231Pa and 230Th profiles in the open ocean water column

231Pa and two Th isotopes (230Th and 228Th) were measured using large-volume water samples from the western North Pacific. Although 231Pa and 230Th have uniform source distributions (by decay of uranium) throughout the oceans, their vertical profiles are discernibly different from each other. The 231Pa profile shows a mid-depth maximum around 2.5 km, while 230Th increases monotonically with depth. This demonstrates that these nuclides follow different pathways for their transport and removal from seawater to depositional sinks. The activity ratio of 230Th/231Pa in seawater increases from one at the surface to four at a depth of 4 to 5 km, which is lower by a factor of 10 than the ratios published for open ocean particulate matter at the same depth horizon. The distribution coefficient, Ksub(D) for Pa, and the fractionation factor, Fsub(Th/Pa), between natural marine particulate matter and seawater are constant with depth at approx. 2 x 106 and 10, respectively. This implies that the adsorptive characteristics of deep open ocean particles are independent of particle composition or the content of manganese. The mean residence time of 231Pa is estimated to be 200 years, which is considerably longer than that of 230Th, but close to that of 210Pb. (author)