Cation exchange capacity and water content of ODP sites from Nankai

Pore fluid chlorinity lower than seawater is often observed in accretionary wedges and one of the possible causes of pore water freshening is the smectite to illite reaction. This reaction occurs during diagenesis in the 80-150°C temperature range. Low chlorinity anomalies observed at the toe of accretionary wedges have thus been interpreted as evidence for lateral fluid migration from inner parts of the wedge and the seismogenic zone. However, temperature conditions in Nankai Trough are locally high enough for the smectite to illite transition to occur in situ. Cation exchange capacity is here used as a proxy for smectite content in the sediment and the amount of interlayer water released during the smectite to illite reaction represents in average 12 water molecules per cation charge. Water and chloride budget calculations show that there is enough smectite to explain the chlorinity anomalies by in situ reactions. The shape of the pore fluid chlorinity profiles can be explained if compaction is also taken into account in the model. Lateral flow is not needed. This argument, based solely on chloride concentration, does not imply that lateral flow is absent. However, previous estimations of lateral fluid fluxes, and of the duration of transient flow events along the de.collement, should be reconsidered.在增生楔（accretionary wedges）中，常观测到孔隙流体氯度低于海水的现象，而孔隙水淡化的潜在成因之一为蒙脱石向伊利石的转变反应（smectite to illite reaction）。该反应发生于80~150℃温度区间的成岩作用（diagenesis）过程中。因此，增生楔前缘观测到的低氯度异常，曾被解释为流体从楔体内部及发震带（seismogenic zone）向侧方运移的证据。然而，南海海槽（Nankai Trough）的局部温度条件已足够高，可原位发生蒙脱石-伊利石转变。本研究以阳离子交换容量（cation exchange capacity）作为沉积物中蒙脱石含量的替代指标，且蒙脱石向伊利石转变过程中释放的层间水（interlayer water），平均每单位阳离子电荷对应释放12个水分子。水-氯收支计算结果表明，足够量的蒙脱石可通过原位反应解释氯度异常现象。若在模型中同时考虑压实作用（compaction），则可复现孔隙流体氯度剖面的形态，无需侧向流体运移。此项仅基于氯浓度的论证，并不代表侧向流体运移完全不存在。不过，此前针对沿滑脱带（decollement）的侧向流体通量及瞬态流动事件持续时长的估算，均需重新考量。