The sensitivity of the Maastrichtian thermohaline circulation to the opening/closing of marine communications between the Arctic and North Pacific oceans is investigated through a set of numerical experiments using the model CLIMBER-2 (Earth Model of Intermediate Complexity). We show here that the opening or closing of an Arctic-Pacific marine gateway induces transitions between different equilibrium states of the thermohaline circulation. Sensitivity tests of the inferred modes of thermohaline circulation to atmospheric CO2 level changes have also been explored. An abrupt switch in deep convection from high northern to high southern latitudes, a change consistent with isotopic evidences, is reproduced by our simulations. The switch is caused by a combination of increased atmospheric CO2 concentration and inflow in the North Pacific of low-salinity Arctic waters when the Arctic-Pacific marine gateway is opened. The state of the gateway (open/closed) may have changed rapidly through variations in sea level that have been inferred for the Maastrichtian period.