Ocean Alkalinity Enhancement (OAE) is a negative emissions technology that aims to chemically sequester CO₂ in the ocean while also counteracting ocean acidification. Yet, experimental work to guide potential implementation at scale is scarce. To address knowledge gaps surrounding durability and application methods in close-to-natural conditions, <em>in situ</em> mesocosm experiments were conducted in subtropical and temperate pelagic ecosystems. In both, we simulated OAE scenarios in a gradient approach and regularly monitored the carbonate system for 30 and 48 days after adding alkaline solutions. In the subtropical experiment, a CO₂-equilibrated approach with 0-2400 umol Kg^-1 of added Total Alkalinity (TA) was implemented to focus on whether the added alkalinity and associated sequestered carbon remained in the system. In the temperate experiment, a non-equilibrated approach with 0-600 umol Kg^-1 added TA was used to observe and follow the ingassing of CO₂. We found that TA was stable over time in both ecosystems, except for the highest +2400 umol Kg^-1 treatment, in which alkalinity loss via secondary precipitation of aragonite started to occur 12 days after manipulation. Furthermore, an increase in DIC associated with CO₂ uptake was barely observed in the non-equilibrated set-up for the timeframe of the experiment. Here we will discuss these equilibrated and non-equilibrated deployment methods of OAE in natural settings and their potential implications for deployment at scale.

Primary Presenter: Julieta Schneider, GEOMAR (jschneider@geomar.de)

Authors:

Julieta Schneider, GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany  (jschneider@geomar.de)

Allanah Paul, GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany  (apaul@geomar.de)

Charly Moras, Southern Cross University, Australia  (c.moras.10@student.scu.edu.au)

Kai Schulz, Southern Cross University, Australia  (kai.schulz@scu.edu.au)

Ulf Riebesell, GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany  (uriebesell@geomar.de)