403
Based on the analysis of 520 samples of ferromanganese crusts from ~250 locations, it was
established that the flux of Co into ferromanganese
crusts is constant over the entire period of growth of
the crusts whereas the flux of Mn into the crusts is
variable. As a consequence, the content of Co in
crusts is inversely proportional to the rate of formation of the crust. It therefore became possible to
derive an empirical equation relating the growth rate
of each layer in a crust to the Co content in that layer
by fitting data derived from the
10
Be dating of 20
ferromanganese crusts. This led to the formulation
of the equation G (mm Ma
-1
) = 1.28/[Co(%) – 0.24]
where G is the growth rate of the layer of the crust
under consideration from which the growth rate and
therefore the age of the crust can be derived. However,
modification of this equation is required in cases
where phosphatization of the crust has occurred. The
great advantage of this method is that it requires no
further radiometric age determinations to date a crust
of unknown age. It is therefore much easier to
calculate the growth rates of manganese crusts using
this method than by the other methods. Frank et al.
(1999a) have confirmed the validity of the Co geochronometer and shown that it provides detailed
information on the growth history of ferromanganese
crusts prior to 10-12 Ma where the
10
Be method can
not be applied.
Growth rates of shallow-water ferromanganese
concretions from Mecklenburg Bay in the Baltic Sea
have also been determined by
210
Pb dating (half life
of
210
Pb is 22.3 years) and by comparison of the
distribution of the Zn concentration (on a mm scale)
in the outer layers of selected concretions with the
distribution of Zn in dated sediment cores from the
same area (Hlawatsch 1999). These data gave growth
rates for the concretions in the range 0.018 - 0.21 mm a
-1
which are 4 to 5 orders of magnitude higher than for
the deep-sea manganese deposits. Liebetrau et al.
(2002) also determined the growth rates of concretions from this area from the distribution of
226
Ra excess /Ba ratios with depth in selected concretions
(half life of
226
Ra is 1622 years) and obtained somewhat
lower growth rates in the range 0.0075 - 0.021 mm a
-1
.
From these data, the maximum age of these
concretions was calculated to be 4,300 ± 300 years
which is close to the period when sea level stabilized
at its present level in the Baltic about 5,500 to 4,500
years ago. By contrast, very little information is
available on the growth rates of submarine hydrothermal manganese crusts at present (Lalou et
al. 1983; Usui and Nishimura 1992; Usui and
Terashima 1997).
11.4.10 Mn Crusts as Paleoceanographic
Indicators
11.4.10.1 Recording Hiatuses in Mn Crusts
Mn crusts may be considered to be condensed
stratigraphic sections that record variations in paleoceanographic conditions with time. High resolution
dating of Mn crusts can therefore be used to record
the occurrence of major paleoceanographic events.
A number of such studies has been undertaken (Segl
et al. 1989; McMurtry et al. 1994; Koschinsky et al.
1996; Frank et al. 1999a). In this section, the results
of two detailed studies of dating deep-sea manganese
crusts using various methods of radiometric dating
are presented. In addition, the distribution of the longlived radiogenic isotopes of Nd, Pb, Be, Hf and Os in
Co-rich Mn crusts has been extensively used to study
the patterns of deep ocean circulation and the provinciality of these crusts over the past 10-15 years. This
topic has been excellently reviewed by Frank (2002).
One of the most thoroughly studied Mn crusts
was collected on top of an abyssal seamount at 4,830
m water depth in the equatorial N. Pacific in 1976
during cruise VA 13/2 of R.V. Valdivia (sample 237
KD) (Friedrich and Schmitz-Wiechowski 1980). It is a
large hemispherical Fe-Mn crust 500 mm in diameter
and up to 250 mm thick partly covering a basalt
substrate. It is quite different in character from the
Co-rich Mn crusts described in section 11.4.4. The
volcanic seamount on which it occurs was formed
about 65 Ma near crest of the East Pacific Rise.
Submarine weathering of glass on the surface of the
submarine basalt led to formation of an 8 mm thick
nontronite layer. The seamount migrated north across
the equatorial high productivity belt during which
time it was above the Carbonate Compensation Depth
(CCD).
The mode of formation of this crust has been
outlined by von Stackelberg et al. (1984). Below a
depth of 40 mm, the crust was characterized by higher
Fe contents which could be attributed to the
dissolution of calcareous tests resulting in an increased supply of iron to the crust. Goethite was the main
mineral formed and could be seen as yellowish-brown
flecks. Calcareous tests were also observed within
the crust. At 40 mm, there is an abrupt change in
composition of the crust with higher Mn/Fe (1.6) and
Ce/La (3.5) ratios and higher contents of Ni (0.45%)
and Cu (0.25%) above that boundary. These changes
were associated with the development of a strong
AABW at 12 Ma. From 10-0 mm, the chemical compo11.4
Manganese Nodules and Crusts
Based on the analysis of 520 samples of ferromanganese crusts from ~250 locations, it was
established that the flux of Co into ferromanganese
crusts is constant over the entire period of growth of
the crusts whereas the flux of Mn into the crusts is
variable. As a consequence, the content of Co in
crusts is inversely proportional to the rate of formation of the crust. It therefore became possible to
derive an empirical equation relating the growth rate
of each layer in a crust to the Co content in that layer
by fitting data derived from the
10
Be dating of 20
ferromanganese crusts. This led to the formulation
of the equation G (mm Ma
-1
) = 1.28/[Co(%) – 0.24]
where G is the growth rate of the layer of the crust
under consideration from which the growth rate and
therefore the age of the crust can be derived. However,
modification of this equation is required in cases
where phosphatization of the crust has occurred. The
great advantage of this method is that it requires no
further radiometric age determinations to date a crust
of unknown age. It is therefore much easier to
calculate the growth rates of manganese crusts using
this method than by the other methods. Frank et al.
(1999a) have confirmed the validity of the Co geochronometer and shown that it provides detailed
information on the growth history of ferromanganese
crusts prior to 10-12 Ma where the
10
Be method can
not be applied.
Growth rates of shallow-water ferromanganese
concretions from Mecklenburg Bay in the Baltic Sea
have also been determined by
210
Pb dating (half life
of
210
Pb is 22.3 years) and by comparison of the
distribution of the Zn concentration (on a mm scale)
in the outer layers of selected concretions with the
distribution of Zn in dated sediment cores from the
same area (Hlawatsch 1999). These data gave growth
rates for the concretions in the range 0.018 - 0.21 mm a
-1
which are 4 to 5 orders of magnitude higher than for
the deep-sea manganese deposits. Liebetrau et al.
(2002) also determined the growth rates of concretions from this area from the distribution of
226
Ra excess /Ba ratios with depth in selected concretions
(half life of
226
Ra is 1622 years) and obtained somewhat
lower growth rates in the range 0.0075 - 0.021 mm a
-1
.
From these data, the maximum age of these
concretions was calculated to be 4,300 ± 300 years
which is close to the period when sea level stabilized
at its present level in the Baltic about 5,500 to 4,500
years ago. By contrast, very little information is
available on the growth rates of submarine hydrothermal manganese crusts at present (Lalou et
al. 1983; Usui and Nishimura 1992; Usui and
Terashima 1997).
11.4.10 Mn Crusts as Paleoceanographic
Indicators
11.4.10.1 Recording Hiatuses in Mn Crusts
Mn crusts may be considered to be condensed
stratigraphic sections that record variations in paleoceanographic conditions with time. High resolution
dating of Mn crusts can therefore be used to record
the occurrence of major paleoceanographic events.
A number of such studies has been undertaken (Segl
et al. 1989; McMurtry et al. 1994; Koschinsky et al.
1996; Frank et al. 1999a). In this section, the results
of two detailed studies of dating deep-sea manganese
crusts using various methods of radiometric dating
are presented. In addition, the distribution of the longlived radiogenic isotopes of Nd, Pb, Be, Hf and Os in
Co-rich Mn crusts has been extensively used to study
the patterns of deep ocean circulation and the provinciality of these crusts over the past 10-15 years. This
topic has been excellently reviewed by Frank (2002).
One of the most thoroughly studied Mn crusts
was collected on top of an abyssal seamount at 4,830
m water depth in the equatorial N. Pacific in 1976
during cruise VA 13/2 of R.V. Valdivia (sample 237
KD) (Friedrich and Schmitz-Wiechowski 1980). It is a
large hemispherical Fe-Mn crust 500 mm in diameter
and up to 250 mm thick partly covering a basalt
substrate. It is quite different in character from the
Co-rich Mn crusts described in section 11.4.4. The
volcanic seamount on which it occurs was formed
about 65 Ma near crest of the East Pacific Rise.
Submarine weathering of glass on the surface of the
submarine basalt led to formation of an 8 mm thick
nontronite layer. The seamount migrated north across
the equatorial high productivity belt during which
time it was above the Carbonate Compensation Depth
(CCD).
The mode of formation of this crust has been
outlined by von Stackelberg et al. (1984). Below a
depth of 40 mm, the crust was characterized by higher
Fe contents which could be attributed to the
dissolution of calcareous tests resulting in an increased supply of iron to the crust. Goethite was the main
mineral formed and could be seen as yellowish-brown
flecks. Calcareous tests were also observed within
the crust. At 40 mm, there is an abrupt change in
composition of the crust with higher Mn/Fe (1.6) and
Ce/La (3.5) ratios and higher contents of Ni (0.45%)
and Cu (0.25%) above that boundary. These changes
were associated with the development of a strong
AABW at 12 Ma. From 10-0 mm, the chemical compo11.4
Manganese Nodules and Crusts