Abstract
Ocean alkalinity enhancement (OAE) is receiving considerable attention as a carbon dioxide (CO2) removal strategy, and novel approaches to increase the total alkalinity (AT) of the surface ocean are being explored. In bivalve aquaculture, calcification during shell growth consumes AT, thus leading to CO2 emissions. After consumption, shells are typically landfilled or incinerated, which can generate additional CO2 emissions. Here, we investigate how the CO2 footprint of shellfish aquaculture can be reduced by using bivalve shells as a resource for mineral-based OAE. The idea is to grind the calcium carbonate (CaCO3) shells to increase the reactive surface area and distribute them into permeable, oxygen-rich sediments, where their dissolution produces AT that could then compensate the CO2 emitted during calcification. To evaluate this concept, we conducted microcosm incubations of sediments amended with crushed mussel shells (∼ 8 wt %), and monitored the sediment geochemistry and sediment-water exchange over 24 weeks. Control sediments exhibited low and constant CaCO3 dissolution rates (Rdiss = 0.9 ± 0.5mmol m-2d-1) and AT fluxes (FAT = 3.2 ± 1.1mmol m-2d-1). In contrast, shell-amended sediments showed markedly higher Rdiss and FAT values, which exhibited a transient response modulated by oxygen and organic matter availability. Initially, shell dissolution was restricted, most likely by oxygen availability due to the intense mineralization of shell-associated organic matter. Subsequently, following gradual sediment reoxygenation, dissolution rates increased, reaching a maximum Rdiss of 22.7 ± 2.6 after 9 weeks, corresponding to a measured FAT of 43.0 ± 6.0mmol m-2d-1. After that, CaCO3 dissolution rates declined as organic matter availability decreased, approaching control rates, with a slightly elevated Rdiss of 2.2 ± 1.1mmol m-2d-1 at the end of the experiment. After 24 weeks, ∼ 6 % of the initial shell mass had dissolved, and extrapolation of the dissolution rate at the end of the experiment suggests that complete dissolution would require ∼ 38 years. Our results suggest that organic matter availability limits CaCO3 dissolution in the permeable sediment investigated. This constraint, however, can be alleviated by targeting environments with high organic matter deposition for in-situ applications, such as sediments beneath mussel farms, thereby promoting mussel aquaculture circularity.