Leg 204 demonstrated the need for a multidisciplinary "hydrocarbon systems" approach toward modeling, integrating, and interpreting a wide range of geological, biogeochemical, and geophysical data in order to understand the formation and evolution of marine gas hydrate deposits and to predict gas hydrate distribution elsewhere. Geophysical and geochemical analyses provide complementary data for understanding the source of gas for gas hydrate formation and the mechanisms whereby it migrates into the GHSZ. Two fundamentally different gas delivery processes coexist in the region of southern Hydrate Ridge to generate the observed gas hydrate distribution. The rich gas hydrate deposit observed near the seafloor at the summit, which contains ~30% gas hydrate by volume and extends to a depth of 20–30 mbsf, results from migration and focusing of methane by geologic structure. At south Hydrate Ridge, the structure that acts as a gas conduit is a 2- to 4-cm-thick volcanic ash–rich layer that is surrounded by less permeable sediments and has been folded to form an anticline. Modeling suggests that an abundant supply of free gas is needed to generate the massive gas hydrates and high-Cl– pore waters observed at the summit. Throughout the region, gas hydrate, which occupies, on average, 2%–8% of the pore space, is present in a depth range that extends from ~30 mbsf to the BSR. The distribution of gas hydrate within this zone is very heterogeneous, both vertically and horizontally. The amount of gas hydrate and whether it fills pore space or forms steeply dipping veins and subhorizontal lenses is determined by a variety of factors, including the rate of microbial methane production, the rate of diffuse fluid flow, lithology, and faulting, with gas hydrate forming preferentially in relatively coarse grained or faulted strata.
Although Leg 204 greatly improved our understanding of gas hydrate processes in accretionary complexes, a number of important questions remain unanswered. Here we give a few examples. (1) The mechanism whereby gas migrates through the gas hydrate stability zone to form shallow, massive gas hydrate or vent into the ocean and the rate at which this occurs remain poorly controlled. In situ time-series observations on and below the seafloor are needed. (2) Questions also remain about the generality of some of the empirical relationships used to convert geophysical observations to in situ gas hydrate quantity. Much of the apparent variability in gas hydrate content derived using different techniques in neighboring holes is likely due to actual heterogeneity in the gas hydrate distribution, although some may be due to uncertainties in the calibration of various gas hydrate proxies. Separating these effects will require acquisition of new data sets from a variety of geologic settings in which multiple proxies are available from the same hole, as well as additional detailed analysis of existing data sets. (3) A better understanding of the response of submarine gas hydrates to oceanographic and tectonic perturbations and resulting temporal variations in the supply of methane require new models that incorporate the heterogeneity found in natural systems. (4) Ambiguity remains about whether a second faint BSR located ~30 m below the current base of the GHSZ is due to the presence of Structure II hydrate or is a relic from a time when the BSR was deeper. (5) Factors controlling the relative roles of microbial oxidation of organic carbon and anaerobic oxidation of methane, in particular the possible existence of threshold metabolite levels in controlling microbial activity and the role of fluid flow in supplying such metabolites, need to be determined through improved microbial sampling and culturing techniques and integration of biological, geochemical, and geophysical data and models.