Increasing Rates of Carbon Burial in Southwest Florida Coastal Wetlands

Abstract

Rates of organic carbon (OC) burial in some coastal wetlands appear to be greater in recent years than they were in the past. Possible explanations include ongoing mineralization of older OC or the influence of an unaccounted‐for artifact of the methods used to measure burial rates. Alternatively, the trend may represent real acceleration in OC burial. We quantified OC burial rates of mangrove and coastal freshwater marshes in southwest Florida through a comparison of rates derived from 210Pb, 137Cs, and surface marker horizons. Age/depth profiles of lignin: OC were used to assess whether down‐core remineralization had depleted the OC pool relative to lignin, and lignin phenols were used to quantify the variability of lignin degradation. Over the past 120 years, OC burial rates at seven sites increased by factors ranging from 1.4 to 6.2. We propose that these increases represent net acceleration. Change in relative sea‐level rise is the most likely large‐scale driver of acceleration, and sediment deposition from large storms can contribute to periodic increases. Mangrove sites had higher OC and lignin burial rates than marsh sites, indicating inherent differences in OC burial factors between the two habitat types. The higher OC burial rates in mangrove soils mean that their encroachment into coastal freshwater marshes has the potential to increase burial rates in those locations even more than might be expected from the acceleration trends. Regionally, these findings suggest that burial represents a substantially growing proportion of the coastal wetland carbon budget.

Reply to Comment by R. Parkinson on “Increasing Rates of Carbon Burial in Southwest Florida Coastal Wetlands” by J. Breithaupt et al.

“…Parkinson argues that our finding of increased OC burial over the past century in Breithaupt et al (2020) is an artifact of using a density normalization to mathematically correct for auto-compaction. We did not use a density correction as it is not applicable to OC burial measurements; therefore, Parkinson's argument is misplaced.…”

Engineering With Nature : the role of mangroves in coastal protection

“…The black mangrove is the most cold-tolerant Florida species and responsible for the majority of the presumed poleward expansions observed. Mangrove expansion and encroachment into salt marshes can increase carbon storage in mangroves forests, which store 2× more carbon per area compared to salt marshes (Doughty et al 2016;Breithaupt et al 2020). High sedimentation rates, significant peat accumulation from biomass production, and minimal decomposition due to a lack of oxygen make mangroves highly efficient at carbon capture and storage.…”

Salinity reduces coastal marsh respiration more than photosynthesis

“…In subtropical and tropical coastal wetlands, mangroves are expanding -latitudinally 25,26 and landward 27 with rising temperatures and rising seas -into marsh-dominated wetlands. This functional shift from herbaceous and succulent marshes to woody mangrove swamps is a major and global transformation that can provide stability and, in the case of landward migration, may provide a pathway toward enhanced accretion and carbon storage compared to coastal freshwater marshes 28 . A similar transition in abandoned, salinized agricultural lands, initially to woody shrublands 29 , could also provide greater opportunity for carbon storage.…”