Dataset: Increased Organic Carbon Burial in Northern Florida Mangrove-Salt Marsh Transition Zones

Blue carbon habitats like salt marshes and mangroves bury large amounts of carbon with limited area; however, they also are increasingly susceptible to current climate change. Combined effects of rising temperatures, decreasing freeze frequencies, and increasing sea-level rise rates are resulting in mangrove replacement of salt marshes along the southern United States. Studies on mangrove migration have revealed conflicting results regarding carbon burial, with some suggesting mangrove migration does not increase burial. Surface soils analyzed here from wetlands along northern Florida Atlantic and Gulf coasts showed higher apparent sedimentation rates in mangrove-dominated sites (1.5-3.2 mm yr^-1) and where mangroves are migrating into the marsh (termed transition sites, 2.3-3.8 mm yr^-1). Average carbon burial rates were higher in transition sites for both coasts (27-47 gC m^-2 yr^-1) compared to the respective mangrove (10-22 gC m^-2 yr^-1) and salt marsh (4-7 gC m^-2 yr^-1) sites. Lignin biomarker data (Λ-6, Λ-8, C/V) indicated mangrove and transition sites had higher lignin inputs from woody vascular plants compared to salt marsh sites, which may slow decadal to centennial-scale decay. Higher amino acid concentrations in mangrove soils relative to mangrove biomass (1.8-2.3 mmol gC^-1 vs. 0.2-0.9 mmol gC^-1) and lower C/N indicated these mangrove sites receive higher algal inputs than the transition and salt marsh sites, attributed to greater tidal inundation in the mangrove sites given their position near the shoreline. Overall, increased accretion, carbon burial, and lignin in mangrove transition sites indicates this migration may increase carbon burial and increase the stability and residence time of buried soil carbon. Future studies on mangrove migration in northern Florida can verify this through replication and elevation analysis.

Although carbon burial rates increased with mangrove expansion at the northern Florida sites, questions remained regarding future changes in these wetlands with a warming climate and sea-level rise. To better understand potential future responses of these wetlands, two studies were undertaken using longer sediment cores at the same northern Florida sites to examine the wetlands development over the mid- to late-Holocene. Carbon measurements within 1-3 meter length vibracores yielded total core stocks of 9.8 x 10³ – 2.1 x 10⁴ gC m^-2 and 7.4 x 10³ – 9.5 x 10³ gC m^-2 for the Atlantic and Gulf coast cores, respectively. Following recent IPCC guidelines, blue carbon stock estimates down to 1-meter were 6.8 x 10³ gC m^-2 - 7.3 x 10³ gC m^-2 and 5.8 x 10³ gC m^-2 - 8.6 x 10³ gC m^-2 for the Atlantic and Gulf cores, respectively. Changes in stable isotopic (δ¹³C, C/N) and lignin biomarker (C/V) indices suggest both coastlines likely experienced salt marsh and mangrove transgressions into former non-blue carbon habitats during the mid- to late-Holocene following changes in relative sea-level rise and climate. This suggests that not all blue carbon was estimated in the Atlantic coast cores when restricting measurements to the 1-meter benchmark and some carbon deposited in a carbonate-dominant system was included in the 1-meter Gulf coast stocks. Constraining blue carbon measurements to include carbon within 1-meter thus may over- or under-estimate blue carbon stocks. If the natural carbon burying capabilities of these habitats are to be utilized in the future, preservation of regions upslope of wetlands is critical to allow for inland migration in response to ongoing sea-level rise.