Current methods overestimate coastal blue carbon potential

Blue carbon ecosystems buffer climate change via sediment carbon capture, gaining elevation and mitigating sea level rise in the process. Carbon sequestration and accretion estimates share a common methodology, whereby dry masses are converted to volume using densities. However, our analysis of 23,302 tidal marsh sediment cohorts shows that these methods overestimate carbon contribution to long‐term sequestration and accretion because they incorporate both dissolved and mineral‐associated organic matter. Neither dissolved nor mineral‐associated organic matter contributes to sediment volume; thus, the volumetric budgets underlying estimates of organic matter contribution to predicted marsh resilience are inflated by up to 380% in the top 25‐cm. This “volumeless” organic matter in surficial, uncompacted sediments is 36% greater than deeper sediments, suggesting that some carbon thought to be sequestered is lost and does not contribute to long‐term storage. Combined, we demonstrate that traditional methods overestimate organic matter contributions to blue carbon stocks and accretion.