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

Abstract

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 explore a thought experiment of ocean alkalinity enhancement (OAE) via hypothetical 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. Adding 20 μmol kg-1 of alkalinity at the river mouth could elevate the total carbon uptake in the Amazon River plume by 0.07–0.1 MtCO2 month-1. Such thought experiments are useful in designing minimalistic field trials and setting achievable goals for monitoring, reporting, and verification purposes.

Monitoring, Reporting, and Verification for Ocean Alkalinity Enhancement

“…For example, an increase in alkalinity of 10 µmol kg −1 would result in a decrease in pCO 2 of around 20 µatm (given temp = 20 • C; salinity = 35; initial TA = 2200 µmol kg −1 ; DIC = 1965 µmol kg −1 , and no secondary precipitation or biotic calcification). Historical carbonate system variability, like the examples given here, can be used in sensitivity studies to assess the detectability of a given OAE perturbation for different observing systems (Mu et al, 2023).…”

“…Measurements of total alkalinity (TA) and DIC are important to quantify the background state of the carbon system, which determines the pCO 2 response per unit change in alkalinity. Further, measurements of TA might help verify that alkalinity has been added effectively, although signal-tonoise ratios may be insufficiently strong to enable robust detection and attribution of TA anomalies (Mu et al, 2023). In addition, pH is an important measurement to ensure that the OAE deployment conforms with water quality limits (usually pH < 9) and that the deployment does not create conditions that induce precipitation.…”

“…Though appealing in its comprehensiveness, the reality of observing all of the parameters needed to quantitatively close a perturbed carbon budget and compare it against an unperturbed counterfactual is likely impossible in the near to medium term, even in the context of highly monitored field trials. The difficulty is inherent in the fact that the patch of water perturbed by the addition of TA is likely to be turbulently dispersed in the ocean, and its signal diluted below the limit of detectability by mixing over the timescale required for CO 2 equilibration (He and Tyka, 2023; Mu et al, 2023;Wang et al, 2023).…”

“…Both cases will constitute CDR as it leads to a net increase in DIC in the ocean reservoir . Geographic patterns of CO 2 ingassing and outgassing are controlled by the ocean's large-scale and subtropical overturning circulations (e.g., Iudicone et al, 2016), mesoscale and submesoscale motions (e.g., Nakano et al, 2011;Ford et al, 2023), variations in winds (e.g., Andersson et al, 2013;Nickford et al, 2022), storms (e.g., Nicholson et al, 2022), upwelling dynamics, local inputs from rivers (e.g., Mu et al, 2023), exchanges with sediments, and biology (e.g., Huang et al, 2023). Outside the tropics, there is pronounced seasonal variability in air-sea CO 2 fluxes mostly driven by phytoplankton blooms that draw down CO 2 in the surface ocean during spring and summer (e.g., Fassbender et al, 2022), and winter mixing that brings carbon-rich waters to the surface.…”

Monitoring, reporting, and verification for ocean alkalinity enhancement

“…Both cases will constitute CDR as it leads to a net increase in DIC in the ocean reservoir . Geographic patterns of CO 2 ingassing and outgassing are controlled by the ocean's large-scale and subtropical overturning circulations (e.g., Iudicone et al, 2016), mesoscale and submesoscale motions (e.g., Nakano et al, 2011;Ford et al, 2023), variations in winds (e.g., Andersson et al, 2013;Nickford et al, 2022), storms (e.g., Nicholson et al, 2022), upwelling dynamics, local inputs from rivers (e.g., Mu et al, 2023), exchanges with sediments, and biology (e.g., Huang et al, 2023). Outside the tropics, there is pronounced seasonal variability in air-sea CO 2 fluxes mostly driven by phytoplankton blooms that draw down CO 2 in the surface ocean during spring and summer (e.g., Fassbender et al, 2022), and winter mixing that brings carbon-rich waters to the surface.…”