The ocean absorbs about a quarter of anthropogenic CO2 emissions each year, which slows climate change but changes ocean chemistry, lowering seawater pH. Ocean-atmosphere CO2 exchange and its consequences are governed by a set of reversible chemical reactions and parameters collectively known as the marine carbonate system. This session welcomes submissions that focus on observing, understanding and modelling the changing marine carbonate system and its role in the global carbon cycle. This includes studies on anthropogenic CO2 uptake and its chemical consequences, the physical and biological carbon pumps, biominerals and the alkalinity cycle, air-sea CO2 fluxes, internal consistency of the carbonate system variables, organic alkalinity, physical chemistry, measurement techniques and data compilations. Studies may be global or regional in scope and they may cover a range of different timescales. Results may be derived from a variety of sources including in situ observations, laboratory experiments, remote sensing, machine learning, and model output.