389
contents of the shallow-water continental margin
concretions was taken to reflect the remobilization of
Mn in the sediment column which resulted in the
fractionation of Mn from Fe, Co, Ni, Cu and REE in the
concretions. It was suggested that the REE were
adsorbed from seawater onto colloidal FeOOH in the
nodules. The high rate of manganese remobilization in
the shallow-water continental margin sediments also
resulted in growth rates of the associated concretions
two orders of magnitude higher than those of the deepsea nodules (Ku and Glasby 1972).
Based on these ideas, Glasby et al. (1987) attempted
to use the Ce/La ratios of deep-sea manganese nodules
as a means of tracing the flow path of the AABW in
the S.W. and Central Pacific Ocean. On this basis, the
Ce/La ratios in the deep-sea nodules should decrease
along the flowpath of the AABW as the oxygen in the
bottom waters is consumed. The data confirmed that
the ΣREE contents of the nodules are strongly
correlated with Fe resulting in higher REE contents in
nodules from the Southwestern Pacific Basin compared to those from the C-C F.Z (La 169 v 93 ppm).
They also showed a systematic decrease in the Ce/La
ratios of the nodules from the Southwestern Pacific
Basin to the C-C F.Z. and on to the Peru Basin as
follows: Area III (9.6) > Area K (8.0) > Area C (3.7) >
Peru Basin (2.0) > Area G (1.5) > Area F (1.4). The Peru
Basin and Areas G and F are situated in the equatorial
South Pacific in regions which do not lie directly under
the flow path of the AABW and which are characterized by sluggish bottom water flow (Nemoto and
Kroenke 1981; Davies 1985). This factor explains the
low Ce/La ratios of these samples. The sequence of
these ratios confirms that the Ce/La ratio of deep-sea
manganese nodules can be used to trace the flow path
of the AABW and therefore that the Ce/La ratios of
manganese nodules can be used as a redox indicator
in the deep-sea environment. Subsequent work on the
REE contents of manganese nodules taken on an E-W
transect across the Southwestern Pacific Basin at 42°S
showed that the highest Ce/La ratios and ΣREE
contents of the nodules occur in the central part of
the basin which is exposed to the strongest flow of
AABW (and therefore the most oxygenated conditions) (Kunzendorf et al. 1993).
More recently, Kasten et al. (1998) have determined
the REE contents of a suite of manganese nodules
from the South Atlantic to see if a similar relationship
between the Ce/La ratios of the nodules and AABW
flow could be observed there. In fact, the Ce/La ratios
of the nodules decrease in the sequence Lazarev Sea,
Weddell Sea (10.4 and 9.7) > East Georgia Basin (6.57.1) > Argentine Basin (5.0) but then increase in the
Brazil Basin (6.2) and Angola Basin (9.8 and 15.1). A
further decrease was observed in the Cape Basin (7.6).
In addition, an extremely high Ce/La ratio of 24.4 had
already been determined for nodules sampled north of
the Nares Abyssal Plain in the North Atlantic. These
data reflect the more complicated pattern of bottom
water flow in the Atlantic compared to the Pacific Ocean.
It is believed that the penetration of more oxygenated
North Atlantic Deep Water (NADW) into the South
Atlantic accounts for the higher Ce/La ratios in the
nodules from the Angola and Brazil Basins. The
influence of AABW could therefore be traced only as
far north as the Argentine Basin. These data confirm
the potential importance of REE studies in deep-sea
manganese nodules in tracing oceanic bottom water
flow.
Of particular interest in this regard is the study of
REE patterns of different types of nodules from
DOMES Site A in the C-C F.Z. by Calvert et al. (1987)
on nodules which had previously been investigated
by Piper and Blueford (1982) and Calvert and Piper
(1984) (see sections 11.4.2 and 11.4.10.2). These
authors demonstrated that there were three endmember types of nodules at this site, granular nodules
from the valley which formed on thin (<20 cm)
Quaternary sedimentary sections and displayed the
highest Mn/Fe, Cu/Ni, todorokite/δMnO 2 ratios and
the lowest Ce/La ratios, smooth nodules from the
uplands which formed on thick (>36 cm) Quaternary
sections and displayed the lowest Mn/Fe, Cu/Ni,
todorokite/δMnO 2 ratios and highest Ce/La ratios and
an intermediate type of nodules. The authors interpreted these differences in Ce/La ratios of the nodules
in terms of variations in the composition of seawater
from which the nodules formed, either from geochemically more evolved AABW in the valley or from more
local sources of seawater in the highlands. However,
other authors have interpreted such local variations
in nodule type in other areas of rugged bottom
topography in the C-C F.Z. in terms of the formation of
three genetic types of nodules, hydrogenous, diagenetic, and mixed hydrogenous/diagenetic (Heye 1978;
Glasby et al. 1982; Skornyakova and Murdmaa 1992).
At DOMES Site A, the extreme hydrogenous endmember is characterized by a Mn/Fe ratio of 1.1, Ni + Cu of
1.0% and a Ce/La ratio of 2.7 and the extreme diagenetic
endmember by a Mn/Fe ratio of 5.2, Ni + Cu of 2.9%
and a Ce/La ratio of 2.5. Although the Ce/La ratios for
these two nodules are quite similar, the highest Ce/La
ratio for the hydrogenous nodules was 3.7 and the
lowest ratio for the diagenetic nodules 1.6 (see Glasby
et al. 1987; Kunzendorf et al. 1993). This suggests that
variations in the REE patterns of nodules at DOMES
11.4
Manganese Nodules and Crusts
contents of the shallow-water continental margin
concretions was taken to reflect the remobilization of
Mn in the sediment column which resulted in the
fractionation of Mn from Fe, Co, Ni, Cu and REE in the
concretions. It was suggested that the REE were
adsorbed from seawater onto colloidal FeOOH in the
nodules. The high rate of manganese remobilization in
the shallow-water continental margin sediments also
resulted in growth rates of the associated concretions
two orders of magnitude higher than those of the deepsea nodules (Ku and Glasby 1972).
Based on these ideas, Glasby et al. (1987) attempted
to use the Ce/La ratios of deep-sea manganese nodules
as a means of tracing the flow path of the AABW in
the S.W. and Central Pacific Ocean. On this basis, the
Ce/La ratios in the deep-sea nodules should decrease
along the flowpath of the AABW as the oxygen in the
bottom waters is consumed. The data confirmed that
the ΣREE contents of the nodules are strongly
correlated with Fe resulting in higher REE contents in
nodules from the Southwestern Pacific Basin compared to those from the C-C F.Z (La 169 v 93 ppm).
They also showed a systematic decrease in the Ce/La
ratios of the nodules from the Southwestern Pacific
Basin to the C-C F.Z. and on to the Peru Basin as
follows: Area III (9.6) > Area K (8.0) > Area C (3.7) >
Peru Basin (2.0) > Area G (1.5) > Area F (1.4). The Peru
Basin and Areas G and F are situated in the equatorial
South Pacific in regions which do not lie directly under
the flow path of the AABW and which are characterized by sluggish bottom water flow (Nemoto and
Kroenke 1981; Davies 1985). This factor explains the
low Ce/La ratios of these samples. The sequence of
these ratios confirms that the Ce/La ratio of deep-sea
manganese nodules can be used to trace the flow path
of the AABW and therefore that the Ce/La ratios of
manganese nodules can be used as a redox indicator
in the deep-sea environment. Subsequent work on the
REE contents of manganese nodules taken on an E-W
transect across the Southwestern Pacific Basin at 42°S
showed that the highest Ce/La ratios and ΣREE
contents of the nodules occur in the central part of
the basin which is exposed to the strongest flow of
AABW (and therefore the most oxygenated conditions) (Kunzendorf et al. 1993).
More recently, Kasten et al. (1998) have determined
the REE contents of a suite of manganese nodules
from the South Atlantic to see if a similar relationship
between the Ce/La ratios of the nodules and AABW
flow could be observed there. In fact, the Ce/La ratios
of the nodules decrease in the sequence Lazarev Sea,
Weddell Sea (10.4 and 9.7) > East Georgia Basin (6.57.1) > Argentine Basin (5.0) but then increase in the
Brazil Basin (6.2) and Angola Basin (9.8 and 15.1). A
further decrease was observed in the Cape Basin (7.6).
In addition, an extremely high Ce/La ratio of 24.4 had
already been determined for nodules sampled north of
the Nares Abyssal Plain in the North Atlantic. These
data reflect the more complicated pattern of bottom
water flow in the Atlantic compared to the Pacific Ocean.
It is believed that the penetration of more oxygenated
North Atlantic Deep Water (NADW) into the South
Atlantic accounts for the higher Ce/La ratios in the
nodules from the Angola and Brazil Basins. The
influence of AABW could therefore be traced only as
far north as the Argentine Basin. These data confirm
the potential importance of REE studies in deep-sea
manganese nodules in tracing oceanic bottom water
flow.
Of particular interest in this regard is the study of
REE patterns of different types of nodules from
DOMES Site A in the C-C F.Z. by Calvert et al. (1987)
on nodules which had previously been investigated
by Piper and Blueford (1982) and Calvert and Piper
(1984) (see sections 11.4.2 and 11.4.10.2). These
authors demonstrated that there were three endmember types of nodules at this site, granular nodules
from the valley which formed on thin (<20 cm)
Quaternary sedimentary sections and displayed the
highest Mn/Fe, Cu/Ni, todorokite/δMnO 2 ratios and
the lowest Ce/La ratios, smooth nodules from the
uplands which formed on thick (>36 cm) Quaternary
sections and displayed the lowest Mn/Fe, Cu/Ni,
todorokite/δMnO 2 ratios and highest Ce/La ratios and
an intermediate type of nodules. The authors interpreted these differences in Ce/La ratios of the nodules
in terms of variations in the composition of seawater
from which the nodules formed, either from geochemically more evolved AABW in the valley or from more
local sources of seawater in the highlands. However,
other authors have interpreted such local variations
in nodule type in other areas of rugged bottom
topography in the C-C F.Z. in terms of the formation of
three genetic types of nodules, hydrogenous, diagenetic, and mixed hydrogenous/diagenetic (Heye 1978;
Glasby et al. 1982; Skornyakova and Murdmaa 1992).
At DOMES Site A, the extreme hydrogenous endmember is characterized by a Mn/Fe ratio of 1.1, Ni + Cu of
1.0% and a Ce/La ratio of 2.7 and the extreme diagenetic
endmember by a Mn/Fe ratio of 5.2, Ni + Cu of 2.9%
and a Ce/La ratio of 2.5. Although the Ce/La ratios for
these two nodules are quite similar, the highest Ce/La
ratio for the hydrogenous nodules was 3.7 and the
lowest ratio for the diagenetic nodules 1.6 (see Glasby
et al. 1987; Kunzendorf et al. 1993). This suggests that
variations in the REE patterns of nodules at DOMES
11.4
Manganese Nodules and Crusts