405
of about 2.9 x 10
6
atoms of
60
Fe cm
-2
to the Earth’s
surface. This amount is compatible with the
60
Fe being
derived from an exploding supernova located about
10 light years from Earth. This event began at ~3.0 Ma
and lasted ~300 ka and therefore coincides with the
onset of N. Hemisphere glaciation at 2.8 Ma. The explosion released as much energy as the sun will do in its
entire lifetime. The authors speculated that the interstellar particles may have acted as nuclei for the
condensation of water and therefore cloud formation
in the atmosphere. This offers the possibility that the
Northern Hemisphere glaciation may have been controlled by interstellar events and not by changes in
CO 2 levels in atmosphere as is more commonly
assumed. A weak
244
Pu signal from this explosion was
also detected in the crust (Wallner et al. 2003).
The VA 13/2 crust has also been used for paleoceanographic studies by a number of authors (Abouchami et al. 1997; Ling et al. 1997; David et al. 2001;
Frank et al. 1999b; van de Fliert et al. 2004a).
Detailed studies have also been carried out on the
Co-rich Mn crust (KK84-RD50 S1-B), a 95 mm thick
crust, which was sampled at a depth of 2,250-2,600 m
from near the summit of Schumann Seamount, one of
the Musicians Seamounts, located 700 km north of
Kaui, Hawaii. The initial study was based on identification of coccolith imprints in the crust from which
the ages of different layers of ages of the individual
layers could be determined (Cowen et al. 1993). These
ages were not well constrained and varied from 1-4
Ma to >10 Ma. However, the minimum age for the upper
27 mm of the crust was shown to be Eocene giving an
average growth rate for the crust of about 0.5 mm Ma
-1
.
Extrapolating this growth rate through the entire 95
mm of the crust gave a very much older age for the
crust than was accepted for Co-rich crusts at that time.
Subsequently, McMurtry et al. (1994) carried out a
very detailed study of this crust using a variety of
techniques including
10
Be profiling of the crust,
determination of
87
Sr/
86
Sr and δ
18
O isotopic ratios of
included phosphatized limestone debris and vein
infillings and Co chronometry of the ferromanganese
layers. Using the
10
Be method, it was possible to date
only the upper 12 mm of this crust with the lower 82
mm of the crust lying beyond the limits of this method.
However, by combining the
10
Be data with results
obtained from microfossil dating (Cowen et al. 1993)
and Co chronometry, it was possible to establish that
formation of the crust began at least in the Eocene
and possibly as far back as the Cretaceous. Within the
crust, eight major disconformities were identified of
which three of the upper disconformities in the crust
were placed at the Plio-Pliocene, the Middle Miocene
and, tentatively, the Paleocene-Eocene boundaries. As
previously described, McMurtry et al. (1994) considered the boundary between the older and younger
crust layers had a minimum Oligocene age (28-33 Ma)
but was possibly of Late Paleocene age (55 Ma) but
this may reflect the fact that these boundaries correspond to different episodes of phosphogenesis from
that described by Halbach and Puteanus (1984) (see
section 11.4.4).
In order to assess the validity of the
87
Sr/
86
Sr
method of dating manganese crusts, von der Haar et
al. (1995) carried out a series of leaching experiments
on the Schumann Seamount crust in which it was
established that Sr was leached from both phosphatic
material and aluminosilicate detritus within the crust.
It was also shown that Sr within the oxides exchanges
with seawater strontium throughout the history of the
deposit. These results cast severe doubt on the validity of the Sr isotopic method for determining the ages
of these crusts (see section 11.4.9). In addition, De
Carlo (1991) analyzed 11 samples at various depths in
the Schumann Seamount crust for REE in an attempt
to reconstruct its growth history (see section 11.4.3).
The results are somewhat surprising in that they
show a steady increase in the Ce/La ratio of the
deposit from the base of the crust (4.5 at a depth of
78-95 mm) to a maximum of 7.0 at 63-68 mm and then
a steady decline to 3.0 at a depth of 0-5 mm. It is not
easy to understand why the most oxidizing conditions
in the water column should prevail at a depth of 6368 mm in the crust corresponding to a Paleocene age
(McMurtry et al. 1994).
In addition,
10
Be dating of seven manganese
nodules from the Southwestern Pacific Basin was
carried out by Mangini et al. (1990b). In one nodule
(143GB), extrapolation of the nodule growth rates
indicated that the nodule began forming at about 14.5
Ma. At 6 Ma, there was a major change in structure of
the nodule. Within the outer layer of the nodule, concentric layers filled with detritus corresponding to ages
of 3.2 and 1.3 Ma. However, there was no corresponding
change in growth rate at any of the intervals, the
growth rate remaining constant with time. This study
confirmed that these ages are important time markers
which had previously been recorded in many Pacific
pelagic sediments and manganese crusts and nodules.
The dates of the hiatuses reported above are
consistent with the ages listed by Frank (2002) for
major paleogeographic changes in the Pacific resulting
from the opening and closing gateways which are
thought to have been responsible for large-scale reorganizations of global ocean circulation and are linked
to changes in global climate. In the Pacific, these include
11.4
Manganese Nodules and Crusts
of about 2.9 x 10
6
atoms of
60
Fe cm
-2
to the Earth’s
surface. This amount is compatible with the
60
Fe being
derived from an exploding supernova located about
10 light years from Earth. This event began at ~3.0 Ma
and lasted ~300 ka and therefore coincides with the
onset of N. Hemisphere glaciation at 2.8 Ma. The explosion released as much energy as the sun will do in its
entire lifetime. The authors speculated that the interstellar particles may have acted as nuclei for the
condensation of water and therefore cloud formation
in the atmosphere. This offers the possibility that the
Northern Hemisphere glaciation may have been controlled by interstellar events and not by changes in
CO 2 levels in atmosphere as is more commonly
assumed. A weak
244
Pu signal from this explosion was
also detected in the crust (Wallner et al. 2003).
The VA 13/2 crust has also been used for paleoceanographic studies by a number of authors (Abouchami et al. 1997; Ling et al. 1997; David et al. 2001;
Frank et al. 1999b; van de Fliert et al. 2004a).
Detailed studies have also been carried out on the
Co-rich Mn crust (KK84-RD50 S1-B), a 95 mm thick
crust, which was sampled at a depth of 2,250-2,600 m
from near the summit of Schumann Seamount, one of
the Musicians Seamounts, located 700 km north of
Kaui, Hawaii. The initial study was based on identification of coccolith imprints in the crust from which
the ages of different layers of ages of the individual
layers could be determined (Cowen et al. 1993). These
ages were not well constrained and varied from 1-4
Ma to >10 Ma. However, the minimum age for the upper
27 mm of the crust was shown to be Eocene giving an
average growth rate for the crust of about 0.5 mm Ma
-1
.
Extrapolating this growth rate through the entire 95
mm of the crust gave a very much older age for the
crust than was accepted for Co-rich crusts at that time.
Subsequently, McMurtry et al. (1994) carried out a
very detailed study of this crust using a variety of
techniques including
10
Be profiling of the crust,
determination of
87
Sr/
86
Sr and δ
18
O isotopic ratios of
included phosphatized limestone debris and vein
infillings and Co chronometry of the ferromanganese
layers. Using the
10
Be method, it was possible to date
only the upper 12 mm of this crust with the lower 82
mm of the crust lying beyond the limits of this method.
However, by combining the
10
Be data with results
obtained from microfossil dating (Cowen et al. 1993)
and Co chronometry, it was possible to establish that
formation of the crust began at least in the Eocene
and possibly as far back as the Cretaceous. Within the
crust, eight major disconformities were identified of
which three of the upper disconformities in the crust
were placed at the Plio-Pliocene, the Middle Miocene
and, tentatively, the Paleocene-Eocene boundaries. As
previously described, McMurtry et al. (1994) considered the boundary between the older and younger
crust layers had a minimum Oligocene age (28-33 Ma)
but was possibly of Late Paleocene age (55 Ma) but
this may reflect the fact that these boundaries correspond to different episodes of phosphogenesis from
that described by Halbach and Puteanus (1984) (see
section 11.4.4).
In order to assess the validity of the
87
Sr/
86
Sr
method of dating manganese crusts, von der Haar et
al. (1995) carried out a series of leaching experiments
on the Schumann Seamount crust in which it was
established that Sr was leached from both phosphatic
material and aluminosilicate detritus within the crust.
It was also shown that Sr within the oxides exchanges
with seawater strontium throughout the history of the
deposit. These results cast severe doubt on the validity of the Sr isotopic method for determining the ages
of these crusts (see section 11.4.9). In addition, De
Carlo (1991) analyzed 11 samples at various depths in
the Schumann Seamount crust for REE in an attempt
to reconstruct its growth history (see section 11.4.3).
The results are somewhat surprising in that they
show a steady increase in the Ce/La ratio of the
deposit from the base of the crust (4.5 at a depth of
78-95 mm) to a maximum of 7.0 at 63-68 mm and then
a steady decline to 3.0 at a depth of 0-5 mm. It is not
easy to understand why the most oxidizing conditions
in the water column should prevail at a depth of 6368 mm in the crust corresponding to a Paleocene age
(McMurtry et al. 1994).
In addition,
10
Be dating of seven manganese
nodules from the Southwestern Pacific Basin was
carried out by Mangini et al. (1990b). In one nodule
(143GB), extrapolation of the nodule growth rates
indicated that the nodule began forming at about 14.5
Ma. At 6 Ma, there was a major change in structure of
the nodule. Within the outer layer of the nodule, concentric layers filled with detritus corresponding to ages
of 3.2 and 1.3 Ma. However, there was no corresponding
change in growth rate at any of the intervals, the
growth rate remaining constant with time. This study
confirmed that these ages are important time markers
which had previously been recorded in many Pacific
pelagic sediments and manganese crusts and nodules.
The dates of the hiatuses reported above are
consistent with the ages listed by Frank (2002) for
major paleogeographic changes in the Pacific resulting
from the opening and closing gateways which are
thought to have been responsible for large-scale reorganizations of global ocean circulation and are linked
to changes in global climate. In the Pacific, these include
11.4
Manganese Nodules and Crusts