Erosion Rates and 36Cl

New to ACE? The Introduction is a good place to start.

In the previous post, we showed that for a 21Ne sample the apparent age increases with erosion rate:

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Let’s repeat this for a 36Cl sample.  From the ACE homepage, go to the 36Cl data section and download the MK03-5-Mo sample listed in the 36Cl template file:

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This sample has no erosion rate, so import it into Excel/OpenOffice:

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Add some identical rows, and then create a new column titled ‘erosion rate’.  In these columns, start with 1E-5 cm / yr and increase in steps for each row:

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Now save this file, import into ACE and compute ages. The result?  In the Sample Browser, select ‘erosion rate’ to sort by, and then ‘age’.  Now click on Plot Sort Attributes:

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For our 36Cl sample. computed ages decrease with assumed erosion rate. Why is this? As for our 21Ne example, insight is given by looking at the Production Rate Profile:

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Because of the moderation of high-energy neutrons just below the surface of the earth and diffusion of low-energy neutrons back into the atmosphere, low-energy neutrons are more concentrated at depth than at the surface (e.g. Phillips et al., 2001).  This means that unlike other nuclides, a 36Cl sample just below the surface has a higher cosmogenic nuclide production rate than the same 36Cl sample at the surface.  The production rate profile suggests that at a depth of about 20 cm, the total production rate is about the same as the surface production rate. Let’s increase the erosion rates in the csv file and see what happens:

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So an erosion rate of 0.002 cm / yr gives a similar computed age as for zero erosion.

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