Uranium is well mixed in the ocean, and its decay produces Pa and Th at a constant activity ratio (0.093). The decay products are rapidly removed by adsorption on settling particles, but not at equal rates. Pa has a residence equivalent to the residence time of deep water in the Atlantic basin (around 1000 yrs) but Th is removed more rapidly (centuries). Thermohaline circulation effectively exports Pa from the Atlantic into the Southern Ocean, while most of the Th remains in Atlantic sediments. As a result, there is a relationship between Pa/Th in Atlantic sediments and the rate of overturning: faster overturning produces lower sediment Pa/Th ratio, while slower overturning increases this ratio. The combination of δC and Pa/Th can therefore provide a more complete insight into past circulation changes.