Schmidt Hammer exposure dating (SHED): Calibration procedures, new exposure age data and an online calculator

Abstract

exposure dating (SHED): Calibration procedures, new exposure age data and an online calculator. Quaternary Geochronology, 44, 55-62. https://doi.Abstract 5 Recent research has established Schmidt Hammer exposure dating (SHED) as an effective method 6 for dating glacial landforms in the UK. This paper presents new data and discussion to clarify and to 7 evaluate calibration procedures. These make a distinction between Schmidt Hammer drift following 8 use (instrument calibration), and variation between both individual Schmidt Hammers and between 9 user strategies when utilising age-calibration curves (age calibration). We show that while test anvil 10 methods are useful for verifying that Schmidt Hammers maintain their standard R-values, they are 11 inappropriate for instrument calibration except for the hardest natural rock surfaces (R-values: ≥ 12 70). A range of surfaces were tested using 3 N-Type Schmidt Hammers, which showed that existing 13 anvil calibration procedures led to consistent overestimation of R-values by up to 17.9%. In contrast, 14 new calibration procedures, which are based on the use of a calibration point which lies within the 15 range of R-values measured in the field [Dortch et al. 2016, Quat. Geochron., 35, 67-68], limit 16 variance to maximum of 4.4% for surfaces typically tested by Quaternary researchers (R-values: 25 -17 60). Moreover, these new calibration procedures are more appropriate for age calibration as they 18 incorporate operator variance through choice of sampling location. New calibration procedures are 19 used to compile an updated age-calibration curve based upon 54 granite surfaces (R 2 = 0.94, p < 20 0.01) from across Scotland, NW England and Ireland. The inclusion of a further 29 terrestrial 21 cosmogenic nuclide (TCN) exposure ages extends the calibration period to 0.8 -23.8 ka, covering 22 the entire post-Last Glacial Maximum (LGM) history of the British-Irish Ice Sheet. To facilitate 23 comparison between studies, an online calculator is made available at http://shed.earth for Schmidt 24Hammer instrument and age calibration and SHED exposure age calculation. The SHED-Earth 25 calculator provides a rapid and accessible means of exposure age calculation to encourage wider and 26 more consistent application of SHED throughout the British Isles. 27 28

Small rock‐slope failures conditioned by Holocene permafrost degradation: a new approach and conceptual model based on Schmidt‐hammer exposure‐age dating, Jotunheimen, southern Norway

“…Furthermore, Shakesby et al (2011) specifically tested the linearity assumption in relation to granite boulders on independently-dated staircases of raised beaches deposited since 10.4 ka in northern Sweden, with the conclusion that the relationship between mean R-value and age was best described by a linear function. The same conclusion can be reached from age-calibration curves in the British Isles (Tomkins et al 2018a) and the Pyrenees (Tomkins et al 2018b), which are based on 54 and 52 10 Be TCNDdated granitic surfaces, respectively, all associated with glacial depositional or erosional landforms (moraine boulders or ice-sculpted bedrock). While the Pyrenean age-calibration curve is clearly non-linear over the full age range of~50 ka, both age-calibration curves evidence linearity over the last~20 ka.…”

“…The methodology has been developed in conjunction with the relatively new calibrated-age dating technique of Schmidt-hammer exposure-age dating (SHD; Shakesby et al 2006Shakesby et al , 2011Matthews & Owen 2011;Matthews & Wilson 2015;Wilson et al 2017). SHD has the potential to estimate the numerical age of rock surface exposure at low cost with comparable accuracy and precision, and greater representativeness, than TCND over the Lateglacial and Holocene (Winkler 2009;Winkler & Matthews 2010;Matthews & Winkler 2011;Tomkins et al 2016Tomkins et al , 2018a.…”

Timing of glacial retreat in the Wicklow Mountains, Ireland, conditioned by glacier size and topography

“…Moreover, given the long timescales of exposure (≥11 ka) and limited climatic variability across the relatively small mountain range (∼220 km 2 ), any differences in surface R ‐values due to lithology will probably be significantly smaller than the effect of variable exposure age. This interpretation is supported by large spatial scale 10 Be‐SH calibration curves from the British Isles (Tomkins et al , , ; n = 54; R 2 = 0.94, p < 0.01) and the Pyrenees (Tomkins et al , ; n = 52; R 2 = 0.96, p < 0.01) which indicate that the primary control on surface R ‐values is cumulative exposure to subaerial weathering. Instrument calibration (correction factor = 1.017) and age calibration (correction factor = 0.992) were performed using the SHED‐Earth online calculator (http://shed.earth) following the recommendations of Dortch et al .…”

“…Instrument calibration (correction factor = 1.017) and age calibration (correction factor = 0.992) were performed using the SHED‐Earth online calculator (http://shed.earth) following the recommendations of Dortch et al . () and Tomkins et al (). SH exposure ages and 1σ uncertainties were calculated based on the arithmetic mean for each surface (mean of 30 R ‐values) and based on the updated calibration curve of Tomkins et al .…”

“…SH exposure ages and 1σ uncertainties were calculated based on the arithmetic mean for each surface (mean of 30 R ‐values) and based on the updated calibration curve of Tomkins et al . (), which includes 10 Be‐dated surfaces from Blackstairs Mountain, Wexford ( n = 2; Ballantyne and Stone, ), and Bloody Foreland, Donegal ( n = 6; Ballantyne et al ., ; Clark et al ., ). These data fit the trend established at calibration sites in Scotland and north‐west England and indicate that errors in SH exposure age estimates due to climatic variability appear unlikely (Barr et al ., ).…”