Ninth international conference on - Marum

Abstracts of oral presentations 29
2004, multibeam and single beam surveys and the deployment of the hydroacoustic lander system GasQuant
were carried out in an active seep area west of the Crimea Peninsula within the CRIMEA project. Multibeam
data (bathymetry and backscatter mapping) and single beam data (bubble seep localization) were used for
mapping the seep distribution and for determining the relation of seeps/m 2 and backscatter values. Backscatter
data are used to quantify the amount of active seeps in the entire mapped area based on the full coverage
backscatter maps derived from the multibeam survey (Fig. 1).
Fig. 1: Backscatter data from the
study area and seep positions (red
dots). The very good correlation
allows using the backscatter data to
calculate the number of seeps/m2
for specific back scatter values and
to extrapolate the number of seeps
for the entire area (21.8 km 2 ).
Flux measurements using the submersible JAGO yielded very accurate single spot fluxes but do not provide
information about the temporal variability of bubble release. To analyse this, the GasQuant lander was deployed.
The data gained show that by far the majority of bubble releasing seeps is only active for less than 10% of the
time (Greinert, JGR 2008). Distinct release patterns could be observed, varying from frequent and periodic
releases to sporadic and periodic, to erratic or single bursts. These patterns are related to the filling and emptying
of gas reservoirs, e.g. underneath bacteria mats or carbonate slabs triggering periodic bubble release on a minute
basis. Tidal cycles were not observed as the Black Sea does not have prominent sea level changes (< 20 cm).
Using the average bubble release activity (only 12% of the time) together with the directly measured fluxes (0.24
to 0.63 mmol/s), the total number of seeps in the study area (2709), and correcting this flux with the mean
activity measured by GasQuant results in a methane flux of 14043 mol/d (224 kg of C).
To estimate the maximum amount of methane that could be transported to the sea surface, we applied a bubble
dissolution model (McGinnis at al., JGR 2006). The amount of methane that reaches the sea surface was
modeled by assuming different bubble size spectra and taking the changing water depth condition in the study
area into account. Based on the most likely initial Gaussian bubble size distribution we can estimate that only 0.2
to 3.3 % (110 and 70m water depth, respectively) of the initially released methane reaches the sea surface
(Figure 2). For the entire study area this sums up to only 1680 mol/d or 20.16 kg of carbon.
Gas hydrate induced fluid flow alteration
J. Hauschildt 1 , V. Unnithan 1 , J. Vogt 1
1 Jacobs University, Bremen
The formation of sub-seafloor gas hydrates in marine environments can be described as a coupled transport and
thermodynamic process inside a host sediment matrix undergoing structural evolution. The transport processes
are driven by the sedimentary load and induced overpressure gradients, controlled by the sediment permeability.
For accurately modelling the resulting fluid flow profile, the alteration of the sediment permeability during
hydrate precipitation has to be taken into account, which affects both the transport of solutes and the sediment
compaction.
We investigated the effect of the flow deflection due to local permeability reduction on the total hydrate
abundance in scenarios of a laterally varying thickness of the hydrate stability field.
The predicted results of the coupled system of the sediment and fluid transport, of the thermodynamic
equilibrium and of the pore pressure were obtained using a semi-implicit two-dimensional Finite Volume