This paper was published in Quaternary Geochronology (v.3, p. 174, 2008) and is intended to be the referenceable documentation for exposure ages and erosion rates calculated here. For basic information about the inner workings, goals, assumptions, and limitations of the calculators, read this first.

Significant changes have been made since the original release, and some parts of the original paper are no longer up-to-date. Updates reflecting these changes are described in this PDF (v2.2), in this PDF (v2.2.1) and here (v2.3).

Version 3 is similar in concept to the 2008 online exposure age calculators (that is, up to version 2.3) that are also available at hess.ess.washington.edu and described in a paper by Balco et al. (2008). The design concept for version 3 is to make the following improvements:

1. Do calculations not just for Be-10 and Al-26, but all commonly measured nuclides produced by spallation and muon interactions.

2. Remove obsolete production rate scaling methods and add more recent methods based on particle transport modeling, including (i) the "LSDn" scaling method of Nat Lifton, which is based on the work of Tatsuhiko Sato, and (ii) potentially, a similar method developed by David Argento.

3. Improve and update various ancillary computation schemes and input parameters, including, among others, muon interaction cross-sections, atmosphere models, and figure generation.

4. While doing all this, improve execution speed so that the code runs fast enough to serve as a back end for online databases that require dynamically calculated exposure ages (e.g., ICE-D:ANTARCTICA).

The difference between this and the system described by Marrero and others (2016) is that the Marrero code was designed to carry out the model-fitting experiments described in Borchers and others (2016). The aim of these experiments was to determine whether or not various scaling models could or could not be fit to production rate calibration data. Clearly for this purpose one must make sure that differences between scaling model predictions and observations are not just due to simplifying assumptions or numerical approximations; thus, their code includes a very comprehensive representation of the physics of nuclide production, does not make simplifying assumptions, and is designed to have very good numerical precision. While all these features are necessary for the purposes of the Borchers study, they make the code quite slow.

In contrast, the aim of the version 3 code described here is to make as many simplifying assumptions and numerical shortcuts as possible while retaining acceptable accuracy for the single application of exposure-dating of surface samples (and its reverse, i.e., estimating production rates from independently dated surface samples). "Acceptable accuracy" is approximately defined as 2-4 per mil, which is small compared to (i) measurement precision for cosmogenic nuclides (in nearly all cases) and (ii) uncertainty in production rate scaling methods. To summarize, the overall design goal for this code is to compute exposure ages and/or production rates as fast as possible while maintaining acceptable precision (and also without doing anything that is really badly unphysical).

The official documentation of "The online exposure age calculator formerly known as the CRONUS-Earth online exposure age calculator", version 3 can be found here.

What the overall format looks like for non-Cl-36 data:

Data to be entered are arranged in blocks, or lines, that are separated by semicolons; so each string of text that ends in a semicolon describes either a sample or a nuclide concentration measurement made on that sample. Here is an example:

This has three lines: one that describes properties of the sample (what the properties are is described in more detail below); one that describes a Be-10 measurement; and one that describes an Al-26 measurement.

The order of the lines doesn't matter, and the nuclide concentration lines don't need to appear right after the sample they pertain to — they are linked to the sample by the fact that the sample name is included in each of the nuclide concentration lines.

So this text block describing two samples with both Be-10 and Al-26 measurements:

Works the same as this block:

And the same as this block:

Although I've shown the data lines as separate lines of text, the separator is actually the semicolon, so this:

Is the same as this, which may often be easier to lay out in spreadsheets:

Tedious and pedantic formatting details for each data line:

A. Sample data line. These have 10 fields, as follows. Here is an example:

B. Nuclide concentration lines.

In general, all the lines describing nuclide concentration measurements have information in the following order: the name of the corresponding sample; the nuclide measured; the mineral in which it is measured; the nuclide concentration and uncertainty; and then standardization information, which varies by nuclide, at the end. The currently available set of nuclide-mineral pairs is as follows:

Exotica such as Ne-21 in sanidine or Be-10 in olivine are not, at present, supported. The following sections detail the format of the data entry lines for Be-10 and Al-26 in quartz; refer to the official documentation for other nuclide-mineral pairs listed above.

B.1. Be-10-in-quartz and Al-26-in-quartz measurement lines. These have 6 fields, as follows. Here are examples of both:

This page provides a list of standardizations to which Be-10 and Al-26 measurements submitted to the online exposure age and erosion rate calculators can be referenced. A 'standardization' means the combination of a particular isotope ratio standard material and an assumed isotope ratio for that material. Each standardization described below is associated with a name that appears in the first column. This name must be entered exactly in the appropriate place in the online calculator input forms. Note that two pieces of information are critical here: the identity of the actual standard material, and the nominal isotope ratio that the material is assumed to have. In a number of cases, the same physical standard material has been used with different assumed isotope ratios.

The goal here is that users not have to renormalize their AMS measurements before submitting them to the online calculators. For all the standardizations listed below, there is a known conversion factor that can be used to convert measurements made using that standardization to be compatible with the online calculators. The calculators will do that conversion internally before calculating exposure ages or erosion rates. Thus, users are NOT required to renormalize data to a particular standardization before submitting it to the calculator — the idea is that the user lists the standardization that was used for a particular sample, and the correction is done internally in a consistent fashion.

The calculator multiplies a nuclide concentration measured using a given standardization with a conversion factor to make it consistent with the 07KNSTD (for Be-10) or KNSTD (for Al-26) standardization.
A table with conversion factors last updated February 25, 2016 is available in this PDF.

If you are using a standard material/isotope ratio pair that is not listed here, and you would like it to be added, please contact Greg Balco.