Sites 677 & 678

Site 677 is located in a band of slightly low heat flow about 3 km S of the Site 504. Leg 111 penetrated 309.4 m of siliceous nannofossil ooze and chalk and a thin stringer of chert before reaching basement. Recovery with the APC was excellent averaging over 100% of the cored interval. Analyses of pore waters suggested that ocean bottom water is slowly recharging through the sediment cover, at rates on the order of a few mm per year. Site 678 is located 2 km NE of Site 677, in a local maximum of heat flow. Time allowed only a brief program of washing and coring to basement at Site 678, primarily for geothermal and geochemical studies of advection of pore waters and diagenesis of the sediments. The compositions of formation waters from the basal sediments are similar at Sites 677 and 678; at both sites, concentrations of dissolved Ca in pore waters were about 6 times greater and those of dissolved Mg were about 5 times less than in seawater. This suggests that the chemical fluxes through the sediments are negligible compared with advective fluxes within basement. -from Authors