In radiocarbon applications, it is often necessary to bind atmospheric radiocarbon curves from different sources. Here, I show an example on how to bind a pre- and post-bomb curve as well as a forecasted curve for which atmospheric radiocarbon data have not been released yet.
As an example, we will consider the case in which we need an atmospheric radiocarbon curve from the years 500 to 2020 AD. This implies binding first the IntCal09 curve with one of the post-bomb curves, and then doing a forecast from the year 2009 to 2020.
Binding pre- and post-bomb curves
SoilR includes already two prebomb datasets:
IntCal13, and four curves for the post-bomb period stored as a list in the object
SoilR also includes a handy function called
bind.C14curves that allows you to bind a pre- and a post-bomb curves. This function has three arguments:
prebomb argument accepts either
IntCal13, and we recommend to use
IntCal13 because it is the curve
currently recognized as official by the radiocarbon community. The
postbomb argument requires the specification of the hemispheric zone as described in Hua et al. (2013).
time.scale argument determines whether the results are reported in a time scale in years before present (BP) or anno domini (AD).
In this example, we will bind the
IntCal13 and the post-bomb curve for the northern hemisphere zone 2, which corresponds to most of the temperate zone in this hemisphere.
The combined dataset goes from 50,000 years before present (-500000 AD) to the year 2009.62. The dataset is therefore very large:
##  5694 3
We are only interested in values of Delta14C starting from the year 500, so we select only the values of interest and plot the results
Forecast of the bomb curve
We use the R package
forecast to predict the values of atmospheric radiocarbon until the year 2020.
The forecast algorith requires equally spaced values for the radiocarbon time series, however the original time series is not equally spaced, for this reason we need to create a new series based on an interpolation procedure using a spline function
yrs=seq(1966,2009.5,by=1/4) # A series of years by quarters nz2=spline(Hua2013$NHZone2[,c(1,4)],xout=yrs) #Spline interpolation of the NH_Zone 2 dataset at a quaterly basis nhz2=ts((nz2$y-1)*1000,start=1966,freq=4) #Transformation into a time-series object
Now, we use the
ets function to fit an exponential smoothing state space model, and then perform a forecast based on this model
m=ets(nhz2) #Fits an exponential smoothing state space model to the time series f2=forecast(m,h=11*4) #Uses the fitted model to forecast 11 years into the future
Now we are ready to combine both curves
bc=data.frame(Year=c(bombcurve[-dim(bombcurve),1], seq(tsp(f2$mean),tsp(f2$mean), by=1/tsp(f2$mean))), Delta14C=c(bombcurve[-dim(bombcurve),2],as.numeric(f2$mean)))
This data.frame is now ready to use in SoilR. We can now plot this final dataset.