indicated by the high concentrations found in the removed material (Table 2). Both coral fragments reveal an almost linear decrease of the Nd/Mn ratio throughout the stepwise cleaning procedure (R2 ¼ 0.89 and 0.96 for samples MD01 2455G 0e5 cm b and MD01 2455G 21e27 cm, respectively), but the slopes DNd/Mn are different ranging between 4.2 and 22. This great variability of the Nd/Mn ratios observed in the two un-cleaned corals suggests that the contaminant Fe-Mn coatings present a broad range geochemical variability. This might reflect variable Fe/Mn ratios assuming Nd is preferentially

Age (ka) 220  12 471  25 750  170

147.1  3.1 0.03276  0.00052 33.0  0.5 148.1  9.5 0.00328  0.00013 59.7  0.3 Calibrated 14C age BP (Reservoir age 450 years)

d234UT (&)

Age* (ka)

145.4  1.6 150.4  4.2 147.2  5.4

150  40 240  120 190  190 rejected for dating 850  200 rejected for dating 3060  90 261  33

1250  41

140.9  7.5

3163  60 313  15 851  99

148.5  3.2 148.2  9.5

scavenged by Fe. This observation is in good agreement with the absence of a correlation observed between the Nd/Ca and Mn/Ca ratios for fully-cleaned samples. A more aggressive chemical cleaning has also been tested for these two fossils corals and for several modern ones (from living corals to corals dated at 250 yr) in order to evaluate the possibility of improving the cleaning

a

300

living corals fossil corals

Nd/Ca = 0 to 300 nmol/mol R2 = 0.27

250

Nd/Ca (nmol/mol)

232

Sample

Gif-1281 Gif-1282 Gif-1284 Gif-1285

200 150 100 Nd/Ca = 0 to 80 nmol/mol R2 = 0.11

50 0 0

b

2

4

10000

Nd (ng/g)

238

Labcode

6 8 10 Mn/Ca ( mol/mol)

12

14

MD01-2455G 0-5 cm b MD01-2455G 21-27 cm modern corals

1000

100

10 Fe-Mn coating

physical oxidative modern cleaning cleaning corals chemical reductive uncleaned cleaning cleaning

Fig. 2. (a) Nd/Ca (nmol/mol) versus Mn/Ca (mmol/mol) for fully cleaned living deep-sea corals (black circles) and fossil deep-sea corals (white squares) investigated in this study and presented in Table 1, (b) evolution of Nd concentration during cleaning protocol for samples MD01-2455G 0e5 cm b (black triangles) and MD01 2455G 21e27 cm (black diamonds) presented in Table 2, compared to modern fully cleaned samples (white circles, not presented in Table 1) The cut off bar on Fig. 2a corresponds to the limit between accepted samples and rejected ones due to a too high Nd/Ca ratio, suggesting a residual contamination. The grey label in Fig. 2b corresponds to Nd concentration range of modern corals.

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procedure and to test if the Nd/Ca variability observed in the fullycleaned corals could be induced by the re-precipitation of Nd at the sample’s surface during the dilute HCl attack cleaning step. The skeleton coral surface has been cleaned in a first step with 10 ml of 0.05M HNO3. After being rinsed with MilliQ water, coral samples were heated for 30 min in a bath of 10 ml of solution with H2O2 and NaOH in order to remove organic material. Coral samples were then rinsed with MilliQ water and finally heated for 30 min in a bath of 8 ml of MilliQ water and 2 ml of ascorbic acid/EDTA (Lomitschka and Mangini, 1999). The reductive properties of ascorbic acid are there to insure the complete dissolution of Fe-Mn oxides and metal ions, which then build chemical complexes with EDTA to hinder reprecipitation. The Nd concentrations have been reported in Fig. 2b for the different stepwise cleaning experiments. The Nd concentrations decrease drastically during the two first steps of the cleaning procedure. No further significant and systematic decrease of the Nd concentration can be observed using further redoxcleaning steps as the broad range of Nd concentrations is in agreement with the range of modern corals. These results suggest that the complex redox-cleaning of samples might be useful in cases to even further remove Fe-Mn oxide and hydroxide coating. Here we have chosen to treat coral fragments only using the first two cleaning steps to avoid introducing further chemicals into the protocol that could potentially by themselves contaminate the sample. However, in one case, cleaned sample MD01-2455G 21e27 cm has a Nd concentration by far higher than those of modern ones supposing a residual contamination is present and thus this sample was excluded for isotopic composition measurements. For this sample a more aggressive chemical cleaning did further decrease significantly the Nd concentration suggesting in fact that more rigorous cleaning may further allow to approach the initial coral Nd composition. 4.3. Nd isotopic composition

eNd values of the 15 living corals and 11 fossils corals are reported in Table 1. Several duplicate analyses of the same sample indicate excellent reproducibility within uncertainty of the measurement. The eNd values obtained on living deep-sea corals present a large range from 9.8 to 14.1. The geographical distributions of the eNd values from the corals are reported in Fig. 3. One living coral from the Strait of Gibraltar

(SEGE 303) shows an eNd value of 9.8. Two living corals located along the Portugal coast (VH97-315) and the Bay of Biscay (B08-04) present eNd values of 12.2 and 11.5, respectively. The living coral located on the Porcupine Bank (CARACOLE PL 127-05) yields an eNd of 13.3. Two living corals located on southwest Rockall Bank at a 747e850 m water depth (MD01-2454G top and CARACOLE KGS 16/132-10) are characterised by an eNd of ‑13.8 and ‑13.3. Living corals collected at a greater water depth (1003e1325 m) on the north Rockall Through (Rosemary Bank) (Pos 241 362 DS 9H and Pos 241 364 DS Dos G) are characterised by a similar eNd ranging from ‑13.3 to 14. The eNd obtained on three living deep-sea corals collected in the Norwegian Fjords also range from 13 to 14.1 (Table 1). On the other hand, two corals located south of Iceland (Bioice 2424H and Bioice 2472) are characterised by more radiogenic isotopic composition (eNd of 11.0 and 10.2). For the nine fossil corals collected in core MD01-2455G from Rockall Bank, the eNd values vary from 11.9 to 12.9 (Table 1). Two fossil deep-sea corals collected in core MD01-2463G from Procupine Seabight are characterised by identical eNd values of 12.3 and 12.7. Consequently, the eNd indicates a mean value of 13.3 for most of the samples, but strong and significant variations occur along with the sample’s location (living specimens) and apparently with time (fossil specimens at one single location). We can also note that no correlations can be observed between the eNd and the Mn/Ca ratio for both living and fossil coral skeletons (R2 ¼ 0.28). eNd were also measured during all the steps of the mechanical and chemical cleaning procedure (Table 2). For sample MD012455G 21e27 cm, the eNd values are almost identical at each step of the cleaning process, and even for the residue of mechanical cleaning. This suggests that the Nd isotopic composition of the FeMn oxide coating and any other contaminating phase is similar to those of the aragonite skeleton of corals. In contrast, for sample MD01-2455G 0e5 cm b, the Nd isotopic composition display significant changes for each step of the cleaning procedure. The uncleaned coral indicates an eNd of 12.5. After mechanical cleaning, the eNd increases slightly to 13.1, which is in accordance with the high eNd of the mechanical residue (11.9). Such changes may reflect that the mechanical cleaning eliminates various contaminates, such as organic matter, fine detrital particles and Fe-Mn oxide coatings. After chemical cleaning, eNd decreases slightly to 12.3 a value identical to the un-cleaned sample, indicating that the two attacks with dilute HCl remove further superficial fractions

Fig. 3. Nd isotopic compositions (eNd) of the living deep-sea corals (white circles) investigated in this study with the depth they were sampled. The main water masses of the North Atlantic Ocean with their Nd isotopic compositions have been also reported. Seawater stations (black diamonds) where eNd has been analysed close to deep-sea corals of this study and used for comparisons are also reported. (from Spivack and Wasserburg, 1988; Tachikawa et al., 2004; Lacan and Jeandel, 2004a,b; Rickli et al., 2009). Water masses are SubArctic Intermediate Water (SAIW), Western North Atlantic Central Water (WNACW), Modified North Atlantic Water(MNAW), Eastern North Atlantic Central Water (ENACW)and Mediterranean Outflow Water(MOW).

K. Copard et al. / Quaternary Science Reviews 29 (2010) 2499e2508

characterised by an Nd isotopic composition that is different than that removed by the mechanical cleaning. This might suggest that the fossil coral skeleton was exposed for a significant period of time at the seabed and was successively contaminated with Nd of low isotopic composition such as modern seawater. 5. Discussion 5.1. Nd concentration in deep-sea corals The Nd concentration of living corals is less than 45 ppb. In comparison, Nd concentrations of deep-sea corals are slightly higher than tropical corals. Tropical corals have values that range between 2 and 5 ppb according to Sholkovitz and Shen (1995), and between 2 and 22 ppb at the Australian Great Barrier Reef, according to Wyndham et al. (2004). The highest values in the latter study occurred at an open ocean location. Recently, in situ laser ablation ICPMS studies of the deep-sea coral Desmophyllum have demonstrated high variability in Nd concentrations ranging from a few ppb to approximately 50 ppb (Montagna, pers comm.), which confirms our results made on large coral fragments. Nd/Ca ratios versus water depth are shown in Fig. 4. An increasing trend of coral Nd/Ca ratios is observed with increasing water-depth. It is not possible to compare directly such Nd/Ca ratios to those obtain on seawater because the investigated living corals have been collected in different hydrological situations in the North Atlantic and the Norwegian Sea. However, qualitatively such results are in agreement with several seawater Nd concentration profiles obtained throughout the North Atlantic Ocean (Elderfield and Greaves, 1982) which indicate an increase of the Nd concentrations at greater water depth in the water column. Consequently, Nd concentrations in corals seem positively correlated to the Nd concentration of seawater. For deep-sea corals located close to a site were Nd concentration measurements of seawater were available (Bay of Biscay and off Norway), we have estimated a distribution coefficient (D) between 3.35 and 5.80 (D ¼ (Nd/ Ca)coral/(Nd/Ca)seawater). Such a distribution coefficient is larger than the one observed for shallow water corals (less than 2.12, (Sholkovitz and Shen, 1995)). However, future investigations are needed to better constrain the distribution coefficient of Nd in deep-sea corals. 5.2. Significance of the eNd in living deep-sea corals For all living deep-sea corals that were analysed in this study, the Mn/Ca and Nd/Ca ratios are very low, implying that Nd isotopic

0

corals 0 surface (open ocean) 10

coral Nd/Ca (nmol/mol) 20 30 40

50

depth (m)

200 400 600 800 1000 1200 1400 Fig. 4. Nd/Ca ratio versus water depth obtained for living deep-sea corals with depth in m. The labeled “surface corals” refers to Nd/Ca ratio ranges for surface corals (Sholkovitz and Shen, 1995; Wyndham et al., 2004). Stippled lines reflects the overall envelop of data to highlight the tendency of increasing Nd/Ca ratios with water depth.

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compositions will likely represent those of the cold-water coral skeletal aragonite. However, keep in mind that a direct comparison of seawater and coral Nd isotopic compositions is hampered by the fact that seawater samples are a snapshot of a complex oceanographic setting, while corals represent a long-term average value over the time of coral calcification and further post-mortem Nd uptake from seawater. Comparing corals and seawater Nd isotopic compositions is here used to test whether corals can capture at basin scale Nd isotopic gradients for water masses at intermediate depth bathing the corals. From south to north, eNd of living deep-sea corals show a gradual increase from values of 9.8 (Strait of Gibraltar) to 14.1 (Norwegian Sea), similar to the available seawater Nd isotopic measurements, as illustrated in Fig. 3 (Piepgras and Wasserburg, 1987; Spivack and Wasserburg, 1988; Lacan and Jeandel, 2004a,b, c, 2005a,b). The eNd of the corals located at 100 m water depth (9.8) in the Strait of Gibraltar is identical to the Nd isotopic composition measured for the MOW (10.1/9.4) by Spivack and Wasserburg (1988) and Tachikawa et al. (2004) (Fig. 5a). The living deep-sea corals (sample B08-04) located in the Bay of Biscay and around a 700e800-m water depth have been collected in the ENACW and display eNd of 11.5. Once again, seawater eNd, as measured by Rickli et al. (2009), is identical to the one obtained from the coral (Fig. 5b). This also holds for corals located on the northern Hatton Bank (Pos 292 5526-1), which are characterised by an eNd (13.1) identical to obtained by Lacan and Jeandel (2005a) for the MNAW (eNd ¼ 13.1) at the entrance of the Nordic seas above the IcelandScotland ridge. The three living L. pertusa corals collected on the Norwegian shelf and Oslo Fjord show eNd values also similar to the Nd isotopic compositions of the Norwegian Sea surface water, but coral and seawater localities are from very different location (Fig. 5c). Moreover, one sample (VH-95-163) seems to have lower Nd isotopic composition compared to seawater. This difference can be due to the fact that the coral is situated very close to continental Nd sources and thus may have received some local inputs from Norway which has Nd isotopic composition between 14 and 14.6 (Revel, 1995; Boundy et al., 1997). The isotopic composition of the deep-sea corals off southern Iceland (10.2 and 11) could unfortunately not be compared to seawater due to the lack of nearby Nd seawater measurements. However, they are similar to the Nd isotopic composition obtained by Lacan and Jeandel (2004a) (eNd ¼ 9.3 to 11.5) for surface seawater entering into the Nordic Sea through the Denmark Strait. Such values are 2e3 eNd units higher than those of the MNAW, implying a local contribution of Icelandic sources, which are mainly composed of young basalts characterised by a mean eNd value around þ8 (Hémond et al., 1993). Considering that basaltic materials are easily weathered on land (Gislason et al., 1996) and within the ocean, the eNd of the water masses off southern Iceland may rapidly take up a more radiogenic Nd isotopic composition as suggested by Lacan and Jeandel (2004a). Such a rapid isotopic exchange along an active volcanic margin has lead to the hypothesise of an “active” boundary exchange process, which would explain the observed radiogenic Nd isotopic composition in corals located offshore Iceland. Living corals at depths of 500e1200 m that were investigated along the slopes of the Rockall Trough and the Porcupine Bank display a narrow range of eNd values between 13.1 and 14.0, with a mean eNd of 13.5, Most of these corals are located in a relatively uniform water mass resulting from winter mixing at about 400e600 m (Holliday et al., 2000; New and Smythe-Wright, 2001;Mienis et al., 2007). This upper layer water-mass results from the mixing of saline Eastern North Atlantic Central Water (ENACW)

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a -15 0

-14

-13

Nd -12 -11

-10

-9

-8

-13

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-10

-9

-8

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-10

-9

-8

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depth (m)

400 600 800 1000 1200 1400

Gibraltar sta 5 MED-15 Alboran sta C SEGE 303

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-14

200 depth (m)

400 600 800

1000 1200 1400

Biscay sta 69/3 and 69/6 B08-04

c -15 0

-14

-13

200

depth (m)

400 600 800

1000 1200 1400

station 25 station 26 VH-95-163 JR-111 Alk 232 BG 1163 Alk 231 BG 1158

Fig. 5. Comparison between deep-sea corals Nd isotopic composition and seawater Nd isotopic composition (a) in the strait of Gibraltar, (b) in the Bay of Biscay and (c) in the Norwegian seas (seawater data from Spivack and Wasserburg, 1988; Lacan and Jeandel, 2004a,b; Tachikawa et al., 2004; Rickli et al., 2009).

with an eNd around 11.6 (Rickli et al., 2009) originating from the South (Bay of Biscay) (Pollard et al., 1996) and a fresher “Modified North Atlantic Water” (MNAW; eNd around 13.1; Lacan and Jeandel, 2005a) originating from the Northwest (Read, 2001). Consequently, the eNd of living deep-sea corals collected at 750 m reflects the Nd isotopic composition of mainly the MNAW, and implies an insignificant contribution of ENACW. The eNd obtained on one living deep-sea coral (Pos 241 364 DS Dos G; 57 25.360 N, 1143.110 W), located in the northern Rockall Trough at the water depth, where it was supposed to have influence from the Mediterranean Overflow Water (MOW), presents an eNd of 14.0  0.2. Such eNd value is much higher than the value measured for the MOW at the outflow (eNd ¼ 9.4  0.2, Tachikawa et al., 2004). This implies that Nd results cannot confirm the presence of a strong influence of MOW at this sample location. Reid (1978) and Ellett et al. (1986) have proposed for the Rockall Trough that MOW flows northward

typically at depths of 1000e1200 m, and that it could reach the Nordic seas. The presence of MOW in the Trough has, however, recently been called into question by McCartney and Mauritzen (2001), who found, on the basis of regional water mass distributions, that the MOW did not penetrate further northward than around the Porcupine Bank. Recently, Lozier and Stewart (2008) have shown that the northward penetration of MOW into the Rockall Trough may be related to the North Atlantic Oscillation (NAO), implying that episodic advances of the MOW could episodically enter into the Trough. Considering the fact that deep-sea corals grow from several years to several decades, it is possible that this coral has integrated a time interval where the MOW did not penetrate into the Rockall Trough, or where the MOW penetration was not sufficiently long so as to imprint its Nd isotopic composition on the deep-sea coral. Such relationship between the eNd of seawater and deep-sea corals demonstrates that solitary and framework-forming deepsea corals (L. pertusa, M. oculata and D. dianthus) register Nd isotopic gradients at a basin scale of bottom waters at the coral locations. Such result was also reported by Robinson and Van de Flierdt (2009) for D. dianthus at the Drake Passage. The observed gradients of Nd isotopic composition are generally assumed to reflect water mass transport and mixing, as well as local to regional sources and sink processes, including boundary exchange. Through the investigation of sedimentary archives such as manganese crusts and nodules, it is well-known that variable basin scale gradients recorded over millions of years reflect variable Nd sources and sinks, as well as the variation of ocean circulation and thus mixing of isotopically distinguishable water masses. On shorter time scales, such as thousands of years, it is also welladmitted that variation in the advection and mixing of water masses characterised by contrasted Nd isotopic composition can be recorded in deep-sea sediments and corals (Vance and Burton, 1999; Rutberg et al., 2000;Van de Flierdt et al., 2006). The rapid growth of corals and their ability to capture seawater eNd now provides first insights regarding temporal variations of the eNd over the last few hundred to a few thousand years ago. In our attempt to contribute to this task, we have investigated 11 fossil deep-sea corals but only five corals are characterised by similar ranges of Nd concentrations (35e65 ppb), Nd/Ca ratios (39e56 nmol/mol) and Mn/Ca ratios (0.8e7.8 mmol/mol) than those of the living deep-sea corals (Table 1). The other 6 samples of deepsea corals present a higher range of Nd contents suggesting that further improvements of the cleaning protocol to extract seawater Nd are needed. One deep-sea coral collected in the Porcupine Seabight (MD012463G top), dated at 261  33 yr (Table 3), reveals an eNd of 12.3, which is close to the values observed today further in the south (Bay of Biscay and off Portugal). Moreover, the four fossil corals from the gravity core MD01-2455G, located on southwest Rockall Bank (close to MD01-2454G living), have eNd values of 11.9 to 12.9, which are significantly higher than those of the modern corals from this area (around 13,8). This suggests a significantly stronger contribution of MOW or other temperate Atlantic middepth water masses within Rockall Trough and at intermediate water depth during the time of coral growth a few hundred years ago. Based on those preliminary records from fossil corals, it is evident that eNd within the eastern North Atlantic at intermediate depth is sensitive to rapid changes in oceanic circulation such as recently suggested by Lozier and Stewart (2008) and Reverdin (2010). Further investigations are need to establish an eNd seawater record of the eastern North Atlantic at high temporal resolution in order to understand the origin of such significant hydrological changes.

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6. Conclusion Nd concentrations, Mn/Ca, Nd/Ca ratios and eNd values have been analysed from 15 living and 11 fossil L. pertusa, D. dianthus and M. oculata corals located along the eastern margin of the North Atlantic Ocean between the Strait of Gibraltar and the Norwegian Sea. Nd isotopic compositions were analysed using a Nd-oxide method on a TIMS. A rigorous cleaning technique was developed in order to remove Nd contamination of the deep-sea corals from manganese-oxide and iron hydroxide coatings. Mn and Nd concentrations were systematically analysed in order to control the efficiency of the applied cleaning protocol. The Nd concentration of living deep-sea corals incorporated in the aragonite skeleton varies between 6 and 43 ppb. A slight increasing trend of the Nd/Ca ratios was observed along with water depth, qualitatively in agreement with Nd behaviour in seawater. Nd isotopic compositions of living deep-sea corals from the Strait of Gibraltar, the Bay of Biscay and the Norwegian Sea vary from 9.8 to 14.1 and match the eNd of the seawater bathing the corals. This suggests that constructional as well as solitary corals from the species L. pertusa, D. dianthus and M. oculata capture nicely the basin scale Nd isotopic gradients of subsurface water masses. From the distribution of the eNd analysed on living corals, we propose that the ENACW originating from the Bay of Biscay is characterised by an eNd around 11.5/12.2. The Nd isotopic composition of living deep-sea corals (14.1) at the water-depth interval where the MOW could potentially flow in the Rockall Trough implies no evidence of the MOW in the Rockall Trough. This implies that the MOW do not penetrate the Rockall Trough or do not penetrate it for a long enough time in order to imprint a Nd isotopic composition on the deep-sea corals that live for at least several years or a few decades. The rapid growth of corals and their ability to capture seawater eNd now provides the first insights regarding temporal variations of the eNd over the past few hundred to a few thousand years. The first results from five fully-cleaned fossils corals (dated by 230Th/U methods between 150  40 and 3060  90 yr) collected on the Porcupine Seabight and the southwest Rockall Bank reveal significantly higher eNd than those of the modern corals located in similar areas. This implies the higher contribution of the MOW or other temperate Atlantic mid-depth water masses (ENACW). This indicates the ability of deep-sea coral archives to trace decadal to multidecadal hydrological changes along the eastern margin of the North Atlantic Ocean. Acknowledgements This work was funded through the French Agence National de Recherche projet (NEWTON: ANR-BLANC06-1-139504), the French Centre National de la Recherche Scientifique (CNRS) and the Commisariat à l’Energie Atomique (CEA). We thank IPEV (Institut Polaire Emile Victor), the members and crew of the GEOMOUND Marion Dufresne cruise, for their excellent work recovering the three gravity cores investigated in this study. In addition, we are grateful to Jan Scholten, who provided deep-sea corals from close to the Rosemary Bank presented here and Hans Pirlet who provided deep-sea coral from Gascogne. We further thank Eline Sallé and Claude Noury for their support with U-series dating and clean laboratory management. This is LSCE contribution #XXX. References Adkins, J.F., Henderson, G.M., Wang, S.L., O’Shea, S., Mokadem, F., 2004. Growth rates of the deep-sea scleractinia Desmophyllum cristagalli and Enallopsammia rostrata. Earth and Planetary Science Letters 227, 481e490.

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