Abstract
In recent years luminescence dating has increasingly been applied to date glaciofluvial sediments, but uncertainties about the degree of bleaching of the luminescence signal at deposition make dating of such sediments challenging. Here we test a new approach for luminescence dating of glaciofluvial sediments, based on the analysis of rock cores drilled from granite cobbles, and compare the luminescence ages generated against independent age control.Luminescence measurements from rock slices in cobble-sized clasts can be used to reconstruct the extent of bleaching, thereby giving greater confidence in the ages produced. This study illustrates that another important advantage of using cobbles is that at depths of 2 mm or more below the cobble surface >90% of the total dose rate arises from the cobble itself, making the dose rate insensitive to the water content of the sediment matrix. Ordinarily, uncertainties in estimating water content during burial are one of the largest sources of uncertainty in luminescence dating methods, and hence reducing the reliance upon the dose rate could be particularly advantageous for glacial deposits, where water contents can potentially be large and highly variable.Measurements of cobbles from Orrisdale Head, Isle of Man, demonstrate that the luminescence signal was completely bleached to depths of up to 12 mm into the cobble. Sampling of orientated cobbles from lithofacies diagnostic of bar-top environments was used to maximise the chances of exposure to sunlight. The upper-faces of these orientated cobble surfaces appear to be bleached to a greater depth than the lowermost faces. Data from 45 rock slices from these cobbles were tightly clustered, yielding a mean age of 20.7 ± 0.3 ka that is in agreement with independent age control. One of the well-bleached cobbles shows evidence of two discrete exposure events, potentially recording both the advance at 26.2 ± 0.8 ka, and retreat at 20.7 ± 0.3 ka, of the Irish Sea Ice Stream.
Highlights
Glacigenic sediments are amongst the most challenging Quaternary deposits for dating
Stimulated luminescence (OSL) dating using sand-sized grains of quartz or feldspar has been applied in recent years, and whilst many ages have been determined that underpin our understanding of ice sheet dynamics over the last glacial cycle (e.g. Svendsen et al, 2004; Ou et al, 2015; Smedley et al, 2016, 2017a; b), the method is challenging
Jenkins et al / Quaternary Science Reviews 192 (2018) 263e273 because when using sand-sized grains, statistical models are required to identify which equivalent dose (De) values are derived from sediment grains that were bleached at deposition, and to exclude those that were not bleached (Galbraith et al, 1999)
Summary
Glacigenic sediments are amongst the most challenging Quaternary deposits for dating. Stimulated luminescence (OSL) dating using sand-sized grains of quartz or feldspar has been applied in recent years, and whilst many ages have been determined that underpin our understanding of ice sheet dynamics over the last glacial cycle In recent years a new luminescence method has been developed, dating buried clasts varying. G.T.H. Jenkins et al / Quaternary Science Reviews 192 (2018) 263e273 because when using sand-sized grains, statistical models are required to identify which equivalent dose (De) values are derived from sediment grains that were bleached at deposition, and to exclude those that were not bleached (Galbraith et al, 1999). This study is the first to apply the newly developed cobble dating method to glaciofluvial sedimentary deposits, and aims to see whether it can be used to circumvent many of the issues associated with luminescence dating of sediments in this environment. The ability of buried clasts to record the extent of bleaching at deposition within a heterogeneously bleached (in this case, glacial) environment will be considered
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