473
expelled from subduction zones are a significant
player in the carbon cycle of the oceans as a
result of the recycling of organic matter in marine
sediments.
Methane plumes have been found to be a
characteristic feature of cold seep areas. The
concentrations of dissolved CH 4 in some areas are
lower than in hydrothermal vent areas at midocean ridges, but in other areas they exceed ridge
values by more than an order of magnitude and
have been successfully used as a tracer for
detecting cold seep venting. The methane and
carbon dioxide that are expelled with the pore
waters are derived mainly from the breakdown of
organic matter in the sediments. Methane together
with H 2 S support abundant chemosynthetic
bacteria, pogonophorans, vestimentifera, and
bivalves which differ from those that inhabit hightemperature vent sites on the mid-ocean ridges
(Suess et al. 1998). As the temperatures of venting
are low compared to hydrothermal vents at the
mid-ocean ridges, metals are not significantly
mobilized.
Typical precipitates for areas of subduction
venting are carbonate and barite crusts and
carbonate-cemented sediment (Dia et al. 1993;
Suess et al. 1998). The formation of carbonate is
related to the microbial oxidation of methane
which causes the precipitation of CaCO 3 from pore
fluids (Wallmann et al. 1997). The source for
barium is thought to be the high concentration of
biogenic barite buried in sediments at high
productivity areas which is mobilized from the
reduced sediment column due to sulfate depletion.
Barite in the cold seep areas forms as a result of
mixing between Ba-rich fluids upwelling from the
sediment with seawater sulfate (Ritger et al. 1987;
Torres et al. 1996).
Ice-like gas hydrates (clathrates) also have been
found at numerous sites along the convergent plate
margins (e.g., Suess et al. 1997). Methane hydrates
are stable in solid form only in a narrow temperature-pressure window (Dickens and Quinby-Hunt
1994; Fig. 14.3 in chapter 14). In theory, 1m
3
of
methane hydrate can contain up to 164 m
3
of
methane gas at standard conditions (Kvenvolden
1993). It has been estimated that the amount of
carbon in gas hydrates considerably exceeds the
total of carbon occurring in all known oil, gas and
coal deposits worldwide (Kvenvolden and
McMenamin 1980; Kvenvolden 1988). This raises
the possibility that gas hydrates may be a future
energy source of global importance.
If disturbed by a thermal anomaly or pressure
change, large volumes of gas hydrates can break
down into water, methane and carbon-dioxide.
The dissolution of metastable gas hydrates is a
natural consequence of tectonic uplift of accretionary prisms at plate margins and is seen to be
partly responsible for the extensive methane
plumes reported from these areas (Suess et al.
1997). Because of the limited stability of gas
hydrates (low temperature and high pressure),
they are of major relevance for the budget of
greenhouse gases (Suess et al. 1997). The
destabilization and dissolution of gas hydrates
due to environmental changes (increase in ocean
bottom water temperature or change of sea level)
could liberate enormous amounts of methane to
the water column and eventually to the atmosphere where they could potentially accelerate
greenhouse warming and have an effect on
future global climate (Leggett 1990).
13.7 Problems
Problem 1
A fluid sample collected at a mid-ocean ridge hydrothermal vent has a temperature of 250°C and Mg
concentration of 15 mmol/kg (Fig. 13.9). If the fluid
contains 100 ppm Fe at 250°C, what is the likely concentration of Fe in the end-member fluid at 350°C?
Problem 2
A high-temperature black smoker has a calculated
end-member Fe concentration of 150 ppm (e.g.,
Table 13.3). If the fluid is venting from the chimney
at a rate of 1 kg/s, how much Fe is being discharged
into the ocean by this vent every year? If the
global flux of black smoker fluids along the
midocean ridges is equivalent to approximately one
vent for every 50 m of ridge-crest, how much Fe
would be discharged along the ridges every year?
Problem 3
A 100 m diameter sulfide deposit on the mid-ocean
ridge has been drilled to a depth of 25 m. The metal
concentrations of the sulfides recovered from the
core are 2 wt.% Cu, 5 wt.% Zn, and 15 wt.% Fe, and
the samples have a bulk density of 3 g/cm
3
. What is
the total metal content of the deposit? If the
13.7
Problems
expelled from subduction zones are a significant
player in the carbon cycle of the oceans as a
result of the recycling of organic matter in marine
sediments.
Methane plumes have been found to be a
characteristic feature of cold seep areas. The
concentrations of dissolved CH 4 in some areas are
lower than in hydrothermal vent areas at midocean ridges, but in other areas they exceed ridge
values by more than an order of magnitude and
have been successfully used as a tracer for
detecting cold seep venting. The methane and
carbon dioxide that are expelled with the pore
waters are derived mainly from the breakdown of
organic matter in the sediments. Methane together
with H 2 S support abundant chemosynthetic
bacteria, pogonophorans, vestimentifera, and
bivalves which differ from those that inhabit hightemperature vent sites on the mid-ocean ridges
(Suess et al. 1998). As the temperatures of venting
are low compared to hydrothermal vents at the
mid-ocean ridges, metals are not significantly
mobilized.
Typical precipitates for areas of subduction
venting are carbonate and barite crusts and
carbonate-cemented sediment (Dia et al. 1993;
Suess et al. 1998). The formation of carbonate is
related to the microbial oxidation of methane
which causes the precipitation of CaCO 3 from pore
fluids (Wallmann et al. 1997). The source for
barium is thought to be the high concentration of
biogenic barite buried in sediments at high
productivity areas which is mobilized from the
reduced sediment column due to sulfate depletion.
Barite in the cold seep areas forms as a result of
mixing between Ba-rich fluids upwelling from the
sediment with seawater sulfate (Ritger et al. 1987;
Torres et al. 1996).
Ice-like gas hydrates (clathrates) also have been
found at numerous sites along the convergent plate
margins (e.g., Suess et al. 1997). Methane hydrates
are stable in solid form only in a narrow temperature-pressure window (Dickens and Quinby-Hunt
1994; Fig. 14.3 in chapter 14). In theory, 1m
3
of
methane hydrate can contain up to 164 m
3
of
methane gas at standard conditions (Kvenvolden
1993). It has been estimated that the amount of
carbon in gas hydrates considerably exceeds the
total of carbon occurring in all known oil, gas and
coal deposits worldwide (Kvenvolden and
McMenamin 1980; Kvenvolden 1988). This raises
the possibility that gas hydrates may be a future
energy source of global importance.
If disturbed by a thermal anomaly or pressure
change, large volumes of gas hydrates can break
down into water, methane and carbon-dioxide.
The dissolution of metastable gas hydrates is a
natural consequence of tectonic uplift of accretionary prisms at plate margins and is seen to be
partly responsible for the extensive methane
plumes reported from these areas (Suess et al.
1997). Because of the limited stability of gas
hydrates (low temperature and high pressure),
they are of major relevance for the budget of
greenhouse gases (Suess et al. 1997). The
destabilization and dissolution of gas hydrates
due to environmental changes (increase in ocean
bottom water temperature or change of sea level)
could liberate enormous amounts of methane to
the water column and eventually to the atmosphere where they could potentially accelerate
greenhouse warming and have an effect on
future global climate (Leggett 1990).
13.7 Problems
Problem 1
A fluid sample collected at a mid-ocean ridge hydrothermal vent has a temperature of 250°C and Mg
concentration of 15 mmol/kg (Fig. 13.9). If the fluid
contains 100 ppm Fe at 250°C, what is the likely concentration of Fe in the end-member fluid at 350°C?
Problem 2
A high-temperature black smoker has a calculated
end-member Fe concentration of 150 ppm (e.g.,
Table 13.3). If the fluid is venting from the chimney
at a rate of 1 kg/s, how much Fe is being discharged
into the ocean by this vent every year? If the
global flux of black smoker fluids along the
midocean ridges is equivalent to approximately one
vent for every 50 m of ridge-crest, how much Fe
would be discharged along the ridges every year?
Problem 3
A 100 m diameter sulfide deposit on the mid-ocean
ridge has been drilled to a depth of 25 m. The metal
concentrations of the sulfides recovered from the
core are 2 wt.% Cu, 5 wt.% Zn, and 15 wt.% Fe, and
the samples have a bulk density of 3 g/cm
3
. What is
the total metal content of the deposit? If the
13.7
Problems