spec.pgram in R: how to check for seasonality with longer phase? I am using 
    spec.pgram(tsobj,spans=6, plot=TRUE)
to obtain a periodogram for my univariate time series of monthly observations which were sampled over 86 years (so I have 1032 observations in total). It's defined as an object of class "ts" with frequency=12. The periodogram nicely depicts the seasonality for 1 year and has further peaks on following years. However, it stops at 6 years. I have reason to believe that there might be a seasonality with longer phase, say 8 years. How can I change the command to obtain values for longer seasonalities?
I am sorry, it sounds like a stupid question and possibly it is, but starring further on the help files, playing around with the available options and googling around didn't brought me any inside.
Thanks alot for your help!

Here is the plot that I obtain:


 A: What do you mean by "the periodogram [...] stops at 6 years". Do you mean cycles that are completed every 6 years? To make it clearer, you may post the graphic that you see or the data (the output from dput(tsobj).
I'm not sure I understood your question but it may be a misunderstanding of the x-axis. 
With monthly data, the x-axis of the plot displayed by spec.pgram ranges from 0 to 6. The peak (if any) at point 0 in the periodogram is related to a long cycle of infinite period (a long-term trend).
The remaining peaks (if any) at points 1, 2, 3, 4, 5, 6 are related to seasonal frequencies, that is, cycles that are completed within one year. The first one is the annual cycle, which is completed once in a year; the second is the semianual and completes two cycles within a year, and so on. The numbers in the 
x-axis (from 0 to 6) indicate the number of cycles that are completed within a year by the corresponding cycle.
Cycles that are completed every, say, 6 years should show up between the vertical lines at x=0 and x=1 in the periodogram. Do you see many peaks in  this range, if so, maybe you should change the period of your data.
