TESTING NEW MATERIALS FOR RADIOCARBON DATING: TERRESTRIAL GASTROPOD SHELLS

The quest for new materials that yield reliable ¹⁴C ages has been ongoing since the advent of the radiocarbon dating technique in the mid 20^th century. All materials (organic and inorganic) that yield reliable ¹⁴C ages have two common characteristics. First, the initial ¹⁴C activity of the material – a plant, for example – was in equilibrium with atmospheric ¹⁴C at the time that it was alive. In other words, the ¹⁴C activity of a plant that lived T years ago was the same as the ¹⁴C activity of the atmosphere T years ago (after accounting for isotopic fractionation). Second, after death, the material must behave as a closed system over geologic timescales; that is, carbon was neither added to nor exchanged with the sample material. If both of these criteria are met, then the material should yield reliable ¹⁴C ages. In this presentation, I will summarize research that has been conducted on terrestrial gastropod shells to determine whether they might be suitable for ¹⁴C dating. Terrestrial gastropods are one of the most successful groups of organisms on Earth. Their distribution includes all continents except Antarctica, and they occupy exceptionally diverse habitats, from marshes and wet meadows to alpine forests and arctic tundra. Their shells are also commonly preserved in Quaternary deposits, including alluvial, fluvial, loess, and wetland deposits. However, terrestrial gastropods are known to ingest limestone (likely in order to obtain sufficient calcium) when building their aragonite shells. Unfortunately for dating purposes, they also incorporate dead carbon from the rocks in their shells, which can cause apparent ¹⁴C ages to be as much as 3,000 ¹⁴C years too old. Recently, we found that many small (<1 cm diameter) terrestrial gastropods avoid this “limestone problem” even when living in areas in which carbonate rocks are readily available. The shells of many of these same taxa also appear to act as closed systems with respect to carbon over geologic timescales in a wide range of environments. Thus, we conclude that shells of some small terrestrial gastropods may indeed yield reliable ¹⁴C ages. Lessons learned from this exercise can be applied to other materials to determine their suitability for dating as well.