Figure F17. Methane concentrations determined from PCS measurements. Approximate phase boundaries are shown between fields where dissolved gas, gas hydrates, and free gas are predicted to be present in the subsurface. Uncertainties (~30%) in the position of these boundaries are a result of a number of factors, including variations in subsurface temperature gradient, gas composition, and pore fluid salinity. Although these boundaries give a first order view of where gas hydrate should be present, data points that fall near boundaries should be interpreted with caution. More detailed analysis of the predicted stability fields for conditions measured during Leg 204 is the subject of postcruise research. Near the summit, all PCS measurements from within the GHSZ indicate the presence of gas hydrate, with the highest concentration near the seafloor. On the flanks and in the basin, hydrate presence within the stability zone is intermittent and the highest concentration is just above the BSR (no PCS measurements are available from the thin hydrate-bearing layer just above the BSR at Site 1251) (see Figs. F14, F18).