Experimental studies on modern aragonitic shells of the gastropods Melanopsis praemorsum and M. tuberculata yielded bulk distribution coefficients of 0.31 for Sr and of 0.48 for Ba, between the shells and their ambient waters. These partition coefficients are about three times lower than their "inorganic" equivalents. H2O leaching experiments of modern shells indicate that Sr and Ba dissolve congruently with no effect on the original Sr and Ba contents of the shells. Conversely, the Na/Ca, Mg/Ca and K/Ca ratios in the shells and in the extrapallial fluids are constant, independent of variations in the habitat water chemistry. This "vital effect" dominates the shell composition regarding the latter three ion ratios. Moreover, dissolution experiments suggest that there is leaching of a significant fraction of Na, Mg and K which is presumably not bound to the mineral's lattice. This preferential dissolution starts before any mineralogical alteration occurs. The laboratory data are supported by a sharp decrease of the Na/Ca ratio observed in aragonitic fossils during early diagenesis, in contrast with its constancy in live specimens. These results clearly demonstrate that Sr and Ba can be used for environmental studies, whereas Na, Mg and K cannot.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology