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

Abstract. Ocean alkalinity enhancement (OAE) is receiving considerable attention as a carbon dioxide (CO₂) 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 CO₂ emissions. After consumption, shells are typically landfilled or incinerated, which can generate additional CO₂ emissions. Here, we investigate whether bivalve shells could be a potential 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 A_T 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 (R_diss = 0.9 ± 0.5 mmol m^-2 d^-1) and A_T fluxes (F_AT = 3.2 ± 1.1 mmol m^-2 d^-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 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 mmol m^-2 d^-1 after 9 weeks, corresponding to a measured F_AT of 43.0 ± 6.0 mmol m^-2 d^-1. After that, CaCO₃ dissolution rates declined as organic matter availability decreased, thus reducing dissolution toward a constant steady-state R_diss of 2.2 ± 1.1 mmol m^-2 d^-1. After 6 months, ~6 % of the initial shell mass had dissolved, and extrapolation of the new quasi-steady-state dissolution rate at the end of the experiment suggests that complete dissolution would take ~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.