The normal, biologically productive ocean is characterized by a gradient of the 13C/12C ratio from surface to deep waters. Here we present stable isotope data from planktonic and benthic micro-fossils across the Cretaceous/Tertiary boundary in the North pacific, which reveal a rapid and complete breakdown in this biologically mediated gradient. The fluxes of barium (a proxy for organic carbon) and CaCO3 also decrease significantly at the time of the major marine plankton extinctions. The implied substantial reduction in oceanic primary productivity persisted for ???0.5 Myr before the carbon isotope gradient was gradually re-established. In addition, the stable isotope and preservational data indicate that environmental change, including cooling, began at least 200 kyr before the Cretaceous/Tertiary boundary, and a peak warming of ???3 ??C occurred 600 kyr after the boundary event. ?? 1989 Nature Publishing Group.