INTERSTITIAL WATER CHEMISTRY OF SEDIMENTS OF THE COSTA-RICA ACCRETIONARY COMPLEX OFF THE NICOYA PENINSULA

Interstitial water analyses from numerous piston, gravity, and Alvin p ush cores show that fluid flow at the Costa Rica Accretionary Prism is spatially limited and can only be defected by visually directed cores in zones of biogenic activity. Most of the sites cored show evidence for normal diagenetic processes in rapidly deposited, organic carbon-r ich sediments, with little evidence for fluid advection. However, in d irected Alvin push cores, obtained from black sediments characterized by the presence of Calyptogena clams or tubeworms, evidence for fluids advected upward from greater depth horizons is shown. These zones are associated with a large mudvolcano on the prism and with the toes of Out Of Sequence Thrusts. As changes from sea water concentrations are still relatively small, substantial mixing with sea water must have oc curred during this upward fluid movement.