Luminescence Dating of Sediment from the University of Newcastle Archaeological Excavations at Mothecombe Beach, Devon

Kinnaird, T., Sanderson, D. and Agate, A. (2012) Luminescence Dating of Sediment from the University of Newcastle Archaeological Excavations at Mothecombe Beach, Devon. Technical Report. Scottish Universities Environmental Research Centre.

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Abstract

This report is concerned with optically stimulated luminescence (OSL) investigations of sediment collected from the University of Newcastle archaeological excavation at Mothecombe Beach (Mothecombe, Devon). Four sediment samples were submitted to the luminescence laboratories at the Scottish Universities Environmental Research Centre (SUERC) for dating. The samples are taken from contexts in association with an archaeological structure of expected post-Roman age. OSL single aliquot regenerative dose (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 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 (HRGS) and thick source beta counting (TSBC) in the laboratory, coupled with water content analysis. Significant differences were noted between the beta dose rate estimates obtained by HRGS and TSBC. These were investigated and shown to arise from contrasting radioactivity levels between the fine sedimentary matrix and associated clasts. This was taken into account when estimating the dose rates used in age evaluation. Quartz was extracted from the finer sediment fractions using standard laboratory procedures, and purity checked with scanning electron microscopy. Equivalent doses were determined on 16 aliquots of quartz per sample using the quartz SAR procedure. 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 sediment samples are taken from colluvial units that occur in association with structures at Mothecombe beach that have been dated to the 5th-7th century AD. The chronology for the site is based on two radiocarbon dates - 450-610 cal. AD and 430-580 cal. AD - and the presence of c. 20 sherds of eastern Mediterranean pottery. Given the associations between the sediment samples and the archaeological features, it was postulated that the sediment should yield luminescence ages in the first millennium AD. However, the quartz SAR ages obtained from the site span from the 3rd millennium BC to the late 16th century. The luminescence ages obtained for the colluvial units that are believed to seal the two archaeological structures are 400 ± 110 BC (colluvial unit 2) and 710 ± 130 BC (colluvial unit 3). If one accepts the chronology implied by the radiocarbon dates, the luminescence dates could be interpreted as recording earlier depositional events in the history of the site (when the colluvial units were last bleached). In this scenario, the deposits, would have to be re-worked, subsequently transported down-slope, and re-deposited in their archaeological contexts without significant exposure to light. In support of this hypothesis, the luminescence ages obtained for the units 2 and 3 are inverted, as one would expect in such a depositional scenario. Interestingly, however, the equivalent dose distributions obtained for the colluvial units are tight, which would normally indicate well-bleached sediment. Resolving the hypotheses concerning the colluvial re-deposition process, and the relationship between these materials and the archaeological features, including the cut features, ceramics and 14C samples, would require further investiagtions on-site and in the laboratory. The OSL investigations at SUERC reported here have verified that there is sufficient quartz in the sediment for dating purposes and that this quartz has suitable behaviour for luminescence dating. Having established the protocols for estimating environmental dose rates on site, through reconciliation of the different contributions from the natural radioactivity of the clasts and matrix, and shown the effectiveness of the quartz SAR OSL technique for dating the sediment samples, it would now be possible to extent the study to date further units at the site if required. For future work, consideration should be given to the use of luminescence profiling methods (including the use of portable OSL instrumentation), to search for, and identify discontinuities in the colluvial sequence, and to target specific well-zeroed material for dating. Coupled with in-situ field gamma spectrometry, this would provide a means to test the hypotheses outlined above.

Item Type:Research Reports or Papers (Technical Report)
Status:Published
Glasgow Author(s) Enlighten ID:Kinnaird, Dr Timothy and Sanderson, Professor David
Authors: Kinnaird, T., Sanderson, D., and Agate, A.
Subjects:G Geography. Anthropology. Recreation > GE Environmental Sciences
College/School:College of Arts > School of Humanities > Archaeology
College of Science and Engineering > Scottish Universities Environmental Research Centre
Publisher:Scottish Universities Environmental Research Centre

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