Turbulent mixing controls the pace of ocean ventilation, the rate at which the ocean interior is filled up with water, heat and chemicals from the ocean’s surface. This process governs the ocean's ability to store atmospheric heat and greenhouse gases, exerting a profound impact on Earth's climate and moderating the rate of human-induced climate change. However, the role of mixing in ocean ventilation remains poorly understood, due to the scarcity of direct observations and the inability of numerical models to capture small-scale turbulent dynamics. Our limited understanding can be rationalised around three major questions: 1) How does the interplay between advective and diffusive processes control ocean ventilation? 2) What is the role of mixing in the water-mass transformations sustaining the ocean’s overturning circulation? 3) What is the relative importance of the two primary regimes of ocean turbulence (the mesoscale and microscale) for the ventilation of climate-critical tracers? REMIX-TUNE addresses these questions through an innovative approach founded on two pillars: 1) Deploying the first large fleet of autonomous profiling floats equipped with microstructure turbulence sensors in key ocean ventilation regions; and 2) pioneering a novel theoretical approach to quantify mesoscale and microscale mixing using float microstructure observations and existing hydrographic data from the Argo programme. With this strategy, REMIX-TUNE will generate the first comprehensive, observation-based global database quantifying the role of mixing in ocean ventilation. This understanding will then be used to develop a new framework to assess and ground-truth the representation of mixing in the next generation of ocean-climate models. Thus, REMIX-TUNE will elicit a step change in our rationalisation of ocean mixing, and its integration into numerical models, and will leave a lasting impact on mixing research by revolutionising the methodologies employed in the field.