One of the largest uncertainties in cosmogenic nuclide dating is erosion rates, because as production rates vary with depth in the earth, cosmogenic nuclide production rates can vary significantly. As ACE uses time stepping, erosion is modelled slightly differently to the time averaged erosion production rate calculation (eg Lal 1991). Essentially ACE works backwards in time from the cosmogenic inventory until the inventory is zero. Therefore, at each timestep ACE ‘adds’ a layer that was eroded during that timestep and determines the change in production rate as a result of the fact that this sample is now at depth.
The following shows how to determine the sensitivity of a sample to erosion rates. We begin with the default samples that ship with ACE:
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Plotting ‘sample id’ against ‘cosmogenic inventory’, the sample with largest cosmogenic inventory is the 21Ne sample 05BT20:
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For purposes here we delete the other samples and save this one to a csv file using the Export Samples button at the bottom of the sample browser:
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Where we called the csv file ’05BT20.csv’ to remind us of the exported sample. Now open this file in a spreadsheet editor such as Excel or OpenOffice:
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The field ‘erosion rate’ is already listed as a column in this file, as we exported the sample with the ‘All’ view attribute. However for other or new files erosion rate can be added as a new column anywhere in the spreadsheet. To examine the sensitivity of this sample to erosion, we assume an erosion rate of somewhere between 0 and 1 mm/ka. From the ACE variables page, we see the the units for erosion rate are cm/yr, so that we want a maximum of 1E-4 cm/yr (1 mm/ka). In the spreadsheet program, duplicate the sample row ten times and for each erosion rate increase the value by 1E-5 cm/yr. This will allow us to see trends in sample age with erosion rate:
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Now delete any unused columns, save the file, and import it back to ACE using the Import Samples button at the bottom of the sample browser. You don’t have to assign different names to each sample with a different erosion rate, as ACE will see that the sample names repeat and allocate each a unique value when it imports the csv file:
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Now we can run each sample with a different erosion rate and calculate the computed ages using the experiment ’21Ne Demonstration Experiment’ that ships with ACE. After the experiment is performed we plot ‘age’ vs ‘erosion rate’ using the Plot tool in the sample browser:
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So why export this sample to a spreadsheet program just to change the erosion rate? ACE is designed to demonstrate the sensitivity of a computed sample age to measurement and theoretical uncertainties such as erosion. If ACE allowed users to type in a different number for the erosion rate and recompute, very little would be learned.