Earth and Planetary Science Letters 201 (2002) 639^647
www.elsevier.com/locate/epsl
Glacial weathering and the hafnium isotope composition
of seawater
Tina van de Flierdt , Martin Frank, Der-Chuen Lee, Alex N. Halliday
Institute for Isotope Geology and Mineral Resources, Department of Earth Sciences, ETH-Zentrum, NO C61,
Sonneggstrasse 5, CH-8092 Zu«rich, Switzerland
Received 18 September 2001; received in revised form 30 January 2002; accepted 4 February 2002
Abstract
The Hf isotope composition of seawater does not match that expected from dissolution of bulk continental crust.
This mismatch is generally considered to be due to retention of unradiogenic Hf in resistant zircons during incomplete
weathering of continental crust. During periods of intense glacial weathering, zircons should break down more
efficiently, resulting in the release of highly unradiogenic Hf to the oceans. We test this hypothesis by comparing Nd
and Hf isotope time series obtained from NW Atlantic ferromanganese crusts. Both isotope systems show a decrease
associated with the onset of northern hemisphere glaciation. The observed changes display distinct trajectories in ONd ^
OHf space, which differ from previously reported arrays of bulk terrestrial material and seawater. Such patterns are
consistent with the release of highly unradiogenic Hf from very old zircons, facilitated by enhanced mechanical
weathering. 4 2002 Elsevier Science B.V. All rights reserved.
Keywords: Hf-177/Hf-176; zircon; sea water; isotope geochemistry; ferromanganese composition
1. Introduction
The isotope composition of Nd in pristine seawater precipitates, such as hydrogenous ferromanganese crusts, re£ects the dissolved isotope
composition of Nd in ambient seawater [1,2].
This in turn has varied in response to changes
in inputs and mixing, the evolution of which can
* Corresponding author. Tel.: +41-1-6320730;
Fax: +41-1-6321179.
E-mail address: tina.vande£[email protected]
(T. van de Flierdt).
be monitored by time-resolved sampling of crusts.
The £ux of hydrothermal Nd is negligible because
of very strong particulate scavenging of rare earth
elements near vents [3,4]. Therefore, the ultimate
sources of Nd in the oceans today are dominated
by inputs from weathering of continental crust.
Consequently, the Nd isotope composition of seawater is sensitive to the amount of detrital input
and the source provenance. The dependence on
the style of weathering is thought to be small
[5^7]. Recently some evidence has been presented
for incongruent weathering e¡ects on Nd [8^10],
but it is not yet clear whether these e¡ects are
signi¢cant for the dissolved Nd isotope composition of seawater.
Less use has been made of Hf isotopes as a
0012-821X / 02 / $ ^ see front matter 4 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 7 3 1 - 8
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tracer in seawater, because at present the inputs
are not well de¢ned and understood. The roles of
hydrothermal £uxes and incongruent weathering
are particularly unclear. The data obtained from
ferromanganese crusts/nodules and deep-sea clays
display an o¡set to higher 176 Hf/177 Hf for a given
143
Nd/144 Nd [11^15] when compared with the corresponding isotope composition of bulk continental rocks, consistent with either incongruent
weathering or hydrothermal contributions [11].
Hydrothermal crusts carry highly radiogenic
(MORB-like) Hf isotopes, indicating that hydrothermal Hf does indeed enter the ocean to some
extent [12]. However, no Hf isotope data are
available so far for seawater pro¢les or vent £uids
to test or quantify this. Furthermore, the Hf isotope compositions recorded in ferromanganese
crusts from the central Paci¢c Ocean have been
remarkably constant for the past 10 Myr [15,16]
and do not show any correspondence with independent estimates of changes in arc volcanism or
ridge activity. Therefore, the most likely cause of
the high 176 Hf/177 Hf of seawater is that the normal erosion of the continents involves incongruent weathering [14]. That is, the Hf isotope composition of river waters should be more radiogenic
relative to the bulk composition of their source
rocks.
Lu and Hf are strongly fractionated between
muds and sands due to the preferential concentration of Hf (V1% by weight) in chemically resistant zircons [6,11,17]. These detrital zircons
have low Lu/Hf and low 176 Hf/177 Hf. They reside
mainly in sands on the continents and continental
shelves and can only enter the deep oceans via
turbidites. Such zircons remain mostly intact
and do not release their unradiogenic Hf to
deep water. The more radiogenic Hf of the remaining (non-zircon) fraction in the source rocks
is incorporated in less resistant minerals. Therefore, Hf is probably preferentially supplied to the
oceans either as suspended ¢ne particulates or in
dissolved form [6,17]. Because of analytical di⁄culties there are no direct river and seawater Hf
isotope measurements with which these theories
could be tested. However, by using ferromanganese crusts one can study the changes in seawater
Hf isotope composition that took place as a func-
tion of time adjacent to continental regions that
underwent a big change in weathering style.
If the retention of zircon on the shelf areas is
responsible for the o¡set of OHf in seawater, a
change in the style of weathering (more intense
mechanical weathering) should result in a more
e⁄cient breakdown of zircons and consequently
larger amounts of unradiogenic Hf would be released to the ocean. Therefore during times of
intense physical erosion such as during glacial
weathering the Hf isotope composition of seawater should be driven towards the bulk rock
composition and lead to seawater Hf and Nd isotope compositions that track bulk continental
crust more closely. Here we test this model by
using isotope records of ferromanganese crusts
from the NW Atlantic, proximal to the areas
where drastic changes in weathering regime accompanied the onset of northern hemisphere glaciation (NHG).
2. Samples and methods
Ferromanganese crusts ALV539 and BM1969.05 were dredged from the New England
Seamount and the San Pablo Seamount (35‡36PN,
58‡47PW, V2700 m depth and 39‡00PN, 60‡57PW,
V1800 m depth, respectively) approximately
120 km SE of the edge of the North American
continental shelf. Both locations are close to the
pathway of the Deep Western Boundary Current,
which is the southern extension of North Atlantic
Deep Water (NADW). The isotope compositions
for Be, Pb, Nd and Hf of both crusts have already
been studied intensively [14,18^21]. To complete
the data set we performed additional Hf isotope
analysis for BM1969.05 because the upper 2 Myr
were missing in the previous work [14], as deduced from Nd isotope measurements on the
same aliquots. Nine samples from the surface
down to 5 mm depth (3.1 Ma) were analysed
for their Hf isotope composition (Table 1). The
sampling was done using a computer-controlled
drilling device. Between 10 and 15 mg of materials
were used in each analysis. Chemical separation
of Hf followed the two-column procedure of Lee
et al. [16]. Measurements were carried out on a
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T. van de Flierdt et al. / Earth and Planetary Science Letters 201 (2002) 639^647
Nu Plasma MC-ICPMS. Repeated analyses of the
JMC475 Hf standard gave an average 176 Hf/177 Hf
of 0.282139 P 18 (n = 56). The integrity of our time
series was checked by measuring Pb isotopes in
the uppermost sample (0^0.5 mm). The 206 Pb/
204
Pb of 19.233 P 0.002 obtained this way shows
good agreement with the previously reported
ratio for the surface of this crust (206 Pb/
204
Pb = 19.240 P 0.004 [21]).
3. Discussion
3.1. Previous work
There is still controversy about the cause of the
major changes in Pb, Nd and Hf isotopes in the
NW Atlantic seawater. A general decline in 207 Pb/
206
Pb, ONd and OHf , all starting around 3^2 Ma,
occurs in the records of both crusts. Possible explanations include major palaeoceanographic
changes caused by the closure of the Panama
Gateway, intensi¢cation of NADW £ow, changes
in water mass contributions (higher Labrador
Seawater component in NADW), and changes in
the amount and/or provenance of material delivered to the North Atlantic Ocean [10,14,18,20^
25].
Table 1
Hf isotope time series for the uppermost part of BM1969.05
Depth
(mm)
Agea
(Ma)
176
Hf/177 Hfb
P 2c S.E.
OHf ð0Þ c
P 2c S.E.
0.0-0.5
0.5-1.0
1.0-1.5
1.5-2.0
2.0-2.5
2.5-3.0
3.0-3.5
3.5-4.0
4.0-5.0
0.15
0.46
0.77
1.08
1.39
1.70
2.01
2.31
2.78
0.282823 P 22
0.282841 P 16
0.282826 P 14
0.282866 P 18
0.282890 P 22
0.282898 P 32
0.282841 P 18
0.282867 P 10
0.282855 P 12
1.80 P 0.76
2.45 P 0.59
1.93 P 0.50
3.33 P 0.65
4.16 P 0.79
4.44 P 1.15
2.42 P 0.63
3.34 P 0.37
2.95 P 0.45
a
Be ages calculated with growth rate of 1.62 P 0.10 mm/Ma
de¢ned by 10 Be/9 Be ratios [19].
b
Repeated analyses of the JMC475 Hf standard gave an
average ratio 176 Hf/177 Hf of 0.282139 P 18 (n = 56); to compare the values with Hf isotopic measurements of [14] all ratios were normalised to their reported JMC475 value of
0.282161 P 9.
c
Calculated with 176 Hf/177 HfCHUR = 0.282772 [47].
641
The scope of our study is not to provide a solution for the (mis)matches in the variations of
Pb, Nd and Hf isotope records but to further
investigate the potential of combined Hf and Nd
isotopes in ferromanganese crusts to trace changes
in the continental weathering style [13,14].
Vervoort et al. [17,26] showed that Hf and Nd
isotopes for nearly all terrestrial samples (oceanic
basalts, sediments, continental basalts, granitoids
and juvenile crustal rocks) form a single array
(Fig. 1). To a ¢rst-order approximation, Hf and
Nd isotope systems behave coherently in the crust
and the mantle [26]. Subsequent variations within
the array are due to Lu^Hf and Sm^Nd fractionations in certain crust and mantle reservoirs but
none of these rather local features results in largescale decoupling of the Hf and Nd systems [26].
Generally speaking, Hf and Nd isotopes in old,
crustal rocks (post 2.7 Ga) with low Sm/Nd and
Lu/Hf ratios plot at the lower (less radiogenic)
end of the terrestrial array while young and juvenile rocks with high Sm/Nd and Lu/Hf ratios plot
at the upper (radiogenic) end of the terrestrial
array.
Albare'de et al. [13] have shown that seawater
Nd and Hf isotope compositions are roughly correlated along an array that is distinct from the
terrestrial array (Fig. 1). This so-called seawater
array is de¢ned by surface scrapings of ferromanganese crusts and nodules from the Paci¢c,
Indian and Atlantic oceans (Fig. 1). The residence
time of both Nd and Hf is on the order of or
slightly shorter than the global ocean mixing
time [15,16,27^32]. Therefore, a general provinciality between the three deep ocean basins can
be observed. The samples of the more radiogenic
upper part of the seawater array are from the
Paci¢c Ocean and the least radiogenic values of
the lower part are from the North Atlantic Ocean
[13] (Fig. 1). This feature is a simple function of
the age of the surrounding continents/source
areas. The Atlantic Ocean is mainly sourced by
Proterozoic and Archaean cratons in Canada and
Greenland whereas the Paci¢c Ocean is rather
dominated by young, arc-derived material. The
o¡set in OHf to more radiogenic values compared
to the values of the terrestrial array is relatively
small for the Paci¢c Ocean but increases towards
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T. van de Flierdt et al. / Earth and Planetary Science Letters 201 (2002) 639^647
Fig. 1. Plot of ONd vs OHf . (a) Terrestrial samples (¢lled triangles: mantle rocks, ¢lled squares: crustal rocks) compiled in [15];
crust and nodule data (open circles: Atlantic Ocean, open triangles: Indian Ocean, open squares: Paci¢c Ocean) after [12^15];
seawater array (solid line) after [13]; terrestrial array (solid line) after [17]. Data for ALV539 are shown as ¢lled diamonds
[14,20]. Data for BM1969.05 are shown as open diamonds [14,20]. Data for ALV539 and BM1969.05 are not time corrected. Arrows indicate the expected evolution of the isotope composition of seawater following increased erosional input (horizontal) and
mechanical weathering causing breakdown of zircon minerals (vertical). (b) Blow up of ALV539 data and (c) BM1969.05 data
(black and grey diamonds). Labelled times correspond to the black diamonds.
the Atlantic values. This decoupling of Nd and Hf
isotopes in seawater is thought to be caused by
the retention and preservation of zircon on the
shelf areas. As a ¢rst approximation, the seawater
array can be regarded as a zircon-de¢cient equivalent of the terrestrial array.
New and published data for BM1969.05 and
ALV539 time series are shown in Fig. 1, where
they plot at the least radiogenic end of the seawater array. They are among the most extreme
examples showing the decoupling between Hf
and Nd isotopes and de¢ne distinct curved trends
within the seawater array (Fig. 1b,c).
3.2. Changes in the weathering style
The pronounced curvatures of the NW Atlantic
ferromanganese crust time series (Fig. 1b,c) can
be explained by a change in weathering style.
The records start with a relatively radiogenic Hf
isotope composition that would correspond to the
‘normal’ weathering e¡ect of a large portion of Hf
released from minerals that are not very resistant.
There is a decrease in ONd in crust ALV539 by
about 1 O unit between 3.5 and 2.1 Ma. The Hf
isotopes show little change during the same period
(Fig. 1b). Furthermore, no systematic relation-
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T. van de Flierdt et al. / Earth and Planetary Science Letters 201 (2002) 639^647
ships can be observed between ONd and OHf prior
to 1.7 Ma for BM1969.05 (Fig. 1c). However,
between about 2 Ma and the present both crusts
show a distinct decrease in OHf by 2.5^4 O units,
while ONd during that same time interval only decreased by V0.5 O units. This timing is similar to
changes of other climate indicators. For example,
there is a controversial debate on deep ocean circulation changes associated with the onset of
NHG. Several arguments have been presented,
ranging from a general increase in NADW production [33,34], or an increase in Labrador Sea
Water contribution [25], to the concept of weaker
NADW production [35,36]. However, it is very
unlikely that the decrease in Hf isotopes in NW
Atlantic seawater, starting V2 Ma, is a result of
ocean circulation changes, because in this case it
should have started simultaneously with the
changes in Nd and Pb isotopes.
The beginning of increased N18 O variations and
the ¢rst major occurrence of ice-rafted detritus in
the North Atlantic was at 2.5^2.4 Ma [37,38], and
an increase in dropstone abundances in sediment
cores from the Arctic Ocean started around 1.7
Ma [39]. This correspondence with the timing of
the change in Hf isotopes provides support for the
hypothesis that northern hemisphere ice growth
and the accompanying changes in weathering
style were responsible for the changes in Hf isotopes in ALV539 and BM1969.05.
In accordance with Piotrowski et al. [14], we
suggest that more intense mechanical weathering
has facilitated more e⁄cient breakdown of usually
very resistant zircons. Although glaciers do not
in£uence the total chemical denudation rates,
they may yield compositionally distinctive chemical £uxes to the oceans. Two important factors
may be: (1) the disruption of mineral lattices by
grinding which increases dissolution rates and (2)
the increase of surface area of mechanically destroyed minerals [40]. These processes make it
very likely that glacial sediments are more easily
weathered. It is important to mention that physical erosion and chemical weathering are interrelated processes, which means that physical erosion
is the essential precondition to make trace elements in robust minerals, such as Hf in zircons,
chemically accessible. In contrast, studies on the
643
surface chemistry of zircons weathered over a
long time in tropical areas (e.g. Amazon basin)
suggest that these zircon grains are very robust
to pure chemical weathering [41].
The more complete breakdown of zircons
would greatly a¡ect the Hf isotope composition
of continental runo¡ and of seawater without a
signi¢cant change in Nd isotopes. This is because
zircons contain the majority of the Hf budget in a
rock (about 1% or 104 ppm Hf in zircon) but
much less Lu (usually 10^100 ppm) and other
rare earth elements. If also Nd would have been
a¡ected by incongruent weathering to any large
extent, the isotope time series should not show
the observed curvature during the past 3 Myr
(Fig. 1b,c). The trend in Nd isotopes can be better
explained by increased net erosional inputs from
the old surrounding continents [21,24,25].
The Hf isotope composition of old zircons derived from the continental crust is extremely unradiogenic. Isotope measurements in 3.6^3.8 Ga
old single zircon grains from Greenland and
north-western Canada yielded OHf values as low
as 388 [26,42]. The more complete destruction of
the old resistant zircons by glacial weathering entails a change from incongruent weathering prior
to V2 Ma (release of radiogenic Hf from easily
weatherable minerals solely) to more congruent
weathering thereafter (release of a more bulk
rock-like Hf isotope composition to the oceans
because Hf from zircons is partly included in the
dissolved material). Hence the data of the crusts
are driven away from the seawater array towards
the bulk crust data array (Fig. 1).
3.3. Changes in the source terrain
Another interpretation that may be considered
is that the changes in Hf isotope composition,
observed in NW Atlantic ferromanganese crust
time series, re£ect changes in the source terrain.
To ¢t the observed data in ALV539 and
BM1969.05, the new source area would have to
be characterised by lower (less radiogenic) ONd
values and lower (less radiogenic) OHf values as
is typical for old continental crust. The problem
with this interpretation lies in the amplitude of
changes in Nd and Hf isotopes. It is evident
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T. van de Flierdt et al. / Earth and Planetary Science Letters 201 (2002) 639^647
from the less steep slope of the seawater array
(Fig. 1) that the bulk of the materials delivered
to the ocean shows the tendency to be more radiogenic in Hf relative to Nd when compared with
typical crustal materials (Fig. 1). While terrestrial
rocks display a range in ONd of V30 O units and
a range in Hf isotopes of V50 O units
(Nd:HfW1:2), manganese crusts and nodules
exhibit only a range of V10 O units, both in Nd
and in Hf isotopes (Nd:HfW1:1). Both arrays in
Fig. 1 are a function of age and lithology. While
the terrestrial array represents the whole range
of mantle and crustal rocks, the seawater array
is the equivalent to the zircon-free part of the
same rock suite. The smaller degree of overall
variation in ONd and OHf in seawater is due to
homogenisation in the oceans. Obviously, if the
weathering and erosion processes have remained
the same, any changes towards an older source
terrain should result in a shift of the data on
the seawater array towards lower ONd and OHf values. However, the Nd isotopes in both crusts
show continuous shifts towards lower values
(0.5^1 O unit changes during the past 2 Ma Fig.
1b,c) but are accompanied by much more drastic
shifts in the Hf isotopes (3^4 O units in the same
time interval, Fig. 1b,c), which is not expected
from a simple change of source regions. Solely
changing the source area towards old continental
crust without the accompanying change in the
weathering mechanism is unlikely to be responsible for the observation in these two ferromanganese crusts.
Another important point to mention is that the
radiogenic isotope compositions of BM1969.05
and ALV539 may have been derived from di¡erent water masses. While ALV539 (2700 m water
depth) de¢nitely grew from NADW or a precursor of it, BM1969.05 may have been sourced by a
di¡erent (intermediate) water mass (1800 m water
depth). This might also explain the observed difference in the ¢ne structure of the two time series
[20]. The Hf data of BM1969.05 are overall more
radiogenic than those of ALV539 and a clear
trend prior to 1.7 Ma is missing. However, besides
these di¡erences there is remarkable agreement
between the Hf, Nd (and Pb) isotope trends since
the onset of NHG. These similar trends point to a
Fig. 2. Lead isotope correlation plot for ALV539 and
BM1969.05 showing apparent intercept ages of radiogenic
components. Data are taken from [21].
common process, e.g. a change in the style of
weathering associated with the onset of enhanced
physical erosion as described above. Such fundamental changes are likely to be visible in every
water mass sourced in the regions of Canada
and Greenland and should consequently be monitored by both crusts even if they grew from mixtures of di¡erent water masses.
3.4. Evidence from Pb isotopes
Assuming that the breakdown of zircon minerals plays a key role in controlling the Hf isotope
composition of ALV539 and BM1969.05, we can
do some simple model calculations for the zirconderived dissolved Pb component in seawater to
check our hypothesis.
Incongruent weathering also a¡ects Pb isotopes, but in a completely di¡erent way. The ¢rst
Pb fraction released during the initial stage of
weathering of a fresh granitic rock has higher
206
Pb/204 Pb, higher 208 Pb/204 Pb and lower 207 Pb/
206
Pb than its bulk composition [10,43,44]. This
is because 206 Pb and also 208 Pb are loosely bound
in U- and Th-rich minerals and grain boundaries
due to radiation damage originating from the decay of 238 U and 232 Th. Intense physical erosion
during glacial periods causes increased exposure
of fresh, previously unweathered rock surfaces
and thus favours the release of radiogenic Pb. It
has been inferred by von Blanckenburg and Na«-
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gler [10] that this is the main reason for the drastic mismatch between the Pb isotope composition
of deep water in the Northern Atlantic (presentday NADW has a 206 Pb/204 Pb v 19 [22]) and that
of the nearby continental landmasses (Archaean
crust from Labrador, Greenland, and the Proterozoic parts of Greenland show bulk 206 Pb/204 Pb
between 13 and 16 [39]). If the changes in Pb
isotope composition over the past few million
years re£ect the breakdown of minerals enriched
in U such as zircon, we can calculate the average
age of these zircons. Zircon incorporates almost
no common lead and therefore its 204 Pb/206 Pb ratio is e¡ectively zero. The correlation between
207
Pb/206 Pb and 204 Pb/206 Pb for ALV539 (Fig. 2,
MSWD for the linear ¢t: 0.44) de¢nes a 207 Pb/
206
Pb intercept at zero 204 Pb/206 Pb that corresponds to an age of 1.68þ0:17
30:20 Ga. This age is
probably biased to low values because the dominant leachable Pb in crustal rocks is labile 206 Pb
from U-rich lattice damaged sites. However, even
as a minimum value it is consistent with the presence of large outcrops of Precambrian crust in the
area bordering the North Atlantic. The data for
crust BM1969.05 (Fig. 2, MSWD = 0.40) do not
de¢ne such a precise intercept, but the resulting
age, 1.30þ0:31
30:39 Ga, is similar.
One can take this a step further and calculate
the OHf value of a hypothetical ‘pure zircon component’. There have certainly been sources other
than zircons contributing to the dissolved Hf and
Pb in the North Atlantic but nevertheless this
value is a useful semi-quantitative test of the hypothesis that the changes in Hf isotopes in seawater have indeed been dominated by zircon destruction. Linear extrapolation of the OHf vs 204 Pb/
206
Pb plot of ALV539 to 204 Pb/206 Pb = 0 yields an
OHf value of 3100 P 20 (MSWD = 1.43) if the
uppermost 2.25 mm (0.75 Myr) are omitted
(Fig. 3). This youngest portion shows no clear
relationship between Hf and Pb isotopes, probably due to the complex processes involved in
the more intense breakdown of the zircons themselves and other silicates, rather than just the initial release of loosely bound radiogenic Pb. The
OHf of 3100 P 20 is similar to reported Hf isotope
measurements in single grains of zircon from
Greenland (3.81 Ga, OHf (0) = 383 to 387 [26])
645
Fig. 3. Plot of OHf vs 204 Pb/206 Pb for ALV539 showing the
extrapolated Hf isotope composition of a hypothetical ‘pure
zircon component’. Data are taken from [14,21].
and north-western Canada (3.60 Ga, OHf (0) =
382 to 388 [42]) and is consistent with an Archaean crustal residence age.
4. Concluding remarks
The presented lines of evidence support the
conclusion that the o¡set in Hf relative to Nd
isotope composition in seawater is caused by incomplete breakdown of zircon, an e¡ect that is
lessened during more intense physical erosion.
Average continental crust of old landmasses
such as the Canadian shield or Greenland, which
are the main source regions for detrital supply to
the NW Atlantic Ocean, host a large amount of
very unradiogenic Hf, which is released to the
ocean mainly during periods of intense mechanical weathering such as NHG. Therefore, the combination of Nd and Hf isotopes in seawater at
locations which are directly sourced by continental regions should be a powerful tool to assess
changes in style and intensity of continental
weathering in the past, which have been related
to major climatic changes.
However, with our restricted data set we cannot
ultimately rule out the possibility that the trends
observed in ALV539 and BM1969.05 are ^ at
least partly ^ due to changes in the source areas.
Further work on other suitable locations or on
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other promising weathering tracers (e.g. Os isotopes [45,46]) is necessary to demonstrate conclusively that incongruent weathering of Hf is responsible for the observed decoupling of Hf and
Nd in NW Atlantic deep water.
Acknowledgements
We are very grateful to Alex Piotrowski and
Friedhelm von Blanckenburg for discussion and
comments on an earlier version of this paper.
The ¢nal version of this paper greatly bene¢ted
from thorough and constructive reviews of Bernhard Peucker-Ehrenbrink and an anonymous reviewer. Yoshihiro Asahara is thanked for assistance with the Hf chemistry. This research was
supported by the Swiss National Fonds and the
ETH.[BARD]
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