Recent coccoliths deposited in the Atlantic Ocean undergo selective dissolution in the calcium carbonate compensation realm, resulting in an increase in the relative proportion of solution-resistant placoliths in the assemblage. Solution of the coccoliths proceeds through gradual selective removal of ultrastructural elements in a sequence characteristic for each taxonomic group.

Selective dissolution of coccoliths allows recognition of 3 zones: (1) a basal dissolution zone termed here the "Mesolytic zone," about 500 m thick, directly overlying the calcium carbonate compensation depth. Sediments in this zone lack planktonic Foraminifera, have a low CaCO₃ content, and contain a coccolith assemblage of low diversity and composed of solution-resistant species, chiefly placoliths. In the southern and equatorial Atlantic these sediments are bathed by Antarctic bottom waters. (2) The middle Oligolytic zone is in the region from 500 to 1,500 m above the calcium carbonate compensation depth. Sediments contain corroded and fragmental tests of planktonic foraminifers and a coccolith assemblage with abundant resistant species and some corroded, less resistant orms. (3) The upper Eolytic zone extends from about 1,500 m above the calcium carbonate-compensation depth to the calcium carbonate-saturation depth. Sediments contain normal planktonic foraminiferal assemblages and diverse, well- to moderately well-preserved coccoliths, with only a few species showing obvious signs of corrosion.

Selective dissolution with depth removes "tropical" species, so that assemblages deposited at greater depths resemble living assemblages from higher latitudes.

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