E. De Man, L. Ivany, and N. Vandenberghe. “Stable Oxygen Isotope Record of the Eocene-Oligocene Transition in the Southern North Sea Basin: Positioning the Oi-1 Event”. Netherlands Journal of Geosciences, vol. 83, Sept. 2004, pp. 193 -97, doi:10.1017/S0016774600020266.