Amazon River Plume Control on Carbonate Chemistry in the Western Tropical Atlantic

The Amazon River Plume (ARP) modulates the carbonate system of the western Tropical Atlantic (WTA) through physical and biological processes, including freshwater dilution, biological carbon uptake, and circulation-driven transport; however, its basin-scale impact has not been quantitatively assessed. Here we combine climatological spatial anomalies of sea surface salinity (SSS), carbonate system variables, and biogeochemical indicators to determine the percentage contribution of offshore ARP waters to regulate carbonate chemistry during wet and dry seasons. We show that ARP-derived freshwater and nutrients drive ∼8%–10% reductions in sea surface fugacity (fCO₂), total alkalinity, dissolved inorganic carbon and carbonate ion concentrations across the WTA during the wet season, closely matching a ∼9% decrease in SSS. These reductions represent a substantial perturbation to carbonate chemistry across the offshore plume-influenced WTA, strengthening the role of the ARP in modulating carbon cycling and atmospheric CO₂ uptake. These findings identify physical dilution, enhanced photosynthesis and nitrogen cycle transformations, as strong first-order controls on carbonate system drawdown, supporting CO₂ uptake in low-SSS waters. ARP-driven reductions in fCO₂ extend far beyond the plume through offshore export by the North Brazil Current and North Equatorial Countercurrent, decreasing fCO₂ by up to 20 μatm near 10°N and modulating carbonate chemistry across the equatorial Atlantic. Together, these results demonstrate that the ARP is a major regulator of sea surface carbonate chemistry in the WTA, enhancing atmospheric CO₂ uptake through coupled physical–biological processes. The spatial anomaly approach provides a robust framework for quantifying river-plume impacts on basin-scale carbon cycling.