556
Answers to Problems
rature, Eu
2+
becomes the stable form of Eu in
hydrothermal fluids resulting in elevated concentrations of Eu relative to the other REE in
submarine hydrothermal manganese crusts
(Sverjensky 1984, Michard and Albarède 1986).
Because the REE are preferentially adsorbed on Fe
oxyhydroxide minerals, it follows that the ΣREE
contents of marine manganese deposits are a
function of the Fe content of the deposit.
For the data presented here, the Ce/La ratios of
the samples lie in the sequence SW Pacific nodule
(5.7) > Co-rich Mn crust (4.5) > C-C F.Z. (3.5) >
Peru Basin (2.0). The SW Pacific nodule therefore
formed under the most oxygenated conditions and
the Peru Basin under the least oxygenated conditions. This reflects the strength of the ocean
bottom currents and the oxygen content of the
bottom waters at these sites. The Vani Mn deposit
is a fossil stratiform hydrothermal deposit from
Milos island, Greece and the submarine hydrothermal crust both display negligible Ce anomalies
which is surprising in view of the fact that they
are clearly submarine hydrothermal deposits
which would be expected to have pronounce
negative Ce anomalies (Fleet 1983). However, the
Baltic Sea concretion displays a slight negative Ce
anomaly reflecting the less oxidizing conditions
prevalent during their formation (Szefer et al.
1998). The Eu anomaly, on the other hand, is
marked for the Vani Mn deposit, minor for the
submarine hydrothermal crust and slight for the
Baltic Sea concretion. These data indicate that the
Vani Mn deposit formed from hydrothermal fluids
which had been well above 250°C at some stage
and the submarine during their evolution. The
slightly positive Eu anomaly for the Baltic Sea
concretion probably reflects the incorporation of
plagioclase into the concretion during its growth.
Answer 5
A growth rate of 0.8 mm Ma
-1
of Co-rich Mn crusts
makes them of the slowest growing minerals on
Earth. This low growth rate for hydrogenous
deep-sea manganese deposits is a reflection of the
extremely low concentration on Mn in deep-sea
water (~0.01 ppb, Fig. 11.1) and the slow rate of
removal of Mn from seawater. Growth rates of
manganese nodules formed on a red clay
substrate have been estimated to be 1-2 mm Ma
-1
,
of nodules formed on a siliceous ooze substrate 38 mm Ma
-1
and on nodules formed on a hemipelagic clay substrate 20-50 mm Ma
-1
(Huh and Ku
1984). Growth rates of nodules formed as a result
of oxic (siliceous ooze) and suboxic (hemipelagic
clay) diagenesis are higher than those of
hydrogenous nodules (red clay) because of an
additional contribution from Mn derived
remobilization of Mn from the sediment pore water
(Sawlan and Murray 1983, Dymond et al. 1984,
Müller et al 1988). This frequently results in
higher growth rates on the underside of the
nodules than on the upper surface (Finney et al.
1984). By contrast, very much higher growth rates
are encountered for ferromanganese concretions
from the Baltic Sea and from submarine hydrothermal manganese crusts. In the case of ferromanganese concretions from Mecklenburg Bay,
this reflects the enhanced migration of Mn from
below the redoxocline to shallower, more oxidizing
environments during the spring and summer in
this part of the Baltic (Liebetrau et al. 2002). In the
case of submarine hydrothermal manganese
crusts, reducing hydrothermal fluids migrate
upwards through the volcanic sands to form a
manganese-cemented hardpan within the upper
several cm of the sea floor (Usui and Nishimura
1992). Dense submetallic layers of pure manganate
minerals can then develop beneath this hardpan to
form the submarine hydrothermal manganese crust
which acts as a trap to ascending hydrothermal
fluids. Several authors have proposed that these
crusts grow downwards on the underside of the
crusts. This conclusion is supported by the younger ages of the underside of submarine hydrothermal manganese crusts from the Izu-Bonin Arc (03,000 years) compared to those from the upper
surfaces (40,000-50,000 years; Usui and Nishimura
1992). The rapid formation of these crusts is
facilitated by the high concentrations of Mn in
submarine hydro-thermal fluids which range from
0.5 to 390 ppm (German and Von Damm 2004).
Chapter 12
Answer 1
ω = 2.7 cm kyr
-1
, burial rate C org. = 0.0125 mg cm
-2
yr
-1
,
degree of organic carbon preservation = 12.5 %.
Answer 2
Sediment traps cannot resolve the influence of
lateral particle transport processes. At best, these
Answers to Problems
rature, Eu
2+
becomes the stable form of Eu in
hydrothermal fluids resulting in elevated concentrations of Eu relative to the other REE in
submarine hydrothermal manganese crusts
(Sverjensky 1984, Michard and Albarède 1986).
Because the REE are preferentially adsorbed on Fe
oxyhydroxide minerals, it follows that the ΣREE
contents of marine manganese deposits are a
function of the Fe content of the deposit.
For the data presented here, the Ce/La ratios of
the samples lie in the sequence SW Pacific nodule
(5.7) > Co-rich Mn crust (4.5) > C-C F.Z. (3.5) >
Peru Basin (2.0). The SW Pacific nodule therefore
formed under the most oxygenated conditions and
the Peru Basin under the least oxygenated conditions. This reflects the strength of the ocean
bottom currents and the oxygen content of the
bottom waters at these sites. The Vani Mn deposit
is a fossil stratiform hydrothermal deposit from
Milos island, Greece and the submarine hydrothermal crust both display negligible Ce anomalies
which is surprising in view of the fact that they
are clearly submarine hydrothermal deposits
which would be expected to have pronounce
negative Ce anomalies (Fleet 1983). However, the
Baltic Sea concretion displays a slight negative Ce
anomaly reflecting the less oxidizing conditions
prevalent during their formation (Szefer et al.
1998). The Eu anomaly, on the other hand, is
marked for the Vani Mn deposit, minor for the
submarine hydrothermal crust and slight for the
Baltic Sea concretion. These data indicate that the
Vani Mn deposit formed from hydrothermal fluids
which had been well above 250°C at some stage
and the submarine during their evolution. The
slightly positive Eu anomaly for the Baltic Sea
concretion probably reflects the incorporation of
plagioclase into the concretion during its growth.
Answer 5
A growth rate of 0.8 mm Ma
-1
of Co-rich Mn crusts
makes them of the slowest growing minerals on
Earth. This low growth rate for hydrogenous
deep-sea manganese deposits is a reflection of the
extremely low concentration on Mn in deep-sea
water (~0.01 ppb, Fig. 11.1) and the slow rate of
removal of Mn from seawater. Growth rates of
manganese nodules formed on a red clay
substrate have been estimated to be 1-2 mm Ma
-1
,
of nodules formed on a siliceous ooze substrate 38 mm Ma
-1
and on nodules formed on a hemipelagic clay substrate 20-50 mm Ma
-1
(Huh and Ku
1984). Growth rates of nodules formed as a result
of oxic (siliceous ooze) and suboxic (hemipelagic
clay) diagenesis are higher than those of
hydrogenous nodules (red clay) because of an
additional contribution from Mn derived
remobilization of Mn from the sediment pore water
(Sawlan and Murray 1983, Dymond et al. 1984,
Müller et al 1988). This frequently results in
higher growth rates on the underside of the
nodules than on the upper surface (Finney et al.
1984). By contrast, very much higher growth rates
are encountered for ferromanganese concretions
from the Baltic Sea and from submarine hydrothermal manganese crusts. In the case of ferromanganese concretions from Mecklenburg Bay,
this reflects the enhanced migration of Mn from
below the redoxocline to shallower, more oxidizing
environments during the spring and summer in
this part of the Baltic (Liebetrau et al. 2002). In the
case of submarine hydrothermal manganese
crusts, reducing hydrothermal fluids migrate
upwards through the volcanic sands to form a
manganese-cemented hardpan within the upper
several cm of the sea floor (Usui and Nishimura
1992). Dense submetallic layers of pure manganate
minerals can then develop beneath this hardpan to
form the submarine hydrothermal manganese crust
which acts as a trap to ascending hydrothermal
fluids. Several authors have proposed that these
crusts grow downwards on the underside of the
crusts. This conclusion is supported by the younger ages of the underside of submarine hydrothermal manganese crusts from the Izu-Bonin Arc (03,000 years) compared to those from the upper
surfaces (40,000-50,000 years; Usui and Nishimura
1992). The rapid formation of these crusts is
facilitated by the high concentrations of Mn in
submarine hydro-thermal fluids which range from
0.5 to 390 ppm (German and Von Damm 2004).
Chapter 12
Answer 1
ω = 2.7 cm kyr
-1
, burial rate C org. = 0.0125 mg cm
-2
yr
-1
,
degree of organic carbon preservation = 12.5 %.
Answer 2
Sediment traps cannot resolve the influence of
lateral particle transport processes. At best, these