USING GEOCHEMICAL AND TAPHONOMIC SIGNATURES OF FRESHWATER MUSSEL SHELLS TO EXPLORE INDUSTRY-RELATED EXTIRPATIONS IN THE NORTH FORK HOLSTON RIVER, VA

The focus of this research is to develop non-invasive techniques to obtain new, independent insights into extirpations (loss of populations) induced by anthropogenic changes. We use freshwater ecosystems of a mercury-polluted river to test the approach. The Olin-Mathieson Chemical Company used mercury to produce chlorine and caustic soda in Saltville, Virginia on the North Fork Holston River from 1950 to 1972. The effluent containing elemental mercury and chloride salts had devastated the freshwater mussel fauna downstream of Saltville by the early 1970's. In a two-pronged approach, we targeted (1) geochemical mercury signatures of empty mussel shells and (2) taphonomic signatures based on quantifying preservational qualities of shells. The ICPMS analysis shows that shells collected upstream from Saltville (above the pollution source) have very low Hg concentrations (<10 ppb). In contrast, the shells collected directly below the pollution source contain significantly higher Hg concentrations (exceeding 160 ppb for some specimens). Shells collected further downstream have notable Hg concentrations, which decrease with distance from Saltville. Taphonomic signatures of shells also confirm that extirpation patterns relate to the pollution source. Shells are most heavily altered and fragmented in areas directly downstream of the contamination point, which have been devoid of extant populations for at least 30 years, and thus, contain shells that are at least 30 years old. In contrast, upstream sites, unaffected directly by the mercury contamination, contain many fresh-dead shells indicating the presence of reproducing populations, which continuously contribute recent mortalities to the death assemblage. Areas further downstream, which have recently been recolonized by mussel populations, represent an assemblage with an intermediate taphonomic signature. The study demonstrates that non-invasive techniques focused on dead mussel shells may offer a powerful, independent source of information about freshwater communities with different extirpation and contamination histories. Geochemical and taphonomic signatures of shell remains should become especially useful tools in documenting the pollution history and ecosystem changes in regions with unknown extirpation histories.