SHED Earth

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.

The first and 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.

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).

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 anvil^10,11. 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.

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.

Users who have tested their Schmidt Hammer using the University of Manchester calibration boulder¹⁷ should input their mean R-Value here. A correction factor (%) is applied to all user R-values.

Users who have not completed age calibration using the University of Manchester calibration boulder should use the default value (R-value: 48.08 ± 0.82). This is the mean R-value generated by the Proceq N-type Schmidt Hammer used to generate the original calibration curve¹. 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 125 boulder or bedrock surfaces reported in previous publications^1,2 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.

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: