Erosion of archaeological sites: Quantifying the threat using optically stimulated luminescence and fallout isotopes

Although visible evidence shows that erosion has damaged many archaeological sites, especially when tilled, there has hitherto been scant attention to its quantitative assessment. Accordingly, the archaeology communities lack insight into whether long-term threats to the stability and integrity of soils at these sites allow these cultural repositories to be preserved for future human generations. Of the techniques that are available to measure erosion rates, few have been tested on the timescales needed. We selected three archaeological sites with high expected erosion rates. We combined optically stimulated luminescence (OSL) dating with analyses of radioactive fallout isotope distributions to assess erosion patterns and rates. An age–depth representation of OSL single-aliquot results was developed to determine past erosion, and to identify stable land surfaces on centennial to millennia timescales. Fall-out isotopes of cesium (Cs) and plutonium (Pu) were suitable for shorter timescales: The ²⁴⁰Pu/²³⁹Pu ratios and a correlation between activities of ^239+240Pu and ¹³⁷Cs demonstrated the weapons testing fallout origin of these isotopes in the ~1952–1966 timeframe. Erosion rates in recent decades ranged from 2 to 6 mm/year on the studied sites. Our results indicate that erosion is not only tied to the past, but keeps on threatening archaeological sites.