Browsing by Author "Boyd, Brandon"
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Item Comparison of sediment accumulation and accretion in impounded and unimpounded marshes of the Delaware Estuary(University of Delaware, 2012) Boyd, BrandonCoastal marshes represent a small land area relative to the ecosystem services they provide. Humans have altered the coastal environment in a number of ways, one being the tidal restriction of marshes. One type of tidal restriction involves marsh impoundments, which are managed by humans to provide ecological benefits including mosquito control and waterfowl habitat. In the state of Delaware, impounded marshes are faced with rising sea level just like their unrestricted counterpart. Currently, coastal managers are concerned about the fate of these systems and are considering what actions (if any) can be taken to preserve them. Accretion, or vertical growth of the marsh platform by accumulation of mineral and organic material, is an important part of understanding marsh elevation change through time. This study compares impounded and unimpounded tidal marshes of coastal Delaware to examine 1) the relative accretionary status of selected managed and natural marshlands, 2) the relative influences of mineral and organic solids accumulation on rates of accretion. Accretion rates were measured using 137Cs and 210Pb radiometric methods, which are commonly used in tidal marsh studies but not always together. This study explores the value of using both radiometric methods and compares the results. Gravimetric, loss-on-ignition, and radionuclide analyses were conducted on over 500 subsamples of 44 marsh cores collected at eleven sites along the western coast of the Delaware Estuary. Radionuclide analysis was performed via gamma spectroscopy, which allows simultaneous measurement of 137Cs and 210Pb activity. Accretion and accumulation rates were calculated using activity-depth profiles of 137 Cs and excess 210Pb, using the 1964 reference horizon and the Constant Initial Concentration model, respectively. The two radionuclide methods were found to provide similar results, suggesting that 137Cs and 210Pb are effective sediment chronometers in the study area. For unimpounded marshes, mineral and organic mass accumulation averaged 0.22 g cm-2 y-1 and 0.06 g cm-2 y-1 , respectively. For impounded marshes, mineral and organic mass accumulation averaged 0.08 g cm-2 y-1 and 0.03 g cm-2 y-1 , respectively. Accretion rates for unimpounded marshes averaged 0.57 cm y-1 and, for unimpounded marshes, 0.28 cm y-1. Overall, rates of accumulation and accretion determined for this study were comparable to rates reported in the literature for U.S. East Coast marshes. Impounded marshes investigated exhibited lower accretion rates and lower mineral sediment inventories than the unimpounded marshes. Impounded and unimpounded marshes were found to show a similar direct relationship between accretion rates and accumulated mass; accretion increased with increasing organic and mineral mass accumulation. However, accretion was more sensitive to organic accumulation than mineral sediment accumulation. In the case of impounded marshes, accretion rates appear to be limited by mineral sediment accumulation. The implication is that these marshes are deficient in suspended mineral sediment supplied by tidal flooding and deposition, perhaps due to the impoundment works. Accretion rates determined for the impounded marshes (0.11--0.72 cm y-1 range, 0.25+-0.16 cm y-1 mean) fell at or below the rate of relative sea-level rise for middle Delaware Estuary (0.36+-0.06 cm y-1 at Reedy Point). Where the rate of marsh accretion is deficient, coastal flooding and inundation related to future sea-level rise will be most pronounced. Coastal managers should consider sediment management among the various adaptation strategies implemented by the state of Delaware to mitigate effects of rising sea level on the impounded marshlands.Item A radiometric study of sediment accumulation and accretion in tidal marshes of Delaware and New Jersey(University of Delaware, 2016) Boyd, BrandonThe 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.