402
11 Manganese: Predominant Role of Nodules and Crusts
manganese nodules from the Peru Basin. These
nodules were shown to have high growth rates of
~110 mm Ma
-1
but with frequent oscillations in growth
rate which were attributed to transitions in the glacial/
interglacial cycle. These climatic variations were
assumed to control the depth of the suboxic/oxic
boundary in the sediment leading to changing growth
rates and the development of distinct laminations
within the nodules. However, von Stackelberg (1997,
2000) subsequently observed that the highest growth
rates in these nodules occur on the underside of large
nodules which repeatedly sink to a level immediately
above the redox boundary where diagenetic recycling
of Mn is at a maximum. These authors therefore suggested that layering of these nodules was the result
of the lifting of these nodules by benthic organisms
within the oxic surface sediments from a diagenetic to
a hydrogenous environment rather than the result of
climatic change as proposed by Böllhofer et al. (1996,
1999).
Han et al. (2003) also established that there are
four orders of basic cyclic growth patterns in a deepsea manganese nodule from the C-C F.Z. based on
analysis of samples taken at intervals of 0.1 mm in the
outer 1.3 mm of the nodule using the
230
Th excess /
232
Th
method. These growth patterns were named laminae
bonds, laminae zones, laminae groups and laminae
pairs and had thicknesses of 402-454, 185-206, 58-67
and 15-18 µm, respectively. This rhythmic growth was
considered to be related to Milankovitch cycles with
the growth cycles of the laminae, bands and zones
corresponding to the periods of eccentricity, obliquity
and precession of the Earth’s orbit, respectively. From
the thicknesses of these laminae, it was was then
possible to establish a net growth rate of the outer
layers of the nodule of 4.5 mm Ma
-1
based on the
Earth’s orbital cycle in close agreement with the rate
of 4.6 mm Ma
-1
determined by
230
Th excess /
232
Th dating
of the nodule. This is an excellent example of the high
resolution that can now be attained using
230
Th/
232
Th
dating.
10
Be dating involves a measurement of the depth
profile of
10
Be in a deep-sea nodule or crust. The half
life of
10
Be is 1.5 Ma which normally permits dating
back to ~10 Ma, although earlier authors dated crusts
back to ~14 Ma by analyzing larger amounts of material
for the older samples (Segl et al. 1989).
10
Be is a
cosmogenic nuclide which is deposited at the sea
surface before being mixed into the oceans. Its
residence time in the ocean is about 1,000 yrs. Because
of its greater half life,
10
Be can be measured to much
greater depths within the nodule or crust (> 40 mm)
than the U decay-series isotopes (1-2 mm). Postdepositional diffusion of
10
Be is not considered a
problem in determining the growth rates of the nodule
or crust (cf. Kusakabe and Ku 1984). The development
of accelerator mass spectrometry (AMS) led to a
reduction in sample sizes and counting times by
several orders of magnitude compared to measurements based on radioactive decay. Much better
resolution can therefore be obtained and dating to 15
Ma is now possible. This predates the age of the lower
Miocene Antarctic glaciation. Discontinuities in the
growth rates of nodules and crusts can also be
determined from breaks in the
10
Be depth profile. As
an example, Koschinsky et al. (1996) determined the
growth rates of the surface layers of two NE Atlantic
manganese crusts to be 3 and 4.5 mm Ma
-1
using this
method. Mangini et al. (1990) and Usui et al. (1993)
also determined the growth rates of S.W. Pacific
nodules to be in the range 1.1-6.0 mm Ma
-1
.
A particularly interesting application of the
10
Be
method involved the dating of entrapped manganese
nodules in an ODP sediment core taken in the Campbell
Nodule Field located at the foot of the Campbell
Plateau in the Southwestern Pacific Basin (Graham et
al. 2004). From the ages of the rims of the buried
nodules, it was concluded that bottom water circulation
was more intense, and therefore sedimentation rates
were lower, during the periods 10-5.5 Ma and 1.5-0 Ma
which facilitated the maintenance of the nodules at
the sediment surface. However, a major limitation of
the
10
Be method is that it does not permit the direct
determination of the ages of the earliest sections of
Co-rich Mn crusts.
10
Be dating must therefore be used
in conjunction with other methods for this purpose.
Sr isotope stratigraphy has also been developed
as a high resolution stratigraphic method for determining the growth rates of hydrogenous and hydrothermal Mn crusts based on a comparison of the Sr
isotope ratios of individual layers in the crust with Sr
isotope curves for seawater (Futa et al. 1988). In
principal, this method can be used to identify hiatuses
in the growth of the crusts to ages in excess of 20 Ma.
However, von der Haar et al. (1995) subsequently
demonstrated that Sr in the crusts is extracted from
both the phosphatic and detrital phases of the crust
during leaching and that the Sr in the Mn oxide phase
exchanges with seawater throughout the history of
the crust. The method therefore appears to be of little
value in dating Mn crusts.
In addition, indirect methods of dating Mn crusts
have been developed such as the one based on the
inverse relationship between the Co content of the
crusts and their rate of accumulation (Manheim and
Lane-Bostwick 1988; Puteanus and Halbach 1988).
11 Manganese: Predominant Role of Nodules and Crusts
manganese nodules from the Peru Basin. These
nodules were shown to have high growth rates of
~110 mm Ma
-1
but with frequent oscillations in growth
rate which were attributed to transitions in the glacial/
interglacial cycle. These climatic variations were
assumed to control the depth of the suboxic/oxic
boundary in the sediment leading to changing growth
rates and the development of distinct laminations
within the nodules. However, von Stackelberg (1997,
2000) subsequently observed that the highest growth
rates in these nodules occur on the underside of large
nodules which repeatedly sink to a level immediately
above the redox boundary where diagenetic recycling
of Mn is at a maximum. These authors therefore suggested that layering of these nodules was the result
of the lifting of these nodules by benthic organisms
within the oxic surface sediments from a diagenetic to
a hydrogenous environment rather than the result of
climatic change as proposed by Böllhofer et al. (1996,
1999).
Han et al. (2003) also established that there are
four orders of basic cyclic growth patterns in a deepsea manganese nodule from the C-C F.Z. based on
analysis of samples taken at intervals of 0.1 mm in the
outer 1.3 mm of the nodule using the
230
Th excess /
232
Th
method. These growth patterns were named laminae
bonds, laminae zones, laminae groups and laminae
pairs and had thicknesses of 402-454, 185-206, 58-67
and 15-18 µm, respectively. This rhythmic growth was
considered to be related to Milankovitch cycles with
the growth cycles of the laminae, bands and zones
corresponding to the periods of eccentricity, obliquity
and precession of the Earth’s orbit, respectively. From
the thicknesses of these laminae, it was was then
possible to establish a net growth rate of the outer
layers of the nodule of 4.5 mm Ma
-1
based on the
Earth’s orbital cycle in close agreement with the rate
of 4.6 mm Ma
-1
determined by
230
Th excess /
232
Th dating
of the nodule. This is an excellent example of the high
resolution that can now be attained using
230
Th/
232
Th
dating.
10
Be dating involves a measurement of the depth
profile of
10
Be in a deep-sea nodule or crust. The half
life of
10
Be is 1.5 Ma which normally permits dating
back to ~10 Ma, although earlier authors dated crusts
back to ~14 Ma by analyzing larger amounts of material
for the older samples (Segl et al. 1989).
10
Be is a
cosmogenic nuclide which is deposited at the sea
surface before being mixed into the oceans. Its
residence time in the ocean is about 1,000 yrs. Because
of its greater half life,
10
Be can be measured to much
greater depths within the nodule or crust (> 40 mm)
than the U decay-series isotopes (1-2 mm). Postdepositional diffusion of
10
Be is not considered a
problem in determining the growth rates of the nodule
or crust (cf. Kusakabe and Ku 1984). The development
of accelerator mass spectrometry (AMS) led to a
reduction in sample sizes and counting times by
several orders of magnitude compared to measurements based on radioactive decay. Much better
resolution can therefore be obtained and dating to 15
Ma is now possible. This predates the age of the lower
Miocene Antarctic glaciation. Discontinuities in the
growth rates of nodules and crusts can also be
determined from breaks in the
10
Be depth profile. As
an example, Koschinsky et al. (1996) determined the
growth rates of the surface layers of two NE Atlantic
manganese crusts to be 3 and 4.5 mm Ma
-1
using this
method. Mangini et al. (1990) and Usui et al. (1993)
also determined the growth rates of S.W. Pacific
nodules to be in the range 1.1-6.0 mm Ma
-1
.
A particularly interesting application of the
10
Be
method involved the dating of entrapped manganese
nodules in an ODP sediment core taken in the Campbell
Nodule Field located at the foot of the Campbell
Plateau in the Southwestern Pacific Basin (Graham et
al. 2004). From the ages of the rims of the buried
nodules, it was concluded that bottom water circulation
was more intense, and therefore sedimentation rates
were lower, during the periods 10-5.5 Ma and 1.5-0 Ma
which facilitated the maintenance of the nodules at
the sediment surface. However, a major limitation of
the
10
Be method is that it does not permit the direct
determination of the ages of the earliest sections of
Co-rich Mn crusts.
10
Be dating must therefore be used
in conjunction with other methods for this purpose.
Sr isotope stratigraphy has also been developed
as a high resolution stratigraphic method for determining the growth rates of hydrogenous and hydrothermal Mn crusts based on a comparison of the Sr
isotope ratios of individual layers in the crust with Sr
isotope curves for seawater (Futa et al. 1988). In
principal, this method can be used to identify hiatuses
in the growth of the crusts to ages in excess of 20 Ma.
However, von der Haar et al. (1995) subsequently
demonstrated that Sr in the crusts is extracted from
both the phosphatic and detrital phases of the crust
during leaching and that the Sr in the Mn oxide phase
exchanges with seawater throughout the history of
the crust. The method therefore appears to be of little
value in dating Mn crusts.
In addition, indirect methods of dating Mn crusts
have been developed such as the one based on the
inverse relationship between the Co content of the
crusts and their rate of accumulation (Manheim and
Lane-Bostwick 1988; Puteanus and Halbach 1988).