Influence of Deep‐Sea Carbonate Sediments on the Long‐Term Durability of Carbon Storage From Ocean Alkalinity Enhancement

Abstract

Ocean Alkalinity Enhancement (OAE) is a marine carbon dioxide removal (CDR) strategy with a theoretical sequestration potential of several Gt . The long‐term durability of OAE‐induced carbon storage depends on the persistence of the added alkalinity in the ocean, which is influenced by sedimentary processes and biogeochemical and physical feedbacks. Using the University of Victoria v2.10 Earth System Model of intermediate complexity, we investigated the millennial‐scale durability of OAE‐induced extra alkalinity and associated additional carbon storage and, specifically, the role of deep‐sea calcium carbonate sediments in limiting the durability. We conducted 10,000‐year simulations for four emissions scenarios, each combined with a global alkalinity addition experiment (0.135 Pmol for 50 years) with and without interactive carbonate sediments. Neither the added alkalinity nor the associated oceanic carbon storage remained fully stable on millennial timescales compared to a baseline simulation. By year 10,000, 35%–59% of the initial alkalinity increase through OAE is lost due to dynamics in deep‐sea sediments, resulting in a reduction of the OAE‐induced carbon storage by 14%–27% compared to simulations without sediments. Enhanced alkalinity reduces the natural sedimentary climate feedback associated with dissolution. Additionally, outgassing to the atmosphere as a response to reduced atmospheric has been identified as a limiting factor for the durability of OAE‐induced carbon storage in the ocean, possibly affecting all CDR methods. Episodic Southern Ocean deep convection events further contributed to variability in Earth system responses during global cooling.