High-resolution numerical assessment of large-scale riverine alkalinity modification scenarios along the southern coast of the United States

River-based alkalinity modifications represent potentially effective approaches for removing atmospheric CO₂ and mitigating anthropogenic climate change. Evaluating their effectiveness requires consideration of downstream impacts on coastal ocean CO₂ air–sea exchange following intervention. In this study, we applied a high-resolution (5 km) regional coupled physical and biogeochemical model (CROCO-PISCES) to assess two carbon dioxide removal approaches, alkalinity enhancement (AE) and enhanced weathering (EW), in the northern portion of the Gulf of Mexico. Alkalinity and dissolved inorganic carbon inputs were added to riverine outflow from the Mississippi and Atchafalaya Rivers according to eight hypothetical scenarios with variable magnitude and timing. In the AE scenarios, simulations showed oceanic CO₂ uptake efficiencies ranging from 58% to 85%, with higher values under modest perturbations and summer additions when shallow mixed layers promoted near-surface retention of added alkalinity. In the EW scenarios, simulations indicated that 12–15% of land-based carbon sequestration was re-emitted to the atmosphere from the ocean, with the amount remaining largely consistent across scenarios, suggesting that the ocean-side leakage is relatively stable in the EW case and represents a relatively small component of the overall EW life cycle. Collectively, these findings demonstrate that the long-term carbon removal efficiency of river-based alkalinity modification will often depend on the ratio between alkalinity and dissolved organic carbon introduced to the coastal ocean.