Description: Commercial utilization of methane hydrate deposits in the seabed: The vast amount of natural gas bound in methane hydrates is considered as future energy resource by a growing number of states and companies in South-East Asia and North America. Successful field production tests showed that gas hydrates can be dissociated in the sub-surface by heat addition and pressure reduction while the released gas is produced via conventional drill wells. Laboratory studies demonstrate that CO2 from coal power plants can be applied to liberate methane from the hydrate structure and produce natural gas while the injected CO2 is safely stored as hydrate in the sub-surface. The commercial exploitation of sub-seabed gas hydrates may start in the next decade pending on the success of field production tests off Japan scheduled for 2012 and 2014. Specific environmental risks are associated with the future utilization of gas hydrates. These include the extinction of special benthic ecosystems relying on methane from hydrates as energy source, the triggering of slope failure, and leakage of greenhouse gases into the marine environment. Suitable measures have to be taken to avoid these risks. An appropriate legal framework should be established at the international level to meet the specific challenges and risks associated with the commercial use of gas hydrates in the marine environment.

Description: During the past quarter century views have changed in marine gas-hydrate research and in its perception by the society at large: (1) Deep-sea drilling has gone from a policy of avoiding gas hydrate to emphasizing deliberate drilling for it. (2) International programs have evolved from exploiting gas hydrates as energy to considering exchange of CO2 for CH4 hydrates as a means of carbon dioxide storage. (3) Lately, due to global change, research has changed from pursuing methane-hydrate reserves to documenting release of methane from destabilization in marginal seas. The first stage generated a wealth of knowledge and laid the foundation for marine gas hydrate research upon which we build today. The second stage is traced to more accurately estimating exploitable hydrate-bound gas and finding recovery technologies, that has lead to the discovery of an innovative option coupling production of methane from CH4-hydrate to storage of CO2 via in the sub-seafloor. Governments worldwide have recognized the potential for carbon dioxide storage and have begun to implement regulations for such environmentally safe carbon capture and storage (CCS). During the third stage, in further exploring global methane hydrate reserves, it has become evident that environmental changes over the past decades may have triggered release of methane from destabilizing hydrate at the seabed as well as diminished oxygen content in the near-bottom of marginal seas. Such scenarios had been proposed for past global warming and now appear to become active again. Exemplary highlights and selected cases studies are documented for each of the evolving stages.

Description: Hydrate Ridge is part of the accretionary complex at the Cascadia margin and is an area of widespread carbonate precipitation induced by the expulsion of methane-rich fluids. All carbonates on Hydrate Ridge are related to the methanecarbon pool either through anaerobic methanotrophy or through methanogenesis. Several petrographically distinct lithologies occur in boulder fields or in massive autochtonous chemoherm complexes which include methane-associated diagenetic mudstones and venting-induced breccias. The mudstones result from methane diagenesis in different sediment horizons and geochemical environments related to very slow methane venting. Cemented bioturbation casts occur as fragments, complex framework or as clasts together with bivalve shells as part of intraformational breccias, which are restricted to chemoherm complexes. Here, fluids ascend from the sub-seafloor and support aragonite-dominated carbonate precipitation near or at the sediment surface. Voids within mudclast breccias are either aragonite-rich indicating a formation near the surface at vent sites or are cemented by dolomite, which indicates formation in deeper parts of the sediment column. Brecciation is caused by tectonic or slump processes. In addition, we recognized a direct relationship between gas hydrates and sediment fracturing as well as the oxygen isotope composition of carbonate lithologies. Such gas hydrate-associated carbonates either show layered megapores and veins as relics of the original gas hydrate fabric or consist of aragonite-cemented intraclast breccias formed by growing and decomposing gas hydrate near the sediment surface. Both rock fabrics and the enrichment of 180 in high Mg-calcite demonstrate carbonate-forming mechanisms of gas hydrate.

Description: he present-day upwelling circulation off Peru, the regional pattern of organic matter in surface sediments and the stable carbon isotope characteristics of Neogene and Quaternary carbonate lithologies suggest a unique feedback mechanism in continental margin deposition and subsequent alteration after burial. In such a scenario, the high bioproductivity, the position of a poleward flowing undercurrent and the rate of subsidence of margin basins appear to be the principal variables controlling this mechanism. Transfer of organic matter from the sea surface to the sea floor is particularly efficient in the upwelling ecosystem off Peru. Preservation and burial are enhanced by high bulk sedimentation rates along the upper continental slope (between 11°–15°S) at depths where the subsurface current velocities decrease below those normally associated with the poleward flow. Burial and preservation are diminished, however, where shallow water depths promote continuous reworking of the bottom sediments by onshore flows and alongshore water movement (between 6°–10°S). The resulting sedimentary facies are distinctly different from each other in that the former process yields an organic-rich (〉 5 wt % Corg) and the latter process yields a calcareous (〉 15 wt % CaCo3) mud facies. The bulk sediment accumulation and individual component fluxes are estimated for both portions of the margin situated between 6° and 15°S latitude and lying in 〈 500 m of water depth. Furthermore, the chemical environment of organic-matter decomposition in the rapidly accumulating carbonate-poor facies is dominated by microbial fermentation and methanogenesis, whereas, the muds containing lesser amounts of organic matter are dominated by microbial sulphate reduction. These differences in facies composition persist throughout the subsequent stages of compaction and diagenesis. Most prominent among these is the formation of ‘organic’ dolomites with distinctly different isotopic signatures and mineral assemblages. The original upwelling facies (i.e., organic-rich muds or calcareous muds), the extent of reworking by subsurface currents, and the subsidence history of the margin basins may be inferred from these sedimentary signatures.