SHED Earth

Welcome to SHED Earth. SHED refers to Schmidt Hammer Exposure Dating: a technique that provides a cost-effective solution for dating the exposure of granite surfaces. This can be used to constrain the timing of past events, which permits a better understanding of the links between climate and landscape evolution.

The approach is based upon a study by Tomkins et al. (2016)1, in which a statistically significant relationship was observed between the exposure ages (derived from terrestrial cosmogenic nuclide dating), and Schmidt Hammer rebound values (R-values) of 25 granitic surfaces from NW England and Scotland. This has recently been updated with 29 additional exposure ages from across Scotland and Ireland2 and is now applicable over the timescale 0.8 - 23.8 thousand years ago, covering the entire post-Last Glacial Maximum (LGM) history of the British-Irish Ice Sheet. In addition, a new curve has recently been made available for the Pyrenees1, based on 52 TCN dated surfaces from across the mountain range. These new data confirm the accuracy and reproducibility of previous research1 and demonstrate that this method can be applied successfully in contrasting climatic regimes. In addition, these studies show that equivalent time-dependent weathering of granite surfaces can occur over significant spatial scales for regions of similar climate. The technique has been demonstrated to be of comparable accuracy and precision to ages derived from TCN dating, but without the significant expense that goes along with the use of such techniques, making it useful for researchers and students alike.

Calibration Curve

The calibration curve used in SHED calculations is region-specific. It is therefore important that you select the curve that is suitable for the location in which your research is taking place, so that the appropriate curve may be selected. If you would like to submit an additional curve to SHED-Earth, in order to allow you to date samples from other regions of the world, please .

regression chart

This is the latest calibration curve for the British Isles and is based on 54 TCN dated surfaces from Scotland, NW England and Ireland. These exposure ages are calculated using the time-dependent Lm scaling scheme3,4, the Loch Lomond production rate 5 and assuming 0 mm ka-1 erosion. For full details, see Tomkins et al. (2017)2. It is anticipated that as new calibration curves are constructed in similar well-dated regions, they will be made available for use on this site.

Production Rate

The largest uncertainty involved in calculating TCN exposure ages is the choice of production rate. As the SHED calibration is based on TCN exposure ages, production rate controls the slope of the calibration curve and in turn, influences SH exposure age estimates. As such, we provide three production rates for calculating SH exposure ages.

For the British Isles SHED curve, the default calibration dataset is the Loch Lomond production rate (LLPR; 4.02 ± 0.18 atoms g-1 a-1; Fabel et al., 20125), which is based on 10Be concentrations from erratic boulders on the terminal moraine of the Younger Dryas Loch Lomond glacier advance, the timing of which is independently constrained by 14C ages derived from a varve chronology (MacLeod et al., 20116). This calibration dataset is the most widely used local production rate in the British Isles.

Secondly, we provide the Glen Roy production rate (GRPR; 4.31 ± 0.21 atoms g-1 a-1; Small and Fabel, 20157) which is derived from assumed ages of tephra within a floating varve chronology (MacLeod et al., 20158). Collectively, the LLPR and GRPR provide upper and lower limits on the range of local production rates from the British Isles.

Finally, we provide the primary calibration dataset of Borchers et al. (2016)9 which is derived from global calibration sites and is the default production rate for both the CRONUS Web Calculator and the online calculators formerly known as the CRONUS-Earth online calculators. . This is the default calibration dataset for the Pyrenees SHED curve as no local production rates are available.

Data Input

R Values

Users should input raw R-value data in the format displayed below. Inputs include samples IDs and locations (latitude/longitude) which are stored in database for monitoring of site usage. User data (R-values and Schmidt Hammer exposure ages) are not recorded. While we encourage users to record 30 R-values per surface to ensure statistically significant results, the tool will also operate on smaller sample sizes. This tool performs instrument and age calibration (see below) and returns calibrated R-values and Schmidt Hammer exposure ages with 1σ uncertainties for each sampled surface. Data should be pasted into the box below in tab-delimited format, which is most easily achieved by copying it directly out of a spreadsheet (download example here). The column order should be name latitude longitude followed by the R-values (recommended 30) for that surface (click the Load Demo Values button at the bottom of this form for an example).


Instrument Calibration Values

In order to ensure that the Schmidt Hammer is functioning correctly and yielding the same R-value on an identical rock surface, even after hundreds or thousands of impacts10, users should test their Schmidt Hammer using a suitable surface before and after sampling. Previous studies have advocated the use of the Proceq test anvil10,11. An implicit assumption in the test anvil calibration procedure is that the difference (%) between the specified anvil standard and the recorded average is consistent throughout the operational range of the Schmidt Hammer i.e. a 20% R-value difference between two Schmidt Hammers as recorded using the test anvil should also be replicated on a range of natural rock surfaces. However, this is not supported by evidence2 as the difference between Schmidt Hammers as recorded using the test anvil is not maintained throughout the tools’ operational range but decreases significantly as the surface R-value decreases. As a result, this method over-estimates R-values for surfaces typically tested by Quaternary researchers2.

For very hard rock surfaces (R-values: ≥ 70), the test anvil method may be more effective, as variation between Schmidt Hammers as recorded on the anvil is probably representative of variability on sampled rock surfaces. However, for the vast majority of rock surfaces tested by Quaternary researchers (R-values: ≤ 60), the anvil method will significantly overestimate R-values2. Thus, users should perform instrument calibration using a calibration surface which is within the range of their sample data and that is of sufficient size12, that is free of surface discontinuities13 and lichen14 and that is easily accessible. Users should input pre- and post-calibration values for their chosen surface and input their raw R-value data in chronological order (related to the time of sampling). R-values will be corrected assuming linear R-value drift1 based on the variance between pre- and post-calibration values and the number of individual R-values. This procedure is most effective when periods between calibration tests are short15.



Age Calibration Value

The goal of SHED-Earth is to encourage researchers and students to test and use our calibration curve on undated landforms and compare results with independent dating methods (TCN, 14C, OSL), in order to evaluate its effectiveness. As such, the standardisation of different Schmidt Hammers15 and different user strategies 16 to a verifiable standard (e.g. University of Manchester calibration boulder) is necessary to minimise potential errors in SHED age estimates.

For the British Isles SHED curve, users should test their Schmidt Hammer using the University of Manchester calibration boulder (R-Value = 48.08 ± 0.82)17 and input their mean R-Value here. For the Pyrenees SHED curve, users should test their Schmidt Hammer using one of the three calibration surfaces provided (R-Values: Maladeta = 52.60 ± 0.74; Bassies = 44.14 ± 0.60; and Carlit = 48.67 ± 0.65) and input their mean R-Value here. A correction factor (%) is applied to all user R-values.

Users who have not completed age calibration using one of these age calibration surfaces should use the default value. As such, no correction for variance between different Schmidt Hammers or between users will be made. This may be appropriate if users have calibrated their Schmidt Hammer using one of the boulder or bedrock surfaces reported in previous publications1,2 although we advocate caution with this approach17. Although variance between Schmidt Hammers is usually small for surfaces with R-values of ≤ 60, variance can exceed ~10% for older Schmidt Hammers and should be accounted for 2. Users can contact or at the University of Manchester for advice on age calibration.

Photograph of boulder location

If you wish to load any of the default values into the Mean R Value (boulder) box, you can do so by clicking the link in the below table. Use the coordinate links to view the location on Google Maps.

Calibration Boulder NameCalibration Curve (Location)R-ValueUncertaintyLocation (Lon,Lat)
University of ManchesterBritish Isles48.08±0.82-2.23484069, 53.46667120
MaladetaPyrenees52.60±0.740.64960200, 42.67894000
BassiesPyrenees44.14±0.601.46471300, 42.74457700
CarlitPyrenees48.67±0.652.05129200, 42.51446200

Demo Data

If you would like to have a go with the website but do not have data to hand, simply click here for to load some demo data into the above form:

Information

Referencing this site

If you use this site in academic works, please reference it accordingly:

Source Code

The source code for this website is available on GitHub.

References
  1. Tomkins, M. D., Huck, J. J., Dortch, J. M., Hughes P. D., Kirkbride, M. & Barr, I. (2017) Schmidt Hammer exposure dating (SHED): Calibration procedures, new exposure age data and an online calculator, in. prep.
  2. McCarroll, D. (1987). The Schmidt hammer in geomorphology: five sources of instrument error. Br. Geomorphol. Res. Group Tech. Bull., 36, 16–27.
  3. Tomkins, M. D., Dortch, J.D., Hughes, P.D., Huck, J.J., Stimson, A., Delmas, A., Calvet, M., and Pallàs, R. (2017) Rapid, inexpensive dating of glacial landforms in the Pyrenees, Geology, In Submission