Agder College, Norway

Gas-hydrates release fresh water upon decomposition in the sampling process from drill cores. The freshening of the pore waters through this sampling effect can be used to estimate hydrate concentration, if it can be separated from the effects of diffusion and advection, which also change pore-water composition during burial diagenesis. Successful in-situ water sampling ahead of the drill bit with tools such as the WSTP (water sampling temperature probe) together with a method of reliably assessing advection rates are required to quantify gas-hydrate abundance in submarine hydrate zones. Ocean Drilling Program (ODP) Leg 164 on the classical Blake Ridge hydrate field off the Carolinas provided the sample material for which a method based on stable chlorine isotopes was developed to estimate advection rates. Chlorine isotopes are suitable to trace advection, because chloride rarely participates in chemical reactions during diagenesis that would cause isotope fractionation. With an advection rate of 0.18 mm/a, a combined diffusion-advection model could be fitted to the measured delta-37Cl isotope profile. This advection rate could then be used to establish a best-fit in-situ chloride model-curve from which hydrate concentrations were estimated that amount to an average of only 3.8% of the pore space at site 997 (hydrate zone between 24 and 451 mbsf).