In these methods, one uses the age of previously determined markers to determine the age of various points in the ice-core.The major advantage of these methods is that they can be completed relatively quickly. The water molecules composed of H2(18O) evaporate less rapidly and condense more readily then water molecules composed of H2(16O). Of the temperature dependent markers the most important is the ratio of 18O to 16O.
The major disadvantage of this method is that one must have a previously age-dated ice core to start with.
A major disadvantage of these types of dating is that they are extremely time consuming.
This process is analagous to the counting of tree rings.
Upon reaching the earth’s surface, a small percentage of carbon-14 containing carbon dioxide is taken up by plants and then incorporation into plant biomolecules via photosynthesis.
It becomes incorporated into the biomolecules of heterotrophic organisms (animals) via the food chain.
Thus each annual layer starts 10Be and 36Cl poor, becomes 10Be and 36Cl rich, and then becomes poor again. Although what is said above is true, this is an exceedingly minor effect.