T. van der Hammen, and B. van Geel. “Charcoal in Soils of the Allerød-Younger Dryas Transition Were the Result of Natural Fires and Not Necessarily the Effect of an Extra-Terrestrial Impact”. Netherlands Journal of Geosciences, vol. 87, Dec. 2008, pp. 359 -61, doi:10.1017/S0016774600023416.