The natural radionuclides 230Th and 231Pa are continuously produced in the water column with a constant initial
Th/Pa activity ratio of 10.8. Small differences in particle reactivity of Th and Pa are responsible for a fractionation of both radionuclides. The total mass flux is believed to be the primary factor controlling the Th/Pa ratio of particulate matter and oceanic sediments. This is well documented by Th/Pa ratios in excess of their production ratio of 10.8 in regions of low particle flux and by ratios lower than 10.8 in areas of high particle flux. Therefore, the Th/Pa ratio has been used to assess past changes in oceanic productivity of dated sediment cores.
In addition to the total mass flux the Th/Pa ratio can also be influenced by the particle composition which might limit its use as a paleoproductivity proxy. Investigations of Th and Pa in the water column in the Atlantic sector of the Southern Ocean (Rutgers van der Loeff and Berger, 1993) have revealed low particulate Th/Pa ratios in regions of very low total mass flux. These low Th/Pa values may be related to high contents of biogenic opal in sinking particles, for which a high scavenging
efficiency for Pa is suggested (Taguchi et al., 1989).
We had the opportunity to verify this hypothesis during Polarstern Expedition Ant XII/4 on a N-S transect across the Antarctic circumpolar current (ACC) in the SE Pacific sector of the Southern Ocean, a region characterized by low total mass flux north and south of the ACC-system. The water column was sampled at eight stations with seven in situ filtration pumps to obtain the dissolved and particulate fractions of Th and Pa. Surface sediment samples were also taken from each station.
First results show similar patterns of vertical dissolved Th and Pa profiles at all stations with Th/Pa ratios mostly between 2 and 3 with no detectable latitudinal effect. In contrast, Th/Pa ratios of surface sediments show an almost linear decrease from high values north of the ACC-system to low values in the south. Because of similar total mass flux north and south of the ACC-system it is more likely that the N-S decrease in the Th/Pa ratio of surface sediments results from a change in the chemical composition of particulate matter, than from a gradient in particle flux.
These preliminary data will be compared with data on the chemical composition of surface sediments and with Th/Pa ratios in suspended material. Implications for the use of the Th/Pa ratio as a paleoproductivity proxy will be discussed.
Rutgers van der Loeff, M.M. & Berger, G.W., Deep-Sea Research 40, 339-357 (1993).
Taguchi, K., Harada, K., & Tsunogai, S., Earth Planet. Sci. Lett. 93, 223-232 (1989).