401
studies by Harada and Nishida (1976, 1978) established
slow growth rates for the outer layers of some deepsea nodules (1.0-6.7 mm Ma
-1
) but higher rates for the
inner layers of some nodules (39 mm Ma
-1
) based on
biostratigraphic analysis of calcareous microfossils.
Cowen et al. (1993) subsequently determined the ages
of different layers of a Mn crust (KK84-RD50 S1B)
from Schumann Seamount in the North Pacific based
on the identification of coccolith imprints at various
depths in the crust (see section 11.4.10.1). In a detailed
study, Pulyaeva (1997) also identified three main
periods of growth of Co-rich Mn crusts from the
Magellan Seamounts in the western Pacific based on
a study of calcareous microfossils. On this basis, it
was established that the crusts began forming intermittently in the Late Cretaceous in the presence of
phosphatized nannoforaminifera. The crusts attained
a composition similar to that of today in the Eocene
but the Miocene-Pliocene was shown to be the most
favourable period for growth of the crusts. Numerous
interruptions in the growth of the crusts were observed
with major regional hiatuses being recorded at the
Cretaceous-Paleocene and Eocene-Oligocene boundaries (see section 11.4.10.1). More recently, Shilov
(2004) has reported on a major programme of dating
manganese nodules from the C-C F.Z. based on age
determinations of radiolaria in the nodules. From this,
he was able to demonstrate that active growth of the
manganese nodule fields started between the Late
Oligocene and Early Miocene at about 23.7 Ma and
that the principal layers which make up 50-80% of the
total nodule volume in individual nodules were formed
in the Middle Eocene (16.6-11.2 Ma) and Late MiocenePleistocene (11.2-1.8 Ma). He also established that the
highest growth rates of the nodules (2.5-3.7 mm Ma
-1
)
occurred during the Pleistocene-Holocene (1.8-0 Ma).
These ages are in agreement with the findings of
Glasby (1978) who recorded the presence of buried
manganese nodules in D.S.D.P. cores associated with
sediments of Oligocene age and older (cf. Usui and Ito
1994; Ito et al. 1998) and by von Stackelberg and
Beiersdorf (1991) who showed that most of the nodules
in the C-C F.Z. southeast of Hawaii began their growth
at Tertiary hiatuses. These ages are much older than
the middle Miocene age (~12-15 Ma) obtained by
Kadko and Burckle (1980) for two nodules from the
C-C F.Z. based on the age of fossil diatoms in the
nodules.
Nonetheless, most modern methods of dating Mn
nodules and crusts rely on radiometric determinations.
There are three principal methods available based on
the following isotope ratios
230
Th/
232
Th,
231
Pa/
230
Th
and
10
Be. The half lives of
230
Th,
231
Pa and
10
Be are
75,200 a, 32,000 a and 1.5 Ma, respectively (Turekian
and Bacon 2004). The
230
Th/
232
Th and
231
Pa/
230
Th
methods can therefore only date samples to ages of
300,000 and 125,000 a (corresponding to 4 half lives of
the isotopes), respectively, which is equivalent to the
outer 1-2 mm of a deep-sea nodule or crust. The
10
Be
method, on the other hand, permits dating back to
~10 Ma which is equivalent to a depth of a few cm in
some samples (Turekian and Bacon 2004). Hein et al.
(2000) have compiled a list of all isotopically-determined growth rates of hydrogenous Mn crusts to that
date.
The
238
U and
235
U decay-series methods are based
on the fact that
230
Th and
231
Pa are generated in
seawater by the decay of U isotopes. The isotopes are
then carried to the sea floor on particles and incorporated into the nodules and crusts. The residence
times for Th and Pa in seawater are short (less than 40
and 160 years respectively). The distribution of
230
Th
and
231
Pa with depth in nodules or crusts can be
measured by counting a particles after separation from
the nodule material and plating on a planchet or more
rapidly by α-track counting on nuclear emulsion plates.
The growth rate of the nodule or crust can then be
calculated from the formula
dC/dt = S dC/dx - λC
or
C (x)
= C 0 exp (-xλ/S)
where C (x) is the concentration at depth x, ë is the
decay constant of the nucleus (=ln 2/τ 1/2 ), τ 1/2 the half
life of the nuclide and S the nodule growth rate.
The
230
Th/
232
Th and
231
Pa/
230
Th methods were the
first radiometric methods to be used for dating of
manganese nodules but their application was limited
because of the short half lives of
230
Th and
231
Pa. In
spite of the limitation of working with samples from
the uppermost 1-2 mm of the sample, high resolution
studies of nodules have been achieved using this
method. For example, Eisenhauer et al. (1992) analyzed
69 samples to a depth of 1.4 mm with a resolution of
0.02 mm in the VA 13/2 Mn crust sample (see section
11.4.10.1). The samples were analyzed by alpha
counting after chromatographic separation of thorium
from uranium and electroplating the elements on
stainless steel holders. However, the development of
the thermal-ionization mass spectrometric (TIMS)
method for determining U and Th isotopes increased
the precision of the measurements and extended the
possibilities of this method (Chabaux et al. 1995, 1997).
In particular, Böllhofer et al. (1996, 1999) used this
method to investigate the growth rates of diagenetic
11.4
Manganese Nodules and Crusts
studies by Harada and Nishida (1976, 1978) established
slow growth rates for the outer layers of some deepsea nodules (1.0-6.7 mm Ma
-1
) but higher rates for the
inner layers of some nodules (39 mm Ma
-1
) based on
biostratigraphic analysis of calcareous microfossils.
Cowen et al. (1993) subsequently determined the ages
of different layers of a Mn crust (KK84-RD50 S1B)
from Schumann Seamount in the North Pacific based
on the identification of coccolith imprints at various
depths in the crust (see section 11.4.10.1). In a detailed
study, Pulyaeva (1997) also identified three main
periods of growth of Co-rich Mn crusts from the
Magellan Seamounts in the western Pacific based on
a study of calcareous microfossils. On this basis, it
was established that the crusts began forming intermittently in the Late Cretaceous in the presence of
phosphatized nannoforaminifera. The crusts attained
a composition similar to that of today in the Eocene
but the Miocene-Pliocene was shown to be the most
favourable period for growth of the crusts. Numerous
interruptions in the growth of the crusts were observed
with major regional hiatuses being recorded at the
Cretaceous-Paleocene and Eocene-Oligocene boundaries (see section 11.4.10.1). More recently, Shilov
(2004) has reported on a major programme of dating
manganese nodules from the C-C F.Z. based on age
determinations of radiolaria in the nodules. From this,
he was able to demonstrate that active growth of the
manganese nodule fields started between the Late
Oligocene and Early Miocene at about 23.7 Ma and
that the principal layers which make up 50-80% of the
total nodule volume in individual nodules were formed
in the Middle Eocene (16.6-11.2 Ma) and Late MiocenePleistocene (11.2-1.8 Ma). He also established that the
highest growth rates of the nodules (2.5-3.7 mm Ma
-1
)
occurred during the Pleistocene-Holocene (1.8-0 Ma).
These ages are in agreement with the findings of
Glasby (1978) who recorded the presence of buried
manganese nodules in D.S.D.P. cores associated with
sediments of Oligocene age and older (cf. Usui and Ito
1994; Ito et al. 1998) and by von Stackelberg and
Beiersdorf (1991) who showed that most of the nodules
in the C-C F.Z. southeast of Hawaii began their growth
at Tertiary hiatuses. These ages are much older than
the middle Miocene age (~12-15 Ma) obtained by
Kadko and Burckle (1980) for two nodules from the
C-C F.Z. based on the age of fossil diatoms in the
nodules.
Nonetheless, most modern methods of dating Mn
nodules and crusts rely on radiometric determinations.
There are three principal methods available based on
the following isotope ratios
230
Th/
232
Th,
231
Pa/
230
Th
and
10
Be. The half lives of
230
Th,
231
Pa and
10
Be are
75,200 a, 32,000 a and 1.5 Ma, respectively (Turekian
and Bacon 2004). The
230
Th/
232
Th and
231
Pa/
230
Th
methods can therefore only date samples to ages of
300,000 and 125,000 a (corresponding to 4 half lives of
the isotopes), respectively, which is equivalent to the
outer 1-2 mm of a deep-sea nodule or crust. The
10
Be
method, on the other hand, permits dating back to
~10 Ma which is equivalent to a depth of a few cm in
some samples (Turekian and Bacon 2004). Hein et al.
(2000) have compiled a list of all isotopically-determined growth rates of hydrogenous Mn crusts to that
date.
The
238
U and
235
U decay-series methods are based
on the fact that
230
Th and
231
Pa are generated in
seawater by the decay of U isotopes. The isotopes are
then carried to the sea floor on particles and incorporated into the nodules and crusts. The residence
times for Th and Pa in seawater are short (less than 40
and 160 years respectively). The distribution of
230
Th
and
231
Pa with depth in nodules or crusts can be
measured by counting a particles after separation from
the nodule material and plating on a planchet or more
rapidly by α-track counting on nuclear emulsion plates.
The growth rate of the nodule or crust can then be
calculated from the formula
dC/dt = S dC/dx - λC
or
C (x)
= C 0 exp (-xλ/S)
where C (x) is the concentration at depth x, ë is the
decay constant of the nucleus (=ln 2/τ 1/2 ), τ 1/2 the half
life of the nuclide and S the nodule growth rate.
The
230
Th/
232
Th and
231
Pa/
230
Th methods were the
first radiometric methods to be used for dating of
manganese nodules but their application was limited
because of the short half lives of
230
Th and
231
Pa. In
spite of the limitation of working with samples from
the uppermost 1-2 mm of the sample, high resolution
studies of nodules have been achieved using this
method. For example, Eisenhauer et al. (1992) analyzed
69 samples to a depth of 1.4 mm with a resolution of
0.02 mm in the VA 13/2 Mn crust sample (see section
11.4.10.1). The samples were analyzed by alpha
counting after chromatographic separation of thorium
from uranium and electroplating the elements on
stainless steel holders. However, the development of
the thermal-ionization mass spectrometric (TIMS)
method for determining U and Th isotopes increased
the precision of the measurements and extended the
possibilities of this method (Chabaux et al. 1995, 1997).
In particular, Böllhofer et al. (1996, 1999) used this
method to investigate the growth rates of diagenetic
11.4
Manganese Nodules and Crusts