Considerations for hypothetical carbon dioxide removal via alkalinity addition in the Amazon River watershed

Abstract

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, ΔTA) and the number of samples: there is a 90 % chance to meet a minimum detectability requirement with ΔTA>15 µmol kg−1 and sample size >40, given background variability of 15–30 µmol kg−1. OAE-induced pCO2 reduction at the Amazon plume surface would range between 0–25 µatm when ΔTA=20 µmol kg−1, decreasing with increasing salinity (S). Adding 20 µmol kg−1 of alkalinity at the river mouth could elevate the total carbon uptake in the Amazon River plume (15<S<35) by at least 0.07–0.1 Mt CO2 per month, and a major portion of the uptake would occur in the saltiest region (S>32) due to its large size, comprising approximately 80 % of the S>15 plume area. However, the lowest-salinity region (S<15) has a greater drop in surface ocean partial pressure of CO2 (pCO2sw) due to its low buffer capacity, potentially allowing for observational detectability of pCO2sw reduction in this region. Reduced outgassing in this part of the plume, while more uncertain, may also be important for total additional CO2 uptake. Such sensitivity tests are useful in designing minimalistic field trials and setting achievable goals for monitoring, reporting, and verification purposes.