Magnesium/calcium, Sr/Ca, and Na/Ca ratios were determined for planktonic forminiferal tests taken from core top samples at water depths of 500 to 4,700 m along the East Pacific Rise and central Pacific. Six species with varying susceptibilities to calcium carbonate dissolution were analyzed by neutron activation and atomic absorption techniques. These natural dissolution experiments were supplemented by laboratory dissolution and exchange experiments. The magnesium content of planktonic foraminifera was found to be highly susceptible to diagensis; for tests of a given species, Mg/Ca ratios decreased with increasing water depth. Sodium/calcium ratios decreased slightly with depth, and Sr/Ca ratios did not change as a result of the partial dissolution of test material with increasing water depth.

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