Methane environments around cold seeps: Examples from Antarctica and the Mediterranean

Abstract

Marine cold seeps occur globally and represent unique pathways for material transport from depth. Methane is one of the major components which can be released at the seafloor into the overlying water column. The sediment-water methane flux can be described by two different transport mechanisms, diffusive flux and advective flux, the latter being mostly associated with gas bubbles. In sediments, diffusive fluxes of methane are often hampered by microbial anaerobic oxidation of methane (AOM), also termed the benthic filter. But if this benthic filter is limited or if stronger fluxes exceed the capacity of this filter, methane is released into the hydrosphere. Once methane reaches the hydrosphere it can be either transported by currents or diffusion, or it can be microbially oxidized. It is indeed known that methane released at cold seeps does not often reach the atmosphere. Yet, the processes in the water column that control the sink of methane are still poorly constrained. The goals of this thesis were multifold: 1) to develop a novel and enhanced method for rapid and accurate methane detection in water samples, 2) to investigate methane seepage at two sites: In South Georgia (Antarctica) the impact of methane seepage on the carbon cycle was investigated, while in the Mediterranean methane seepage was studied around an active mud volcano with a focus on comparatively evaluating advective versus diffusive methane input into the water column. This thesis provides insights into the behavior of methane around seep sites. Although the terminal sink of methane in both case studies was not explicitly determined, the available data showed how methane is distributed in the water column. Furthermore, new findings were made at the mud volcano about the impact of degassing mud, enabled by the new sampling method.