Luminescence (optical) dating of cobbles in fluvioglacial deposits, Isle of Man

Here is another important paper, with a major advance that needs to be applauded -- the ability to date cobbles and boulders contained within fluvioglacial sediments (or glaciofluvial sediments, as we are supposed to call them these days...).  The authors have based their study on the Isle of Man, and have shown that the retreating or wasting  Irish Sea Ice Stream front evacuated this area around 20,000 years ago.  This contributes to the emerging narrative.  There are still some anomalies and things that do not quite fit, but that's only to be expected.

As in the paper by Glasser et al (2018) there is a recognition that the edge of the Irish Sea Ice Stream wrapped itself around the SW coasts of Pembrokeshire and pushed into Cardigan Bay.  Bur again I take exception to the easternmost arrow or flowline in the big lobe south of St George's Channel.  Ice does not flow parallel to ice edges.  In Pembrokeshire the ice did ot flow NE towards SW.  It flowed NW towards SE, as we would expect from the laws of physics.  The same would have been true of the ice edge further south.

But that's a minor niggle.  Very useful stuff.

Luminescence (optical) dating of cobbles in fluvioglacial deposits, Isle of Man

https://livrepository.liverpool.ac.uk/3022823/1/jenkinsetal2018.pdf

A new approach for luminescence dating glaciofluvial deposits - High precision optical dating of cobbles
G.T.H. Jenkins, G.A.T. Duller, H.M. Roberts, R.C. Chiverrell, N.F. Glasser
Quaternary Science Reviews 192 (2018) 263-273

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.