I don't see how an 11 year cycle could prove that the seasons cycle has not changed in the past 100 years. I am almost 100% positive that the cycles have even changed in the past 100 years. I in no way can believe that they haven't, especially when the cycle has changed, even throughout the United States, and other countries in the past century even.
Which cycle are you referring to? The 11 year solar cycle, or the arctic day/night cycle?
What I was making reference to, is that the background effects of the 11 year solar cycle in the ice-core data correlate to the annual hoarfrost layers, and this pattern continues backward in time down the icecore layers at least to the Holocene.
There are other correlations to the Vostok ice core data and also volcanic dust markers that reference specific volcanic events which correlate to markers found in seafloor cores.