Physical controls of cold seep methane emission

In situ methane emission measurements from sediments are combined with water column backscatter anomalies recorded with an Acoustic Doppler Current Profiler (ADCP) integrated on a benthic observatory. During cruise SO191 to the Hikurangi Margin (New Zealand), the Fluid Flux Observatory (FLUFO) was deployed at a cold seep site at Omakere Ridge. The first 26 hours of in situ incubation revealed low to moderate methane fluxes (0.01 to 0.4 mmol m^-2 d^-1). In the following sampling sequence, however, the methane concentration in the flux chamber reached 3-fold higher concentrations. Simultaneous to the sudden methane increase, a significant backscatter anomaly was recorded and persisted for 30 min and covered the entire depth range (100 m) of the upward looking ADCP. Data analyses revealed that a plume outburst likely occurred during this time. A second data set was obtained during lander deployments at Rock Garden where visual observations by ROV confirmed the transient pattern of free gas injection into the water column. Acoustic flares and methane concentration increases in the bottom water hint towards a pressure (tidal) induced discharge mechanism. The presented data demonstrate the temporal and spatial variability of seabed methane emission, and very short methane signal lifetime in the water column (hours) due to turbulent diffusion. Both have to be considered when methane budgets are extrapolated from single methane emission rates.