Post-run analysis

By default, Bacon will perform millions of MCMC iterations for each age-model run, although only a fraction of these will be stored. In most cases the remaining MCMC iterations will be well mixed (the upper left panel of the fit of the iterations shows no strange features such as sudden systematic drops or rises). However if the iterations seem not well mixed, or if too few remain (say less than a few hundred), then you could check the Gelman and Rubin Reduction Factor1. Too high differences (high Factors) between runs indicate poor MCMC mixing. Robust MCMC mixing is indicated by a Gelman and Rubin Reduction factor below the 1.05 safety threshold.

For example, try the default core, running it five times with a very small sample size of ssize=100:

Baconvergence("MSB2K", thick=5, runs=5, ssize=100, coredir=tempfile())
 Did 5 Bacon runs.
 Gelman and Rubin Reduction Factor 1.10078680880009 (smaller and closer to 1 is better).
 Probably not a robust MCMC run! Too much difference between runs, above the 1.05 threshold. Increase sample size?

greyscale plots

Once a robust, reliable and realistic age-depth model has been produced, the fun starts. Greyscale plots for example can be used to show not just one age-depth curve but the entire MCMC run output. If pollen or other ‘proxies’ have been analysed across a range of depths of your core, then these proxies can be plotted on the time-scale as grey-scale ‘ghosts’ where less certain sections are plotted in lighter grey than more certain sections. Bacon looks for a file in the core’s folder, starting with the core’s name and ending in _proxies.csv, e.g., Bacon_runs/MSB2K/MSB2K_proxies.csv. This file should have columns separated by commas, with the first column being the depth, followed by columns for the proxies. The first row should contain the names of the columns. To produce a proxy ghost of the seventh proxy of MSB2K:

proxy.ghost(7)

proxy.ghost graph of the 7th proxy in core MSB2K

Ghost graphs can also be produced for the accumulation rate throughout the core (accrate.depth.ghost) or over time (accrate.age.ghost):

accrate.depth.ghost()
accrate.age.ghost()

ghost-graph of the accumulation rate of core MSB2K against depth

ghost-graph of the accumulation rate of core MSB2K over time

Other analyses

R provides a very versatile environment to query the age-model output. To get the age estimate of any single core depth:

Bacon.hist(20)
## 
  |                                                                            
  |                                                                      |   0%
## mean (red): 4851.4 cal yr BP, median (green): 4843.3 cal yr BP
## 95% range (blue): 4766.7 to 4953.3 cal yr BP

You can also store the iterations of the age estimates of that depth in a new variable and then query it:

a.d20 <- Bacon.Age.d(20)
summary(a.d20)
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##    4667    4826    4843    4851    4875    5035
hist(a.d20)

Or calculate how much time has passed between 30 and 20 cm depth:

a.d30 <- Bacon.Age.d(30)
a.d20 <- Bacon.Age.d(20)
summary(a.d30-a.d20)
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##   216.8   385.2   427.1   429.7   475.0   651.1
hist(a.d30-a.d20)

Accumulation rates at specific depths or ages can also be investigated:

acc.d20 <- accrate.depth(20)
summary(acc.d20)
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##  0.1065 11.0272 20.7150 23.3210 32.3215 77.6965
acc.a4500 <- accrate.age(4500)
summary(acc.a4500)
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##  0.3918 14.8440 21.9517 24.9250 33.6193 63.7062

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  1. Brooks, S.P., Gelman, A., 1998. General methods for monitoring convergence of iterative simulations. Journal of Computational and Graphical Statistics 7, 434-455↩︎