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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 ultra-structural elements in a sequence characteristic for each taxonomic group.
Selective dissolution of coccoliths permits recognition of 3 zones. (1) A basal dissolution zone about 500 m thick immediately overlies 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 composed of solution resistant species, chiefly placoliths. In the southern and equatorial Atlantic, these sediments are bathed by Antarctic bottom waters. (2) A middle zone in the region from 500 to 1,500 m above the calcium carbonate-compensation depth contains corroded and fragmental tests of planktonic Foraminifera and a coccolith assemblage, with abundant resistant species and some corroded, less resistant forms. (3) An upper dissolution zone extends from about 1,50 m above the calcium carbonate compensation depth to the calcium carbonate compensation 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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