INTRODUCTION

Methane is an important link within the global carbon cycle and has become a major focus for scientific investigations over the last decades, especially since the discovery of large deposits of methane hydrates in continental margins. The majority of recent methane production is biogenic, i.e. produced either by thermogenic transformation of organic material or by methanogenesis as the final step in fermentation of organic matter carried out by methanogenic archaea in anoxic habitats (Reeburgh, 1996). There are also abiotic sources of methane, e.g. at mid-oceanic ridges, where serpentinization takes place. In marine environments, the bulk of the methane is produced in shelf and upper continental-margin sediments, which receive large amounts of organic matter from deposition (Reeburgh, 1996). As methane builds up, it migrates upwards and may reach the sediment surface. Here, its ebullition and oxidation can lead to the formation of complex geostructures, such as pockmarks or carbonate chimneys and platforms, as well as large-scale topographies, such as mud volcanoes and carbonate mounds (Ivanov et al., 1991; Milkov, 2000). In most of the deeper continental margin and the abyssal plain sediments, methane production is low, as only 1-5 % of the surface primary production reaches the bathyal and abyssal seabed, due to degradation processes in the water column (Gage & Tyler, 1996).