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Sediments deposited off the Nicoya Peninsula advect large volumes of water as they enter the Costa Rica subduction zone. Seismic reflection data, together with results from Ocean Drilling Program Leg 170, show that hemipelagic mud comprises the upper ∼135 m of the sediment column (ranging from 0 to 210 m). The lower ∼215 m of the sediment column (ranging from 0 to 470 m) is pelagic carbonate ooze. We analyzed samples from 60 shallow (<7 m) cores to characterize the spatial variability of sediment composition on the incoming Cocos Plate. The bulk hemipelagic sediment is 10 wt% opal and 60 wt% smectite on average, with no significant variations along strike; the pelagic chalk contains approximately 2 wt% opal and <1 wt% smectite. Initially, most of the water (96%) in the subducting sediment is stored in pore spaces, but the pore water is expelled during the early stages of subduction by compaction and tectonic consolidation. Approximately 3.6% of the sediment's total water volume enters the subduction zone as interlayer water in smectite; only 0.4% of the water is bound in opal. Once subducting strata reach depths greater than 6 km (more than 30 km inboard of the subduction front), porosity drops to less than 15%, and temperature rises to greater than 60°C. Under those conditions, discrete pulses of opal and smectite dehydration should create local compartments of fluid overpressure, which probably influence fluid flow patterns and reduce effective stress along the plate boundary fault. [ABSTRACT FROM AUTHOR]