The Benefits of Undrained Swamps: Late Holocene Variability in Carbon Sequestration by Salt Marshes in Delaware, North Carolina, and Florida.

Abstract

Saltmarshes sequester large quantities of carbon due to the anaerobic conditions in coastal wetland soils. Thick saltmarsh peat layers can also record past environmental conditions at the coast. This study analyzed stable isotope ratios of organic carbon in order to elucidate the mechanisms by which carbon burial rates varied in response to climate and environmental perturbations over the last 2,000 years. Sediment cores from saltmarshes in Delaware, North Carolina, and northeastern Florida from were analyzed for carbon concentration and carbon isotopic composition. The largest isotopic shift (~10 per mille) resulted from paleoecological transition from C3 to C4 vegetation at the Delaware study site, but numerous smaller amplitude variations in δ13Corg and [Corg] are observed that did not result from changes in local marsh vegetation type. Shifts in δ13Corg and [C] of varying amplitude and duration coincide with decadal to multi-centennial climate perturbations such as the Medieval Warm Period, Little Ice Age, and the Maunder sunspot minimum. Greater temporal coherence is observed between the Delaware and North Carolina records, although the slower accretion rate of the Florida reduced its sampling resolution, hampering comparison with the other two sites. The findings indicate that relatively small changes in Late Holocene climate, as well as local environmental factors, cause buried carbon concentrations of East Coast saltmarshes to vary by a factor of two or more. The results also illustrate the potential usefulness and limitations of carbon isotopes in saltmarsh sediments for the reconstruction of past hydroclimate conditions. Overall, the findings imply that carbon sequestration rates of coastal marshes will continue to change in the future.