A comparison of incoming solar radiation at 45-65oN with the marine Oxygen Isotope Chronology. During glacials 16O is deposited preferentially in the continental ice sheets and 18O remains behind in the (lower) oceans, thus the δ 18O/16O ratio is numerically higher during glaciations (plotted "down" in lower figure). The abrupt change from high to low ratios are the deglaciations. Note that these coincide with periods of high incoming solar radiation, and the minima of incoming solar radiation match cooling intervals leading to glaciations.
Broecker, Wallece S. and Denton, George H. 1990.