Governing onboard carbon capture residues: a precautionary framework for sea disposal of calcium-looping by-products

International shipping faces increasing pressure to reduce greenhouse gas (GHG) emissions, and onboard carbon capture and storage (OCCS) is emerging as a transitional option for decarbonising parts of the existing fleet. Calcium looping (CaL)-inspired OCCS is technically attractive because it can mineralise captured CO2 into CaCO3-rich residues, but this same feature creates a distinct governance challenge: the captured carbon is no longer only a gas or liquid stream to be offloaded, but a substantial solid or slurry material generated at sea. This study uses a qualitative legal-doctrinal and policy-analysis approach, supported by technology scouting and science-based risk screening, to examine how CaL residues should be classified and governed under the MARPOL–London Convention/Protocol interface. The analysis identifies four interlinked governance gaps: carbon-crediting and traceability, regulatory fit, environmental thresholds and assessment metrics, and institutional responsibility across flag, coastal, and port States. The findings show that CaL residues cannot be treated simply as ordinary operational waste, because their physical state, chemical composition, carbon-accounting role, and potential marine-placement pathways raise questions beyond the current scope of MARPOL Annex V. At the same time, intentional release to seawater would engage LC/LP precautionary logic for deliberate marine inputs and possible geoengineering-style activities. The article proposes a staged precautionary framework that makes legal classification the first decision gate, distinguishes dry solids, wet slurry, and dewatered filter cake, requires auditable custody transfer for climate crediting, recognises current port-reception capacity limitations, and limits any marine placement to exceptional, time-bound, independently reviewed research pathways. The study contributes to marine policy by translating emerging OCCS residue risks into a practical governance framework for future IMO rule-making and port-State implementation.