A cross-calibration of chlorine isotopic measurements and suitability of seawater as the international reference material

Chemical Geology 207 (2004) 1 – 12 www.elsevier.com/locate/chemgeo

A cross-calibration of chlorine isotopic measurements and suitability of seawater as the international reference material Arnaud Godon a,*, Nathalie Jendrzejewski a, Hans G.M. Eggenkamp b,c, David A. Banks d, Magali Ader a,b, Max L. Coleman b, Francßoise Pineau a a

Laboratoire de Ge´ochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Universite´ Paris 7, UMR 7047, 2, Place Jussieu, Tour 54-64 1er e´tage, 75251 Paris Cedex 05, France b Postgraduate Research Institute for Sedimentology, The University of Reading, Whiteknights, Reading RG6 6AB, UK c Department of Geochemistry, Utrecht University, 3508 TA Utrecht, The Netherlands d School of Earth Sciences, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK Received 29 April 2003; received in revised form 27 October 2003; accepted 26 November 2003

Abstract A collection of 24 seawaters from various worldwide locations and differing depth was culled to measure their chlorine isotopic composition (d37Cl). These samples cover all the oceans and large seas: Atlantic, Pacific, Indian and Antarctic oceans, Mediterranean and Red seas. This collection includes nine seawaters from three depth profiles down to 4560 mbsl. The standard deviation (2r) of the d37Cl of this collection is F 0.08x , which is in fact as large as our precision of measurement ( F 0.10x ). Thus, within error, oceanic waters seem to be an homogeneous reservoir. According to our results, any seawater could be representative of Standard Mean Ocean Chloride (SMOC) and could be used as a reference standard. An extended international cross-calibration over a large range of d37Cl has been completed. For this purpose, 13 geological fluid samples of various chemical compositions and a manufactured CH3Cl gas sample, with d37Cl from about 6xto + 6xhave been compared. Data were collected by gas source isotope ratio mass spectrometry (IRMS) at the Paris, Reading and Utrecht laboratories and by thermal ionization mass spectrometry (TIMS) at the Leeds laboratory. Comparison of IRMS values over the range 5.3xto + 1.4xplots on the Y = X line, showing a very good agreement between the three laboratories. On 11 samples, the trend line between Paris and Reading Universities is: d37ClReading= (1.007 F 0.009)d37ClParis (0.040 F 0.025), with a correlation coefficient: R2 = 0.999. TIMS values from Leeds University have been compared to IRMS values from Paris University over the range 3.0xto + 6.0x . On six samples, the agreement between these two laboratories, using different techniques is good: d37ClLeeds=(1.052 F 0.038)d37ClParis + (0.058 F0.099), with a correlation coefficient: R2 = 0.995. The present study completes a previous cross-calibration between the Leeds and Reading laboratories to compare TIMS and IRMS results (Anal. Chem. 72 (2000) 2261). Both studies allow a comparison of IRMS and TIMS techniques between d37 Cl values from 4.4x to + 6.0x and show a good agreement: d37ClTIMS=(1.039 F 0.023)d37ClIRMS+(0.059 F 0.056), with a correlation coefficient: R2 = 0.996.

* Corresponding author. Fax: +33-1-44-27-28-30. E-mail addresses: [email protected] (A. Godon), [email protected] (N. Jendrzejewski), [email protected] (H.G.M. Eggenkamp), [email protected] (D.A. Banks), [email protected] 0009-2541/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2003.11.019

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A. Godon et al. / Chemical Geology 207 (2004) 1–12

Our study shows that, for fluid samples, if chlorine isotopic compositions are near 0x , their measurements either by IRMS or TIMS will give comparable results within less than F 0.10x , while for d37Cl values as far as 10x(either positive or negative) from SMOC, both techniques will agree within less than F 0.30x . D 2004 Elsevier B.V. All rights reserved. Keywords: Chlorine stable isotopes; Mass spectrometry; d37Cl; IRMS; TIMS; SMOC

1. Introduction The relative abundances of the stable isotopes of chlorine (35Cl and 37Cl; natural abundances 75.77% and 24.23%, respectively; Shields et al., 1962 and recommended by the Commission on Atomic Weights and Isotopic Abundances (IUPAC, 1998)) can fractionate strongly during geologic processes and natural variations vary from 14xto + 16x(e.g.: Banks et al., 2000; Kaufmann, 1984; Gaudette, 1990; Eggenkamp, 1994; Magenheim et al., 1994; Volpe and Spivack, 1994; Lev and Vocke, 1999; Godon, 2000; Godon et al., in press; Hesse et al., 2000; Stewart, 2000; Willmore et al., 2002 and references therein). The extreme negative value was found by Gaudette (1990) in layers containing volcanic ash particles in samples of ice core and snow pit from Antarctica. The d37Cl of + 16xwas measured by Lev and Vocke (1999) in a study on diagenetically altered black shale sequence. The isotopic ratio 37Cl/35Cl is typically expressed as d37Cl (in x ) relative to the seawater chloride isotopic composition taken as an international reference, Standard Mean Ocean Chloride (SMOC). Kaufmann (1984) and Kaufmann et al. (1984) showed on a small collection (n = 8) of seawater samples that oceanic waters seem to be isotopically homogeneous for chlorine. Nevertheless, as explicitly stated by Rosenbaum et al. (2000) and Xiao et al. (2002), there is no real Mean Seawater available (i.e.: a mixture of seawaters from various locations). In fact each laboratory uses its own seawater aliquot for calibration, assuming that its reference sample is representative of SMOC. Consequently, Xiao et al. (2002) recently proposed an NaCl salt collected from a purified seawater as an international standard. To provide a more complete examination of seawater variability, we present data from 24 seawaters

from 18 different localities, with several seawaters at different depths from three locations. Beyond the problem of a common reference material, chlorine isotopic compositions can be measured also by two completely different methods, gas source dual inlet isotope ratio mass spectrometry (IRMS; e.g.: Long et al., 1993; Eggenkamp, 1994; Holt et al., 1997; Jendrzejewski et al., 1997; Godon, 2000; Hesse et al., 2000; Ader et al., 2001) and by thermal ionization mass spectrometry (TIMS; e.g.: Xiao and Zhang, 1992; Magenheim et al., 1994; Volpe and Spivack, 1994; Banks et al., 2000; Rosenbaum et al., 2000; Stewart, 2000). These techniques differ as IRMS is based on the ionization of CH3Cl gas to produce CH3Cl+, while TIMS measurements are made on CsCl salt, generating Cs2Cl+ ions during heating of the filament. To use data from different laboratories one has to make sure that they can be compared. The way to do this is to report all the data against the same reference: seawater in the case of chlorine. The second essential condition is to check that different laboratories and different people find the same values for this seawater and the samples measured against it. This is the purpose of the second part of this study. A comparison of IRMS and TIMS techniques was made recently with five samples including: Sargasso seawater (GPS-1), a pure CsCl(s) solution, water and NaCl(s) solution from the Dead Sea, and a brine from North Sea Oil Field. However, all these samples were either very pure (CsCl) or of similar chemical compositions (seawater, NaCl solution or brine). Their d37 Cl values are between 4.39xand + 0.34x , of which four range between 0.45xand + 0.34x(Rosenbaum et al., 2000). Although TIMS measurements are shown to be dependent on the amount of Cl loaded on the filament, when this is taken into account, their results show that within error, both techniques are in good agreement.

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Our study is both an interlaboratory and intertechniques cross-calibration. We compared the d37Cl data obtained by IRMS at the laboratory of Ge´ochimie des Isotopes Stables, at Paris University-IPGP (France) to the IRMS measurements from the Department of Geochemistry, at Utrecht University (The Netherlands) and the Postgraduate Research Institute for Sedimentology, at Reading University (England) as well as the values determined by TIMS at the School of Earth sciences, at Leeds University (England). Spread over a large isotopic range, approximately from 6xto + 6x , 14 samples of various types and chemical compositions (salts, fluids and gas) have been compared. This range represents more than a third of the total range of natural variations observed to date ( 14xto + 16x; see beginning of section for references) and is within the range of most published d37Cl data. This study complements the comparison of Rosenbaum et al. (2000) between IRMS and TIMS and extends it to very positive d37Cl values.

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2. Sample description 2.1. Selected seawaters The seawater sample Atlantique 2 (Atl 2) was sampled in the North Atlantic (Gorringe) at 36j43VN and 11j36VW at surface sea level (Fig. 1). This seawater is used at Paris University as a reference material to represent SMOC. A collection of 23 other seawaters (essentially from the sea surface, off-shore or close to the coast) was assembled (Fig. 1; Tables 1 and 2). These samples, from 18 localities, cover all the oceans and large seas: Atlantic, Pacific, Indian and Antarctic oceans, Mediterranean and Red seas (Fig. 1). The collection also includes nine seawaters from three depth profiles down to 4560 mbsl (Table 2). Five of them were recovered above the Southwest Indian Ridge (SWIR), between the Gallieni and Atlantis II fracture zones (34j10VS and 55j37VE), and two

Fig. 1. Location map of the seawater samples, listed in Tables 1 and 2. Large open circles: this study; small dark spots: from Kaufmann (1984). EDUL 1 and 2 (Me´vel et al., 1997a,b), and CRC (Rintoul and Trull, 2001; Trull et al., 2001a,b) correspond to depth profiles. Atl 1: Atlantique 1 (21j40VN and 45j15VW), Atl 2: Atlantique 2 (36j43VN and 11j36VW), GPS-1: Sargasso Sea seawater (approximately 24 – 25jN and 60 – 65jW), IAPSO: international chemical standard of seawater, KB: Kimmeridge Bay and WHOI: seawater collected in front of the Woods Hole Oceanographic Institution. Names of seawater samples close to the coast are given from the nearest city from the sampling sites. The position of the seawater sample Augustine has been precisely measured by GPS at 59j24VN and 153j30VW.

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Table 1 d37Cl values (in x ) of the various oceanic waters determined by IRMS at Paris University Number of d37Cl 2r replicates (x ) (x ) Atlantic Ocean Atlantique 2 (Gorringe, 5 mbsl) 128 Kimmeridge Bay (England, surface) 5 GPS-1 (Sargasso Sea, surface) 1 Atlantique 1 (North MAR, surface) 2 IAPSO (North Atlantic, 1000 mbsl) 2 WHOI (USA, surface) 2 Pigeon Bouillante (Guadeloupe, surface) 3

0.00 0.00 + 0.03 0.05 0.00 0.07 + 0.01

0.01 0.07 0.05 0.08 0.10 0.05 0.04

Atlantic/Indian Oceans Le Cap (South Africa, surface)

2

0.01

0.02

Mediterranean Sea St Cyprien (France, surface)

3

+ 0.04

0.09

Red Sea Tarabin (Egypt, surface)

1

+ 0.06

0.03

Pacific Ocean Kamakura (Japan, surface) Miyajima (Japan, surface) Chejudo (South Korea, surface) Augustine (USA, surface) Santa Cruz (USA, surface)

1 1 1 1 1

+ 0.01 + 0.08 0.02 + 0.02 0.01

0.03 0.05 0.23 0.04 0.12

All these data are reported versus the Atl 2 reference seawater. For replicates, 2r represents the external reproducibility, while it is the analytical precision for single analysis. Location (Fig. 1) and depth are indicated in between brackets; mbsl: meters below sea level. GPS1: Sargasso Sea seawater, MAR: Mid-Atlantic Ridge, IAPSO: international chemical standard of seawater (salinity: 34.993 x ), and WHOI: seawater collected in front of the Woods Hole Oceanographic Institution.

others (down to 3655 mbsl) at the Atlantis II fracture zone (31j41VS and 57j57VE) (Me´vel et al., 1997a,b). According to both helium isotopes and methane tracers, these seawaters did not sample any hydrothermal plume (unpublished data, P. Jean-Baptiste and J.-L. Charlou, personal communication, 2003). The two samples (down to 2000 mbsl) of the last profile are from the Antarctic Ocean (54j59VS and 141j44VE; Rintoul and Trull, 2001; Trull et al., 2001a,b). Among this collection, there are the reference seawaters from Reading and Leeds Universities, Kimmeridge Bay (English Channel coast, surface sea level), labelled KB, and GPS-1 (North Atlantic Sargasso Sea, approximately 24 – 25jN and 60 – 65jW, surface sea level), respectively.

2.2. Samples selected for the cross-calibration In addition to the reference seawaters (described above), we first selected a tank of manufactured CH3Cl gas (Air Liquide, # N30, 99.90% pure), named Tank, to check with a high purity CH3Cl gas sample, for any isotopic shift between two IRMS laboratories (Paris and Reading Universities). For this purpose, six glass capsules containing CH3Cl were prepared on the same day and under the same conditions, by sampling 50 to 107 Amol of CH3Cl gas from the tank. This CH3Cl gas tank is also used as an internal standard gas reference for the dual inlet gaseous mass spectrometry at Paris University. We then selected natural samples of various types (salts, fluids and gas), leading to different bulk chemical compositions, to deal with potential interference effects occurring during IRMS and/or TIMS analysis. Four interstitial fluids expelled from submarine mud volcanoes at the Manon site, Barbados accretionary prism (Godon et al., in press), named MMV1, MMV2, MMV3 and MMV4, as well as a water extract from the Plainview meteorite (a brecciated H5 stone chondrite), named PVMW, were selected for their negative d37Cl values.

Table 2 ) of seawater from three depth profiles (Fig. 1) d37Cl values (in x Depth (mbsl)

Temperature (jC)

Salinity (g/l)

d37Cl (x )

2r (x )

Indian Ocean (EDUL 1): 34j10VS and 55j37VE CTD 12B-13 22 16.2 35.6 CTD 12B-9 2389 2.2 34.8 CTD 12B-7 3610 1.1 34.7 CTD 12B-2 4482 1.0 34.7 CTD 12B-1 4560 1.0 34.7

+ 0.02 0.03 0.01 + 0.09 0.04

0.08 0.06 0.15 0.05 0.14

Indian Ocean (EDUL 2): 31j41VS and 57j57VE CTD 5-12 1013 6.3 34.5 CTD 5-1 3655 1.4 34.8

0.04 + 0.01

0.07 0.08

Antarctic Ocean: 54j59VS and 141j44VE CRC J23 10 3.6765 33.7854 CRC J1 2000 1.2103 34.6803

+ 0.03 0.01

0.10 0.11

All these data are reported versus the Atl 2 reference seawater. Each measurement represents up to three successive mass spectrometric analyses of one capsule, thus 2r is the analytical precision. Depth (mbsl: meters below sea level), temperature and salinity data are from Me´vel et al. (1997a) for CTD samples and from T.W. Trull (personal communication, 1999) for CRC samples.

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One sample of millimeter grain-sized marine salt, as well as one brackish solution (both from France), and one saline karst water (Tunisia), respectively labelled MS, B and TKW, were chosen to test different types of samples with approximately the same isotopic composition. A few grains of the salt MS were first dissolved in deionized water before each analysis. Also, to avoid any precipitation of salt from the brine sample B, and thus to prevent any isotopic heterogeneity, the sample was diluted with pure water. The Tunisian karst water was collected from a flowing source. Finally, two condensates of volcanic gas, named VGC1 (Soufrie`re de Guadeloupe, French West Indies) and VGC2 (Merapi, Indonesia) extend the calibration to positive d37Cl. The volcanic gas from Guadeloupe, VGC1, was cryogenically recovered, while VGC2 from Merapi was trapped at source and condensed in a Giggenbach bottle.

3. Analytical techniques 3.1. Isotope ratio mass spectrometry (IRMS) This technique was first developed by Kaufmann (1984) and is fully detailed by Long et al. (1993). Similar systems were built at Utrecht and then at Reading Universities (Eggenkamp, 1994). The technique used at Paris University is derived from Eggenkamp (1994), with minor changes (Godon, 2000). This method is detailed below, but is substantially identical to those used in other laboratories. To ensure good yields of AgCl precipitation, the liquid samples were acidified to pH 2.2 and KNO3 was added to adjust the ionic strength of the sample solutions. Then AgCl was quantitatively precipitated and recovered by filtration on a 0.7 Am Whatman GF/ F glass fibre filter. This filter was placed in a blind glass tube, excess CH3I was added and sealed under vacuum ( < 10 2 mbar) to produce CH3Cl by reaction at 80 jC for 48 h. The CH3Cl was then dried with a semipermeable Nafion membrane (30 cm length, 0.61 mm internal diameter, 3.175 mm outer diameter and counter flux of dry pure He at 35 ml/min) and twice purified with a Pe´richrom IGC-11 gas chromatograph (with dry pure He at 130 jC, 2.1 bar and 15 ml/min) in two identical packed columns (Porapak-Q 80 – 100 mesh, 2 m length and 3.175 mm outer diameter). A

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thermal conductivity detector is used to check the progress of this purification and to detect any leaks. The pure CH3Cl is separated cryogenically and then transferred to a cold finger, under vacuum, to check reaction yield using a calibrated pressure gauge (Keller). The CH3Cl is then transferred to a sample tube, adapted for use on the introduction line of the mass spectrometer. The d37Cl measurements were made on CH3Cl+ in a triple collector gas-source dual-inlet mass spectrometer (VG Optima). We reduced the resistance of the third collector to 109 V to avoid signal saturation of this collector. Thus the signal of the minor (mass 52) is near 8  10 10 A, while the major (mass 50) is between 2 and 3  10 9 A. At least two measurements of seawater (Atl 2), representative of SMOC, were made each day (first and last analysis), with typically three to seven samples in between. This procedure checks for instrumental drift during the day, and allows direct comparison between the samples and the seawater reference. Each reported isotopic result corresponds to the average of 10 measurements of the mass/charge ratio (m/z), 52/50 (effectively the 37/35 isotopic ratio), alternately of an internal reference tank gas and of the unknown gas sample. Any measurement more than F 2r from the average value is rejected and therefore only 1 in 20 should be discarded. In fact, 1 of the 10 measurements is occasionally rejected this way. The isotopic composition of the sample is compared to the daily average of seawater compositions, to make a correction for the blank and for the instrumental background (mainly the gas chromatograph and the mass spectrometer). The average mass-spectrometer precision for a single analysis for the Paris laboratory is F 0.01x(2r) on 30 Amol of Cl. However, it is important to note that although this is a good measure of analytical uncertainty, it is not a true standard deviation. This is because each reading is not an independent variable. In order to compensate for drift in readings, it is usual to determine the ratio of the sample 52/50 to the mean of that ratio of the reference immediately preceding and after. This produces a smoothing effect on the data set since most reference readings are used twice and reduces the apparent value of r. The results are measured and reported versus Atl 2 seawater for Paris University data and KB seawater for Reading University data, respectively

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(Tables 1 – 3). The mean reproducibility is usually F 0.10 x(2r). 3.2. Thermal ionization mass spectrometry (TIMS) This technique was first proposed by Xiao and Zhang (1992) and developments have been made to reduce matrix effects (Magenheim et al., 1994; Xiao et al., 1995; Stewart, 2000). The technique used at Leeds University is described by Banks et al. (2000), and only the main steps are presented here. The solution was run through cation exchange columns on different resins firstly in Ca and Ba forms, to remove F and sulphates, respectively, and then in Cs form to produce a solution of CsCl. This later is loaded on a degassed Ta filament, dried and coated with graphite to aid ionization. The absolute m/z 303/301 ratio (equivalent to the 37 Cl/35Cl) was measured with Cs2Cl+ ions on a thermal ionization mass spectrometer (VG Micromass 30). The mass discrimination is reduced by the high mass of Cs2Cl+ and therefore the small relative difference in masses of the two ions. At least 25 blocks of 12 isotopic ratio measurements were made and the data processed to remove erroneous values. The reference seawater GPS-1 was analysed with each

sample set, and for over 100 analyses has a precision of F 0.18x(2r) for 0.3 Amol (10 Ag) of Cl. The average uncertainty for a sample run was F 0.14x , and thus the overall uncertainty of determination relative to GPS-1 was F 0.23x . Rosenbaum et al. (2000) pointed out an increase in the measured Cl isotopic ratio as a function of the amount of sample loaded on the filament (an average of 1.6x /Amol of Cl loaded). Accordingly, to avoid any isotopic shift, our measurements of the seawater standard and the samples were determined on similar Cl quantities.

4. Results 4.1. IRMS seawater data from Paris University Chlorine isotopic measurements of the seawater standard used at Paris University (Atl 2) were very consistent over time. Over a 4-year period, 128 measurements of Atl 2 were performed versus our internal CH3Cl tank gas. They show a reproducibility of F 0.01x(at 95% confidence, Table 1). The various surface, subsurface or deep seawaters analysed against Atl 2 show a remarkably small range of d37Cl values. KB and GPS-1 seawaters have been

Table 3 Interlaboratory cross-calibration for d37Cl measurements from Reading, Paris, Utrecht and Leeds universities

Atl 2 KB GPS-1 Tank MMV1 MMV2 MMV3 MMV4 PVMW MS B TKW VGC1 VGC2

d37Cl (x ) (IRMS) Reading

n

d37Cl (x ) (IRMS) Paris

0.03 F 0.10 standard + 0.04 F 0.10 + 1.15 F 0.17 0.84 F 0.12 3.21 F 0.10 4.45 F 0.08 5.29 F 0.08

1 1 4 1 1 2 2

0.11 F 0.15 + 0.25 F 0.06

3 3

+ 1.34 F 0.05

3

standard 0.01 F 0.07 + 0.03 F 0.05 + 1.15 F 0.01 0.84 F 0.03 3.21 F 0.01 4.31 F 0.00 5.24 F 0.08 3.03 F 0.22 + 0.13 F 0.26 + 0.28 F 0.15 0.00 F 0.14 + 1.37 F 0.11 + 5.44 F 0.16

n

5 1 2 2 2 2 2 1 4 3 1 2 1

d37Cl (x ) (IRMS) Utrecht

n

d37Cl (x ) (TIMS) Leeds

n

standard 0.99 F 0.09 3.18 F 0.04 4.39 F 0.19 5.34 F 0.03

2 2 2 2 2.92 F 0.32

4

+ 0.04 F 0.14 + 0.22 F 0.34 + 1.36 F 0.22 + 5.94 F 0.38

4 4 4 4

As any seawater is representative of the seawater (see text for explanation), the data are reported in xversus SMOC and n is the number of replicates. For replicates, 2r represents the external reproducibility, while it is the analytical precision for single analysis. Atl 2: Atlantique 2 seawater, KB: Kimmeridge Bay seawater, GPS-1: Sargasso Sea seawater, Tank: manufactured CH3Cl tank gas, MMV1, MMV2, MMV3 and MMV4 are interstitial fluids from Manon mud volcanoes (Godon et al., 2004), PVMW: water extract from the Plainview meteorite, MS: grains of marine salt, B: NaCl brine, TKW: saline karst water, VGC1 and VGC2: volcanic gas condensates. N.B.: the sea salt MS could be heterogeneous from grain to grain.

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measured at 0.00 F 0.07x(n = 5) and at + 0.03 F 0.05x(n = 1), respectively (Table 1). Surface or subsurface seawaters, including the samples from the top of the depth profiles, have a spread of d37Cl values between 0.07xto + 0.08x(Tables 1 and 2; n = 16) equivalent to the range shown by deep seawaters (>100 mbsl) from 0.04xto + 0.09x (Table 2; n = 8). No correlation of d37Cl was observed with either the depth of the seawater, its temperature, or its salinity. Whatever their location or depth, all these seawaters have d37Cl at 0.00x, within error. 4.2. Cross-calibration IRMS results At Paris University, all samples were analysed by IRMS versus the reference seawater Atl 2. They were all measured in at least one other laboratory using IRMS or TIMS (Table 3). The whole range of IRMS d37Cl values is from 5.34xto + 5.44x(Table 3). The d37Cl of the CH3Cl from the tank gas was measured at + 1.15 F 0.17x(2r) at Reading University, and its value from Paris University was determined at + 1.15 F 0.01x(Table 3). The perfect agreement between Paris and Reading Universities data for both the reference seawaters (as stated before) and the CH3Cl tank gas was interpreted, early in this study, as a validation of the technique installed at Paris University. The four interstitial fluids from Manon mud volcanoes were analysed in all three laboratories and have negative d37 Cl values from 0.84xdown to 5.34x(Table 3). The d37Cl of MMV1 is in perfect agreement between the Paris and Reading laboratories, while its value from the Utrecht laboratory is slightly more negative (however, still within or close to the limit of the error bars). MMV2 and MMV4 have d37Cl values in perfect agreement between all the three IRMS-user laboratories. The MMV3 d37Cl value agrees between the Reading and Utrecht laboratories, but its value measured in Paris is slightly higher compared to the result obtained in Reading. Nevertheless, this shift and the one observed for MMV1 between Paris and Utrecht are small ( < 0.1x ). In the case of MMV1, Utrecht analysis gives the lowest d37Cl value, while in the case of MMV3, the lowest d37Cl value was measured in Reading. The salt MS shows the largest reproducibilities obtained for each laboratories, and both d37Cl mea-

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surements from the Reading and Paris laboratories are within error, close to 0x(Table 3). The brine sample ) B has slightly positive d37Cl values (up to + 0.28x in perfect agreement in both laboratories. Sample VGC1 has positive d37Cl values ( + 1.34x and + 1.37x ) attesting a perfect agreement between the two laboratories (Reading and Paris, respectively; Table 3). 4.3. Cross-calibration TIMS results In addition to the reference seawater GPS-1, five other samples were analysed by TIMS and the d37Cl obtained are consistent with IRMS results. The whole range of TIMS d37Cl values is from 2.92xto + 5.94x(Table 3). The TIMS d37Cl value of the sample PVMW, at 2.92 F 0.32xis in perfect agreement with the IRMS determination at the Paris Laboratory, at 3.03 F 0.22x (Table 3). The d37Cl values of the samples B and TKW are, within error, in good agreement with the IRMS data from the Paris laboratory. Nevertheless, even considering the error bars, the TIMS value of the sample B is still 0.01xlower than the IRMS value from the Reading University. The TIMS d37Cl value of the sample VGC1, at + 1.36 F 0.22xis in perfect agreement with the two other determinations by IRMS at the Paris and Reading laboratories (Table 3). The TIMS d37Cl value of the sample VGC2, at + 5.94 F 0.38xis in agreement with the IRMS from the Paris Laboratory, at + 5.44 F 0.16x(Table 3). Without considering the error bars, there are some small differences between TIMS and IRMS determinations for the samples B, TKW and VGC2. However, the observed differences between TIMS and IRMS data are not systematic: the sample B has a lower TIMS value (of roughly 0.2x) while TKW and VGC2 have a higher one (of roughly 0.2xand 0.5x , respectively).

5. Discussion 5.1. Seawater as a reference Employing both IRMS and TIMS techniques, all the standard seawater values used in this study are

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identical within error, and measurements of any one sample versus Atl 2 can be directly compared to the values determined versus KB or versus GPS-1. Thus, the good agreement between KB and GPS-1 established in the previous comparison (Rosenbaum et al., 2000) is confirmed here. Moreover, we can directly compare and check for any bias in the data obtained at Paris University with those from Reading, Leeds, and by extension Utrecht Universities, even if the results are not given versus the same reference seawater. With eight seawaters mostly from Pacific and South Atlantic, from the surface down to 600 mbsl, Kaufmann (1984) and Kaufmann et al. (1984) concluded that the seawater was a homogeneous reservoir for chlorine stable isotopes and could be used as a standard, i.e.: SMOC. Compared to their collection, we enlarged the number of samples by 24 more seawaters and took different locations and depth profiles to complete these previous studies. We can confirm that it is realistic to use one single seawater to represent SMOC. We can also deduce from our study that the temperature or the salinity (both being linked with depth) do not seem to change the chlorine isotopic composition of the seawater. Moreover, very deep seawaters (down to 4560 mbsl) recovered near the SWIR (EDUL profiles, Fig. 1, Table 2) do not seem to be affected by the presence of the mid-ocean ridge. As these deep seawaters are not contaminated by any hydrothermal plume, no effect on d37Cl was expected. To sum up, we have taken into account a lot of parameters that could affect seawater and change its isotopic signature: the temperature of the seawater, its salinity, even maybe its age if we consider that deep water should not have the same age as surface water of the same geographic location, the size of the water reservoir, the direct influence of the atmosphere, oceanic and continental crust with off-shore surface seawaters, deep samples close to the ridge or coastline, respectively. We have shown that any seawater from the open sea or ocean has a constant chlorine isotopic ratio. The standard deviation (2r) of our collection of 24 seawaters is F 0.08 xand is as large as our precision ( F 0.10 x ). Thus, all these seawaters have a very small range of d37Cl values (0.15x), centred at 0x(Tables 1 and 2). Accordingly, such seawaters could be used as representative of SMOC for both IRMS and TIMS measurements. According to our

results, with a precision of F 0.10x(2r), there is no need to prepare a mixture of seawaters to obtain a SMOC. Consequently, the NaCl salt collected from a purified seawater, named ISL 354 and proposed as an international reference standard by Xiao et al. (2002) may be useful but seems not to be necessary. Xiao et al. (2002) reported high d37Cl values for three seawaters from the Central Indian Ridge between + 0.59xand + 0.94x. No such isotopic values were found in our collection of 24 seawaters, even for the samples from the SWIR (EDUL profiles), which are close to the Central Indian Ridge (Fig. 1; Tables 1 and 2). Thus, an explanation for their positive d37Cl values on these seawater samples is still an open question. The three anomalous seawaters reported by Xiao et al. (2002) may have been influenced by the presence of a hydrothermal plume, while our samples did not. If so, their samples could be very important in terms of Earth degassing and global chlorine cycle. It is then important to confirm these values and to investigate seawaters with exotic d37Cl compared to SMOC. 5.2. The interlaboratory cross-calibration IRMS values from Paris, Reading and Utrecht Universities have been compared over a large range of d37Cl values, from 5.3xto + 1.4x(Table 3; Fig. 2). Most of the samples have d37Cl values in good agreement in all three laboratories. Only the pore fluids MMV1 and MMV3 data show a small isotopic difference of 0.03xand 0.06x , between the Paris Laboratory and the Utrecht and Reading laboratories, respectively. However, these differences are very small and may not be of significance if we consider the fact that the reproducibility for both Paris determinations ( F 0.03xfor MMV1 and F 0.00x for MMV3) are below the mean reproducibility of IRMS technique ( F 0.10x). With such a mean reproducibility, all IRMS results would be within error. Also, due to the fluid nature of the samples from Manon mud volcanoes, heterogeneities are unlikely. Evaporation of the sample is also unlikely because the chlorine content, measured before the isotopic determination, matched well between all the laboratories. Moreover, if it is only evaporation of the water of the sample, this process should not produce any isotopic fractionation effect for chlorine. The salt MS has isotopic values that are just in agreement

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Fig. 2. Interlaboratory cross-calibration for IRMS and TIMS d37Cl measurements. Uncertainties of the measurements are often smaller than the symbol size. See Table 3 for the abbreviations of the samples. N.B. the sea salt MS could be heterogeneous within and between grains. The dashed line is Y = X.

within error. However, the sample MS is a millimeter grain-sized marine salt and it is likely that there are isotopic heterogeneities both between and within grains (Ader et al., 2001). Considering that the values for the brine sample B are in perfect agreement, we interpreted the small differences for the sample MS due to heterogeneities rather than a difference of measurements between the two laboratories. Thus, within error, all the IRMS data plot on the line Y = X in Fig. 2, which means a very good agreement between the data obtained in these three laboratories. For example, on 11 samples, the trend line between Paris and Reading universities is: d37 Cl Reading = ( 1.007 F 0.009)d37ClParis ( 0.040 F 0.025), with a correlation coefficient: R2 = 0.999. Most of the d37Cl data published to date are usually between 2xand + 2x . However, some samples can show strong enrichment in 37Cl (e.g.: Banks et al., 2000; Eggenkamp, 1994; Magenheim et al., 1994; Volpe and Spivack, 1994; Lev and Vocke, 1999; Godon, 2000; Stewart, 2000; Willmore et al., 2002 and references therein), and thus further investigation is necessary to verify such good agreement for more positive d37Cl values.

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A comparison between Leeds University data (TIMS) and Paris University determinations (IRMS) were obtained on six samples for a large range of d37Cl values, from 3.0xto + 6.0x(Table 3; Fig. 3). Despite the slightly lower precision of TIMS analysis, the agreement is good: d37ClLeeds=(1.052 F 0.038) d37ClParis+(0.058 F 0.099), with a correlation coefficient: R2 = 0.995. Because only one sample with a very high positive d37Cl value was compared, further studies are needed to complete the comparison of intermediate positive values between + 1.4xand + 5.5x . It is also important to add negative d37Cl values below 3.0xto improve the comparison. Notably, TIMS data from Leeds University and IRMS values from Reading University have been previously compared (Rosenbaum et al., 2000). Their d37Cl values range from 4.4xto + 0.4x , but four of the five samples are in fact close to 0x(between 0.45xand + 0.34x ). Moreover, these samples have very similar chemical compositions, always as chloride ions in

Fig. 3. Calibration of TIMS versus IRMS over a large range of d37Cl values. The data have been calculated versus Atl 2 or KB seawaters for IRMS measurements and versus GPS-1 seawater for TIMS data. These seawater chloride isotope values are indistinguishable from each other, all at 0x , and thus the results are given in xversus SMOC. Triangles: this study, vertical filled rectangles: from Rosenbaum et al. (2000). The error on the measurements is often smaller than the symbol size. See Table 3 for the abbreviations of the samples. The dashed line is Y = X.

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aqueous solution: seawater, NaCl and CsCl solutions, or brine. As Reading and Paris universities data for such a range ( 4.4xto + 0.4x) show a perfect agreement (Fig. 2), we can use the previous study of Rosenbaum et al. (2000) to extend our calibration. 5.3. An extended comparison TIMS versus IRMS Adding the former data from Rosenbaum et al. (2000) to our data allows a comparison of IRMS and TIMS techniques between d37Cl values from 4.4x to + 6.0x(Fig. 3). Extending the range of comparison is not the only contribution of our study: other chemical compositions, such as a water extract from a meteorite (PVMW) and volcanic gas condensates (VGC1 and VGC2) were also compared (Table 3; Fig. 3). The overall agreement is good: d37ClTIMS= (1.039 F 0.023)d37ClIRMS+(0.059 F 0.056), with a correlation coefficient: R2 = 0.996. This means that d37Cl values near 0xwill agree within less than F 0.10x , while values as far as 10x(either positive or negative) from SMOC, will show agreement between both techniques within less than F 0.30x . This is an acceptable order of difference, to use both techniques complementarily, especially for series of samples showing large chlorine isotopic variations, or for samples with very low chlorine content. For the latter, TIMS is recommended, because it consumes less sample than IRMS, even if it is less precise. For further studies, it will be very important to compare IRMS and TIMS techniques for nonsoluble solid samples requiring a more complicated extraction procedure to prepare Cl for analysis and leading to more chemically complicated solutions. This is usually done by pyrohydrolysis (Magenheim et al., 1994; Boudreau et al., 1997; Musashi et al., 1998; Stewart, 2000; Willmore et al., 2002) but can also be performed by alkali fusion (Eggenkamp, 1994; Godon, 2000; Godon et al., 2004b).

6. Conclusions Whatever the locations of the seawater samples, their distance from the coast, their depth (surface, subsurface or deep water), their salinity, temperature or age, the size of the water reservoir, the potential influence of another geochemical reservoir (such as

the atmosphere, the oceanic or continental crust), the standard deviation (2r) of the d37Cl data from our collection of 24 seawaters is F 0.08xwith a mean value centred at 0x . This reproducibility is as large as our mean reproducibility ( F 0.10x). All these seawaters are thus representative of SMOC and can be used as an international reference material. For this level of precision, there is no need to define and generate another specific international standard for chlorine stable isotopes. Measured by IRMS at the Paris, Reading and Utrecht laboratories and/or by TIMS at the Leeds laboratory, 14 fluid or gas samples with d37Cl from about 6xto + 6xhave been compared. No significant differences between the three IRMS-user laboratories (Paris, Reading and Utrecht) were found on 11 samples ranging from 5.3xto + 1.4x . Also, no bias was found between TIMS values from the Leeds University and IRMS data from the Paris Laboratory, on six samples ranging from 3.0xto + 6.0x. Our results extend a previous comparison study (Rosenbaum et al., 2000) and confirm the good agreement between both techniques for a large range of d37Cl from 4.4xto + 6.0xand on various chemical compositions. Thus, d37Cl values measured on natural fluid samples either by IRMS or TIMS will agree within less than F 0.10xif near 0x , while values as far as 10x(either positive or negative) from SMOC, will show agreement between both techniques within less than F 0.30x .

Acknowledgements We are grateful to the Marion Dufresne crew on EDUL cruise (1997), the staff of the Volcanological Observatory of La Guadeloupe, T. W. Trull, P. Agrinier, C. Laverne, P. Cartigny, M. Girard, C. Me´vel, J. Rosenbaum, B. Yardley, M. Castrec-Rouelle and J. Boule`gue, who provided some of the samples presented in this paper. Samples provided by T. W. Trull were collected with support of Australian SAZ Project (ASAC 1156). We would also like to thank P. Jean-Baptiste and J.-L. Charlou for sharing with us He isotopes and methane data and their expertise about hydrothermal plumes; E. Petit and J.-J. Bourrand for their technical help, M. Girard for the mass spectrometer adaptation and to our secretary S. Panzolini.

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