Comparison of the Composition of Fossil and Recent Mollusk Shells

Abstract

Unrecrystallized shells of five species of mollusks were collected from Miocene, Pliocene and Pleistocene deposits and from a variety of Recent environments. The manganese, iron, magnesium, strontium and barium contents, and the mineralogy of these shells, were determined. The average magnesium and manganese contents of the aragonitic fossil shells are lower, and the strontium, barium, and iron contents higher than in Recent shells of the same species. The mean strontium and magnesium contents are lower in the fossil shells of the single calcitic species studied, whereas the remainder of the elements in the shells of this species do not differ statistically. The differences in composition of fossil and Recent shells are attributed to the effects of weathering and diagenesis. Since the trace-element content of both aragonite and calcite can change without recrystallization, geochemical and paleoecological studies based on assumptions of unchanging composition of carbonate skeletons should be used with caution.