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 Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, Université Paris 7, UMR 7047, 2,
Place Jussieu, Tour 54-64 1er étage, 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
2
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.
A. Godon et al. / Chemical Geology 207 (2004) 1–12
Our study is both an interlaboratory and intertechniques cross-calibration. We compared the d37Cl
data obtained by IRMS at the laboratory of Géochimie 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.
3
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 (Mével 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.
4
A. Godon et al. / Chemical Geology 207 (2004) 1–12
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) (Mével 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 Mével et al. (1997a) for CTD samples and from T.W. Trull
(personal communication, 1999) for CRC samples.
A. Godon et al. / Chemical Geology 207 (2004) 1–12
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 (Soufrière 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 Périchrom 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
5
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
6
A. Godon et al. / Chemical Geology 207 (2004) 1–12
(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.
A. Godon et al. / Chemical Geology 207 (2004) 1–12
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-
7
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
8
A. Godon et al. / Chemical Geology 207 (2004) 1–12
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
A. Godon et al. / Chemical Geology 207 (2004) 1–12
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.
9
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.
10
A. Godon et al. / Chemical Geology 207 (2004) 1–12
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.
Mével, J. Rosenbaum, B. Yardley, M. Castrec-Rouelle
and J. Boulègue, 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.
A. Godon et al. / Chemical Geology 207 (2004) 1–12
We also thank C. Gorge for ion chromatography
facilities at the Laboratoire de Géochimie et Cosmochimie. We are also grateful to C. Göpel, A. Boudreau,
M. Stewart and an anonymous reviewer for their very
helpful suggestions and constructive comments. Part
of this work was supported by INSU and by the
University of Paris 7-Denis Diderot. This is an IPGP
contribution number 1938 and a CNRS contribution
number 348. [CA]
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