Pore fluids were extracted on shipboard from 12 deep-sea sediment cores taken during all east to west traverse from Africa to the Mid-Atlantic Ridge between latitudes 18N and 20 degrees N. They were analyzed for Cl, SO 4 Mg, Ca, K, Sr, SiO 2 , B, Li, and Mn. Excepting Cl, SO 4 , and Ca, which are very close to sea water composition, other components examined are slightly but systematically different from normal sea water. Constancy of Cl in samples of varying clay: CaCO 3 , ratios negate semipermeable membrane behavior of compacting clays, at least for the top 10 meters of sediment. K and B enrichments mirrored by Mg depletions suggest authigenic chlorite formation at the expense of illite K-feldspar and perhaps montmorillonite. The rather constant SiO 2 level and its enrichment over that of bottom water appear indicative of control by clay-silica equilibria. Minor variations of all elements along depths of cores become magnified and more erratic in the Ridge area, due to heterogeneity of mineralogy or perhaps local hydrothermal effects.