A radiometric study of sediment accumulation and accretion in tidal marshes of Delaware and New Jersey

Sommerfield, ChristopherThe goal of this dissertation research was to develop new insight into the application of radionuclide methods (210Pb, 137Cs, 241Am) in tidal marshes for studies of geochronology and geomorphology. A comprehensive radionuclide dataset was generated by gamma spectroscopy of samples from eighteen marsh soil cores collected in two Mid-Atlantic estuaries, the Delaware Bay and the Barnegat Bay. The research was carried out in four separate studies, all of which involved different aspects of radionuclide measurements as applied to tidal marsh deposits. The mineral-rich estuarine marshes of the Delaware Bay provided a contrast to the organic-rich coastal marshes of the Barnegat Bay. The close proximity of these sites in southern and central New Jersey minimized climatic and oceanographic variation that must be taken into account when comparing marsh accretionary processes for different regions. Additionally, the atmospheric flux of 210Pb, 137Cs, 241Am to the marshes can be assumed to be nearly identical considering the proximity of the study sites, which allowed for direct comparison of radionuclide inventories and chronologies. Marsh sites in both estuaries were dominated by the halophyte Spartina alterniflora minimizing biotic variations in the soil forming process. The marshes of Delaware Bay were found to have higher rates of mineral sediment accumulation compared to the Barnegat Bay marshes. This difference was at least partly due to the larger supply of mineral sediment in Delaware Bay, a consequence of the estuarine turbidity maximum and its entrapment of sediment supplied to the estuary. Barnegat Bay had more frequent and longer flooding but mineral sediment accumulation in marshes there was low. Barnegat Bay marsh accretion rates were found to be equal to or less than rates of relative sea-level rise, whereas Delaware Bay marshes were found to outpace relative sea-level rise. The applicability of radionuclide chronology models (210Pb, 137Cs), originally developed for marine and lacustrine deposits, were objectively tested for salt marshes in the study area. The biophysical nature of marsh accretion – the in situ accumulation of organic matter and tidal deposition of mineral sediments – requires careful consider of the models’ assumptions. The range of soil compositions from sites sampled for this research provided a unique case study of model performance in both organic- and mineral-rich marshes, and recommendations for best practices were offered. Part of this analysis involved separation and radionuclide analysis of size-fractions of marsh surface soils, the results of which raise questions concerning traditional assumptions of how 210Pb and 137Cs are delivered to the marsh surface. Detection of 241Am in marsh deposits, a consequence of the decay of fallout 241Pu, provides a new chronological tool that will become increasingly useful as 137Cs decays to extinction. The results of this study have implication for numerical modeling of marsh accretion, chronology of mixed sediments, and contaminant transport and burial in the marsh-estuary system.University of Delaware, School of Marine Science and PolicyPh.D