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The abundance and diversity of calcareous plankton species in suspension in the modern ocean are determined by the influence of global temperatures on species diversity and the combined effect of atmospheric and hydrospheric circulation on organic productivity in the photic zone. The species composition and dominance of individual species within deep-sea sedimentary assemblages are, however, primarily a function of the resistance of calcareous tests to solution in the calcium carbonate undersaturated ocean deep-water. Resistance to solution is determined by the composition of the skeletal elements, i.e., calcitic or aragonitic, a low concentration of pores, the tendency to develop thickened tests with age in some species of planktonic foraminifera (Berger and Parker, 1970; Berger, 1971), and the presence or absence of a protective organic sheath in both the planktonic foraminifera and the calcareous nannoplankton (Berger, 1970; Mclntyre and Mclntyre, 1971). The influence of species selective dissolution and the resulting modification of recent sedimentary assemblages of planktonic foraminifera is already documented for parts of the ocean (Berger, 1970, 1971). Modern calcareous nannoplankton assemblages which are similarly modified are recognized by Mclntyre and Mclntyre (1971) in sediments from the Atlantic and Indian oceans and their results have been substantiated and extended by Schneidermann (1972). The affects of species selective dissolution are also evident in fossil foraminiferal and calcareous nannoplankton assemblages in deep-sea sediments. Depending on the degree of solution foraminiferal-nannofossil assemblages subjected to dissolution are characterized by a decreasing ratio of planktonic to benthonic foraminiferal tests, the fragmentation of planktonic foraminiferal tests, low species diversity, and/or species with more massive tests (Cita 1971). Ultimately planktonic foraminiferal tests become rare or are totally absent. The dissolution of planktonic foraminiferal tests is accompanied by the enrichment of the sediment in the more resistant calcareous nannofossils and leads to the formation of nannoplankton chalk or ooze. Species selective dissolution in Eocene nannofossil assemblages is indicated by an increase in the percentage of some species of Discoaster (Ramsay, 1972a). The data presented in this report show that Miocene and Pliocene nannofossil assemblages are similarly affected. Solution-induced species patterns in ancient deep-sea sequences provide information on past rates of calcium carbonate solution in the oceans. Although most interpretations are based on analyses of planktonic foraminiferal assemblages, calcareous nannofossils are potentially equally useful and possibly even superior indicators of solution at depth. Apart from their abundance, they are also more resistant to solution than planktonic foraminiferal tests and therefore provide information on sediments in which these organisms are either rare or absent as a result of solution.

Fluctuation in the Past Rates of Carbonate Solution in Site 149: A Comparison with Other Ocean Basins and an Interpretation of Their Significance