ABSTRACT

Rare earth elements (REE), Sr, Ba, Fe, Mg, Ca, Na, and K concentrations in three kinds of living foraminiferal tests collected from Iriomote Island, Japan were measured in order to obtain some qualitative aspects of trace element behavior during biogenic CaCO₃ formation. The obtained results were examined using the sample/seawater concentration ratios, which can be considered as a measure of total effect of element incorporation into biogenic CaCO₃ in marine environments. On a sample/seawater concentration ratio vs. ionic radius diagram, the foraminifera/seawater ratios of trivalent REEs and divalent Mg, Ca and Sr fall on their respective curves having peaks near the Ca ionic radius. This is consistent with the most widely accepted theory that solid/liquid partition coefficients are largely controlled by the sizes of the host site in the crystal and the substituting cation, such that a cation that has the most suitable size for the site can enter the crystal most easily. On the other hand, the foraminifera/seawater ratios of Fe and Ba show higher values than those expected from the crystal structure control model, which might be caused by insoluble precipitates such as Fe-hydroxides and barite. It is interesting that the concentrations of REEs in foraminifera are correlated with those of Fe, which suggests coprecipitation of REE with Fe-hydroxide. Such coprecipitation of REE might have occurred in seawater, followed by uptake of Fe-hydroxides by foraminiferal food micro-organisms. After digestion of such food, the dissolved REEs in intracellular fluid might have fractionated as CaCO₃ crystallization proceeds, and the combined crystal structure and ionic radius effect becomes conspicuous forming a peak on the foraminifera/seawater ratio vs. ionic radius diagram.