Rapid racemization of aspartic acid in mollusc shells and potential for dating over recent centuries

Abstract

DATING of deposits and materials less than 350 years old is hindered by the very poor time resolution of the radiocarbon method over this period^1,2. Fluctuations in atmospheric ¹⁴C levels result in the existence of several possible calendric ages for any given radiocarbon age for terrestrial samples. In the marine record, these fluctuations are dampened. However, only a small change in marine radiocarbon ages occurs over this period (from AD 1700 to 1950, radiocarbon ages become only 100 yr younger²). Consequently, age resolution is poor. Furthermore, there is an uncertainty in the amount of the correction for the reservoir effect, the apparent radiocarbon age of modern marine carbon (typically ~400yr), which reflects the average residence time of carbon in the oceans. Amino-acid racemization/epimerization analysis has been used primarily for dating samples older than the limit of radiocarbon dating (40,000–50,000 yr BP)^3,4. Here I show that aspartic acid (Asp) in mollusc shells has racemized particularly rapidly over the past few centuries. Analyses done on land snails from deposits in the Negev Desert, eolianites in Madeira, cave deposits in Jamaica and from museum collections indicate racemization rates of 2–5% per century. Asp racemization thus provides a means of dating recent faunal assemblages of molluscs and other biogenic carbonates, as well as recent terrestrial, fluvial and marine sedimentary sequences that contain molluscs.