558
Answers to Problems
Chapter 14
Answer 1
No! Defined by its stability field the potential
volume for gas hydrate is much larger in the deep
sea than on the upper slope; however, the
availability of sufficient methane is a critical
requirement for gas hydrate formation. Due to
high plankton productivity continental slopes
have, in general, higher organic carbon concentrations than deep sea areas. Higher amounts of
methane and gas hydrate are therefore restricted
to continental margins.
Answer 2
In principle there is no difference which methane
molecule is used to stabilize the clathrate structure. Hydrates formed from biogenic methane are
thought to be more widespread in marine sediments, and these can be formed from methane gas
produced in situ. In contrast, the presence of
thermogenic methane requires gas migration form
deeper sediment sources (e.g. in mud volcanoes).
Thermogenic methane is also accompanied by the
presence of higher hydrocarbons, which may
result in the formation of gas hydrate structure II,
whereas methane hydrate from biogenic methane
is restricted to structure I.
Answer 3
A methane concentration of 300 mM is clearly
above methane saturation and in a depth of 90
mbsf well in the stability for methane hydrate. It is
obvious that the same concentration of methane
in 250 mbsf is outside the stability field where it
occurs as free methane gas. A methane concentration of 10 mM is clearly below the methane
solubility at both 90 and 250 mbsf, which means
that methane hydrate cannot be formed and the
methane occurs as a dissolved phase in the pore
water.
Answer 4
The chloride concentration is typical for an open
marine environment, suggesting no dilution by
gas hydrate dissociation. More important, the
high sulfate concentration indicates that there is
no methane in the sediment. Even if methane were
migrating to this region, it would be consumed by
anaerobic methane oxidation (AOM).
Answer 5
Around 10 % of the pore space are filled with gas
hydrate.
Answer 6
Since chlorides and other ions from marine pore
water cannot be incorporated into the clathrate
structure, those constituents are enriched in
residual pore fluids when hydrate is formed in
marine sediments. When hydrate formation is
faster than the rate of salt removal by diffusion
and advection, the excess salt would accumulate
in the pore water resulting in the formation of a
brine.
Chapter 15
Answer 1
The pH-value is 7.6.
Answer 2
The log pCO 2 is -4.1. The lower partial pressure of
CO 2 in ocean surface water is usually caused by
algal photosynthesis.
Answer 3
A steady state penetration depth is expected at
about 12.5 cm.
Answer 4
Sulfate and methane have different diffusion
coefficients. Methane diffusion is about 50 %
faster. Different porosities at different depths of a
sediment profile are possible.
Answers to Problems
Chapter 14
Answer 1
No! Defined by its stability field the potential
volume for gas hydrate is much larger in the deep
sea than on the upper slope; however, the
availability of sufficient methane is a critical
requirement for gas hydrate formation. Due to
high plankton productivity continental slopes
have, in general, higher organic carbon concentrations than deep sea areas. Higher amounts of
methane and gas hydrate are therefore restricted
to continental margins.
Answer 2
In principle there is no difference which methane
molecule is used to stabilize the clathrate structure. Hydrates formed from biogenic methane are
thought to be more widespread in marine sediments, and these can be formed from methane gas
produced in situ. In contrast, the presence of
thermogenic methane requires gas migration form
deeper sediment sources (e.g. in mud volcanoes).
Thermogenic methane is also accompanied by the
presence of higher hydrocarbons, which may
result in the formation of gas hydrate structure II,
whereas methane hydrate from biogenic methane
is restricted to structure I.
Answer 3
A methane concentration of 300 mM is clearly
above methane saturation and in a depth of 90
mbsf well in the stability for methane hydrate. It is
obvious that the same concentration of methane
in 250 mbsf is outside the stability field where it
occurs as free methane gas. A methane concentration of 10 mM is clearly below the methane
solubility at both 90 and 250 mbsf, which means
that methane hydrate cannot be formed and the
methane occurs as a dissolved phase in the pore
water.
Answer 4
The chloride concentration is typical for an open
marine environment, suggesting no dilution by
gas hydrate dissociation. More important, the
high sulfate concentration indicates that there is
no methane in the sediment. Even if methane were
migrating to this region, it would be consumed by
anaerobic methane oxidation (AOM).
Answer 5
Around 10 % of the pore space are filled with gas
hydrate.
Answer 6
Since chlorides and other ions from marine pore
water cannot be incorporated into the clathrate
structure, those constituents are enriched in
residual pore fluids when hydrate is formed in
marine sediments. When hydrate formation is
faster than the rate of salt removal by diffusion
and advection, the excess salt would accumulate
in the pore water resulting in the formation of a
brine.
Chapter 15
Answer 1
The pH-value is 7.6.
Answer 2
The log pCO 2 is -4.1. The lower partial pressure of
CO 2 in ocean surface water is usually caused by
algal photosynthesis.
Answer 3
A steady state penetration depth is expected at
about 12.5 cm.
Answer 4
Sulfate and methane have different diffusion
coefficients. Methane diffusion is about 50 %
faster. Different porosities at different depths of a
sediment profile are possible.