The Schmidt hammer as a Holocene calibrated-age dating technique: Testing the form of the R-value-age relationship and defining the predicted-age errors

Abstract

Most recent developments of Schmidt-hammer exposure-age dating (SHD) as a calibrated-age dating tool have been limited by the use of locations at two age control points. This has necessitated: (1) making assumptions about a linear R-value—age relationship; and (2) basing predictions of age errors only on R-value variance at the two age control points. This paper analyses 9900 R-values obtained from multiple-age control points on a sequence of glacio-isostatically raised shorelines with well constrained ages (Skuleskogen High Coast World Heritage Site and environs, Gulf of Bothnia, northern Sweden) ranging from modern to 10.5 ka in age. The sampling design involved 30 measurements on granite beach boulders from 30 sites at 11 age control points. The R-value—age relationship is best described by a linear function (ordinary least squares, OLS) with an R2 of 0.89 ( p < 0.0001). Predicted-age errors are calculated using two approaches: (1) OLS regression, which takes account of the uncertainties associated with the regression line and the age control points; and (2) weighted total least squares (WTLS) regression which, in addition, takes account of possible shoreline age errors and the heteroscedasticity of R-value residuals with increasing age. With both approaches, 95% predicted-age errors are relatively modest: ±355—455 years and ±300—515 years, respectively, with larger R-value variance and hence uncertainty for older surfaces, and the possibility of further improvement from a more selective field sampling scheme that differentiates between age-related and non-age-related rock-surface variability. Our results also indicate that for certain lithologies and weathering environments, it is possible to construct reliable calibrated-age curves from relatively few age control points.

Geomorphology and weathering characteristics of erratic boulder trains on Tierra del Fuego, southernmost South America: Implications for dating of glacial deposits

“…Rebound values (R-values) produced by the impact of the hammer decline with increased rock-surface weathering, and so can be used as a relative measure of exposure history (McCarroll, 1991;McCarroll and Nesje, 1993;Nesje et al, 1994;Goudie, 2006;Shakesby et al, 2011). We used an N-type Schmidt hammer to analyse 50 boulders from two of the boulder trains thought to be from different glacial cycles (BI 1 and RC 1).…”

Improved Schmidt-hammer exposure ages for active and relict pronival ramparts in southern Norway, and their palaeoenvironmental implications

“…All rock surfaces used in our study have mean R-values between 33 and 59 (see Tables 1 and 2) and hence rock surfaces that yield R-values of 60 can be regarded as twice as hard as those with R-values of 30. The second assumption was tested by Shakesby et al (2011) on raised beaches in northern Sweden, where many control points of Holocene age were available. Although weathering rates are likely to slow down over longer timescales (Colman, 1981;Colman and Dethier, 1986), linear rates can be expected over the Holocene timescale where relatively resistant lithologies are subject to relatively slow rates of subaerial chemical weathering in periglacial environments (André, 1996(André, , 2002Nicholson, 2008Nicholson, , 2009Fig.…”

“…This requires at least two surfaces of known age (control points) which are comparable in respect of lithology of the rock surfaces being dated (Matthews and Owen, 2010;Shakesby et al, 2011). The calibration equations proved a weakness in the previous application of SHD to pronival ramparts in southern Norway .…”

Age and origin of ice-cored moraines in Jotunheimen and Breheimen, southern Norway: insights from Schmidt-hammer exposure-age dating

“…), beach ridges (Shakesby et al . ), rock glaciers (Kellerer‐Pirklbauer ; Matthews et al . ) and flood berms (Matthews and McEwen ).…”