The chemical composition of seawater has changed profoundly over Earth’s history, reflecting the combined influence of tectonics, continental growth, ocean–crust interaction, and biological evolution. In the Archean, when continents were small and atmospheric oxygen was low, limited continental weathering resulted in low oceanic alkalinity and restricted carbonate formation. At the same time, seawater chemistry was shaped by two external controls: continental runoff, which set the long-term chlorinity and salinity budget of the ocean, and hydrothermal interaction with the oceanic crust, which modified non-conservative ions and alkalinity. Under these conditions, phosphorus availability in the pre–Great Oxidation Event ocean was likely limited, owing to weak continental inputs and efficient scavenging of phosphate by iron oxides in ferruginous waters. The progressive emergence of continents increased the delivery of alkalinity and nutrients, weakened iron–phosphate coupling, and supported higher marine productivity, contributing to the rise of atmospheric oxygen. Today, the ocean continues to play a central role in the carbon cycle, with physical circulation controlling short-term CO₂ uptake and biological processes governing long-term carbon storage. The modern ocean operates as a chemically buffered reactor in which alkalinity, redox state, nutrients, and circulation jointly control carbon storage through physical and biological pumps. This seminar explores how the coevolution of ocean chemistry, continental growth, and nutrient cycling has regulated Earth’s climate and habitability through deep time.