Appendix 2 : luminescence analysis and dating of sediments from archaeological sites and valley fill sequences

Abstract

This report describes Optically Stimulated Luminescence (OSL) investigations to provide a temporal framework to underpin investigations into the early Holocene topography of the Ramla and Marsalforn valleys, Gozo, and excavations at the Neolithic temple sites of Ġgantija, Gozo and Skorba, Malta, and the Punic-Roman site of Tal-Istabal, Qormi, on the outskirts of Valletta in Malta. These chronologies are also of benefit in understanding the development and history of these World Heritage Site monuments. Although the local limestone geology was not expected to generate quartz-rich sediments, micromorphology conducted by the FRAGSUS team had shown the presence of sand sized quartz within local soils and sediments. Field and laboratory profile measurements were conducted on 60 samples from seven sequences to provide initial assessments of the brightness of luminescence signals from different silicate fractions, and the stratigraphic relationships between the range of signals measured, to assess the prospects of being able to determine robust age quantifications from these materials, and guide the collection of larger tube samples for OSL dating. Laboratory profiling measurements confirmed the presence of quartz in prepared sediments with generally bright luminescence signals, and opening the way for Single Aliquot Regenerative (SAR) OSL dating of quartz fractions from 12 tube samples collected from five sites, with accompanying field and laboratory dose rate measurements. At both the Ġgantija Temple (Gozo) and Skorba Temple/settlement (Malta), profile samples collected from below the modern agricultural soils show photon counts and apparent doses that increase steadily with depth, indicating that these buried soils accumulated gradually without subsequent disturbance. The OSL equivalent dose measurements showed no significant variation between aliquots, again indicating that the quartz minerals had been zeroed prior to deposition without subsequent disturbance, allowing robust ages to be determined. For both these locations, the OSL dates for the bottom of the sequences indicating the onset of soil accumulation were consistent at 8560±630 bc (Ġgantija) and 8780±710 bc (Skorba). At Ġgantija the top of this buried soil gives a Bronze Age OSL date of 1140±250 bc, whereas the OSL date for the top of the buried soil at Skorba is 7760±560 bc, predating the known Neolithic activity at the site. At Ramla and Marsalforn Valleys, OSL investigations were conducted on sequences of buried palaeosols, hillwash and alluvial deposits. For Marsalforn Valley, the profile samples show a slight increase in photon counts and dose with depth, suggesting a gradual build-up of material. However, the OSL samples show evidence of multiple dose components, including high dose residuals, consistent with variations in light exposure during the reworking of soils. The upper sample could not be reliably dated, and the lower two samples generate the same age within uncertainties (1560±240 bc and 1480±340 bc). The Ramla Valley profiles are complex, showing relatively high photon counts in the upper samples decreasing with depth, and no clear trend in the apparent dose estimates. The OSL samples all show evidence of multiple dose components, including both high dose residuals and modern OSL dates in the nineteenth and early twentieth centuries ad. The investigations reported here are the first applications of luminescence techniques on these sites. The results show that quartz OSL is an applicable approach to investigations of these sites, and that luminescence profiling techniques using field instruments and laboratory methods are highly informative. There is clearly the potential to apply these techniques to establish more detailed chronologies in future work.