Modern Changes and Millennial-Scale Development of Blue Carbon Pools in Northern Florida

Salt marshes and mangroves are recognized globally as blue carbon habitats, burying large amounts of carbon within relatively limited area. Recent temperature increases have driven mangroves poleward into salt marsh habitats, potentially altering their carbon burial. Studies on mangrove migration have revealed conflicting results regarding carbon burial, suggesting mangrove migration may not increase burial. Surface soils from northern Florida Atlantic and Gulf coast sites showed that mangrove-dominated sites had higher sedimentation (1.5-3.0 mm yr^-1) and carbon burial rates (244-435 gC m^-2yr^-1) over the last ~100 years. Lignin biomarker (Λ-6, Λ-8, C/V), bulk soil carbon/nitrogen ( δ¹³C and C/N), and amino acid (THAA, AA N%, and Hyp) data indicate this may be related to lignin input increases from woody vascular plants combined with high amino acid inputs from algal sources. Additionally, the further a mangrove site was from tidal sources, the greater its carbon burial ability. This was likely, in part, due to wave and tidal erosive activities.

To better understand the potential carbon stocks of these wetlands over millennial timescales, longer soil cores (1-3 m) were used as records for blue carbon development over the mid- to late-Holocene. Both coasts had changes in bulk and biomarker parameters that suggested wetlands migrated inland as sea-level rose throughout the Holocene; however, there were slight differences in the habitats that may have developed, transitioning out of more upland habitats (e.g. maritime hammocks and freshwater marshes) prior to the development of modern-day blue carbon habitats. Additionally, the Gulf coast sites had lower carbon stocks normalized to core depth ( 2.2 x 10⁵gC m^-1- 3.1 x 10⁵gC m^-1) compared to the Atlantic coast cores (2.4 x 10⁵gC m^{-`</sup>- 4.9 x 10<sup>5</sup>gC m<sup>-`}), opposing suggestions that carbon stocks are higher along the Gulf when just considering the top 1 m. Carbon stocks from both coasts, however, would have been influenced by the aforesaid wetland migration and thus may partially represent carbon buried in non-blue carbon habitats. In order to better understand the extent of carbon stocks in blue carbon habitats, further research should consider the depths chosen to calculate carbon stocks and whether all of that carbon was deposited in a blue carbon habitat.