RADIOCARBON RESERVOIR EFFECT (RRE) CORRECTION OF PLEISTOCENE AND HOLOCENE AQUATIC MOLLUSK SHELLS FROM THE SCIOTO RIVER, OHIO

Freshwater mollusk shells are repositories of information for geologists, archaeologists, and biologists alike. Geologists find mollusk shells in fluvial deposits, and want to use them as a means to date the strata. Archaeologists have similar interests, but with more human applications, such as dating shells found in middens to track their use over time. Biologists want to know how mollusk populations change over long periods of time to inform restoration efforts. The one thing these all have in common is time.

Time can be quantified and recorded in many ways, but the focus of this research is time as it is recorded by aquatic mollusks’ intake of radiocarbon (C-14) and how to correct for the effect of C-14-deficient freshwater systems. This lack of C-14 in freshwater systems is often caused by the presence of carbonate bedrock in the watershed. All of the C-14 in carbonate bedrock has decayed away, so when it is dissolved by groundwater, that carbon is brought into streams via groundwater discharge. This results in the surface water systems having proportionally less C-14 than the atmosphere.

This causes the organisms that live in these freshwater systems to appear older than they actually are when radiocarbon dated. This is known as Radiocarbon Reservoir Effects (RREs). RREs of freshwater systems are known to vary regionally, thus in order to date freshwater mollusks in a given region, one must first establish a regional RRE correction. In the case of this research, the study area is the Scioto River - with field sites just south of Columbus, Ohio.

To determine the RRE correction, we used the matched pairs method (1 terrestrial organic to 2 mollusk shells), along with the crush and homogenize method (multi-year aliquots), to account for and decrease 1) potential temporal variability and 2) sampling bias. RRE determinations were made for six matched pairs ranging from 2,500 to 12,000 years old, and after regression analysis yielded RRE corrections between 1,100 ± 35 and 1,400 ± 15 C-14 years, with a weighted mean of 1,260 ± 120 C-14 years. These results display remarkable consistency both temporally and across species. These findings are significant as they suggest that freshwater RREs are remarkably consistent over time in a given study area and that different genera of aquatic mollusks yield similar results.