Accumulation of sediment and radionuclides in tidal marshes of the Murderkill River Estuary, Delaware
Date
2010
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Publisher
University of Delaware
Abstract
This thesis examines patterns and rates of sediment accumulation and tidal marsh accretion in the Murderkill River estuary, Delaware, with special emphasis placed on changes in accumulation rates and sediment physical properties associated with historical land-use practices, such as mosquito ditching. Over 90% of United States Atlantic East coast salt marshes have been ditched to some degree, but little quantitative work has been done to examine the specific effects on marsh sedimentary processes. An understanding how these ditches have affected sediment delivery to and retention on the marsh platform will provide insight into how ditched marshes are likely to respond to changes in sea level, sediment availability and vegetative growth. To investigate historical changes in sediment composition of the marsh sediment column, the specific contributions of mineral and organic solids and water/entrapped gas were determined from measurements of sediment dry-bulk density and loss-on-ignition. Additionally, grain-size analysis was conducted to determine the textural composition of sedimentary particles delivered to the marsh. Downcore profiles of the radionuclides 210Pb and 137Cs were used to determine sediment accumulation and marsh accretion rates, to develop chronologies for the sediment column, and also as indicators of sediment transport pathways within the marsh. In addition, an historical investigation was conducted to learn more about past land-use practices and sources of human disturbance in the estuarine system. Spatial and temporal variations in the relative contributions of organic and mineral matter to total sediment volume show that marsh accumulation in the Murderkill is dominated by mineral matter and that the organic contribution has varied little over the past century. At two upriver sites, changes in dry-bulk density, loss-on-ignition and grain size were observed and interpreted to correspond to the transition from freshwater marsh to brackish marsh. Although the cause of this change is unknown, it cannot be directly associated with mosquito ditching in the lower estuary due to the distance separating these sites from ditched areas. Vertical accretion rates (0.3−0.7 cm/yr) determined for undisturbed marsh sites are comparable to rates computed for other salt marshes of the greater Delaware Estuary. Radionuclide focusing factors for the marsh sites indicate that the Murderkill is a well-mixed estuary, consistent with results from a previous study of hydrodynamics of the Murderkill River. Temporal changes in accumulation (organic vs. mineral) were observed at one inter-ditch site, but the change cannot be directly associated with ditching activities because similar trends are not evident in other inter-ditch cores. Ditches filled rapidly with mineral mud after being excavated but have not matched the adjacent marsh elevation, possibly because they are acting as the preferred tidal pathways into and out of the marsh. Ditches have not significantly affected the median grain size and sorting of inter-ditch or ditch sites compared to a similarly located non-ditched location. There is no evidence to suggest ditches have deprived the marsh platform of suspended sediment in order to infill. The sedimentary record suggests that the effects of ditching on the adjacent marsh platform are subtle and not always similar among depositional sites. Thus, the results of this study suggest that there is no specific sedimentary response to ditching in ditched or non-ditched areas of the same marsh.