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Methane
production and oxidation rates Radiotracer
measurement of methane production (MP) will follow
the published method of Crill and Martens (1986)
and the method for methane oxidation (MO) will
follow Reeburgh (1980), both as modified by Hoehler
et al. (1994). For both measurements, 3 ml
horizontal syringe cores of sediment will be
removed at multiple depths from the large cores and
stoppered without headspace using one-hole stoppers
that have had the hole filled with a silicone plug.
These will be amended with 25 ml of 14CH4 (gaseous,
~5x105 cpm) or NaH14CO3 (aqueous ~5x106 cpm) for MO
and MP, respectively, via line injection through
the silicone septa. The subcores will be incubated
at the in situ temperature in the dark for suitable
periods of time (1-2 days) and the samples will
then be expelled into serum vials containing NaOH
solution and stoppered. This will stop any further
activity and preserve the sample. The
serum vials for both rate measurements will be
sequentially stripped of their methane and then
CO2. While the samples are in base, methane will be
stripped by a stream of flowing nitrogen gas. The
gas will passes through a tube furnace filled with
cuprox at 800°C to combust the methane to CO2
which is then trapped in a solution of
scintillation cocktail (Fisher Scintiverse II),
phenethylamine and methanol. This solution will be
used directly for scintillation counting of the 14C
activity. The sample will then be acidified and the
CO2 stripped and trapped in the same cocktail after
first bubbling through a tube of 0.5 M zinc acetate
solution adjusted to pH 4.0 to trap sulfide
liberated by the acid. Rates
will be calculated as follows, Rate =
[C]×a ×s / A×t . Here
[C] is the reactant concentration (methane
for MO, or CO2 for MP), a is the activity of the
product, A is the activity of the reactant added, t
is the incubation time, and s is the isotope
fractionation factor associated with the process.
Crill,
P. M. and C. S. Martens. 1986. Methane production
from bicarbonate and acetate in an anoxic marine
sediment. Geochim. Cosmochim. Acta
50:2089-2097. Hoehler,
T. M., M.J. Alperin, D.B. Albert and C.S. Martens.
1994. Field and laboratory studies of methane
oxidation in an anoxic marine sediment: Evidence
for a methanogen-sulfate reducer consortium. Global
Biogeochem. Cycles 8:451-463. Reeburgh,
W. S. 1980. Anaerobic methane oxidation: Rate depth
distributions in Skan Bay sediments. Earth Planet.
Sci. Lett. 47:345-352. |
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