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Earth and Planetary Science Letters 158 (1998) 121–130
Sources of Pb for Indian Ocean ferromanganese crusts:
a record of Himalayan erosion?
M. Frank Ł , R.K. O’Nions
Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
Received 4 December 1997; accepted 17 March 1998
Abstract
A high resolution Pb isotope time-series for the last 26 Ma, dated by 10 Be=9 Be chronology, is reported for a north
Indian Ocean ferromanganese crust. This record is compared with available Pb isotope time-series of six other crusts
from the Atlantic, Indian and Pacific Oceans, each of which is based on 10 Be=9 Be chronology. The seven Pb isotope
records reveal some remarkable features. In contrast to the Nd isotope time-series of these crusts which show a long-term
(¾60 Ma) provinciality between the three main ocean basins, the Pb isotopes only show comparable provinciality over
the last ¾5 Ma. Prior to about 15 Ma ago no distinct Indian Ocean Pb isotope signal existed. Within this established
framework of Pb isotope distribution in the oceans the 208 Pb=206 Pb data for the north Indian Ocean crust reported here
are anomalous. The 208 Pb=206 Pb ratio is particularly high and exceeds a value of 2.08 during the time interval from 20
to 8 Ma ago. Consideration of potential sources of Pb in the Indian Ocean which might provide such high 208 Pb=206 Pb
ratios suggests that this crust most probably has recorded a time-varying erosional input of Pb from the Himalayas. The
timing of the isotopic shift is in good agreement with maximum Himalayan exhumation rates deduced from crystallisation
and cooling ages of synorogenic granites (20–14 Ma) and the sedimentation history of the Bengal Fan.  1998 Elsevier
Science B.V. All rights reserved.
Keywords: Himalayan Orogeny; erosion; Pb-208=Pb-206; ferromanganese composition; crust; Indian Ocean
1. Introduction
Ferromanganese crusts (hereafter called crusts)
scavenge and incorporate trace metals from ambient
seawater and thereby record its isotopic composition
with respect to Nd and Pb [1–10]. Nd and Pb isotope profiles of crusts, dated using 10 Be=9 Be ratios,
make it possible to relate isotopic variations in sea
water at a particular location to changes in the supply and distribution of Nd and Pb in the oceans. It
Ł Corresponding author. Tel.: C44 1865 272055; Fax: C44 1865
272072; E-mail: [email protected]
is now evident that the present-day provinciality of
eNd values in the three main ocean basins has been
a persistent feature over the last ¾60 Ma [10] and
Nd isotope stratification in the Pacific Ocean has
existed for ¾30 Ma [6] despite the major paleogeographic changes that have occurred over this period
of time. These include, for example, the closing of
the Panama Gateway to deep-water circulation starting from about 12 to 8 Ma ago [11] to its final
closure about 3 Ma ago [12], and the opening of
the Drake Passage about 23 Ma ago [13]. Any variations arising from changes in ocean circulation are
superimposed on the long-term eNd signature typical
0012-821X/98/$19.00  1998 Elsevier Science B.V. All rights reserved.
PII S 0 0 1 2 - 8 2 1 X ( 9 8 ) 0 0 0 5 5 - 7
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M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130
for each ocean basin. Notable amongst these changes
are the decrease in eNd during the last about 5 Ma
in the western North Atlantic Ocean and the difference in eNd between the Atlantic and the Pacific
Oceans which has increased continuously from about
6 eNd prior to 20 Ma ago to about 9–10 eNd in the
present-day ocean [10].
On a global scale, changes in rates of erosion
such as those accompanying the main period of Himalayan exhumation, which started about 24 Ma
ago [14], have been considered a possible cause for
the long-term increase in the marine 87 Sr=86 Sr ratio
(cf. Ref. [15]). However, in marked contrast, it has
been shown recently that Himalayan erosion products have not had a significant impact on either the
global nor local Indian Ocean deep-water budget of
Nd [10]. There is much less information available on
Pb isotope input into the oceans, but the pre-anthropogenic distribution of Pb isotopes (of the last about
300 ka) is well-known and also shows a clear interocean provinciality [3,4]. The distinction between
the three ocean basins with respect to Pb isotopes
might be expected to have been even greater than
for Nd over the last 60 Ma because of the shorter
average oceanic residence time of Pb (about 80–100
years [16,17]), compared to Nd (about 1000 years
[1,18,19]), but as shown below this was not the case.
In this study the Pb isotope time-series of crust
SS-663 (northern Indian Ocean) and six other crusts
for which Nd, Pb and Be isotope profiles have been
published previously [6–8,10], provide new constraints on the Pb isotopic evolution of the Atlantic,
Indian and Pacific Oceans over the last ¾60 Ma. The
results of this study suggest that material supplied by
erosion of the Himalayas has had a major influence
on the Pb isotope composition of northern Indian
Ocean deep-water during the last about 26 Ma.
Table 1
Results of the 10 Be=9 Be measurements and growth rate determinations for crust SS-663
Depth (mm)
10 Be=9 Be
Surface
0.5–1.2
3.5–4.5
7.0–8.5
14.7–16.0
22.0–23.5
31.5–33.0
8:75 š 0:70
7:86 š 0:27
4:35 š 0:14
2:60 š 0:10
0:70 š 0:04
0:28 š 0:03
0:04 š 0:02
(1E-8)
Age (Ma)
0.00
0:23 š 0:01
1:52 š 0:05
2:64 š 0:10
5:49 š 0:32
7:52 š 0:75
11:67 š 4:73
the initial 10 Be=9 Be ratio at the growth surface of
the crust has remained constant. This is in good
agreement with growth rates between 2.2 and 3.4
mm Ma 1 derived from decay profiles of 230 Thexcess
and 230 Thexcess =232 Th of the uppermost 2 mm of this
crust [22]. A high-resolution (37 sample) Pb isotope profile has been obtained from the outer 56
mm of the crust using TIMS techniques described
2. Results
10
Be=9 Be ratios have been determined on a
depth profile comprising seven samples of ferromanganese crust SS-663 from the northern Indian
Ocean (12º570 S, 76º060 E, water depth 5250 m) using procedures established on the Oxford ISOLAB
[20,21] (Table 1). An average growth rate of 2:8š0:1
mm Ma 1 (Fig. 1) has been estimated assuming that
Fig. 1. 10 Be=9 Be ratios in crust SS-663 from the northern Indian
Ocean versus depth beneath the growth surface. The ages are
calculated with a half life for 10 Be of 1.5 Ma.
M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130
123
Table 2
Pb isotope results of crust SS-663
Depth (mm)
Age (Ma) a
206 Pb=204 Pb b
207 Pb=204 Pb b
208 Pb=204 Pb b
0.0–0.4
1.1–1.5
2.0–2.4
2.6–3.0
3.4–3.8
3.4–3.8 c
4.4–4.8
5.5–5.9
6.7–7.1
6.7–7.1 d
8.2–8.6
10.0–10.4
11.3–11.7
12.7–13.1
12.6–13.0 d
14.8–15.2
15.9–16.3
17.0–17.4
18.0–18.4
19.5–19.9
19.9–20.3
21.0–21.4
22.1–22.5
23.2–23.6
24.5–24.9
25.6–26.0
28.3–28.7
30.0–30.4
32.2–32.8
33.8–34.2
34.9–35.3
36.6–37.0
38.9–39.3
41.5–41.9
47.8–48.2
50.7–51.3
55.0–56.0
0.0
0.5
0.8
1.0
1.3
1.3
1.6
2.0
2.5
2.5
3.0
3.6
4.1
4.6
4.6
5.4
5.8
6.1
6.5
7.0
7.2
7.6
8.0
8.4
8.8
9.2
10.2
10.8
11.6
12.1
12.5
13.1
14.0
14.9
17.1
18.2
19.8
18.881
18.903
18.904
18.879
18.912
18.902
18.887
18.864
18.785
18.859
18.853
18.828
18.831
18.880
18.851
18.837
18.815
18.813
18.784
18.764
18.779
18.749
18.777
18.763
18.733
18.746
18.773
18.769
18.787
18.723
18.756
18.749
18.676
18.638
18.616
18.621
18.655
15.712
15.715
15.716
15.690
15.723
15.714
15.718
15.696
15.679
15.693
15.698
15.673
15.685
15.735
15.699
15.710
15.700
15.707
15.679
15.669
15.691
15.677
15.721
15.712
15.675
15.686
15.701
15.702
15.731
15.702
15.708
15.730
15.675
15.673
15.653
15.663
15.664
39.167
39.160
39.173
39.093
39.192
39.168
39.174
39.096
38.992
39.095
39.117
39.034
39.076
39.240
39.115
39.139
39.101
39.126
39.045
39.000
39.056
39.003
39.148
39.120
39.001
39.025
39.084
39.091
39.182
39.047
39.101
39.135
38.926
38.877
38.776
38.808
38.851
a Ages are calculated by extrapolation of growth rates defined by 10 Be=9 Be ratios in the outer part of the crust. Ages in italics were
estimated from extrapolation of the 10 Be=9 Be-derived growth rates beyond 12 Ma.
b Repeated measurements of the NIST SRM981 Pb standard during the period of this study yields 206 Pb=204 Pb D 16:902 š 0:019,
207 Pb=204 Pb D 15:451 š 0:023, 208 Pb=204 Pb D 36:576 š 0:070 .n D 14/. Errors represent the external 2σ standard deviation and Pb
isotope ratios have been adjusted relative to the accepted value [41]. The 2σ errors of repeat measurements of samples during the period
of this study were better than for the repeated standard measurements (š0.012 for 206 Pb=204 Pb, š0.015 for 207 Pb=204 Pb and š0.044 for
208 Pb=204 Pb; n D 18) but nevertheless the external 2σ error was applied.
c Repeat measurement of the same sample solution.
d Repeat measurements of sample aliquots.
previously [23,24]. The high-resolution Pb isotope
profile (Table 2, Fig. 2) is in very good agreement
with a published low-resolution profile for the same
crust [10] and has been extended using the low-
ermost three samples from this source. Assuming
that crust SS-663 has grown at the rate defined by
the 10 Be=9 Be ratios over the entire profile, then the
Pb isotope profile in Fig. 2 has recorded the com-
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M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130
position of Indian Ocean deep-water from 26 Ma
ago until present. The application of a modified Coconstant flux-based dating technique for this crust
[Frank et al., in prep.] shows good agreement with
the 10 Be=9 Be-based results for the last 10 Ma, but
suggests that the growth rate before 10 Ma ago was
somewhat lower, which results in an age of about 30
Ma for the base of the crust.
The Pb isotope variations for crust SS-663 are displayed in Fig. 2 in terms of 206 Pb=204 Pb, 207 Pb=206 Pb,
and 208 Pb=206 Pb ratios together with the Pb isotope
time-series available for other crusts [6–8,10]. A particular and outstanding feature of crust SS-663 is the
large increase of the 208 Pb=206 Pb ratio from the base
of the crust to a broad peak between 20 and 8 Ma
years ago with a maximum ratio of 2.088 at about 13
Ma ago followed by a general decrease until present.
3. Discussion
3.1. Mixing of Nd and Pb in the ocean
The Pb isotope time-series available for six crusts
from the Atlantic, Indian and Pacific Oceans, each
based on their respective 10 Be=9 Be chronologies, are
compared with crust SS-663 in Fig. 2. In contrast to
the Nd isotope time-series obtained for these crusts
[6,7,10], the Pb isotope profiles show much less
provinciality over the last 60 Ma. This is, perhaps,
contrary to expectations from residence times of
Nd and Pb in the oceans. A similar observation
was made for crusts from the Pacific Ocean, where
the Pb isotopic composition of surface scrapings
have a remarkably low variance compared to that
of the compositions of the surrounding continental
and island arc rocks [4]. If there is no other, and
unaccounted for, source of Pb which determines
the low variance of Pb isotope composition in the
125
Table 3
Correlation coefficients between eNd and Pb isotopic ratios in
crusts over the last 30 Ma
Crust
BM1969.05
ALV539
109D-C
SS663
VA13=2
CD29-2
D11-1
206 Pb=204 Pb
0.90
0.93
0.69
0.81
0.54
0.94
0.87
207 Pb=206 Pb
0.89
0.92
0.64
0.79
0.66
0.70
0.92
208 Pb=206 Pb
0.77
0.87
0.39
0.52
0.27
0.23
0.59
The values of crusts CD29-2 and D11-1 only represent the
unphosphatized sections. Bold face values are significant at the
99% level.
Pacific, then these data imply that an efficient mixing
process must exist for Pb in the Pacific; it may also
suggest that the average residence time of Pb in the
oceans is longer than previously thought.
A longer residence time for Pb would imply a
larger advective length scale within the thermohaline circulation. Comparison of Nd and Pb isotope
variations in crusts lends some support to this view.
From the 30 Ma isotope records of the seven crusts
available, the correlation coefficient between eNd and
208,207,206
Pb=204 Pb changes sign when going from
the North Atlantic through the Indian Ocean into
the Pacific (Table 3, Fig. 3). In the two North Atlantic crusts, eNd and 206 Pb=204 Pb show a strong
negative correlation and the opposite is true for eNd
and the 208 Pb=206 Pb and 207 Pb=206 Pb ratios. These
relationships reflect the variability of the input of
radiogenic Pb and unradiogenic Nd from continental
North America into the North Atlantic. Crust 109
D-C from the Indian sector of the Southern Ocean
shows the same pattern of correlations, albeit at a
lower statistical significance. These characteristics
may be inherited from the influence of North Atlantic Deep-water (NADW) which is mixed with
Fig. 2. Comparison of (a) 206 Pb=204 Pb, (b) 207 Pb=206 Pb, and (c) 208 Pb=206 Pb ratios of ferromanganese crusts with ages based on
10 Be=9 Be chronology. The North Atlantic crusts have diamond symbols [7,10], the southern Indian Ocean crust open squares [10],
northern Indian Ocean crust closed squares and bold lines, and deep Pacific crust open circles [6]. In (a) the filled circles mark the data
of the Pacific seamount crusts [6] and in (b) and (c) the filled circles mark the low-resolution data of sections older than 32 Ma of the
Pacific seamount crusts [6]. Those profiles without any symbols represent the high-resolution records of the two Pacific seamount crusts
[8]. The shaded area highlights the last 5 Ma which is the time over which separate Atlantic, Indian and Pacific Ocean signatures of the
Pb isotopes in the water column have existed. The lightly shaded area in (c) indicates a period of maximum 208 Pb=206 Pb ratios in the
northern Indian and Pacific crusts (20–8 Ma ago).
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M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130
servations support the notion that the Pb isotopic signal produced in the North Atlantic is traceable along
the path of the thermohaline circulation through the
Atlantic Ocean [4] at least into the Indian sector of
the Southern Ocean [3]. In the Pacific Ocean input of
material with an unradiogenic Pb and radiogenic Nd
isotopic composition has exerted a different control
to that observed in the Atlantic Ocean.
3.2. Pb isotopic evolution of the ocean
Fig. 3. 206 Pb=204 Pb vs. eNd and 207 Pb=206 Pb vs. eNd for all seven
crust profiles. Sources for the Pb isotope data and symbols are
the same as in Fig. 2.
Circum Polar Deep-water (CDW) on its way to the
location of crust 109 D-C. In contrast, at the location of crust VA13=2, which grew from deep-water
in the equatorial Pacific, the sign of the correlation
coefficients changes. There is a weak positive correlation between eNd and 206 Pb=204 Pb and either a
weak negative or no correlation with 207 Pb=206 Pb
and 208 Pb=206 Pb ratios, respectively. The two intermediate water depth records in the equatorial Pacific
(CD29-2 and D11-1) display a strong positive correlation between eNd and 206 Pb=204 Pb and a significant
negative correlation between eNd and 207 Pb=206 Pb.
The correlation between eNd and the 208 Pb=206 Pb ratio is not significant. In the northern Indian Ocean
crust SS-663 the correlation pattern of the Nd and Pb
isotopes is similar to that of the intermediate water
depth records from the equatorial Pacific. These ob-
In contrast to the distinctive eNd distribution
between the Atlantic and Indian Oceans [10],
206
Pb=204 Pb and 207 Pb=206 Pb have been distinct only
from ¾5 Ma ago (Fig. 2). Prior to that time no clear
provinciality is evident. The cause of the major shift
in Atlantic Pb isotopes from 5 Ma ago is not fully understood. It is possible that the closure of the Panama
Gateway may have contributed to the development of
a strong separate Pb isotopic signature in the western
North Atlantic [7,10]. However, the development of
the Atlantic signature was probably further amplified by an increase in detrital input into the Atlantic
Ocean during the course of the Northern Hemisphere
glaciation starting 3.2–2.6 Ma ago [25,26]. This material would have generally high 206 Pb=204 Pb values
and low 208 Pb=206 Pb and 207 Pb=206 Pb values derived
from continental North America. This influence is
reflected in the steep gradient of Pb isotope change
in the two North Atlantic crusts and is supported by
the correlated Nd isotopic records [7,10]. In further
support of this view, recently published Nd and Pb
isotope records of authigenic components in sediment cores from the Arctic Ocean [27] are similar to
those from the western North Atlantic crusts and emphasize the importance of detrital material supplied
by the glaciers from North America in determining
the Pb and Nd isotopic composition of the deep
North Atlantic Ocean.
From 45 to 5 Ma ago the Pb isotopic signatures
of the Atlantic and Pacific Ocean crusts were persistently different. There is an indication that before
about 45 Ma ago the Pb isotope ratios of the Pacific
crusts D11-1 and CD29-2, although clearly distinct
for each crust [8], were more similar to Atlantic-type
values [6,8]. The similarity of those Pb isotope ratios
in the Atlantic and Pacific crusts (Fig. 2) is consistent with an efficient exchange of water masses
M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130
through an open ocean between North and South
America at that time although more work is required
to address this issue fully. The Pb isotopic signal of
Pacific deep-water crust VA13=2 remained separate
from the Atlantic-type values, possibly due to a hydrothermal influence at that time [Frank et al., in
prep.]
Crust 109 D-C from the southern part of the Indian Ocean, south of the Mid-Indian Ridge, which is
at present bathed either in CDW or Antarctic Bottom Water (AABW) [28,29], has 206 Pb=204 Pb and
207
Pb=206 Pb ratios which are indistinguishable from
values for Atlantic Ocean crusts prior to 5 Ma ago.
They correspond quite well to the present-day signature of the CDW in the Atlantic and Indian sectors of
the Southern Ocean [3]. In marked contrast, crust SS663 has recorded 206 Pb=204 Pb and 207 Pb=206 Pb ratios
indistinguishable from the Pacific time-series prior
to ¾15 Ma ago which may well reflect a relatively
unrestricted circulation between the Pacific and Indian Oceans. In addition, the relationship between
208
Pb=206 Pb and 207 Pb=206 Pb in this crust suggests a
secular change from a more Pacific-type character
26–16 Ma ago to one located in the field of the
Indonesian Arcs after 16 Ma ago [10]. The evolution
and weathering of the Indonesian Arcs and the closure of the Indonesian Passage for deep-water flow
¾21 Ma ago [30] were most probably responsible
for these changes.
3.3. Pb sources for the northern Indian Ocean:
Himalayan erosion?
The most striking feature of the Pb isotope timeseries of the seven crusts summarized in Fig. 2 is
the high 208 Pb=206 Pb ratios of northern Indian Ocean
crust SS-663 with the broad peak in the period from
20 to 8 Ma ago which shows even higher values than
those recorded by the Pacific crusts. The 208 Pb=206 Pb
ratios and to a lesser extent also the 207 Pb=206 Pb
ratios in this crust are anomalous relative to other
Indian Ocean Pb isotope data which are intermediate between the Atlantic and Pacific values — they
monitor an input of Pb from a source with a distinct
Pb isotope composition. An obvious candidate to be
considered is the erosional input associated with the
Himalayan uplift which occurred at approximately
the same time. Although direct information about
127
the Pb isotopic composition of the input into the
Indian Ocean originating from Himalayan weathering is unavailable at present, there is some evidence
in support of this view. It has been estimated that
about 80% of the detritus deposited in the Bengal
Fan sediments over the last about 20 Ma has been
derived from the high-grade metasedimentary rocks
of the High Himalayan Crystalline sequence [31].
Pb isotope analyses of these metasediments and associated leucogranites in Ref. [32] and compiled in
Ref. [33] from Refs. [34–36] form arrays in plots
of 207 Pb=204 Pb vs. 206 Pb=204 Pb and 208 Pb=204 Pb vs.
206
Pb=204 Pb which are distinct from all ferromanganese crust records, with the array of SS-663 being
the closest (Fig. 4). These observations show that
the variability of the Pb isotope record of crust SS663 has been controlled by Pb derived either from
weathering of the Himalayan rocks or a source with
comparable Pb isotope composition.
An examination of other potential sources of Pb
input into the Indian Ocean has so far failed to reveal any with such unusual Pb isotope compositions.
It has long been known that Indian Ocean basalts
have in general relatively high 208 Pb=204 Pb and
207
Pb=204 Pb ratios [37] compared with other oceanic
basalts. Hart [38] has viewed these as part of a largescale isotope anomaly in the Southern Hemisphere
mantle, the so-called Dupal anomaly. A comparison
of Pb data from Indian Ocean basalts showing this
anomaly with the record of crust SS-663 suggests
that these basalts are an unlikely source. Although
some basalt samples do have ratios as high as those
in crust SS-663, the corresponding 206 Pb=204 Pb ratios
are much too low. Similarly volcanics from the Indonesian Arcs have Pb isotope compositions which
cannot account for the 208 Pb=204 Pb and 207 Pb=204 Pb
variability of SS-663 whilst the Pb data of this crust
are in good agreement with the pelagic sediment arrays from the Indian Ocean compiled in [4]. Pb isotope data from eastern Africa are scarce and highly
variable. Although it remains possible that a further
search may yield other options, a Himalayan source
is certainly viable, particularly because the Bengal
and Indus Fans are the most important detrital inputs
into the northern Indian Ocean.
Assuming that the 10 Be=9 Be dating of crust SS663 is correct back to 26 Ma ago, then our data
suggest that the maximum erosional input from the
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M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130
Himalayas occurred between about 20 and 8 Ma ago
(Fig. 2) whilst the Co-based dating [Frank et al., in
prep.] gives an age of about 25 Ma for the beginning
of the 208 Pb=206 Pb peak. Both estimates correspond
quite well to ages derived from crystallization and
cooling ages of various synorogenic granites in the
Himalaya, which suggest that a phase of rapid exhumation and concomitant erosion started about 24
Ma ago and reached its maximum between 20 and
14 Ma ago [14]. During this period up to 30 km
of crustal thickness are estimated to have been removed from the High Himalaya by erosion [39]. The
ferromanganese crust data also concur with results
derived from isotopic and clay mineralogical inves-
tigations of Bengal Fan sediments deposited during
the last 20 Ma, which show that the sedimentation
rates of the Bengal Fan decreased significantly at
about 7.4 Ma ago, parallel with a change from a
physical to a more chemical weathering regime [40].
Pb isotope ratios obtained from the surfaces of Indian
Ocean crusts, which represent the pre-anthropogenic
Pb isotope distribution in the oceanic deep-water,
yield a similar but less pronounced trend as that
shown in Fig. 4 when compared to the data from
the Pacific and Atlantic [4] and thus corroborate the
importance of Himalayan erosion also for today’s Pb
isotopic signature of the Indian Ocean.
eNd values for the Bengal Fan sediments (18–0
Ma) are 16 š 2 [40], which is much lower than the
values of 7 to 8 found for the two Indian Ocean
crusts [10]. Thus, Nd derived from Himalayan erosion does not appear to have contributed in a major
way to the Nd budget of northern Indian Ocean
deep-water although a small negative shift of about
0.6 eNd coincides with the 208 Pb=206 Pb maximum in
crust SS-663. Thus, whereas large quantities of Nd
and Pb from the Himalayas end up in the sediments
of the Bengal Fan, apparently dissolved Nd from this
source is too low to have a dominating effect on
Indian Ocean deep-water (or for that matter surface
water) isotopic composition [10] as obviously is the
case for Pb. Thus, there is an apparent decoupling
between Nd and Pb. Whether or not Nd is retained
more efficiently in the Bengal Fan sediments than
is Pb due to different adsorption behaviour depends
on the mass balance between Pb and Nd in the erosional input on the one hand and the dissolved Pb
and Nd in Indian Ocean deep-water on the other. In
addition, more particle reactive Pb is more efficiently
scavenged from the water column and therefore a
local isotopic signal such as from the Himalayas
may only be recorded close to the source of input
whereas the Nd isotopic signal might be mixed and
diluted.
4. Conclusions
207 Pb=204 Pb
206 Pb=204 Pb
208 Pb=204 Pb
Fig. 4.
vs.
and
vs.
206 Pb=204 Pb ratios for all seven crust profiles. Sources for the
Pb isotope data are the same as in Fig. 2. The arrays for the
Himalayan gneisses and leucogranites are taken from Ref. [32]
and the compilation in Ref. [33].
Time-series of seven ferromanganese crusts
demonstrate that there was no distinctive Indian
Ocean Pb isotope signal prior to about 15 Ma ago.
The presently observed separation into Pacific, In-
M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130
dian and Atlantic-type values has only existed for
the last about 5 Ma. This is contrary to the expectations from the pronounced long-term (¾60 Ma)
provinciality of Nd isotope time-series when comparing published estimates of the respective oceanic
residence times of both elements. It is suggested that
Pb isotopes have been more sensitive to paleoceanographic changes such as the development of the
Indonesian Arcs and the stepwise evolution from an
equator-dominated global circulation system during
the Tertiary towards the present mode.
Particularly high 208 Pb=206 Pb ratios in a crust
from the northern Indian Ocean are suggested to represent a record of varying erosional input from the
uplift of the Himalyas. The start of the peak ratios at
about 25–20 Ma ago is in good agreement with maximum exhumation rates deduced from crystallisation
and cooling ages of synorogenic Himalayan granites
(20–14 Ma) and the termination of the peak ratios
at about 7.4 Ma agrees well with a marked decrease
of the sedimentation rates in the Bengal Fan. The
fact that the Nd isotopes in the same crust do not
show a strong influence of a Himalayan contribution
can possibly be explained by a more efficient lateral
mixing of Nd in the Indian Ocean compared with Pb
which shows a strong impact close to the source of
the input signal. On the other hand the mass balance
between Nd and Pb in the eroded material and the
Indian Ocean deep-water may be responsible for the
apparent decoupling of Pb and Nd isotopes.
Acknowledgements
This work was funded by a HCM grant of the
EU and a contract within the TMR network program ‘The Marine Geochemical Record of Continental Tectonics and Erosion’ of the EU to M.F.
We acknowledge J.R. Hein, US Geological Survey,
Menlo Park, CA and V. Banakar, National Institute
of Oceanography, Goa, India for kindly providing
material of crust SS-663. We wish to thank Balz
Kamber and Friedhelm von Blanckenburg for discussions and their help with the mass spectrometer
and Mike Searle for discussions. This paper benefitted from reviews of A.N. Halliday, B. White and an
anonymous reviewer. [RV]
129
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