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Examples of such inclusions are a decrease (or increase) in temperature over a period of years that can be determined from flora and fauna found in the oceanic core and a decrease (increase) in the 18O enrichment over this same period of years. These are the primary "inclusions" that are compared.
I apologize for my use of nondescript terminology here.
The major disadvantage of this dating method is that these isotopes also tend to diffuse over time.
In these methods, one uses the age of previously determined markers to determine the age of various points in the ice-core.
This process is analagous to the counting of tree rings.
Once such markers of seasonal variations are found, they can be used to find the number of years that the ice-core accumulated over.
Both of these isotopes are produced by cosmic rays and solar irradiation impinging on the upper atmosphere, and both are quickly washed from the atmosphere by precipitation.
Of the irradiation dependent markers the two most important are 10Be and 36Cl.
Furthermore the alkaline precipitants of the ice ages limits this measure to approximately 8000 BC.
These eruptions leave a distinct marker within the snow which washed the atmosphere.
We can then use recorded volcanic eruptions to calibrate the age of the ice-core.
This depletion is a temperature dependent process so in winter the precipitation is more enriched in H2(16O) than is the case in the summer.
As the water vapor travels towards the poles it becomes increasingly poorer in H2(18O) since the heavier molecules tend to precipitate out first.
References Of the four distinct methods for determining the ages of ice cores, the first three are direct experimental tests and the fourth rests on somewhat uncertain theories.