Feldspar SARA IRSL dating of very low dose rate aeolian sediments from Sandwick South, Unst, Shetland

Abstract

a b s t r a c tYoung sediments, with low sensitivity and low dose rates, are challenging for luminescence dating. Here, we present work on the site of Sandwick South, a Norse settlement, in which these challenges were present. Field gamma dose rates below 0.1 mGy a À1 , and total dose rates of 0.4e0.5 mGy a À1 , combined with expected ages of less than 1 ka, resulted in a requirement for quantitative determination of equivalent doses of 0.2e0.4 Gy. The bedrock geology of the area are metagabbros, which explain both the exceptionally low dose rates and the lack of autochthonous quartz. Luminescence profiling during fieldwork revealed stratigraphically progressive OSL and IRSL signals, indicating phases in the sediments with dating potential. While laboratory characterisation recovered some quartz, its low yields and luminescence sensitivity precluded application of quartz methods. Electron microscopy confirmed the presence of feldspars, which were separated and used for IRSL Single Aliquot Regeneration Additive (SARA) analysis. Counting times for both high resolution gamma spectrometry (HRGS) and thick source beta counting (TSBC) measurements were extended by 1 order of magnitude, resulting in overall uncertainties of <6% for these low dose rates. Dose estimates were obtained using an adapted SARA protocol, incorporating long overnight preheats before first measurement, with the aim of mitigating shortterm fading effects. Using these procedures, archaeologically meaningful dates were obtained from this difficult material, which are internally consistent, coherent with stratigraphy, and concordant with the radiocarbon constraints of the associated archaeological settlement. The dates demonstrate sand accumulation in the early to mid-13th century AD and also in the 18th century AD, which are contemporary with disruptive sand movements registered in other coastal regions of the Northeast Atlantic and North Sea regions. The approaches adopted here have provided solutions to the challenging conditions of this young, insensitive material and can therefore be considered to extend the range and applicability of luminescence dating methods.

Dynamic beach response to changing storminess of Unst, Shetland: implications for landing places exploited by Norse communities

“…These sand movements can be successfully dated using OSL and our results help to build a picture of changing environmental conditions experienced at Lunda Wick in the latter stages of the Medieval Climatic Anomaly (MCA) and the transition into the Little Ice Age (LIA), around 1250 CE. The timing of sand blows is consistent with Kinnaird et al (2015)'s findings at Sandwick and other work carried out in Shetland, as well as the general trend towards storminess in the British Isles understood at this time (e.g. Lamb, 1972;Lamb & Frydendahl, 1991;Burbidge et al, 2001;Sommerville, 2007;Bampton et al, 2017).…”

“…Significant blown sands are present in the Zone 3 section, with some evidence of both unconformities (OSL 1 and 2 area separated by ~3000 years in a relatively small section), and thick deposits formed at similar times. OSL4 and 5, taken within a deep stratigraphic unit, are approximately the same age and are dated to 1270 -1480 CE, somewhat later than the mid-13 th century dates of blown sands found at Sandwick (Kinnaird et al, 2015), but broadly consistent with the crossing of key environmental thresholds associated with the climate changes around the MCA-LIA transition, beginning in the mid-13 th century. This also is broadly consistent with the phase of discrete sands units separated by thin soils found in the Zone 1 section.…”

“…Long et al, 1998;Lowe & Boardman, 1998;Bromhead & Ibsen, 2006;Westley et al, 2011;Dawson, 2013;Graham et al 2017). In the case of Unst, work at Sandwick by Kinnaird et al (2015) for example, identifies periods of sand blows dating to around the mid-13 th century, concurrent with the late Norse period. Thus a lack of evidence for former landing sites may be due to an 'absence of evidence' rather than 'evidence for absence'.…”

Quantitative dating of Pleistocene deposits of the Kyrenia Range, northern Cyprus: implications for timing, rates of uplift and driving mechanisms

“…the fast component) precluded application of quartz-based methods. Electron microscopy confirmed the presence of feldspars, which were separated and used instead of quartz for IRSL and elevated temperature post-IR IRSL single aliquot regeneration (SAR) dating (Buylaert et al 2009;Thomsen et al 2011;Kinnaird et al 2015).…”

Changes and continuities in a Mediterranean landscape: a new interdisciplinary approach to understanding historic character in western Catalonia

“…Field profiling was undertaken using a SUERC portable luminescence reader, which records both infra-red stimulated luminescence (IRSL) and OSL from bulk sediments (Sanderson & Murphy, 2010). Laboratory profiling and conventional quantitative quartz OSL dating were completed at the SUERC laboratories in accordance with established protocols (Ghilardi et al, 2015;Kinnaird, Sanderson, & Bigelow, 2015). A full description of the methods and results of the dating programme are published in a separate paper (Kinnaird, Bolòs, Turner, & Turner, 2017).…”