557
values give some indication of the mass accumulation at the sea floor.
Answer 3
Without the justification from variogram analysis
that a statistically verifiable regional structure
exists within the database, any result of regionalization by kriging will be extremely questionable. The pure kriging method is only a weighting process. The necessary boundary conditions,
like the range of a structure, can only be determined by geostatistical methods.
Answer 4
Coupled benthic pelagic models are useful to
apply, because they enable a better understanding
of the interaction of global biogeochemical cycles.
For example, the impact of variations of mineralization processes in the sediment can be investigated straightforward in terms of potential
changes in ocean water chemistry and hence,
climate feedbacks. On the other hand, their degree
of complexity and spatial resolution are rather
limited to date and hence, predictions are not very
precise when compared to data-based regionalization approaches.
Chapter 13
Answer 1
A 350°C end-member fluid would have a Mg
concentratrion of 0 mmol kg
-1
, and seawater has a
Mg concentration of 52 mmol kg
-1
(Fig. 13.9).
Therefore, the 250°C fluid, with a Mg concentration of 15 mmol kg
-1
, contains about 71% of the
high-temperature end-member and about 29% of
mixed seawater. If the 250°C fluid contains 100
ppm Fe, then the high-temperature end-member
must contain about 140 ppm Fe (0.71 x 140 ppm =
100 ppm).
Answer 2
A concentration of 150 ppm Fe is equivalent to
150 mg of Fe per kilogram of fluid. A black smoker
with a flux of 1 kg s
-1
is discharging 3.2 · 10
7
kg of
hydrothermal fluid every year containing 4.8 · 10
9
mg of Fe (150 mg kg
-1
Fe · 3.2 · 10
7
kg = 4.8 · 10
9
mg
Fe or 4.8 t Fe). One black smoker every 50 m along
the 55,000 km of mid-ocean ridge is equivalent to
1,100,000 black smoker vents. This number of
vents along the world’s mid-ocean ridges could
deliver more than 5 million tonnes of Fe to the
seafloor every year. (Note: this calculation neglects any component of diffuse flow along the
ridges. Less than 10% of the fluids vented at the
ridge axis are discharged at black smoker temperatures. Therefore the total number of black
smoker vents is likely much smaller. For a more
complete analysis of metal fluxes at ridge crests,
see Elderfield and Schultz 1996).
Answer 3
A sulfide deposit with a diameter of 100 m and a
thickness of 25 m will contain a total volume of
sulfide equivalent to 196,250 m
3
. For a bulk
density of 3 g/cm
3
, the total amount of sulfide
present is 588,750 tonnes. The deposit therefore
contains 11,775 tonnes of Cu metal (2 wt.%),
29,437 tonnes of Zn (5 wt.%), and 88,312 tonnes
of Fe (15 wt.%). A vent complex with 100 black
smokers (each discharging 4.8 tonnes Fe per
year) and a depositional efficiency of 1% could
deposit this much Fe in 18,400 years [88,312
tonnes Fe per (4.8 tonnes/year · 100 vents · 1/100
efficiency) = 18,400 years].
Answer 4
The pressure at 1000 m depth is 100 bars (10 m =
1 bar). The maximum temperature of venting is
determined by the pressure on the two-phase
curve for seawater (Fig. 13.8). At 1000 m or 100
bars, a hydrothermal fluid venting at the seafloor
cannot be hotter than about 310°C. At 500 m
depth or 50 bars, the maximum temperature will be
about 270°C.
Answer 5
Many reactions can be written, but the majority
of methane and carbon dioxide are produced in
marine sediments by the oxidation of buried
organic matter, following reactions of the type:
C org + 2H 2 O = CO 2 + 2H 2 and CO 2 + 4H 2 = CH 4 + 2H 2 O.
At 5°C, solid methane hydrates will be stable at a
pressure of about 45 bars (45 atm, Fig. 13.14),
which is equivalent to a depth of 450 m. At 1°C,
the hydrate will form at about 300 m depth.
values give some indication of the mass accumulation at the sea floor.
Answer 3
Without the justification from variogram analysis
that a statistically verifiable regional structure
exists within the database, any result of regionalization by kriging will be extremely questionable. The pure kriging method is only a weighting process. The necessary boundary conditions,
like the range of a structure, can only be determined by geostatistical methods.
Answer 4
Coupled benthic pelagic models are useful to
apply, because they enable a better understanding
of the interaction of global biogeochemical cycles.
For example, the impact of variations of mineralization processes in the sediment can be investigated straightforward in terms of potential
changes in ocean water chemistry and hence,
climate feedbacks. On the other hand, their degree
of complexity and spatial resolution are rather
limited to date and hence, predictions are not very
precise when compared to data-based regionalization approaches.
Chapter 13
Answer 1
A 350°C end-member fluid would have a Mg
concentratrion of 0 mmol kg
-1
, and seawater has a
Mg concentration of 52 mmol kg
-1
(Fig. 13.9).
Therefore, the 250°C fluid, with a Mg concentration of 15 mmol kg
-1
, contains about 71% of the
high-temperature end-member and about 29% of
mixed seawater. If the 250°C fluid contains 100
ppm Fe, then the high-temperature end-member
must contain about 140 ppm Fe (0.71 x 140 ppm =
100 ppm).
Answer 2
A concentration of 150 ppm Fe is equivalent to
150 mg of Fe per kilogram of fluid. A black smoker
with a flux of 1 kg s
-1
is discharging 3.2 · 10
7
kg of
hydrothermal fluid every year containing 4.8 · 10
9
mg of Fe (150 mg kg
-1
Fe · 3.2 · 10
7
kg = 4.8 · 10
9
mg
Fe or 4.8 t Fe). One black smoker every 50 m along
the 55,000 km of mid-ocean ridge is equivalent to
1,100,000 black smoker vents. This number of
vents along the world’s mid-ocean ridges could
deliver more than 5 million tonnes of Fe to the
seafloor every year. (Note: this calculation neglects any component of diffuse flow along the
ridges. Less than 10% of the fluids vented at the
ridge axis are discharged at black smoker temperatures. Therefore the total number of black
smoker vents is likely much smaller. For a more
complete analysis of metal fluxes at ridge crests,
see Elderfield and Schultz 1996).
Answer 3
A sulfide deposit with a diameter of 100 m and a
thickness of 25 m will contain a total volume of
sulfide equivalent to 196,250 m
3
. For a bulk
density of 3 g/cm
3
, the total amount of sulfide
present is 588,750 tonnes. The deposit therefore
contains 11,775 tonnes of Cu metal (2 wt.%),
29,437 tonnes of Zn (5 wt.%), and 88,312 tonnes
of Fe (15 wt.%). A vent complex with 100 black
smokers (each discharging 4.8 tonnes Fe per
year) and a depositional efficiency of 1% could
deposit this much Fe in 18,400 years [88,312
tonnes Fe per (4.8 tonnes/year · 100 vents · 1/100
efficiency) = 18,400 years].
Answer 4
The pressure at 1000 m depth is 100 bars (10 m =
1 bar). The maximum temperature of venting is
determined by the pressure on the two-phase
curve for seawater (Fig. 13.8). At 1000 m or 100
bars, a hydrothermal fluid venting at the seafloor
cannot be hotter than about 310°C. At 500 m
depth or 50 bars, the maximum temperature will be
about 270°C.
Answer 5
Many reactions can be written, but the majority
of methane and carbon dioxide are produced in
marine sediments by the oxidation of buried
organic matter, following reactions of the type:
C org + 2H 2 O = CO 2 + 2H 2 and CO 2 + 4H 2 = CH 4 + 2H 2 O.
At 5°C, solid methane hydrates will be stable at a
pressure of about 45 bars (45 atm, Fig. 13.14),
which is equivalent to a depth of 450 m. At 1°C,
the hydrate will form at about 300 m depth.