| Summary: | The present-day distribution of gas hydrates is world-wide, in permafrost of polar regions and along outer continental margins, and they are one of the more “hot” topics of modern geo-marine research. In fact, not only GH are considered by industry as a viable energy source for the future, but their possible involvement in margin stability and climate change attracts the interest of all researchers.In order to assess the real relevance of GH dissociation on both margin stability and climate changes, it could be useful to look into the sedimentary record for possible evidence both of mass wasting processes due to GH dissociation and of past, global peaks of GH dissociation. To do that, specific evidence of the occurrence of ephemeral products such as GH in ancient sedimentary sequences are needed. The presence of gas in sediments can leave “solid" evidence as authigenic carbonates generated by aerobic or anaerobic degradation of methane. “Cold seep carbonates” can be considered the specific proxy needed to identify sedimentary sequences flushed by CH4 rich fluids, bearing, however, in mind that not all the gas-charged fluids circulating in the sedimentary column originate from the dissociation of GH . Also the presence of brecciated structures, chaoticization of sediments by soft-sediment deformations and mud diapirism, have been considered as a convincing evidence of the past presence of GH as such features are widespread in the sediments hosting GH along present-day continental margins.Two possible lines of evidence can so be suggested to assess the past presence of GH in ancient sedimentary sequences:1)The presence of carbonate rocks resulting from diagenetic processes linked to biological degradation of gas previously trapped as GH. 2)The chaoticization of sediments generated by the large changes in pore fluid volumes due GH formation and dissociation. As a whole, lithological products of the dissociation of GH are , recently named clathrites. 3)Petrological, mineralogical and geochemical characteristics of ...
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