Sodium and Manganese in Recent and Fossil Shelis

Abstract

To study the behaviors of trace elements in shells during the process of fossilization, sodium and manganese in shells belonging to Pelecypoda collected from Oligocene, Miocene, Pliocene, Pleistocene and Recent deposits in the northeastern Japan were determined by nondestructive neutron activation analysis.
The crystal-form of calcium carbonate and the shell structure have close connection with the manganese content in Recent shells, i. e., calcite type shells with foliated structure were higher in manganese content than aragonite type shells and calcite type shells with homogeneous or complex prismatic structure. Whereas, no significant difference in average content of sodium was observed between calcite and aragonite type shells, and the same is true for shells in fresh water and marine environments.
From Recent to Pleistocene, the sodium content of shells decreases greatly with decreasing residual organic matters in shells. And this trend becomes weak before Pliocene, whereas the manganese content in shells, especially calcitic shells, increases greatly with increasing age even before Pliocene. These results suggest that sodium was removed from shells together with decomposed organic matters and manganese was accumulated by adsorption in the remainders of shells from the surrounding water during the progress of fossilization of shells.