INTRODUCTION

The chemistry of interstitial water is strongly influenced by tectonic, sedimentological, mineralogical, and diagenetic processes. Dissolved chloride concentrations (Cl^–) and the stable isotope compositions (D and ¹⁸O values) of waters are valuable in establishing the origin and evolution of fluids in marine sediments (e.g., Dählmann and de Lange, 2003; Hesse, 2003). In the absence of gas hydrates, chloride is not involved in geochemical reactions in shallow sediments. Water is the carrier of dissolved ions, and its isotopic signal reflects processes such as dehydration and gas hydrate formation involving uptake and release of water. These are the only processes that affect the hydrogen isotopic signal because water is the main hydrogen reservoir. Oxygen, however, is a major component of minerals; thus, isotopic exchange of these phases with pore water may provide valuable information on diagenetic processes.

At Hydrate Ridge, offshore Oregon (USA), the Cl^– distribution in interstitial waters within the gas hydrate stability zone (GHSZ) is clearly associated with gas hydrate dynamics, whereas at depth, the Cl^– signal reflects dehydration reactions (Torres et al., 2004a, 2004b). The migration of deep fluids to the GHSZ is a process that delivers dissolved methane for the formation of gas hydrates; thus, defining the geological and geochemical histories recorded in the interstitial waters is an essential approach for further understanding of gas hydrate systems. Here we present hydrogen and oxygen isotope data of interstitial waters from the Cascadia accretionary complex collected during Ocean Drilling Program (ODP) Leg 204 to identify key geological and geochemical processes operating within this accretionary margin.