555
Answers to Problems
higher nitrate availability during the upwelling
season, less nitrate utilization.
δ
15
N = 8‰ => f = 0.07
quick depletion of nitrate, low productivity.
Answer 5
The glacial pH was about 0.2 units higher. Hence,
the carbonate ion concentration was higher and
the atmospheric pCO2 was lower during the last
glacial, in accordance with ice core observations.
Chapter 11
Answer 1
To be meaningful, an E H , pH diagram must reflect
the environmental conditions of the system
under consideration. The E H , pH diagram of Mn
presented in Fig. 11.2 was calculated for the
following conditions pertaining to deep seawater
from the Angola Basin; temperature 2°C, density
(1.023 g cm
-3
), Ca 427 ppm, Mg 1,284 ppm, Na
10,760 ppm, K 390 ppm, Cl 19,350 ppm, Mn 0.012
ppb and Fe 0.021 ppb (Glasby and Schulz 1999).
These data give a realistic value for the concentration of Mn in deep seawater and permit calculation of the degree of complexation of dissolved
Mn species with anions in solution. As such, this
diagram reflects as accurately as possible the
environmental conditions in which deep-sea
manganese nodules form in this region. By
contrast, the E H -pH diagram of Mn taken from
Garrels and Christ (1965) is plotted for a temperature of 25°C at a pressure of 1 atmosphere in
pure water in which the activity of dissolved Mn
is taken to be is 10
-6
. This, in no way, reflects the
conditions under which deep-sea manganese
nodules form. An additional problem is that there
are no standard free energy data for the finegrained manganese oxide minerals such as 10 Å
manganate and δMnO 2 which make up manganese
nodules. This is a major limitation in using these
diagrams to infer the conditions of formation of
marine manganese deposits.
Answer 2
See answer to Problem 1. Again, there are no
standard free energy data for the fine-grained iron
oxyhydroxide minerals such as akagenéite (βFeOOH) which make up marine manganese
deposits.
Answer 3
a) Diffusion coefficient in seawater D
SW
for Mn
2+
at 10°C is 4.46E-10 m
2
s
-1
. Tortuosity for a
sediment with a porosity of φ = 0.84 is θ
2
= 1.45.
Diffusion coefficient for Mn
2+
in pore water under
these conditions is D sed = 3.3E-10 m
2
s
-1
. The
steepest gradient in the depth interval between
0.75 and 1.5 cm is 18.85 mol m
-3
m
-1
. The upward
flux of Mn
2+
is J sed = 0.84 · 3.3E-10 · 18.85 mol m
-2
s
-1
= 5.2E-9 mol m
-2
s
-1
or 0.45 mmol m
-2
d
-1
.
b) The highest concentrations of Mn
2+
in pore
water were measured at between 4 and 6 cm below
the sediment surface. This is therefore the depth
at which Mn
2+
is released.
c) Yes. The gradient between 0.25 and 0.75 cm
under sediment surface is 10.54 mol m
-3
m
-1
which
is only 56 % of the steepest gradient.
Answer 4
The NASC-normalized REE patterns for hydrogenous manganese deposits generally display a
positive Ce anomaly which is a function of the
oxygen content of the ambient water mass. The
Ce/La ratio of these deposits can therefore be
used as a redox indicator (Glasby et al. 1987,
Kunzendorf et al. 1993, Kasten et al. 1998, Szefer
et al. 1998). The NASC-normalized REE patterns
for submarine hydrothermal manganese deposits,
on the other hand, sometimes display a negative
Ce anomaly and a positive Eu anomaly. In
principle, submarine hydrothermal manganese
crusts should display a negative Ce anomaly
typical of submarine hydrothermal fluids and
seawater (de Baar et al. 1985). In practice, many
hydrothermal crusts contain an additional
hydrogenous component characterized by a
positive Ce anomaly (Scholten et al 2004). From
the observed Ce anomaly, it is possible to calculate the relative contribution of these two components in the crust (Fleet 1983, Scholten et al.
2004, Fig. 12.14). However, this calculation ignores
the possible presence of volcaniclastic material
which frequently acts as a cementing agent in the
crust and which can significantly increase the REE
content of the crust (Scholten et al. 2004). A
positive Eu anomaly, on the other hand, indicates
that the hydrothermal fluid has exceeded 250°C at
some stage during its evolution. At this tempe-
Answers to Problems
higher nitrate availability during the upwelling
season, less nitrate utilization.
δ
15
N = 8‰ => f = 0.07
quick depletion of nitrate, low productivity.
Answer 5
The glacial pH was about 0.2 units higher. Hence,
the carbonate ion concentration was higher and
the atmospheric pCO2 was lower during the last
glacial, in accordance with ice core observations.
Chapter 11
Answer 1
To be meaningful, an E H , pH diagram must reflect
the environmental conditions of the system
under consideration. The E H , pH diagram of Mn
presented in Fig. 11.2 was calculated for the
following conditions pertaining to deep seawater
from the Angola Basin; temperature 2°C, density
(1.023 g cm
-3
), Ca 427 ppm, Mg 1,284 ppm, Na
10,760 ppm, K 390 ppm, Cl 19,350 ppm, Mn 0.012
ppb and Fe 0.021 ppb (Glasby and Schulz 1999).
These data give a realistic value for the concentration of Mn in deep seawater and permit calculation of the degree of complexation of dissolved
Mn species with anions in solution. As such, this
diagram reflects as accurately as possible the
environmental conditions in which deep-sea
manganese nodules form in this region. By
contrast, the E H -pH diagram of Mn taken from
Garrels and Christ (1965) is plotted for a temperature of 25°C at a pressure of 1 atmosphere in
pure water in which the activity of dissolved Mn
is taken to be is 10
-6
. This, in no way, reflects the
conditions under which deep-sea manganese
nodules form. An additional problem is that there
are no standard free energy data for the finegrained manganese oxide minerals such as 10 Å
manganate and δMnO 2 which make up manganese
nodules. This is a major limitation in using these
diagrams to infer the conditions of formation of
marine manganese deposits.
Answer 2
See answer to Problem 1. Again, there are no
standard free energy data for the fine-grained iron
oxyhydroxide minerals such as akagenéite (βFeOOH) which make up marine manganese
deposits.
Answer 3
a) Diffusion coefficient in seawater D
SW
for Mn
2+
at 10°C is 4.46E-10 m
2
s
-1
. Tortuosity for a
sediment with a porosity of φ = 0.84 is θ
2
= 1.45.
Diffusion coefficient for Mn
2+
in pore water under
these conditions is D sed = 3.3E-10 m
2
s
-1
. The
steepest gradient in the depth interval between
0.75 and 1.5 cm is 18.85 mol m
-3
m
-1
. The upward
flux of Mn
2+
is J sed = 0.84 · 3.3E-10 · 18.85 mol m
-2
s
-1
= 5.2E-9 mol m
-2
s
-1
or 0.45 mmol m
-2
d
-1
.
b) The highest concentrations of Mn
2+
in pore
water were measured at between 4 and 6 cm below
the sediment surface. This is therefore the depth
at which Mn
2+
is released.
c) Yes. The gradient between 0.25 and 0.75 cm
under sediment surface is 10.54 mol m
-3
m
-1
which
is only 56 % of the steepest gradient.
Answer 4
The NASC-normalized REE patterns for hydrogenous manganese deposits generally display a
positive Ce anomaly which is a function of the
oxygen content of the ambient water mass. The
Ce/La ratio of these deposits can therefore be
used as a redox indicator (Glasby et al. 1987,
Kunzendorf et al. 1993, Kasten et al. 1998, Szefer
et al. 1998). The NASC-normalized REE patterns
for submarine hydrothermal manganese deposits,
on the other hand, sometimes display a negative
Ce anomaly and a positive Eu anomaly. In
principle, submarine hydrothermal manganese
crusts should display a negative Ce anomaly
typical of submarine hydrothermal fluids and
seawater (de Baar et al. 1985). In practice, many
hydrothermal crusts contain an additional
hydrogenous component characterized by a
positive Ce anomaly (Scholten et al 2004). From
the observed Ce anomaly, it is possible to calculate the relative contribution of these two components in the crust (Fleet 1983, Scholten et al.
2004, Fig. 12.14). However, this calculation ignores
the possible presence of volcaniclastic material
which frequently acts as a cementing agent in the
crust and which can significantly increase the REE
content of the crust (Scholten et al. 2004). A
positive Eu anomaly, on the other hand, indicates
that the hydrothermal fluid has exceeded 250°C at
some stage during its evolution. At this tempe-