I would think the space between the arches is due to cloud cover. On a clear day the sun would record an arch on the paper, and on a cloudy day it would record nothing, so in a sense it really is like tree rings in that they reflect changes in weather from season to season.
I've seen these before, but I think yours is one of the best ones I've seen so far. What happens if you fix it? I guess you lose the colour, but do you get a more permanent image?
Marco -
Doh! I will edit. Sorry about that. I type it myself so infrequently!
Yes, no souping in the developer. That would make it BLACK.
No fix. It will bleach it back too much. I've done some POP printing using regular old DOP stock and a UV source. Works and you get some pretty funky purple/brown/peach colors. Even plain hypo bleached them back a lot.
The bottom most arc started with Dec 21, 2007. The top most arc would be March 19, 2008. The gaps are due to long periods of overcast days. This is about right for the region, high thin clouds during the winter so lots of bright, cold days. Getting closer to spring and we have more overcast and thunderstorm season begins.