A transect of four sites drilled by Integrated Ocean Drilling Program Expedition 311 provides an ideal data set to investigate the distribution of gas hydrates across the northern Cascadia convergent margin. We quantify gas hydrate saturation ( fraction of pore space occupied by gas hydrate) in a joint interpretation of pore water chlorinity data and downhole logs of porosity and electrical resistivity. The estimated saturation profiles define a gas hydrate occurrence zone (GHOZ), the depth interval where gas hydrates are actually found. In three of the Expedition 311 transect sites (U1326, U1325, and U1327), the top of the GHOZ systematically deepens moving landward of the deformation front of the Cascadia accretionary wedge. The farthest site from the deformation front ( U1329) shows no clear evidence of gas hydrates. We apply a simple diagenetic model to explain the observed landward deepening of the GHOZ. The model computes the methane concentration in the pore fluid for a given in situ bacterial methane production, sedimentation rate, and fluid advection velocity. Model results show that lower rates of sedimentation or fluid advection result in slower increases in methane concentration with depth and deeper tops of the GHOZ. Sedimentation rates in the Expedition 311 sites decrease landward, and fluid advection rates due to the dewatering of the accretionary wedge are expected to decrease moving landward of the deformation front as well. A combination of these two mechanisms can explain the deepening of the top of the GHOZ observed in the Expedition 311 transect sites.
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