DEEP ocean circulation may be a significant factor in determining global climate(1-5). Increases in the flux of warm, saline waters from low to high latitudes would enhance the poleward transport of heat and, thus, help maintain the warm conditions at high latitudes typical of globally warm ’greenhouse’ periods, But controversy exists(1,2) as to whether the ocean’s thermohaline circulation can transport enough heat to bring about the temperature distributions of these times, such as the mid-Cretaceous and early Eocene, Here we present stable-isotope records of ocean temperature and salinity that indicate that bottom waters in Late Cretaceous oceans of the Southern Hemisphere became cooler and less saline at the same time (about 70 Myr ago) as widespread biotic changes(3,6-8). These findings support the idea that changes in deep ocean circulation can act as a climate switch.