Sediment 231Pa/230Th has the potential to estimate the strength of the Atlantic ocean deep circulation as a result of the difference in particle reactivity between 231Pa and 230Th in the water column. 230Th is the most particle-reactive, with very short residence times in the water column (a few decades) which reduce considerably its potential redistribution by horizontal transport. In contrast, the residence time of 231Pa (100-200 yrs) is similar to the estimated mean residence time of deep water in the modern Atlantic. Consequently, the meridional circulation in the modern Atlantic is estimated to export to the Southern Ocean approximately half the 231Pa produced in deep water , which would explain that 231Pa/230Th in Atlantic sediments is only half the 231Pa/230Th production ratio [Yu, et al., 1996]. Past decreases in the rate of the Atlantic meridional overturning circulation (AMOC) would translate into higher 231Pa/230Th activity ratio in Atlantic sediments, which would reach the production ratio of 0.093 for a total shutdown of the AMOC. Because of the longer residence time of deep water estimated for the Pacific Ocean, sediment 231Pa/Th ratios from Pacific sediments could not be used as a potential paleo-circulation proxy.
There is a need for distinguishing between the influence of particle chemical composition (e.g., biogenic opal) and that of circulation. Coverage of sediment 231Pa/230Th data is very poor. There is also a strong need for much more water column observations for better understanding the relation between 231Pa/230Th in sediment and the concentrations of 231Pa and 230Th in water (e.g., in order to better understand the effects of particle chemical composition).