The isotopes ²³¹Pa and ²³⁰Th are products of the decay of the uranium isotopes ²³⁵U and ²³⁴U, respectively, dissolved in seawater and well mixed owing to uranium’s long residence time, yielding a Pa:Th ratio that is constant and distributed uniformly. ²³¹Pa and ²³⁰Th are adsorbed onto settling mineral particles, Th more efficiently than Pa, so Pa has a longer seawater residence time (~200 years) than Th (~30 years), which allows Pa to move laterally more than Th prior to deposition. The ²³¹Pa deficit in the Pa:Th ratio in the mineral content of ocean sediment can be used to calculate the extent to which the water was moving laterally (and removing Pa) during the period of deposition and is, therefore, an indicator of past ocean circulation.