Dating samples

In ACE samples are dated by applying ‘experiments’, which can be thought of as combination of internally consistent experimental parameters (atmospheric scaling, geomagnetic variability etc) with HLSL cosmogenic production rates.  The following section shows how to create new experiments, but here we will just use a default experiment supplied with ACE.  Details for this experiment are provided below.

First we import some data.  We choose the 3He dataset of Blard et al (2006).  This is how this dataset appears in the ACE main window.  As these samples have independent ages, we need to check the ‘Include Calibration Samples‘ checkbox to view them.  We have also typed ‘Blard’ in the Search panel to only show data from Blard et al (2006):

Click on image to expand

Click on image to expand

The ‘Date Samples‘ button is the second from left at the base of the Sample Browser. This button will apply a pre-existing experiment to the data in the sample browser.  In this way single samples can be dated by typing their id in the Search panel. By default ACE comes with a calibrated experiment for 3He, called ‘3He Demonstration Expt’. To see the details of this experiments we switch to the Experiment Browser:

Click on image to expand

Click on image to expand

The default 3He experiment uses Desilets and Zreda (2003) scaling, and when this scaling is used to calibrate independently dated samples it predicts an HLSL production rate of 123.1 ± 6.2 atoms g-1 yr-1.  Now we will apply this experiment on the Blard et al (2006) samples by clicking on the ‘Date Samples‘ button:

A panel opens allowing us to choose which experiment we want run the samples in the sample browser on.  The samples in the sample browser is given in the ‘To Eligible Samples’ dialog, and the list of experiments is in the pulldown menu. Note that to be listed here an experiment must be calibrated, so that HLSL production rates are already known.  For more on this, see the following section of the quick start guide. From the pulldown menu we choose ‘3He Demonstration Expt':

When you click on ‘OK’, the experiment will operate on the samples.  Note that for very old samples, or very small timesteps, this may take a long time.  To indicate the progress of the experiment a progress bar pops up which tracks the progress of the experiment:

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Click on image to expand

When all samples have been processed, the ages computed by this experiment are shown in the Sample Browser:

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Click on image to expand

Then these samples can be exported to a csv file (using the Export Samples dialog button) or analyzed using the ACE graphics output (see below for an example), or with CALVIN using the Analyze Samples dialog button.

Why does ACE use experiments to date samples and not just compute an age?  As the following section shows, with ACE all kinds of different experiments can be created so that the sensitivity of computed ages to differences in experimental design can be examined. For example, the figure below shows how experiments with different scalings affect the computed age for sample MK4, a sample with an independently dated age of 41 ± 3 ka:

By making different experiments, you can assess how sensitive your samples are to assumptions about the theory of cosmogenic nuclide dating (in this case, scaling).  The way to create different experiments is shown in the next section.

To watch a demonstration of this procedure click here (requires Quicktime)

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