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Progressive structural breakdown of planktonic foraminiferal shells was obtained by three types of laboratory dissolution experiments. First, individual specimens of Globorotalia truncatulinoides and Orbulina universa were dissolved in sea water baths at a pH of 6.5. Percent dissolution, determined by weight loss, ranged from a few percent to 83 percent. Second, a single specimen of G. truncatulinoides and a suite of specimens of Pulleniatina obliquiloculata were subjected to repeated exposures in an acetic acid bath at a pH of 6. Third, four aliquots of a death assemblage of planktonic foraminifera were dissolved to varying degrees in the pH-stat of Morse (1974) at a pH of 7.53 at 5°C. Changes in the species composition were noted with increasing dissolution. The sequential breakdown of shell ultrastructures in each experiment was observed in a scanning electron microscope. For G. truncatulinoides, this breakdown was similar in each experimental method and closely resembles those occurring in nature.

Tests were carried out to determine specimen orientation during laboratory dissolution. It was observed that G. truncatulinoides assumed a preferred spiral-side resting position after being stirred in a cylindrical container. In some of our experiments this preferred orientation determined the final shell morphology by selectively exposing certain parts of the shell to dissolution.

Natural species assemblages from four surface sediment samples of various depths in the west-central North Atlantic reflected changes in species composition caused by natural dissolution. However, the trends of relative resistance to dissolution of individual species in these natural assemblages showed only minor agreement with the trends of species dissolved in the laboratory.

The effects of natural dissolution was studied for Pulleniatina obliquiloculata, because this species is highly dissolution-resistant and possesses a relatively homogeneous surface texture (cortex) in the adult stage. This species is potentially useful as an indicator of the degree of dissolution of planktonic foraminiferal assemblages on the sea floor.

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