Long-term incubations reveal geochemical controls on wood biomass preservation at the anoxic sediment-water interface

Ocean-based carbon dioxide removal (CDR) strategies increasingly consider biomass burial in anoxic marine basins as a pathway for long-term carbon storage. To evaluate its stability and environmental impact, we conducted 14-month bottle incubations testing three configurations of terrestrial wood under anoxic conditions: suspended in the water column, placed on the sediment surface, and buried within sediments. Across all treatments, wood remained highly preserved: >97.5% carbon retained, based on C content, ~98.3% via carbon budgets, and >96% based on dry mass retained. Geochemical signals indicated that sulfate reduction was the dominant terminal pathway: dissolved inorganic carbon and alkalinity increased in nearly a 1:1 stoichiometry, methane yields were low relative to Dissolved Inorganic Carbon (DIC), and sulfide was presumably sequestered into iron sulfide minerals. Microbial community analyses corroborated these patterns, showing enrichment of fermenters and sulfate reducers with only minor contributions from methanogens. Dissolved organic carbon (DOC) dynamics further revealed sediment contact as a key control on DOC retention, with suspended wood yielding the most significant DOC accumulation. We observed no mobilization of toxic metals and only minor changes in major cations during the 14 months. Together, these results demonstrate that geochemical and microbial processes act in concert to preserve biomass carbon under anoxic conditions, highlighting marine burial as a durable, low-risk CDR strategy.