Luminescence dating of wind blown sands from archaeological sites in northern Scotland
The sheltered bays of the Orkney Islands are backed by extensive dune systems that commonly contain archaeological sites, many of which now protrude from cliffed sections due to coastal erosion. In an area where other dating techniques are often precluded due to a lack of organic dating material, this research establishes that optically stimulated luminescence (OSL) dating can provide a viable and robust alternative by dating wind blown sands within the sites to constrain the archaeological age. Since this is the case, the OSL chronology can also be applied to natural sites where no archaeological chronology exists and so be used in palaeoenvironmental reconstruction. Six periods of increased sand movement are recorded in the Orkney Islands at the sites sampled; the Neolithic, the Bronze Age, two periods in the Iron Age, the Viking/Medieval period and the Little Ice Age. The phases of sand movement identified using OSL in the Orkney Islands are also identified at other sites in Scotland and north-east Europe and support the chronologies derived from other environmental indicators such as tree rings and peat bogs. It is suggested that at least two events may be related to the deterioration in climate subsequent to the eruption of Hekla 4 in the late Neolithic and Hekla 3 in the late Bronze Age. However, OSL dating requires that the latent luminescence signal within quartz and feldspar is rapidly reduced to near zero by exposure to light, yet the rate and extent of bleaching depends on the length of time that the grains are exposed and the light intensity at the time of exposure. Bleaching experiments used here confirmed that not only is there a difference in the rate of bleaching between quartz and feldspar depending on the light intensity, but that there is also a difference in the rate of bleaching between samples from different geological areas and this needs to be taken into account in future research. The residual levels from the Orcadian modern beach sands (<0.5 Gy) suggest that some samples collected for OSL dating may be partially bleached and a new technique, the psi (?) ratio, is proposed here to identify partially bleached sediments.