Freshwater mussel shells (Unionidae) describe anthropogenic changes to trace element cycling within a North American river

Bivalve shells provide an unparalleled opportunity for understanding the history of bioavailable trace elements in aquatic systems. The present study analyzed the elements Al, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, U, V and Zn in freshwater mussel shells collected from a large floodplain river. Shells were collected fresh, sampled from a historic archive, and retrieved from pre-Columbian archeological sites. The elements As, Co, Cu and Ni varied with time over the course of the 20th century. When compared to the pre-Columbian shells, 20th century shell concentrations for these elements were either consistently higher (Co, Cu and Ni) or lower (As). The 20th century shells also had consistently lower concentrations of Mn and Zn when compared to the pre-Columbian period, however diagenesis is the most likely cause of this difference in Mn. The elements Cd and Fe had little spatial or temporal variation in this data set. Several elements (Al, Cr, Hg, Pb, Se, U, and V) were below method detection limits in most shells. This study demonstrated that mussel shells can be used as archives of environmental history in river systems.