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<p>The Amazon River plume plays a critical role in shaping the carbonate chemistry over a vast area in the western tropical North Atlantic. We conduct a sensitivity analysis of hypothetical ocean alkalinity enhancement (OAE) via quicklime addition in the Amazon River watershed, examining the response of carbonate chemistry and air–sea carbon dioxide flux to the alkalinity addition. Through a series of sensitivity tests, we show that the detectability of the OAE-induced alkalinity increment depends on the perturbation strength (or size of the alkalinity addition, <span class="inline-formula">ΔTA</span>) and the number of samples: there is a 90 % chance to meet a minimum detectability requirement with <span class="inline-formula">ΔTA&gt;15</span> <span class="inline-formula">µmol kg⁻¹</span> and sample size <span class="inline-formula">&gt;40</span>, given background variability of 15–30 <span class="inline-formula">µmol kg⁻¹</span>. OAE-induced <span class="inline-formula"><i>p</i>CO₂</span> reduction at the Amazon plume surface would range between 0–25 <span class="inline-formula">µatm</span> when <span class="inline-formula">ΔTA=20</span> <span class="inline-formula">µmol kg⁻¹</span>, decreasing with increasing salinity (<span class="inline-formula"><i>S</i></span>). Adding 20 <span class="inline-formula">µmol kg⁻¹</span> of alkalinity at the river mouth could elevate the total carbon uptake in the Amazon River plume (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">15</mn><mo>&lt;</mo><mi>S</mi><mo>&lt;</mo><mn mathvariant="normal">35</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="59pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="0aef546cad9c3e9dc5cd859dec5d81a5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-1963-2023-ie00001.svg" width="59pt" height="10pt" src="bg-20-1963-2023-ie00001.png"/></svg:svg></span></span>) by at least 0.07–0.1 <span class="inline-formula">Mt CO₂ per month</span>, and a major portion of the uptake would occur in the saltiest region (<span class="inline-formula"><i>S</i>&gt;32</span>) due to its large size, comprising approximately 80 % of the <span class="inline-formula"><i>S</i>&gt;15</span> plume area. However, the lowest-salinity region (<span class="inline-formula"><i>S</i>&lt;15</span>) has a greater drop in surface ocean partial pressure of <span class="inline-formula">CO₂</span> (<span class="inline-formula"><i>p</i>CO₂</span><span class="inline-formula">^sw</span>) due to its low buffer capacity, potentially allowing for observational detectability of <span class="inline-formula"><i>p</i>CO₂</span><span class="inline-formula">^sw</span> reduction in this region. Reduced outgassing in this part of the plume, while more uncertain, may also be important for total additional <span class="inline-formula">CO₂</span> uptake. Such sensitivity tests are useful in designing minimalistic field trials and setting achievable goals for monitoring, reporting, and verification purposes.</p>