Ice cores provide ice properties as a function of depth. Climate is a function of time.
Climatic properties are 'mapped' on to successive ice layers.
Part of the problem in reading the ice cores then is to know how climatic conditions are transformed into ice properties. The other part is relating time of formation to current depth - this is the issue of dating the ice core. Also with other ice cores Periodicity Johnsen, et al. Technique was maximum entropy grequency analysis. This adds to their confidence in the time scale for this period.