<p>Understanding the ocean’s chemical composition is key to assessing the carbon cycle and its climate impact, especially through its control on calcium carbonate saturation and preservation. While rivers and hydrothermal systems are recognized contributors to ocean chemistry, the role of coastal aquifers has been underestimated. This study shows that long-term submarine groundwater discharge (SGD) is a major source of solute fluxes to the ocean, especially of calcium and alkalinity, while removing sodium and potassium. Through analysis of elemental and isotopic ocean budgets, we find that fluxes from long-term SGD rival those from rivers in magnitude. These fluxes influence the residence times of major elements and significantly affect the global carbon budget. Moreover, they are dynamically linked to sea-level changes: falling sea levels enhance SGD-driven solute inputs, whereas rising sea levels suppress them. These patterns suggest that SGD fluxes may regulate long-term climate trends, including rates of global cooling or warming.</p>