University of Houston, United States

During Leg 166 of the ODP, two transects of holes were drilled perpendicular to the western margin of Great Bahama Bank (GBB) in order to investigate the possibility that active fluid movement is taking place through the sediments into the platform. It had been previously suggested that water was being recharged through the margins of GBB as a result of the temperature differences between the platform interior and the adjacent seaways providing the geochemical potential for the alteration and cementation of carbonate sediments. Strong support for this hypothesis was evident in the form of the geochemical gradients of conservative and non-conservative elements in the interstitial pore water collected during Leg 166. In the upper portions of all the holes (0 to 60 m) there is an absence of geochemical gradients. In this region, which we are calling the "flushed" zone, continual advection of seawater caused the gradients produced by diagenetic reactions and diffusion from underlying sources to be removed. Geothermal measurements are also consistent with this hypothesis. Close spacing of Sites 1003, 1004, and 1005 allowed identification of a tongue of water entering into Great Bahama Bank. At the present time we have not identified the mechanism for flushing, but note that the observations are consistent with the advection of cold water into the platform by the Kohout convection process. In addition to the shallow advection we observed changes in water chemistry across stratigraphic horizons that also indicate an overall horizontal flow component controlled by the sequence stratigraphic framework.