(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.

在卡斯卡迪亚增生复合体中进行钻探，使我们能够评估脱水反应和天然气水合物分解对孔隙水淡化的贡献。观察到的孔隙水随深度和距棱镜趾的距离而变淡的现象，与由沉积物温度和年龄的增加驱动的蒙脱石向伊利石转化的增强转化一致。尽管这些地点含有天然气水合物（如离散的低氯含量峰值所示），但在海冰脊上钻探的最西端地点并未显示出随深度变淡的趋势。锶数据揭示，所有混合样品均含有与俯冲海洋地壳反应而改性的深层流体。因此，我们推断，在最西端地点，沉积物的增生过于近期，尚未发生显著的伊利石化。我们的数据表明，溶解氯含量随深度的平滑下降一般不能用来推断沉积边缘是否存在天然气水合物或估计其数量。