Looking for evidence of the past presence of gas hydrates in ancient sedimentary successions.

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 clathrites are still imperfectly known as very few examples of present-day carbonate rocks precipitated within or in proximity to gas hydrate have been described. In ancient sediments, clathrites are concealed by other types of cold seep carbonate rocks that share with them an origin due to bacterial degradation of methane. The question is: how is possible to evaluate if that particular methane was caged in the gas hydrate structure before reaching the site where it was degraded by bacteria triggering carbonate precipitation? Methane-derived carbonate preserve the geochemical signatures of the interstitial fluids from which they are derived, registering not only the involvement of methane in their formation, but also, within certain limits, the oxygen isotopic signature of the water in which they formed. During hydrate formation, water molecules containing the 18O isotope are preferentially incorporated in the hydrate structure leaving the residual brines enriched in the light 16O isotope. The reverse happens during hydrate dissociation when an input of gas-driven,18O- enriched water, flushes the surrounding sediments. The carbonate phases resulting from the bacterial degradation of methane are so viable to register these changes in isotopic composition of pore-waters and to preserve it in the sedimentary record.However, as the oxygen isotopic composition of pore waters is influenced also by other factors unrelated to the presence of hydrates (evaporation, temperature, clay-dehydration) it is not correct at the moment to identify a clathrite only on the basis of its oxygen isotope composition. Only if the stratigraphic and structural setting of the carbonates are perfectly known and additional evidence as mineralogical composition, petrological features (as fluid inclusion) and chaotic structures of the enclosed sediments are present, the definition of a seep-carbonate as a clathrite can be proposed.