409
water into the C-C F.Z. Fig. 11.24 illustrates the principal
pathways of oceanic bottom water flow mentioned
above.
In an early study of oceanic bottom water flow
within the C-C F.Z., Johnson (1972) deployed free-fall
bottom current meters in an area north of the Clipperton
Fracture Zone where substantial sediment erosion was
known to occur. The limited data showed that the
bottom currents were generally slow (<10 cm sec
-1
)
but fluctuated markedly due to a strong semi-diurnal
tidal component. It was also established that the
currents flowed mainly to the east with minor variations due to topographic effects. In addition, data
from a 14-day record of bottom current measurements
taken at 210 m above the sea floor revealed an averge
bottom water flow of 2.0 cm sec
-1
in an ENE direction
with peak velocities of up to 16.5 cm sec
-1
at semidiurnal
periods. These data showed that peak velocities of
bottom water transport were strong enough to erode
and transport sediment in the area (Amos et al. 1977).
More detailed studies in the C-C F.Z. were carried
out during the period 1986 to 1989 in which a grid of
bottom-moored instruments was deployed (Kontar
and Sokov 1994). In general, the bottom currents
were shown to be weak with velocities not greater
than 4 cm sec
-1
. However, a benthic storm lasting 10 days
was recorded during these observations in which bottom
currents with average velocities of 10 -15 cm sec
-1
were
measured. It was thought that these storms were related to periods when the sea surface was elevated by
more than 4 cm above the norm which would lead to
the excess kinetic energy at the sea surface being
transmitted to the seafloor at depths > 3,000 m (Kontar
and Sokov 1994). Other measurements of oceanic
bottom water flow in the DOMES areas of the C-C F.Z.
have been reported by Hayes (1979, 1998).
Within the Samoan Passage, the most rapid drift
deposition occurred in the Late Oligocene-Early
Miocene and in the Late Miocene-Early Pliocene
(Lonsdale 1981). Both these periods were characterized
by intensified bottom circulation These times correspond roughly with periods of enhanced sediment
erosion in the C-C F.Z. (van Andel et al. 1975) in accordance with the ages of the nodules in the C-C F.Z. as
reported by Shilov (2004) (see section 11.4.9).
Mangini et al. (1982) also established that there
had been a marked increase in deep circulation in the
Pacific at ~70 ka corresponding to the end of interglacial
cycle stage 5 based on
230
Th and
231
Pa dating in 9 out
of 13 sediment cores from the C-C F.Z. and in all 4
cores from the Aitutaki Passage. These results
supported the idea that the onset of glacial events
increased ocean bottom water flow in the C-C F.Z.,
although this effect may have been masked by
bioturbation in the sediment cores in some cases.
In a detailed study of the DOMES Site A in the
C-C F.Z., Piper and Blueford (1982) described an
apparently anomalous situation in which small, polynucleate nodules with smooth surface texture and
δMnO 2 as the principal manganese oxide phase formed
in Quaternary sediments with variable thickness >40
cm located on the side of an W-E-trending valley
whereas large nodules with granular surface texture
and todorokite as the principal manganese oxide phase
formed on Tertiary sediments at sites of active erosion
on the sea floor. This is contrary to the normal trend
where the diagenetic influence on nodules growth is
more pronounced in those areas with higher sedimentation rates (see section 11.4.2). However, this area
is unusual in that the topographic fabric lies W-E. It
was therefore thought that the AABW would be
channelled through the valley creating an erosional
area on its southern flanks. In other areas of the C-C
F.Z., however, the alignment of the topographic fabric
may lie normal to the flow path of the AABW.
The data presented above paint a picture of relatively weak bottom currents in the C-C F.Z. resulting
from the limited inflow of bottom water through the
Clarion Passage. However, physical oceanographic
data suggest that there are at least three components
to the bottom currents, namely AABW flow, tidal
effects and benthic storms. At present, it is not possible
to quantify the relative importance of these components, although it is believed that the intensity of
bottom water flow has fluctuated through time, for
example during glaciations. Nonetheless, there is
indirect evidence that the C-C F.Z. has been a dynamic
and fluctating environment during the entire period of
formation of deep-sea manganese nodules. Of particular interest is the observation of Knoop et al. (1998)
that the relative influence of oxic diagenesis declines
and that of suboxic diagenesis increases along the
flow path of the AABW through the C-C F.Z. This
may explain the trend of decreasing Ce/La ratios of
deep-sea manganese nodules along the flow path of
the AABW in the equatorial North Pacific as noted in
section 11.4.3. Skornyakova and Murdmaa (1992) have
also suggested that benthic storms play a key role in
the formation of diagenetic nodules in the C-C F.Z.
because they resuspend the active surface layers of
the radiolarian ooze resulting in the burial of the
nodules under a thin layer of semi-fluid sediment. This
then acts as a source of transition elements such as
Mn, Ni, Cu and Zn to the nodules and leads to the
development of the rhythmic structure characteristic
of these nodules. By contrast, hydrogenous nodules
11.4
Manganese Nodules and Crusts
water into the C-C F.Z. Fig. 11.24 illustrates the principal
pathways of oceanic bottom water flow mentioned
above.
In an early study of oceanic bottom water flow
within the C-C F.Z., Johnson (1972) deployed free-fall
bottom current meters in an area north of the Clipperton
Fracture Zone where substantial sediment erosion was
known to occur. The limited data showed that the
bottom currents were generally slow (<10 cm sec
-1
)
but fluctuated markedly due to a strong semi-diurnal
tidal component. It was also established that the
currents flowed mainly to the east with minor variations due to topographic effects. In addition, data
from a 14-day record of bottom current measurements
taken at 210 m above the sea floor revealed an averge
bottom water flow of 2.0 cm sec
-1
in an ENE direction
with peak velocities of up to 16.5 cm sec
-1
at semidiurnal
periods. These data showed that peak velocities of
bottom water transport were strong enough to erode
and transport sediment in the area (Amos et al. 1977).
More detailed studies in the C-C F.Z. were carried
out during the period 1986 to 1989 in which a grid of
bottom-moored instruments was deployed (Kontar
and Sokov 1994). In general, the bottom currents
were shown to be weak with velocities not greater
than 4 cm sec
-1
. However, a benthic storm lasting 10 days
was recorded during these observations in which bottom
currents with average velocities of 10 -15 cm sec
-1
were
measured. It was thought that these storms were related to periods when the sea surface was elevated by
more than 4 cm above the norm which would lead to
the excess kinetic energy at the sea surface being
transmitted to the seafloor at depths > 3,000 m (Kontar
and Sokov 1994). Other measurements of oceanic
bottom water flow in the DOMES areas of the C-C F.Z.
have been reported by Hayes (1979, 1998).
Within the Samoan Passage, the most rapid drift
deposition occurred in the Late Oligocene-Early
Miocene and in the Late Miocene-Early Pliocene
(Lonsdale 1981). Both these periods were characterized
by intensified bottom circulation These times correspond roughly with periods of enhanced sediment
erosion in the C-C F.Z. (van Andel et al. 1975) in accordance with the ages of the nodules in the C-C F.Z. as
reported by Shilov (2004) (see section 11.4.9).
Mangini et al. (1982) also established that there
had been a marked increase in deep circulation in the
Pacific at ~70 ka corresponding to the end of interglacial
cycle stage 5 based on
230
Th and
231
Pa dating in 9 out
of 13 sediment cores from the C-C F.Z. and in all 4
cores from the Aitutaki Passage. These results
supported the idea that the onset of glacial events
increased ocean bottom water flow in the C-C F.Z.,
although this effect may have been masked by
bioturbation in the sediment cores in some cases.
In a detailed study of the DOMES Site A in the
C-C F.Z., Piper and Blueford (1982) described an
apparently anomalous situation in which small, polynucleate nodules with smooth surface texture and
δMnO 2 as the principal manganese oxide phase formed
in Quaternary sediments with variable thickness >40
cm located on the side of an W-E-trending valley
whereas large nodules with granular surface texture
and todorokite as the principal manganese oxide phase
formed on Tertiary sediments at sites of active erosion
on the sea floor. This is contrary to the normal trend
where the diagenetic influence on nodules growth is
more pronounced in those areas with higher sedimentation rates (see section 11.4.2). However, this area
is unusual in that the topographic fabric lies W-E. It
was therefore thought that the AABW would be
channelled through the valley creating an erosional
area on its southern flanks. In other areas of the C-C
F.Z., however, the alignment of the topographic fabric
may lie normal to the flow path of the AABW.
The data presented above paint a picture of relatively weak bottom currents in the C-C F.Z. resulting
from the limited inflow of bottom water through the
Clarion Passage. However, physical oceanographic
data suggest that there are at least three components
to the bottom currents, namely AABW flow, tidal
effects and benthic storms. At present, it is not possible
to quantify the relative importance of these components, although it is believed that the intensity of
bottom water flow has fluctuated through time, for
example during glaciations. Nonetheless, there is
indirect evidence that the C-C F.Z. has been a dynamic
and fluctating environment during the entire period of
formation of deep-sea manganese nodules. Of particular interest is the observation of Knoop et al. (1998)
that the relative influence of oxic diagenesis declines
and that of suboxic diagenesis increases along the
flow path of the AABW through the C-C F.Z. This
may explain the trend of decreasing Ce/La ratios of
deep-sea manganese nodules along the flow path of
the AABW in the equatorial North Pacific as noted in
section 11.4.3. Skornyakova and Murdmaa (1992) have
also suggested that benthic storms play a key role in
the formation of diagenetic nodules in the C-C F.Z.
because they resuspend the active surface layers of
the radiolarian ooze resulting in the burial of the
nodules under a thin layer of semi-fluid sediment. This
then acts as a source of transition elements such as
Mn, Ni, Cu and Zn to the nodules and leads to the
development of the rhythmic structure characteristic
of these nodules. By contrast, hydrogenous nodules
11.4
Manganese Nodules and Crusts