Volumeless Organic Matter

Blue carbon ecosystems buffer climate change via sediment carbon capture up to two orders of magnitude faster than terrestrial ecosystems on a per-area basis, 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 self-packing densities. However, our analysis of >23,300 tidal marsh data points from the Coastal Carbon Atlas shows that these methods overestimate organic carbon contribution to long-term sequestration and accretion because they incorporate both dissolved and mineral-associated organic matter. Dissolved and mineral-associated organic matter in surficial uncompacted (0-25 cm) sediments is 36% greater than deeper compacted sediments, suggesting that some of the carbon thought to be sequestered is lost, most likely through porewater flushing, sediment autocompaction, and decomposition, and does not contribute to long-term carbon storage. Neither dissolved nor mineral-associated organic matter contribute 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. Combined, we demonstrate that traditional methods, which are often applied across blue carbon ecosystems, overestimate organic matter contributions to tidal marsh carbon stocks and accretion.