Luminescence Dating of Sediments from the Baile Sear Wheelhouse, North Uist

This report provides an Optically Stimulated Luminescence (OSL) chronology for the construction, occupation and abandonment sequence of the Baile Sear Wheelhouse structure in North Uist. The structure was exposed following coastal erosion in 2005, and was excavated by SCAPE with community support in 2009 and 2010. The site was visited by David Sanderson in September 2010 to collect OSL samples. The sediments associated with the monument and its immediate environment were reviewed, and luminescence profiling was undertaken to evaluate the sequence, and to identify sampling positions for OSL dating. Luminescence profiling, using portable field equipment, was used to guide sampling and interpretation. Field gamma spectrometry was used to record the environmental dose rates in the positions of the OSL dating tube samples. Luminescence profiling was undertaken in 7 sections. Profiles 1 and 2 examined substrates below the wheelhouse structure, and an underlying burnt earth surface which represents an earlier structure. The dating questions associated with this material, relate to the age of the earlier structure and the construction of the wheelhouse. Profiles 3 and 4 examined the stratigraphy of sediments immediately outside the southern wheelhouse wall, which record a collapse of the southern wall, and subsequent repacking. The dating questions here relate to the timing of the collapse and repacking events relative to wheelhouse construction and the occupation evidence within the southern chamber. Profile 5 examined the infill of the southern interior area, including materials associated with part of the occupation period and deliberate abandonment of this part of the wheelhouse. The chronology of this abandonment relative to the wall collapse is of interest. Profiles 6 and 7 examined the infill of the central and northern interior areas, including occupation evidence, a hearth sequence, and aeolian sands associated with the final abandonment. In total 58 profiling samples and 11 tube samples with associated field dosimetry were collected, from these 7 profiles, which together represent all of the key phases of activity associated with the construction, use and abandonment of the Baile Sear wheelhouse. The wheelhouse itself is located within a broader sequence of mounds in the vicinity, and which may, together with the other monuments in the surrounding landscape have also contributed to the archaeological materials in proximity to the site. OSL analyses of the profiling samples, both in the field, and in the laboratory have been reported elsewhere. This report concerns the OSL dating measurements conducted using the Single Aliquot Regenerative (SAR) method from 10 of the OSL dating samples, and there use to determine a chronology for the site. Sample preparation and analysis was undertaken at the Scottish Universities Environmental Research Centre (SUERC). OSL SAR dating utilises extracted quartz from the samples to determine the radiation dose experienced by the sediments since their last zeroing event, assumed to be by exposure to light prior to final deposition. This is combined with dose rate analysis based on field and laboratory measurements of environmental radioactivity. The age is determined as the ratio of dose divided by dose rate. In this work, dose rates for the bulk sediment were quantified using high resolution gamma spectrometry and thick source beta counting in the laboratory, coupled with water content analysis and in-situ gamma dose rate measurements. Quartz was extracted using standard laboratory procedures, and purity checked with scanning electron microscopy. Equivalent doses were determined, initially, on 16 aliquots of quartz per sample using the quartz single-aliquot regenerative (SAR) procedure, with additional investigation of further sets of 16 to build statistical power for selected pre-heating groups. Radial plotting methods were used to appraise sample homogeneity, and robust statistics were used, for aliquots satisfying SAR acceptance criteria, to estimate equivalent doses. The quartz OSL SAR age estimates have been assessed relative to the stratigraphy of the wheelhouse, in combination with profiling data using both conventional assimilation methods and also a Bayesian approach. Both approaches lead to similar conclusions concerning the overall chronology of site construction, occupation and abandonment as summarised here. (i) OSL profiling verifies that material associated with the earliest Iron Age occupation of the wheelhouse and preceding structure(s) in the vicinity can be found, outside the wheelhouse, in midden deposits which surround and overlie the surviving structures. This material goes back to approximately 400 BC, and some of it predates the wheelhouse construction. (ii) The preceding burnt surface represents a structure of short-lived duration, which spanned the period first/second century BC to the first century AD. Material associated within this structure and its use may be present in the midden deposits around the wheelhouse. (iii) The most probable period for construction of the wheelhouse is in the first/second century AD. (iv) In the monument, material dating to the earliest occupation of the wheelhouse is preserved in the northern chamber, and its associated pits. OSL profiling indicates that the northern chamber was in use during all period of occupation, from the Iron Age construction of the wheelhouse (c AD 150) through to the first millennium AD abandonment of the site (c AD 900-1000). (v) In the southern chamber, the earliest occupation phases are poorly represented in surviving debris. The collapse of the southern wall (c AD 600-750), the rebuild (c AD 750), the last occupation the southern chamber (c AD 750), and abandonment of the southern chamber (c AD 750) all occur in the later part of the first millennium AD, significantly after the wheelhouse construction and earlier phases of occupation. (vi) After the southern chamber was abandoned, the central and northern chambers remained in use. The central chamber contains a hearth sequence, which indicates occupation of this chamber through a later phase of occupation (~ 700AD) until the Norse period (~ AD 940). OSL profiles provide some evidence of earlier occupation in the central area. (vii) Final abandonment of the site is registered by clean abandonment sands starting from the tenth century AD, consistent with abandonment in the Norse period, and with the upper occupation in the northern and central chambers. Later evidence of aeolian activity through the Little Ice Age (up to 16th century AD) is also recorded in the upper sand fills of the central and northern chambers. The combination of field and laboratory luminescence profiling techniques, and quartz OSL SAR dating, has provided a chronological framework to interpret the complete sequence of construction, occupation and phased abandonment of the wheelhouse structure. To our knowledge, this is the first time that field and laboratory luminescence profiling techniques coupled to quantitative OSL SAR analysis and Bayesian assimilation methods have been applied to define the complete sequence of construction, occupation and abandonment of a site of this type. A further review of other artefact, environmental and dating evidence, would be useful as the rest of the post-excavation work advances