The Southern Ocean Hub: For Nutrients, Micronutrients, and Their Isotopes in the Global Ocean

The sustenance of marine primary productivity depends on the supply of macro- and micronutrients to photosynthesizers in the ocean’s sunlit surface. Without supply from the deep, sinking particles would deplete the upper ocean of these vital elements within decades. Over the last 20 years, it has been recognized that the Southern Ocean, where nutrient-rich deep waters are brought to the surface and the water masses that fill much of the upper ocean are formed, plays a pivotal role in replenishing upper-ocean nutrients. Photosynthesizers that grow and take up nutrients within the Southern Ocean circulation “hub” thus have an outsize influence on global-​scale distributions of macronutrients and many micronutrients. The GEOTRACES program has contributed observations of the concentration and stable isotope composition of “nutrient-​type” metals like zinc, cadmium, and nickel, within the Southern Ocean and beyond it, that are driving a sea change in our understanding of their marine cycles. Simultaneously, our understanding of Southern Ocean circulation has been refined, with recognition of the importance of longitudinal variability and subtropical overturning. Here, we aim to bring together these two strands of progress, review insights gained into marine micronutrient cycling, and consider the questions that remain to be resolved.