The causes of mass extinctions and the importance of major bio-events in the history of life are subjects of considerable scientific interest. A large amount of geological, geochemical, and paleontological information now exists for the Cretaceous/Tertiary (K/T) boundary (66 Myr BP). These data are used here to constrain a newly developed time-dependent biogeochemical cycle model that is designed to study transient behavior of the earth system. Model results suggest significant fluctuations in ocean alkalinity, atmospheric CO2, and global temperatures at the K/T boundary, brought about by the extinction of calcareous plankton and reduction in pelagic CaCO3 and organic carbon sedimentation rates. Oxygenisotope analyses and other paleoclimatic data provide some evidence that such climatic fluctuations may have occurred, but stabilizing feedback processes may have acted to reduce the ocean-alkalinity and carbon dioxide fluctuations.