Smectite diagenesis, pore-water freshening, and fluid flow at the toe of the Nankai wedge

The presence of low-chloride fluids in the lowermost sediments drilled at Ocean Drilling Program Site 808, at the Nankai accretionary wedge, has been considered as prime evidence for long-distance, lateral fluid flow from depth. Here, we re-evaluate the potential role of in situ reaction of smectite (S) to illite (I) in the genesis of this low chloride anomaly. This reaction is known to be occurring at Site 808, with both the S content and S to I ratio in the mixed layer clays decreasing substantially with depth. We show that the bulk of the chloride anomaly can generate by in situ clay dehydration, particularly if pre-reaction smectite abundances (A_i) approach Ο 10-15% of the bulk sediment. The A_i values, however, are not well constrained. At A_i values < 10-15%, an additional source of low-Cl fluid centered close to the décollement could be required. Thus, there remains the important possibility that the observed low-Cl anomaly is a compound effect of both lateral flow and in situ smectite dehydration.