Whole foraminifera shell weights for G. sacculifer and N. dutertrei in Caribbean Sea cores covering a depth range of 680 to 3620 m show a decrease in weight of 3.6 ¿ 0.4 ¿g between glacial time and the Holocene. This difference likely reflects a thickening of foraminifera shells as a result of the higher carbonate ion concentration of glacial surface water. The fact that the shells from the core at 680 m depth are considerably lighter than those from the core at 1030 m depth suggests either that there is an up-water column increase in the offset between the pore water and the bottom water carbonate ion concentration or that factors other than surface water carbonate ion concentration influence the initial shell weight. Of particular interest is a strong preservation event corresponding to the deglacial interval at 680 m depth (the present-day core of the intermediate water mass).