The faunal variations described above are associated with differences in preservation that range from moderate in the intervals of high diversity (above and below the clay-rich layer) to poor in the interval of low species richness (within the clay-rich layer) (Fig. F2; Pl. P3). Such overall moderate to poor preservation is an effect of differential dissolution, which is more significant within the clay-rich layer. In general, planktonic foraminiferal species vary in their susceptibility to dissolution and in the structural changes that occur during dissolution. In the studied samples, common to abundant keel and wall fragments of morozovellids are present throughout. Igorinids (except for I. albeari) are characterized by heavy overgrowth (shell thickening) (see Pl. P4) and exhibit a calcite crust that covers the outside of the test, inhibiting dissolution. The abundance of igorinids, especially in the clay-rich layer, is emphasized by the rarity and/or absence of the other genera and by the low number of total specimens (often <300 specimens) in the residues that contain abundant crystals of phillipsite. Among the igorinids, I. tadjikistanensis, I. pusilla, I. albeari "chubby", and I. pusilla "high trochospire" are the most dissolution-resistant taxa and are the only species recorded in the clay-rich layer. Conversely, I. albeari, morozovellids, acarininids, globanomalinids, subbotinids, and chiloguembelinids are quite common below the clay-rich layer, they almost disappear within it, and they reappear in low abundance above the clay-rich layer and are interpreted as more dissolution-susceptible taxa.
The assemblages below the clay-rich layer are different in composition: acarininids, igorinids, and I. albeari dominate in the shallower holes (1209A and 1210A), whereas morozovellids, subbotinids, and globanomalinids are more abundant in the deeper holes (1212B and 1211B). Such variations might be related to the different water depths at the studied sites (Table T1), the different susceptibilities to dissolution of the planktonic foraminiferal species, and the variations in the degree of calcite dissolution in the water column.