Dissolved and particulate Protactinium-231 (231Pa) was analyzed for samples from the BONUS GoodHope (BGH) IPY-GEOTRACES cruise in the SE Atlantic sector of the Southern Ocean (36°S-13°E to 57°S-0°, Feb.–Mar. 2008). The inflowing waters from the Atlantic Ocean fuel the Antarctic Circumpolar Current in dissolved 231Pa which is mostly removed from seawater by the biogenic silica produced by diatoms in the Southern Ocean. This scavenging flux of the 231Pa induces a meridional gradient of the 231Pa concentration and of the Th–Pa fractionation factor (FTh/Pa).

We propose a first direct estimate of the 231Pa partition coefficient between suspended opal and seawater of about 1.42 ± 0.55 × 106 g/g. This partition coefficient could directly apply to simulate the particulate Pa concentration from the dissolved fraction and the opal concentration. We apply to 231Pa the isopycnal advection-diffusion-scavenging model built for Th isotopes across the ACC. This additional constraint on the model does not modify the isopycnal eddy diffusion estimate of about 1900 ± 180 m2/s at different isopycnal surfaces but suggests a higher particle settling velocity, about 1000 m/y, in the northern part of the ACC than solely derived from Th isotopes. The most different feature is the reduction by half of the estimated uncertainties among the different transport parameters just by the addition of a new constraining parameter. Moreover, this study confirms that 231Pa budget of the Atlantic Sector of Southern Ocean cannot be balanced by considering meridional transport only, and need to consider eastward export to scavenge Pa in the whole area of the Opal Belt.