Elemental and isotopic chloride geochemistry and fluid flow in the Nankai trough

Abstract

The first transect of pore fluid Cl- profiles across the deformation front of an accretionary prism and underthrust section, Nankai Trough, and the first Cl isotopic profile in this environment are presented. These demonstrate that there is channelized horizontal episodic flow through most of the section arcward of the deformation front and in the lowermost section seaward of the deformation front. In the underthrust sediments, channels appear to be continuous. Fluid velocities, within the channels, based on Cl- gradients, are 13 ± 5cm/year. The near balance of depth-integrated flow and the subduction of pore fluid indicates that flow episodes are frequent. δ37Cl values are lower than seawater throughout the section, averaging -6.1 per mil. This demonstrates fluid/mineral exchange of Cl and suggests that subducted solids carry enriched δ37Cl that is efficiently recycled from deeper levels in the subduction zone back into seawater and may be responsible for the seawater/mantle δ37Cl fractionation.