(Table 1) Depth to basement, accreted melange, bottom simulating reflector and sediment penetration in meters below seafloor

Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and distance from the prism toe is consistent with enhanced conversion of smectite to illite, driven by increase in temperature and age of accreted sediments. Although they contain gas hydrate -as evidenced by discrete low chloride spikes- the westernmost sites drilled on Hydrate Ridge show no freshening trend with depth. Strontium data reveal that all the mélange samples contain deep fluids modified by reaction with the subducting oceanic crust. Thus we infer that, at the westernmost sites, accretion is too recent for the sediments to have undergone significant illitization. Our data demonstrate that a smooth decrease in dissolved chloride with depth cannot generally be used to infer the presence or to estimate the amount of gas hydrate in accretionary margins.

对卡斯卡增生复合体（Cascadia accretionary complex）开展钻探工作，可用于评估脱水反应与天然气水合物（gas hydrate）分解对孔隙水淡化的贡献。观测到的孔隙水淡化程度随深度及距增生楔趾部（prism toe）的距离增加而增强，这与温度升高及沉积地层增生时代增加所驱动的蒙脱石（smectite）向伊利石（illite）转化作用增强一致。尽管水合物脊（Hydrate Ridge）最西侧的钻探点位以离散的低氯浓度峰值证实了天然气水合物的赋存，但这些点位的孔隙水并未呈现出随深度的淡化趋势。锶数据显示，所有混杂岩（mélange）样品均含有与俯冲洋壳（subducting oceanic crust）发生反应改造后的深部流体。据此我们推断，在最西侧的钻探点位处，沉积地层的增生年代过新，尚未发生显著的伊利石化（illitization）作用。本研究数据表明，通常情况下无法通过溶解态氯离子（dissolved chloride）随深度的平稳降低，来推断增生型大陆边缘（accretionary margins）中天然气水合物的存在，亦无法估算其赋存总量。