Physical vertical fluxes decouple iron and manganese supply in the Southern Ocean

Sub-nanomolar concentrations of dissolved iron and manganese seasonally limit phytoplankton growth in the Southern Ocean, as both are essential cofactors for metabolic processes. These trace metals are supplied from the subsurface to the surface through physical processes, including winter
entrainment, diapycnal diffusion and Ekman pumping, where they are made available for biological uptake. Whilst the quantification of these processes for iron has been previously reported, no large scale quantification exists for manganese in the Southern Ocean. We analysed regridded dissolved iron and manganese data from all available Southern Ocean cruises, to quantify these physical supply
processes. Here we show that winter entrainment (18.58±4.82μmolm⁻²yr⁻¹manganese;29.78±5.23
μmol m⁻²yr⁻¹ iron), followed by diapycnal diffusion (1.59 ± 1.51 μmol m⁻²yr⁻¹ manganese; 0.55±1.58 μmol m⁻²yr⁻¹ iron), dominates the supply across the Southern Ocean. Matched low iron: manganese supply ratios along the zero meridian and near the Antarctic Peninsula suggest potential
iron–manganese co-limitation, with the latter consistent with field observations in the Drake Passage. Accurately reproducing this spatial variability in models will improve projections of trace metal control on the Southern Ocean carbon pump.