The capacity of wetlands to mitigate greenhouse gas (GHG) emissions is the sum of two services–the protection of vulnerable organic stocks from remineralisation, and the capacity to sequester GHGs relative to their anthropogenic replacements. Organic carbon accumulation (CA) down through the sediment column is often taken as the measure of sequestration because of its capacity to record long-term variability and trends. However, we demonstrate that: i) CA is not equivalent to sequestration as net ecosystem production (NEP) for open systems; it requires the subtraction of the initial deposition rate of labile allochthonous carbon sources; ii) CA also requires subtraction of intrinsically allochthonous recalcitrants down through the sediment column, and together with subtraction of autochthonous recalcitrants from organic stock services; iii) CA as a climatic mitigation service also requires a diagenetic correction, as the annual deposition of labile organic carbon continues to remineralise over the long-term; and iv) preserving of a wetland has a significantly greater mitigation potential than restoring one. To address the above concerns, a global diagenetic solution is proposed, applied, and tested for a tropical seagrass and mangrove. As expected traditional CA estimates were disproportionately larger than their respective cal. NEPs and together with stocks fell within the ranges reported in the literature, with a final carbon accreditation highly dependent on the choice of their anthropogenic replacements. The review demonstrates that mitigation concepts and measurements for natural carbon sequestration solutions require re-evaluation to avoid GHG emissions above their capacity or reduce the ability to fulfil emission targets.