Ninth international conference on - Marum

Abstracts of oral presentations 17
Two representative locations (water depth 12m and 85m) close to the former Rhine estuary have been selected
for further geochemical examinations. After deploying a digital horizontal sonar system at the lake floor to allow
for exact positioning, water and sediment samples were taken within the pockmark with niskin bottles from a
rosette and multicorer. In addition, sediment samples and gas samples were taken by divers across the shallow
pockmark. Gas concentration and isotope ratios of dissolved gas in water were measured using standard GC-FID
and GC-irMS techniques. Methane concentration in sediment samples was measured directly after coring by
means of by a diffusion-based methane sensor.
Preliminary results of the isotope analysis of methane (δ 13 C, δD) and CO2 (δ 13 C) as well as the absence of
higher molecular weight alkanes strongly indicate a bacterial formation of the gases rather than a thermogenic
origin of the methane. The results of the analysis of free gas of the sediments indicate the methyl formation as
dominant pathway. However, up to now available data suggest a certain difference in the gases located at the
deep pockmark, the corresponding reference site and gas sampled at the shallow pockmark.
In the sediments of the deep pockmark high methane concentrations were recorded, however with no significant
differences between the pockmark and control cores. At the shallow pockmark sediment cores could be
positioned more precisely. Here methane concentrations vary within meters in order of magnitude.
Several autonomous devices for gas flow measurements using different approaches were deployed in March
2008 at selected gas emanating pockmarks. Data are not yet available but will hopefully presented
.
Fig 1: Sonar image of a large pockmark (diameter 16m) at Lake Constance. Note the gas bubbles ascending
from the structure
δ 13 C-methane (‰)
-110
-90
-70
-50
-30
artificial
methane
fermentation
mix
-10
-400 -300 -200 -100
mix
CO 2 reduction
thermal
δD-methane (‰)
MUC 1 inside deep pockmark
MUC 2 reference site deep pockmark
stab tubes shallow pockmark
TT(m)
TT(h)
atmosphere
Fig. 2: Carbon and deuterium isotopical composition of selected samples in a diagnostic diagram