We measured the dissolved barium concentrations in the water column along the Southern Ocean I9S meridional transect (115°E; Australian-Antarctic Basin) of the World Ocean Circulation Experiment (WOCE), which was sampled in January - February 2012. Dissolved Ba concentrations ranged from 36 nM (in the subtropical surface waters close to Australia) to 100 nM (at the greatest depths). We compare these results with existing data for 3 other meridional transects in the Weddell-Enderby and Australian-Antarctic basins. While this work confirms the well known remarkable correlation of dissolved Ba with silicic acid (Si), an essential nutrient for diatoms, it also highlights the fact that Ba and silicic acid are uncoupled in surface waters and that regressions differ zonally, reflecting the different biogeochemical behaviour of both elements. This reflects the well known fact that both elements are involved in different biogeochemical cycles. Observations of dissolved Ba and Si along the deep layers of the thermohaline circulation reveal that dissolution of the SiO2 (opal) diatom skeletons returns Si to the seawater solution more efficiently than Ba is returned to the solution from the dissolution of particulate Ba carriers. This condition reflects the efficiency of the Southern Ocean ecosystem to maintain Si within its geographical domain, and probably explains why diatom growth is so successful in these waters. We also studied the changes in dissolved Ba and Si in identified water masses along the Antarctic circumpolar travel path from the Weddell-Enderby Basin to the Australian-Antarctic Basin. For the mid-depth North Atlantic Deep Water mass, which flows freely between the two basins, Ba and Si concentrations clearly increase along the circulation path. For Antarctic Bottom Water a west to east increase is also observed for Si, but not for Ba, reflecting the fact that bottom waters in both basins originate from different source regions with different Ba and Si characteristics and do not exchange as easily as middepth waters due to the presence of the Kerguelen Plateau separating the Weddell- Enderby and the Australian-Antarctic Basin.