Authigenic carbonate and clay formation in marine sediments are important but poorly constrained processes that link major element cycling to the ocean alkalinity budget. Here, we investigated the distributions of dissolved potassium (K), magnesium (Mg), calcium (Ca), and strontium (Sr) in the Mediterranean water column, together with pore fluids, solid sediments, and brine pools from the eastern basin. Compared with Atlantic end-member waters, Mediterranean surface waters displayed elevated potential alkalinity and salinity-normalized Ca concentrations, consistent with the combined effects of riverine inputs and CaCO₃ dissolution. Strontium concentrations were significantly higher in the western basin than in the eastern basin, possibly due to sedimentary removal in the eastern basin. Porewater profiles further demonstrated that sediments act as sinks for Ca and Sr, with a Ca:Sr removal ratio of 1:0.011, indicating authigenic CaCO₃ precipitation in surface sediments, predominantly as aragonite, likely favored by elevated temperatures and high CaCO₃ saturation states. Meanwhile, sedimentary uptake of Mg and K points to the formation of authigenic clay minerals. Together, these observations indicate that authigenic carbonate and clay formation in sediments constitute important sinks for major elements. Tentative budgets of major elements and alkalinity are nearly balanced at the basin scale, underscoring the Mediterranean Sea as a semi-enclosed basin that offers a unique perspective on regional major element cycling.