Available online at www.sciencedirect.com
Geochimica et Cosmochimica Acta 73 (2009) 91–101
www.elsevier.com/locate/gca
The hafnium isotope composition of Pacific Ocean water
Bettina Zimmermann a, Don Porcelli b,*, Martin Frank c, Jörg Rickli a,
Der-Chuen Lee d, Alex N. Halliday b
a
Institute of Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH Zentrum, Clausiusstrasse 25,
CH-8092 Zurich, Switzerland
b
Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
c
IFM-GEOMAR, Leibniz Institute of Marine Sciences, 24148 Kiel, Germany
d
Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan, ROC
Received 19 May 2008; accepted in revised form 10 September 2008; available online 22 October 2008
Abstract
The first Hf isotope data for seawater are reported for a series of stations in the Northwestern Pacific and define a range
from eHf = 3.5 ± 1.4 to 8.6 ± 1.6. Most samples have values within error of the average of eHf = 5.9, but significant variations
are found in intermediate waters at a depth of 600 m, as well as in deep waters. The Nd and Hf isotope compositions of the
deep waters fall within the range of values found for surfaces of hydrogenetic ferromanganese crusts in the region, confirming
that Hf in the Fe-Mn crusts has been derived from the overlying water column, which thus provide an archive of past seawater
compositions. Although the seawater samples are generally close to the global eNd–eHf correlation obtained from ferromanganese crusts, there are significant deviations from this correlation indicating that there is some additional decoupling between
Nd and Hf isotope signals, most likely caused by local water mass mixing and differences in residence times. This is not
resolved in the crust samples, which integrate seawater signals over 104 years. The combined use of these two isotope systems
in seawater therefore provides an additional dimension for tracing water masses in the oceans. Studies of the distribution of
oceanic Hf isotope compositions that have been confined to deep water and boundary waters, as recorded in seafloor ferromanganese crusts, can now be extended and aimed at characterising the entire present-day water column. Average Hf concentrations measured in this study are somewhat lower than previously reported, suggesting a shorter residence time for
Hf in the global oceans, although the uncertainty in the extent of Hf removal from the water column during estuarine mixing
as well as a lack of data on hydrothermal and dust inputs remains a limit on how well the residence time can be defined.
Ó 2008 Elsevier Ltd. All rights reserved.
1. INTRODUCTION
age and Lu/Hf fractionation. Similarly, 147Sm decays to
Nd (t1/2 = 1.06 1011 a), producing variations in
143
Nd/144Nd. Fractionations of Lu/Hf and Sm/Nd ratios
are generally well-correlated in common petrological processes, so that 176Hf/177Hf and 143Nd/144Nd ratios (typically
reported as eHf and eNd, which represent parts in 104 deviations from the CHUR standard; see Table 1) show a
strong positive correlation for most crustal and mantle-derived rocks (Fig. 1; Patchett et al., 1981; Vervoort et al.,
1999). Measured Hf isotope compositions range from the
most unradiogenic values of eHf = 30 in old granitic rocks
(Vervoort and Patchett, 1996) and even less radiogenic values of eHf as low as 40 to 50 in some sedimentary rocks
containing high fractions of zircons (Vervoort et al., 1999),
143
The isotopic compositions of hafnium (Hf) and neodymium (Nd), which exhibit large variations in inputs to the
oceans, have been widely used to document changes in
oceanographic circulation patterns and weathering inputs
resulting from tectonic and climatic factors (see Frank,
2002; Goldstein and Hemming, 2003). The decay of 176Lu
(t1/2 = 3.71 1010 a) produces 176Hf, resulting in wide variations in 176Hf/177Hf within crustal rocks as a function of
*
Corresponding author. Fax: +44 1865 272072.
E-mail address: [email protected] (D. Porcelli).
0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.gca.2008.09.033
92
B. Zimmermann et al. / Geochimica et Cosmochimica Acta 73 (2009) 91–101
Fig. 1. A comparison between Nd and Hf isotopes indicates that
rocks from the continental crust (grey squares and array bounded
by solid line) and the mantle (grey triangles and array bounded by
dashed line) fall on a linear correlation that reflects coherent
fractionation between Lu/Hf and Sm/Nd during magmatic processes (data compiled in Vervoort et al., 1999). In contrast, marine
authigenic deposits, e.g. Fe-Mn crusts and Mn-nodules (Godfrey
et al., 1997; Albarède et al., 1998; Lee et al., 1999; Piotrowski et al.,
2000; David et al., 2001; van de Flierdt et al., 2004a,b, 2006), fall on
a separate correlation, with a more radiogenic Hf (higher eHf
values) isotope composition for a given eNd value. It has been
assumed that these deposits reflect seawater values, with the
displacement of Nd and Hf isotopes from crustal rock values
arising from incongruent weathering effects (see text). The inserted
box represents Fig. 4.
to +13 to +16 in young island arc volcanics (White and
Patchett, 1984), and to radiogenic values as high as +25
in MORB (Patchett and Tatsumoto, 1980a; Salters and
Hart, 1991; Salters and White, 1998). In seawater, the isotopic composition of Nd varies both between and within
ocean basins due to differences in regional inputs, since
the average oceanic residence time of Nd in the oceans of
500 to 1000 years (Tachikawa et al., 1999, Tachikawa
et al., 2003; Lacan and Jeandel, 2005) is shorter than the
time required for homogenizing oceanic Nd compositions.
The resulting variations between different water masses
have proven useful as fingerprints for studying global circulation patterns (see review by Goldstein and Hemming,
2003).
In contrast to Nd, only a limited amount of Hf concentration data are available for seawater (Godfrey et al., 1996;
McKelvey and Orians 1998; Firdaus et al., 2008), and due
to analytical difficulties associated with measuring very
small amounts of Hf, there are no direct measurements of
seawater Hf isotope compositions. Significant variations
in seawater Hf isotope compositions are expected, since
estimates of the oceanic residence time of Hf are somewhat
longer than for Nd but still shorter than the global ocean
mixing time of 1500 years (Broecker and Peng, 1982;
Godfrey et al., 1996; McKelvey and Orians, 1998; White
et al., 1986). Records of seawater compositions have been
sought through measurements of hydrogenetic ferromanganese (Fe-Mn) crusts, which grew from direct precipitation
of metals from seawater. These crusts have been shown to
take up seawater Nd, and presumably Hf, during formation
(White et al., 1986; Godfrey et al., 1997; Albarède et al.,
1998; Lee et al., 1999; Piotrowski et al., 2000; David
et al., 2001; van de Flierdt et al., 2002, 2004a), although
the correspondence between such Hf isotope compositions
and those of the overlying water column has not been directly confirmed.
The eHf and eNd measurements from Fe-Mn-crusts and
Mn-nodules fall along an array which is offset from the terrestrial Hf-Nd array towards higher eHf for a given eNd,
which has been explained as a result of the so-called zircon
effect (Fig. 1; Albarède et al., 1998; Piotrowski et al., 2000;
David et al., 2001; van de Flierdt et al., 2007). Zircons concentrate Hf, and with very low Lu/Hf ratios maintain
unradiogenic eHf values relative to their host rocks. Preferential dissolution of more easily weathered mineral phases
with higher Lu/Hf ratios results in Hf isotope compositions
of weathering solutions and river waters, and thus ultimately of seawater, that are more radiogenic than those
of the Hf source rocks (Bayon et al., 2006). In contrast, riverine Nd isotope compositions appear to reflect source rock
compositions (e.g. Goldstein et al., 1984). It has been argued that seawater compositions are probably dominated
by such riverine inputs, and other sources of seawater Hf,
such as eolian dust and hydrothermal inputs, are of minor
importance (Piotrowski et al., 2000; van de Flierdt et al.,
2002, 2004a,b, 2007). This is supported by recent riverine
Hf isotope data (Bayon et al., 2006). In contrast, it has been
suggested that the ocean’s Hf budget may also be affected
by hydrothermal Hf (White et al., 1986; Godfrey et al.,
1997; Bau and Koschinsky, 2006) contributing to elevated
eHf values in Fe-Mn-crusts and Mn-nodules. Indeed, it
has been proposed that the flux of riverine Hf to the oceans
is reduced by the preferential removal of Hf relative to Nd
during estuarine mixing because of the stronger association
of the former with colloids that are removed by flocculation
(Bau and Koschinsky, 2006).
In this study, the first measurements of the isotopic composition of Hf in seawater are presented along with Nd isotope data, allowing a direct comparison between Hf and Nd
isotope compositions found in Mn crusts and those found
in overlying waters.
2. SAMPLING AND EXPERIMENTAL METHODS
2.1. Sampling
Water samples were collected from six stations in the
western Pacific Ocean, to the east of Japan, during the
2002 IOC Contaminant Baseline Survey (Fig. 2). Measures
et al. (2006) provide details of the cruise and sampling, as
well as a detailed analysis of the hydrography of the region,
which is only briefly summarized here. Circulation of the
upper layers is dominated by the Subarctic Current, which
forms the boundary of the Subarctic Gyre, and the Kuroshio Current, which forms the boundary of the Subtropical
Gyre and leaves the coast to travel eastwards at 36oN as
the Kuroshio Extension. Between these currents is the
Mixed Water Region, where waters are mixed through
the formation of eddies. The cruise track crossed between
the Subtropical Gyre (Stations 6 and 7) and Subarctic Gyre
(Stations 2 and 3), traversing the Subarctic Current, the
Hafnium isotopes in Pacific Ocean water
93
Table 1
Hf and Nd isotope data.
Sampling station
Depth (m)
176
eHfa,c
143
eNdb,c
Station1
34°280 N 146°590 E
05-May-2002
10
100
600
1500
10
100
600
1200
4000
5000
10
100
600
1500
10
100
600
1500
10
100
600
1500
10
100
600
1200
3500
5000
–
–
0.282872
0.282949
0.282929d
–
–
3.5 ± 1.4
6.2 ± 1.4
5.6 ± 1.6
0.283003
0.282914
0.282925
0.282910
0.282962
8.2 ± 1.6
5.0 ± 1.6
5.4 ± 1.6
4.9 ± 0.8
6.7 ± 1.4
0.512427
0.512420
0.512466
0.512603
0.512509
0.512529
0.512508
0.512508
0.512532
0.512479
0.512499
0.512500
0.512484
0.512492
0.512441
0.512425
0.512472
0.512476
0.512403
0.512432
0.512448
0.512473
0.512521
0.512490
0.512452
0.512534
0.512441
0.512394
4.1 ± 0.5
4.3 ± 0.5
3.4 ± 0.5
0.7 ± 0.5
2.5 ± 0.5
2.1 ± 0.5
2.5 ± 0.5
2.5 ± 0.5
3.4 ± 0.5
3.8 ± 0.5
2.7 ± 0.5
2.7 ± 0.5
3.0 ± 0.5
2.8 ± 0.5
3.9 ± 0.5
4.2 ± 0.5
3.2 ± 0.5
3.2 ± 0.5
4.6 ± 0.5
4.0 ± 0.5
3.7 ± 0.5
3.2 ± 0.5
2.3 ± 0.5
2.9 ± 0.5
3.6 ± 0.5
2.0 ± 0.5
3.8 ± 0.5
4.8 ± 0.5
Station 2
44°000 N 155°000 E
11-May-2002
Station 3
50°000 N 167°000 E
14-May-2002
Station 4
39°220 N 170°350 E
17-May-2002
Station 6
30°300 N 170°350 E
20-May-2002
Station 7
24°150 N 170°200 E
22-May-2002
Hf/177Hf
0.282915
0.282915
0.282904
d
d
0.282881
0.282938
–
–
0.282930
0.282944
–
–
0.282976
0.283007
–
0.283015
5.0 ± 1.4
5.0 ± 1.4
4.7 ± 1.4
3.8 ± 1.4
5.9 ± 1.4
–
–
5.6 ± 1.4
6.1 ± 1.4
–
–
7.2 ± 1.6
8.3 ± 1.6
–
8.6 ± 1.6
Nd/144Nd
d
d
d
All data is for filtered waters.
a
eHf = [(176Hf/177Hfsample 176Hf/177HfCHUR)/176Hf/177HfCHUR]104, where (176Hf/177Hf)CHUR = 0.282169 (Nowell et al., 1998).
b
eNd = [(143Nd/144Nd)sample/(143Nd/144Nd)CHUR 1]104; where (143Nd/144Nd)CHUR = 0.512638.
c
Including external reproducibility.
d
Waters from 10 and 100 m combined.
Mixed Water Zone (Station 4), and the Kuroshio Extension
(Station 1). The mixed layer in the sampling stations ranged
from 23 m (Station 6) to 110 m (Station 3). The North Pacific Intermediate Water (NPIW), defined by a salinity minimum at 300–1000 m, is seen throughout much of the
region but not at the Subarctic Gyre stations. Low salinity
intrusions were also seen in the Kuroshio Extension station
at 400 m and 700 m. Salinities increase again at greater
depths, and below 2000 m Pacific Deep water (PDW) has a
fairly uniform potential temperature and salinity. The deep
waters are supplied by Antarctic Bottom Water (AABW)
flowing from the Antarctic Circumpolar Current.
Samples were collected using Go-Flo samplers mounted
on a rosette equipped with a conductivity-temperature-depth
(CTD) instrument. Upon collection, water was transferred to
acid-washed polyethylene bottles, then filtered within a few
hours after collection using a peristaltic pump through precleaned 142 mm diameter 0.45 lm membrane filters. The
samples were acidified to pH <2 and stored in acid-washed
polyethylene containers for transport to the laboratory.
2.2. Hafnium concentration and isotope measurements
in
For Hf concentration determinations, a spike enriched
Hf was added to 2 l sample aliquots and allowed to
178
equilibrate for five days. Hafnium was then concentrated
by Fe co-precipitation using 0.033 g Fe in the form of
FeCl3, and then separated from Fe, Mg, Ca and other elements using a modified version of the cation column chemistry of Patchett and Tatsumoto (1980b). Samples were
dissolved in 3 ml of 1 M HCl + 0.05 M HF and loaded onto
4 ml columns (50W-X8 resin). The Hf was eluted with 2 ml
of 1 M HCl + 0.05 M HF. Samples were measured by MCICPMS in a solution of 1.5% HNO3 and 0.5% HF. All concentration measurements were performed on the Faraday
cups. Mass fractionation of Hf was monitored via determination of the 182W/183W ratio of an added W standard
(NIST 3163). Direct internal mass bias determination of
spiked samples can also be carried out using 179Hf/177Hf
but requires an iterative routine to separate natural and
spike contributions on 179Hf and 177Hf. Hafnium concentration data determined with W as an external mass bias
monitor and using the iterative routine for 179Hf/177Hf
yielded data that were identical within error. Hafnium concentrations were calculated from measured 179Hf and 178Hf
peaks, which are both free of direct interferences. For the
determination of the external precision of the measurements, five 2 l aliquots of the same 20 l bottle were separately processed and measured on the MC-ICPMS. The
1r standard deviation of the five concentration measure-
94
B. Zimmermann et al. / Geochimica et Cosmochimica Acta 73 (2009) 91–101
Fig. 2. Sampling locations. Waters for this study were collected from profiles at six sampling stations as part of the 2002 IOC Contaminant
Baseline Survey (Measures et al., 2006). The black arrows schematically show the Kuroshio/Oyashio surface water circulation patterns (e.g.
Measures et al., 2006). The Stations 2 and 3 are within the Western Subarctic Gyre, Stations 6 and 7 are within the Subtropical Gyre, Station 1
is within the Kuroshio Extension of the Kuroshio Current, and Station 4 is within the Mixed Water Region between the two gyres. Also
shown (white circles) are sampling locations for which data have been previously reported for Nd isotopes (TPS24 and TPS 47, Piepgras and
Jacobsen, 1988; CM5, LM 9, LM 6/11, and LM 2, Amakawa et al., 2004) and for Hf concentrations (K1, K2, KNOT, and 35N, Firdaus et al.,
2008). Station 2 of this study is at approximately the same location as the CM5 and KNOT sites of the previous studies.
ments was 2.5% of the average concentration. A 2.5% 1r
error has therefore been assigned to all concentration
measurements.
For isotope measurements, Hf was separated from 20 l
sample aliquots by Fe co-precipitation using 0.17 g Fe
in the form of FeCl3. A combination of the modified cation
column by Patchett and Tatsumoto (1980b) and the 2-column chemistry described by Lee et al. (1999) was used to
further separate Hf. Each aliquot was loaded in 6 ml 1 M
HCl + 0.05 M HF onto columns with 7.8 ml of AG 50WX8 resin, and eluted with 5 ml 1 M HCl + 0.05 M HF.
After passing 20 ml of 6 M HCl through the column, the
REE fraction was eluted with 32 ml 6 M HCl. For each
sample, the Hf of three 20 l aliquots was then combined,
dried down, and redissolved in 4 M HF to be loaded onto
an anion column with 1 ml AG 1-X8. The columns were
washed with 12 ml 4 M HF, and then Hf was collected
with 6 ml 6 M HCl + 1 M HF. After drying down, the samples were redissolved in 1 ml 2 M HCl + 0.1 M HF. A final
column with 0.7 ml of Eichrom Ln resin was used to separate Hf from Zr, as well as from any remaining Fe, Ti, Mg,
Ca, Br, Cl, Lu, and Yb. The column was washed with
2.7 ml of 2 M HCl + 0.1 M HF, and Hf was eluted using
7 ml 2 M HCl + 0.1 M HF. Samples were measured by
MC-ICPMS in 1.5% HNO3 + 0.5% HF. Neodymium was
separated from the REE split from the cation column
following the method described by Cohen et al. (1988).
Samples for Nd isotopic composition measurements were
dissolved in 0.1 M HNO3 for measurement by MC-ICPMS.
The total procedural blank for the Hf isotopic composition was 80 pg, which corresponds between 1.3% and
3.5% of the available amounts of Hf in the samples and
thus has a negligible effect on the measured Hf isotope ratios. Yields for the precipitation and chemical separation
procedure of Hf were 80%. Each column separation step
as well as the precipitation of the seawater resulted in a loss
of 5% of the sample.
Hafnium and Nd isotope ratios were measured on the
Nu instruments MC-ICPMS at ETH Zürich. Given the
very low amounts of Hf available (1–6 ng of total Hf), the
samples were dissolved in 200–400 ll of 0.5 M HNO3/
0.1 M HF solutions and were measured with the time-resolved software of the mass spectrometer in order to completely utilize all available material. At uptake rates of the
nebulizer of 80 and 100 ll/min this resulted in total beam
sizes between 1 and 6 V of Hf, which were stable for
1–5 min and resulted in internal errors of the measurements
generally better than 1 eHf unit. The samples were measured
alternating with JMC 475 standard solutions at concentrations corresponding to the Hf concentrations in the respective sample solutions. Measured 176Hf/177Hf was corrected
for instrumental mass bias using a 179Hf/177Hf ratio of
0.7325 using the exponential law and interferences of Yb
and Lu on 176Hf were corrected by monitoring 172Yb and
Hafnium isotopes in Pacific Ocean water
95
175
Lu and applying the same mass fractionation as for Hf.
The naturally invariable 178Hf/177Hf isotopic ratio (normalized 179Hf/177Hf) was monitored to check for potential matrix and interference problems. The 178Hf/177Hf ratios of all
samples presented here were within the external reproducibility of the repeated JMC 475 standard measurements
All 176Hf/177Hf results were normalized to 0.282160 for
JMC 475 (Nowell et al., 1998). The errors for the results
of presented 176Hf/177Hf measurements represent the 2r
standard deviation of repetitive Hf standard measurements
(JMC 475) with similar concentrations to the sample solution. In those cases where the within-run error (2r standard
error of the mean) was larger than the 2r error of the external reproducibility, this error was used.
For Nd isotopic composition measurements, the instrumental mass bias was corrected using the exponential law
and a 146Nd/144Nd ratio of 0.7219. The measured and mass
bias corrected 143Nd/144Nd was normalized to 0.511833, the
averaged 143Nd/9uNd of an in-house Nd standard, which
has been cross-calibrated to the commonly used La Jolla
standard (van de Flierdt et al., 2004b). Due to the higher
Nd concentrations compared to Hf in Pacific seawater, at
least 40 ng total Nd were available for all isotopic ratio
measurements, resulting in higher precision and a better
external reproducibility of 0.2–0.4 eNd units for repeated
standard measurements (2r standard deviation during
one measuring session).
3. RESULTS AND DISCUSSION
3.1. Hafnium isotope compositions and Hf–Nd isotope
relationships
The eHf values for the Pacific waters are shown in
Fig. 3. The average eHf value is 5.9 ± 1.5 (one standard
deviation), and only 5 of the 18 samples are not within error of this value. Three exceptions are associated with
waters from 600 m water depth; low values of
eHf = 3.5 ± 1.4 at Station 1 and 3.8 ± 1.4 at Station 4,
and a high value of 8.2 ± 1.6 at Station 2. The other samples from this depth with intermediate values of 5.0 ± 1.4
(Station 3) and 5.6 ± 1.4 (Station 6) are within error identical to the low values, while the third sample with a value
of eHf = 7.2 ± 1.6 (Station 7) is indistinguishable from the
highest measured value, and so it is possible that waters at
this depth at Stations 1, 3, 4, and 6 represent a less radiogenic water mass, while waters at Stations 2 and 7 represent waters entraining more radiogenic Hf. The other
values that are resolvable from the average are the deep
waters at Station 7; eHf = 8.3 ± 1.6 at 1200 m and
eHf = 8.6 ± 1.6 at 5000 m. The latter is clearly different
from waters at that depth from Station 2, with
eHf = 4.9 ± 0.8, indicating that there is heterogeneity in
the deepest waters in this region.
The data for the deeper waters can be compared to those
from ferromanganese crusts in the region that presumably
had incorporated Hf from the immediately overlying waters
at the time of formation. David et al. (2001) reported a value of eHf = 5.4 ± 0.8 for a sample from 2600 m at a location close to Station 3 at 48o530 N, 168o060 E. This is
Fig. 3. Hf isotope data for Pacific water samples. Most samples are
within error of the average value of eHf = 5.9 ± 1.5 (the standard
deviation is marked by the grey field). Exceptions to this are the
somewhat lower values for waters from 600 m at both Stations 1
and 4, and the higher values for Station 2, 600 m, and waters from
both 1200 and 5000 m at Station 7.
indistinguishable from the average value of the waters measured here, and from the deep water samples from Station
2. van de Flierdt et al. (2004) reported that a sample with
an age of 0.18 Ma from a Mn crust found near Kamchatka,
recovered from 1800 to 1500 m at 51o280 N 167o380 E, had
eHf = 8.0 ± 0.3 (average of 2 duplicates), a value consistent
with data (when renormalized to the same standard and
CHUR values) from Godfrey et al. (1997) for Mn crusts
from the same area. The radiogenic Hf isotope value reported here for the deep water sample from Station 7 is
96
B. Zimmermann et al. / Geochimica et Cosmochimica Acta 73 (2009) 91–101
indistinguishable from this value, although it is a considerable distance away. Overall, the values measured for the
deep water samples in this study are therefore represented
by ferromanganese crusts in the region. In the absence of
data for both a crust and overlying water from the same
location, these similarities are the strongest evidence to date
that ferromanganese crusts faithfully record deep water Hf
isotope compositions.
The Hf and Nd isotope compositions of the Pacific
water samples are plotted in Fig. 4. All of the waters have
Hf isotope compositions that are more radiogenic than
crust and mantle rocks with similar Nd isotope compositions, and thus plot above the crust-mantle array. There
is a remarkable overlap between the Pacific water data
and those for Fe-Mn crusts, confirming the above conclusion that these deposits contain Hf (as well as Nd) derived
from the overlying waters.
3.2. Neodymium isotope compositions and Pacific water
masses
Neodymium isotope data for the samples are shown in
Table 1 and Fig. 5. Data for the distribution of Nd isotopes
in the northwestern Pacific have previously been published
by Amakawa et al (2004) and Piepgras and Jacobsen
(1988), at locations shown in Fig. 2. As discussed by Amakawa et al. (2004), there are significant variations in surface
waters across the Pacific, indicating that inputs from local
sources are important. The values found in this study from
10 m water depth range from low values of eNd = 2.3 to
2.7 (Stations 2, 3, and 7) to higher values of 3.9 to
4.6 (Stations 1, 4, and 6). These values overlap with those
of Amakawa et al. (2004) of 2.8 to 5.6; the lower value
was measured of the same location as Station 2 of the pres-
Fig. 4. The relationship between Hf and Nd isotope compositions
in Pacific waters. All samples have more radiogenic Hf than mantle
and crustal rocks with similar Nd isotope compositions, and plot
around the global correlation of marine Fe-Mn crust and nodules
(the seawater array), which is shown for comparison. The data for
Pacific hydrogenetic ferromanganese crusts from throughout the
ocean basin are shown as open diamonds (Godfrey et al., 1997;
Albarède et al., 1998; David et al., 2001; van de Flierdt et al.,
2004a,b); the water samples, representing the northwest Pacific,
generally fall within the range of the crusts, strongly supporting the
presumption that the Hf isotope signature in crusts represents that
of the overlying waters.
ent study, where a similar value of 2.5 ± 0.5 was found.
Farther north, at TPS 47 (between Stations 2 and 3;
Fig. 2), Piepgras and Jacobsen (1998) found 0.1 ± 1.0,
while a value of 2.7 ± 0.5 was found at Station 2 (Table
1), suggesting that there are local effects causing substantial
spatial heterogeneities. Neodymium in waters from 100 m
at all stations is isotopically indistinguishable from overlying waters (see Table 2).
Waters from 600 m depth have a narrower range of eNd
values, from 2.5 ± 0.5 to 3.7 ± 0.5, and are generally
not resolvable from one another. Therefore, no correlation
is evident between Nd isotope ratios and the Hf isotope
variations seen at this depth. These Nd isotope values are
consistent with those measured at similar depths of 2.6
and 3.7 at TPS 47 and TPS 24, respectively (Piepgras
and Jacobsen, 1988). However, Amakawa et al. (2004)
did find that waters from 500 to 700 m had increasingly
more radiogenic values for eNd further north, with
6.0 ± 0.5 and 5.3 ± 0.5 at stations LM 2 and LM 6/
11, respectively, near Station 1, and values of 3.1 ± 0.4
at LM 9 in the mixed water region and 2.9 ± 0.7 at
CM5 near Station 2 (Fig. 2). Overall, the Nd isotope compositions measured in this study are consistent with previously reported values for this depth in this region, but
define a somewhat smaller range. The Nd isotope data do
not suggest that there is a systematic difference between
locations where the NPIW is clearly identified in the Subtropical Gyre, and where it is absent in the Subarctic Gyre.
3.3. Decoupling of Hf and Nd isotopes in Pacific seawater
It might be expected that Hf isotopic variations are similarly generated by different continental inputs, although
the specific sources of the variations cannot be identified
without further direct information on the composition of
the particular potential continental sources and the distribution of Hf isotopes in different regional currents. It is
possible that the value of eHf = 3.5 ± 1.4 measured at
600 m at Station 1 represents a distinctive value for the area
of the Kuroshio Current, and water with similar sources
also contributes to waters collected at that depth at Station
4 in the mixed water region where eHf = 3.8 ± 1.4. The data
from 600 m depth from Station 2 are amongst those showing the largest difference between eNd and eHf (>10). These
are the same samples with Hf isotope compositions that can
be statistically distinguished from the rest of the samples
(see Section 3.1). Obviously, the 600 m sample of Station
2 is supplied by somewhat different sources of Hf, which
might indicate some injection of weathering signals originating from the Sea of Okhotsk to the otherwise less radiogenic Hf isotope composition at this depth at Stations 1 and
4. The hydrographic data do not show evidence for the
presence of a different water mass at 600 m at Station 2
and only indicate that this water depth is occupied by
NPIW (Measures et al., 2006). As with observations for
Nd isotopes in the North Atlantic Ocean (Lacan and
Jeandel, 2005) an exchange process with the shelves might
be responsible for a change in isotopic composition of Hf
of this water mass without altering its hydrographic
characteristics.
Hafnium isotopes in Pacific Ocean water
97
Fig. 5. Nd isotope characteristics of waters from the 6 profiles of this study. The data from Amakawa et al. (2004) for the same location as
Station 2 is shown for comparison (open circles). Waters down to a depth of 1200 m are generally within error of eNd = 3, with somewhat
lower values found in shallow waters in stations 1, 4, and 6. Lower values are also found in the deepest waters in Stations 2 and 7. Somewhat
higher values are found at 10 and 1200 m in Station 7.
Waters at 1200 m depth generally fall in the range of eNd
between 2.0 ± 0.5 and 3.2 ± 0.5, with the exception of
that from Station 1, where eNd = 0.7 ± 0.5. The somewhat higher value for eHf = 8.3 ± 1.6 found at Station 7
therefore does not correspond to a different eNd value,
and the unusual eNd value for Station 1 does not correspond
to a distinctive eHf value. The sources of Hf and Nd for
waters at this depth therefore exhibit some decoupling between the two isotopic systems.
For deeper waters at Station 2, eNd = 3.4 ± 0.5
(4000 m) and 3.8 ± 0.5 (5000 m), in agreement with the
values measured by Amakawa et al. (2004) at their CM5
location of 3.5 to 3.9. At Station 7, similar values were
found of eNd = 3.8 ± 0.5 (3500 m) and 4.8 ± 0.5
(5000 m), in agreement with the values measured by Piepgras and Jacobsen (1988) at their TPS 24 location of 3.4 to
5.0. However, an even greater range in Nd isotope compositions has been found for waters >2000 m at other locations, from the most radiogenic value of eNd = 3.0 ± 0.4
at TPS 47 (Piepgras and Jacobsen, 1988) to
eNd = 6.0 ± 0.2 at LM 9 (Amakawa et al., 2004), indicating that there is a range of Nd isotope compositions in Pa-
cific deep waters over this region. A similar lack of
homogeneity in Hf isotopes therefore is not surprising.
The two deepest data points at Station 7 also show the
largest difference between eNd and eHf (>10). The deepest
sample of Station 7 is the one that plots furthest away
from the seawater array in Fig. 4, whereas the shallower
samples of Station 7 plot close to the seawater array.
Either the deepest waters at this location have received
contributions from a source area in which the incongruent
weathering was very pronounced, which would seem unlikely or, alternatively, Nd and Hf were not supplied from
same area, such that the relatively negative Nd isotope value is explainable by relatively strong contributions from
Antarctic Bottom Water, whereas the Hf with its supposed
shorter residence time is influenced by supply areas where
weathering of highly radiogenic Pacific source rocks prevails. It has also been noted that Hf isotope compositions
in ferromanganese crusts across the Pacific suggest that
deep water becomes more radiogenic northwards, following Nd, as Antarctic Bottom Water is mixed with Hf
and Nd from young continental sources (David et al.,
2001).
98
B. Zimmermann et al. / Geochimica et Cosmochimica Acta 73 (2009) 91–101
Table 2
Hf and Nd concentrations together with salinities and potential
temperatures.
Station
Depth
[m]
Salinity
Potential
temp. (°C)
Hf conc.a
(pM)
Station 1
10
100
600
1500
10
100
600
1200
4000
5000
10
100
600
1500
10
100
600
1500
10
100
600
1500
10
100
600
1200
3500
5000
34.81
34.81
34.47
34.50
32.97
33.09
34.26
34.48
34.69
34.69
33.12
33.10
34.28
34.54
34.04
34.00
34.15
34.50
34.81
34.62
34.03
34.51
35.46
35.27
34.09
34.47
34.68
34.70
20.53
19.11
6.58
2.34
3.68
1.60
3.02
2.24
1.14
1.08
2.92
1.61
3.07
2.00
11.88
8.87
4.21
2.28
20.62
16.56
7.03
2.39
24.43
22.22
7.52
3.11
1.23
1.00
0.15
0.11
0.29
0.37
0.22
0.19
0.24
0.29
0.45
0.50
0.28
0.20
0.35
0.43
0.20
0.21
0.33
0.34
0.13
0.22
0.30
0.41
0.12
0.25
0.24
0.36
0.47
0.52
Station 2
Station 3
Station 4
Station 6
Station 7
All data is for filtered waters.
a
Errors are ±5%.
in the same area, and for depths below 3500 m found substantially higher concentrations between 0.68 and 0.92 pM
at the same location as Station 2 and at three other nearby
stations, suggesting that Hf enrichment continues toward
the seafloor due either to vertical particulate transport of
Hf, release from the pore waters of the sediments, or advection of more Hf-rich waters. However, the data in this study
do not confirm these enrichments. The profiles for Station 7
is compared to the Hf concentration profile by Firdaus
et al. (2008) for a station (35N) about midway between Stations 2 and 7, as well as the profile measured by McKelvey
and Orians (1998) much further east at 55oN, 145oW.
Again, much lower concentrations were found for the profile in this study, with greater concentrations found below
3500 m by Firdaus et al. (2008) and below 2000 m by
McKelvey and Orians (1998). Interestingly, Godfrey et al.
(1996) also found that Hf concentrations were relatively
uniform for depths below 1500 m (with one exceptionally
high concentration at mid-depth) at a site in the north
Atlantic. The reason for the discrepancy of the deep water
data with the earlier studies is unclear and only an intercalibration exercise between all involved laboratories, such as
within the international GEOTRACES programme, will be
able to provide unambiguous information on its causes.
The systematically lower Hf concentrations found for
much of the water column in this study suggests a lower
overall oceanic Hf budget and thus a shorter residence time
than previously reported. As a general consideration, David
et al. (2001) argued that the variations in present-day seawater inferred from ferromanganese crusts suggest that
Hf is not well-mixed in the ocean, and so the residence time
cannot be longer than the ocean circulation time of 1500
years. The residence time can be calculated from:
Hf
3.4. Hafnium concentrations and the oceanic Hf residence
time
Hafnium concentration profiles for all stations are shown
in Fig. 6 and exhibit a consistent and systematic behaviour.
As noted in previous work (Godfrey et al., 1997; McKelvey
and Orians, 1998; Firdaus et al., 2008), Hafnium concentrations generally increase smoothly with depth. Surface concentrations were measured for waters from 10 to 100 m.
The topmost surface samples show some variability in that
the concentrations from the stations north of 36°N apparently have systematically higher surface concentrations than
the southern ones. Although only a small effect, this could
be related to dissolution of Asian dust at the northern stations or, alternatively a more efficient removal by scavenging in the southern stations. The data of the upper 1500 m
water depth show a consistent increase from values of
0.1–0.28 pM at the surface to values between 0.29 and
0.43 pM at 1500 m and absolute concentrations agree very
well with the previously published work.
There are only two deep stations below 1500 m and all
four deep samples show a narrow range of concentrations
between 0.45 and 0.52 pM for water depths between 3500
and 5000 m. These deep water concentrations are substantially lower than previously reported. Firdaus et al. (2008)
have recently published Hf concentration data for stations
s ¼ Hf
Hf
C SW M SW
C SW
¼ Hf
3:6 104 years
water
C RW
F RW fR
C RW f R
ð1Þ
where HfCSW and HFCRW are the concentrations of Hf in seawater and river water, MSW is the total mass of seawater
(1.35 1021 kg), waterFRW is the total river discharge
(3.74 1016 kg/yr), and fR is the fraction of riverine Hf that
is not removed from the water column within estuaries and so
is added to the deep ocean basins. Significant inputs by dust
and hydrothermal vents have been discounted, at least in the
Pacific, based on mass balance arguments and the coherence
of Hf and Nd isotopes over time (van de Flierdt et al.,
2004a,b). Based on limited river data, Gaillardet et al.
(2005) lists a weighted average river concentration of
33 pM Hf. Assuming that the concentration of Hf increases
linearly from 1200 to 5000 m (Fig. 5) provides an average
concentration of 0.40 pM. Using this as an average for
seawater (and so assuming that more shallow depletions
do not represent a significant overall volume), then
Hf
CSW/HfCRW = 0.012. If there is no removal of riverine Hf
in estuaries (fR = 1), then t = 435yr. Firdaus et al. (2008) used
similar values except for an average seawater concentration
that was twice as high, and so obtained a residence time of
870 yrs. Godfrey et al. (1996) used the same high average value for seawater, but using a different set of river data (including for the Amazon River) with an average of 20 pM,
obtained a residence time of 1500 yrs. All of these estimates
Hafnium isotopes in Pacific Ocean water
99
Fig. 6. Hafnium concentration data of this study (open squares), along with salinity and potential temperature profiles (Measures et al., 2006)
for Stations 1–4, 6, and 7. Hafnium concentrations generally increase with depth. Also shown are data (solid circles) by Firdaus et al. (2008)
collected in March 2005 at the same location as Station 2 (KNOT), at a location near Station 3 at 51°N, 165°E (K1), and between Stations 2
and 7 at 35°N, 160°E (35 N). In the plot of Station 7 we also show the data of McKelvey and Orians (1998) for a station further west at 55°N,
145°W (solid squares), where higher concentrations were found at all depths. In general, there is a very good agreement between these profiles
in the upper 1200 m but at depths below 4000 m the Hf concentrations of our study are substantially lower than those found in the earlier
studies. The reason for this discrepancy is unclear (see text).
assume that fR = 1. However, there is the strong possibility of
substantial losses of Hf (Bau and Koschinsky, 2006), which
would reduce the value for fR and increase the residence time
accordingly. Taking the lower seawater concentration of
0.40 pM and an intermediate value of 27 pM for average river water, then if the maximum residence time is 1500 yrs,
fR P 0.36; that is, at least 36% of riverine Hf must reach
the deep ocean basins, in agreement with recent considerations by van de Flierdt et al. (2007). This, of course, can include release from sediments after initial losses from the
water column. Therefore, while the present data suggest a
lower average seawater value than previously used, the dom-
inant uncertainty in constraining the seawater Hf residence
time is the extent of estuarine removal.
These values can be compared with the Nd residence
time of 500 to 1000 yrs (Tachikawa et al., 1999, Tachikawa et al., 2003; Lacan and Jeandel, 2005). A tight correlation between Hf and Nd would suggest very similar
residence times between these two elements, consistent with
the shorter Hf residence time estimates. Although there is
insufficient seawater data to assess this, reasonably coherent
variations in Hf and Nd isotopes with time in the Pacific recorded in ferromanganese crusts supports this (van de Flierdt et al., 2004b). However, as discussed in Section 3.3,
100
B. Zimmermann et al. / Geochimica et Cosmochimica Acta 73 (2009) 91–101
there are some patterns suggesting some level of additional
decoupling in the relation between the two isotope systems
in the seawater data of this study, which are not seen in
samples from ferromanganese crusts because the resolution
of the time interval by sampling is 104 years,. This could
originate either from different behaviours during weathering or estuarine transport, or from a difference in oceanic
residence times, as discussed above.
4. CONCLUSIONS
The first Hf isotope data for seawater are reported here.
The relationships between Nd and Hf isotopes in northwestern Pacific waters fall within the range of values found
for hydrogenetic ferromanganese crusts in the region, confirming that Hf in the crusts has been derived from the overlying water column, which thus provide a reliable archive of
past seawater compositions. Although the seawater samples
generally fall around the global eNd–eHf correlation for
crusts, there are some significant deviations, indicating that
there is some additional decoupling between the seawater
Nd and Hf isotope signals, most likely linked to the difference in residence times. The combined use of these two isotope systems therefore provides an additional dimension for
tracing water masses and weathering inputs in the oceans.
Studies of the distribution of Hf oceanic isotope compositions that have been confined to deep water and boundary
waters as recorded in seafloor ferromanganese crusts can
now be extended and aimed at characterising the entire
present water column. Average Hf concentrations measured in this study are somewhat lower than previously reported, suggesting a shorter residence time for Hf in the
global oceans, although uncertainty in the extent of Hf removal from the water column during estuarine mixing remains a limit on how well the residence time can be defined.
ACKNOWLEDGMENTS
We thank Chris Measures, Greg Cutter, and Bill Landing for enabling large volume sampling and participation of B.Z. in the 2002
IOC Contaminant Baseline Survey cruise. Funding for this project
was provided by the Schweizer National Fonds (SNF) and the
ETH. We acknowledge Felix Oberli, C. Stirling, H. Williams, S.
Woodland, M. Rehkämper, D.-C. Lee, H. Baur, M. Maier, U. Menet, D. Niederer, B. Rütsche, and A. Süsli, for their help in keeping
the MC-ICPMS running smoothly and their support in the clean labs
and with the computers. We also thank Associate Editor Richard J.
Walker for handling the manuscript and Vincent Salters, Germain
Bayon and an anonymous reviewer for their constructive reviews.
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Associate editor: Richard J. Walker