Manganese (Mn) is an essential element for photosynthetic life, yet concentrations in Southern Ocean open waters are very low, resulting from biological uptake along with limited external inputs. At southern latitudes, waters overlying the Antarctic shelf are expected to have much higher Mn concentrations due to their proximity to external sources such as sediment and sea ice. In this study, we investigated the potential export of Mn-rich Antarctic shelf waters toward depleted open Southern Ocean waters. Our results showed that while high Mn concentrations were observed over the shelf, strong biological uptake decreased dissolved Mn concentrations in surface waters north of the Southern Antarctic Circumpolar Current Front (< 0.1 nM), limiting export of shelf Mn to the open Southern Ocean. Conversely, in bottom waters, mixing between Mn-rich Antarctic Bottom Waters and Mn-depleted Low Circumpolar Deep Waters combined with scavenging processes led to a decrease in dissolved Mn concentrations with distance from the coast. Subsurface dissolved Mn maxima represented a potential reservoir for surface waters (0.3 – 0.6 nM). However, these high subsurface values decreased with distance from the coast, suggesting these features may result from external sources near the shelf in addition to particle remineralization. Overall, these results imply that the lower-than-expected lateral export of trace metal-enriched waters contributes to the extremely low (< 0.1 nM) and potentially co-limiting Mn concentrations previously reported further north in this Southern Ocean region.

Antarctic Bottom Water (AABW) production supplies the deep limb of the global overturning circulation and ventilates the deep ocean. While the Weddell and Ross Seas are recognised as key sites for AABW production, additional sources have been discovered in coastal polynya regions around East Antarctica, Vincennes Bay being the latest. Vincennes Bay, despite encompassing two distinct polynya regions, is considered the weakest source, producing Dense Shelf Water (DSW) only just dense enough to contribute to the lighter density classes of AABW found offshore. Importantly, the network of local glaciers and upstream Totten Ice Shelf system are all reportedly thinning and the freshwater input from such melting is likely to influence water mass structure. Accordingly, Vincennes Bay presents an interesting test case for DSW/AABW sensitivity to climate-driven changes in Antarctic coastal oceanography. Here we provide the first detailed observations of the Vincennes Bay shelf region and surrounds, using CTD data from instrumented elephant seals in late summer/early fall. We find that Vincennes Bay has East Antarctica’s warmest recorded intrusions of modified Circumpolar Deep Water (mCDW), intrusions that both hinder sea-ice production and contribute salt to new DSW formation. Warm mCDW is also observed to be driving basal melt in Vincennes Bay, as seal CTD data provide the first direct observational evidence for inflow of basal melt to this region. As the most marginal of AABW sources, Vincennes Bay is a particularly useful region for assessment of the sensitivity of AABW production to changes in climate.

The ocean’s permanent pycnocline is a layer of elevated stratification that separates the well-ventilated upper ocean from the more slowly-renewed deep ocean. Despite its pivotal role in organizing ocean circulation, the processes governing the formation of the permanent pycnocline remain little understood. Two factors in particular are generally overlooked: the presence of a zonal channel in the Southern Ocean, and the nonlinear interplay between temperature and salinity distributions. Here we assess the mechanism generating the Southern Ocean’s permanent pycnocline through the analysis of a high-resolution, realistic, global sea ice-ocean model. We show that the permanent pycnocline is formed by seasonal sea ice-ocean interactions in two distinct ice-covered regions, fringing the Antarctic continental slope and the winter sea ice edge. In both areas, persistent sea ice melt leads to the formation of strong, salinity-based stratification at the base of the surface mixed layer in winter. The resulting sheets of high stratification subsequently descend into the ocean interior at fronts of the Antarctic Circumpolar Current, and are projected equatorward into the Southern Hemisphere basins along density surfaces. Our findings thus highlight the crucial role of localized sea ice-ocean interactions in configuring the vertical structure of the Southern Ocean.