1. INTRODUCTION
I. INTRODUCTION
1.A. Overall description
This document describes the function of MATLAB/Octave code that drives the version 3 online exposure age calculator at the following addresses:
Calculate exposure ages: http://hess.ess.washington.edu/math/v3/v3_age_in.html
This page allows one to compute surface exposure ages using a default production rate calibration.
Production rate calibration: http://hess.ess.washington.edu/math/v3/v3_cal_in.html
This page allows one to compute exposure ages using a production rate calibration that is different from the default.
The production rate calibration workflow is as follows. First, enter calibration data (i.e., information about samples whose exposure age is already independently known) in this page. One can obtain published production rate calibration data in the correct format from http://calibration.ice-d.org. Second, submit the data. The server returns a separate page that shows information about the production rate calibration and allows input of data from unknown-age sites. Third, enter data the data for the unknown-age site(s) and submit it. This will compute exposure ages for the unknown-age sites using the production rate calibration established in the first step, instead of the default calibration. There is more information in the 'workflow' section below.
1.B. Design parameters
This does two things: computes exposure ages from cosmogenic-nuclide concentrations in surface samples, and computes cosmogenic-nuclide production rates from cosmogenic-nuclide concentrations in surface samples whose exposure age is already independently known.
It can be used for relatively thin (e.g., less than ca. 20 cm thick) surface samples. It includes many simplifications that would yield inaccurate results if applied to subsurface samples.
It accounts for three pathways for cosmogenic-nuclide production: fast neutron spallation, fast muon interactions, and negative muon capture. Thus, it does calculations for all commonly measured nuclide/target pairs where production is by these pathways, including 10Be, 26Al, 21Ne, 14C, and 3He in quartz, and 3He in pyroxene and olivine.
Nuclide production by thermal neutron capture is not included in the main multiple-nuclide calculator (so there is no consideration of 3He production by thermal neutron capture via lithium), but there is a separate calculator implementation that only calculates 36Cl exposure ages, and this implementation includes all additional pathways for 36Cl production.
Likewise, the main multiple-nuclide calculator does not consider nucleogenic production for any nuclide, but this is included in the 36Cl calculator .
At present, there are no calculations for several other nuclide-mineral pairs where production is predominantly spallogenic but measurements are extremely rare, for example 21Ne in pyroxene, olivine, and feldspar, 10Be in olivine and pyroxene, and 14C in olivine and feldspar.
The code is similar in concept to the 2008 online exposure age calculators (that is, up to version 2.3) that were available at hess.ess.washington.edu for a long time and were described in a paper by Balco and others (2008). The design concept for version 3 was to make the following improvements:
1. Do calculations not just for 10Be and 26Al, but all commonly measured nuclides.
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., the ICE-D database) .
The difference between this and the system described by Shasta Marrero and others (in a 2016 paper; the Marrero code is now the "CRONUS-Earth online exposure age calculator") 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).
1.C. Workflow.
There are two possible workflows for calculating exposure ages using the online calculator. You can either calculate exposure ages of unknown-age samples using the default production rate calibration data set, or you can recalibrate production rates using a different calibration data set and then use the new calibration to calculate exposure ages for your unknowns.
Exposure ages with default production rate calibration
The default exposure age workflow is very simple. Assemble all needed data about your samples, format the data in v3 input form, paste it into the main exposure age data entry page, and press 'Calculate now.' A new browser tab that contains the results will open. Note: successive calculations do not open additional tabs, they overwrite the data in the existing results tab. So make sure to save the results somewhere else before starting the next calculation. If you want a lot of example data in v3 input form, try consulting, for example, the ICE-D:ALPINE database.
Exposure ages calculated with a different production rate calibration
The production rate calibration --> exposure age workflow is more complicated.
Step 1 is to open the production rate calibration data entry page. Here you will enter a production rate calibration data set. A production rate calibration data set is is a set of cosmogenic-nuclide measurements from samples whose exposure age is independently known. Thus, one can choose a production rate that most closely reproduces the true ages of the calibration samples. This process is called "production rate calibration."
A production rate calibration data set looks just like a set of data from unknown-age samples in the normal workflow, with one difference: there is additional information about the independently determined age of all the samples. Thus, each sample in a production rate calibration data set must have an 'independent age' line formatted as described in the v3 input format. For examples of correctly formatted production rate calibration data, see the ICE-D:PRODUCTION RATE CALIBRATION DATA database. If all the samples do not have an independent age line, the server will return nothing.
Note: a production rate calibration data set must only have data for exactly one nuclide-mineral pair (e.g., Be-10/quartz or Al-26/quartz). It is not possible to recalibrate multiple nuclide/mineral systems at the same time. Failure to understand this may result in spurious two-nuclide diagrams later in the process. Entering multiple nuclide data should generate an error.
Note: the production rate calibration workflow is not available for 36Cl. It is possible to envision a future in which one could calibrate one of the 36Cl production pathways (e.g., K, Ca spallation, thermal neutron capture) at a time, but at the moment it is too complicated to make that work. For 36Cl you are stuck with the default production rate calibration.
After you have entered a production rate calibration data set, press 'Calculate now.' A new browser tab will open. This will not contain exposure age results. Instead, it will look like the normal exposure-age input form, except that there is additional diagnostic information about fitting the new calibrated production rate parameters to the data set that you entered. Below the diagnostic information, there is an input block for data from your unknown-age samples.
Then, step 2 is to enter the data for your unknown-age samples here, and press 'calculate now.' A new tab will open containing the exposure age results calculated with the new production rate calibration.
Note for those who are familiar with HTML: the new input page with the production rate calibration has the new production rate parameters hard-coded into it as hidden form inputs. Thus, you can save a local copy of the web page on your computer and just open it again when you want to do exposure-age calculations with the new calibration. It is not necessary to go through step 1 all over again.
Note: as indicated above, if you recalibrate the production rate for one nuclide but then enter multiple-nuclide data, you may get spurious or inconsistent results. For example, suppose you calibrate the Be-10-in-quartz production rate, but then you enter data for both Be-10 and Al-26. The resulting Be-10 exposure ages will use the new production rate calibration, but the Al-26 exposure ages will continue to use the default calibration. This may result in spurious two-nuclide diagrams, so if you care about the two-nuclide relationships, be careful. If you want to compute recalibrated ages for multiple nuclides you need to go through the workflow for each nuclide separately. At present there is no way to carry multiple nuclide recalibrations into the multiple-nuclide diagrams.
1.D. Is this still the "CRONUS-Earth online exposure age calculator?"
No. The Marrero et al. "CRONUScalc" code has that distinction now. Although of course some code used here is inherited from the 2008 online calculator, the CRONUS project (which, in any case, has been over for several years as of this writing) was not involved in developing the version 3 code.
This situation does require a new name for the former CRONUS-Earth online exposure age calculators. In homage to the late Prince, both the v2.3 and v3 code running on hess.ess.washington.edu should henceforth be referred to as "The online exposure age calculator formerly known as the CRONUS-Earth online exposure age calculator."