Geochimica et Cosmochimica Acta, Vol. 63, No. 11/12, pp. 1689 –1708, 1999
Copyright © 1999 Elsevier Science Ltd
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0016-7037/99 $20.00 ⫹ .00
Pergamon
PII S0016-7037(99)00079-4
60 Myr records of major elements and Pb–Nd isotopes from hydrogenous
ferromanganese crusts: Reconstruction of seawater paleochemistry
M. FRANK,1,* R. K. O’NIONS,1 J. R. HEIN,2 and V. K. BANAKAR3
1
Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
U.S. Geological Survey, 345 Middlefield Road, MS-999, Menlo Park, California 94024, USA
3
National Institute of Oceanography, Dona Paula, 403004 Goa, India
2
(Received June 24, 1998; accepted in revised form November 24, 1998)
Abstract—We compare the time series of major element geochemical and Pb- and Nd-isotopic composition
obtained for seven hydrogenous ferromanganese crusts from the Atlantic, Indian, and Pacific Oceans which
cover the last 60 Myr.
Average crust growth rates and age– depth relationships were determined directly for the last about 10 Myr
using 10Be/9Be profiles. In the absence of other information these were extrapolated to the base of the crusts
assuming constant growth rates and constant initial 10Be/9Be ratios due to the lack of additional information.
Co contents have also been used previously to estimate growth rates in Co-rich Pacific and Atlantic seamount
crusts (Puteanus and Halbach, 1988). A comparison of 10Be/9Be- and Co-based dating of three Co-rich crusts
supports the validity of this approach and confirms the earlier chronologies derived from extrapolated
10
Be/9Be-based growth rates back to 60 Ma. Our data show that the flux of Co into Co-poor crusts has been
considerably lower. The relationship between growth rate and Co content for the Co-poor crusts developed
from these data is in good agreement with a previous study of a wider range of marine deposits (Manheim,
1986). The results suggest that the Co content provides detailed information on the growth history of
ferromanganese crusts, particularly prior to 10 –12 Ma where the 10Be-based method is not applicable.
The distributions of Pb and Nd isotopes in the deep oceans over the last 60 Myr are expected to be
controlled by two main factors: (a) variations of oceanic mixing patterns and flow paths of water masses with
distinct isotopic signatures related to major paleogeographic changes and (b) variability of supply rates or
provenance of detrital material delivered to the ocean, linked to climate change (glaciations) or major tectonic
uplift. The major element profiles of crusts in this study show neither systematic features which are common
to crusts with similar isotope records nor do they generally show coherent relationships to the isotope records
within a single crust. Consequently, any interpretation of time series of major element concentrations of a
single crust in terms of paleoceanographic variations must be considered with caution. This is because local
processes appear to have dominated over more basin-wide paleoceanographic effects. In this study Co is the
only element which shows a relationship to Pb and Nd isotopes in Pacific crusts. A possible link to changes
of Pacific deep water properties associated with an enhanced northward advection of Antarctic bottom water
from about 14 Ma is consistent with the Pb but not with the Nd isotopic results. The self-consistent profiles
of the Pb and Nd isotopes suggest that postdepositional diagenetic processes in hydrogenous crusts, including
phosphatization events, have been insignificant for particle reactive elements such as Pb, Be, and Nd. Isotope
time series of Pb and Nd show no systematic relationships with major element contents of the crusts, which
supports their use as tracers of paleo-seawater isotopic composition. Copyright © 1999 Elsevier Science Ltd
Diagenetic deposits on the other hand derive a substantial part
of their metals through remobilisation via sediment pore waters
and reprecipitation close to the sediment water interface. These
ferromanganese encrustations grow at lower rates of 10’s to
100’s mm/Ma. In contrast, hydrogenous deposits precipitate
from ambient seawater at very low rates between 1 and 15
mm/Ma (cf. Segl et al., 1984; Manheim, 1986; Puteanus and
Halbach, 1988). They form at locations protected from high
sedimentation rates, either as nodules on pelagic sediments or
as crusts on hard substrates such as basalt or hyaloclastite. As
such they are effective archives of ocean paleochemistry.
A prerequisite for paleoceanographic studies involving ferromanganese crusts and nodules is a knowledge of their growth
rates. Attempts to date ferromanganese encrustations have employed various approaches including 87Sr/86Sr ratios (Hein et
al., 1993; Burton et al., 1997), magnetostratigraphy (Joshima
and Usui, 1998), and radioactive tracers incorporated into ferromanganese encrustations at their growth surfaces. The most
1. INTRODUCTION
The major element composition of ferromanganese crusts has
been studied extensively to address both genetic differences
between crusts as well as variations in the paleoceanographic
conditions prevailing during their formation. Three main genetic types of ferromanganese encrustations have been distinguished from the relative abundances of Mn, Fe, Ni, Co, and
Cu (Bonatti et al., 1972); these are hydrothermal, diagenetic,
and hydrogenetic. Hydrothermal deposits form where the supply rates of Mn and Fe oxides are high at ocean ridges and
volcanic arcs and grow rapidly at rates above 1 m/Ma such that
Mn and Fe dilute other components to low concentrations.
*Address reprint requests to M. Frank at present address: Institute for
Isotope Geology and Mineral Resources, Department of Earth Sciences, ETH Zürich, NO C61, Sonneggstrasse 5, CH-8092 Zürich,
Switzerland; Tel. (⫹41) 1 632 3764; Fax: (⫹41) 1 632 1179; (E-mail:
[email protected] or [email protected]).
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M. Frank et al.
Fig. 1. World map showing the locations of the seven ferromanganese crusts discussed.
precise growth rate estimates have been obtained using U and
Th isotopes which are, however, restricted to the last about 400
kyr (Segl et al., 1984; Banakar and Borole, 1991; Eisenhauer et
al., 1992; Chabaux et al., 1995; 1997; Bollhöfer et al., 1996;
Abouchami et al., 1997). The application of cosmogenic 10Be,
which appears to give reliable results when normalized to
stable 9Be, extends chronologies back to ⬃10 Ma (Segl et al.,
1984; McMurtry et al., 1994; Koschinsky et al., 1996; Ling et
al., 1997; O’Nions et al., 1998; Frank and O’Nions, 1998).
Beyond 10 Ma the only other geochemical tool available to
estimate growth rates of ferromanganese crusts, which was
assessed in any detail, is that based on constant flux models of
Co incorporation (Halbach et al., 1983; Manheim, 1986; Puteanus and Halbach, 1988; Manheim and Lane-Bostwick, 1988).
A large number of studies of the bulk element composition
of hydrogenous ferromanganese crusts (hereafter called crusts)
have led to the recognition of three main groups of co-genetic
elements (Aplin and Cronan, 1985; De Carlo et al., 1987; Hein
et al., 1988; 1990; Koschinsky and Halbach, 1995; Banakar et
al., 1997; Wen et al., 1997). These are a manganophile group
which includes associated elements such as Co, Ni, Zn, and Mg
which are interpreted to be incorporated directly from seawater;
an aluminosilicate group related to authigenic or detrital aluminosilicates which includes the elements Si, Al, Fe, K, and
sometimes Ti and Ba; a biogenic group consisting of elements
such as Ba, Cu, Ca, P, Mg, Sr, and sometimes Ni, released
during dissolution of biogenic material. The ratio between Al
and Si was suggested to be an indicator of opal productivity
(Alvarez et al., 1990) and may be included in the biogenic
group. A fourth group also including the elements Ti and Fe
(Aplin and Cronan, 1985; Hein et al., 1990) has occasionally
been recognised. Phosphatization commonly affected the older
parts of crusts from water depths less than about 3000 m
(Halbach et al., 1989; Hein et al., 1993) and involves an
enrichment in P and Ca associated with the carbonate fluoroapatite phase (CFA). In these phosphatized sections Fe, Si, Al,
Ti, Co, and Mn are depleted relative to the unphosphatized
sections, whereas Cu and Ni, for example, are enriched (Koschinsky et al., 1997).
Hydrogenous crusts record the trace metal isotopic composition of ambient seawater (Goldstein and O’Nions, 1981;
Albarède and Goldstein, 1992; Abouchami and Goldstein,
1995; von Blanckenburg et al., 1996a, 1996b; Burton et al.,
1997; Christensen et al., 1997; Albarède et al., 1997; Ling et
al., 1997; Abouchami et al., 1997; 1998). Pb and Nd isotope
time series obtained from crusts dated by 10Be/9Be ratios (Segl
et al., 1984; Ling et al., 1997; O’Nions et al., 1998; Frank and
O’Nions, 1998) have been used to reconstruct the isotopic
composition of seawater at particular locations in the oceans
through time and to assess their response to paleoceanographic
and paleogeographic changes that occurred over the last 60
Myr. A particular focus of these discussions has been the
importance of the opening and closing of oceanic gateways
such as the Isthmus of Panama (Burton et al., 1997; O’Nions et
al., 1998) and of changes in supply rates of detrital material to
the ocean (Ling et al., 1997; Abouchami et al., 1997; Frank and
O’Nions, 1998).
The main aims of this study are twofold. The first is to
compare the Nd and Pb isotope and major element variations in
crusts from the Atlantic, Indian, and Pacific Oceans and evaluate their applicability as tracers of deep water composition.
The second aim is to reassess the Co content of crusts as a
chronometer. New data are reported on element contents of
crusts for which Pb and Nd isotope records are available and
Co contents are obtained for crusts with 10Be/9Be records.
2. SAMPLING AND ANALYTICAL PROCEDURE
Seven crusts with published Nd, Pb, and Be isotope profiles are used
as the basis for this study (Ling et al., 1997; Burton et al., 1997;
Christensen et al., 1997; O’Nions et al., 1998; Frank and O’Nions,
1998) (Fig. 1; Table 1). Two of the crusts were recovered from
seamounts in the NW Atlantic (BM1969.05 and ALV 539) at 1800 m
and 2700 m water depth, respectively, and two are from seamounts in
the Pacific Ocean (CD29-2 and D11-1) at 2390 –1970 m and 1870 –
1690 m water depth, respectively. A third from the Pacific Ocean,
VA13/2, was recovered from 4830 m water depth and two crusts are
from the Indian Ocean (109D-C and SS663) from 5689 to 5178 m and
5250 m water depth, respectively. Crusts CD29-2 and D11-1 are
Co-rich crusts as defined by Halbach et al. (1983) and Puteanus and
Halbach (1988).
Element abundance profiles of Al, Si, P, Ca, Ti, Mn, Fe, Co, Ni, Cu,
and Ba have been measured using a Cambridge Instruments Microscan
9 electron probe on polished sections of crusts cut perpendicular to
their growth layers. Slabs of the crusts were embedded in epoxy resin
60 Myr records of seawater chemistry
1691
Table 1. Locations of the crusts.
Source of isotope dataa
Cruise
Sample
Latitude
Longitude
Water depth
(m)
—
ALV 539
Antipode
SS-XI
VA13/2
F7-86-HW
F10-89-CP
BM1969.05
2-1
109D-C
SS663
237 KD
CD29-2
D11-1
39°00⬘N
35°00⬘N
27°58⬘S
12°57⬘S
09°25⬘N
16°42⬘N
11°39⬘N
60°57⬘W
59°00⬘W
60°48⬘E
76°06⬘E
146°03⬘W
168°14⬘W
161°41⬘E
1800
2700
5689–5178
5250
4830
2390–1970
1870–1690
(Nd ⫹ Pb)
(Be)
Thickness
(mm)
Avg. growth rate
(mm/Ma)
Max. ageb
(Ma)
1
3
3
3⫹6
2
2.5
2.5
3
3
3
6
4
2
2
130
80
30
67
209
105
147
1.62
2.37
1.60
2.80
3.57
2.10
2.53
80
41
24
26
58
55
58
a
1 Burton et al. (1997); 2 Ling et al. (1997); 3 O’Nions et al. (1998); 4 Segl et al. (1984); 5 Christensen et al. (1997); 6 Frank and O’Nions (1998).
The ages of the bases of the crusts given in the last column were calculated by extrapolating the growth rates derived from 10Be/9Be ratios in the
upper sections of the respective crusts. For comparability with other publications samples 2-1 of cruise ALV 539 and 237 KD of cruise VA13/2 are
identified with the respective cruise names in the text.
b
for sectioning and measurement and the electron probe microanalyses
(EPMA) were made with a 20 ␮m beam diameter in order to minimise
specimen damage. The relative error for each element is approximately
1% (1␴) for concentrations in the range 10 –30 wt%, but increases to
about 10% (1␴) for concentrations between 0.1 and 0.5 wt%. Spot
analyses of the crusts were made at a resolution of 0.1 mm perpendicular to the microstructural growth banding wherever identifiable. The
porous nature of the crusts together with high water contents resulted in
oxide totals mostly less than 80%. Analyses with totals less than 40%
were rejected. Those analyses with totals between 40% and 80% were
summed to 100% applying a water correction using representative
water content determinations (Table 2) following Hein et al. (1992).
Repeat measurements of element profiles were made for several crusts
and found to be indistinguishable from each other in overall structure.
Spot analyses with Si contents exceeding the respective hydrogenous
background concentration of each crust, which varies between 2 and
6.2 wt%, were not included for paleoceanographic interpretation as
they are considered to reflect pure aluminosilicates. For development of
a relationship between Co concentration and growth rate (Sec. 4) the
analyses with high Si contents were included.
3. MAJOR ELEMENTS
Major element abundances and ratios are presented in Fig. 2.
Either Fe, Ti, or Co has been used for the ratio denominators.
Fe and Ti were chosen because their abundances mostly reflect
the aluminosilicate group. Co belongs to the manganophile
group and is a growth rate indicator. In previous studies, the
Mn/Fe ratio has been considered to reflect water depth (Halbach and Puteanus, 1984) and deep water oxygenation (von
Stackelberg et al., 1984). Variations of Ba/Ti and Ni/Co ratios
are taken to reflect biogenic phases. Age models of the crusts
are discussed in detail in Sec. 4. Complete data sets for these
and all other elements measured are available from the authors
on request.
The two Pacific seamount crusts CD29 and D11-1 are phosphatized in the sections older than 25 and 18.5 Ma, respectively. The sharp transitions between phosphatized and unphosphatized sections, which are clearly resolved in the abundances
of the CFA phase elements Ca and P, are much less evident, if
at all, in the concentrations of the other major elements and
element ratios (Figs. 2a,b). However, the general distribution of
element abundances between phosphatized and unphosphatized
parts of the crusts reported by Koschinsky et al. (1997) is
confirmed by these measurements.
In the unphosphatized parts of CD29-2 and D11-1 Mn is
positively correlated with Co and Ni and negatively correlated
with Fe. However, Al, Si, and Ti are a well-resolved aluminosilicate group only in CD29-2 (see Appendix for correlation
matrices). D11-1 displays a clear pattern of decreasing contents
of Fe and Ti and Ni/Co ratios, as well as an increase of Co and
Mn/Fe ratios with time. Fe shows a minimum and Co and
Mn/Fe ratios show peak values around 5 Ma. In crust CD29 the
patterns are less clear but Ti, Co, and Ni/Co ratios, together
with the Ba/Ti ratios, show the same trends as D11-1, whereas
in contrast Fe shows a peak and Mn/Fe ratios a minimum
around 5 Ma.
The major element time series of deep water crust VA13/2
(Fig. 2c) resemble those of D11-1 over the last ⬃25 Myr in that
Mn/Fe ratios and Co show a marked increase and Fe, Ni/Co,
and Ba/Ti ratios a decrease starting about 14 Ma. In contrast to
D11-1, however, Ti increased together with Co and Mn/Fe
ratios. There are no clear peaks ⬃5 Ma but uniformly higher
values from about 10 Ma until present.
The major elements in the NW Atlantic crusts ALV 539 and
BM1969.05 show less clearly resolved structures than the Pacific crusts (Figs. 2d,e). BM1969.05 displays a more or less
constant elemental composition for the last about 50 Myr with
only the Ni/Co ratios showing some peaks. Prior to 50 Ma
contents of Ti and Co were higher and Fe was lower. None of
the genetic groupings of elements described above are evident
in this crust. In ALV 539 a well-resolved increase of Ti at about
4 Ma is accompanied only by increases in Ba/Ti and Ni/Co
ratios which, however, also display a considerable variability in
the older sections of this crust. Furthermore Mn/Fe ratios
decrease continuously from around 2 at the base of the crust to
a value below 1 at the top. Such a trend has previously been
ascribed to subsidence of the seafloor with time (Halbach and
Puteanus, 1984). However, a similar trend is not observed in
the Mn/Fe data of crust BM1969.05.
The elemental composition of crust SS663 (Fig. 2f) shows
pronounced manganophile (Mn, Co, Ni) and aluminosilicate
(Fe, Si, Al) groupings with shifts in Fe, Mn/Fe ratio, Co, and Ti
at about 14 Ma similar to, but less well-resolved than, those
observed in Pacific deep water crust VA13/2. There is also a
marked change of those trends at about 7 Ma that is not
observed in VA13/2. Crust 109D-C from the southern Indian
Ocean (Fig. 2g) shows generally uniform major element patterns, with the exception of an overall decrease of Ti through-
1692
M. Frank et al.
Table 2. Semiquantitative XRD mineralogy and hygroscopic water contents.*
Depth
(mm)
Hygroscopic
water (%)
(␦-MnO2)
(%)
CFA
(%)
main comp.
—
0–130
—
0–15
15–23
23–58
58–base
18.0
15.1
20.1
11.1
97
99
97
94
—
—
—
—
0–14
14–26
26–base
15.7
15.0
11.3
97
92
92
—
—
—
0–20
20–32
32–44
44–47
47–base
11.7
10.2
7.9
6.9
5.9
99
96
96
95
88–94
—
—
—
—
—
0–10
10–20
62–90
95–110
222–232
232–240
—
—
—
—
—
—
main comp.
main comp.
main comp.
—
—
mainly nontronite
—
—
—
—
—
—
0–9
9–49
49–76
76–base
18.4
24.7
13.9
22.0
96
99
89
93
—
—
10
7
0–9
9–35
35–43
43–65
65–95
95–135
135–base
14.7
13.1
13.6
10.5
13.4
8.1
15.0
98
⬃99
100
80
76
71
84
—
—
—
20
24
29
16
Quartz
(%)
Goethite
(%)
BM1969.05
—
traces
ALV539
1
—
1
—
1
2
—
3
109D-C
1
—
2
—
3
—
SS663
1
—
1
—
1
2
1
2
1
2
VA13/2
traces
—
—
—
traces
common
—
main comp.
—
main comp.
traces
—
CD29-2
2
—
1
—
1
—
—
—
D11-1
2
—
⬍1
—
—
—
—
—
—
—
—
—
—
—
K-feldspar
(%)
Plagioclase
(%)
Calcite
(%)
—
⫹ minor smectite
—
—
—
—
—
—
—
—
2
2
—
—
1
1
—
—
1
6
5
—
—
—
—
1
traces
1
1–6
—
2
1
1
1
—
—
—
—
⫹1% clay
—
—
—
—
traces
common
—
—
—
—
traces
common
—
—
—
—
—
traces
—
—
—
—
2
traces
traces
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
*All data were acquired by XRD and represent new data except CD29-2, which is from Hein et al. (1990) and the data of VA13/2, which were
taken from Stackelberg et al. (1984). No data on hygroscopic water contents were available for crusts VA13/2 and BM1969.05 for which a value of
20% was estimated.
out the crust, and a decrease of Co and Mn/Fe ratio and increase
in Fe and Ni/Co ratio at about 1.2 Ma.
3.1. Variations Between Genetic Types of Crusts
Over Time
On the basis of transition metal variations Bonatti et al.
(1972) have distinguished among hydrothermal, diagenetic,
hydrogenetic, and mixed-type ferromanganese nodules. Assuming that these distinctions are also applicable for crusts, the
ones considered here are generally located in Bonatti’s field of
hydrogenetic growth (Fig. 3) confirming that they are hydrogenous in origin. In detail, the crusts have recorded relative
variations in Fe, Mn, and (Co ⫹ Ni ⫹ Cu) within the hydrogenetic field during their growth.
The older part of Atlantic crust ALV 539 plots centrally in
the hydrogenetic field and then shifts towards a more iron-rich
composition in the section younger than ⬃11 Ma. A second
Atlantic crust, BM1969.05, plots in the lower part of the
hydrogenetic field without showing any obvious trends (Fig. 3).
The two Indian Ocean deep water crusts also plot within the
field of hydrogenetic growth, but SS663 shows a trend towards
a lower iron abundance during the last 10 Myr, whereas
109D-C, which displays a generally low variability, tends towards a higher relative iron abundance outside the hydrogenetic window in the last 1.2 Myr. This shift would be consistent
with a local hydrothermal influence, however the crust’s location is remote from the Southwest Indian Ridge.
The two Pacific seamount crusts CD29-2 and D11-1 are
located in the Mn-rich part of the hydrogenetic field, despite the
phosphatization of their older parts. They appear to show a
trend towards lower Fe and Mn abundances from the older
phosphatized parts to the younger parts. This seems to be in
opposition to results on the distribution of elements between
phosphatized and unphosphatized sections obtained by Koschinsky et al. (1997), but is explained by a relatively strong
depletion of Co coinciding with a weak enrichment of Cu and
Ni in the phosphatized sections of the crusts. Pacific deep water
crust VA13/2 is the only one lying significantly outside the
60 Myr records of seawater chemistry
Fig. 2. Time series of the concentrations of selected major elements and element ratios versus age for: (a) Crusts CD29-2
and (b) D11-1. The upper shaded periods between 1 and 7 Ma mark the maxima of ␧Nd and 206Pb/204Pb ratios and minima
of 207Pb/206Pb and 208Pb/206Pb ratios (Ling et al., 1997). The shaded periods between 25 and 18.5 Ma, respectively, and the
bases of the crusts mark the phosphatized parts. (c) Crust VA13/2. The shaded area marks a period of major change in
isotopic and elemental composition from 14 Ma until present. (d) and (e) As above for the NW Atlantic crusts BM1969.05
and ALV 539. The shaded area marks the period of strong decreases of ␧Nd, 207Pb/206Pb and 208Pb/206Pb ratios and increases
of the 206Pb/204Pb ratio (Burton et al., 1997; O’Nions et al., 1998). (f) and (g) Indian Ocean crusts 109D-C and SS663. The
shaded period between 0 and 1.2 Ma on the plot of 109D-C marks a major change in the major element composition and
the shaded period on the plot of SS663 marks the period between 20 and 7.4 Ma which was interpreted to represent
maximum Himalayan exhumation and erosion rates deduced from the 208Pb/206Pb profile in this crust (Frank and O’Nions,
1998). All data represent five times running averages.
1693
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M. Frank et al.
Fig. 2. Continued
60 Myr records of seawater chemistry
Fig. 3. Ternary diagrams of Mn, Fe, and (Cu ⫹ Co ⫹ Ni) ⫻ 10 following Bonatti et al. (1972) for all crusts of this study.
At the base of each diagram, the shaded area corresponds to hydrothermal growth conditions and the vertically hatched areas
correspond to hydrogenetic growth. The other shaded areas define distinct compositions during the growth of the crusts and
the arrows indicate possible trends. Ages are derived from a combination of 10Be/9Be and Co chronology (see text).
1695
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M. Frank et al.
field of hydrogenetic growth in parts of the section older than
⬃14 Myr, suggesting a period of increased hydrothermal influence (Fig. 3). This pattern is in good agreement with results
for this crust obtained by von Stackelberg et al. (1984). Interestingly a section of major change in mineralogical composition from vernadite to goethite as main ferromanganese mineral
below a depth of 95 mm (corresponding to an age of ⬃19 Ma)
in crust VA13/2 does not correspond to the values outside the
hydrogenetic field in Fig. 3. However, the Pb-, and to a lesser
extent also the Nd-isotopic composition in the goethite-dominated section are shifted towards more Mid Ocean Ridge Basalt
(MORB)-like values, suggesting a change in provenance of the
isotopes (Ling et al., 1997).
4. CO AND
10
BE/9BE CHRONOLOGIES
The growth rates of the seven crusts considered here are
derived from 10Be/9Be ratios measured as a function of depth
beneath growth surfaces (Table 1). At ages greater than 10 –12
Ma there is no viable alternative dating method that may be
employed, and ages have to be estimated from extrapolation of
growth rates based on 10Be/9Be ratios. One possible test of the
validity of such extrapolations beyond 10 Ma is to use Co
concentration profiles referenced to growth rates derived from
Be- and U-series isotope dating.
A relationship between Co content and growth rate in various types of ferromanganese crusts and nodules as well as
pelagic sediments was derived by Manheim (1986) and Manheim and Lane-Bostwick (1988):
growth rate (mm/Ma) ⫽
0.68
,
Cow1.67
(1)
where Cow is the Co concentration in wt% less a detrital
background concentration of 0.0012 wt%. A method similar to
this was applied by McMurtry et al. (1994).
A relationship between Co content and growth rate more
specifically for Co-rich central Pacific seamount crusts was
developed by Halbach et al. (1983), Halbach and Puteanus
(1984), and Puteanus and Halbach (1988) applying growth
rates determined from 10Be and 230Thexcess profiles:
growth rate (mm/Ma) ⫽
1.28
,
关Co兴 ⫺ 0.24
(2)
where [Co] is the Co concentration in wt%. This equation was
considered to be valid for Co contents between 0.24 and 2.0
wt% (Puteanus and Halbach, 1988) and these authors also
proposed a modified form of Eqn. 2 for the older phosphatized
sections of Pacific seamount crusts:
Co(x)⬘⫽Co(x)m
Mn/Co(x)
Mn/Co(b)
(3)
and
Co(x)⬘
Co(x)⬙⫽
,
1⫺0.05⌬P
(4)
where Co(x)⬘ is the Co concentration of layer x of the phosphatized part corrected for dilution by the CFA phase. Co(x)m and
Mn/Co(x) are the measured Co content and Mn/Co ratio of layer
Fig. 4. Age– depth plots derived from 10Be/9Be ratios and Co content
for the Pacific seamount crusts: (a) D11-1, (b) CD29-2, and (c) Atlantic
seamount crust ALV 539. 10Be/9Be-derived growth rates (dashed lines)
older than 10 Ma were extrapolated to the base of the crusts assuming
constant growth rates and constant initial 10Be/9Be ratios (Ling et al.,
1997; O’Nions et al., 1998). They show an excellent match with
Co-based growth rate estimates (solid lines) using Eqns. 2– 4 (Puteanus
and Halbach, 1988). The data used for the growth rate calculations are
five times running averages of all Co contents including those analyses
which have higher than background Si contents. Mn/Co(b) values of 35
and 40 and average phosphate contents for the unphosphatized sections
of 0.36 and 0.38 wt % were used for CD29-2 and D11-1, respectively.
The lower dashed line in (c) defines the maximum growth rate estimate
of 3.3 mm/Ma based on the 10Be/9Be record when including all data
given in O’Nions et al. (1998).
x of the phosphatized part, Mn/Co(b) is the measured Mn/Co
ratio at the boundary layer between unphosphatized and phosphatized parts of the crust, and ⌬P is the difference between the
phosphate content of layer x of the phosphatized part and the
average value of the unphosphatized part. Co(x)⬙ is the Co
content in layer x of the phosphatized part that is inserted in
place of [Co] into Eqn. 2 to calculate the growth rate of the
phosphatized section. Co(x)⬙ is in effect corrected for dilution
by the CFA phase and a partial dissolution of the crust by
phosphate-rich pore waters.
Age– depth relationships calculated from the high resolution
Co concentration records of central Pacific seamount crusts
CD29-2 and D11-1, using Eqn. 2 for the unphosphatized parts
and Eqns. 2– 4 for the phosphatized parts are shown in Figs. 4a
and 4b and are compared with those derived from 10Be/9Be
ratios (Ling et al., 1997). The age– depth relationships derived
from Co contents for the two crusts are consistent and within
⬃3 Myr of their 10Be/9Be-derived growth rates. This is remarkable given that the 10Be/9Be-derived growth rates obtained for
the upper parts were simply extrapolated to the base of the
crusts. These data support the claim that Co chronology is a
powerful tool for estimating growth rates and ages of central
Pacific seamount crusts (Halbach et al., 1983; Halbach and
Puteanus, 1984; Puteanus and Halbach, 1988) and corroborate
that these two Pacific crusts grew over a period of 50 – 60 Myr.
60 Myr records of seawater chemistry
It should be recalled that both approaches would fail to account
for any growth hiatus or erosion in the sections older than
10 –12 Ma (Hein et al., 1992; McMurtry et al., 1994). Thus the
Co-derived total ages should be considered as minimum estimates. The results also support the view that nonphosphatized
parts of Pacific seamount crusts have started to grow about 25
Ma (Segl et al., 1984; Puteanus and Halbach, 1988); this
corresponds to the late Oligocene/early Miocene phosphatization event suggested by Hein et al. (1993) using 87Sr/86Sr ratios
of the phosphate phases.
The relationship described in Eqns. 2– 4 has also been used
to date crusts in the Atlantic Ocean (Koschinsky et al., 1995;
1996) but the general applicability of these calculations to
crusts from the Atlantic is less obvious, as results for the two
crusts of this study show. Growth rates derived from 10Be/9Be
ratios (O’Nions et al., 1998) and Co are shown for a nonphosphatized crust, ALV 539, from a seamount in the NW Atlantic
(Fig. 4c). The agreement between the two estimates is less good
than for the Pacific seamount crusts, but does suggest that the
Co method, as developed for Co-rich Pacific seamount crusts,
may also be applicable for relatively Co-rich crusts in the
western Atlantic Ocean, even though, in this case, the water
depth of 2700 m is far below the oxygen minimum layer
(OML), which is probably responsible for the high Co supply
to the Pacific Co-rich crusts (Puteanus and Halbach, 1988;
Koschinsky and Halbach, 1995; Koschinsky et al., 1997). The
two age estimates are in better agreement for the upper 25 mm
of the crust when two 10Be/9Be data points regarded as outliers
(O’Nions et al., 1998) are included.
A second crust from the NW Atlantic, BM1969.05, from a
location close to ALV 539 at a water depth of 1830 m, grew at
a similar average rate of 1.6 mm/Ma during the last 7 Myr as
derived from the 10Be/9Be ratios (O’Nions et al., 1998). However, it only contains 30%–50% of the Co concentration, resulting in a ⬃50% lower Co flux compared with ALV 539.
Given that the two crusts are located close by and BM1969.05
grew in shallower water, and therefore closer to the OML than
ALV 539, this is difficult to understand. It may be that small
scale differences of the circulation pattern in the NW Atlantic
Ocean or other local environmental conditions at the location of
this crust are responsible for the observed difference in Co
supply. Until these possibilities are resolved the Co chronometer should be applied with caution in the Atlantic. Based on the
failed attempts to date crusts, including ALV 539, using 87Sr/
86
Sr ratios of the ferromanganese phase (Ingram et al., 1990;
VonderHaar et al., 1995; Ling et al., 1997; O’Nions et al.,
1998) and the consistency of the 10Be/9Be-derived datings
(Segl et al., 1984; Ling et al., 1997; O’Nions et al., 1998; Frank
and O’Nions, 1998) the apparent agreement between the
growth rate estimates for BM1969.05 derived from Eqn. 2 and
the only so far reported internally consistent 87Sr/86Sr dating
(Burton et al., 1997) displayed in Fig. 6 is suggested to be
fortuitous.
Equation 2 is inapplicable to Co-poor crusts from Pacific and
Indian Ocean deep water (below ⬃3000 m) such as VA13/2,
SS663, and 109D-C (Table 1) and Co-poor crusts from shallower depths including BM1969.05 from the Atlantic as shown
above. In each case Eqn. 2 yields growth rates which are too
high when compared with dating results based on 10Be/9Be
ratios. However, there is no obvious reason why the mechanism
1697
Fig. 5. Relationship between growth rates derived from 10Be/9Be
ratios and Co contents for deep water crusts and other crusts with low
Co contents, for which Eqn. 2 developed for Co-rich central Pacific
seamount crusts (Puteanus and Halbach, 1988) is not applicable. The
solid logarithmic fit shows a significant correlation at the 99% confidence level between Co concentration and growth rate. The dashed line
represents the relationship given by Manheim (1986). The Co concentrations determined by microprobe analyses tend to be somewhat
higher than the ones measured by AAS (Segl et al., 1984; von Stackelberg et al., 1984) which is indicated by the long error bars of crust
VA13/2. Only the microprobe data were used for fitting.
of Co incorporation into these crusts should differ from that of
Co-rich seamount crusts. The flux of Co into Co-rich crusts was
shown to be near constant at a value of ⬃3 ␮g/cm2 kyr
(Halbach et al., 1983) which was suggested to be a consequence of its short oceanic residence time and the chemical
processes by which Co is fixed to the colloidal particle surfaces.
Vernadite (␦-MnO2), which is the main manganese mineral in
hydrogenous crusts, efficiently scavenges Co2⫹ from the Mnand Co-enriched OML in the upper water column, which is
subsequently oxidised to Co3⫹ (Puteanus and Halbach, 1988;
Koschinsky and Halbach, 1995; Koschinsky et al., 1997). Thus
the principal difference between the Co-rich Pacific seamount
crusts and those from elsewhere in the ocean appears to be the
absence of a high Co supply rate via the OML.
In order to gain information on the growth history of crusts
with low Co contents, all available 10Be/9Be-derived growth
rates and the corresponding average Co concentrations are used
to develop a relationship between Co content and growth rate
of Co-poor crusts. These data also allow the calculation of the
average Co flux into Co-poor crusts. In Fig. 5 the average Co
contents for the three deep-water crusts and BM1969.05, as
well as several other Co-poor crusts from the Atlantic and
Pacific and Indian Oceans are plotted versus growth rate derived from 10Be/9Be ratios (Frank et al., submitted; Reynolds et
al., submitted). The data clearly show that there is a correlation
between Co content and growth rate. A logarithmic fit to the
data yields
growth rate (mm/Ma) ⫽
0.25
,
关Co兴 2.69
(5)
which agrees well with Eqn. 1 given by Manheim (1986)
considering the relatively large scatter of the data in Fig. 5. The
fit to our data is very sensitive to the one value at ⬃10 mm/Ma
1698
M. Frank et al.
Fig. 6. Age– depth plots comparing 10Be/9Be-derived (dashed line) and Co-derived (bold line) growth rates applying Eqn.
1 (Manheim, 1986). 10Be/9Be-derived growth rates older than 10 Ma were extrapolated assuming constant growth rates and
constant initial 10Be/9Be ratios (Segl et al., 1984; O’Nions et al., 1998; Frank and O’Nions, 1998): (a) SS663, (b) 109D-C.
The lower dashed line marks the maximum growth rate estimate including all 10Be/9Be data for this crust (O’Nions et al.,
1998); (c) BM1969.05. Also shown for this crust are the 87Sr/86Sr dating results of Burton et al. (1997) (open squares) and
the age model resulting from application of Eqn 2 (lower solid line); (d) VA13/2. The lower solid line is the age model from
application of Eqn 2. The dashed curves branching off the extrapolated 10Be/9Be-derived growth rates in (b), (c), and (d)
represent the finally applied age models after correction for the differences between Co content and 10Be/9Be-derived
growth rates before 11, 48, and 14 Ma, respectively (see Table 3).
due to missing data between 6 and 10 mm/Ma. Without more
data in this range of growth rates to confirm the fit, we consider
Eqn. 1 more reliable for Co-poor crusts because it is based on
a larger number of samples which cover a much wider range of
growth rates (Manheim, 1986). The average Co flux into the
Co-poor crusts used in Fig. 5 is only 1.9 ␮g/cm2 kyr. The
difference in Co flux between Co-rich and Co-poor crusts,
which is probably caused by differences in Co supply rate via
the OML, may explain why Eqn. 1 does not reproduce growth
rates of Co-rich Pacific seamount crusts correctly (Manheim,
1986), where Eqn. 2 is obviously more reliable (Puteanus and
Halbach, 1988).
Equation 1 is applied to the Co concentration profiles of the
three deep water crusts and crust BM1969.05 in Fig. 6. The
agreement between growth rates from 10Be/9Be ratios and Eqn.
1 for Indian Ocean crust SS663 is good over the last 10 Myr.
The Co-derived growth rate of SS663 in the section older than
10 Ma also agrees well with the extrapolation of the 10Be/9Bederived growth rate and supports the chronology employed in
the study of Frank and O’Nions (1998). In contrast, the Co-
60 Myr records of seawater chemistry
1699
Fig. 7. Comparison of: (a) ␧Nd, (b) 206Pb/204Pb, (c) 207Pb/206Pb, and (d) 208Pb/206Pb ratios and of the studied crusts based
on a combination of 10Be/9Be and Co chronology (see text and Table 3). The sources of the data are given in Table 1.
Diamond symbols—NW Atlantic crusts, filled squares—Southern Indian Ocean, open squares—Central Indian Ocean, open
circles— deep Pacific, filled circles—low resolution data of the sections older than 32 Ma of the Pacific seamount crusts
(Ling et al., 1997), lines without symbols— high resolution record of the two Pacific seamount crusts (Christensen et al.,
1997). The shaded area marks the period of about the last 5 Myr which represents the time for which a separated Atlantic,
Indian, and Pacific Ocean signature of the Pb isotopes in the water column has existed. The lightly shaded area in (d) marks
the period of maximum 208Pb/206Pb ratios in central Indian Ocean crust SS663 (20 –7.4 Ma).
derived growth rate of 109D-C changes from a high value of
6 –7 mm/Ma prior to 11 Ma to an average value of 1.6 mm/Ma,
which is in very good agreement with the published value
(O’Nions et al., 1998). The Co data also suggest that the growth
rate has varied considerably from 11 to 0 Ma in this crust and
that the lowermost 10Be/9Be value in this crust is indeed
disturbed. The Co data in combination with the 10Be/9Be chronology indicate an age at the base of this crust of about 15 Ma
rather than 20 Ma, which was estimated from extrapolation of
the 10Be/9Be-derived growth rate (O’Nions et al., 1998). The
Nd and Pb isotope time series of 109D-C shown in Fig. 7 are
based on the age depth relationship given in Fig. 6b (Table 3).
Application of Eqn 1 to Atlantic crust BM1969.05 results in
a relatively good agreement between the Co- and 10Be/9Bederived growth rates for the last 7 Myr, although the Co-based
growth rate is about 30% higher (1.6 mm/Ma compared to 2.2
mm/Ma). The Co-based age model suggests a more or less
constant growth rate from the surface to 75 mm depth, resulting
in a total departure of up to 12 Myr from the extrapolated
10
Be/9Be-based model (Fig. 6c). It also indicates that the
1700
M. Frank et al.
Table 3. Corrected ages according to results of the Co chronometer.*
Depth
(mm)
21–21.5
23–24
26.5–26.8
29.5–30
79–81
98–99
120–121
40–42
49–51
58.5–60
66.5–67.5
74–75.5
81–82.5
88.5–91
96.5–98
104–105
122–124
134–136
147–149
159–161
174–176
192–194
207–209
Published age
(Ma)
109D-C
13.7
15.2
17.2
19.2
BM1969.05
49.4
60.8
74.4
VA13/2
13.6
16.0
18.5
20.6
22.6
24.5
26.5
28.6
30.6
35.5
38.7
42.2
45.4
49.4
54.2
58.2
Corrected age
(Ma)
11.9
12.3
12.9
14.0
48.9
50.4
52.5
13.2
14.1
15.0
15.6
16.5
17.2
17.9
18.5
19.0
20.0
21.0
21.9
22.8
24.0
25.5
26.5
*Published ages for crusts 109D-C and BM1969.05 are taken from
O’Nions et al. (1998) and for VA13/2 from Ling et al. (1997).
growth rates were very low between 120 and 125 mm depth
(⬃0.7 mm/Ma), but between 80 and 120 mm depth they were
higher than 10 mm/Ma, which yields an age of the base of the
crust based on Co alone of ⬃45 Ma. Although the absolute
value of the Co-derived growth rate does not match the estimate from 10Be/9Be ratios, the relatively uniform Co content
suggests that the 10Be/9Be-based growth rate can be extrapolated until 75 mm depth, corresponding to an age of ⬃48 Ma.
Below this depth the significantly higher Co-derived growth
rates are considered more realistic, which results in the age
model in Fig. 6c yielding an age at the base of the crust of about
⬃60 Ma (Table 3). The isotope time series for this crust in Fig.
7 are based on this age model.
Crust VA13/2 from the Pacific yields Co-derived growth
rates using Eqn. 1 which are lower than those from 10Be/9Be
ratios (Fig. 6d). Nevertheless the Co data do suggest significantly higher growth rates prior to 14 Ma (38 mm depth). In
support of this, von Stackelberg et al. (1984) have reported a
pronounced change not only in chemical composition but also
in microstructure of this crust below 40 mm depth. Extrapolation of the 10Be/9Be-based growth rate beyond 38 mm depth in
crust VA13/2 appears unjustified and the growth rates derived
from Co are adopted for the section below this depth. Accordingly the age at the base of the crust at 21 cm depth is probably
only 26 Ma rather than 58 Ma as extrapolation of the 10Be/
9
Be-derived growth rate would suggest (Ling et al., 1997). The
age model resulting from combination of 10Be/9Be ratios and
Co data (lower dashed line in Fig. 6d, Table 3) was applied for
the isotope time series in Fig. 7.
The data currently available suggest that Eqn. 1 yields the
better estimate of growth rates if average Co concentrations are
less than 0.7– 0.8 wt%, whereas Eqn. 2 is more appropriate at
higher concentrations. The results of this study support the
view that Co-derived growth rates represent an independent
tool to provide valuable and detailed information about the
growth history of ferromanganese crusts, particularly beyond
the 10 –12 Ma limit of direct dating by 10Be/9Be ratios.
5. MAJOR ELEMENT AND ND AND PB ISOTOPE
TIME SERIES
An important objective of this study is to compare time
series for major elements and Pb and Nd isotopes within the
same crusts. In this way it will be possible to ascertain whether
provenance changes recorded by the isotopes are accompanied
by changes in major element composition. Previous studies
have linked variations in major elements to changes in the
carbonate compensation depth (Halbach and Puteanus, 1984),
to ocean circulation changes (Hein et al., 1992), and for the NE
Atlantic to variability of biological productivity and the
Messinian salinity crisis (Koschinsky et al., 1996). In contrast,
Wen et al. (1997) suggested from a comparison of records
obtained from two Pacific crusts that time series of major
elements may have been controlled more by local environmental conditions and seawater geochemistry than by global or
basin-wide processes.
In this study, the average element contents and element ratios
were calculated for each depth section of the crusts previously
sampled for Pb and Nd isotopes. Element and isotope data of
each crust were correlated (see Appendix). For Pacific crusts
D11-1 and CD29-2, for which high depth resolution Pb isotope
profiles are available (Christensen et al., 1997), spline fits to the
data with 0.5 Myr resolution were obtained for both the isotope
and element data. The unphosphatized sections of the two
Pacific seamount crusts CD29-2 (25– 0 Ma) and D11-1 (18.5– 0
Ma) and the section of deep water crust VA13/2 representing
the last ⬃19 Myr show a consistent continuous increase in ␧Nd
and 206Pb/204Pb ratio until about 2–5 Ma (Ling et al., 1997;
Christensen et al., 1997) (Fig. 7). This is followed by a decrease
in both parameters until the present day (Ling et al., 1997;
Abouchami et al., 1997). Comparison of the Nd and Pb isotope
profiles for each of the three crusts shows a positive correlation
between ␧Nd and 206Pb/204Pb ratio, but a negative one with the
207
Pb/206Pb ratio. This is probably related to the influence of
Nd and Pb originating from weathering of young Pacific island
arc rocks (von Blanckenburg et al., 1996b; Frank and O’Nions,
1998). The corresponding element and element ratio time series
in the same sections (Fig. 2a– c) exhibit some well-resolved
features, which however, differ between the three crusts. The
time series of Mn/Fe ratio and Co in D11-1 peak at 3–5 Ma as
do ␧Nd and the 206Pb/204Pb ratio, whereas the Fe and Ti profiles
are at a minimum at this time. In contrast, CD29-2 shows a
minimum in the Mn/Fe ratio, high Fe and Co, but low Ti
contents over the same period of time. In both crusts ␧Nd and
206
Pb/204Pb ratio are negatively and 207Pb/206Pb ratio positively correlated with element ratios and contents such as Ni/Co
and Al/Si and Ba. This might be a consequence of variations in
biogenic particle flux-induced scavenging within the equatorial
high productivity area, which was shown to influence the major
element composition of manganese nodules (Cronan and Hod-
60 Myr records of seawater chemistry
kinson, 1994). The element abundance patterns for VA13/2
(Fig. 2c) show a pronounced change about 14 Ma, which also
corresponds to changes in isotope composition. In this crust
␧Nd and 206Pb/204Pb ratio are positively correlated with Mn,
Co, Ni, and Ti and negatively correlated with Fe and Ba
contents. In summary, of those transition metals presented here
only Co shows similar trends in all three Pacific crusts and
correlates positively with ␧Nd and 206Pb/204Pb ratios. This is
discussed in further detail below (Sec. 6).
The correlations of element contents and ratios with Nd and
Pb isotopes of the complete profiles of D11-1 and CD29-2,
including the phosphatized parts, differ from the unphosphatized sections. Thus Nd and Pb isotopes are uncorrelated in
D11-1 and in CD29-2 show a change of sign compared to the
unphosphatized sections (Appendix). Furthermore, for D11-1
there is in effect no correlation between any of the element
contents or ratios with the Pb isotopes in the complete section
of the crust. Correlations between CFA phase elements Ca and
P and other elements such as Mn, Ti, and Co are much higher
than in the unphosphatized section for this crust. The opposite
situation is observed in CD29-2: here the correlation between
Pb isotopes and element contents is generally improved,
whereas none of the transition metal contents or ratios correlate
with Ca and P when the data from the phosphatized section are
included. This demonstrates that phosphatization exerted considerable influence on the major element composition of crusts,
whereas it apparently did not significantly affect Pb and Nd
isotopes (see also Sec. 5.1).
In the two NW Atlantic crusts Nd and Pb isotope time series
show a general increase in 206Pb/204Pb ratio and decrease in
207
Pb/206Pb, 208Pb/206Pb ratios and ␧Nd from the base of both
crusts until about 3– 4 Ma. These trends have been accentuated
from 3 to 4 Ma to present (Burton et al., 1997; O’Nions et al.,
1998). The correlation coefficients between ␧Nd and all Pb
isotope ratios in the Atlantic crusts are significant (0.77 ⬍ r
⬍ 0.93) and of opposite sign to those for the Pacific Ocean
crusts (Frank and O’Nions, 1998). The progressive closure of
the Panama Gateway for deep water circulation from about 13
to 5 Ma (Duque-Caro, 1990; Collins et al., 1996) and the
increased input of radiogenic Pb and unradiogenic Nd with
detrital material from continental North America associated
with the onset of Northern Hemisphere Glaciation (Shackleton
et al., 1984; Raymo, 1994) have probably been the most important factors controlling the Nd- and Pb-isotopic composition
in the NW Atlantic Ocean.
In contrast to the close correspondence between the Nd and
Pb isotopes in the two NW Atlantic crusts, their transition metal
contents and ratios differ markedly (Fig. 2d,e). The correlations
are poor between elements and between elements and Pb and
Nd isotopes for crust ALV 539 and insignificant for
BM1969.05. ALV 539 shows a marked increase in Ti by a
factor of 2 starting at about 4 Ma, which correlates with the
pronounced changes in the Nd and Pb isotopes over this same
period. With the exception of Ni/Co, this increase is not accompanied by any other element such as Al and Si. Nevertheless it could reflect an increased input of terrigenous Ti into the
N Atlantic during the onset of northern hemisphere glaciation
although this is not observed in BM1969.05.
In the Indian Ocean, ␧Nd in crust SS663 shows a positive
correlation with the 206Pb/204Pb ratio and a negative correlation
1701
with the 207Pb/206Pb ratio, similar to the Pacific crusts (Frank
and O’Nions, 1998). When compared with the element data ␧Nd
shows a positive correlation only with Ni and a negative
correlation with the Ni/Co ratio. Pb isotope ratios are weakly
correlated with a number of element contents, but no correlation coefficient exceeds r ⫽ 0.65 (Appendix). Crust SS663
also shows a pronounced peak in the 208Pb/206Pb ratio between
20 and 7.4 Ma, which has been interpreted to reflect the
changing intensity of Himalayan erosion (Frank and O’Nions,
1998). This pattern, and in particular the drop of the 208Pb/
206
Pb ratio at 7.4 Ma, which coincides with a pronounced
decrease in supply of detrital material to the Bengal Fan (Derry
and France-Lanord, 1996) are apparently reflected by a decrease of Co (growth rate increase) and an increase in Fe and
Ni/Co ratio in this same crust (Fig. 2f). The second Indian
Ocean crust, 109D-C, shows only small variations in its Nd and
Pb isotope time series, which points to the presence of a
relatively stable circumpolar deep water (CDW) during its
entire 15 Myr period of growth. ␧Nd is negatively correlated
with the 206Pb/204Pb ratio and positively with the 207Pb/206Pb
ratio (Frank and O’Nions, 1998). Of the elements only the
Ni/Co ratio is significantly correlated with Nd and Pb isotopes.
In summary, there are significant correlations between the
time series of some element contents and Nd and Pb isotopes in
most of the crusts studied here, but these differ markedly from
one crust to another (see Appendix). It is apparent that the
element contents have been mainly controlled by different
factors from those responsible for Nd and Pb isotope variations.
Crusts from similar or neighbouring locations, which should
have experienced and recorded the same paleoceanographic
changes, display strong local control on element contents during crust growth. For the two Pacific seamount crusts differences in the intensity of surface water productivity in the
equatorial Pacific, which was shown to affect the elemental
composition of nodules (Cronan and Hodkinson, 1994), the
movement of the Pacific Plate relative to the equatorial high
productivity belt, or the thickness of the OML may have
influenced their major element composition. The deep water
crust VA13/2 was not influenced by changes in the OML and
its composition was probably more controlled by the supply of
dissolved metals in the bottom waters. The differences in the
element composition of the two NW Atlantic crusts are difficult
to understand, but it is possible that regional differences of
water mass chemistry and subsidence history of the two locations have been responsible. In general, the interplay of more
than one controlling factor on the major elemental composition
combined with the related complex dilution processes make it
very difficult, if not impossible at many locations, to isolate and
distinguish between these controlling factors. The results presented indicate that basin-wide paleoceanographic signals may
be recorded by Pb and Nd isotope time series in crusts but are
mostly not resolvable from their local records of major element
composition.
5.1. Diagenetic Alteration and Postdepositional
Isotope Exchange
The available major element and isotope data have implications not only for paleoceanographic changes but also for
possible effects of diagenesis and postdepositional exchange of
1702
M. Frank et al.
crusts with sea water. Manganese nodules always have a diagenetic component which is, for example, evident from the
presence of authigenic minerals such as zeolites, whereas crusts
are not growing in direct contact with potential diagenetic
sources of metals from sediments and the amount of authigenic
minerals is usually insignificant. Nevertheless it is important to
question whether diagenesis has influenced the isotope and
major element patterns of the crusts, although depth profiles of
10
Be and 230Th even in nodules mostly give consistent results
in terms of dating (Krishnaswami et al., 1982; Bollhöfer et al.,
1996).
The consistent patterns of Pb and Nd isotopes in crusts
within ocean basins and the 10Be/9Be chronologies demonstrate
that major large scale remobilisation cannot have occurred. The
most obvious diagenetic process affecting the crusts in this
study is phosphatization of the basal sections older than 25 and
18.5 Ma of the two Pacific seamount crusts, CD29-2 and
D11-1, respectively. Phosphatization is a well-known feature
which has also been found in some Atlantic crusts (Hein et al.,
1993; McMurtry et al., 1994; Bau et al., 1996; Koschinsky et
al., 1995; 1996; 1997). Between 60 and 32 Ma 206Pb/204Pb and
207
Pb/206Pb ratios of the crusts D11-1 and CD29-2 shifted
markedly towards Atlantic-type values, whereas the Pb isotope
ratios in deep water crust VA13/2 have remained at Pacifictype values over the entire 26 Myr period recorded in this crust
(Ling et al., 1997; Christensen et al., 1997). It might be possible
that the Pb isotope shift in the phosphatized sections of D11-1
and CD29-2 results from diagenesis of the ferromanganese
phases during phosphatization.
However, the shift in Pb isotope ratios in D11-1 and CD29-2
occurred at ⬃32 Ma, some 8 –10 Myr before the end of the
period of phosphatization (Ling et al., 1997; Christensen et al.,
1997) which argues against the shift in Pb isotopes time series
being related to phosphatization and remineralization. In addition, even in the phosphatized parts, high-resolution Pb isotopic
profiles measured by laser ablation ICP-MS on crusts CD29
and D11-1 show a variability with clear shifts on less than 1
Myr time scales (Christensen et al., 1997). These variations in
the Pb isotope time series are coherent between two locations
3000 km apart from each other (Christensen et al., 1997) which
strongly supports the origin of the Pb and Nd isotopes from
ambient sea water in the phosphatized sections.
Other crusts in this study show no sign of phosphatization or
indeed any evidence for diagenetic alteration (which is supported by the Bonatti diagrams in Fig. 3). The possibility that
some elements have been mobile in the crusts cannot be excluded, even though their Be, Nd, and Pb isotope compositions
appear to have been unaffected. This is evident from comparison of the two NW Atlantic crusts which show very similar
and nearly contemporaneous Pb- and Nd-isotopic variations
(Burton et al., 1997; O’Nions et al., 1998) although their
elemental composition, thickness, and porosity is very different. This difference in elemental composition might be interpreted in terms of diagenesis, but it is difficult to imagine a
major remobilisation of, for example, Mn and Fe which did not
affect the coherent isotopic pattern. It is therefore suggested in
agreement with Wen et al. (1997) that the observed differences
in major element composition have mainly been caused by
variations of local water mass chemistry and depositional conditions.
The possibility that diffusion of dissolved elements from sea
water into porous ferromanganese nodules and crusts has been
discussed as an alternative explanation to radioactive decay and
continuous growth for depth profiles of 10Be, 230Th, or 231Pa,
which decrease exponentially with depth in the crusts and
nodules (Ku et al., 1979). A model was developed by these
authors, which describes the radionuclide profiles by a combination of radioactive decay within the crusts and redistribution
and mixing by postdepositional exchange with sea water.
Whereas there is still debate about diffusion of less reactive
elements such as U, which is mobile under oxidizing conditions
(Chabaux et al., 1995, 1997; Neff et al., 1998; 1999), diffusion
appears to be unimportant for highly particle reactive elements
such as Be, Nd, Pb, Th, and Pa (Henderson and Burton, 1999).
Continuous isotopic exchange between a crust and seawater
should, as discussed above, tend to homogenize and overprint
any isotopic variability. All isotope profiles measured should
show exponential decreases as predicted by diffusion processes—the consistent patterns in Pb and Nd isotope time series
within ocean basins (Fig. 7) should not exist.
Exponential depth profiles of Be and Th isotopes also display
changes in slope (Segl et al., 1984; Ling et al., 1997;
Abouchami et al., 1997) and gaps (Eisenhauer et al., 1992)
which must be interpreted as changes of growth rate and
interruptions of growth, respectively. Clearly the well-correlated Pb and Nd isotope time series from the Pacific basin do
not resemble diffusion patterns, but represent variability in the
Pb and Nd isotopic composition of Pacific deep water. For the
NW Atlantic a deep water origin of the isotope records is
supported by the similarity of their patterns to Pb and Nd
isotope records derived from ferromanganese micronodules
buried within Arctic Ocean sediments (Winter et al., 1997).
This suggests the onset of Northern Hemisphere Glaciation and
associated increased sediment input into both basins as a common cause for the observed isotope variations.
6. CO CONTENTS AND PACIFIC OCEAN
PALEOCEANOGRAPHY
Co is the only element that shows a significant positive
correlation with ␧Nd and the 206Pb/204Pb ratio (and a negative
correlation with the 207Pb/206Pb ratio) in all three Pacific crusts.
This corresponds to a negative correlation of growth rate with
␧Nd and the 206Pb/204Pb ratio and a positive correlation with the
208
Pb/206Pb and 207Pb/206Pb ratios. The abrupt increase in the
Mn/Fe ratio and other manganophile elements (such as Co) in
deep water crust VA13/2 about 14 Ma has been previously
interpreted by von Stackelberg et al. (1984) as a consequence of
an increase of deep water oxygen content associated with a
strengthening of advection of Antarctic bottom water (AABW)
northward flow at about that time (van Andel et al., 1975;
Ciesielski et al., 1982). The present day AABW has average
206
Pb/204Pb, 207Pb/206Pb, and 208Pb/206Pb values of 18.8,
0.8328, and 2.062 respectively (Abouchami and Goldstein,
1995), and an ␧Nd value of ⫺7 to ⫺6 (Albarède and Goldstein,
1992; Albarède et al., 1997). The near constant Pb and Nd
isotopic composition of crust 109D-C from the Indian sector of
the Southern Ocean suggests that these present day values have
been representative of the Indian and Pacific sectors of the deep
Southern Ocean over at least the last ⬃15 Myr. The slight
60 Myr records of seawater chemistry
increase in 206Pb/204Pb ratio and decreases in 208Pb/206Pb and
207
Pb/206Pb ratios starting at about 14 Ma, evident in the three
Pacific crusts, are consistent with the interpretation of enhanced
AABW intensity. Any similar influence in the Pb isotope
record of crust SS663 from the Central Indian Ocean is obscured by the superimposed signal originating from Himalayan
erosion (Frank and O’Nions, 1998).
In contrast to Pb, the ␧Nd time series in the three Pacific and
the Central Indian Ocean crusts show a shift towards more
positive values over the last about 14 Myr. This cannot be
explained by enhanced flow of AABW if this deep water mass
has indeed remained at values between ⫺7 and ⫺6 over the last
20 Myr. To reach ␧Nd values of ⫺3 in the period from 14 to 0
Ma would require the influence of a water mass with an ␧Nd
value even more positive than ⫺3, which cannot be AABW.
We therefore argue that the increase in Co concentration and
decrease in growth rate, which started at about 14 Ma in the
Pacific, and possibly the Central Indian Ocean, may indeed be
related to a stronger influence of AABW since that time. On the
other hand the Pb and Nd isotopic variations on approximately
the same time scales must have a different origin. Taking into
account the isotopic signature of the Island arc rocks, particularly of the Indonesian Island Arcs which started to develop at
about 20 –15 Ma, we suggest that weathering of these sources
is the most likely explanation for the observed Pb- and Ndisotopic variability in the Pacific crusts.
1703
that postdepositional diagenetic processes other than the phosphatization have either not occurred or failed to influence the
isotopic records of particle reactive elements in crusts.
An observed correlation between growth rates derived from
Co contents and Pb and Nd isotope records in the Pacific and
Indian Oceans is not explained by enhanced Antarctic Bottom
Water flow from about 14 Ma, because only the trends of the Pb
but not of the Nd isotopes are consistent with a stronger
influence of a water mass derived from the Southern Ocean.
Whereas the Co content and growth rate records may reflect an
enhanced influence of AABW, the Pb and Nd isotope records
in the Pacific Ocean have more likely been controlled by the
evolution and weathering of the Indonesian Island Arcs over
the last 20 Myr.
Acknowledgments—This work was funded by an H.C.M. grant of the
E.U. and a contract within the T.M.R. network program “The Marine
Record of Continental Tectonics and Erosion” of the E.U. to
M.F. V.K.B. wants to thank the director of the NIO and R. R. Nair,
former head of the Geological Oceanography Division of the NIO, for
the encouragement to collaborative work. We thank U. von Stackelberg, Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover,
Germany for providing a sample of crust 237KD from cruise VA13/2.
We also wish to thank N. Charnley for assistance with the microprobe
analyses, D. Sharrock for his help with the statistics, and M. Maggiulli
and A. Koschinsky for discussions. D. S. Cronan and F. Manheim
provided thoughtful reviews.
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