I understand that completely. My question is how compressed are the layers after the weight of 100,000 years of snow have been on top of them? I would not think that individual years could be distinguished, just broader patterns - more like earth cores than like tree rings. Dig down in the earth and you can see the eons, but only very rarely was there a specific layer that can be certain to have formed in one year (like a volcanic layer). Instead you can see how the earth changed in different periods for sure, but can't determine specific years because of the compression.
I think they're still working on the problem from the little I've read. The electrical conductivity was a new one for me. They put electrodes on opposite sides of a core and measure the conductivity. Because the pH is different between summer and winter (which I also didn't know), they can find a yearly boundary. This should only be limited by the fineness of the probes.
Good observation
Here are some data from the Vostok Ice Cores --- Vostok ice data
BP = before present.
" Depth, m - Yr BP - mean age-- Co2, ppmv
3120.61--- 324711 --322827--- 288.4
3123.51 ---325400-- 323485 ---298.7
3129.91--- 327237 --324991--- 285.8 "
Looks to me that the largest CO2 number that I noticed is an average of several hundred years.
The current 370 ppmv CO2 is 24 percent higher than an average of several hundred years --- seems to me that to get an average of 298.7 over several hundred years, some of the data had to be higher, --- and thus, much closer to current data.