Ocean acidification is the hydrogen ion increase caused by the oceanic uptake of anthropogenic CO₂, and is a focal point in marine biogeochemistry, in part, because this chemical reaction reduces calcium carbonate (CaCO₃) saturation states (Ω) to levels that are corrosive (i.e., Ω ≤ 1) to shell-forming marine organisms. However, other processes can drive CaCO₃ corrosivity; specifically, the addition of tidewater glacial melt. Carbonate system data collected in May and September from 2009 through 2012 in Prince William Sound (PWS), a semienclosed inland sea located on the south-central coast of Alaska and ringed with fjords containing tidewater glaciers, reveal the unique impact of glacial melt on CaCO₃ corrosivity. Initial limited sampling was expanded in September 2011 to span large portions of the western and central sound, and included two fjords proximal to tidewater glaciers: Icy Bay and Columbia Bay. The observed conditions in these fjords affected CaCO₃ corrosivity in the upper water column (< 50 m) in PWS in two ways: (1) as spring-time formation sites of mode water with near-corrosive Ω levels seen below the mixed layer over a portion of the sound, and (2) as point sources for surface plumes of glacial melt with corrosive Ω levels (Ω for aragonite and calcite down to 0.60 and 1.02, respectively) and carbon dioxide partial pressures (pCO₂) well below atmospheric levels. CaCO₃ corrosivity in glacial melt plumes is poorly reflected by pCO₂ or pH_T, indicating that either one of these carbonate parameters alone would fail to track Ω in PWS. The unique Ω and pCO₂ conditions in the glacial melt plumes enhances atmospheric CO₂ uptake, which, if not offset by mixing or primary productivity, would rapidly exacerbate CaCO₃ corrosivity in a positive feedback. The cumulative effects of glacial melt and air–sea gas exchange are likely responsible for the seasonal reduction of Ω in PWS, making PWS highly sensitive to increasing atmospheric CO₂ and amplified CaCO₃ corrosivity.