Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in the subsurface What have we learned in the past decade?

The most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the Ocean Drilling Program (ODP), notably legs 112, 141, 146 and 164, and to a lesser extent legs 131, 160, 170 and 186. Leg 164, in particular, was dedicated to gas-hydrate drilling in the Blake Ridge gas-hydrate field in the West Atlantic in an unprecedented multidisciplinary research effort [Paull et al., 1996. Proc. Ocean Drill. Program, Initial Rep. 164, 623 pp.; Paull C.K., Matsumoto, R., Wallace, P.J., Dillon, W.P. (Eds.), 2000a. Proc. Ocean Drill. Program Sci. Results 164, Ocean Drilling Program, College Station, TX, 459 pp.]. Most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion-modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones. This started with qualitative evidence for advective flow from drilling the (hydrate-bearing) Peru and (hydrate-free) Barbados active margins, continued with the Nankai Trough accretionary prism, the Japan Trench Slope and the Cascadia and Costa Rica active margins and culminated in the quantitative advection-diffusion model of Egeberg and Dickens [Chem. Geol. 153 (1999) 53.] for the passive margin setting of the Blake Ridge. Advective flow regimes are different at active and passive margins, as there is a tendency for the flow at active margins to be focussed along landward-dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step-pattern of the pore water profiles. For passive margins, but also for some active margin sites, advection is from sources below the drilled section, either through subvertical faults (passive margins) or from the décollement zone (active margins). We have learned that the well-known coupled pore water anomalies that are ascribed to hydrate dissociation—downward chlorinity decrease combined with δ18O increase [Hesse and Harrison, 1981, Earth Planet. Sci. Lett. 55 (1981) 453.]—need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low-δ18O fluids. However, if the anomalies show up, hydrates are present almost invariably (with the exception of advected low-Cl−/high-δ18O waters). Coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate-affected pore water profiles at passive margins. Instrumental for modeling are chlorine isotopes, which provide an effective tool to assess advection rates. The Egeberg and Dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection, diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions. Modeling the more complex pore water profiles of active margins is a challenge for the future. Compared to geophysical methods to estimate hydrate concentration, the geochemical method gives minimum amounts.