Rock surface IRSL dating of buried cobbles from an alpine dry-stone structure in Val di Sole, Italy

Abstract

Here, we investigate the application of rock surface IRSL dating to chronology restrain archaeological structures related to upland pastoralism. We applied the method to cobbles collected from archaeological units in an excavation of a dry-stone structure in Val di Sole in the Italian Alps. At this site, archaeological finds and previous radiocarbon analyses have dated an initial human occupation of the site to the Early Bronze Age (ca. 2200–1600 BC), and a possible second occupation to the Middle Bronze Age (ca. 1600–1350 BC). These archaeological units have later been buried by colluvial sediments. Theoretically, the luminescence-depth profiles from rock surfaces from inside such structures could record the exposure and burial of these archaeological units. We collected buried gneiss cobbles from these archaeological units and measured rock slices and chips from 1 to 4 cm long cores with a low-temperature pIR-IRSL protocol to investigate the signal resetting in these cobbles. Only the IRSL50 signal was deemed appropriate for dating. Measured luminescence-depth profiles demonstrate varying levels of signal resetting before burial. Dating of two paragneiss cobbles from the lower unit yielded corrected burial ages of 1450-700 BC and 19 ka. The older date is clearly not associated with human occupation; the younger date slightly underestimates the Early Bronze Age occupation, which was confirmed by new radiocarbon dating of charcoal (1731-1452 and 2124-1773 cal. BC). The burial of the upper archaeological unit was dated to AD 1000, based on ages derived from the bottom surface of an orthogneiss cobble and the top surface of a paragneiss cobble. This is slightly younger than two new radiocarbon ages (426-596 and 537-654 cal. AD) from charcoal fragments sampled from the same unit. This new chronological data show longer exposure of the upper archaeological unit than was previously known. Furthermore, the paragneiss cobble from the upper unit has been exposed to sufficient heat to reset the IRSL50 and pIR-IRSL290 signals throughout the cobble; an event which can be dated to AD 100–1500 BC. Comparisons between fading-corrected IRSL50 ages and pIR-IRSL290 ages from the heated cobble are in agreement, which suggests that the conventional g-value approach accurately corrects for signal loss during burial. Overall, our research suggests that rock surface IRSL dating can provide complementary chronological data for archaeological settings.

Resources

• https://www.sciencedirect.com/science/article/pii/S1871101421000625
• Ageby_et_al_Rock_Surface_IRSL_dating_Val_di_Sole.pdf

Bibliography

Ageby, L., Brill, D., Rethemeyer, J., Brückner, H., Klasen, N. (2021): Rock surface IRSL dating of buried cobbles from an alpine dry-stone structure in Val di Sole, Italy. – In: Quaternary Geochronology, Vol. Volume 66, DOI: https://doi.org/10.1016/j.quageo.2021.101212