Methane ebullition (bubbling) in the Rhone River delta of Lake Geneva
The Rhone River delta of Lake Geneva is a complex system consisting of a deep (up to 40 m) and long (~16 km) canyon extending from the current river mouth, as well as several former canyons lying to the east. We hypothesized that the proximal part of the active canyon is a strong source of methane (CH4) to Lake Geneva and possibly the atmosphere via ebullition, mostly due to the organic loading of the present day location of the Rhone River. We also hypothesized that the older canyons would contribute much less to the CH4 budget of the delta area. Using a bubble-size calibrated echosounder we discovered that ebullition is prevalent throughout the proximal active delta, and also along the tops of the canyon walls in the middle and distal parts of the active canyon. Ebullition is present but to a much lesser degree in two of the older canyons. Preliminary data will also be shown for the work performed within the elemo project using the MIR submersibles. In order to understand what sedimentalogical features influence CH4 bubbling, cores were taken from the MIR submersibles along 3 transects in the active canyon and one transect in the two older canyons. Porosity, grain size, CH4, and organic matter content were compared at bubbling and non-bubbling areas, as well as between canyons. An in situ mass spectrometer (Tethys) was used aboard one of the MIRs on a transect along the active canyon floor to map dissolved CH4. High levels of CH4 were observed and water sampling in the deep canyon complimented these findings.