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Chemical analyses have been performed on pore solutions from more than 25 drillholes in the North and South Atlantic Ocean. Several holes penetrated Tertiary-Mesozoic sediments and bottomed in basalt. The data indicate that in the central areas of the Atlantic Ocean the salinity and chlorinity of pore fluids approach the values for bottom waters and vary less than about 1-2% with depth, with a few exceptions. Diagenetic changes in major inorganic ions are relatively minor, regardless of depth, lithology, or proximity to basalt bottom. Significant effects include chiefly loss of magnesium, partial loss of sulfate, and corresponding increases in alkalinity. Nearer the continents, fluid compositions range over somewhat wider limits, especially in the direction of lower salinity. With due allowance for molecular diffusion and other disturbing effects, the data offer no evidence that the oceans varied appreciably in either chloride, salinity, or ionic composition during Tertiary time.

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