BARIUM IN MOLLUSK SHELLS – A DUAL PROXY OF ENVIRONMENTAL CONDITIONS

Many trace element signals in mollusk shells are not robust proxies of environmental conditions. Reports on Mg/Ca, Sr/Ca, U/Ca and Mn/Ca have shown that these proxies can be complicated by vital or kinetic effects. Ba/Ca profiles on the other hand have been shown to be similar in several different species of bivalves and highly reproducible between shells growing in the same region. The typical signal is a flat ‘background’ signal episodically interrupted by sharp peaks. The background signal records ambient dissolved Ba/Ca of the water in which the mollusk grew. There is typically an inverse relationship between Ba/Ca in water and salinity, making the background Ba/Ca proxy in mollusk shells a potential salinity proxy. The driving mechanism of the peak has yet to be understood; the main hypothesis is that the peaks are related to ingestion of Ba rich phytoplankton or direct ingestion of barite. Developing the Ba/Ca peak proxy further could possibly lead to a paleo-productivity proxy. In this paper we give examples of Ba/Ca profiles from several different groups of marine and freshwater modern bivalves as well as from Eocene Venericardia from the Gosport Sand (Alabama, USA) and Pliocene Mercenaria from the Duplin Formation in South Carolina. The Eocene fossil specimens show profiles similar to modern bivalves, with low background Ba/Ca indicating full marine salinities. However, the much younger fossil specimen (Mercenaria sp. from the Pliocene) showed clear signs of ‘elemental diagenesis’, with background Ba/Ca levels about 10 times higher than modern specimens (despite original mineralogy of aragonite and typical stable isotope profiles). This highlights the potentials and problems of using this developing proxy on fossil specimens.