three cores the DOC remains relatively constant throughout the column at approximately
30 mg C 1 -1 , except in one core where no DOC could be detected between 15 cm and 45
cm. Sulphate concentrations decreased down the profile to levels below 1mM at 80 cm
consistent with its utilisation in the surface layers of the sediment by sulphate-reducing
bacteria. In one case quantities of VOC increased down the core, indicating the production of possible substrates for the terminal bacterial processes, although the activity of
these groups would normally keep the concentrations low, possibly below detection
levels. The VOC was mainly being detected in the zone where sulphate levels should be
sufficiently low to ensure the predominance of methanogenesis. At concentrations of
sulphate above 5mM methane concentrations of less than 200 µmoles kg - 1 were measured, but below 1mM sulphate up to 2000 µmoles kg -1 (a ten-fold increase) were measured.
Figure 5 : Methane concentrations, DOC, VOC and sulphate concentrations for three cores from Holyhead
harbour.
The results of methane analysis of Western Irish Sea cores indicated that more methane
was present compared to the Holyhead harbour cores, with as much as 45000µmoles kg'1
(Fig 6), but because of the increased hydrostatic pressure at 100 m compared to 10 m,
these values were not unreasonably high. Gas bubble formation within the sediments is
dependent upon one gas component concentration exceeding its saturation point which is
itself dependent upon the hydrostatic pressure of the system (Anderson and Hampton,
1980 a). Some seasonal effect may be involved in determining the methane concentrations
in the cores but insufficient data is available for that analysis. Although core blackening
was a common feature of Holyhead harbour cores it was noticed in only three cores from
the Western Irish sea, the same cores showed the highest levels of methane. Although we
have not investigated net gas fluxes from either sediment system, bubble and pore tube
formation has been noted, especially within Holyhead harbour sediments.
The Holyhead harbour microcosm experiments (Fig 7a) indicated that raised temperature stimulated methanogenesis. H2/CO2 was seen to stimulate methane production in
40
30 mg C 1 -1 , except in one core where no DOC could be detected between 15 cm and 45
cm. Sulphate concentrations decreased down the profile to levels below 1mM at 80 cm
consistent with its utilisation in the surface layers of the sediment by sulphate-reducing
bacteria. In one case quantities of VOC increased down the core, indicating the production of possible substrates for the terminal bacterial processes, although the activity of
these groups would normally keep the concentrations low, possibly below detection
levels. The VOC was mainly being detected in the zone where sulphate levels should be
sufficiently low to ensure the predominance of methanogenesis. At concentrations of
sulphate above 5mM methane concentrations of less than 200 µmoles kg - 1 were measured, but below 1mM sulphate up to 2000 µmoles kg -1 (a ten-fold increase) were measured.
Figure 5 : Methane concentrations, DOC, VOC and sulphate concentrations for three cores from Holyhead
harbour.
The results of methane analysis of Western Irish Sea cores indicated that more methane
was present compared to the Holyhead harbour cores, with as much as 45000µmoles kg'1
(Fig 6), but because of the increased hydrostatic pressure at 100 m compared to 10 m,
these values were not unreasonably high. Gas bubble formation within the sediments is
dependent upon one gas component concentration exceeding its saturation point which is
itself dependent upon the hydrostatic pressure of the system (Anderson and Hampton,
1980 a). Some seasonal effect may be involved in determining the methane concentrations
in the cores but insufficient data is available for that analysis. Although core blackening
was a common feature of Holyhead harbour cores it was noticed in only three cores from
the Western Irish sea, the same cores showed the highest levels of methane. Although we
have not investigated net gas fluxes from either sediment system, bubble and pore tube
formation has been noted, especially within Holyhead harbour sediments.
The Holyhead harbour microcosm experiments (Fig 7a) indicated that raised temperature stimulated methanogenesis. H2/CO2 was seen to stimulate methane production in
40