DO LOCAL EXTINCTIONS CORRELATE WITH TAPHONOMIC SIGNATURES OF FRESHWATER MUSSEL SHELLS IN THE NORTH FORK HOLSTON RIVER, VA?

From 1950 to 1972 the Olin-Mathieson Chemical Company used mercury in the production of chlorine and caustic soda in Saltville, Virginia on the North Fork Holston River. It is now estimated that as much as 1,814 metric tons of salt and 34 kg of mercury were deposited per day into the plant’s settling ponds, which eventually caused the contamination of the river. The elemental mercury and chloride salts devastated the freshwater mussel fauna downstream of Saltville. This study explores whether taphonomic signatures can be correlated with the time of a local extinction. The North Fork Holston River provides a prime study area because essentially all freshwater mussel populations downstream of Saltville were wiped out by 1973. Areas that are devoid of active populations today, and thus contain shells that are at least 30 years old, can be compared to upstream sites unaffected directly by the mercury contamination to establish end members on a taphonomic scale. In addition, areas that have recently been recolonized by active populations illustrate a still different type of assemblage; intermediate in terms of its taphonomic history. Using systematically collected samples of shells, the taphonomic alteration was estimated using a series of taphonomic rank variables. The taphonomic index computed from those variables can be used to identify freshwater communities with different timing of extinction. This method of examining taphonomic signatures may be an especially useful tool in studying environmental and ecosystem changes in regions with unknown extinction history.