This document investigates the homogeneity of magnesium isotopes in seawater by analyzing 40 seawater samples from the Gulf of Mexico and one sample from Hawaii using mass spectrometry. The results indicate the Mg isotopic composition of seawater is homogeneous both vertically and horizontally, with average values of d26Mg = -0.832 ± 0.068 and d25Mg = -0.432 ± 0.053 for the Gulf of Mexico samples and identical values for the Hawaii sample. Given seawater's homogeneous isotopic composition and ready availability, the document concludes it could serve as an excellent geostandard for assessing accuracy in high-precision magnesium isotope analysis.

1. Homogeneous magnesium isotopic composition of seawater: an
excellent geostandard for Mg isotope analysis
Ming-Xing Ling1,2
*, Fatemeh Sedaghatpour2
, Fang-Zhen Teng2
**, Phillip D. Hays2
,
Josiah Strauss3
and Weidong Sun4
1
State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,
Guangzhou 510640, P.R. China
2
Isotope Laboratory, Department of Geosciences and Arkansas Center for Space and Planetary Sciences, University of
Arkansas, Fayetteville AR 72701, USA
3
Water Research Centre, School of Civil and Environmental Engineering, University of New South Wales, Sydney 2052,
Australia
4
Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences,
Guangzhou 510640, P.R. China
The magnesium (Mg) isotopic compositions of 40 seawater samples from the Gulf of Mexico and of one seawater
sample from the southwest Hawaii area were determined by multi-collector inductively coupled plasma mass spec-
trometry (MC-ICP-MS) to investigate the homogeneity of Mg isotopes in seawater. The results indicate that the Mg
isotopic composition of seawater from the Gulf of Mexico is homogeneous, both vertically and horizontally, with
average values for d26
Mg = À0.832 Æ 0.068 and d25
Mg = À0.432 Æ 0.053 (n = 40, 2SD) – identical to those of seawater
from Hawaii (average d26
Mg = À0.829 Æ 0.037 and d25
Mg = À0.427 Æ 0.033) and to the average literature values of sea-
water worldwide (d26
Mg = À0.83 Æ 0.11 and d25
Mg = À0.43 Æ 0.06, n = 49, 2SD). Collectively, global seawater has a
homogeneous Mg isotopic composition with d26
Mg = À0.83 Æ 0.09 and d25
Mg = À0.43 Æ 0.06 (2SD, n = 90).
The magnesium isotopic composition of seawater is principally controlled by river water input, carbonate precip-
itation and oceanic hydrothermal interactions. The homogeneous Mg isotopic composition of seawater indicates a
steady-state budget in terms of Mg isotopes in oceans, consistent with a long Mg residence time (~13 Ma). Consid-
ering that seawater is homogeneous, readily available in large amounts, can be easily accessed and processed for
isotopic analysis, and has an isotopic composition near the middle of the natural range of variation, it is an excel-
lent geostandard for accuracy assessment to rule out analytical artifacts during high-precision Mg isotopic analysis.
Copyright © 2011 John Wiley & Sons, Ltd.
Magnesium is a major element in the Earth’s mantle (MgO =
37.8 wt.%)[1]
and continental crust (MgO = 4.66 wt.%),[2]
and
it is also abundant in the oceans (Mg% = ~0.13 wt.%).[3]
Mag-
nesium has three stable isotopes, 24
Mg, 25
Mg and 26
Mg, for
which the representative bulk isotopic compositions are
78.99%, 10.00% and 11.01% (atom%), respectively.[4]
Early
Mg stable isotopic studies were limited because of the low
precision (~1%) caused by large instrumental mass fraction-
ation effects.[5]
The advent of multi-collector inductively
coupled plasma mass spectrometry (MC-ICP-MS) a decade
ago has enabled Mg isotope measurements to be made at a
precision of at least one order of magnitude better than with
other techniques.[5,6]
For example, a precision of Æ0.07%
(26
Mg/24
Mg, 2SD) can be routinely achieved for MC-ICP-
MS Mg isotopic analysis,[7]
which is sufficient to discriminate
mass-dependent Mg isotopic variations in natural samples.[6]
Recent high-precision Mg isotopic studies have significantly
increased our knowledge of Mg isotope geochemistry by docu-
menting the Mg isotopic variations in the mantle and crustal
rocks,[5,7–18]
and illuminating the processes that may produce
these variations, e.g. igneous differentiation, metamorphic
dehydration, and continental weathering.[7,9,11,14,15,17,19]
Nevertheless, debate continues regarding: (1) whether
the earth has a chondritic Mg isotopic composition or
not,[7–9,13,18,20]
and (2) the accuracy of Mg isotopic composi-
tions reported by different labs for certain geostandards, such
as BCR-1, BCR-2 and San Carlos olivine.[5,9,10,12,13,16,18,20–25]
Both debates may reflect the presence of MC-ICP-MS analyti-
cal artifacts in high-precision Mg isotopic analyses con-
ducted at different laboratories.[7–9,18,20]
Analyses of
geostandards BCR-1 and BCR-2 in different laboratories
yielded significantly different Mg isotopic compositions
(Fig. 1)[5,9,12,13,20–23,26]
(e.g. the d26
Mg of BCR-1 varied from
À0.58 to À0.09 (Fig. 1(A)) and the d26
Mg of BCR-2 varied
from À0.33 to À0.12 (Fig. 1(B)) – value ranges of seven and
* Correspondence to: M.-X. Ling, State Key Laboratory of Iso-
tope Geochemistry, Guangzhou Institute of Geochemistry,
Chinese Academy of Sciences, Guangzhou 510640, P.R.
China.
E-mail: mxling@gig.ac.cn
** Correspondence to: F.-Z. Teng, Isotope Laboratory, Depart-
ment of Geosciences and Arkansas Center for Space and
Planetary Sciences, University of Arkansas, Fayetteville
AR 72701, USA.
E-mail: fteng@uark.edu
Copyright © 2011 John Wiley & Sons, Ltd.Rapid Commun. Mass Spectrom. 2011, 25, 2828–2836
Research Article
Received: 2 May 2011 Revised: 2 July 2011 Accepted: 2 July 2011 Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2011, 25, 2828–2836
(wileyonlinelibrary.com) DOI: 10.1002/rcm.5172
2828

2. three times, respectively, the typical reported precision –
although both standards are expected to have a similar,
homogeneous composition. Furthermore, the Mg isotopic
composition of San Carlos olivine was also widely deter-
mined but its value varied greatly in different labora-
tories.[9,12,16,20,25]
The reported d26
Mg values of San Carlos
olivine grains vary from À0.68 to À0.06 (Fig. 1(C)). This
inconsistency may reflect analytical artifacts and/or sample
heterogeneity since different labs worked on different San
Carlos olivine grains; however, analyses of different aliquots
of homogeneous powder made by homogenizing a large
amount of San Carlos olivine also yielded different d26
Mg
values, ranging from À0.55 to À0.19[10,18,23]
(Fig. 1(D)), indi-
cating that heterogeneity is not the primary problem.
To achieve universal comparability for Mg isotopic data
and to enable assessment of accuracy of data from different
laboratories, a set of well-accepted standards is necessary.
Although the pure Mg standard, Cambridge-1,[27]
has been
widely analyzed in different labs, yielding similar d26
Mg
values,[7,13–15,18,23–25,27–29]
its use also does not test for sample
preparation accuracy and reproducibility, since the pure Mg
standard needs no sample preparation as is necessary for
natural samples. Reliance on this standard for inter-laboratory
comparison leaves the Mg isotope science community open to
major problems in accuracy assessment and comparability
because sample preparation processes, e.g. sample dissolution
and column chemistry have the greatest potential for intro-
ducing analytical artifacts and therefore are the analytical
stages most important to monitor.
Here we test the possibility of using seawater as a potential
geostandard for Mg isotopic analysis. Seawater matrix
element concentrations are much more comparable with
those of natural rocks than a pure Mg standard (Table 1). In
addition, a seawater sample can be easily accessed, is readily
available in large amounts and potentially homogeneous as
shown by previous case studies (Fig. 2 and Table 2). In order
to investigate the homogeneity of Mg isotopes in seawater
and the suitability of seawater as a geostandard for future
Mg isotopic analysis, we chose 40 seawater samples from dif-
ferent locations and depths from the Gulf of Mexico, as well
as a seawater sample from Hawaii to run high-precision Mg
isotopic analyses. Our results show that the Mg isotopic com-
position of seawater is homogeneous and hence it can be used
as a geostandard for assessment of accuracy.
EXPERIMENTAL
Samples
Forty seawater samples were collected from 15 sampling sta-
tions in the northwestern Gulf of Mexico, including the Texas
shelf (20 samples from 5 stations collected in August 2007),
Louisiana shelf (10 samples from 5 stations collected in July
2008) and Louisiana shelf break (10 samples from 5 stations
collected in August 2008) (Fig. 3). The shelf break is the
boundary where the continental shelf meets the continental
slope. At least one surface sample and one bottom sample
with depths ranging from 8 m to 112 m were collected from
each sampling station (Table 3). One seawater sample from
southwestern Hawaii was also analyzed for comparison.
The seawater samples were pre-treated by filtration to
remove particles and phyto- or zooplankton, and then
B
BCR-2
-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0
Liu et al., 2010
Young et al., 2009
Chakrabarti &
Jacobsen, 2010
D
SC olivine
(Same olivine powder)
-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1
Wiechert & Halliday, 2007
Huang et al., 2009
Teng et al., 2007
Handler et al., 2009
Pearson et al., 2006
SC olivine
(Different mineral grains)
C
A
BCR-1
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
Young & Galy, 2004
Huang et al., 2009
Bourdon et al., 2010
Wiechert & Halliday, 2007
Teng et al., 2007
Chakrabarti &
Jacobsen, 2010
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
Tipper et al., 2008
Bizzarro et al., 2005
Bourdon et al., 2010
Wombacher et al., 2009
Huang et al., 2009
Teng et al., 2007
Baker et al., 2005
26Mg 26Mg
Figure 1. Magnesium isotopic compositions of geostandards: (A) BCR-1, (B) BCR-2,
(C) different grains of San Carlos olivine, and (D) different aliquots of homogeneous
powder made by homogenizing a large amount of San Carlos olivine, distributed by
S.B. Jacobsen group from Harvard University.
Homogeneous magnesium isotopic composition of seawater
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2829

3. preserved at a temperature below 18
C prior to column
chemistry (the series of chemical procedures related to the
separation of Mg from the sample solutions).
Analytical methods
The magnesium isotopic analyses were carried out at the Iso-
tope Laboratory of the University of Arkansas, Fayetteville,
AR, USA. Chemical procedures were conducted in a class
10 000 clean laboratory, equipped with a class 100 laminar-
flow exhaust hood.[15]
Optima grade acids or distilled acids,
prepared by sub-boiling distillation from trace-metal grade
acids, and Milli-QW
water (Millipore, Billerica, MA, USA)
with a resistivity of 18.2 MΩÁcm were used throughout all
chemical procedures.[9,15]
Column chemistry and MC-ICP-MS analysis
Volumes of 40 mL of solutions for each of the pre-treated sea-
water samples were evaporated to dryness on a hotplate at
80
C. The dried residue was dissolved in 500 mL 1 N
HNO3. Magnesium was separated by cation-exchange using
columns filled with 200–400 mesh AG50W-X8 resin (Bio-
Rad, Hercules, CA, USA), which was pre-cleaned with 20
times column volume of 6 N HCl, 5 times column volume
of 1 N HNO3 and Milli-QW
water. A 100 mL sample solution
was loaded onto the columns and Mg was eluted with 1 N
HNO3.[9,15]
The procedures for eluting Mg were determined
using both pure Mg standard solutions and different repre-
sentative prepared rock-digestion solutions, in order to
achieve 100% recovery of Mg and thus avoid potential frac-
tionation during cation exchange.[9]
The separation procedure
was conducted twice successively, to ensure pure Mg solution
recovery.
The magnesium isotope ratios were determined by a
Nu Plasma (Wrexham, UK) MC-ICP-MS instrument in
low-resolution mode, in which the 26
Mg, 25
Mg and 24
Mg
isotopes were measured simultaneously in separate Faraday
cups.[7]
The purified Mg solutions were introduced by a
quartz cyclonic spray chamber (Elemental Scientific Inc.,
Omaha, NE, USA) to the plasma and analyzed using the
sample-standard bracketing method.[5]
Ratio measurements
for all sample solutions were repeated four times within
a session.[10]
The Mg isotopic results are reported in
standard d-notation in per mil relative to DSM3:[27]
dX
Mg ¼ 103
Â
X
Mg=24
Mgð Þsample
XMg=24
Mgð ÞDSM3
À 1
,
where X refers to 25 or 26.
Precision and accuracy
The precision and accuracy of analyses in the University of
Arkansas laboratory were evaluated by repeated, full-
procedural analyses of samples and standards with dif-
ferent matrices, including a synthetic multi-element standard
solution (IL-Mg-1, concentration ratios of Mg:Fe:Al:Ca:Na:K:
Ti = 1:1:1:1:1:1:0.1), rocks, minerals, seawater and Cambridge-1
pure Mg standard solution, reported in detail by Teng
et al.[7,9]
and Li et al.[15]
The in-run 26
Mg/24
Mg ratio precision
for a single measurement in one block of 40 ratios was less
than Æ0.02% (2SD). The internal precision on the measured
26
Mg/24
Mg ratio was Æ0.1% (2SD), based on ≥4 repeat
runs of the same sample solution during a single analytical
session. The external precision for d26
Mg and d25
Mg was
approximately Æ0.07% and Æ0.06% (2SD), respectively, as
shown by replicate analyses of synthetic solution, mineral,
and rock standards over 2 years.[7]
For example, multiple
analyses of the synthetic multi-element standard solution
(IL-Mg-1) yielded a d25
Mg value of À0.01 Æ 0.06 and a
d26
Mg value of À0.01 Æ 0.07 (2SD, n = 13),[7]
which is con-
sistent with the expected value of 0. Furthermore, the re-
sults for samples and standards such as Cambridge-1 Mg
standard solution, Kilbourne Hole olivine, SUNY MORB
and Allende chondrite, etc., following the full procedures
in this lab, are accurate and consistent with published
values.[7,10,15]
During the course of this study, a seawater
sample from southwestern Hawaii was also analyzed
together with other seawater samples. The duplicate
Table 1. Comparison of Mg contents and the ratios of other major elements to Mg in major igneous rocks and reservoirs of
the earth
UCC MCC LCC PM Granite Basalt Peridotite Seawater
Mg (%) 1.50 2.17 4.37 22.80 0.45 4.36 26.90 0.13
Ca/Mg 1.72 1.73 1.57 0.11 3.10 1.87 0.01 0.32
Na/Mg 1.62 1.16 0.45 0.01 6.69 0.38 0.0006 8.35
K/Mg 1.55 0.88 0.12 0.001 8.22 0.10 9.26E-05 0.29
UCC: upper continental crust,[2]
MCC: middle continental crust,[2]
LCC: lower continental crust,[2]
PM: primitive mantle.[1]
Granite: USGS granite standard (G-2), Basalt: USGS basalt standard (BHVO-2), Peridotite: Geological Survey of Japan peri-
dotite standard (JP-1), data are from the recommended values of these standards. Seawater data are from Brown et al.[3]
Average = -0.83 0.11
(2SD, n=49, data from literature)
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
26Mg
Figure 2. Magnesium isotopic composition of seawater sam-
ples compiled from literature results. Data are from Table 2.
Error bars in this figure represent 2SD uncertainties.
M.-X. Ling et al.
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2830

4. Table 2. Magnesium isotopic composition of seawater compiled from literature results
No. Sample name or location d26
Mg 2SD* d25
Mg 2SD References
1 North Atlantic À0.824 0.040 À0.416 0.080 [41]
2 Mediterranean Sea À0.864 0.122 À0.434 0.105 [5]
3 Atlantic À0.799 0.037 À0.407 0.007
4 Mediterranean Sea À0.851 0.244 À0.422 0.121
Seawater À0.84 0.13 À0.43 0.15 [28]
Seawater À0.77 0.10 À0.39 0.04 [42]
Seawater À0.82 0.06 À0.42 0.03 [43]
5 Average#
À0.81 0.07 À0.41 0.04
IAPSO standard À0.75 0.13 À0.39 0.07 [25]
IAPSO standard À0.83 0.05 À0.40 0.04
6 Average#
À0.79 0.11 À0.40 0.01
7 IAPSO standard À0.79 0.10 À0.41 0.06 [26]
8 NASS-5 standard À0.84 0.16 À0.43 0.07
Hawaii SW À0.83 0.07 À0.42 0.05 [8]
Hawaii SW À0.86 0.02 À0.45 0.02 [15]
Hawaii SW À0.83 0.06 À0.43 0.03 [7]
9 Average#
À0.84 0.03 À0.43 0.03
10 Bermuda À0.79 0.18 À0.41 0.09 [44]
11 IAPSO standard À0.74 0.07 À0.37 0.04 [45]
12 Northwest Pacific À0.79 0.08 À0.37 0.05
13 Northwest Pacific À0.85 0.15 À0.47 0.09
14 Northwest Pacific À1.00 0.28 À0.51 0.01
15 Northwest Pacific À0.97 0.18 À0.49 0.12
16 Northwest Pacific À0.69 0.22 À0.38 0.13
17 Northwest Pacific À0.77 0.40 À0.39 0.16
18 Northwest Pacific À0.86 0.30 À0.34 0.12
19 Northwest Pacific À0.73 0.29 À0.40 0.14
20 Seawater À0.83 0.09 À0.44 0.08 [46]
21 IAPSO standard À0.89 0.18 [47]
BCR-403 À0.89 0.06 À0.47 0.11 [24]
BCR-403 À0.96 0.03 À0.46 0.03
BCR-403 À0.89 0.14 À0.51 0.08
BCR-403 À0.82 0.14 À0.42 0.08
BCR-403 À0.87 0.14 À0.47 0.08
22 Average#
À0.89 0.10 À0.47 0.06
23 IAPSO standard À0.80 0.05 À0.42 0.02 [48]
24 W. Dutch Wadden Sea À0.79 0.03 À0.42 0.02
25 IAPSO standard À0.79 0.26 À0.40 0.15 [23]
26 Bermuda À0.82 0.29 À0.41 0.18
27 North of Newfoundland À0.84 0.16 À0.44 0.09 [49]
28 North of Newfoundland À0.82 0.06 À0.42 0.03
29 North Atlantic À0.87 0.09 À0.44 0.03
30 North Atlantic À0.80 0.09 À0.42 0.09
31 North Atlantic À0.87 0.03 À0.45 0.06
32 North Atlantic, BATS À0.83 0.09 À0.43 0.06
33 North Atlantic, BATS À0.86 0.06 À0.44 0.06
34 North Atlantic, BATS À0.86 0.06 À0.44 0.06
35 North Atlantic, BATS À0.81 0.09 À0.41 0.06
36 North Atlantic, BATS À0.87 0.09 À0.46 0.06
37 North Atlantic, BATS À0.86 0.09 À0.44 0.06
38 Southern Ocean À0.86 0.06 À0.45 0.06
39 North Pacific, St. PAPA À0.84 0.09 À0.44 0.06
40 North Pacific, St. PAPA À0.79 0.03 À0.40 0.00
41 North Pacific, St. PAPA À0.84 0.06 À0.43 0.06
42 North Pacific, St. P26 À0.79 0.09 À0.40 0.06
43 North Pacific, St. P26 À0.80 0.09 À0.41 0.06
44 North Pacific, St. P26 À0.80 0.03 À0.40 0.06
45 North Pacific, St. P26 À0.77 0.09 À0.39 0.03
46 North Pacific, St. P26 À0.80 0.13 À0.42 0.03
47 North Pacific, St. P26 À0.87 0.09 À0.46 0.06
(Continues)
Homogeneous magnesium isotopic composition of seawater
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2831

5. (repeated measurement of different aliquot of the same Mg-
cut) and replicate (repeated column chemistry and measure-
ment) analyses yielded an average d26
Mg = À0.829 Æ 0.037
and d25
Mg = À0.427 Æ 0.033 (2SD, n = 13) (Table 3), which is
consistent with our previously published data (Table 2).[7,8,15]
RESULTS
The Mg isotopic composition of seawater from the Gulf of
Mexico ranges from À0.907 to À0.765 for d26
Mg with an average
of À0.832 Æ 0.068 (2SD, n = 40) (Fig. 4 and Table 3), and
from À0.501 to À0.361 for d25
Mg with an average of
À0.432 Æ 0.053 (2SD, n = 40) (Table 3). Seawater from south-
western Hawaii has d26
Mg ranging from À0.872 to À0.813
with an average of À0.829 Æ 0.037 (2SD, n = 13), and d25
Mg
from À0.449 to À0.394 with an average of À0.427 Æ 0.033
(2SD, n = 13) (Table 3), identical to previously reported values
of the same seawater sample in this laboratory[7,8,15]
(Table 2).
In addition, the Mg isotopic composition of seawater from the
Gulf of Mexico shows no variation as a function of depth, lati-
tude, longitude, locations of continental shelf or shelf break
(Fig. 5 and Table 3). Overall, the Mg isotopic composition of
seawater analyzed in this study is very homogeneous, and
identical to the average value of those compiled from pre-
viously published results (d26
Mg = À0.83 Æ 0.11, d25
Mg =
À0.43 Æ 0.06, 2SD, n = 49) (Table 2).
DISCUSSIONS AND CONCLUSION
The residence time for dissolved species in oceans is defined
as the average time that a substance remains before removal
through precipitation or adsorption processes and subse-
quent deposition,[30]
and it is also called the replacement
time.[31]
The residence time is calculated as total component
mass in seawater divided by input or output rate.[30]
The resi-
dence times of different major seawater constituents vary by
over six order of magnitudes.[31]
Among the abundant ele-
ments in seawater, Mg has a residence time of ~13 Ma, mod-
erately close to that of Ca (10 Ma).[31]
The mixing time for the
earth’ oceans is estimated at 1 kya to 2 kya.[32]
The short mix-
ing time, long residence time, and balance of input and removal
functions can result in a homogeneous distribution of Mg in
the oceans with a global average content of ~0.13%.[3]
Our
work, together with previous studies, indicates a homoge-
neous distribution of Mg isotopes in the oceans.
Table 2. (Continued)
No. Sample name or location d26
Mg 2SD* d25
Mg 2SD References
48 North Pacific, St. P26 À0.82 0.06 À0.44 0.00
49 North Pacific, St. P26 À0.80 0.06 À0.41 0.06
Total average À0.83 0.11 À0.43 0.06
*2SD of total average is calculated using all the Mg isotopic data in this table. 2SD values of some data are calculated from
2SE in literature results.
#
Average of results from the same sample is used to plot in Figs. 2 and 6.
Houston
Austin
New Orleans
Texas
Louisiana
Mississippi
Gulf of Mexico
31D
6C BC12
3B
9B
400401
417418
404405
412413
406407
BR3D
BR5A
BR4D
BR5C
BR6D
100km
Continental
shelf
Shelf slope
Baton Rouge
Figure 3. Sketch map of the northwest Gulf of Mexico and sampling locations
(modified from Google Earth). The red diamonds represent sampling locations.
The locations in Texas shelf and Louisiana shelf are labeled with station names
whereas those in the Louisiana shelf break are marked as sample names (station
name unavailable) corresponding to Table 3.
M.-X. Ling et al.
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2832

6. Table 3. Sampling details and Mg isotopic compositions of seawater in this study
Locations Station name* Sample Latitude Longitude Depth
(m)
d26
Mg 2SD d25
Mg 2SD
LA-Shelfx
3B 33 28.5816 À90.7780 0 À0.809 0.066 À0.431 0.073
254 28.5816 À90.7780 20 À0.819 0.071 À0.427 0.054
9B 132 28.7214 À90.5516 16 À0.831 0.071 À0.421 0.054
7 28.7214 À90.5516 0 À0.841 0.071 À0.427 0.054
6 C 125 28.9234 À92.2287 0 À0.785 0.071 À0.424 0.054
23 28.9234 À92.2287 25 À0.876 0.071 À0.465 0.054
BC12 150 28.9946 À92.0045 0 À0.840 0.071 À0.454 0.054
143 28.9946 À92.0045 20 À0.777 0.061 À0.416 0.063
31D 53 29.3994 À92.5977 0 À0.794 0.061 À0.397 0.063
102 29.3994 À92.5977 8 À0.833 0.071 À0.452 0.054
LA-Shelf
Break
406 27.8078 À93.0541 0 À0.832 0.067 À0.450 0.060
407 27.8078 À93.0541 69 À0.824 0.071 À0.415 0.054
404 27.9544 À92.0146 0 À0.873 0.067 À0.416 0.060
405 27.9544 À92.0146 60 À0.852 0.067 À0.438 0.060
412 27.8527 À92.9203 0 À0.795 0.061 À0.409 0.063
Duplicate†
À0.840 0.084 À0.397 0.063
Average À0.818 0.084 À0.403 0.063
413 27.8527 À92.9203 100 À0.803 0.061 À0.405 0.063
400 28.3174 À90.5659 0 À0.793 0.061 À0.406 0.063
401 28.3174 À90.5659 60 À0.785 0.061 À0.398 0.063
417 28.0909 À91.1491 0 À0.841 0.061 À0.401 0.063
418 28.0909 À91.1491 112 À0.803 0.061 À0.426 0.063
TX-Shelfx
BR4D BR4D-BOT1 28.5588 À95.5835 20 À0.843 0.067 À0.381 0.060
Duplicate À0.835 0.068 À0.446 0.050
Average À0.839 0.068 À0.413 0.060
BR4D-BOT2 28.5588 À95.5835 20 À0.835 0.075 À0.470 0.064
BR4D-SFC1 28.5588 À95.5835 0 À0.860 0.080 À0.435 0.069
BR4D-SFC2 28.5588 À95.5835 0 À0.839 0.080 À0.473 0.069
BR3D BR3D-BOT1 28.6244 À95.4323 18 À0.876 0.075 À0.478 0.064
BR3D-BOT2 28.6244 À95.4323 18 À0.805 0.072 À0.421 0.053
BR3D-SFC1 28.6244 À95.4323 0 À0.844 0.072 À0.456 0.053
BR3D-SFC2 28.6244 À95.4323 0 À0.872 0.075 À0.484 0.064
BR6D BR6D-BOT1 28.4251 À95.8739 20 À0.861 0.078 À0.456 0.038
BR6D-BOT2 28.4251 À95.8739 20 À0.870 0.072 À0.438 0.053
BR6D-SFC1 28.4251 À95.8739 0 À0.880 0.075 À0.458 0.064
BR6D-SFC2 28.4251 À95.8739 0 À0.774 0.072 À0.398 0.053
BR5A BR5A-BOT1 28.6419 À95.8168 9 À0.853 0.080 À0.430 0.069
BR5A-BOT2 28.6419 À95.8168 9 À0.843 0.067 À0.396 0.060
BR5A-SFC1 28.6419 À95.8168 0 À0.781 0.072 À0.361 0.053
Duplicate À0.832 0.084 À0.501 0.063
Duplicate À0.856 0.075 À0.452 0.064
Average À0.823 0.084 À0.438 0.064
BR5A-SFC2 28.6419 À95.8168 0 À0.809 0.072 À0.390 0.053
BR5C BR5C-BOT1 28.5520 À95.7459 16 À0.765 0.084 À0.409 0.063
Duplicate À0.780 0.080 À0.394 0.069
Average À0.772 0.084 À0.402 0.069
BR5C-BOT2 28.5520 À95.7459 16 À0.888 0.080 À0.458 0.069
BR5C-SFC1 28.5520 À95.7459 0 À0.807 0.080 À0.457 0.069
BR5C-SFC2 28.5520 À95.7459 0 À0.907 0.058 À0.474 0.060
Total average À0.832 0.068 À0.432 0.053
Hawaii Seawater À0.815 0.078 À0.416 0.038
Duplicate À0.847 0.062 À0.448 0.055
Duplicate À0.813 0.072 À0.417 0.053
Duplicate À0.813 0.067 À0.417 0.060
Duplicate À0.813 0.071 À0.417 0.054
Duplicate À0.813 0.084 À0.417 0.063
Duplicate À0.872 0.075 À0.440 0.064
Replicate†
À0.836 0.068 À0.426 0.050
Replicate À0.824 0.068 À0.445 0.050
Replicate À0.847 0.058 À0.443 0.060
(Continues)
Homogeneous magnesium isotopic composition of seawater
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2833

7. Controlling factors on Mg content and isotopic composition
of seawater
The magnesium content of seawater is controlled by river
water, groundwater, carbonate precipitation,[31]
oceanic
hydrothermal interactions,[33,34]
ion-exchange reactions with
clays[33]
and dust of continental origin; of these contributory
factors river water input is the main source of Mg in sea-
water.[28,34,35]
Magnesium in rivers is mainly derived from
the weathering of continental rocks and is controlled by
watershed lithology.[36]
Under the steady-state seawater Mg
conditions thought to have predominated through much of
geological time,[35]
river input was balanced by the important
Mg removal – carbonate precipitation[31]
and hydrothermal
interactions, e.g. circulation at the mid-ocean ridge (MOR)
spreading centers,[34], and references cited therein
where Mg is
exchanged with Ca,[33]
to remove Mg from seawater. At
present MOR hydrothermal alteration is thought to be the
predominant Mg removal process; however, syndepositional
dolomite precipitation is thought to have been the predom-
inant control on seawater Mg concentrations and on Mg/Ca
ratios over a large extent of the Phanerozoic Eon, out-
weighing removal of Mg at the MOR and providing the
balance for the input of riverine Mg.[35]
Consistent with the predominant controls on seawater Mg
concentrations, the Mg isotopic composition of seawater
largely depends on river-water input, carbonate precipitation
and oceanic hydrothermal interactions. The magnesium iso-
topic compositions of river waters are highly heteroge-
neous.[19,37,38]
The flux-weighted d26
Mg of global runoff has
been estimated at À1.09%, based on sampling of 45 rivers
across the globe,[37]
which is lower than the average d26
Mg
of seawater at À0.83%. Carbonate precipitation and clay for-
mation may slightly modify the Mg isotopic composition of
Depth(m)LatitudeLongitude
-97
-96
-95
-94
-93
-92
-91
-1.00 -0.95 -0.90 -0.85 -0.80 -0.75 -0.70 -0.65
27.5
28.0
28.5
29.0
29.5
120
100
80
60
40
20
0
C
B
A
26
Mg
Figure 5. Lack of correlations between Mg isotopic
composition and sampling depth, latitude and longitude of
seawater in the Gulf of Mexico: (A) d26
Mg vs. depth,
(B) d26
Mg vs. latitude, and (C) d26
Mg vs. longitude.
Table 3. (Continued)
Locations Station name* Sample Latitude Longitude Depth
(m)
d26
Mg 2SD d25
Mg 2SD
Hawaii Replicate À0.813 0.080 À0.417 0.069
Replicate À0.834 0.067 À0.394 0.060
Replicate À0.838 0.066 À0.449 0.073
Average À0.829 0.037 À0.427 0.033
x
LA-Shelf = Louisiana shelf. TX-Shelf = Texas shelf.
†
Duplicate = repeated measurement of different aliquot of the same Mg-cut. Replicate = repeated column chemistry and
measurement of the same sample.
*The station names of Louisiana Shelf Break are not available.
-1.00
-0.95
-0.90
-0.85
-0.80
-0.75
-0.70
-0.65
Average = -0.832 0.068
(2SD, n=40, this study)
26
Mg
Figure 4. Magnesium isotopic composition of seawater
samples from the Gulf of Mexico.
M.-X. Ling et al.
wileyonlinelibrary.com/journal/rcm Copyright © 2011 John Wiley Sons, Ltd. Rapid Commun. Mass Spectrom. 2011, 25, 2828–2836
2834

8. seawater at time scales matching riverine input.[5]
Further-
more, although Mg in calcite has a consistently lower d26
Mg
value than Mg in dolomite by approximately 2%,[5]
consider-
ing that the Mg content of calcite is significantly lower than
that of dolomite, dolomite precipitation may be an important
control on seawater Mg isotopic composition. The extent to
which oceanic hydrothermal interactions affect Mg isotopic
composition of seawater remains poorly understood. Tipper
et al.[37]
proposed that hydrothermal circulation must prefer-
entially scavenge light Mg isotopes from seawater. The fact
that the seawater all over the world has a homogeneous Mg
isotopic composition implies a steady-state oceanic Mg iso-
tope budget.
Seawater as a geostandard for Mg isotopic analysis
All the seawater samples from the Gulf of Mexico and Hawaii
analyzed in this study have homogeneous Mg isotopic com-
position, identical to the averages based on literature data
(Fig. 6). Furthermore, seawater samples from the Gulf of
Mexico were sampled from a wide range of locations (latitude:
27.8078 to 29.3994, longitude: –95.8739 to À90.5516), with
depths ranging from 8 m to 112 m. The results indicate that
the Mg isotopic compositions of seawater are homogeneous
both vertically and horizontally (Fig. 5).
Based on the results in this study and literature data (Fig. 6),
global seawater samples have homogeneous Mg isotopic
compositions with d26
Mg = À0.83 Æ 0.09 and d25
Mg = À0.43
0.06 (2SD, n = 90). In addition, the ratios of seawater matrix
elements to Mg are similar to those of natural samples
(Table 1). This, together with the fact that seawater is readily
accessible in large amounts, can be easily processed for isoto-
pic analysis, and has an isotopic composition near the middle
of natural range of variation, makes it an excellent geostan-
dard for future inter-laboratory accuracy assessment on
high-precision Mg isotopic analysis.
Furthermore, the homogeneous Mg isotopic composition of
seawater may also contribute to further research of seawater
paleo-temperature, since there are possibly small changes in
seawater Mg isotopic composition corresponding to Mg/Ca
variation of echinoderms,[34]
while the Mg content or Mg/Ca
ratio is largely controlled by temperature.[39,40]
Thus, the evo-
lution of seawater Mg isotopic composition over time may
be an efficient indicator of seawater temperature in the past.
Acknowledgements
We thank Yan Xiao for help in the lab and Bing Shen and one
anonymous referee for constructive comments. This work
was supported by NSF (EAR-0838227 and EAR-1056713),
Arkansas Space Grant Consortium (SW19002), Natural
Science Foundation of China (NSFC) (Nos. 41103006 and
41090370) and the CAS/SAFEA International Partnership
Program for Creative Research Teams. This is contribution
No. IS-1374 from GIGCAS.
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