Ice sheet mass loss is one of the clearest manifestations of climate change, with Antarctica discharging mass into the ocean via the melting of glacial ice or through calving. This calving produces icebergs which can modify ocean water properties, often at great distances from source. This affects upper ocean physics and primary productivity, with implications for atmospheric carbon drawdown. Detailed understanding of iceberg modification of ocean waters has hitherto been hindered by a lack of proximal measurements. Here, unique measurements of a giant iceberg from an underwater glider quantifies meltwater effects on the physical and biological processes in the upper layers of the Southern Ocean, a region disproportionately important for global heat and carbon sequestration. Iceberg basal melting erodes seasonally-produced Winter Water (WW) layer stratification, normally forming a strong potential energy barrier to vertical exchange of surface and deep waters, whilst freshwater runoff increases and shoals near-surface stratification. Nutrient-rich deeper waters, incorporating terrigenous loaded meltwater, are ventilated to below this stratification maxima, providing a potential mechanism for alleviating critical biological limiting components. Regional historical hydrographic data demonstrates similar stratification changes during the passage of another large iceberg, suggesting that they may be an important pathway of aseasonal WW modification.