Towards circularity in shellfish aquaculture: stimulating mussel shell dissolution in marine sediments

Ocean alkalinity enhancement (OAE) is receiving considerable attention as a carbon dioxide (CO 2 ) removal strategy, and novel approaches to increase the total alkalinity ( A T ) of the surface ocean are being explored. In bivalve aquaculture, calcification during shell growth consumes A T , thus leading to CO 2 emissions. After consumption, shells are typically landfilled or incinerated, which can generate additional CO 2 emissions. Here, we investigate how the CO 2 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 (CaCO 3 ) shells to increase the reactive surface area and distribute them into permeable, oxygen-rich sediments, where their dissolution produces A T that could then compensate the CO 2 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 CaCO 3 dissolution rates ( R diss = 0.9 ± 0.5 mmolm-2d-1) and A T fluxes ( F AT = 3.2 ± 1.1 mmolm-2d-1). In contrast, shell-amended sediments showed markedly higher R diss and F AT 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 R diss of 22.7 ± 2.6 mmolm-2d-1 after 9 weeks, corresponding to a measured F AT of 43.0 ± 6.0 mmolm-2d-1. After that, CaCO 3 dissolution rates declined as organic matter availability decreased, approaching control rates, with a slightly elevated R diss of 2.2 ± 1.1 mmolm-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 CaCO 3 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.