Shellfish carbonate in sediments: assessing historical baselines, dissolution dynamics, and carbon storage beneath shellfish farms, Marlborough, New Zealand

Inorganic carbon (IC) in the form of calcium carbonate (CaCO₃) produced by shellfish is a key but often overlooked component of estuarine sediments, now threatened by the decline of calcifying biogenic habitats. While biocalcification can be potentially a net emitter of CO₂, shellfish shells also preserve paleobiological records that can inform pre-impact baselines and support ecosystem restoration alongside aquaculture developments. However, global stressors—including overharvesting, benthic disturbance, and increased aquaculture—have distorted the sedimentary record of CaCO₃. We analysed sediment cores beneath three ca.40-year-old green-lipped mussel Perna canaliculus farms to assess shell deposition rates before and after aquaculture development. Mussel shells, including those from fouling species Mytilus galloprovincialis, dominated both periods, but post-farming deposition rates increased by an average of 8-fold. Old shells in deeper sediments were from historic wild mussel reefs reduced to ca.3% of former biomass by the 1970s. We found statistically significant yet minor differences in OC and IC content between old and recent shells. These results demonstrate how shell accumulation beneath farms preserves valuable ecological archives and enhances benthic carbon stocks. Our findings offer a novel tool for aquaculture managers to reconstruct baseline conditions, monitor carbon contributions, and inform the use of shell waste in coastal rehabilitation strategies on soft sediments.