410
11 Manganese: Predominant Role of Nodules and Crusts
in the area form under the influence of strong, stable
bottom currents which ensure long-term contact of
the nodule surface with seawater.
In the Peru Basin, two main flow paths into the
basin have been recognized, both northwards from
the South East Pacific Basin into the Chile Basin and
thence into the Peru Basin (Lonsdale 1976). Inflow of
oceanic bottom water into the Peru Basin from the
northwest across the East Pacific Rise is blocked by
the Galapagos Rise.
11.4.10.3 Application of Isotopic Studies of
Co-rich Mn Crusts to the Study of
Paleocean Circulation
The application of isotopic studies has also led to
considerable advances in understanding the paleoceanographic conditions of formation of ferromanganese deposits in the Pacific Ocean and elsewhere
(Frank 2002). Detailed studies on the isotopic composition of various elements have been carried out on a
number of ferromanganese crusts throughout the
Pacific. These include studies on Nd (Abouchami et
al. 1997; Ling et al. 1997; O’Nions et al. 1998; Frank et
al. 1999a,b; van de Fliert et al. 2004a,b,c), Pb
(Abouchami et al. 1997; Christensen et al. 1997; Ling
et al. 1997; O’Nions et al. 1998; Frank et al. 1999a,b;
van de Fliert et al. 2003, 2004a,b,c), Hf (David et al.
1999; van de Fliert et al. 2004a,b,c), Os (Burton et al.
1999), Th (Rehkämper et al. 2004) and Fe (Beard et al.
2003; Lavasseur et al. 2004). Frank (2002) has plotted
the time slice data for the Nd and Pb isotope
distributions in the world ocean at 8 Ma, 6 Ma, 5 Ma,
1.5 Ma and the Present in which only five samples
are shown for the entire Pacific Ocean reflecting the
extremely sparse database on which to draw any
conclusions.
As an example, we may consider the application
of Nd isotopes of ferromanganese crusts to the study
of paleoceanographic conditions in the Pacific. Nd is
removed from the ocean water column by particle
scavenging and has a global ocean residence time in
deep waters of 600-2,000 years (Frank 2002). Its
isotopic distribution in seawater is characteristic of
individual water masses and is related to the
weathering of Nd from different types continental
crust with different ages. In the Pacific, the island arc
rocks surrounding the ocean are composed of young
mantle-derived material with high (radiogenic) ε Nd
values of up to + 20. The isotopic ratios are expressed
as ε Nd values because of the very small variations in
the
143
Nd/
144
Nd ratios in seawater which are mostly
in the fourth or fifth decimal place (Frank 2002).
Ling et al. (1997) analyzed the Nd isotopic composition of three ferromanganese crusts taken from different water depths (4,800, 2,300 and 1,800 m) in the central
Pacific. The ε Nd pattern of the deepest crust over the
past 20 Ma was shown to be similar to that of the two
shallower crusts but about 1.0-1.5 units more negative.
This was taken to indicate the long-term stratification
of the Pacific over this period. O’Nions et al. (1998)
subsequently confirmed the provinciality of Pacific
Ocean water relative to Atlantic and Indian Ocean
water based on an analysis of ε Nd values of crusts
from the Indian and Atlantic Oceans over this period.
Ling et al. (1997) also demonstrated a steady increase
in the ε Nd values of these crusts from 10 to 3 Ma
followed by a decrease to the present. This was
interpreted to be the result of the progressively restricted access of Atlantic Ocean water to the Pacific via
the Panama gateway which finally closed at 5-3 Ma
leading to intensification of NADW (North Atlantic
Deep Water) and greater production of AABW in the
Pacific (Burton et al. 1997). However, this conclusion
was disputed by Frank et al. (1999b) who argued that
the contribution of Atlantic waters through the
Panama gateway in the 3-4 Ma prior to its closure would
have been very small. Instead, these authors proposed
that this decrease in ε Nd values during the last 3-4 Ma
was related to an increased input of aeolian material
into the Pacific (Frank 2002).
Van de Fliert et al. (2004b) also used Nd and Pb
isotopic profiles in two carefully chosen ferromanganese crusts from the Lord Howe Rise in the S.W.
Pacific and the Nova Trough in the equatorial Pacific
to identify the relative influence of the equatorial Pacific
and southwest Pacific bottom waters on the isotopic
compositions of the crusts. The results were then used
as a time series to reconstruct Cenozoic circulation
patterns for the region. From this, it was possible to
establish the influence of the progressive build up of
Antarctic Circumpolar Current (ACC) from 38 Ma to
21/17 Ma as a result of the opening of the Tasmanian
gateway and the Drake Passage and the development
of the East Antarctic Ice Sheet from 17 Ma onwards. A
major change in the Nd isotopic composition in the
Lord Howe Rise crust from ~10 Ma onwards was also
observed which was thought to reflect the increasing
influence of equatorial Pacific deep water and was
attributed to the closure of the Indonesian Seaway
and reorganization of oceanic circulation in the S.W.
Pacific. Abouchami et al. (1997) also showed that the
Nd isotopic composition of the crusts has been more
or less constant during the last glacial stages.
The above observations demonstrate not only the
importance of isotopic studies on marine ferromanga-
11 Manganese: Predominant Role of Nodules and Crusts
in the area form under the influence of strong, stable
bottom currents which ensure long-term contact of
the nodule surface with seawater.
In the Peru Basin, two main flow paths into the
basin have been recognized, both northwards from
the South East Pacific Basin into the Chile Basin and
thence into the Peru Basin (Lonsdale 1976). Inflow of
oceanic bottom water into the Peru Basin from the
northwest across the East Pacific Rise is blocked by
the Galapagos Rise.
11.4.10.3 Application of Isotopic Studies of
Co-rich Mn Crusts to the Study of
Paleocean Circulation
The application of isotopic studies has also led to
considerable advances in understanding the paleoceanographic conditions of formation of ferromanganese deposits in the Pacific Ocean and elsewhere
(Frank 2002). Detailed studies on the isotopic composition of various elements have been carried out on a
number of ferromanganese crusts throughout the
Pacific. These include studies on Nd (Abouchami et
al. 1997; Ling et al. 1997; O’Nions et al. 1998; Frank et
al. 1999a,b; van de Fliert et al. 2004a,b,c), Pb
(Abouchami et al. 1997; Christensen et al. 1997; Ling
et al. 1997; O’Nions et al. 1998; Frank et al. 1999a,b;
van de Fliert et al. 2003, 2004a,b,c), Hf (David et al.
1999; van de Fliert et al. 2004a,b,c), Os (Burton et al.
1999), Th (Rehkämper et al. 2004) and Fe (Beard et al.
2003; Lavasseur et al. 2004). Frank (2002) has plotted
the time slice data for the Nd and Pb isotope
distributions in the world ocean at 8 Ma, 6 Ma, 5 Ma,
1.5 Ma and the Present in which only five samples
are shown for the entire Pacific Ocean reflecting the
extremely sparse database on which to draw any
conclusions.
As an example, we may consider the application
of Nd isotopes of ferromanganese crusts to the study
of paleoceanographic conditions in the Pacific. Nd is
removed from the ocean water column by particle
scavenging and has a global ocean residence time in
deep waters of 600-2,000 years (Frank 2002). Its
isotopic distribution in seawater is characteristic of
individual water masses and is related to the
weathering of Nd from different types continental
crust with different ages. In the Pacific, the island arc
rocks surrounding the ocean are composed of young
mantle-derived material with high (radiogenic) ε Nd
values of up to + 20. The isotopic ratios are expressed
as ε Nd values because of the very small variations in
the
143
Nd/
144
Nd ratios in seawater which are mostly
in the fourth or fifth decimal place (Frank 2002).
Ling et al. (1997) analyzed the Nd isotopic composition of three ferromanganese crusts taken from different water depths (4,800, 2,300 and 1,800 m) in the central
Pacific. The ε Nd pattern of the deepest crust over the
past 20 Ma was shown to be similar to that of the two
shallower crusts but about 1.0-1.5 units more negative.
This was taken to indicate the long-term stratification
of the Pacific over this period. O’Nions et al. (1998)
subsequently confirmed the provinciality of Pacific
Ocean water relative to Atlantic and Indian Ocean
water based on an analysis of ε Nd values of crusts
from the Indian and Atlantic Oceans over this period.
Ling et al. (1997) also demonstrated a steady increase
in the ε Nd values of these crusts from 10 to 3 Ma
followed by a decrease to the present. This was
interpreted to be the result of the progressively restricted access of Atlantic Ocean water to the Pacific via
the Panama gateway which finally closed at 5-3 Ma
leading to intensification of NADW (North Atlantic
Deep Water) and greater production of AABW in the
Pacific (Burton et al. 1997). However, this conclusion
was disputed by Frank et al. (1999b) who argued that
the contribution of Atlantic waters through the
Panama gateway in the 3-4 Ma prior to its closure would
have been very small. Instead, these authors proposed
that this decrease in ε Nd values during the last 3-4 Ma
was related to an increased input of aeolian material
into the Pacific (Frank 2002).
Van de Fliert et al. (2004b) also used Nd and Pb
isotopic profiles in two carefully chosen ferromanganese crusts from the Lord Howe Rise in the S.W.
Pacific and the Nova Trough in the equatorial Pacific
to identify the relative influence of the equatorial Pacific
and southwest Pacific bottom waters on the isotopic
compositions of the crusts. The results were then used
as a time series to reconstruct Cenozoic circulation
patterns for the region. From this, it was possible to
establish the influence of the progressive build up of
Antarctic Circumpolar Current (ACC) from 38 Ma to
21/17 Ma as a result of the opening of the Tasmanian
gateway and the Drake Passage and the development
of the East Antarctic Ice Sheet from 17 Ma onwards. A
major change in the Nd isotopic composition in the
Lord Howe Rise crust from ~10 Ma onwards was also
observed which was thought to reflect the increasing
influence of equatorial Pacific deep water and was
attributed to the closure of the Indonesian Seaway
and reorganization of oceanic circulation in the S.W.
Pacific. Abouchami et al. (1997) also showed that the
Nd isotopic composition of the crusts has been more
or less constant during the last glacial stages.
The above observations demonstrate not only the
importance of isotopic studies on marine ferromanga-